Mutant G-protein coupled receptors and methods for selecting them

Abstract

The invention relates to mutant G-protein coupled receptors with increased conformational stability, and methods of use thereof. In some aspects, polynucleotides encoding the mutant G-protein coupled receptors are provided. In some aspects, host cells comprising the polynucleotides are provided. In some aspects, the invention relates to crystallized forms of the mutant G-protein coupled receptors, and methods of preparing the same.

Claims

1. A mutant GPCR wherein (i) the mutant GPCR is a mutant β-adrenergic receptor which, when compared to the corresponding wild-type adrenergic receptor, has increased conformational thermostability and has at least one amino acid replacement at a position which corresponds to any of the following positions according to the turkey β-adrenergic receptor defined as SEQ ID NO: 1: G67A, G98A, V230A, D322A, Y227A, A234L, A282L, A334L, R68S, M90V, F338M, I129V, V230A, or F327A; or wherein (ii) the mutant GPCR is a mutant adenosine receptor which, when compared to the corresponding wild-type adenosine receptor, has increased conformational stability and has at least one amino acid replacement at a position which corresponds to any of the following positions according to the human adenosine A2a receptor defined as SEQ ID NO: 5: G114A, G118A, L167A, A184L, R199A, A203L, L208A, Q210A, S213A, E219A, R220A, S223A, T224A, Q226A, K227A, H230A, L241A, P260A, S263A, L267A, L272A, T279A, N284A, Q311A, P313A, or K315A; or wherein (iii) the mutant GPCR is a mutant neurotensin receptor which, when compared to the corresponding wild-type neurotensin receptor, has increased conformational thermostability and has at least one amino acid replacement at a position which corresponds to any of the following positions according to the rat neurotensin receptor defined as SEQ ID NO: 9: A69L, A73L, A86L, A90L, H103A, V165A, E166A, G215A, V229A, M250A, I253A, A177L, R183A, I260A, T279A, T294A, G306A, L308A, V309A, L310A, V313A, F342A, F358A, V360A, S362A, N370A, S373A, F380A, A385L, P389A, G390A, or R395A.

2. A mutant GPCR according to claim 1, which is in a solubilized form.

3. A mutant GPCR according to claim 1, which is immobilized to a solid support.

Description

(1) The invention will now be described in more detail with respect to the following Figures and Examples wherein:

(2) FIG. 1 Amino acid changes in βAR that lead to thermostability. Stability quotient indicates the % remaining binding activity of the mutants after heating the sample for 30 min at 32° C. All values are normalized to βAR.sub.34-424 (50%, showed as a discontinuous line) to remove any experimental variability between assays. Bars show the stability for each mutant. The letters on the x-axis indicate the amino acid present in the mutant. The original amino acid and its position in βAR.sub.34-424 is indicated below. Bars corresponding to the same amino acid in βAR.sub.34-424 are in the same colour with arrows indicating the best mutations. Errors were calculated from duplicate measurements; the best mutants were subsequently re-assayed to determine the Tm for each individual mutation and to give an accurate rank order of stability for each mutant (see Example 1).

(3) FIG. 2 Side chains in rhodopsin that are at equivalent positions to the thermostable mutations in βAR.sub.34-424. The equivalent amino acid residues in rhodopsin to the amino acid residues mutated in βAR.sub.34-424 were located in the rhodopsin structure, based upon an alignment among rhodopsin, β1 adrenergic receptor, neurotensin receptor, and adenosine A.sub.2a receptor (data not shown). Side chains in the same transmembrane helix are shown as space filling models in the same colour. The name and position of the amino acid residues are those in rhodopsin.

(4) FIG. 3 Evolution of thermostability in βAR. Starting from βAR-m10-8, combinations of mutations were rearranged systematically to find the optimum combination of mutations (see also Table 2).

(5) FIG. 4 Stability of βAR-m23 and βAR.sub.34-424 in the apo-state or containing the bound antagonist [.sup.3H]-DHA. To determine Tm in the absence of ligand (apo-state, discontinuous lines), detergent-solubilised receptors were incubated for 30 minutes at the temperatures indicated before carrying out the binding assay. For the Tm determination of the antagonist-bound form (continuous lines), detergent-solubilised receptors were pre-incubated with [.sup.3H]-DHA, followed by incubation at the temperatures indicated. βAR-m23 (circles), and βAR.sub.34-424 (squares). Data points are from duplicates measurements in a representative experiment.

(6) FIGS. 5a-c Competition binding of agonists to βAR-m23 and βAR.sub.34-424. Binding assays were performed on receptors partially purified in DDM; βAR-m23 (triangles) and βAR.sub.34-424 (squares). [.sup.3H]-DHA was used at a concentration three times greater than the K.sub.D of partially purified receptor (see Methods). [.sup.3H]-DHA binding was competed with increasing concentrations of the agonists, norepinephrine (FIG. 5a) and isoprenaline (FIG. 5b), or with an antagonist, alprenolol (FIG. 5c). Log EC.sub.50 and corresponding EC.sub.50 values for the different ligands were calculated by nonlinear regression using GraphPad Prism software and the error for log EC.sub.50s were lower than 10%. The EC.sub.50s for ligand binding to βAR.sub.34-424 and βAR-m23 are: norepinephrine, βAR.sub.34-424 1.5 μM, βAR-m23 3.7 mM; isoprenaline, βAR.sub.34-424 330 nM, βAR-m23 20 μM; alprenolol, βAR 78 nM, βAR-m23 112 nM.

