WO2011033322A2

WO2011033322A2 - Crystal structure

Info

Publication number: WO2011033322A2
Application number: PCT/GB2011/000021
Authority: WO
Inventors: Andrew Stephen DORÉ; Nathan Jacob Robertson; Benjamin Gerald Tehan; Fiona Hamilton Marshall
Original assignee: Heptares Therapeutics Ltd
Current assignee: Nxera Pharma UK Ltd
Priority date: 2011-01-07
Filing date: 2011-01-07
Publication date: 2011-03-24
Anticipated expiration: 2013-07-07
Also published as: WO2011033322A3

Abstract

The invention provides a method of predicting a three dimensional structural representation of a target protein of unknown structure, or part thereof, comprising: providing the coordinates of lhe adenosine A_2A receptor structure listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; and predicting the three-dimensional structural representation of the target protein, or part thereof, by modelling the structural representation on all or the selected coordinates of the adenosine A_2A receptor. The invention also provides the use of the adenosine A_2A receptor coordinates to select or design one or more binding partners of adenosine A_2A receptor.

Description

CRYSTAL STRUCTURE

The present invention relates to protein crystal structures and their use in identifying protein binding partners and in protein structure determination. In particular, it relates to the crystal structure of an adenosine A2A receptor and uses thereof.

The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

G protein-coupled receptors (GPCRs) are integral membrane proteins mediating the signalling of a diverse set of ligands including neurotransmitters and metabolites. In humans, there are approximately 370 non-sensory receptors, representing the site of action for ~30% of clinically used drugs. Activation of the receptor results in a conformational change propagated to the intracellular surface where the receptor interacts with heterotrimeric G proteins to regulate signalling to ion channels and enzyme pathways. GPCRs can also signal independently of G proteins through β-arrestin and are known to exist as dimers. This multimeric state is likely central to their function and also delivery to the membrane, however, the molecular mechanism and details of GPCR oligomerization remains poorly understood.

The adenosine A2A receptor is one of 4 GPCRs (A_{1 t} A2A, A₂B_> A₃) activated by adenosine. Adenosine represents an important modulator of the central nervous system and periphery. In the brain adenosine controls neuronal excitability and the psychoactive effects of caffeine are mediated by adenosine receptors. A_¾ receptors are located in the striatum and are considered a target for neurodegenerative disease¹. A2A receptors are also expressed on the vasculature and immune cells where they have vasodilatory and anti-inflammatory effects ^{2 3}. There is growing evidence that drugs acting at adenosine receptors represent promising approaches in a wide range of diseases ³. Mechanistic understanding of ligand binding and activation, as well as our ability to design drugs for GPCRs is hampered by the lack of structural information. Structures of rhodopsin in conformationally distinct states have indicated that receptor activation results in an outward tilt of transmembrane helix (TM) 6 and a rearrangement of hydrogen bonding networks connecting the ligand binding pocket to the cytoplasmic face. The first non- rhodopsin GPCR structure to be obtained was the β₂^Γβηβ^ίο receptor in complex with an antibody fragment bound to the third intracellular loop (ICL) 3 - a critical domain of the receptor which mediates G protein coupling". A higher resolution structure of the p₂-AR was obtained by fusing T4 lysozyme into ICL3⁵ and the same methodology was used to obtain the first structure of the adenosine A_¾ receptor (A2A-T L)⁶. The conformation of these receptors remains unclear since insertion of the T4 lysozyme alters the pharmacology and prevents signalling. A structure has also been obtained for the p adrenergic receptor (3iAR) using a mutagenesis approach to stabilise the antagonist state⁷. This was the first non- rhodopsin structure to clearly show features of the cytoplasmic regions of the receptor and revealed the presence of a short well defined helix in ICL2. However, in this structure ICL3 was truncated to assist in crystallisation.

The inventors have now solved the structure of the adenosine A^ receptor in complex with the inverse agonist ZM-241 ,385. The structure of the adenosine AZA receptor described here provides new insight into the structural features which define the GPCR inactive state, the regions which interact with signal transduction proteins, and how receptors interact to form signalling complexes.

The coordinates of the adenosine Α2Α receptor can be utilised and manipulated in many different ways with wide ranging applications including the fitting of binding partners, homology modelling and structure solution, analysis of ligand interactions and drug discovery.

Accordingly, a first aspect of the invention provides a method of predicting a three dimensional structural representation of a target protein of unknown structure, or part thereof, comprising:

providing the coordinates of the human adenosine A2A receptor structure listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; and

predicting the three-dimensional structural representation of the target protein, or part thereof, by modelling the structural representation on all or the selected coordinates of the adenosine A2A receptor.

By a 'three dimensional structural representation' we include a computer generated representation or a physical representation. Typically, in all aspects of the invention which, feature a structural representation, the representation is computer generated. Computer representations can be generated or displayed by commercially available software programs. Examples of software programs include but are not limited to QUANTA (Accelrys .COPYRIGHT.2001 , 2002), O (Jones et al., Acta Crystallogr. A47, pp. 1 10-1 19 (1991 )) and RIBBONS (Carson, J. Appl. Crystallogr., 24, pp. 9589-961 (1991 )), which are incorporated herein by reference. Examples of representations include any of a wire-frame model, a chicken-wire model, a ball-and-stick model, a space-filling model, a stick model, a ribbon model, a snake model, an arrow and cylinder model, an electron density map or a molecular surface model. Certain software programs may also imbue these three dimensional representations with physico-chemical attributes which are known from the chemical composition of the molecule, such as residue charge, hydrophobicity, torsional and rotational degrees of freedom for the residue or segment, etc. Examples of software programs for calculating chemical energies are described below.

Typically, the coordinates of the adenosine A^ receptor structure used in the invention are those listed in Table (i) or Table (ii), preferably those listed in Table (i). However, it is appreciated that it is not necessary to have recourse to the original coordinates listed in Table (i) or Table (ii), and that any equivalent geometric representation derived from or, obtained by reference to the original coordinates may be used. For example, as described in Example 4, the A^ - StaR2-ZM-241 ,385 crystal diffraction data can be processed, scaled and model coordinates built and successfully refined (as measured by R _WOR /RFREE statistical validation) in five different space groups. One such group is P1 , which contains 4 copies of the A₂A - StaR2-ZM-241 ,385 molecule and the coordinates of one of the molecules are listed in Table (ii). It is possible to generate coordinates corresponding to the other three molecules by applying appropriate rotation and translation matrices to the coordinates of Table (ii) (see Example 4), and it will be appreciated that any such generated coordinates may be used.

Thus, for the avoidance of doubt, by 'the coordinates of the adenosine A₂A receptor structure listed in Table (i) or Table (ii)', we include any equivalent representation wherein the. original coordinates have been reparameterised in some way. For example, the coordinates in Table (i) or Table (ii) may undergo any mathematical transformation known in the art, such as a geometric transformation, and the resulting transformed coordinates can be used. For example, the coordinates of Table (i) or Table (ii) may be transposed to a different origin and/or axes or may be rotated about an axis. Furthermore, it is possible to use the coordinates to calculate the psi and phi backbone torsion angles (as displayed on a Ramachandran plot) and the chi sidechain torsion angles for each residue in the protein. These angles together with the corresponding bond lengths, enable the construction of a geometric representation of the protein which may be used based on the parameters of psi, phi and chi angles and bond lengths. Thus while the coordinates used are typically those in Table (i) or Table (ii), the inventors recognise that any equivalent geometric representation of the adenosine A2A receptor structure, based on the coordinates listed in Table (i) or Table (ii), may be used.

Additionally, it is appreciated that changing the number and/or positions of the ligand molecule of the Tables does not generally affect the usefulness of the coordinates in the aspects of the invention. Thus, it is also within the scope of the invention if the number and/or positions of ligand molecules of the coordinates of Table (i) or Table (ii) is varied.

It will be appreciated that in all aspects of the invention which utilise the coordinates of the adenosine A2A receptor, it is not necessary to utilise all the coordinates of Table (i) or Table (ii), but merely a portion of them, e.g. a set of coordinates representing atoms of particular interest in relation to a particular use. Such a portion of coordinates is referred to herein as 'selected coordinates'.

By 'selected coordinates', we include at least 5, 10 or 20 non-hydrogen protein atoms of the adenosine A2A receptor structure, more preferably at least 50, 100, 200, 300, 400, 500, 600, 700, 800 or 900 atoms and even more preferably at least 1000, 1 100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100 or 2200 non-hydrogen atoms. Preferably the, selected coordinates pertain to at least 5, 10, 20 or 30 different amino acid residues (i.e. at least one atom from 5, 10, 20 or 30 different residues may be present), more preferably at least 40, 50, 60, 70, 80 or 90 residues, and even more preferably at least 100, 150, 200, 250 or 300 residues. Optionally, the selected coordinates may include one or more ligand atoms as set out in Table (i) or Table (ii). Alternatively, the selected coordinates may exclude one or more atoms of the ligand.

In one example, the selected coordinates may comprise atoms of one or more amino acid residues that contribute to the main chain or side chain atoms of a binding region of the adenosine A2A receptor. For example, amino acid residues contributing to the ligand binding site include amino acid residues Tyr 9, Glu 13, He 60, Phe 62, Ala 63, He 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278, according to the numbering of the adenosine A_∑A receptor sequence as set out in Figure 15, all of which are within 7 A of the ZM-241 ,385 ligand. Thus the selected coordinates may comprise one or more atoms from any one or more (e.g. at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, or 39 amino acid residues) of amino acid residues Tyr 9, Glu 13, lie 60, Phe 62, Ala 63, He 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278, according to the numbering of the adenosine A_¾ receptor as set out in Figure 15. Typically, coordinates of all of the atoms of the side chain are selected.

In a further example, the selected coordinates may comprise atoms of one or more amino acid residues which constitute a dimerisation interface. An interesting observation of the adenosine A_∑A structure is a flat hydrophobic face between TM1 and TM4 believed to represent a dimerisation interface. The interface is defined by the following residues: amino acid residues Val 25, Cys 28, Trp 29, Val 31 , Trp 32, Ser 35 and Gin 38 in TM1 , amino acid residues Tyr 43, Phe 44, Val 46, Ser 47, Ala 49, Ala 50, Ala 51 , lie 53, Leu 54, Val 57, Leu 58, Pro 61 , Phe 62, lie 64 and Thr 65 in TM2, amino acid residues Thr 68, Gly 69, Phe 70, Cys 71 , Ala 72, Ala 73, and Cys 74 in ECL1 , amino acid residues His 75, Gly 76, Phe 79, He 80, Phe 83, Val 86 and Leu 87 in TM3, and amino acid residues Thr 119, Ala 121 , Ala 122, Gly 123, lie 125, Ala 126, He 127, Trp 129, Val 130, Phe 133, Leu 137 and Met 140 in TM4, according to the numbering of the adenosine A^ receptor sequence as set out in Figure 15. Thus the selected coordinates may comprise one or more atoms from any one or more of amino acid residues Val 25, Cys 28, Trp 29, Val 31 , Trp 32, Ser 35, Gin 38, Tyr 43, Phe 44, Val 46, Ser 47, Ala 49, Ala 50, Ala 51 , lie 53, Leu 54, Val 57, Leu 58, Pro 61 , Phe 62, lie 64, Thr 65, Thr 68, Gly 69, Phe 70, Cys 71 , Ala 72, Ala 73, Cys 74, His 75, Gly 76, Phe 79, lie. 80, Phe 83, Val 86, Leu 87, Thr 1 19, Ala 121 , Ala 122, Gly 123, lie 125, Ala 126, lie 127, Trp 129, Val 130, Phe 133, Leu 137 and Met 140, according to the numbering of the adenosine A2A receptor as set out in Figure 15 (e.g. at least 5, 10, 15, 20, 25, 30, 35, 40 or 45 of said amino acid residues).

In another example, the selected coordinates may comprise atoms of one or more amino acids which define the conserved DRY motif in helix 3 of GPCRs. The DRY motif has been implicated both in G protein coupling and in the regulation of receptor activation (Rovati et al 2007, Mol Pharmacol 71(4): 959). Thus, the selected coordinates may comprise atoms of one or more of amino acid resides Asp 101 , Arg 102 and Tyr 103, according to the numbering of the adenosine A_∑A receptor as set out in Figure 15 In another example, the selected coordinates may comprise atoms of one or more amino acids in ICL3 which mediates coupling of the GPCR to G proteins, and other signalling molecules such as beta-arrestin and GPCR activation. The present structure provides the first view of this hydrophobic G protein binding architecture in the human adenosine A∑A receptor. Thus, the selected coordinates may comprise atoms of one or more of (e.g: at least 2, 3, 4 or 5 of, or all 6 of) amino acid residues Gin 214, Pro 215, Leu 216, Pro 217, Gly 218 and Glu 219, according to the numbering of the adenosine A2A receptor as set out in Figure 15, which together connect TM5 to TM6. In addition to the six residues of ICL3 itself, other amino acid residues are believed to be involved in a potential bonding network across the loop and to contribute to G-protein binding. For example, both the C-terminus of TM5 and the N-terminus of TM6 project ~15 A into the cytoplasm, and so these regions may also mediate coupling to G proteins and other signalling proteins. Thus, in yet further example, the selected coordinates may comprise atoms or one or more amino acid residues Phe 201 , Ala 202, Ala 203, Ala 204, Arg 205, Arg 206, Gin 207, Leu 208, Lys 209, Gin 210, Met 211 , Glu 212, Ser 213, Gin 214, Pro 215, Leu 216, Pro 217, Gly 218, Glu 219, Arg 220, Ala 221 , Arg 222, Ser 223, Thr 224, Leu 225, Gin 226, Lys 227, Glu 228, Val 229, His 230, Ala 231 , Ala 232 and Lys 233, according to the numbering of the adenosine A_¾ receptor as set out in Figure 15 (e.g. at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 or 32 of, or all 33 of said residues).

In a further example, the selected coordinates may comprise atoms of one or more of the amino acids that define the intact ionic lock. The present adenosine A2A structure, co- crystallised in complex with the inverse agonist ZM-241 ,385, is the only adenosine A2A GPCR structure to have an ionic lock formed. Thus, the selected coordinates may comprise atoms of one or more of amino acid residues Glu 228, Arg 102, Asp 101 and Tyr 1 12, according to the numbering of the adenosine A_¾ receptor as set out in Figure 15.

In a further example, the selected coordinates may comprise atoms of an amino acid residue known as the rotamer toggle switch, which, consistent with rhodopsin inverse agonist in the protein data bank, is also found in its 'inactive' rotameric state (Chi1 = -60°). Thus, the selected coordinates may comprise atoms of amino acid Trp 246, according to the numbering of the adenosine A2A receptor as set out in Figure 15.

In yet a further example, the selected coordinates may comprise atoms of amino acid residues which form a slightly extended anti-parallel β-sheet on the extracellular surface of the adenosine receptor, namely Gly 69 - Ala 72 of ECL1 and Gin 163 - Cys 166 of ECL2. Thus, the selected coordinates may comprise atoms of one or more of amino acid residues Gly 69, Phe 70, Cys 71 , Ala 72, Gin 163, Val 164, Ala 165 and Cys 166, according to the numbering of the adenosine A_∑A receptor as set out in Figure 15.

It is appreciated that the selected coordinates may comprise any atoms of particular interest including atoms mentioned in any one or more of the above examples, or as listed in Example 1 below.

It is appreciated that the selected coordinates may correspond to atoms from a particular structural region (e.g. helix and/or loop) of the adenosine A_¾ receptor. By the helices and loop regions of the adenosine A2A receptor we mean the following:

Helix 1 Residues 7-32

Helix 2 Residues 39-67

Helix 3 Residues 73-107

Helix 4 Residues 1 19-140

Helix 5 Residues 174-213

Helix 6 Residues 220-258

Helix 7 Residues 266-291

ICL1 Residues 33-38

ECL1 Residues 68-72

ICL2 Residues 108- 18

ECL2 Residues 141 -173

IC_.L3 Residues 214-219

ECL3 Residues 259-265

However, it will be appreciated that there are different criteria for which residues are considered to be in a helical conformation depending on phi and psi angles. Moreover, when comparing the adenosine A2A receptor to other structures, some residues may be missing in one or other of the structures and some residues may be considered helical in one structure but not the other. Further, the loop regions may be defined as amino acid structures that join alpha helices (as above) or may be defined as amino acid structures that are predicted to be outside of the membrane. Therefore the limits above are not to be construed as absolute, but rather may vary according to the criteria used. Nevertheless, for the purposes of the comparisons set out below, we have used the above-mentioned definitions of helices and loops.

Preferably, the selected coordinates include at least 2% or 5% C-a atoms, and more preferably at least 10% C-a atoms. Alternatively or additionally, the selected coordinates include at least 10% and more preferably at least 20% or 30% backbone atoms selected from any combination of the nitrogen, C-a, carbonyl C and carbonyl oxygen atoms.

It is appreciated that the coordinates of the adenosine A∑A receptor used in the invention may be optionally varied and a subset of the coordinates or the varied coordinates may be selected (and constitute selected coordinates). Indeed, such variation may be necessary in various aspects of the invention, for example in the modelling of protein structures and in the fitting of various binding partners to the adenosine A_¾ receptor structure. Protein structure variability and similarity is routinely expressed and measured by the root mean square deviation (rmsd), which measures the difference in positioning in space between two sets of atoms. The rmsd measures distance between equivalent atoms after their optimal superposition. The rmsd can be calculated over all atoms, over residue backbone atoms (i.e. the nitrogen-carbon-carbon backbone atoms of the protein amino acid residues), main chain atoms only (i.e. the nitrogen-carbon-oxygen-carbon backbone atoms of the protein amino acid residues), side chain atoms only or over C-a atoms only.

The least-squares algorithms used to calculate rmsd are well known in the art and include those described by Rossman and Argos (J Biol Chem, (1975) 250:7525), Kabsch (Acta Cryst (1976) A92:922; Acta Cryst (1978) A34:827-828), Hendrickson (Acta Cryst (1979) A35: 158), cLachan (J Mol Biol (1979) 128:49) and Kearsley (Acta Cryst (1989) A45:208). Both algorithms based on iteration in which one molecule is moved relative to the other, such as that described by Ferro and Hermans (Acta Cryst (1977) A33:345-347), and algorithms which locate the best fit directly (e.g. Kabsch's methods) may be used. Methods of comparing proteins structures are also discussed in Methods of Enzymology, vol 1 15: 397- 420.

Typically, rmsd values are calculated using coordinate fitting computer programs and any suitable computer program known in the art may be used, for example MNYFIT (part of a collection of programs called COMPOSER, Sutcliffe et al (1987) Protein Eng 1 :377-384). Other programs also include LSQMAN (Kleywegt & Jones (1994) A super position, CCP4/ESF-EACBM, Newsletter on Protein Crystallography, 31 : 9-14), LSQ AB (Collaborative Computational Project 4. The CCP4 Suite: Programs for Protein Crystallography, Acta Cryst (1994) D50760-763), QUANTA (Jones et al, Acta Cryst (1991 ) A47:1 10-119 and commercially available from Accelrys, San Diego, CA), Insight (Commercially available from Accelrys, San Diego, CA), Sybyl® (commercially available from Tripos, Inc., St Louis) and O (Jones er a/., Acta Cryst (1991 ) A47: 110-1 19).