(7) FIGS. 6a-c Stability of βAR-m23 and βAR.sub.34-424 in five different detergents. Samples of βAR.sub.34-424 (FIG. 6a), and βAR-m23 (FIG. 6b) solubilized in DDM were partially purified on Ni-NTA agarose columns allowing the exchange into various different detergents: DDM (squares), DM (triangles), OG (inverted triangles), LDAO (diamonds) and NG (circles). βAR is so unstable in OG, NG and LDAO that it was not possible to measure any activity after purification at 6° C. Assays were carried out as described in the Methods and the Tm is shown at the intersection between the curves and the discontinuous line. Results are from duplicate measurements in a representative experiment performed in parallel. (FIG. 6c) Photomicrograph of a crystal of βAR-m23 mutant, which showed good order by X-ray diffraction.

(8) FIG. 7 Curve of thermostability of βAR.sub.34-424 (Tm). Binding assays were performed using [.sup.3H]-dihydroalprenolol (DHA) as radioligand as described under “Methods”. Samples were heated for 30 minutes at different temperatures before the assay. Tm represents the temperature at which the binding decreased to the 50%, value showed as a discontinuous line. Data points are from duplicates of one single experiment. This experiment has been repeated several times with similar results.

(9) FIGS. 8a-b Saturation binding assays of membranes of βAR.sub.34-424 and βAR-m23. Binding assays were performed as described in “Methods” using [.sup.3H]-dihydroalprenolol (DHA) as radioligand; βAR.sub.34-424 (FIG. 8a) and βAR-m23 (FIG. 8b). Scatchard plots are shown as insets along with the corresponding values for B.sub.max and K.sub.D. Data points are from duplicates of two independent experiments for each protein. Data were analyzed by nonlinear regression using Prism software (GraphPad).

(10) FIGS. 9A-B Alignment of the turkey β-adrenergic receptor with human β1, β2 and β3 receptors.

(11) FIGS. 10A-B Alignment of human adenosine receptors.

(12) FIGS. 11A-B Alignment of neurotensin receptors.

(13) FIG. 12 Flow chart showing the two different assay formats of ligand (+) and ligand (−) used to determine receptor thermostability.

(14) FIGS. 13A-F Pharmacological profile of thermostable mutant adenosine A2a receptor, Rant21. Saturation binding of (FIG. 13A) antagonist and (FIG. 13B) agonist to solubilised receptors. (FIGS. 13C-F) Inhibition of [.sup.3H]ZM241385 binding by increasing concentrations of antagonists (FIG. 13C) XAC and (FIG. 13D) Theophylline, and agonists (FIG. 13E) NECA and (FIG. 13F) R-PIA; binding of [.sup.3H]ZM241385 (10 nM) in the absence of unlabelled ligand was set to 100%. Each solubilised receptor was incubated with ligands for one hour on ice in binding buffer (50 mM Tris pH7.5 and 0.025% DDM) containing 400 mM NaCl (FIG. 13A, FIG. 13C-F). Data shown are from two independent experiments with each data point measured in triplicate. K.sub.D and K.sub.i values are given in Table (iii).

(15) FIGS. 14A-B Thermostable mutants show a decreased dependence on lipids (FIG. 14A) and an increased survival at higher concentration of DDM (FIG. 14B) upon heating compared to the wild-type receptor. Receptors were solubilised in 1% DDM (diluted in 50 mM Tris pH7.5 and 400 mM NaCl) and immobilised on Ni-NTA agarose for the IMAC step. Exchange of buffer containing the appropriate concentration of DDM and/or lipids was performed during washes and elution from the Ni-NTA beads.

(16) FIG. 15 Mapping of the M90V, Y227A, A282L and F338M m23 mutations in turkey beta1 adrenergic receptor onto homologous residues (M82, Y219, C265 and A321 respectively) in the human beta2 adrenergic receptor structure (Rasmussen et al (2007) Nature 15; 383-387; pdb accession codes 2R4R and 2R4S) reveals their position at a helical interface and helical kink respectively. Amino acid residues in equivalent positions to the thermostabilising mutations in the turkey β1 adrenergic receptor are shown as labelled space filling models.

(17) FIG. 16 Mapping of m23 mutations in turkey beta1 adrenergic receptor onto homologous residues in the human beta2 adrenergic receptor structure (Cherezov et al (2007) Science, 318:1258-65; pdb accession code 2RH1). The Cα trace of the β2AR is shown with the fusion moiety (T4 lysozyme) removed. The six mutations in PAR-m23 (R68S, M90V, Y227A, A282L, F327A, F338M) are equivalent to amino acid residues K60, M82, Y219, C265, L310, F321 in the human β2AR. Lys60 is on the intracellular end of Helix 1 and points into the lipid-water interface. Met82 is near the middle of Helix 2 and points into the ligand binding pocket; the nearest distance between the substrate carazolol and the Met side chain is 5.7 Å. Tyr219 is towards the intracellular end of helix 5 and is at the helix5-helix 6 interface. Cys265 is at the end of the loop region between helices 5 and 6 and points away from the transmembrane regions. Leu310 and Phe321 are both in helix 7 and both point out into the lipid bilayer.