In, for example, the programs LSQKAB and O, the user can define the residues in the two proteins that are to be paired for the purpose of the calculation. Alternatively, the pairing of residues can be determined by generating a sequence alignment of the two proteins as is well known in the art. The atomic coordinates can then be superimposed according to this alignment and an rmsd value calculated. The program Sequoia (Bruns ef a/ (1999) J Mol Biol 288(3):427-439) performs the alignment of homologous protein sequences, and the superposition of homologous protein atomic coordinates. Once aligned, the rmsd can be calculated using programs detailed above. When the sequences are identical or highly similar, the structural alignment of proteins can be done manually or automatically as outlined above. Another approach would be to generate a superposition of protein atomic coordinates without considering the sequence. We have conducted an rmsd analysis of all atoms and of residue backbone atoms (i.e. the nitrogen-carbon-carbon backbone atoms of the protein) between the adenosine A2A receptor (molecule A') and the Jaakola et al A∑A structure¹³ (PDB code: 3EML) using a Maestro script as shown in Example 3. Similar scripts can be used to calculate rmsd values for any other selected coordinates. Rmsd values have been calculated on all atoms and on residue backbone atoms in the complete structure (residues 7-148, 158-208 and 222-305; excluding ICL3 which is not present in 3EML and excluding residues 149-157 which are not visible in the electron density (chain break) for both the present A2A structure and 3E L) and on selected regions of interest as discussed below. Conducting an rmsd analysis of residue backbone atoms between the present A2A structure (molecule A; excluding residues 149-157 and 209-221 ) and the Jaakola et al A2A structure gave an rmsd value of 1.285 A. Thus in one embodiment, the coordinates or selected coordinates of Table (i) or Table (ii) may be optionally varied within an rmsd of residue backbone atoms (i.e. the nitrogen-carbon-carbon backbone atoms of the protein) of not more than 1.285 A. Preferably, the coordinates or selected coordinates are varied within an rmsd of residue backbone atoms of not more than 1.2 A, 1.1 A, 1.0 A, 0.9 A or 0.8 A and more preferably not more than 0.7 A, 0.6 A, 0.5 A, 0.4 A, 0.3 A, 0.2 A or 0.1 A.

It is appreciated that rmsd can also be calculated over all atoms. For example, we conducted an rmsd analysis of all atoms between the present A2A structure (molecule A; excluding residues 149-157 and 209-221 ) and the Jaakola et al A_∑A structure, which gave an rmsd value of 1.815 A. Thus, in one embodiment, the coordinates or selected coordinates of Table (i) or Table (ii) may be optionally varied within an rmsd of all atoms of not more than 1.815 A. Preferably, the coordinates or selected coordinates are varied within an rmsd of all atoms of not more than 1.8 A, 1.7 A, 1.6 A, 1.5 A, 1.4 A, 1.3 A, 1.2 A, 1.1 A, 1.0 A, 0.9 A, 0.8 A, 0.7 A, 0.6 A, 0.5 A, 0.4 A, 0.3 A, 0.2 A or 0.1 A.

We have conducted an rmsd analysis of all atoms and residue backbone atoms between the present adenosine A2A receptor (molecule A') and the Jaakola ef al A2A structure within the active site (i.e. residues Tyr 9, Glu 13, lie 60, Phe 62, Ala 63, He 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278). The rmsd value for all atoms is 1.636 A and for residue backbone atoms is 0.566 A. Thus in an embodiment, where the coordinates or selected coordinates used in the invention are optionally varied within the active site, they are varied within an rmsd of al| atoms of not more than 1.636 A (such as not more than 1.6 A, 1.5 A, 1.4 A, 1.3 A, 1.2 A, 1.1 A, 1.0 A, 0.9 A, 0.8 A, 0.7 A, 0.6 A, 0.5 A, 0.4 A, 0.3 A, 0.2 A or 0.1 A) and/or within an rmsd of residue backbone atoms of not more than 0.566 A (such as not more than 0.5 A or 0.4 A or 0.3 A or 0.2 A or 0.1 A).

We have conducted an rmsd analysis of all atoms and residue backbone atoms between the present adenosine A_M receptor (molecule A') and the Jaakola et al A_∑A structure within the dimerisation interface (i.e. residues amino acid residues Val 25, Cys 28, Trp 29, Val 31 , Trp 32, Ser 35, Gin 38, Tyr 43, Phe 44, Val 46, Ser 47, Ala 49, Ala 50, Ala 51 , He 53, Leu 54, Val 57, Leu 58, Pro 61 , Phe 62, lie 64, Thr 65, Thr 68, Gly 69, Phe 70, Cys 71 , Ala 72, Ala 73, Cys 74, His 75, Gly 76, Phe 79, lie 80, Phe 83, Val 86, Leu 87, Thr 1 19, Ala 121 , Ala 122, Gly 123, He 125, Ala 126, lie 127, Trp 129, Val 130, Phe 133, Leu 137 and Met 140). The rmsd value for all atoms is 1.039 A and for residue backbone atoms is 0.835 A. Thus in an embodiment, where the coordinates or selected coordinates used in the invention are optionally varied within the dimerisation interface, they are varied within an rmsd of all atoms of not more than 1 .039 A (such as not more than 1 .0 A, 0.9 A, 0.8 A, 0.7 A, 0.6 A, 0.5 A, 0.4 A, 0.3 A, 0.2 A or 0.1 A) and/or within an rmsd of residue backbone atoms of not more than 0.835 A (such as not more than 0.8 A or 0.7 A or 0.6 A or 0.5 A or 0.4 A or 0.3 A or 0.2 A or 0.1 A).

We have conducted an rmsd analysis of all atoms and residue backbone atoms between the present adenosine A^ receptor (molecule A') and the Jaakola et al A¾ structure within the DRY motif (i.e. residues Asp 138, Arg 139 and Tyr 140). The rmsd value for all atoms is 1 .806 A and for residue backbone atoms is 0.900 A. Thus in an embodiment, where the coordinates or selected coordinates used in the invention are optionally varied within the DRY motif, they are varied within an rmsd of all atoms of not more than 1 .806 A (such as not more than 1 .8 A, 1 .7 A, 1 .6 A, 1 .5 A, .4 A, .3 A, 1 .2 A, 1 .1 A, 1 .0 A, 0.9 A, 0.8 A, 0.7 A, 0.6 A, 0.5 A, 0.4 A, 0.3 A, 0.2 A or 0.1 A) and/or within an rmsd of residue backbone atoms of not more than 0.900 A (such as not more than 0.8 A or 0.7 A or 0.6 A or 0.5 A or 0.4 A or 0.3 A or 0.2 A or 0.1 A).

In this aspect of the invention, the coordinates of the adenosine A_∑A receptor structure are used to predict a three dimensional representation of a target protein of unknown structure, or part thereof, by modelling. By "modelling", we mean the prediction of structures using computer-assisted or other de novo prediction of structure, based upon manipulation of the coordinate data from Table (i) or Table (ii) or selected coordinates thereof.

The target protein may be any protein that shares sufficient sequence identity to the human adenosine A^ receptor such that its structure can be modelled by using the adenosine A¾ receptor coordinates of Table (i) or Table (ii). It will be appreciated that if a structural representation of only a part of the target protein is being modelled, for example a particular domain, the target protein only has to share sufficient sequence identity to the adenosine A_∑A receptor over that part. It has been shown for soluble protein domains that their three dimensional structure is broadly conserved above 20% amino acid sequence identity and well conserved above 30% identity, with the level of structural conservation increasing as amino acid sequence identity increases up to 100% (Ginalski, K. Curr Op Struc Biol (2006) 16, 172-1 77). Thus, it is preferred if the target protein, or part thereof, shares at least 20% amino acid sequence identity with the human adenosine A2A receptor sequence provided in Figure 16, and more preferably at least 30%, 40%, 50%, 60%, 70%, 80% or 90% sequence identity, and yet more preferably at least 95% or 99% sequence identity.

It will be appreciated therefore that the target protein may be an adenosine receptor analogue or homologue.

Analogues are defined as proteins with similar three-dimensional structures and/or functions with little evidence of a common ancestor at a sequence level. Homologues are proteins with evidence of a common ancestor, i.e. likely to be the result of evolutionary divergence and are divided into remote, medium and close sub-divisions based on the degree (usually expressed as a percentage) of sequence identity.

By a human adenosine A^ receptor homologue, we include a protein with at least 20%, 25%, 30%, 35%, 40%, 45% or at least 50% amino acid sequence identity with the sequence of adenosine A_¾ receptor provided in Figure 16, preferably at least 55%, 60%, 65%, 70%, 75% or 80% amino acid sequence identity and more preferably 85%, 90%, 95% or 99% amino acid sequence identity. This includes polymorphic forms of adenosine A2A receptors, e.g. mutants and adenosine A2A receptors from other species as well as other adenosine receptors such as A^ A_¾, A₂B, A₃. Thus an adenosine A2A receptor homologue would include a human adenosine A^ A₂B or A₃ receptor.

Sequence identity may be measured by the use of algorithms such as BLAST or PSI-BLAST (Altschul et al, NAR (1997), 25, 3389-3402) or methods based on Hidden Markov Models (Eddy S et al, J Comput Biol (1995) Spring 2 (1 ) 9-23). Typically, the percent sequence identity between two polypeptides may be determined using any suitable computer program, for example the GAP program of the University of Wisconsin Genetic Computing Group and it will be appreciated that percent identity is calculated in relation to polypeptides whose sequence has been aligned optimally. The alignment may alternatively be carried out using the Clustal W program (Thompson et a!., 1994). The parameters used may be as follows: Fast pairwise alignment parameters: K-tuple(word) size; 1 , window size; 5, gap penalty; 3, number of top diagonals; 5. Scoring method: x percent. Multiple alignment parameters: gap open penalty; 10, gap extension penalty; 0.05. Scoring matrix: BLOSUM. In one embodiment the target protein is an integral membrane protein. By "integral membrane protein" we mean a protein that is permanently integrated into the membrane and can only be removed using detergents, non-polar solvents or denaturing agents that physically disrupt the lipid bilayer. Examples include receptors such as GPCRs, the T-cell receptor complex and growth factor receptors; transmembrane ion channels such as ligand- gated and voltage gated channels; transmembrane transporters such as neurotransmitter transporters; enzymes; carrier proteins; and ion pumps.

The amino acid sequences (and the nucleotide sequences of the cDNAs which encode them) of many membrane proteins are readily available, for example by reference to GenBank. For example, Foord et al supra gives the human gene symbols and human, mouse and rat gene IDs from Entrez Gene (http://www.ncbi. nlm.nih.qov/entrez) for GPCRs. It should be noted, also, that because the sequence of the human genome is substantially complete, the amino acid sequences of human membrane proteins can be deduced therefrom. In a preferred embodiment, the target protein is a GPCR.

Suitable GPCRs include, but are not limited to adenosine receptors, β-adrenergic receptors, neurotensin receptors (NTR) and muscarinic receptors. Other suitable GPCRs are well known in the art and include those listed in Hopkins & Groom supra. In addition, the International Union of Pharmacology produce a list of GPCRs (Foord et al (2005) Pharmacol. Rev. 57, 279-288, incorporated herein by reference and this list is periodically updated at http://www.iuphar-db.org/GPCR/ReceptorFamiliesForward). It will be noted that GPCRs are. divided into different classes, principally based on their amino acid sequence similarities. They are also divided into families by reference to the natural ligands to which they bind. All GPCRs are included in the scope of the invention and their structure may be modelled by using the coordinates of the adenosine A_¾ receptor.

Although the target protein may be derived from any source, it is particularly preferred if it is from a eukaryotic source. It is particularly preferred if it is derived from a vertebrate source such as a mammal. It is particularly preferred if the target protein is derived from rat, mouse, rabbit or dog or non-human primate or man.

Typically, modelling a structural representation of a target is done by homology modelling whereby homologous regions between the adenosine A2A receptor and the target protein are matched and the coordinate data of the adenosine A2A receptor used to predict a structural representation of the target protein. The term "homologous regions" describes amino acid residues in two sequences that are identical or have similar (e.g. aliphatic, aromatic, polar, negatively charged, or positively charged) side-chain chemical groups. Identical and similar residues in homologous regions are sometimes described as being respectively "invariant" and "conserved" by those skilled in the art.

Typically, the method involves comparing the amino acid sequences of adenosine receptor with a target protein by aligning the amino acid sequences. Amino acids in the sequences are then compared and groups of amino acids that are homologous (conveniently referred to as "corresponding regions") are grouped together. This method detects conserved regions of the polypeptides and accounts for amino acid insertions or deletions.

Homology between amino acid sequences can be determined using commercially available algorithms known in the art. For example, the programs BLAST, gapped BLAST, BLASTN, PSI-BLAST, BLAST 2 and WU- BLAST (provided by the National Center for Biotechnology Information) can be used to align homologous regions of two, or more, amino acid sequences. These may be used with default parameters to determine the degree of homology between the amino acid sequence of the adenosine A2A receptor and other target proteins which are to be modelled.

Preferred for use according to the present invention is the WU-BLAST (Washington University BLAST) version 2.0 software. WU-BLAST version 2.0 executable programs for several UNIX platforms can be downloaded from ftp ://blast. wustl. edu/blast/executables. This program is based on WU-BLAST version 1.4, which in turn is based on the public domain NCBI-BLAST version 1.4 (Altschul and Gish, 1996, Local alignment statistics, Doolittle ed., Methods in Enzymology 266: 460-480; Altschul et al., 1990, Basic local alignment search tool, Journal of Molecular Biology 215: 403-410; Gish and States, 1993, Identification of protein coding regions by database similarity search, Nature Genetics 3: 266- 272; Karlin and Altschul, 1993, Applications and statistics for multiple high-scoring segments in molecular sequences, Proc. Natl. Acad. Sci. USA 90: 5873-5877; all of which are, incorporated by reference herein).

In all search programs in the suite the gapped alignment routines are integral to the database search itself. Gapping can be turned off if desired. The default penalty (Q) for a gap of length one is Q=9 for proteins and BLASTP, and Q=10 for BLASTN, but may be changed to any integer. The default per-residue penalty for extending a gap (R) is R=2 for proteins and BLASTP, and R=10 for BLASTN, but may be changed to any integer. Any combination of values for Q and R can be used in order to align sequences so as to maximize overlap and identity while minimizing sequence gaps. The default amino acid comparison matrix is BLOSUM62, but other amino acid comparison matrices such as PAM can be utilized.

Once the amino acid sequences of adenosine A₂A receptor and the target protein of unknown structure have been aligned, the structures of the conserved amino acids in the structural representation of the adenosine A^ receptor may be transferred to the corresponding amino acids of the target protein. For example, a tyrosine in the amino acid sequence of adenosine A2A receptor may be replaced by a phenylalanine, the corresponding homologous amino acid in the amino acid sequence of the target protein.

The structures of amino acids located in non-conserved regions may be assigned manually by using standard peptide geometries or by molecular simulation techniques, such as molecular dynamics. The final step in the process is accomplished by refining the entire structure using molecular dynamics and/or energy minimization. Typically, the predicted three dimensional structural representation will be one in which favourable interactions are formed within the target protein and/or so that a low energy conformation is formed ("High resolution structure prediction and the crystallographic phase problem" Qian et al (2007) Nature 450; 259-264; "State of the art in studying protein folding and protein structure production using molecular dynamics methods" Lee et al (2001 ) J of Mol Graph & Modelling 19(1 ): 146-149). Whereas it is preferred to base homology modelling on homologous amino acid sequences, it is appreciated that some proteins have low sequence identity (e.g. family B and C GPCRs) and at the same time are very similar in structure. Therefore, where at least part of the structure of the target protein is known, homologous regions can also be identified by comparing structures directly.

Homology modelling as such is a technique well known in the art (see e.g. Greer, {Science,. Vol. 228, (1985), 1055), and Blundell et al {Eur. J. Biochem, Vol. 172, (1988), 513)). The techniques described in these references, as well as other homology modelling techniques generally available in the art, may be used in performing the present invention. Typically, homology modelling is performed using computer programs, for example SWISS- MODEL available through the Swiss Institute for Bioinformatics in Geneva, Switzerland; WHATIF available on E BL servers; Schnare et al. (1996) J. Mol. Biol, 256: 701 -719; Blundell et al. (1987) Nature 326: 347-352; Fetrow and Bryant (1993) Bio/Technology 1 1 :479-484; Greer (1991 ) Methods in Enzymology 202: 239-252; and Johnson et al (1994) Crit. Rev. Biochem. Mol Biol. 29:1 -68. An example of homology modelling is described in Szklarz G. D (1997) Life Sci. 61 : 2507-2520.

Thus, in an embodiment of the first aspect of the invention, the method further comprises aligning the amino acid sequence of the target protein of unknown structure with the amino acid sequence of adenosine A2A receptor listed in Figure 16 to match homologous regions of the amino acid sequences, and subsequently modelling the structural representation of the target protein by modelling the structural representation of the matched homologous regions of the target protein on the corresponding regions of the adenosine A∑A receptor to obtain a three dimensional structural representation for the target protein that substantially preserves the structural representation of the matched homologous regions.

The invention therefore provides a method of predicting a three dimensional structural representation of a target protein of unknown structure, or part thereof, comprising:

providing the coordinates of the adenosine A2A receptor structure listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof;

aligning the amino acid sequence of a target protein of unknown structure or part thereof with the amino acid sequence of adenosine A2A receptor listed in Figure 16 or part thereof to match homologous regions of the amino acid sequences;

modelling the structure of the matched homologous regions of the target protein on the corresponding regions of the adenosine A_M receptor structure as defined by Table (i) or

Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; and

predicting a three dimensional structural representation for the target protein which substantially preserves the structure of the matched homologous regions.

The coordinate data of Table (i) or Table (ii), or selected coordinates thereof, will be particularly advantageous for homology modelling of other GPCRs. For example, since the protein sequence of adenosine A2A receptor and another GPCR can be aligned relative to each other, it is possible to predict structural representations of the structures of other GPCRs, particularly in the regions of the transmembrane helices and ligand binding region, using the adenosine receptor coordinates.

The coordinate data of the adenosine A¾ receptor can also be used to predict the crystal structure of target proteins where X-ray diffraction data or NMR spectroscopic data of the protein has been generated and requires interpretation in order to provide a structure.

A second aspect of the invention provides a method of predicting the three dimensional structural representation of a target protein of unknown structure, or part thereof, comprising: providing the coordinates of the adenosine receptor structure listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; and either (a) positioning the coordinates in the crystal unit cell of the protein so as to predict its structural representation, or (b) assigning NMR spectra peaks of the protein by manipulating the coordinates.

Thus, where X-ray crystallographic or NMR spectroscopic data is provided for a target protein of unknown structure, the coordinate data of Table (i) or Table (ii) may be used to interpret that data to predict a likely structure using techniques well known in the art including phasing, in the case of X-ray crystallography, and assisting peak assignments in the case of NMR spectra.

A three dimensional structural representation of any part of any target protein that is sufficiently similar to any portion of the adenosine A2A receptor can be predicted by this method. Typically, the target protein or part thereof has at least 20% amino acid sequence identity with any portion of adenosine A_¾ receptor, such as at least 30% amino acid sequence identity or at least 40% or 50% or 60% or 70% or 80% or 90% sequence identity. For example, the coordinates may be used to predict the three-dimensional representations of other crystal forms of adenosine A_∑A receptor, other adenosine A2A receptors, adenosine A2A receptor mutants or co-complexes of an adenosine A_∑A receptor. Other suitable target proteins are as defined with respect to the first aspect of the invention.

One method that may be employed for these purposes is molecular replacement which is; well known in the art and described, for example, in Evans & McCoy (Acta Cryst, 2008, D64:M0), McCoy (Acta Cryst, 2007, D63:32-42) and McCoy ef al (J of App Cryst, 2007, 40:658-674). Molecular replacement enables the solution of the crystallographic phase problem by providing initial estimates of the phases of the new structure from a previously. known structure, as opposed to the other major methods for solving the phase problem, i.e. experimental methods (which measure the phase from isomorphous or anomalous differences) or direct methods (which use mathematical relationships between reflection triplets and quartets to bootstrap a phase set for all reflections from phases for a small or random 'seed' set of reflections.) Compared to molecular replacement, such methods are time consuming and generally hinder the solution of crystal structures. Thus molecular replacement provides an accurate structural form for an unknown crystal more quickly and efficiently than attempting to determine such information ab initio. Accordingly, the invention involves generating a preliminary model of a target protein whose structure coordinates are unknown, by orienting and positioning the relevant portion of the adenosine A2A receptor according to Table (i) or Table (ii) within the unit cell of a crystal of the target protein so as best to account for the observed X-ray diffraction pattern of the crystal of the target protein. Phases can be calculated from this model and combined with the observed X-ray diffraction pattern amplitudes to generate an electron density map of the target protein's structure. This, in turn, can be subjected to any well-known model building, and structure refinement techniques to provide a final, accurate structural representation of the target protein (E. Lattman, "Use of the Rotation and Translation Functions", in Meth. Enzymol., 115, pp. 55-77 (1985); M. G. Rossmann, ed., "The Molecular Replacement Method", Int. Sci. Rev. Sen, No. 13, Gordon & Breach, New York (1972)).

Thus the invention includes a method of predicting a three dimensional structural representation of a target protein of unknown structure, or part thereof, comprising: providing the coordinates of the adenosine A2A receptor structure, optionally varied by a root mean square deviation of residue backbone atoms of not more than 1 .285 A, or selected coordinates thereof; providing an X-ray diffraction pattern of the target protein; and using the coordinates to predict at least part of the structure coordinates of the target protein.

In an embodiment, the X-ray diffraction pattern of the target protein is provided by crystallising the target protein unknown structure; and generating an X-ray diffraction pattern from the crystallised target protein. Thus, the invention also provides a method of method of predicting a three dimensional structural representation of a target protein of unknown structure comprising the steps of (a) crystallising the target protein; (b) generating an X-ray diffraction pattern from the crystallised target protein; (c) applying the coordinates of the adenosine A_¾ receptor structure, optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, to the X- ray diffraction pattern to generate a three-dimensional electron density map of the target protein, or part thereof; and (d) predicting a three dimensional structural representation of the target protein from the three-dimensional electron density map. Examples of computer programs known in the art for performing molecular replacement include CNX (Brunger AT.; Adams P. D.; Rice L. M., Current Opinion in Structural Biology, Volume 8, Issue 5, October 1998, Pages 606-611 (also commercially available from Accelrys San Diego, CA), MOLREP (A.Vagin, A.Teplyakov, MOLREP: an automated program for molecular replacement, J Appl Cryst (1997) 30, 1022-1025, part of the CCP4 suite), AMoRe (Navaza, J. (1994). AMoRe: an automated package for molecular replacement. Acta Cryst A50, 157- 163), or PHASER (part of the CCP4 suite).

Preferred selected coordinates of the adenosine A2A receptor are as defined above with respect to the first aspect of the invention.

The invention may also be used to assign peaks of NMR spectra of target proteins, by manipulation of the data of Table (i) or Table (ii) (J Magn Reson (2002) 157(1 ): 1 19-23).

The coordinates of the adenosine A_¾ receptor of Table (i) or Table (ii) optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof may be used in the provision, design, modification or analysis of binding partners of adenosine A^ receptors. Such a use will be important in drug design.

By adenosine A2A receptor we mean any adenosine A_¾ receptor which has at least 75% sequence identity with human adenosine A2A receptor as well as adenosine A_¾ receptors from other species and mutants thereof. Preferably, the adenosine A2A receptor has at least 80% amino acid sequence identity to human adenosine A2A receptor, and more preferably at least 85%, 90%, 95% or 99% amino acid sequence identity. By "binding partner" we mean any molecule that binds to an adenosine A¾ receptor. Preferably, the molecule binds selectively to the adenosine A_∑A receptor. For example, it is preferred if the binding partner has a K_d value (dissociation constant) which is at least five or ten times lower (i.e. higher affinity) than for at least one other adenosine receptor (A,, A₂B or A₃), and preferably more than 100 or 500 times lower. More preferably, the binding partner of an adenosine A₂A receptor has a K_d value more than 1000 or 5000 times lower than for at least one other adenosine receptor. However, it will be appreciated that the limits will vary dependent upon the nature of the binding partner. Thus, typically, for small molecule binding partners, the binding partner typically has a K_d value which is at least 10 times or 50 times or 100 times lower than for at least one other adenosine receptor. Typically, for antibody binding partners, the binding partner typically has a K_d value which is at least 500 or 1000 times lower than for at least one other adenosine receptor.

K_d values can be determined readily using methods well known in the art and as described, for example, below. At equilibrium Kd=[R][L]/[RL]

where the terms in brackets represent the concentration of

• Receptor-ligand complexes [RL],

• unbound receptor [R], and

• unbound ("free") ligand [L).

I

In order to determine the K_d the value of these terms must be known. Since the concentration of receptor is not usually known then the Hill-Langmuir equation is used where Fractional occupancy= [L]/[L] + K_d. In order to experimentally determine a K_d then, the concentration of free ligand and bound ligand at equilibrium must be known. Typically, this can be done by using a radio-labelled or fluorescently labelled ligand which is incubated with the receptor (present in whole cells or homogenised membranes) until equilibrium is reached. The amount of free ligand vs bound ligand must then be determined by separating the signal from bound vs free ligand. In the case of a radioligand this can be done by centrifugation or filtration to separate bound ligand present on whole cells or membranes from free ligand in solution. Alternatively a scintillation proximity assay is used. In this assay the receptor (in membranes) is bound to a bead containing scintillant and a signal is only detected by the proximity of the radioligand bound to the receptor immobilised on the bead.

The binding partner may be any of a polypeptide; an anticalin; a peptide; an antibody; a chimeric antibody; a single chain antibody; an aptamer; a darpin; a Fab, F(ab')₂, Fv, ScFv or dAb antibody fragment; a small molecule; a natural product; an affibody; a peptidomimetic; a nucleic acid; a peptide nucleic acid molecule; a lipid; a carbohydrate; a protein based on a modular framework including ankyrin repeat proteins, armadillo repeat proteins, leucine rich proteins, tetrariopeptide repeat proteins or Designed Ankyrin Repeat Proteins (DARPins); a protein based on lipocalin or fibronectin domains or Affilin scaffolds based on either human gamma crystalline or human ubiquitin; a G protein; an RGS protein; an arrestin; a GPCR kinase; a receptor tyrosine kinase; a RAMP; a NSF; a GPCR; an NMDA receptor subunit NR1 or NR2a; calcyon; or a fragment or derivative thereof that binds to adenosine A_¾ receptor. Typically, the binding partner is a small molecule.

It will be appreciated that the coordinates of the invention will also be useful in the analysis of solvent and ion interactions with an adenosine A_¾ receptor, which are important factors in drug design. Thus the binding partner may be a solvent molecule, for example water or acetonitrile, or an ion, for example a sodium ion or a protein.

It is particularly preferred if the binding partner is a small molecule with a molecule weight less than 5000 daltons, for example less than 4000, 3000, 2000 or 1000 daltons, or with a molecule weight less than 500 daltons, for example less than 450 daltons, 400 daltons, 350 daltons, 300 daltons, 250 daltons, 200 daltons, 150 daltons, 100 daltons, 50 daltons or 10, daltons.

It is further preferred if the binding partner causes a change (i.e a modulation) in the level of biological activity of the adenosine receptor, i.e. it has functional agonist or antagonist activity, and therefore may have the potential to be a candidate drug. Thus, the binding partner may be any of a full agonist, a partial agonist, an inverse agonist or an antagonist of adenosine A receptor. The binding partner may bind to the orthosteric site, e.g. as defined by the ZM241385 binding site, or it may bind to an allosteric binding site. It is also appreciated that the binding partner may be one that modulates the ability of the adenosine AZA receptor to dimerise. For example, the binding partner may bind to the dimerisation interface or bind to another region of the adenosine A^ receptor which nevertheless modulates dimerisation.

Accordingly, a third aspect of the invention provides a method for selecting or designing one or more binding partners of adenosine A_¾ receptor comprising using molecular modelling means to select or design one or more binding partners of the adenosine A₂A receptor, wherein the three-dimensional structural representation of at least part of the human adenosine A₂A receptor, as defined by the coordinates of adenosine receptor of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A or selected coordinates thereof, is compared with a three-dimensional- structural representation of one or more candidate binding partners, and one or more binding partners that are predicted to interact with adenosine A2A receptor are selected.

In order to provide a three-dimensional structural representation of a candidate binding partner, the binding partner structural representation may be modelled in three dimensions using commercially available software for this purpose or, if its crystal structure is available, the coordinates of the structure may be used to provide a structural representation of the binding partner. The design of binding partners that bind to an adenosine receptor generally involves consideration of two factors.

First, the binding partner must be capable of physically and structurally associating with parts or all of an adenosine A₂A receptor binding region (e.g. ligand binding site or an allosteric binding site or dimerisation interface). Non-covalent molecular interactions important in this association include hydrogen bonding, van der Waals interactions, hydrophobic interactions and electrostatic interactions.

Second, the binding partner must be able to assume a conformation that allows it to associate with an adenosine A2A receptor binding region directly. Although certain portions of the binding partner will not directly participate in these associations, those portions of the binding partner may still influence the overall conformation of the molecule. This, in turn, may have a significant impact on potency. Such conformational requirements include the overall three-dimensional structure and orientation of the binding partner in relation to all or a portion of the binding region, or the spacing between functional groups of a binding partner comprising several binding partners that directly interact with the adenosine A2A receptor. This is particularly relevant where the binding partner is a protein.

Thus it will be appreciated that selected coordinates which represent a binding region of the adenosine A_¾ receptor, e.g. atoms from amino acid residues contributing to the ligand binding site including amino acid residues Tyr 9, Glu 13, lie 60, Phe 62, Ala 63, lie 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278, may be used, or atoms from amino acid residues contributing to the dimerisation interface including amino acid residues Val 25, Cys 28, Trp 29, Val 31 , Trp 32, Ser 35, Gin 38, Tyr 43, Phe 44, Val 46, Ser 47, Ala 49, Ala 50, Ala 51 , lie 53, Leu 54, Val 57, Leu 58, Pro 61 , Phe 62, lie 64, Thr 65, Thr 68, Gly 69, Phe 70, Cys 71 , Ala 72, Ala 73, Cys 74, His 75, Gly 76, Phe 79, lie 80, Phe 83, Val 86, Leu 87, Thr 1 19, Ala 121 , Ala 122, Gly 123, lie 125, Ala 126, lie 127, Trp 129, Val 130, Phe 133, Leu 137 and Met 140 may be used Selected coordinates representing an extracellular face would be useful to select or design for binding partners such as antibodies, and selected coordinates representing an intracellular face would be useful to select or design for agents which modulate (e.g. prevent) binding to natural binding partners such as G proteins. Additional preferences for the selected coordinates are as defined above with respect to the first aspect of the invention.

Designing of binding partners can generally be achieved in two ways, either by the step wise assembly of a binding partner or by the de novo synthesis of a binding partner. As is described in more detail below, binding partners can also be identified by virtual screening. With respect to the step-wise assembly of a binding partner, several methods may be used. Typically the process begins by visual inspection of, for example, any of the binding regions on a computer representation of the adenosine Aza, receptor as defined by the coordinates in Table (i) or Table (ii) optionally varied within a rmsd of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof. Selected binding partners, or fragments or moieties thereof may then be positioned in a variety of orientations, or docked, within the binding region. Docking may be accomplished using software such as QUANTA and Sybyl (Tripos Associates, St. Louis, Mo.), followed by, or performed simultaneously with, energy minimization, rigid-body minimization (Gshwend, supra) and molecular dynamics with standard molecular mechanics force fields, such as CHARMM and AMBER.

Specialized computer programs may also assist in the process of selecting binding partners or fragments or moieties thereof, as are known in the art and as detailed in WO2008/068534 incorporated herein by reference. Once suitable binding partners or fragments have been selected, they may be assembled into a single compound or complex. Assembly may be preceded by visual inspection of the relationship of the fragments to each other on the three-dimensional image displayed on a computer screen in relation to the structure coordinates of the adenosine Aa_A receptor. This would be followed by manual model building using software such as QUANTA or Sybyl. Useful programs known in the art (see, for example WO2008/068534 incorporated herein by reference) may aid connecting the individual chemical entities or fragments. Thus the invention includes a method of designing a binding partner of an adenosine A2A receptor comprising the steps of: (a) providing a structural representation of an adenosine A2A receptor binding region as defined by the coordinates of the human adenosine A₂A receptor of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A or selected coordinates thereof (b) using computational means to dock a three dimensional structural representation of a first binding partner in part of the binding region; (c) docking at least a second binding partner in another part of the binding region; (d) quantifying the interaction energy between the first or second binding partner and part of the binding region; (e) repeating steps (b) to (d) with another first and second binding partner, selecting a first and a second binding partner based on the quantified interaction energy of all of said first and second binding partners; (f) optionally, visually inspecting the relationship of the first and second binding partner to each other in relation to the binding region; and (g) assembling the first and second binding partners into a one binding partner that interacts with the binding region by model building.

As an alternative to the step-wise assembly of binding partners, binding partners may be designed as a whole or "de novo" using either an empty binding region or optionally including some portion(s) of a known binding partner(s). There are many de novo ligand design methods including: 1. LUDI (H.-J. Bohm, "The Computer Program LUDI: A New Method for the De Novo Design of Enzyme Inhibitors", J. Comp. Aid. Molec. Design, 6, pp. 61-78 (1992)). LUDI is available from Molecular Simulations Incorporated, San Diego, Calif; 2. LEGEND (Y. Nishibata et al., Tetrahedron, 47, p. 8985 (1991)). LEGEND is available from. Molecular Simulations Incorporated, San Diego, Calif; 3. LeapFrog (available from Tripos Associates, St. Louis, Mo.); and 4. SPROUT (V. Gillet et al., "SPROUT: A Program for Structure Generation)", J. Comput. Aided Mol. Design, 7, pp. 127-153 (1993)). SPROUT is available from the University of Leeds, UK.

Other molecular modelling techniques may also be employed in accordance with this invention (see, e.g., N. C. Cohen et al., "Molecular Modeling Software and Methods for Medicinal Chemistry, J. Med. Chem., 33, pp. 883-894 (1990); see also, M. A. Navia and M. A. Murcko, "The Use of Structural Information in Drug Design", Current Opinions in Structural Biology, 2, pp. 202-210 (1992); L. M. Balbes et al., "A Perspective of Modern Methods in Computer-Aided Drug Design", in Reviews in Computational Chemistry, Vol. 5, K. B. Lipkowitz and D. B. Boyd, Eds., VCH, New York, pp. 337-380 (1994); see also, W. C. Guida, "Software For Structure-Based Drug Design", Curr. Opin. Struct. Biology, 4, pp. 777-781 (1994)). In addition to the methods described above in relation to the design of binding partners, other computer-based methods are available to select for binding partners that interact with adenosine A2A receptor .

For example the invention involves the computational screening of small molecule databases for binding partners that can bind in whole, or in part, to the adenosine A¾ receptor. In this screening, the quality of fit of such binding partners to a binding region of an adenosine A2A receptor site as defined by the coordinates of the human adenosine A_¾ receptor of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A or selected coordinates thereof, may be judged either by shape complementarity or by estimated interaction energy (E. C. Meng et al., J. Comp. Chem., 13, pp. 505-524 (1992)). For example, selection may involve using a computer for selecting an orientation of a binding partner with a favourable shape complementarity in a binding region comprising the steps of: (a) providing the coordinates of adenosine A2A receptor of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A or selected coordinates thereof and a three-dimensional structural representation of one or more candidate binding partners; (b) employing computational means to dock a first binding partner in the binding region; (c) quantitating the contact score of the binding partner in different orientations; and (d) selecting an orientation with the highest contact score.

The docking may be facilitated by the contact score. The method may further comprise the step of generating a three-dimensional structural representation of the binding region and binding partner bound therein prior to step (b).

The method may further comprise the steps of: (e) repeating steps (b) through (d) with a second binding partner; and (f) selecting at least one of the first or second binding partner that has a higher contact score based on the quantitated contact score of the first or second binding partner.

In another embodiment, selection may involve using a computer for selecting an orientation of a binding partner that interacts favourably with a binding region comprising; a) providing the coordinates of the human adenosine A₂A receptor of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A or selected coordinates thereof; b) employing computational means to dock a first binding partner in the binding region; c) quantitating the interaction energy between the binding partner and all or part of a binding region for different orientations of the binding partner; and d) selecting the orientation of the binding partner with the most favorable interaction energy.

The docking may be facilitated by the quantitated interaction energy and energy minimization with or without molecular dynamics simulations may be performed simultaneously with or following step (b). The method may further comprise the steps of; (e) repeating steps (b) through (d) with a second binding partner; and (f) selecting at least one of the first or second binding partner that interacts more favourably with a binding region based on the quantitated interaction energy of the first or second binding partner. In another embodiment, selection may involve screening a binding partner to associate at a deformation energy of binding of less than -7 kcal/mol with an adenosine A_∑A receptor binding region comprising: (a) providing the coordinates of adenosine A¾ receptor of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1 .285 A or selected coordinates thereof and employing computational means which utilise coordinates to dock the binding partner into a binding region; (b) quantifying the deformation energy of binding between the binding partner and the binding region; and (d) selecting a binding partner that associates with an adenosine A2A receptor binding region at a deformation energy of binding of less than -7 kcal/mol. It is appreciated that in some instances high throughput screening of binding partners is preferred and that methods of the invention may be used as "library screening" methods, a term well known to those skilled in the art. Thus, the binding partner may be a library of binding partners. For example, the library may be a peptide or protein library produced, for example, by ribosome display or an antibody library prepared either in vivo, ex vivo or in vitro. Methodologies for preparing and screening such libraries are known in the art.

Determination of the three-dimensional structure of the adenosine A2A receptor provides important information about the binding sites of adenosine A∑_A receptors, particularly when comparisons are made with other adenosine receptors. This information may then be used for rational design and modification of adenosine A2A receptor binding partners, e.g. by computational techniques which identify possible binding ligands for the binding sites, by

2B enabling linked-fragment approaches to drug design, and by enabling the identification and location of bound ligands using X-ray crystallographic analysis. These techniques are discussed in more detail below. Thus as a result of the determination of the adenosine A₂A receptor three-dimensional structure, more purely computational techniques for rational drug design may also be used to design structures whose interaction with adenosine A_¾ receptor is better understood (for an overview of these techniques see e.g. Walters et al (Drug Discovery Today, Vol.3, No.4, (1998), 160-178; Abagyan, R.; Totrov, . Curr. Opin. Chem. Biol. 2001 , 5, 375-382). For example, automated ligand-receptor docking programs (discussed e.g. by Jones et al. in Current Opinion in Biotechnology, Vol.6, (1995), 652-656 and Halperin, I.; Ma, B.; Wolfson, H. ; Nussinov, R. Proteins 2002, 47, 409-443), which require accurate information on the atomic coordinates of target receptors may be used. The aspects of the invention described herein which utilize the adenosine A2A receptor structure in silico may be equally applied to both the human adenosine A2A receptor structure of of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; and, by predicting the three-dimensional structural representation of a target protein, or part thereof, by modelling the structural representation on all or the selected coordinates of the adenosine A_M receptor or selected coordinates thereof, to the models of target proteins obtained by the first and second aspects of the invention. Thus having determined a conformation of a target protein, for example an adenosine A2A receptor, by the methods described above, such a conformation may be used in a computer-based method of rational drug design as described herein. In addition, the availability of the structure of the adenosine A2A receptor will allow the generation of highly predictive pharmacophore models for virtual library screening or ligand design.