(18) FIGS. 17A-C Multiple sequence alignment of human beta-2AR, rat NTR1, turkey beta-1 AR, human Adenosine A2aR and human muscarinic M1 receptors. In each sequence, thermostabilising mutations are marked with a box. Mutations occurring in two or more sequences are denoted with a star.

(19) FIG. 18 Mapping of turkey beta1AR mutation I55A (human beta2AR 147) onto human beta2AR structure (pdb accession code 2RH1). Mutation is at the interface between 3 helices (H1, H2 kink, H7 kink). Left: side view; right: top view.

(20) FIG. 19 Mapping of turkey beta1AR V89L mutation (human beta2AR V81) onto human beta2AR structure (pdb accession code 2RH1). Mutation is in the kink in helix 2. The helices are numbered and the bound antagonist is shown as a space filling model. Amino acid residues in equivalent positions to the thermostabilising mutations in the turkey β1 adrenergic receptor are shown as space filling models and are arrowed for clarity. Left: side view; right: top view.

(21) FIG. 20 Mapping of turkey beta1AR M90V mutation (human beta2AR M82) onto human beta2AR structure (pdb accession code 2RH1). Mutation is in kink in helix 2 oriented towards the binding pocket. The helices are numbered and the bound antagonist is shown as a space filling model. Amino acid residues in equivalent positions to the thermostabilising mutations in the turkey β1 adrenergic receptor are shown as space filling models and are arrowed for clarity. Left: side view; right: top view.

(22) FIG. 21 Mapping of turkey beta1AR I129V mutation (human beta2AR I121) onto human beta2AR structure (pdb accession code 2RH1). Mutation is opposite a kink in helix 5. The helices are numbered and the bound antagonist is shown as a space filling model. Amino acid residues in equivalent positions to the thermostabilising mutations in the turkey β1 adrenergic receptor are shown as space filling models and are arrowed for clarity. Left: side view; right: bottom view.

(23) FIG. 22 Mapping of turkey beta1AR F338M mutation (human beta2AR F321) onto human beta2AR structure (pdb accession code 2RH1). Mutation is in kink in helix 7. The helices are numbered and the bound antagonist is shown as a space filling model. Amino acid residues in equivalent positions to the thermostabilising mutations in the turkey β1 adrenergic receptor are shown as space filling models and are arrowed for clarity. Left: side view; right: top view.

(24) FIG. 23 Mapping of turkey beta1AR Y227A mutation (human beta2AR Y219) onto human beta2AR structure (pdb accession code 2RH1). Mutation is at helix-helix interface. The helices are numbered and the bound antagonist is shown as a space filling model. Amino acid residues in equivalent positions to the thermostabilising mutations in the turkey β1 adrenergic receptor are shown as space filling models and are arrowed for clarity. Left: side view; right: bottom view.

(25) FIG. 24 Mapping of turkey beta1AR A282L mutation (human beta2AR C265) onto human beta2AR structure (pdb accession code 2RH1). Mutation is in loop region. The helices are numbered and the bound antagonist is shown as a space filling model. Amino acid residues in equivalent positions to the thermostabilising mutations in the turkey β1 adrenergic receptor are shown as space filling models and are arrowed for clarity. Left: side view; right: top view.

(26) FIG. 25 Mapping of turkey beta1 AR R68S mutation (human beta2AR K60) onto human beta2AR structure (pdb accession code 2RH1). Mutation is at the lipid-water boundary, pointing into the solvent. The helices are numbered and the bound antagonist is shown as a space filling model. Amino acid residues in equivalent positions to the thermostabilising mutations in the turkey β1 adrenergic receptor are shown as space filling models and are arrowed for clarity. Left: side view; right: angled top view.

(27) FIG. 26 Comparison of the thermostabilities of three β adrenergic receptors (turkey β1 (.square-solid.), human β1 (.Math.) and human β2 (.circle-solid.)) and two thermostabilised receptors (turkey β1-m23 (.box-tangle-solidup.) and human β2-m23 (.diamond-solid.)). The six thermostabilising mutations in β1-m23 (R68S, M90V, Y227A, A282L, F327A, F338M) were all transferred directly to the human β2 receptor (K60S, M82V, Y219A, C265L, L310A, F321M) making β2-m23, based upon the alignment in FIGS. 9A-B. The resulting mutants were transiently expressed in mammalian cells, solubilised in 0.1% dodecylmaltoside and assayed for thermostability in the minus-ligand format (heating the apo-state, quenching on ice, adding 3H-DHA). The apparent Tms for turkey β1 and β2-m23 were 23° C. and 45° C. respectively, giving a ΔTm of 22° C. as seen previously in E. coli expressed receptor. The Tms for human β2 and β2-m23 were 29° C. and 41° C. respectively, showing that the apo receptor was stabilised by 12° C. This exemplifies the principle of the transferability of thermostabilising mutations from one receptor to another receptor, which in this case are 59% identical. The human β1 receptor (Tm˜12° C.) is much less stable than the turkey β1 receptor.