Accordingly, a fourth aspect of the invention provides a method for the analysis of the interaction of one or more binding partners with adenosine A2A receptor, comprising: providing a three dimensional structural representation of adenosine A^ receptor as defined by the coordinates of the human adenosine A_2A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; providing a three dimensional structural representation of one or more binding partners to be fitted to the structural representation of adenosine A2A receptor or selected coordinates thereof; and fitting the one of more binding partners to said structure.

This method of the invention is generally applicable for the analysis of known binding partners of adenosine A2A receptor, the development or discovery of binding partners of adenosine A2A receptor, the modification of binding partners of adenosine A2A receptor e.g. to improve or modify one or more of their properties, and the like. Moreover, the methods of the invention are useful in identifying binding partners that are selective for adenosine A2A receptors over other adenosine receptors. For example, comparing corresponding binding regions between adenosine AZA receptors and other adenosine receptors will facilitate the design of adenosine A2A specific binding partners.

It will be desirable to model a sufficient number of atoms of the adenosine A2A receptor as defined by the coordinates of Table (i) or Table (ii) optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, which represent a binding region, e.g. atoms from amino acid residues contributing to the ligand binding site including amino acid residues Tyr 9, Glu 13, lie 60, Phe 62, Ala 63, lie 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278, or atoms from amino acid residues contributing to the dimerisation interface including amino acid residues Val 25, Cys 28, Trp 29, Val 31 , Trp 32, Ser 35, Gin 38, Tyr 43, Phe 44, Val 46, Ser 47, Ala 49, Ala 50, Ala 51 , lie 53, Leu 54, Val 57, Leu 58, Pro 61 , Phe 62, lie 64, Thr 65, Thr 68, Gly 69, Phe 70, Cys 71 , Ala 72, Ala 73, Cys 74, His 75, Gly 76, Phe 79, lie 80, Phe 83, Val 86, Leu 87, Thr 119, Ala 121 , Ala 122, Gly 123, lie 125, Ala 126, lie 127, Trp 129, Val 130, Phe 133, Leu 137 and Met 140. Although every different binding partner bound by adenosine A^ receptor may interact with different parts of a binding region of the protein, the structure of the adenosine A_¾ receptor allows the identification of a number of particular sites which are likely to be involved in many of the interactions of adenosine A2A receptor with a drug candidate. Additional preferred selected coordinates are as described as above with respect to the first aspect of the invention.

In order to provide a three-dimensional structural representation of a binding partner to be fitted to the adenosine A2A receptor structure, the binding partner structural representation may be modelled in three dimensions using commercially available software for this purpose or, if its crystal structure is available, the coordinates of the structure may be used to provide a structural representation of the binding partner for fitting to the adenosine A_¾ receptor structure of the invention.

By "fitting", is meant determining by automatic, or semi-automatic means, interactions between one or more atoms of a candidate binding partner and at least one atom of the adenosine A2A receptor structure of the invention, and calculating the extent to which such interactions are stable. Interactions include attraction and repulsion, brought about by charge, steric, lipophilic, considerations and the like. Charge and steric interactions of this type can be modelled computationally. An example of such computation would be via a force field such as Amber (Cornell et a/. A Second Generation Force Field for the Simulation of Proteins, Nucleic Acids, and Organic Molecules, Journal of the American Chemical Society, (1995), 1 17(19), 5179-97) which would assign partial charges to atoms on the protein and binding partner and evaluate the electrostatic interaction energy between a protein and binding partner atom using the Coulomb potential. The Amber force field would also assign van der Waals energy terms to assess the attractive and repulsive steric interactions between two atoms. Lipophilic interactions can be modeled using a variety of means. For example the ChemScore function (Eldridge M D; Murray C W; Auton T R; Paolini G V; Mee R P Empirical scoring functions: I. The development of a fast empirical scoring function to estimate the binding affinity of binding partners in receptor complexes, Journal of computer- aided molecular design (1997 Sep), 11 (5), 425-45) assigns protein and binding partner atoms as hydrophobic or polar, and a favourable energy term is specified for the interaction between two hydrophobic atoms. Other methods of assessing the hydrophobic contributions to ligand binding are available and these would be known to one skilled in the art. Other methods of assessing interactions are available and would be known to one skilled in the art of designing molecules. Various computer-based methods for fitting are described further herein.

More specifically, the interaction of a binding partner with the adenosine A2A receptor structure of the invention can be examined through the use of computer modelling using a docking program such as GOLD (Jones et al., J. Mol. Biol., 245, 43-53 (1995), Jones et al., J. Mol. Biol., 267, 727-748 (1997)), GRAMM (Vakser, I.A., Proteins , Suppl., 1 :226-230 (1997)), DOCK (Kuntz et al, (1982) J. Mol. Biol., 161 , 269-288; Makino et al, (1997) J.Comput.Chem., 18, 1812-1825), AUTODOCK (Goodsell et al, (1990) Proteins, 8, 195-202, Morris et al, (1998) J.Comput.Chem., 19, 1639- 1662.), FlexX, (Rarey et al, (1996) J.Mol.Biol., 261 , 470-489) or ICM (Abagyan et al, (1994) J.Comput.Chem., 15, 488-506). This procedure can include computer fitting of binding partners to the adenosine A¾ receptor structure to ascertain how well the shape and the chemical structure of the binding partner will bind to an adenosine A2A receptor.

Thus the invention includes a method for the analysis of the interaction of one or more binding partners with adenosine A2A receptor comprising (a) constructing a computer representation of a binding region of the adenosine A_¾ receptor as defined by the coordinates of the human adenosine A2A receptor of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A or selected coordinates thereof (b) selecting a binding partner to be evaluated by a method selected from the group consisting of assembling said binding partner; selecting a binding partner from a small molecule database; de novo ligand design of the binding partner; and modifying a known agonist or inhibitor, or a portion thereof, of an adenosine A^ receptor or homologue thereof; (c) employing computational means to dock said binding partner to be evaluated in a binding region in order to provide an energy-minimized configuration of the binding partner in a binding region; and (d) evaluating the results of said docking to quantify the interaction energy between said binding partner and the binding region.

Also computer-assisted, manual examination of the binding region structure of the adenosine A₂A receptor may be performed. The use of programs such as GRID (Goodford, (1985) J. Med. Chem., 28, 849- 857) - a program that determines probable interaction sites between molecules with various functional groups and an enzyme surface - may also be used to analyse a binding region to predict, for example, the types of modifications which will alter the rate of metabolism of a binding partner. Computer programs can be employed to estimate the attraction, repulsion, and steric hindrance of the adenosine A_¾ receptor structure and a binding partner.

If more than one adenosine A2A receptor binding region is characterized and a plurality of respective smaller molecular fragments are designed or selected, a binding partner may be formed by linking the respective small molecular fragments into a single binding partner, which maintains the relative positions and orientations of the respective small molecular fragments at the binding sites. The single larger binding partner may be formed as a real molecule or by computer modelling. Detailed structural information can then be obtained about the binding of the binding partner to adenosine A2A receptor, and in the light of this information adjustments can be made to the structure or functionality of the binding partner, e.g. to alter its interaction with adenosine A2A receptor. The above steps may be repeated and re- repeated as necessary.

Thus, the three dimensional structural representation of the one or more binding partners of the third and fourth aspects of the invention may be obtained by: providing structural representations of a plurality of molecular fragments; fitting the structural representation of each of the molecular fragments to the coordinates of the human adenosine A_¾ receptor structural representation of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue C-a atoms of not more than 1.285 A, or selected coordinates thereof; and assembling the representations of the molecular fragments into one or more representations of single molecules to provide the three-dimensional structural representation of one or more candidate binding partners.

Typically the binding partner or molecule fragment is fitted to at least 5 or 10 non-hydrogen atoms of the adenosine A∑A receptor structure, preferably at least 20, 30, 40, 50, 60, 70, 80 or 90 non-hydrogen atoms and more preferably at least 100, 150, 200, 250, 300, 350, 400, 450, or 500 atoms and even more preferably at least 600, 700, 800, 900, 1000, 1 100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100 or 2200 non-hydrogen atoms. The invention includes screening methods to identify drugs or lead compounds of use in treating a disease or condition. For example, large numbers of binding partners, for example in a chemical database, can be screened for their ability to bind to adenosine A_∑A receptor.

It is appreciated that in the methods described herein, which may be drug screening methods, a term well known to those skilled in the art, the binding partner may be a drug-like compound or lead compound for the development of a drug-like compound.

The term "drug-like compound" is well known to those skilled in the art, and may include the meaning of a compound that has characteristics that may make it suitable for use in medicine, for example as the active ingredient in a medicament. Thus, for example, a druglike compound may be a molecule that may be synthesised by the techniques of organic chemistry, less preferably by techniques of molecular biology or biochemistry, and is preferably a small molecule, which may be of less than 5000 daltons (such as less than 500 daltons) and which may be water-soluble. A drug-like compound may additionally exhibit features of selective interaction with a particular protein or proteins and be bioavailable and/or able to penetrate target cellular membranes or the blood:brain barrier, but it will be appreciated that these features are not essential.

The term "lead compound" is similarly well known to those skilled in the art, and may include the meaning that the compound, whilst not itself suitable for use as a drug (for example because it is only weakly potent against its intended target, non-selective in its action, unstable, poorly soluble, difficult to synthesise or has poor bioavailability) may provide a starting-point for the design of other compounds that may have more desirable characteristics.

Thus in one embodiment of the methods of third and fourth aspects of the invention, the methods further comprise modifying the structural representation of the binding partner so as to increase or decrease their interaction with adenosine A_2A receptor. For example, once a binding partner has been designed or selected by the above methods, the efficiency with which that binding partner may bind to an adenosine A receptor may be tested and optimised, for example by computational evaluation. For example, a binding partner designed or selected as binding to an adenosine A_M receptor may be further computationally optimised so that in its bound state it would preferably lack repulsive electrostatic interaction with the target adenosine A2A receptor and with the surrounding water molecules. Such non-complementary electrostatic interactions include repulsive charge-charge, dipole-dipole and charge-dipole interactions.

Furthermore, it is often desired that binding partners demonstrate a relatively small difference in energy between the bound and free states (i.e., a small deformation energy of binding). Thus, binding partners may be designed with a deformation energy of binding of not greater than about 10 kcal/mole, more preferably, not greater than 7 kcal/mole. Binding partners may interact with the binding region in more than one conformation that is similar in overall binding energy. In those cases, the deformation energy of binding is taken to be the difference between the energy of the free binding partner and the average energy of the conformations observed when the binding partner binds to the protein.

Specific computer software is available in the art to evaluate compound deformation energy and electrostatic interactions as detailed in WO2008/068534 (see, for example, page 34) incorporated herein by reference. By modifying the structural representation we include, for example, adding molecular scaffolding, adding or varying functional groups, or connecting the molecule with other molecules (e.g. using a fragment linking approach) such that the chemical structure of the binding partner is changed while its original binding to adenosine A₂A receptor capability is increased or decreased. Such optimisation is regularly undertaken during drug development programmes to e.g. enhance potency, promote pharmacological acceptability, increase chemical stability etc. of lead compounds.

Examples of modifications include substitutions or removal of groups containing residues which interact with the amino acid side chain groups of the adenosine A2A receptor structure of the invention, as described further in relation to the β-adrenergic receptor in WO2008/068534 (see for example, page 35), incorporated herein by reference.

The potential binding effect of a binding partner on adenosine A¾ receptor may be analysed prior to its actual synthesis and testing by the use of computer modeling techniques. If the theoretical structure of the given entity suggests insufficient interaction and association between it and the adenosine A2A receptor, testing of the entity is obviated. However, if computer modelling indicates a strong interaction, the molecule may then be synthesized and tested for its ability to bind to an adenosine A receptor. In this manner, synthesis of inoperative compounds may be avoided.

Thus in a further embodiment of the third and fourth aspects of the invention, the methods further comprise the steps of obtaining or synthesising the one or more binding partners of an adenosine A₂A receptor; and optionally contacting the one or more binding partners with an adenosine A∑A receptor to determine the ability of the one or more binding partners to interact with the adenosine A2A receptor.

Various methods known in the art may be used to determine binding between an adenosine AZA receptor and a binding partner including those described in WO2008/068534 (see for example, pages 35-36) incorporated herein by reference.

Once computer modelling has indicated that a binding partner has a strong interaction, it is appreciated that it may be desirable to crystallise a complex of the adenosine A_2A receptor with that binding partner and analyse its interaction further by X-ray crystallography. Thus in a further embodiment of the third and fourth aspects of the invention, the methods further comprise the steps of obtaining or synthesising the one or more binding partners of an adenosine receptor; forming one or more complexes of the adenosine A_¾ receptor and the one or more binding partners; and analysing the one or more complexes by X-ray crystallography to determine the ability of the one or more binding partners to interact with adenosine A∑A receptor.

Thus, it will be appreciated that another particularly useful drug design technique enabled by this invention is iterative drug design. Iterative drug design is a method for optimizing associations between a protein and a binding partner by determining and evaluating the three-dimensional structures of successive sets of protein/compound complexes, and is described further in WO2008/068534 (see, for example, pages 36 - 37), incorporated herein by reference.

The ability of a binding partner to modify adenosine A^ receptor function may also be tested. For example the ability of a binding partner to modulate an adenosine A2A receptor function could be tested by a number of well known standard methods, described extensively in the prior art.

In addition to in silico analysis and design, the interaction of one or more binding partners with an adenosine A^ receptor may be analysed directly by X-ray crystallography experiments, wherein the coordinates of the human adenosine A∑A receptor of Table (i) or Table (ii) optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, are used to analyse the a crystal complex of the adenosine A_2A receptor and binding partner. This can provide high resolution information of the interaction and can also provide insights into a mechanism by which a binding partner exerts an agonistic or antagonistic function.

Accordingly, a fifth aspect of the invention provides a method for the analysis of the interaction of one or more binding partners with adenosine A receptor, comprising: obtaining or synthesising one or more binding partners; forming one or more crystallised complexes of an adenosine A_2A receptor and a binding partner; and analysing the one or more complexes by X-ray crystallography by employing the coordinates of the human adenosine A_¾ receptor structure, of Table (i) or Table (ii) optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, to determine the ability of the one or more binding partners to interact with the adenosine A2A receptor . Preferences for the selected coordinates in this and all subsequent aspects of the invention are as defined above with respect to the first aspect of the invention.

The analysis of such structures may employ X-ray crystallographic diffraction data from the complex and the coordinates of the human adenosine A2A receptor structure, of Table (i) or Table (ii) optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, to generate a difference Fourier electron density map of the complex. The difference Fourier electron density map may then be analysed. In one embodiment, the one or more crystallised complexes are formed by soaking a crystal of adenosine A_2A receptor with the binding partner to form a complex. Alternatively, the complexes may be obtained by cocrystallising the adenosine A2A receptor with the binding partner. For example a purified adenosine A2A receptor protein sample is incubated over a period of time (usually >1 hr) with a potential binding partner and the complex can then be screened for crystallization conditions. Alternatively, protein crystals containing a first binding partner can be back-soaked to remove this binding partner by placing the crystals into a stabilising solution in which the binding partner is not present. The resultant crystals can then be transferred into a second solution containing a second binding partner and used to produce an X-ray diffraction pattern of adenosine A^ receptor complexed with the second binding partner.

The complexes can be analysed using X-ray diffraction methods, e.g. according to the approach described by Greer et al.,(J of Medicinal Chemistry, Vol. 37, (1994), 1035-1054), and difference Fourier electron density maps can be calculated based on X-ray diffraction patterns of soaked or co-crystallized adenosine A2A receptor and the solved structure of uncomplexed adenosine A_2A receptor . This is described further in WO2008/068534 (see, for example, pages 38 - 39), incorporated herein by reference.

This information may thus be used to optimise known classes of adenosine A₂A receptor binding partners and to design and synthesize novel classes of adenosine A2A receptor binding partners, particularly those which have agonistic or antagonistic properties, and to design drugs with modified adenosine A_2A receptor interactions.

In one approach, the structure of a binding partner bound to an adenosine A∑A receptor may be determined by experiment. This will provide a starting point in the analysis of the binding partner bound to adenosine A2A receptor thus providing those of skill in the art with a detailed insight as to how that particular binding partner interacts with adenosine A^ receptor and the mechanism by which it exerts any function effect. Many of the techniques and approaches applied to structure-based drug design described above rely at some stage on X-ray analysis to identify the binding position of a binding partner in a ligand-protein complex. A common way of doing this is to perform X-ray crystallography on the complex, produce a difference Fourier electron density map, and associate a particular pattern of electron density with the binding partner. However, in order to produce the map (as explained e.g. by Blundell et al., in Protein Crystallography, Academic Press, New York, London and San Francisco, (1976)), it is necessary to know beforehand the protein three dimensional structure (or at least a set of structure factors for the protein crystal). Therefore, determination of the adenosine A^ receptor structure also allows difference Fourier electron density maps of adenosine A2A receptor -binding partner complexes to be produced, determination of the binding position of the binding partner and hence may greatly assist the process of rational drug design.

Accordingly, a sixth aspect of the invention provides a method for predicting the three dimensional structure of a binding partner of unknown structure, or part thereof, which binds to adenosine A_¾ receptor, comprising: providing the coordinates of the adenosine A2A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; providing an X-ray diffraction pattern of adenosine A2A receptor complexed with the binding partner; and using the coordinates to predict at least part of the structure coordinates of the binding partner.

In one embodiment, the X-ray diffraction pattern is obtained from a crystal formed by soaking a crystal of adenosine A2A receptor with the binding partner to form a complex. Alternatively, the X-ray diffraction pattern is obtained from a crystal formed by cocrystallising the adenosine A2A receptor with the binding partner as described above. Alternatively, protein crystals containing a first binding partner can be back-soaked to remove this binding partner and the resultant crystals transferred into a second solution containing a second binding partner as described above.

A mixture of compounds may be soaked or co-crystallized with an adenosine A^ receptor crystal, wherein only one or some of the compounds may be expected to bind to the adenosine A_∑A receptor. The mixture of compounds may comprise a ligand known to bind to adenosine A∑A receptor. As well as the structure of the complex, the identity of the complexing compound(s) is/are then determined. Preferably, the methods of the previous aspects of the invention are computer-based. For example, typically the methods of the previous aspects of the invention make use of the computer systems and computer-readable storage mediums of the ninth and tenth aspects of the invention. A seventh aspect of the invention provides a method for producing a binding partner of adenosine Aa_, receptor comprising: identifying a binding partner according to the third, fourth, fifth or sixth aspects of the invention and synthesising the binding partner.

The binding partner may be synthesised using any suitable technique known in the art including, for example, the techniques of synthetic chemistry, organic chemistry and molecular biology.