(28) FIG. 27 Percentage identity of the turkey β1 adrenergic receptor, human adenosine receptor and rat neurotensin receptor to human β adrenergic receptors, human adenosine receptors and human neurotensin receptors, respectively.

(29) FIGS. 28A-B Alignment of neurotensin receptors.

(30) FIG. 29 shows a list of A2aR stabilising mutations. Mutants were expressed in E. coli, solubilised in 2% DDM+10% glycerol and tested for ligand-binding, using the agonist [.sup.3H]-NECA and the antagonist [.sup.3H]-ZM241385. Concentrations of radioligands were 6-10-fold above their K.sub.D measured for the wild-type receptor. Expression of active receptor was evaluated by ligand binding at 4° C. Stability was assayed by heating the solubilised receptor in its apo-state at 30° C. for 30 minutes and then measuring residual binding activity. Under these conditions, wild-type activity decays to 50% (S.D.=15%). Data obtained for expression and stability were normalised to wild-type values. Mutations included in subsequent rounds of mutagenesis were those whose expression was ≥30-40% and stability ≥130-140% compared to the wild-type. Bold lines indicate cluster of mutations.

EXAMPLE 1

Conformational Stabilisation of the β-Adrenergic Receptor in Detergent-Resistant Form

(31) Summary

(32) There are over 500 non-odorant G protein-coupled receptors (GPCRs) encoded by the human genome, many of which are predicted to be potential therapeutic targets, but there is only one structure available, that of bovine rhodopsin, to represent the whole of the family. There are many reasons for the lack of progress in GPCR structure determination, but we hypothesise that improving the detergent-stability of these receptors and simultaneously locking them into one preferred conformation will greatly improve the chances of crystallisation. A generic strategy for the isolation of detergent-solubilised thermostable mutants of a GPCR, the β-adrenergic receptor, was developed based upon alanine scanning mutagenesis followed by an assay for receptor stability. Out of 318 mutants tested, 15 showed a measurable increase in stability. After optimisation of the amino acid residue at the site of each initial mutation, an optimally stable receptor was constructed by combining specific mutations. The most stable mutant receptor, βAR-m23, contained 6 point mutations that led to a Tm 21° C. higher than the native protein and, in the presence of bound antagonist, βARm23 was as stable as bovine rhodopsin. In addition, βAR-m23 was significantly more stable in a wide range of detergents ideal for crystallisation and was preferentially in an antagonist conformation in the absence of ligand.

(33) Results

(34) Selection of Single Mutations that Increase the Thermostability of the β1 Adrenergic Receptor

(35) βAR from turkey erythrocytes is an ideal target for structural studies because it is well characterised and is expressed at high-levels in insect cells using the baculovirus expression system[10,11]. The best overexpression of βAR is obtained using a truncated version of the receptor containing residues 34-424 (βAR.sub.34-424) [9] and this was used as the starting point for this work. Alanine scanning mutagenesis was used to define amino residues in βAR.sub.34-424 that, when mutated, altered the thermostability of the receptor; if an alanine was present in the sequence it was mutated to a leucine residue. A total of 318 mutations were made to amino acid residues 37-369, a region that encompasses all seven transmembrane domains and 23 amino acid residues at the C terminus; mutations at 15 amino residues were not obtained due to strong secondary structure in the DNA template. After sequencing each mutant to ensure the presence of only the desired mutation, the receptors were functionally expressed in E. coli and assayed for stability.

(36) The assay for thermostability was performed on unpurified detergent-solubilised receptors by heating the receptors at 32° C. for 30 minutes, quenching the reaction on ice and then performing a radioligand binding assay, using the antagonist [.sup.3H]-dihydroalprenolol, to determine the number of remaining functional βAR.sub.34-424 molecules compared to the unheated control. Heating the unmutated βAR.sub.34-424 at 32° C. for 30 min before the assay reduced binding to approximately 50% of the unheated control (FIG. 7); all the data for the mutants were normalised by including the unmutated βAR.sub.34-424 as a control in every assay performed. In the first round of screening, eighteen mutants showed an apparent increase in stability, maintaining more than 75% of antagonist binding after heating and being expressed in E. coli to at least 50% of the native βAR.sub.34-424 levels. In view of the possibility of increasing further the stability of these mutants, each of the 18 residues was mutated to 2-5 alternative amino acid residues of varying size or charge (FIG. 1). Out of these 18 mutants, 12 were not improved by further changes, 5 had better thermostability if another amino acid was present and one mutation from the first screen turned out to be a false positive. In addition, three residues that were not stabilised upon mutation to alanine (V89, S151, L221) were mutated to a range of other amino acid residues; the two positions that when mutated to alanine did not affect thermostability, were also unaffected by other changes. In contrast, V89 showed less thermostability when mutated to alanine, but thermostability increased when it was mutated to Leu. Thus the initial alanine scanning successfully gave two-thirds of the best amino acid residues of those tested for any given position.