It will be appreciated that it may be desirable to test the binding partner in an in vivo or in vitro biological system in order to determine its binding and/or activity and/or its effectiveness. For example, its binding to an adenosine A_¾ receptor may be assessed using any suitable binding assay known in the art including the examples described above. Alternatively, is ability to modulate the adenosine A∑A receptor's ability to form dimers may be assessed. Moreover, its effect on adenosine A_2A receptor function in an in vivo or in vitro assay may be tested. For example, the effect of the binding partner on the adenosine A^ receptor signalling pathway may be determined. For example, the activity may be measured by using a reporter polynucleotide to measure the activity of the adenosine A∑A receptor signalling pathway. By a reporter polynucleotide we include genes which encode a reporter protein whose activity may easily be assayed, for example β-galactosidase, chloramphenicol acetyl transferase (CAT) gene, luciferase or Green Fluorescent Protein (see, for example, Tan et al, 1996 EMBO J 15(17): 4629-42). Several techniques are available in the art to detect and measure expression of a reporter polynucleotide which would be suitable for use in the present invention. Many of these are available in kits both for determining expression in vitro and in vivo. Alternatively, signalling may be assayed by the analysis of downstream targets. For example, a particular protein whose expression is known to be under the control of a specific signalling pathway may be quantified. Protein levels in biological samples can be determined using any suitable method known in the art. For example, protein concentration can be studied by a range of antibody based methods including immunoassays, such as ELISAs, western blotting and radioimmunoassays.

An eight aspect of the invention provides a binding partner produced by the method of the seventh aspect of the invention.

Following identification of a binding partner, it may be manufactured and/or used in the preparation, i.e. manufacture or formulation, of a composition such as a medicament, pharmaceutical composition or drug. These may be administered to individuals.

Accordingly, the invention includes a method for producing a medicament, pharmaceutical composition or drug, the process comprising: (a) providing a binding partner according to the eighth aspect of the invention and (b) preparing a medicament, pharmaceutical composition or drug containing the binding partner.

The medicaments may be used to treat any disorder or condition ameliorated by modulation of the A2A receptor. Examples include: heart failure (such as acute decompensated heart failure and congestive heart failure); kidney failure (e.g. caused by heart failure); oedema; cancer (such as prostate, rectal, renal, ovarian, endometrial, thyroid, pancreatic, particularly breast, colon, bladder, brain, glia, melanoma, pineal gland and, more particularly, lung cancer (e.g. Lewis lung carcinoma)); diabetes; diarrhea; macular degeneration (such as macular degeneration caused by angiogenesis (e.g. retinal angiogenesis)); or, particularly (e.g. for disorders or conditions ameliorated by the inhibition of the A_2a receptor), a disease of the central nervous system such as depression, a cognitive function disease, a neurodegenerative disease (such as Parkinson's disease, Huntington's disease, Alzheimer's disease, amyotrophic lateral sclerosis) and psychoses; an attention related disorder (such as attention deficit disorder (ADD) and attention deficit hyperactivity disorder (ADHD)); extra pyramidal syndrome (e.g. dystonia, akathisia, pseudoparkinsonism and tardive dyskinesia); a disorder of abnormal movement (such as restless leg syndrome (RLS) and periodic limb movement in sleep (PLMS)); cirrhosis; liver fibrosis; fatty liver; dermal fibrosis (e.g. in diseases such as scleroderma); a sleep disorder; stroke; brain injury and neuroinflammation (e.g. migraine or any disorder or condition caused by ischemia, stroke, head injury or CNS inflammation); addictive behaviour.

The invention also provides systems, particularly a computer system, intended to generate structures and/or perform optimisation of binding partner which interact with adenosine A_2A receptor, adenosine A2A receptor homologues or analogues, complexes of adenosine A∑A receptor with binding partners, or complexes of adenosine A2A receptor homologues or analogues with binding partners.

Accordingly, a ninth aspect of the invention provides a computer system, intended to generate three dimensional structural representations of adenosine A_¾ receptor, adenosine A2A receptor homologues or analogues, complexes of adenosine A∑A receptor with binding partners, or complexes of adenosine A_¾ receptor homologues or analogues with binding partners, or, to analyse or optimise binding of binding partners to said adenosine A^ receptor or homologues or analogues, or complexes thereof, the system containing computer-readable data comprising one or more of:

(a) the coordinates of the adenosine A2A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof;

(b) the coordinates of a target adenosine A^ receptor homologue or analogue generated by homology modelling of the target based on the data in (a);

(c) the coordinates of a binding partner generated by interpreting X-ray crystal I ographic data or NMR data by reference to the coordinates of the adenosine A2A receptor structure, of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, and

(d) structure factor data derivable from the coordinates of (a), (b) or (c).

For example the computer system may comprise: (i) a computer-readable data storage. medium comprising data storage material encoded with the computer-readable data; (ii) a working memory for storing instructions for processing said computer-readable data; and (iii) a central-processing unit coupled to said working memory and to said computer-readable data storage medium for processing said computer-readable data and thereby generating structures and/or performing rational drug design. The computer system may further comprise a display coupled to the central-processing unit for displaying structural representations. The invention also provides such systems containing atomic coordinate data of target proteins of unknown structure wherein such data has been generated according to the methods of the invention described herein based on the starting data provided in Table (i) or Table (ii) optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof.

Such data is useful for a number of purposes, including the generation of structures to analyse the mechanisms of action of binding partners and/or to perform rational drug design of binding partners which interact with adenosine A¾_, receptors, such as compounds which are agonists or antagonists.

A tenth aspect of the invention provides a computer-readable storage medium, comprising a data storage material encoded with computer readable data, wherein the data comprises one or more of:

(a) the coordinates of the human adenosine A₂A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof;

(c) the coordinates of a binding partner generated by interpreting X-ray crystallographic data or NMR data by reference to the coordinates of the adenosine A¾ receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, and

(d) structure factor data derivable from the coordinates of (a), (b) or (c).

The invention also includes a computer-readable storage medium comprising a data storage material encoded with a first set of computer-readable data comprising a Fourier transform of at least a portion of the structural coordinates of adenosine A¾ receptor, of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; which data, when combined with a second set of machine readable data comprising an X-ray diffraction pattern of a molecule or molecular complex of unknown structure e.g. a target protein of unknown structure, using a machine programmed with the instructions for using said first set of data and said second set of data, can determine at least a portion of the structure coordinates corresponding to the second set of machine readable data. The invention also provides a computer-readable data storage medium comprising a data storage material encoded with a first set of computer-readable data comprising the structural coordinates of adenosine receptor, of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1 .285 A, or selected coordinates thereof; which, when combined with a second set of machine readable data comprising an X-ray diffraction pattern of a molecule or molecular complex of unknown structure, e.g. a target protein of unknown structure, using a machine programmed with the instructions for using said first set of data and said second set of data, can determine at least a portion of the electron density corresponding to the second set of machine readable data.

It will be appreciated the that the computer-readable storage media of the invention may comprise a data storage material encoded with any of the data generated by carrying out any of the methods of the invention relating to structure solution and selection/design of binding partners to adenosine A2A receptor and drug design.

The invention also includes a method of preparing the computer-readable storage media of the invention comprising encoding a data storage material with the computer-readable data. As used herein, "computer readable media" refers to any medium or media, which can be read and accessed directly by a computer. Such media include, but are not limited to: magnetic storage media such as floppy discs, hard disc storage medium and magnetic tape; optical storage media such as optical discs or CD-ROM; electrical storage media such as RAM and ROM; and hybrids of these categories such as magnetic/optical storage media.

By providing such computer readable media, the atomic coordinate data of the invention can be routinely accessed to model adenosine A2A receptor or selected coordinates thereof. For example, RASMOL (Sayle et al., TIBS, Vol. 20, (1995), 374) is a publicly available computer software package, which allows access and analysis of atomic coordinate data for structure determination and/or rational drug design.

As used herein, "a computer system" refers to the hardware means, software means and data storage means used to analyse the atomic coordinate data of the invention. The minimum hardware means of the computer-based systems of the present invention comprises a central processing unit (CPU), input means, output means and data storage means. Desirably a monitor is provided to visualize structure data. The data storage means may be RAM or means for accessing computer readable media of the invention. Examples of such systems are microcomputer workstations available from Silicon Graphics Incorporated and Sun Microsystems running Unix based, Windows XP or IBM OS/2 operating systems. An eleventh aspect of the invention provides a method for providing data for generating three dimensional structural representations of adenosine A2A receptor, adenosine A2A receptor homologues or analogues, complexes of adenosine A∑A receptor with binding partners, or complexes of adenosine A_∑A receptor homologues or analogues with binding partners, or, for analysing or optimising binding of binding partners to said adenosine A2A receptor or homologues or analogues, or complexes thereof, the method comprising:

(i) establishing communication with a remote device containing computer-readable data comprising at least one of:

(a) the coordinates of the human adenosine A2A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof;

(b) the coordinates of a target adenosine A2A receptor homologue or analogue generated by homology modelling of the target based on the data in (a);

(c) the coordinates of a binding partner generated by interpreting X-ray crystallographic data or NMR data by reference to the coordinates of the human adenosine A_∑A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, and

(d) structure factor data derivable from the coordinates of (a), (b) or (c); and

(ii) receiving said computer-readable data from said remote device.

The computer-readable data received from said remote device, particularly when in the form of the coordinates of the adenosine A2A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, may be used in the methods of the invention described herein, e.g. for the analysis of a binding partner structure with an adenosine A2A receptor structure. Thus the remote device may comprise e.g. a computer system or computer readable media of one of the previous aspects of the invention. The device may be in a different country or jurisdiction from where the computer-readable data is received. The communication may be via the internet, intranet, e-mail etc, transmitted through wires or by wireless means such as by terrestrial radio or by satellite. Typically the communication will be electronic in nature, but some or all of the communication pathway may be optical, for example, over optical fibers. A twelfth aspect of the invention provides a method of obtaining a three dimensional structural representation of a crystal of an adenosine Az<_\ receptor, which method comprises providing the coordinates of the human adenosine A_∑A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, and generating a three-dimensional structural representation of said coordinates.

For example, the structural representation may be a physical representation or a computer generated representation. Examples of representations are described above and include, for example, any of a wire-frame model, a chicken-wire model, a ball-and-stick model, a space- filling model, a stick model, a ribbon model, a snake model, an arrow and cylinder model, an electron density map or a molecular surface model.

Computer representations can be generated or displayed by commercially available software programs including for example QUANTA (Accelrys .COPYRIGHT.2001 , 2002), O (Jones et al., Acta Crystallogr. A47, pp. 1 10-1 19 (1991)) and RIBBONS (Carson, J. Appl. Crystallogr., 24, pp. 9589-961 (1991 )).

Typically, the computer used to generate the representation comprises (i) a computer- readable data storage medium comprising a data storage material encoded with computer- readable data, wherein said data comprise the coordinates of the adenosine A∑A receptor structure, of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; and (ii) instructions for processing the computer-readable data into a three-dimensional structural representation. The computer may further comprise a display for displaying said three- dimensional representation. A thirteenth aspect of the invention provides a method of predicting one or more sites of interaction of an adenosine A₂A receptor or a homologue thereof, the method comprising: providing the coordinates of the human adenosine A¾ receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; and analysing said coordinates to predict one or more sites of interaction.

For example, a binding region of an adenosine A2A receptor for a particular binding partner can be predicted by modelling where the structure of the binding partner is known. Typically, the fitting and docking methods described above would be used. This method may be used, for example, to predict the site of interaction of a G protein of known structure as described in viz Gray JJ (2006) Curr Op Struc Biol Vol 16, pp 183-193. A fourteenth aspect of the invention provides a method for assessing the activation state of a structure for adenosine AM receptor, comprising: providing the coordinates of the human adenosine A∑A receptor structure, of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; performing a statistical and/or topological analysis of the coordinates; and comparing the results of the analysis with the results of an analysis of coordinates of proteins of known activation states.

For example, protein structures may be compared for similarity by statistical and/or topological analyses (suitable analyses are known in the art and include, for example those described in Grindley et al (1993) J Mol Biol Vol 229: 707-721 and Holm & Sander (1997) Nucl Acids Res Vol 25: 231-234). Highly similar scores would indicate a shared conformational and therefore functional state eg the inactive antagonist state in this case.

One example of statistical analysis is multivariate analysis which is well known in the art and can be done using techniques including principal components analysis, hierarchical cluster analysis, genetic algorithms and neural networks.

By performing a multivariate analysis of the coordinate data of the adenosine A_∑A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A or selected coordinates thereof, and comparing the result of the analysis with the results of the analysis performed on coordinates of proteins with known activation states, it is possible to determine the activation state of the coordinate set analysed. For example, the activation state may be classified as 'active' or 'inactive'. A fifteenth aspect of the invention provides a method of producing a protein with a binding region that has substrate specificity substantially identical to that of adenosine A_2A receptor, the method comprising

a) aligning the amino acid sequence of a target protein with the amino acid sequence of an adenosine A_2A receptor ;

b) identifying the amino acid residues in the target protein that correspond to any one or more of the following positions according to the numbering of the adenosine Aa_, receptor as set out in Figure 15: Tyr 9, Glu 13, lie 60, Phe 62, Ala 63, lie 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278; and

c) making one or more mutations in the amino acid sequence of the target protein to replace one or more identified amino acid residues with the corresponding residue in the adenosine A2A receptor .

By "an amino acid residue that corresponds to" we include an amino acid residue that aligns to the given amino acid residue in adenosine A2A receptor when the adenosine A2A receptor and target protein are aligned using e.g. MacVector and CLUSTALW. For example, amino acid residues contributing to the ligand binding site of adenosine A∑A receptor include amino acid residues Tyr 9, Glu 13, lie 60, Phe 62, Ala 63, lie 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278. Thus a binding site of a particular protein may be engineered using well known molecular biology techniques to contain any one or more of these residues to give it the same substrate specificity. This technique is well known in the art and is described in, for example, Ikuta et al (J Biol Chem (2001 ) 276, 27548-27554) where the authors modified the active site of cdk2, for which they could obtain structural data, to resemble that of cdk4, for which no X-ray structure was available. Preferably, all 40 amino acids in the target portion which correspond to amino acid residues Tyr 9, Glu 13, lie 60, Phe 62, Ala 63, lie 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278 of the adenosine A2A receptor are, if different, replaced. However, it will be appreciated that only 39, 38, 37, 36, 35, 34, 33, 32, 31 , 30, 29, 28, 27, 26, 25, 24, 23, 22, 21 , 20, 19, 18, 17, 16, 15, 14, 13, 12, 1 1 , 10, 9, 8, 7, 6, 5, 4, 3, 2 or 1 amino acid residues may be replaced. Preferences for the target protein are as defined above with respect to the first aspect of the invention.

A sixteenth aspect of the invention provides a method of predicting the location of internal and/or external parts of the structure of adenosine A2A receptor or a homologue thereof, the method comprising: providing the coordinates of the adenosine A_¾ receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof and analysing said coordinates to predict the location of internal and/or external parts of the structure.

For example, from the three dimensional representation, it is possible to read off external parts of the structure, eg surface residues, as well as internal parts, eg residues within the protein core. It will be appreciated that the identification of external protein sequences will be especially useful in the generation of antibodies against an adenosine A¾ receptor.

The crystallisation of the adenosine A_∑A receptor has led to many interesting observations about its structure, including its ligand binding site and its dimerisation interface. Thus it will be appreciated that the invention allows for the generation of mutant adenosine A2A receptors wherein residues corresponding to these areas of interest are mutated to determine their effect on adenosine A_2A receptor function, ligand binding specificity, and dimerisation capability. Accordingly, a seventeenth aspect of the invention provides a mutant adenosine A2A receptor which, when compared to the corresponding wild-type adenosine receptor, has a different amino acid at a position which corresponds to any one or more of the following positions according to the numbering of the human adenosine A2A receptor as set out in Figure 15: Tyr 9, Glu 13, lie 60, Phe 62, Ala 63, lie 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278. As mentioned above, these amino acids define the ligand binding site in the human adenosine A_∑A receptor.

The invention also provides a mutant adenosine A2A receptor which, when compared to the corresponding wild-type adenosine A2A receptor, has a different amino acid at a position which corresponds to any one or more of the following positions according to the numbering of the human adenosine A_∑A receptor as set out in Figure 5: Val 25, Cys 28, Trp 29, Val 31 , Trp 32, Ser 35, Gin 38, Tyr 43, Phe 44, Val 46, Ser 47, Ala 49, Ala 50, Ala 51 , lie 53, Leu 54, Val 57, Leu 58, Pro 61 , Phe 62, lie 64, Thr 65, Thr 68, Gly 69, Phe 70, Cys 71 , Ala 72, Ala 73, Cys 74, His 75, Gly 76, Phe 79, lie 80, Phe 83, Val 86, Leu 87, Thr 119, Ala 121 , Ala 122, Gly 123, lie 125, Ala 126, lie 127, Trp 129, Val 130, Phe 133, Leu 137 and Met 140. As mentioned above, these amino acids define the dimerisation interface in the human adenosine A^ receptor. It is particularly preferred if the mutant adenosine receptor of the invention is one which has at least 20% amino acid sequence identity when compared to the given human adenosine A2A receptor, as determined using MacVector and CLUSTALW. Preferably, the mutant adenosine receptor has at least 30%, 40%, 50%, 60%, 70%, 80%, 85%, 90%, 95% or 99% amino acid sequence identity.

The mutant adenosine receptor may be a mutant of any adenosine receptor provided that it is mutated at one or more of the amino acid positions as stated by reference to the given human adenosine A2A receptor amino acid sequence. Thus, the invention includes a mutant human adenosi ne A2A receptor in which, compared to its parent, one or more of these amino acid residues have been replaced by another amino acid residue. The invention also includes mutant adenosine receptors from other sources in which one or more corresponding amino acids in the parent receptor are replaced by another amino acid residue. For the avoidance of doubt the parent may be an adenosine receptor which has a naturally-occurring sequence, or it may be a truncated form or it may be a fusion, either to the naturally-occurring protein or to a fragment thereof, or it may contain mutations compared to the naturally-occurring sequence, providing that it retains ligand-binding ability.

In an embodiment, the mutant adenosine receptor of the invention has a combination of 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 mutations as described above.

By "corresponding amino acid residue" we include the meaning of the amino acid residue in another adenosine receptor which aligns to the given amino acid residue in the human adenosine A∑A receptor when the human adenosine A_¾ receptor and the other adenosine receptor are compared using MacVector and CLUSTALW.

Residues in proteins can be mutated using standard molecular biology techniques as are well known in the art. An eighteenth aspect of the invention provides a method of making an adenosine A2A receptor crystal comprising: providing purified adenosine A2A receptor; and crystallising the adenosine A_¾ receptor either by using the sitting drop or hanging drop vapour diffusion technique, using a precipitant solution comprising Tris-HCI and 30 - 48% PEG. Preferably, the Tris-HCI buffer has a concentration of between 0.01 and 0.3 such as 0.1 M. It will be appreciated that any similar buffer may be used in place of a Tris-HCI buffer at a concentration of between 0.01 and 0.3 M. Any PEG from PEG400 to PEG5000 may be used, such as from PEG400 to PEG 200 (e.g. PEG 1000).

In a particularly preferred embodiment, the precipitant solution comprises 0.1 M Tris-HCI (pH 8.0 - 8.75), 32-42% PEG 1000, 0.01 -0.3 M MgCI₂, 0.3% w/v n-nonyl - β-D glucopyranoside, 0.1 % w/v 1-Butanol, and 0.05% CYMAL-6 at 277K.

In a further embodiment, the precipitant solution comprises 35-40% PEG1000. A nineteenth aspect of the invention provides a crystal of adenosine A_M receptor having the structure defined by the coordinates of the human adenosine A_2A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof. Typically, the crystal has a resolution of 3.3 A or better.