(37) The position and environment predicted for each of the 16 amino residues that gave the best increases in thermostability when mutated were determined by aligning the PAR sequence with that of rhodopsin whose structure is known (FIG. 2). Fourteen of these residues were predicted to be present in transmembrane α-helices, with five of the residues predicted to be lipid-facing, 4 being deeply buried and the remainder were predicted to be at the interfaces between the helices. Some of these residues would be expected to interact with each other in the βAR structure, such as the consecutive amino acids G67 and R68 (V63 and Q64 in rhodopsin), or the amino acids within the cluster Y227, R229, V230 and A234 in helix 5 (Y223, Q225, L226 and V230 in rhodopsin). Other amino acid residues that could interact in βAR were Q194A in external loop 2 and D322A in external loop 3 (G182 and P285 in rhodopsin, respectively).

(38) The increase in stability that each individual mutation gave to βAR.sub.34-424 was determined by measuring the Tm for each mutant (results not shown); Tm in this context is the temperature that gave a 50% decrease in functional binding after heating the receptor for 30 minutes. Each mutation increased the Tm of βAR.sub.34-424 by 1-3° C., with the exception of M90A and Y227A that increased the Tm by 8° C.

(39) Combining Mutations to Make an Optimally Stable Receptor

(40) Initially, mutations that improved thermostability that were adjacent to one another in the primary amino sequence of βAR were combined. Constructions containing the mutations G67A and R68S, or different combinations of the mutations at the end of helix 5 (Y227A, R229Q, V230A and A234L) were expressed and assayed; the Tm values (results not shown) were only 1-3° C. higher than the Tm for βAR.sub.34-424 and one mutant was actually slightly less stable, suggesting that combining mutations that are adjacent to one another in the primary amino acid sequence does not greatly improve thermostability. Subsequently, mutations predicted to be distant from one another in the structure were combined. PCR reactions were performed using various mixes of primers to combine up to 5 different mutations in a random manner and then tested for thermostability (Table 1). The best of these combinations increased the Tm more than 10° C. compared to the Tm of βAR.sub.34-424. In some cases, there was a clear additive effect upon the Tm with the sequential incorporation of individual mutations. This is seen in a series of 3 mutants, m4-1, m4-7 and m4-2, with the addition of V230A to m4-1 increasing the Tm by 2° C. and the additional mutation D332A in m4-7 increasing the Tm a further 3° C. Mutants that contained Y227A and M90A all showed an increase in Tm of 10° C. or more. Just these two mutations together increased the Tm of βAR.sub.34-424 by 13° C. (m7-5), however, the total antagonist binding was less than 50% of βAR.sub.34-424 suggesting impaired expression of this mutant. The addition of F338M to m7-5 did not increase the thermostability, but it increased levels of functional expression in E. coli.

(41) TABLE-US-00001 TABLE 1 Combinations of mutations by PCR. PCR Receptor Mutations T.sub.m (° C.) βAR.sub.34-424 31.7 ± 0.1 4 m4-1 G67A, G98A 35.5 ± 0.9 m4-2 G67A, G98A, V230A, D322A 40.9 ± 0.9 m4-6 G98A, D322A 35.0 ± 0.2 m4-7 G67A, G98A, V230A 38.0 ± 1.2 6 m6-1 Y227A, A234L, A282L, A334L 41.6 ± 0.9 m6-4 R68S, Y227A, A234L, A282L 41.6 ± 0.1 m6-5 R68S, A234L, A282L, A334L 41.9 ± 0.5 m6-9 R68S, Y227A, A234L, A282L, A334L 43.7 ± 0.4 m6-10 R68S, Y227A, A282L, A334L 47.4 ± 1.1 m6-11 R68S, A282L, A334L 39.1 ± 0.5 7 m7-1 M90V, A282L, F338M 43.0 ± 0.8 m7-2 M90V, A282L 38.9 ± 0.6 m7-5 M90V, Y227A 45.2 ± 1.0 m7-6 M90V, I129V 40.0 ± 0.6 m7-7 M90V, Y227A, F338M 45.2 ± 2.0 10 m10-4 R68S, M90V, V230A, A334L 46.9 ± 1.0 m10-8 R68S, M90V, V230A, F327A, A334L 47.3 ± 1.4 10 PCR reactions were performed combining different pairs of primers that contained the selected mutations. Successful PCR reactions are shown in the table. The stability of these new mutants was assayed as described in FIG. 7 and the Tm calculated. The results are shown as the mean ± S.E. from duplicates.

(42) The most thermostable mutants obtained, which were still expressed at high levels in E. coli, were m6-10, m7-7 and m10-8. These mutants contained collectively a total of 10 different mutations, with 8 mutations occurring in at least two of the mutants. A second round of mutagenesis was performed using m10-8 as the template and adding or replacing mutations present in m6-10 and m7-7 (FIG. 3); some of these mutations were very close in the primary amino acid sequence of βAR and therefore were not additive as noted above, but many mutations improved the Tm further (Table 2). For example, exchanging two mutations in m10-8, to create m18, raised the Tm to 49.6° C. and adding A282L to make m23 increased the Tm a further 3° C. to 52.8° C. This produced the most thermostable βAR.sub.34-424 mutant so far and will be referred to as βAR-m23.

(43) TABLE-US-00002 TABLE 2 Improvement of best combination of mutations. These new mutants were obtained mixing the changes present in mutants m6-10, m7-7 and m10-8 by PCR. The stability of these new mutants was assayed as described in FIG. 7 and the Tm calculated. The results are shown as the mean ± S.E. from duplicates. Mutations T.sub.m (° C.) m17 R68S M90V Y227A V230A — F327A A334L — 48.2 ± 1.4 m18 R68S M90V Y227A V230A A282L F327A — F338M 49.6 ± 0/9 m19 R68S M90V Y227A — A282L F327A — F338M 49.0 ± 0.8 m20 R68S M90V — — — F327A A334L — 48.4 ± 0.7 m21 R68S M90V Y227A — — F327A A334L — 47.0 ± 1.3 m22 R68S M90V Y227A F327A A334L — 47.4 ± 0.5 m23 R68S M90V Y227A — A282L F327A — F338M 52.8 ± 1.4

(44) The thermostability assays used to develop βAR.sub.34-424 mutants were performed by heating the receptor in the absence of the antagonist, but it is well known that bound ligand stabilises receptors. Therefore, stability assays for βAR.sub.34-424 and βAR-m23 were repeated with antagonist bound to the receptors during the heating step (FIG. 4).

(45) As expected, the Tm of the receptor that contained bound antagonist during the incubation was higher than that for the receptor without antagonist. For βAR.sub.34-424 the Tm was 6° C. higher with bound antagonist and for βAR-m23 the Tm increased 2° C. to 55° C.; the smaller increase in thermostability observed for βAR-m23 when antagonist binds suggests that the receptor is already in a more stable conformation similar to the antagonist bound state than βAR.sub.34-424 (see also below). The Tm of βAR-m23 with antagonist bound is very similar to the Tm of dark-state rhodopsin in dodecylmaltoside (DDM)[12], whose structure has been solved by two independent laboratories[13,14]. This suggested that βAR-m23 is sufficiently stable for crystallisation.

(46) Characterization of βAR-m23

(47) The three characteristic activities measured for βAR-m23 and βAR.sub.34-424 to identify the effect of the six mutations were the affinity of antagonist binding, the relative efficacies of agonist binding and the ability of βAR-m23 to couple to G proteins. Saturation binding experiments to membranes using the antagonist [.sup.3H]-dihydroalprenolol (FIG. 8) showed that the affinity of binding to βAR-m23 (K.sub.D 6.5±0.2 nM, n=2) was slightly lower than for βAR.sub.34-424 (K.sub.D 2.8±0.1 nM, n=2), suggesting that there are no large perturbations in the structure of βARm23 in the antagonist-bound conformation. This is consistent with the observation that none of the mutations in βAR-m23 correspond with amino acids believed to be implicated in ligand binding. In contrast to antagonist binding, the efficacy of agonist binding by βAR-m23 is 3 orders of magnitude weaker than for βAR.sub.34-424 (FIG. 5). The potency of the agonist isoprenaline is consistently lower in βAR-m23 and βAR.sub.34-424 than for the native agonist norepinephrine, indicating that the agonist-bound conformation for the two receptors is likely to be similar. However, the large decrease in agonist efficacy in βAR-m23 compared to βAR.sub.34-424 indicates that the 6 mutations in βAR-m23 have locked the receptor preferentially in an antagonist-bound conformation. From a crystallisation perspective, this is an added bonus to thermostabilisation, because it is essential to have a conformationally homogeneous protein population for the production of diffraction-quality crystals.

(48) All of the thermostability assays used to derive βAR-m23 were performed on receptors solubilised in DDM. The aim of the thermostabilisation process was to produce a receptor that is ideal for crystallography, which means being stable in a variety of different detergents and not just DDM. We therefore tested the stability of βAR-m23 and βAR in a variety of different detergents, concentrating on small detergents that are preferentially used in crystallising integral membrane proteins.

(49) Membranes prepared from E. coli expressing βAR-m23 or βAR.sub.34-424 were solubilised in DDM, bound to Ni-NTA agarose then washed with either DDM, decylmaltoside (DM), octylglucoside (OG), lauryldimethylamine oxide (LDAO) or nonylglucoside (NG). Stability assays were performed on the receptors in each of the different detergents (FIG. 6). βAR.sub.34-424 was only stable in DDM and DM, with no active receptors eluting from the resin washed with OG, NG or LDAO. In contrast, functional βAR-m23 was still present in all detergents and the Tm could be determined. As expected, the smaller detergents were considerably more denaturing than either DDM (Tm 52° C.) or DM (Tm 48° C.), with T.sub.ms of 25° C. (NG), 23° C. (LDAO) and 17° C. (OG). The difference in Tm between βAR-m23 and βAR.sub.34-424 is about 20° C., irrespective of whether the receptors were solubilised in either DDM or DM; it is therefore not surprising that no active βAR.sub.34-424 could be found in even NG, because the predicted Tm would be about 5° C., thus resulting in rapid inactivation of the receptor under the conditions used for purification. The selection strategy used for the generation of βAR-m23 was chosen deliberately to be based upon thermostability, because it is far simpler to apply than selecting for stability in detergents of increasing harshness. However, it is clear that increasing the thermostability of βAR.sub.34-424 also resulted in increasing tolerance to small detergents ideal for crystallising integral membrane proteins.

(50) Crystallisation of Mutant GPCR

(51) Earlier attempts to crystallise several different constructs of turkey beta-adrenegic receptor failed. Despite experimenting with a variety of conditions, using both the native sequence and several truncated and loop-deleted constructs, over many years, no crystals were obtained.