The space group of the crystal may be either P1 or I222. Thus, in one embodiment the crystal has P1 symmetry with unit cell dimensions a=101.27 (± 15) A, b=101.26 (± 15) A, c=101 .20 (± 15) A, wherein σ= 1 12.45 (± 10)°, β = 103.30 (± 10)° and Y = 1 12.90 (± 10)°.

In another embodiment, the crystal has I222 symmetry with unit cell dimensions a=1 1 1.94 (± 10)° A, b=1 12.55 (± 10)° A, c=125.68 (+ 10/-25) A, wherein a = 90°, β = 90° and γ = 90°.

The invention also includes a co-crystal of adenosine receptor having the structure defined by the coordinates of the adenosine A∑A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than .285 A, or selected coordinates thereof, and a binding partner. Typically, the crystal has a resolution of 3.3 A or better. The invention includes the use of the coordinates of the human adenosine A∑A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof to solve the structure of target proteins of unknown structure. The invention includes the use of the coordinates of the adenosine A^ receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof to identify binding partners of an adenosine A2A receptor. The invention includes the use of the coordinates of the adenosine A2A receptor structure of Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof in methods of drug design where the drugs are aimed at modifying the activity of the adenosine A2A receptor. The invention will now be described in more detail with the aid of the following Figures and Examples.

Figure 1 : Overall Structure of the Adenosine A2A Receptor, (a) Secondary structure of the thermostabilised human _a2A receptor depicted in rainbow colouration (blue to red) from the N- terminus to the C-terminus (N- and C-terminal residues marked), the ligand ZM-241 ,385 is represented as a stick model (blue), oxygen atoms are red and nitrogen atoms deep blue (identical colour scheme used for all subsequent representations). The visible extra- and intracellular loops are labelled. The disordered portion of ECL2 in the crystal structure (Pro149-Gly158) is shown as a dashed line. Dotted lines denote approximate boundaries of the plasma membrane (b) View as in (a), but rotated 90° around the horizontal with extracellular loops in the foreground.

Figure 2: Comparison of A_2A-StaR2 with A2A-T4L and Rhodopsin. (a) Superposition of A^- StaR2 (pale green) with A2A- L (dark red - 3EML) using consensus Ca atoms of TM 1-7, note the difference in TM5/6 helical trajectory. (Inset) Close-up of interactions across the DRY motif of the A; A-StaR2 and A2A-T L demonstrating ionic lock formation in the A2A-StaR2 (Potential hydrogen bonds represented as dashed red lines), (b) Superposition of A₂A-StaR2 (pale green) with bovine rhodopsin (orange -1 F88) using consensus Ca atoms of TM 1 -7. (Inset) close up of interactions across the A2A-StaR2 DRY and Rhodopsin ERY motif illustrating the similarity of the ionic lock (second subscript residue numbers refer to rhodopsin 1 F88).

Figure 3: The A2A-StaR2 Ligand Binding site, (a) Overview of the ligand binding cavity of A2A-StaR2 with ZM-241 ,385. Residues contributing to the binding pocket for ZM-241 ,385 are in pink, the receptor in green and ZM-241 ,385 in blue. Hydrogen bonds are dashed red lines. (b) Comparison of the ligand binding site and loop regions of A2A-StaR2 (green) and A2A-T4L (dark red) with ZM-241 ,385 in blue and tan respectively. Differences in the extracellular regions are evident, in particular the antiparallel β-sheet formed between ECL1 and 2 in the A2A-StaR2 (red asterisk) and the change in helical trajectory (dashed grey lines) for the N- terminus of TM3 propagating from Phe83 3.31.

Figure 4: ICL3 Region of the A_¾ Receptor, (a) Secondary structure cartoon of TM5 and 6 of the A2A-StaR2 receptor (pale green) with the IC3 loop represented as a stick model. The ICL3 is comprised of a well ordered stretch of 6 residues of sequence Q-P-L-P-G-E connecting the two TM helices and maintained by a potential H-bonding network (dashed red lines) involving Arg2226.24/ Met2115.72/ Gln2145.75/ Pro2176.19. (b) View of the putative G protein binding face of ICL3 on the opposite side (180 ° rotation) of TM5 and 6 to that viewed in (a).

Figure 5: Model of an Aaa, Homodimer in an Inactive and Activated State and Heterotrimeric G Protein, (a) Electrostatic surface of A2A-StaR2 with band of charge at the membrane boundaries, rotated 90 ° and 180 ⁰ to show the hydrophobic dimerization interface, (b) Cartoon of the docked homodimer, (monomers are in rainbow colouration (blue to red) from the N- to C-terminus) from the side and from the extracellular surface showing the TM arrangement of the dimer interface and putative inter molecular β-sheet. (c) FRET between A^ dimers native, single mutants F62D and F79D and double mutants, (d) Model of the active A∑_A homodimer (pale green), and ground state (dark grey) in the membrane. The symmetry of the A¾ homodimer amplifies the movement of TM5 and 6 and ICL3 increasing the diameter for G protein binding.

Figure 6: Thermostability of the adenosine A_2A receptor alone, in combination with T4 lysozyme fusion or with thermostabilising mutations. Receptors were solubilised from transiently transfected cells using 0.025%DDM. Samples were heated at the specified temperature for 30 minutes, quenched on ice and the amount of receptor remaining was determined by a single-point binding assay using 100nM [³H]-Z -241 ,385. Data is shown for the wild type A_2a receptor (circles), A_2A-T4L (squares), Α2Α -StaR1 (previously Rant21 , upright triangles), A_¾ -T4 engineered to include the A^-StaRI mutations in combination with the T4L fusion (A54L^{2 52}/T88A^{3 36}/K122A^{4 43} Λ 239Α⁶"¹) (inverted triangles) and vs A₂₃ -StaR2 (diamonds).

Figure 7: Sequence of the wild-type A2A receptor in relation to the secondary structure determined from the structure of A_2A-StaR2. Thermostabilising residues are shown in red circles and the glycosylation site mutation N154A in blue. Disulphide bonds are in yellow. Numbers refer to the first and last residue in each helix (blue boxes) with the Ballesteros- Weinstein numbering in superscript.

Figure 8: Comparison of the pharmacology of A2A-StaR2 and A2A- with the wild type A2A receptor. Radioligand competition binding assays for a range of antagonists (closed circles) and agonists (open circles) were carried out with [³H]-ZM-241 ,385 on membranes from cells transiently transfected with receptors, a) Comparison of A2A-StaR2 with wild type receptor and b) Comparison of A2A-T4 with wild type receptor. pK_D values are listed in Table 2. Solid lines represent Deming regressions for antagonist affinities at wild-type and crystaliographic constructs.

Figure 9: (a) Electron density of the ionic-lock and DRY motif from the refined A2A-StaR2- ZM-241 ,385 co-crystal structure. Specific residues are individually labelled and density calculated from the refined model with the 2F₀ - F_c map rendered at 1.5 σ, and F₀ - F_c map rendered at 3.0 σ (b) F₀ - F_c omit map (no contribution from the ligand) rendered at 3.0 σ for the ligand ZM-241 ,385 - note surrounding density has been omitted for clarity, (c) Electron density example of the ligand binding pocket of the A_2A-StaR2 ZM-241 ,385 co-crystal structure. Specific residues are individually labelled and density calculated from the refined model (with contribution from the ligand) with the 2F₀ - F_c map rendered at 1.5 σ, and F₀ - F_c map rendered at 3.0 σ.

Figure 10: View of each stabilising mutation from StaR2 (green) compared to the corresponding residue in the 3EML structure (blue): a) Ala54^{2 5} (3EML); Leu54 (A_2A StaR2); b) Thr88 ^3.36 (3EML); Ala88 (A_2A StaR2); c) Arg107^{3 55} (3EML); Arg107 (A_2A StaR2); d) Lys122^4.43 (3EML); Ala122 (A_2A StaR2); e) Leu202⁵⁶³ (3EML); Ala202 (A_2A StaR2); f) Leu235^6.37 (3EML); Ala235 (A_2A StaR2); g) Val239 ¹ (3EML); Ala239 (A_2A StaR2); h) Ser277^{7. 2} (3EML); Ala277 (A^ StaR2).

Figure 11 : Effect of thermostabilising mutations in the A_2A-StaR2 structure, (a) Side view of A₂A receptor showing the mutated Leu54^{2 52} in green surrounded by the highly hydrophobic region comprising of Phe62^{2 60}, Phe79^{3 27}, Phe83^{3 31}, Trp129^{4 50} and Phe133 ⁵⁴; (b) Mutation of Arg107^{3 55} to Ala may facilitate hydrogen bond formation between the backbone Ca of Ile106 and Gln207 to effect thermostability; (c) Mutation of residues Thr88 ^{3 36} and Ser277^{7 42} shown in space fill, may energetically favour the highly conserved Trp246^{6 48} micro-switch of the A_2A-StaR2 (found in the CWxP motif of TM6) to sit in the inactive rotameric (Chi1 -60°) state, indicative of the ground state. The Thr88A ^{3 36} and Ser277Ala ⁷ mutations are positioned below the ligand binding pocket on TM3 and TM7 respectively.

Figure 12: (a) Omit electron density map of the A_2A-StaR2 ICL3 loop resolved between helices 5 and 6. Density calculated without the contribution of residues 210-22 (encompassing the ICL3 loop, C-terminus of helix 5 and N-terminus of helix 6). 2F₀ - F_c map rendered at 1.5 σ, and F₀ - Fc map rendered at 2.2 σ. Residue positions for the ICL3 loop are marked, (b) View as in (a) - adapted from main text - showing the final molecular model of the ICL3 loop and equivalent positions of residues in comparison to the omit density shown in (a).

Figure 13: Crystal structures of A_2A-StaR2 (pale green colouration), A2A- 4L - pdb code: 3EML (pink colouration) and AR - pdb code: 2VT4 (yellow colouration) superposed and represented as ribbons for comparison of interactions between TM3 (E/DRY motif), ICL2 and Thr ^{2 39} Specific residues are represented as sticks and residues from PiAR are labelled in grey; residues from A2A are labelled in black. ICL2 displays a high degree of structural conservation between the three receptor models, particularly in the positioning of the Tyr ^{3 60} side chain poised for interaction with Thr ^{2 39} which plays a pivotal role in stabilising the receptor (see main text). Figure 14: Crystal packing. Packing arrangement of A2A-StaR2 monomers within the I222 unit cell. The -25 A-wide regions between receptor molecules in the z direction of the crystal lattice does not appear to contain any density; this is characteristic of regions in membrane protein crystals that are thought to contain high concentrations of detergent molecules creating a "phase" mediating contacts between the adjacent receptors, a) Dimensions of Unit cell; b) View along cell edge - a ; c) View along cell edge - c ; d) View along cell edge - b

Figure 15: Nucleotide and amino acid sequence of the human A2A receptor as crystallised.

Figure 16: Amino acid sequence of wild type human A∑A receptor.

Example 1 : Adenosine A_2A receptor structure reveals a mechanism for dimerization Summary

G protein-coupled receptors play a key role in transducing extracellular signals to the cell interior and are known to function as dimers. Structural information on these receptors has been sparse and the mechanism of receptor dimerization unknown. Here we report the structure of the adenosine A^ receptor in complex with the inverse agonist ZM-241 ,385, the first structure other than rhodopsin to reveal the features of a ground state receptor, in particular, the salt bridge known as the ionic lock is present between helices 3 and 6. The complete third intracellular loop is visible as a highly structured 6 residue motif formed between extended transmembrane helices 5 and 6. A hydrophobic putative dimerisation interface is also found on the monomer involving helices 1 and 4. In-silico docking and modelling of the homodimeric A2A receptor, combined with experimental analyses, provides insight into the molecular mode of A2A dimerization.

Introduction

G protein-coupled receptors (GPCRs) are integral membrane proteins mediating the signalling of a diverse set of ligands including neurotransmitters and metabolites. In humans, there are approximately 370 non-sensory receptors, representing the site of action for -30% of clinically used drugs. Activation of the receptor results in a conformational change propagated to the intracellular surface where the receptor interacts with heterotrimeric G proteins to regulate signalling to ion channels and enzyme pathways. GPCRs can also signal independently of G proteins through β-arrestin and are known to exist as dimers. This multimeric state is likely central to their function and also delivery to the membrane, however, the molecular mechanism and details of GPCR oligomerization remains poorly understood.

The adenosine A_∑A receptor is one of 4 GPCRs (A^ A2A, A₂B, A₃) activated by adenosine. Adenosine represents an important modulator of the central nervous system and periphery. In the brain adenosine controls neuronal excitability and the psychoactive effects of caffeine are mediated by adenosine receptors. A_∑A receptors are located in the striatum and are considered a target for neurodegenerative disease ¹. Α2Α receptors are also expressed on the vasculature and immune cells where they have vasodilatory and anti-inflammatory effects ^{2 3}. There is growing evidence that drugs acting at adenosine receptors represent promising approaches in a wide range of diseases ³.

Mechanistic understanding of ligand binding and activation, as well as our ability to design drugs for GPCRs is hampered by the lack of structural information. Structures of rhodopsin in conformationally distinct states have indicated that receptor activation results in an outward tilt of transmembrane helix (TM) 6 and a rearrangement of hydrogen bonding networks connecting the ligand binding pocket to the cytoplasmic face. The first non-rhodopsin GPCR structure to be obtained was the p₂-adrenergic receptor in complex with an antibody fragment bound to the third intracellular loop (ICL) 3 - a critical domain of the receptor which mediates G protein coupling⁴. A higher resolution structure of the β ²AR was obtained by fusing T4 lysozyme into ICL3⁵ and the same methodology was used to obtain the first structure of the adenosine A2A receptor (A2A-T4L)⁶. The conformation of these receptors remains unclear since insertion of the T4 lysozyme alters the pharmacology and prevents signalling. A structure has also been obtained for the p adrenergic receptor (PiAR) using a mutagenesis approach to stabilise the antagonist state⁷. This was the first non-rhodopsin structure to clearly show features of the cytoplasmic regions of the receptor and revealed the presence of a short well defined helix in ICL2. However, in this structure ICL3 was truncated to assist in crystallisation. The structure of the adenosine A_2A receptor described here provides new insight into the structural features which define the GPCR inactive state, the regions which interact with signal transduction proteins, and how receptors interact to form signalling complexes.

Results and Discussion Thermostabilisation of the A₂A Receptor

To obtain a thermally stable receptor with a pre-defined conformation a technique of conformational stabilisation was employed, as previously used for the iAR⁸ and neurotensin receptor. Such thermostabilised receptors are known as StaRs for 'stabilised receptors' ⁹. The A₂A receptor was previously stabilised in both agonist and inverse agonist conformations¹⁰, however the stabilised inverse agonist receptor known as Rant21 or A2A- StaR1 (containing the stabilising mutations A54L^{2 52}, T88A^{3 36}, K122A^{4 43} , V239A^{6 41}; superscripts refer to Ballesteros-Weinstein numbering) was not considered of sufficient stability for structural studies. Further mutagenesis in the presence of the inverse agonist ligand ZM-241 ,385 20 resulted in the identification of an additional 4 stabilising mutations (R107A^{3 55}, L202A^{5 63}, L235A^{6 37}, S277A^{7 2}) giving an apparent thermostability of 47°C in 0.1 % decylmaltoside (Figure 6) resulting in A2A-StaR2. For crystallisation A_2A-StaR2 was truncated at the C-terminus by 96 amino acids up to Ala316 and included a C-terminal decameric His-tag for purification. An N154A mutation was introduced to remove the glycosylation site (Figure 7).

Pharmacology of the inverse agonist state

The engineered receptor A2A-StaR2 bound ZM-241 ,385 (K_D 1.9nM) and a range of structurally diverse set of antagonists with a similar affinity to the wild type receptor⁹. In contrast, the affinities of agonists including NECA and CGS21860 were reduced by greater than 100-fold and the receptor no longer activated G proteins. This pharmacology is consistent with that expected for the inverse agonist conformation and is similar to the change in pharmacology observed for the stabilised 1AR-m23⁸. This profile differs from A2A- T4L which has a high agonist affinity⁶ more consistent with the active conformation (Figure 8). The thermostabilising residues Thr88^{3 36} and Ser277^{7 42} lie at the bottom of the predicted agonist binding pocket and have previously been shown to play a role in agonist binding and activation^12'13. Mutation of these residues is highly stabilising to the antagonist but not the agonist conformation suggesting that they play a key role in the conformational selection of the receptor. The reduced agonist binding of A₂A-StaR2 is likely to be in part a direct effect of these mutations as well as the conformational stabilisation of the inverse agonist state as previously shown for piAR-m23⁸.

Structure of the A₂A Receptor The purified A_2A-StaR2 was crystallised by vapour diffusion in sitting drops (see supplementary methods) and data collected on 124 at the Diamond Light Source (Oxfordshire, UK) corresponding to a 99.9% complete dataset to 3.3A. The structure was solved by molecular replacement with 3EML with 1 copy of the A_2A-StaR2 per crystallographic asymmetric unit. Statistics for data collection and refinement are given in Table 1.

Table 2

Comparison of the pharmacology of A2A-StaR2 and A_2A-T4 with the wild type A_¾ receptor. Affinity values for a range of agonist and antagonists from radioligand competition binding assays using [³H]-ZM241 ,385 with membranes prepared from HEK293 cells transiently transfected with receptors.

Table 3.

The Ca(StaR2)-Ca(3EML) distances (angstroms) measured for each stabilising mutation by comparing the aligned structures of A_2A-StaR2 and 3EML.

The overall structure is shown in Figure 1 with residues 7-149 and 158-305 and the ligand ZM-241 ,385 (Figure 9). The A_2A-StaR2 maintains the general global architecture of a seven transmembrane (TM) receptor with the TM a helices comprised of residues 7^{1 .33}-32^{1 .58} (TM1 ),

₃₉2.37_₆₇2 . 0^ ₀₇3.55 ₁ g 746.36 _2 .22,

258^{6 60} (TM6), 266^{7 3} -291^{7 5S} (TM7). The structure of the A2A-StaR2 contains a complete model of the native GPCR ICL3 loop structure between TM5 and 6, and is comprised of a six residue motif Gln214-ProLeuProGlyGlu219 between helices which would extend beyond the membrane into the cytoplasm. The structure of A2A-StaR2 is overall similar to the structure of the previously determined A^ receptor fusion protein structure where T4 lysozyme replaces the ICL3 loop (A2A- 4L, PDB code 3EML). The best agreement between the two structures occurs in TMs 1 , 2, 3, 4 and 7, where comparison of equivalent Ca atoms has an r.m.s.d of 0.51 A. However, superposition of TM5 and TM6 of the A2A-StaR2 with A2A-T4L (residues 174-203 and 222-258) reveals significant differences in Cp(r.m.s.d = 1.62A). Analyses of the 8 stabilising mutations that form the A^-StaF^ do not appear to perturb the core receptor structure in comparison to A2A-T L (Figure 10) but may strengthen helical interactions (Figure 1 1).