(52) However, once the stabilising mutations from βAR-m23 were transferred into the constructs, several different crystals were obtained in different detergents and different conditions.

(53) The crystals that have been most studied so far were obtained using the purified beta-36 construct (amino acid residues 34-367 of the turkey beta receptor containing the following changes: point mutations C116L and C358A; the 6 thermostabilising point mutations in m23; replacement of amino acid residues 244-278 with the sequence ASKRK; a C terminal His6 tag) expressed in insect cells using the baculovirus expression system, after transferring the receptor into the detergent octyl-thioglucoside. The precipitant used was PEG600 or PEG1000 and the crystals obtained are elongated plates.

(54) Experiments have also been carried out to see whether, once the crystallisation conditions had been defined using the stabilised receptor, it was possible to get crystals using the original non-stabilised construct. It was possible that similar or perhaps very small crystals could have been obtained, but, in fact, the “wild type” (i.e. the starting structure from which the mutagenesis began) never gave any crystals.

(55) The crystals are plate-shaped with space group C2 and diffract well, though the cell dimensions do vary depending on the freezing conditions used.

(56) In general, once a GPCR has been stabilised it may be subjected to a variety of well-known techniques for structure determination. The most common technique for crystallising membrane proteins is by vapour diffusion (20, 21), usually using initially a few thousand crystallisation conditions set up using commercial robotic devices (22). However, sometimes the crystals formed by vapour diffusion are small and disordered, so additional techniques may then be employed. One technique involves the co-crystallisation (by vapour diffusion) of the membrane protein with antibodies that bind specifically to conformational epitopes on the proteins' surface (23, 24); this increases the hydrophilic surface of the protein and can form strong crystal contacts. A second alternative is to use a different crystallisation matrix that is commonly called either lipidic cubic phase or lipidic mesophase (25, 26), which has also been developed into a robotic platform (27). This has proven very successful for producing high quality crystals of proteins with only small hydrophilic surfaces e.g. bacteriorhodopsin (28). Membrane protein structures can also be determined to high-resolution by electron crystallography (29).

(57) The evolution of βAR-m23 from βAR.sub.34-424 by a combination of alanine scanning mutagenesis and the selection of thermostable mutants has resulted in a GPCR that is ideal for crystallography. The Tm for βAR-m23 is 21° C. higher than for βAR.sub.34-424 and, in the presence of antagonist, βAR-m23 has a similar stability to rhodopsin. The increased Tm of βAR-m23 has resulted in an increased stability in a variety of small detergents that inactivate βAR.sub.34-424. In addition, the selection strategy employed resulted in a receptor that is preferentially in the antagonist-bound conformation, which will also improve the chances of obtaining crystals, because the population of receptor conformations will be more homogeneous than for wild type βAR.sub.34-424. Thus we have achieved a process of conformational stabilisation in a single selection procedure.

(58) It is not at all clear why the particular mutations we have introduced lead to the thermostabilisation of the receptor. Equivalent positions in rhodopsin suggest that the amino acid residues mutated could be pointing into the lipid bilayer, into the centre of the receptor or at the interfaces between these two environments. Given the difficulties in trying to understand the complexities of the thermostabilisation of soluble proteins[15], it seems unlikely that membrane proteins will be any easier to comprehend; we found that there was no particular pattern in the amino acid residues in βAR that, when mutated, led to thermostability. However, since nearly 5% of the mutants produced were more stable than the native receptor, alanine scanning mutagenesis represents an efficient strategy to rapidly identify thermostable mutants.

(59) The procedure we have used to generate βAR-m23 is equally applicable to any membrane protein that has a convenient assay for detecting activity in the detergent solubilized form. While we have selected for stability as a function of temperature as the most convenient primary parameter, the procedure can easily be extended to test primarily for stability, for example, in a harsh detergent, an extreme of pH or in the presence of chaotropic salts. Conformational stabilisation of a variety of human receptors, channels and transporters will make them far more amenable to crystallography and will also allow the improvement in resolution of membrane proteins that have already been crystallised. It is to be hoped that conformational stabilisation will allow membrane protein crystallisation to become a far more tractable problem with a greater probability of rapid success than is currently the case. This should allow routine crystallisation of human membrane proteins in the pharmaceutical industry, resulting in valuable structural insights into drug development.

(60) Methods

(61) Materials. The truncated β1 adrenergic receptor from turkey (βAR.sub.34-424)[9] was kindly provided by Dr Tony Warne (MRC Laboratory of Molecular Biology, Cambridge, UK). This βAR construct encoding residues 34-424 contains the mutation C116L to improve expression[11], and a C-terminal tag of 10 histidines for purification. 1-[4,6-propyl-.sup.3H]-dihydroalprenolol ([.sup.3H]-DHA) was supplied by Amersham Bioscience, (+) L-norepinephrine bitartrate salt, (−) isoprenaline hydrochloride, (−) alprenolol tartrate salt and s-propranolol hydrochloride were from Sigma.