In A2A-T4L the C-terminal portion of TM5 displaced out of the helical bundle and moves laterally towards TM6 by comparison to A2A-StaR2, a difference which propagates along TM5 from Val186 ^{5 47}. In addition, the A2A-StaR2 TM5 C-terminus contains 2 extra helical turns leading into the IC3 loop (Figure 2a), similar to the extended helices observed in squid rhodopsin¹⁴. TM6 of the A2A-T4L is also moved laterally towards TM7 across residues Lys233^{6 35} to Ile237^{6 39} in comparison to the A2A-StaR2. The intracellular end of TM6 in A - T4L is rotated towards TM5 by pivoting ~42° at Val229 ^{6 31} away from the helical bundle, whilst TM6 of A2A-StaR2 continues to helix pack with TM5 (Figure 2a). The global position of TM5 and 6 of the A2A-StaR2 are also in closer agreement to the ground state of rhodopsin (PDB code: 1 F88) (Figure 2b) than A2A-T4L. An outward movement of TM6 is observed in the rhodopsin active conformation as demonstrated by structural and biophysical studies ¹⁵. Insertion of T4 lysozyme into ICL3 of both the A2A and β2 receptors increases the affinity of agonists⁶ as would be expected in an active agonist like conformation. This may be a result of the T4-fusion inducing a movement in TM5 and 6 which changes the properties of the ligand binding site. One of the most highly conserved sequence motifs in GPCRs is the E/DRY motif in TM3. In bovine rhodopsin the side chain of Arg135^{3 50} within the E/DRY motif hydrogen bonds to the side chain of Glu247^6'30 at the N-terminus of T 6 to form the so-called 'ionic lock' ¹⁶, part of a network of interactions bridging TM3 and TM6 and stabilising the inactive-state conformation. Inverse agonists are considered to preferentially bind to and stabilise this inactive conformation thus reducing any basal activity. During activation the ionic lock is broken allowing the outward movement of TM6. It is notable that, other than rhodopsin, none of the GPCR structures reported to date (β1 , β2 and A2A-T4L) have an ionic lock formed. In contrast, the A2A-StaR2 structure, stabilised in the presence of the inverse agonist ZM- 241 ,385 and co-crystallised in complex with this ligand has the potential ionic lock in place, consistent with an inactive conformation and pharmacology. The side chain of Glu228^6'30 and Arg102^{3 50} are in a similar conformation to that found in dark-state rhodopsin (Figure 2a and Figure 9a) and the absence of the lock in the A2A-T4L structure (see Figure 2b) appears to be the result of an outward movement and rotation in TM6 resulting from the 1CL3 T4 lysozyme fusion. Consistent with rhodopsin inverse agonist structures in the protein data bank, the highly conserved Trp246^{6 48}, known as the rotamer toggle switch of the A^-StaF^ (found in the CWxP motif of TM6) is also found in its "inactive" rotameric state (Chi1 = -60°). The stabilising mutations Thr88Ala ^{3 36} and Ser277Ala ^{7 42} are in close proximity to Trp246^{6 48} and may promote the inverse agonist conformation by facilitating the inactive position of this residue (Figure 11 ). Comparison between the extracellular regions of A_2A-StaR2 and A2A- 4L structures show a number of significant differences. The extracellular surface of the A_¾ receptor consists primarily of the second and third extracellular loops (ECL2 and ECL3) with ECL2 ordered through disulphide linkages to ECL1⁶. Interestingly, changes at the top of the ligand binding site involving a rotamer change in His264^{6 66} and movement of the CysProAspCys motif (still maintaining the disulphide link between Cys259^{6 61} and Cys262^{6 64}) away from the entrance of the ligand binding cavity presents a more "open" entrance to allow access of the ligand to the binding pocket. Another interesting feature of the extracellular surface of the A_2A-StaR2 is a slightly extended anti-parallel β-sheet formed by Gly69^{2 67} - Ala72²⁷⁰ of ECL1 and Gln163^{5 25}-Cys166^{5 27} of ECL2 (see Figure 3a) similar to that observed in rhodopsin (pdb code : 1 F88). The exact cause of the differences in the extracellular domains of the two A2A structures remains unclear. Whilst it is possible that this is due to crystal contacts between A_2A monomers, or to modifications of either receptor, such differences may also reflect the true ground state receptor conformation. Ligand binding mode of ZM-241, 385

The structure of the A_2A-StaR2 receptor presents a highly open extracellular configuration exposing the entrance to the ligand binding cavity (Figure 3a). Equivalent Ca atoms of ECL1 of A_2A-StaR2 are a difference of ~ 3 A away from the entrance to the ligand binding pocket compared to ECL1 of A2A-T4L This difference propagates from a kink at the top of TM3 at Phe83 ^{3 31} (Figure 3b). In maintaining the disulphide bond between Cys71^{2 69} and Cys159^{5 20}, this difference in ECL1 also results in a ~3 A difference of ECL2 (residues 157-164) laterally and towards the plasma membrane. The helical portion of ECL2 remains suitably positioned to supply the π stacking of Phe168^{5 29} against the triazolotriazine component of ZM-241 ,385. This bicyclic component is located between the aromatic Phe168^{5 29} and a hydrophobic surface supplied by Ile274^{7 39} including, to a lesser extent, Leu249^{6 51}. Finally the carboxamide carbonyl of Asn253^{6 55} hydrogen bonds to the NH2 group of ZM-241 ,385 (Figure 3a).

Further inspection of the ligand binding site shows the furan and bicyclic triazolotriazine portion of the ligand is rotated in comparison to the A2A-T4L structure, pivoting at N1 (triazolotriazine moiety) of the ligand and propagating to a movement of the furan moiety -1 A closer towards Leu249 ^{6 51} thereby increasing the previously reported ligand distance to the central Trp246^{6 48} micro switch by 0.5 A. The furan ring of ZM-241 ,385, is located deep in the ligand binding cavity and positioned towards TM5 and TM7 to make a hydrophobic interaction with Trp246^{6 48} and His250^{6 52} as well as a polar contact to Asn253^{6 55}.

The phenol group of the ligand is found in a cleft formed by Glu13^{1 39}, Ala63^{2 64}, Ile66^{2 64}, Ser67^{2 65} , Leu267^{7 32} , Met270^{7 35}, Ile274^{7 39}, His278^{7 43} and Tyr271^{7 36} at the extracellular ends of helices 1 , 2 and 7 with Tyr271^{7 36} displaying a rotation towards helix 1 to incorporate this conformation of the phenolic moiety. The phenolic hydroxyl is also poised to make an additional H-bond with the backbone carbonyl of Ala63^{2 64} which itself chelates a water molecule in the previously reported Aa_, structure⁵.

The observed differences in ligand position between the A₂A-StaR2 and A2A-T4L structures may be due to the conformational and binding-site differences discussed above affecting the energetics of binding, but may also be attributable at least in part to alternative binding modes of this relatively flexible ligand.

Cytoplasmic Loop Structure

A striking new feature of the A2A-StaR2 structure is the extended nature of TM5 and TM6 which project ~15A into the cytoplasm. TM5 extends through to Ser213⁵⁷⁴ and is connected to TM6 by 6 residues (Gln214, Pro215, Leu216, Pro217, Gly218, Glu219) with the helix of TM6 commencing at Arg220^{6 22} (see Figure 4a and Figure 12). The ordered and extended nature of the ICL3 (also seen in squid rhodopsin) is contributed to by Pro215 and Pro217 flanking Leu216, but also through a network of potential hydrogen bonds. Arg222^{6 24} is positioned to form a H-bond to the main chain carbonyl of et21 1^{5 72} , which itself forms a putative H-bond bridging network via Gln214^{5 75} to the main chain carbonyl of Pro217^{6 19} and Leu216^{6 8}. Furthermore, the mutations R222A and Q214A have significant destabilising effects on the receptor confirming their structural role in ICL3 (data not shown). Cross linking, chimera/mutagenesis and peptide competition studies on rhodopsin and other GPCRs have demonstrated that the ICL3 region is directly associated with, and plays a critical role in, G protein binding and activation. In the opsin structure, the inner side of ΤΜ5 ΓΜ6 provides a hydrophobic interaction surface for the carboxy terminus of the G protein a-subunit¹⁵. The A2A-StaR2 structure provides the first view of this hydrophobic G protein binding architecture in the human receptor (Figure 4b) displaying a strikingly similar hydrophobic platform contributed by Met211^{5 72}, Leu216^{6 18} , Ala221^{6 23} , Leu225^{6 27}.

The ICL2 loop has been shown to play an important role in G protein coupling and activation. ICL2 in the A2A-StaR2 structure forms a short helical structure similar to that found in iAR and A2A-T4L but not in P2AR-T4L or the pAR-Fab complex where lattice contacts disrupt the loop. An additional helical turn of ICL2 is observed in the A2A-StaR2 compared to A2A-T L similar to that seen in P1AR. The hydroxyl group of Tyr1 12^{3 60} in the ICL2 loop forms a putative hydrogen bond to Asp101³⁴⁹ in the DRY motif at the bottom of TM3, A feature also observed in the A2A-T4L and p1AR structures. Additionally the hydroxyl of Thr41^{2 39} (36.5% conserved across all GPCRs) could also hydrogen bond to Asp10l³⁴⁹. Mutagenesis of Tyr112^{3 60} in A2A, and the equivalent tyrosine across a range of receptors, is found to cause an increase in constitutive activity further emphasising the role of this residue in G protein coupling.

Model of a GPCR Homodimer

The A_2A receptor is known to exist primarily as a homodimer and can also complex with other GPCRs such as the dopamine D₂ receptor. Analysis of the electrostatic surface potential of the A2A-StaR2 receptor reveals a pronounced continuous band of positive surface charge around the extra and intracellular extremities, composed of outward facing charged residues for interaction with the phosphate head-groups on the inner and outer surface of the plasma membrane lipids, a startling feature of the structure is that these regions of charged amino acids are completely absent on the face of the receptor between TM1 and TM4 presenting a flat hydrophobic putative dimerization interface (Figure 5a). Rigid body protein-protein docking and molecular dynamics simulations in the membrane environment (see methods) between two A_¾ monomers results in consistent and stable A2A dimer formation in a helix 1-4 interface (Figure 5b) burying this hydrophobic surface and, importantly, completing a continuous charge band around the homodimer pair. Analysis of the modelled interface which buries -2200A² of molecular surface between the A2A monomers shows a group of hydrophobic aromatics from each monomer along the centre of the dimer interface consisting of Tyr43^{2 41}, Phe62^{2 60}, Phe79^{3 7}, Phe83^{3 31}, Trp129^{4 5}, Phe133^{4 54}. Additionally, the dimer appears locked in place by Trp29^{1 55} and Trp32^{1 58} from helix 1 of each monomer which sit adjacent to the N-terminus of helix 4 and hold it packed against helices 2 and 3. Inspection of the extracellular surface of the homodimer shows that the monomers complete an anti-parallel inter-molecular β-sheet formed by Gly69 ⁶⁷ - Ala72^{2 0} of ECL1 and Gln163⁵²⁵-Cys166^{5 27} of ECL2. A2A homodimerization can be clearly demonstrated using fluorescence resonance energy transfer (FRET). The introduction of mutations in the proposed dimer interface which would result in unfavourable charge interactions between contact residues in TM2 and TM3 (Phe62^{2 60} Asp and Phe79^{3 27} Asp) caused a significant reduction in FRET despite having no effect on the level of receptor expression, providing experimental evidence in support of the mechanism of dimerization (Figure 5c). The structure of GPCR dimers are controversial since different approaches such as cross-linking studies, mutagenesis and modelling have implicated a wide variety of potential interfaces. Interactions involving helix 4 have been previously identified in 2D crystals¹⁷ of squid rhodopsin and in cross linking studies between dopamine dimers. In a recent modelling paper based on the A2A-T4 structure a number of different possible models were suggested involving helix 1 as an interface.

Applying our knowledge of the structural changes which take place upon receptor activation derived from the rhodopsin/opsin structures¹⁵ allows us to speculate on a mechanism by which activation of the A2A dimer can lead to G protein signalling (Figure 5d). The transition from ground state to activated receptor would result in an outward movement of the symmetrical extended ICL3 loops with helix 6 displaying the largest swing. This movement would increase the available diameter of potential G protein binding surface from -42 to ~51A thereby facilitating the docking of a single heterotrimeric G protein into the cradle formed between intracellular loops of the receptor homodimer and subsequent activation of the signalling cascade. Methods

Purification and Crystallisation. A2A-StaR2 was expressed using the baculovirus system and purified in 0.1 % decylmaltoside, with detergent exchange to n-nonyl- 3-d-glucopyranoside via IMAC. Crystals were grown by vapour diffusion in sitting drops after addition of an equal volume of reservoir solution (0.1 M Tris-HCI (pH 8.0-8.75), 32-42% PEG 1000, 0.25M MgCI₂, 0.3% w/v n-nonyl- ?-D- glucopyranoside, 0.1 % w/v 1-Butanol, and 0.05% CYMAL-6 at 277K).

Data Collection, Structure Solution and Refinement.

Diffraction data from two cryo-cooled crystals were collected at I24 of the Diamond Light Source , Oxford, UK at a wavelength of 0.9777 A using a Dectris Pilatus 6M detector. Images were integrated and scaled using the XDS ¹⁸ and SCALA ¹⁹ in the CCP4 program suite ²⁰. The number of observed reflections corresponds to a 99.9% complete dataset to 3.3 A. Molecular replacement was performed using the PHASER program²¹ searching for one copy of 3EML (without T4 insertion)⁶. Crystallographic modelling and rebuilding was performed using COOT²² , and refinement performed using PHENIX ²³. All figures were produced using PyMOL.

The refined model comprises 2,250 protein atoms and the ligand ZM-241 ,385 (Table 1). Of the 292 amino acid residues, 90.94% are in the preferred region of the Ramachandran plot, 8.36% in the allowed region and 0.70% in the outlier region.

Rigid Body Docking

Rigid body docking was carried out with the program GRAMM, according to best practice in the technical reference, low resolution runs were carried out to determine potential areas of global minimum before high resolution runs were conducted to obtain more accurate predictions.

Molecular Dynamics Simulation

The A_2A dimer system was setup within VMD with a POPC membrane, solvent and counter ions added to give ~90K atoms. The system was then minimized and equilibrated within NAMD before undertaking the 5ns molecular dynamics simulation in 1fs steps. The dimer interface contact area was calculated using the solvent accessible surface area (sasa) procedure within VMD.

Ligand-Binding and thermostability assays.

Transfected HE 293T cells were resuspended in ice cold buffer [50mM Tris pH7.4; 400m NaCI; 1 % DDM and protease inhibitors (Complete, Roche)]. After incubation for 1 h at 4°C, samples were centrifuged (16,000xg, 20min, 4°C) and the supernatant was detergent exchanged into 0.1% decylmaltoside (DM) from Ni-NTA resin.

Thermostability was assessed by incubating with [³H]-ZM241385 radioligand (100nM) at increasing temperatures for 30 min followed by a 5 minutes incubation on ice. Receptor bound and free radioligand were separated by gel filtration as previously described¹

Membrane radioligand binding

Membranes from transfected HEK293 cells were incubated with [³H]-ZM241385 as previously described⁹ in the presence or absence competing compounds. After 90 min incubation at room temperature assays were terminated by rapid filtration and bound ligand measured by scintillation spectroscopy.

Purification and crystallization

A2A-StaR2 with a C-terminal decahistidine tag was expressed in Trichoplusia ni (Tni) cells using the FastBac expression system (Invitrogen). Membranes were pelleted by centrifugation at 235,000 g for 1 hour, and subsequently solubilized by addition of 1.5% decyl- -D-maltopyranoside (DM). The solubilised material was applied to a 5 ml Ni-NTA superflow cartridge (Qiagen) pre-equilibrated with buffer with the addition of 0.15% DM. The column was washed at 1 ml/min with 10 column volumes of the same buffer and then eluted with a linear gradient ( 5 column volumes) imidazole in buffer supplemented with 0.15% DM. In cases where a detergent exchange step was required, upon immobilization of the protein onto the Ni-NTA matrix, the column was washed with 20 column volumes of buffer B with the addition of CMC levels of the exchange detergent. Protein was detected with an on-line detector to monitor A₂so and column fractions were collected and analyzed by SDS PAGE gel. Fractions containing the ca. 35 kDa protein were pooled and concentrated using a YM50 Amicon ultrafiltration membrane to a final volume of 200 μΙ. The protein sample was applied to a 10/30 S200 size exclusion column pre-equilibrated with buffer with the addition of 0.1% DM (or exchange detergent) and eluted at 0.5 ml/min. Fractions containing the ca. 35 kDa protein were pooled and concentrated using a YM50 Amicon ultrafiltration membrane to a final concentration of 10 mg/ml and stored at -80°C.

Data collection, structure solution and refinement

A total of 20,064 crystallization trials were set-up. Adenosine A2A StaR2 - ZM-241 ,385 co- crystals were grown by the vapour diffusion method in sitting drops (space group I222 a = 11 1.93A b = 1 12.55 A c = 125.68A α = β= γ = 90.0 ⁰ ). Drops were prepared by mixing 280μΜ Adenosine A2A StaR - ZM-241 ,385 complex prepared as outlined previously, with equal volumes of 0.1 M Tris-HCI (8.0-8.75), 32-42% PEG 1000, 0.25M MgCI₂, 0.3% n-nonyl- β-d-glucopyranoside , 0.1 % w/v 1-Butanol, and 0.05% CYMAL-6. Crystals appeared after 24 h and took three weeks to reach maximum dimensions of 100 x 500 x 50μΜ.

Diffraction data from two cryo-cooled native crystals was collected at I24 of the Diamond Light Source (DLS) in Oxfordshire, UK at a wavelength of 0.9777 A using a Dectris Pilatus 6M detector. Images were integrated and scaled using the programs XDS¹⁸ and SCALA¹⁹ in the CCP4 suite of programs²⁰. The number of observed reflections corresponds to a 99.9% complete dataset to 3.29 A resolution with high redundancy. Molecular replacement was performed using the PHASER²¹ searching for one copy of a human Adenosine A2A receptor (pdb code: 3EML). Crystallographic modelling and manual rebuilding was performed using the program COOT²² , and crystallographic refinement performed using simulated annealing, restrained refinement and TLS in the PHENIX package²³. The refined model comprises 2,250 protein atoms and 24 ligand atoms (Table 1 ). Of the 292 amino acid residues, 90.94% are in the preferred region of the Ramachandran plot, 8.36% in the allowed region and 0.70% in the outlier region.

Rigid Body Docking

Rigid body docking was carried out with the program GRAMM, according to best practice in the technical reference with low resolution runs performed first to determine potential areas of global minimum before high resolution runs were conducted to obtain more accurate predictions. The high resolution runs were performed using the generic mode with a grid spacing of 3A, a VDW sphere radius of atoms and an all representation in 10° angles of rotation. Overall 1000 high resolution solutions were then analysed and filtered according to their angle, offset and displacement within a membrane environment. The ~50 best resultant solutions were then analysed further within CHARMm, by minimizing with helical constraints, calculating the monomer/monomer contact surface area and the interaction energy at the contact surface. Molecular Dynamics Simulation

The predicted dimeric form of A2A, that was favourably reproduced via the rigid body protein- protein docking experiment, was used for our 5ns molecular dynamics simulation within NAMD. The system was initially setup in VMD within a 90 A x 90A POPC membrane and surrounding solvent giving ~90K atoms. Counter ions were then added to ensure overall neutrality so the particle mesh ewald (PME) electrostatics could be used in the simulation. The system was then initially minimized using ABNR minimization for 20K steps in NAMD whilst holding the dimer rigid, a further minimization was then undertaken with constraints on the dimer. The system was then heated to 310°K in two 20ps steps, using langevin dynamics, holding all backbone atoms and then Ca atoms within the TM regions constrained. Langevin piston pressure control was then used to equilibrate the pressure of the system to 1 atm for 100ps whilst holding the Ca atoms within the TM regions constrained before a further unconstrained equilibration of 100ps was performed. The final simulation for 5ns was conducted with no constraints in 1fs steps. Analysis of the dimer simulation was done by measuring the monomer/monomer contact area over the timeframe of the simulation. The contact area was calculated for each frame of the simulation using the solvent accessible surface area (sasa) procedure within VMD.

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16. Vogel, R. ef al. Functional role of the "ionic lock"-an interhelical hydrogen-bond network in family A heptahelical receptors. J Mol Biol 380, 648-655, doi:S0022- 2836(08)00576-7 [pii]22 10.1016/j.jmb.2008.05.022 (2008). 17. Davies, A., Gowen, B. E., Krebs, A. M., Schertler, G. F. & Saibil, H. R. Three- dimensional structure of an invertebrate rhodopsin and basis for ordered alignment in the photoreceptor membrane. J Mol Biol 314, 455-463, doi:10.1006/jmbi.2001.5167

S0022-2836(01 )95167-8 [pii] (2001 ). 18. Kabsch, W. Integration, scaling, space-group assignment and post-refinement. Acta Crystallogr D Biol Crystallogr 66, 133-144, doi:S0907444909047374 [pii] 10.1 107/S0907444909047374 (2010).

19. Evans, P. Scaling and assessment of data quality. Acta Crystallogr D Biol Crystallogr 62, 72-82, doi:S0907444905036693 [pii] 10.1 107/S0907444905036693 (2006).

20. COLLABORATIVE COMPUTATIONAL PROJECT, N. "The CCP4 Suite: Programs for Protein Crystallography". . Acta Crystallogr D Biol Crystallogr D50, 760-763 (1994). 21. McCoy, A. J. et al. Phaser crystallographic software. J Appl Crystallogr 40, 658-674, doi: 10.1107/S0021889807021206 (2007).

22. Emsley, P., Lohkamp, B., Scott, W. & Cowtan, K. Features and Development of Coot. Acta Crystallogr D Biol Crystallogr (in the press) (2010).

23. Adams, P. D. et al. PHENIX: A comprehensive Python-based system for macromolecular structure solution. Acta Crystallogr D Biol Crystallogr 66, 213-221 , doi:S0907444909052925 [pii] 10.1107/S0907444909052925 (2010). Example 2: Crystallisation of a mutant human adenosine Ag^ receptor Adenosine A2A StaR2 - ZM-241 ,385 co-crystals were grown by the vapour diffusion method in sitting drops (space group P1 a = 101.27 A b = 101.26 A c = 101.20 A a = 1 12.45 ⁰ β = 103.26 ° γ = 112.90 ° and space group I222 a = 1 11.935 A b = 112.551 A c = 125.678 A α = β = γ = 90.00 ⁰ ). Drops were prepared by mixing 280μΜ Adenosine A^ StaR2 - ZM- 241 ,385 complex prepared in 0.3% n-nonyl-p-d-glucopyranoside, 10μΜ ZM-241 ,385, 0.1 M NaCI, buffer solution with equal volumes of 0.1 M Tris-HCI (8.0-9.0), 35-40% PEG 1000, 0.25M MgCI₂, 0.3% w/v n-nonyl-p-d-glucopyranoside, 0.1 % w/v 1-Butanol, and 0.05% CYMAL-6 at 277.15 K. Crystals appeared after 24 h and took three weeks to reach maximum dimensions of 100 x 500 x 50μΜ.

Example 3: RMSD calculations

The RMSDs were calculated as indicated: Both molecules were initially read into Maestro and their sequences were aligned using the 'Pairwise Alignment' algorithm contained within the 'Multiple Sequence Viewer' toolbar within Maestro. Manual adjustment within the 'Multiple Sequence Viewer' using the 'Grab and drag' tool was performed on the region between 149- 157, where the residues are not visible due to poor electron density, to ensure correct alignment of identically numbered residues. The identical residues in common between the two structures were then selected within the 'Multiple Sequence Viewer' using the 'Select and slide' tool, corresponding to residues 7-148, 158-208, 222-305 and excluding residues 54, 88, 107, 122, 202, 235, 239 and 277 (mutated for thermostabilisation). For the superposition the 'Superposition' tool was selected from the 'Tools' menu in the main window of Maestro. The 'Superimpose by ASL' tab was selected and the 'Select' button was pressed, in the 'Atom Selection' pop up box that appears 'Selection' was pressed to select the highlighted atoms. Additionally for the backbone RMSD, the 'Residue' tab was selected 'Backbone/side chain' was highlighted, the 'Backbone' checkbox was checked and 'Intersect' was pressed to select only the intersection between the backbone and the highlighted atoms. Finally OK' was selected. The RMSD is then returned in the box at the bottom of the 'Superposition' tool.

Example 4: Transformation of crystal structure coordinates

The A_2A-StaR2-ZM-241 ,385 crystal diffraction data can be processed, scaled and model coordinates built and successfully refined (as measured by RWORK /RFREE statistical validation) in five different space groups. The highest symmetry space group is 1222, where 1 copy of the A2A-StaR2-ZM-2 1 ,385 molecule is present in the asymmetric unit (ASU).

The cell constants for the I222 crystal system are as follows:

Crystal symmetry space group = "I 2 2 2" (No. 23)

Unit cell a = 111.935 (+10) A b = 112.551 (±10) A c = 125.678 (+10 / -25) A

p=90.0 ⁰ β=90.0 ° y=90.0 °

Coordinates are given as chain A' - protein; chain B' - ligand in Table (i). Applying the symmetry operations for this space group as given in the International Tables for Crystallography (International Tables for Crystallography - Volume A: Space-group symmetry, First online edition (2006), ISBN: 978-0-7923-6590-7) then generates the 8 copies that comprise the unit cell, and their spatial arrangement relative to one another.

The diffraction data can also be successfully processed (using a program, such as XDS or an equivalent program known in the art), scaled (using a program such as Scala from the CCP4 package or equivalent software known in the art) and model coordinates refined in the non-isomorphic subgroups 1112, 1121 , 1211 (using the unit cell lengths and angles as given for I222 - above) with two copies of the A2A-StaR2-ZM-241 ,385 ASU ^'1 related by non- crystallographic symmetry (NCS) and which mimic the arrangement described by crystallographic symmetry operators in the I222 crystal system. The A2A-StaR2 crystal diffraction data can also be processed (using a program, such as XDS or an equivalent program known in the art), scaled (using a program such as Scala from the CCP4 package or equivalent software known in the art) and model co-ordinates built and successfully refined (as measured by RWORK /RFREE statistical validation) in the low symmetry space group P1.

The cell constants for the P1 crystal system are as follows: Crystal symmetry space group = "P 1 " (No. 1 ) Unit cell a = 101.2683029 (±15) A b = 101.2558975 (±15) A c = 101.1939011 (±15) A

p=1 12.4477005 (±10) ° β=103.2590027 (±10) ° y=1 12.8952026 (±10) ° The coordinates of the first molecule (chain A - protein; chain B ligand) are provided in Table (ii). The coordinates are related to the three other copies by NCS operators as listed below. Applying the rotation and translation matrices below to chain A/B would permit generation of the 3 other copies present in the P1 crystal system, subsequently denoted chain C/D, E/F, G/H. The subsequent arrangement of the A2A-StaR2-ZM-241 ,385 molecules in the P1 system relative to one another then recreates the crystallographic symmetry arrangement of molecules in the I222 spacegroup.

NCS operator 1 and Rotation / Translation Matrices for coordinate generation :

Reference selection: "chain A and (resseq 7: 149 or resseq 158:305 )"

Other selection: "chain C and (resseq 7:149 or resseq 158:305 )"

Number of atom pairs: 2250

Rotation=-0.228778, -0.51 1662, 0.828168,

-0.511695, -0.66052, -0.549438,

0.828148, -0.549469, -0.110702

Translation^ 64.515, 37.6563, 82.275

RMS difference with respect to the reference: 0.000862

NCS operator 2 and Rotation / Translation Matrices for coordinate generation :

Reference selection: "chain A and (resseq 7:149 or resseq 158:305 )"

Other selection: "chain E and (resseq 7: 149 or resseq 158:305 )"

Number of atom pairs: 2250

Rotation=-0.389535, 0.921012, 0.000105715,

0.921011 , 0.389535, -0.00147121 ,

-0.00139618, -0.000475723, -0.999999

Translation=-97.2848, 131.697, 85.811 1

RMS difference with respect to the reference: 0.000987

NCS operator 3 and Rotation / Translation Matrices for coordinate generation :

Reference selection: "chain A and (resseq 7: 149 or resseq 158:305 )"

Other selection: "chain G and (resseq 7: 149 or resseq 158:305 )"

Number of atom pairs: 2250

Rotation= -0.38229, -0.408271 , -0.828957,

-0.410327, -0.728795, 0.54817,

-0.827941 , 0.549703, 0.1 1 1086

Translation=-37.1731 , -46.7621 , -80.1479 RMS difference with respect to the reference: 0.000943

Tables (i) - (ii)

Table (i) shows the x, y and z co-ordinates by amino acid residue of each non-hydrogen atom in the polypeptide structure for molecule A' in addition to the antagonist ZM-241 ,385 atoms B'. These coordinates represent one crystallised form of the A2A-StaR2-Z -241 ,385 complex in the Orthorhombic space group number 23 (I222) with one copy of the A2A-StaR2- ZM-241 ,385 complex present in the asymmetric unit.

Table (ii) shows the x, y and z coordinates by amino acid residue of each non-hydrogen atom in the polypeptide structure for molecule A in addition to the antagonist ZM-241 ,385 atom B. These coordinates represent another crystallised form of the A2A-StaR²-ZM-241 ,385 complex present in the asymmetric unit.

The fourth column of the tables indicates whether the atom is from an amino acid residue of the protein (by e-letter amino acid code e.g. TRP, GLU, ALA etc) or the ZM-241 ,385 ligand (ZMA). Parameters used for the modelling are listed in the REMARK section, for the I222 crystal system before Table (i) and for the P1 crystal system before Table (ii).

Parameters for modelling the I222 crystal system A2A-StaR2-ZM-241 ,385 complex

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Claims

1. A method of predicting a three dimensional structural representation of a target protein of unknown structure, or part thereof, comprising:

providing the coordinates of the human adenosine A_2A receptor structure listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; and

predicting the three-dimensional structural representation of the target protein, or part thereof, by modelling the structural representation on all or the selected coordinates of the adenosine A_¾ receptor.

2. A method according to Claim 1 further comprising aligning the amino acid sequence of the target protein of unknown structure with the amino acid sequence of adenosine A2A receptor listed in Figure 16 to match homologous regions of the amino acid sequences prior to predicting the structural representation, and wherein modelling the structural representation comprises modelling the structural representation of the matched homologous regions of the target protein on the corresponding regions of the adenosine A^ receptor to obtain a three dimensional structural representation for the target protein that substantially preserves the structural representation of the matched homologous regions.

3. A method of predicting the three dimensional structural representation of a target protein of unknown structure, or part thereof, comprising:

providing the coordinates of the human adenosine A_¾ receptor structure listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; and either (a) positioning the coordinates in the crystal unit cell of the protein so as to predict its structural representation, or (b) assigning NMR spectra peaks of the protein by manipulating the coordinates.

4. A method of predicting a three dimensional structural representation of a target protein of unknown structure, or part thereof, comprising:

providing the coordinates of the human adenosine A2A receptor structure listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof;

providing an X-ray diffraction pattern of the target protein; and using the coordinates to predict at least part of the structure coordinates of the target protein.

5. A method according to any of Claimsl - 4, wherein the target protein is a GPCR, such as any of an adrenergic receptor, an adenosine receptor, a muscarinic receptor or a neurotensin receptor.

6. A method for selecting or designing one or more binding partners of adenosine A¾ receptor comprising using molecular modelling means to select or design one or more binding partners of adenosine A^ receptor , wherein the three-dimensional structural representation of at least part of adenosine A_M receptor , as defined by the coordinates of the human adenosine A∑A receptor listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, is compared with a three-dimensional structural representation of one or more candidate binding partners, and one or more binding partners that are predicted to interact with adenosine Α2Α receptor are selected.

7. A method for producing a binding partner of adenosine A2A receptor comprising: identifying a binding partner according to the method of Claim6, and

synthesising the binding partner.

8. A binding partner produced by the method of Claim7.

9. A method of predicting the three dimensional structure of a binding partner of unknown structure, or part thereof, which binds to adenosine A2A receptor , comprising:

providing the coordinates of the human adenosine A2A receptor structure, listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof;

providing an X-ray diffraction pattern of adenosine A2A receptor complexed with the binding partner; and

using said coordinates to predict at least part of the structure coordinates of the binding partner.

10. A method according to Claim9, wherein the X-ray diffraction pattern is obtained from a crystal formed either by (a) soaking a crystal of adenosine A2A receptor with the binding partner to form a complex, or (b) mixing adenosine A_M receptor with the binding partner and crystallising an adenosine A2A receptor -binding partner complex.

1 1. A method for producing a medicament, pharmaceutical composition or drug, the process comprising: (a) providing a binding partner according to Claim 8 and (b) preparing a medicament, pharmaceutical composition or drug containing the binding partner.

12. A method of obtaining a three dimensional structural representation of a crystal of a adenosine A2A receptor , which method comprises providing the coordinates of the human adenosine A_2A receptor structure, listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1 .285 A, or selected coordinates thereof, and generating a three-dimensional structural representation of said coordinates.

13. A method for assessing the activation state of a structure for adenosine A¾ receptor, comprising: providing the coordinates of the adenosine AM receptor structure, listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; performing a statistical and/or a topological analysis on the coordinates;and comparing the results of the analysis with the results of an analysis of coordinates of proteins of known activation states.

14. A computer system, intended to generate three dimensional structural representations of adenosine A2A receptor , adenosine A2A receptor homologues or analogues, complexes of adenosine A^ receptor with binding partners, or complexes of adenosine A_∑A receptor homologues or analogues with binding partners, or, to analyse or optimise binding of binding partners to said adenosine Α2Α receptor or homologues or analogues, or complexes thereof, the system containing computer-readable data comprising one or more of:

(a) the coordinates of the human adenosine A2A receptor structure, listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof;

(b) the coordinates of a target adenosine A_∑A receptor homologue or analogue generated by homology modelling of the target based on the data in (a);

(c) the coordinates of a binding partner generated by interpreting X-ray crystallographic data or NMR data by reference to the coordinates of the adenosine A_2A receptor structure, listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, and

(d) structure factor data derivable from the coordinates of (a), (b) or (c).

15. A computer-readable storage medium, comprising a data storage material encoded with computer readable data, wherein the data comprises one or more of

(c) the coordinates of a binding partner generated by interpreting X-ray crystallographic data or NMR data by reference to the coordinates of the human adenosine A2A receptor structure, listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof, and

(d) structure factor data derivable from the coordinates of (a), (b) or (c).

16. A computer-readable storage medium comprising a data storage material encoded with a first set of computer-readable data comprising a Fourier transform of at least a portion of the structural coordinates of human adenosine A_∑A receptor listed in Table (i) or Table (ii), optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A, or selected coordinates thereof; which data, when combined with a second set of machine readable data comprising an X-ray diffraction pattern of a molecule or molecular complex of unknown structure, using a machine programmed with the instructions for using said first set of data and said second set of data, can determine at least a portion of the structure coordinates corresponding to the second set of machine readable data.

17. A method of producing a protein with a binding region that has substrate specificity substantially identical to that of adenosine A_¾ receptor , the method comprising

a) aligning the amino acid sequence of a target protein with the amino acid sequence of an adenosine A₂A receptor ;

b) identifying the amino acid residues in the target protein that correspond to any one or more of the following positions according to the numbering of the adenosine A¾ receptor as set out in Figure 15: Tyr 9, Glu 13, lie 60, Phe 62, Ala 63, lie 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278 and

c) making one or more mutations in the amino acid sequence of the target protein to replace one or more identified amino acid residues with the corresponding residue in the adenosine A2A receptor.

18. A mutant adenosine A2A receptor which, when compared to the corresponding wild- type adenosine receptor, has a different amino acid at a position which corresponds to any one or more of the following positions according to the numbering of the human adenosine AZA receptor as set out in Figure 15: Tyr 9, Glu 13, He 60, Phe 62, Ala 63, He 64, Tyr 65, lie 66, Ser 67, Thr 68, Ala 81 , Val 84, Leu 85, Ala 88, Leu 167, Phe 168, Glu 169, Asp 170, Met 174, Met 177, Asn 181 , Phe 182, Val 186, Trp 246, Leu 249, His 250, lie 252, Asn 253, Trp 256, His 264, Ala 265, Pro 266, Leu 267, Met 270, Tyr 271 , Leu 272, lie 274, Val 275, Ala 277 and His 278.

19. A mutant adenosine A2A receptor which, when compared to the corresponding wild- type adenosine A_¾ receptor, has a different amino acid at a position which corresponds to any one or more of the following positions according to the numbering of the human adenosine A_¾ receptor as set out in Figure 15: Val 25, Cys 28, Trp 29, Val 31 , Trp 32, Ser 35, Gin 38, Tyr 43, Phe 44, Val 46, Ser 47, Ala 49, Ala 50, Ala 51 , lie 53, Leu 54, Val 57, Leu 58, Pro 61 , Phe 62, He 64, Thr 65, Thr 68, Gly 69, Phe 70, Cys 71 , Ala 72, Ala 73, Cys 74, His 75, Gly 76, Phe 79, lie 80, Phe 83, Val 86, Leu 87, Thr 1 19, Ala 121 , Ala 122, Gly 123, lie 125, Ala 126, lie 127, Trp 129, Val 130, Phe 133, Leu 137 and Met 140.

20. A method of making an adenosine A_M receptor crystal comprising:

providing purified adenosine A_∑A receptor ; and

crystallising the adenosine A_¾ receptor either by using the sitting drop or hanging drop vapour diffusion technique, using a precipitant solution comprising Tris-HCI and PEG, such as a precipitant solution comprising 0.01 - 0.3 M Tris-HCI, 30-48% PEG1000, 0.25M MgCI₂, 0.3% w/v n-nonyl - β-D- glucopyranoside, 0.1 % w/v 1 -Butanol and 0.05% CYMAL-6.

21. A method according to Claim 19, wherein the precipitant solution comprises 0.1 M tris- HCI (pH8.0 - 8.75), 35 - 40% PEG 000, 0.25M MgCI₂, 0.3% w/v n-nonyl - β-D- glucopyranoside, 0.1 % w/v 1 -Butanol and 0.05% CYMAL-6.

22. A crystal of adenosine AZA receptor having the structure defined by the coordinates of the human adenosine A2A receptor structure listed in Table (i) or Table (ii) optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A or selected coordinates thereof.

23. A crystal according to Claim 22, which has P1 symmetry and unit cell dimensions a=101.27 (± 15) A, b=101.26 (± 15) A, c=101.20 (± 15) A, wherein a= 1 12.45 (± 10)°, β = 103.30 (± 10)° and γ = 112.90 (± 10)°.

24. A crystal according to Claim 22 which has I222 symmetry and unit cell dimensions a=1 1 1.94 (± 10)° A, b=1 12.55 (± 10)° A, c=125.68 (+ 10/-25) A, wherein a = 90°, β = 90° and Y = 90°.

25. A co-crystal of adenosine A2A receptor having the structure defined by the coordinates of the adenosine A¾ receptor structure listed in Table (i) or Table (ii) optionally varied by a root mean square deviation of residue backbone atoms of not more than 1.285 A or selected coordinates thereof, and a binding partner.

26. A crystal according to any of Claims 22-25 having a resolution of 3.3 A or better.