(62) Mutagenesis of DAR. The βAR cDNA was ligated into pRGIII to allow the functional expression of βAR in E. coli as a MalE fusion protein[16]. Mutants were generated by PCR using the expression plasmid as template using the QuikChange II methodology (Stratagene). PCR reactions were transformed into XL10-Gold ultracompetent cells (Stratagene) and individual clones were fully sequenced to check that only the desired mutation was present. Different mutations were combined randomly by PCR by including all the pairs of primers that introduced the following mutations: Mut4, G67A, G068A, V230A, D322A and F327A; Mut6, R068S, Y227A, A234L, A282L and A334L; Mut7, M90V, I129V, Y227A, A282L and F338M; Mut10, R68S, M90V, V230A, F327A and A334L. The PCR mixes were transformed and the clones sequenced to determine exactly which mutations were introduced.

(63) Protein expression and membrane preparations. Expression of βAR and the mutants was performed in XL10 cells (Stratagene). Cultures of 50 ml of 2×TY medium containing ampicillin (100 μg/ml) were grown at 37° C. with shaking until OD.sub.600=3 and then induced with 0.4 mM IPTG. Induced cultures were incubated at 25° C. for 4 h and then cells were harvested by centrifugation at 13,000×g for 1 min (aliquots of 2 ml) and stored at −20° C. For the assays, cells were broken by freeze-thaw (five cycles), resuspended in 500 pa of buffer [20 mM Tris pH 8, 0.4 M NaCl, 1 mM EDTA and protease inhibitors (Complete™, Roche)]. After an incubation for 1 h at 4° C. with 100 μg/ml lysozyme and DNase I (Sigma), samples were solubilized with 2% DDM on ice for 30 minutes. Insoluble material was removed by centrifugation (15,000×g, 2 min, 4° C.) and the supernatant was used directly in radioligand binding assays.

(64) For large-scale membrane preparations, 2L and 6L of E. coli culture of βAR and Mut23, respectively, were grown as described above. Cells were harvested by centrifugation at 5,000×g for 20 min, frozen in liquid nitrogen and stored at −80° C. Pellets were resuspended in 10 ml of 20 mM Tris pH 7.5 containing 1× protease inhibitor cocktail (Complete™ EDTA-free, Roche); 1 mg DNase I (Sigma) was added and the final volume was made to 100 ml. Cells were broken by a French press (2 passages, 20,000 psi), and centrifuged at 12,000×g for 45 min at 4° C. to remove cell debris. The supernatant (membranes) was centrifuged at 200,000×g for 30 min at 4° C.; the membrane pellet was resuspended in 15 ml of 20 mM Tris pH 7.5 and stored in 1 ml aliquots at −80° C. after flash-freezing in liquid nitrogen. The protein concentration was determined by the amido black method[17]. These samples were used in radioligand binding assays after thawing and being solubilized in 2% DDM as above.

(65) For competition assays, as well as testing different detergents, DDM-solubilized βAR was partially purified with Ni-NTA agarose (Qiagen). 200 μl of Ni-NTA agarose was added to 2 ml of solubilized samples (10 mg/ml of membrane protein) in 20 mM Tris pH 8, 0.4 M NaCl, 20 mM imidazole pH 8 and incubated for 1 h at 4° C. After incubation, samples were centrifuged at 13,000×g for 30 sec and washed twice with 250 μl of buffer (20 mM Tris pH 8, 0.4 M NaCl, 20 mM imidazole) containing detergent (either 0.1% DDM, 0.1% DM, 0.1% LDAO, 0.3% NG or 0.7% OG).

(66) Receptors were eluted in 2×100 μl of buffer (0.4 M NaCl, 1 mM EDTA, 250 mM imidazole pH 8, plus the relevant detergent). The K.sub.D for [.sup.3H]-DHA binding to semipurified βAR.sub.34-424 and βAR-m23 was, respectively 3.7 nM and 12.5 nM and the final concentration of [.sup.3H]-DHA used in the competition assays was 3 times the K.sub.D ie 12 nM for βAR.sub.34-424 and 40 nM for βAR-m23.

(67) Radioligand binding and thermostability assays. Single point binding assays contained 20 mM Tris pH 8, 0.4 M NaCl, 1 mM EDTA, 0.1% DDM (or corresponding detergent) with 50 nM [.sup.3H]-DHA and 20-100 μg membrane protein in a final volume of 120 μl; equilibration was for 1 h at 4° C. Thermostability was assessed by incubating the binding assay mix, with or without [.sup.3H]-DHA at the specified temperature for 30 minutes; reactions were placed on ice and [.sup.3H]-DHA added as necessary and equilibrated for a further hour. Receptor-bound and free radioligand were separated by gel filtration as described previously[18]. Non-specific binding was determined in the presence of 1 μM of s-propranolol. Saturation curves were obtained using a range of [.sup.3H]-DHA concentration from 0.4 nM to 100 nM. Competition assays were performed using a concentration of [.sup.3H]-DHA of 12 nM for βAR.sub.34-424 and 40 nM for βAR-m23 (ie three times the K.sub.D) and various concentrations of unlabeled ligands (0-100 mM). Radioactivity was counted on a Beckman LS6000 liquid scintillation counter and data were analyzed by nonlinear regression using Prism software (GraphPad).

(68) Location of βAR-m23 thermostable mutations in rhodopsin structure. The pdb file for the rhodopsin structure, accession code 1GZM[14], was downloaded from the Protein Data Bank website (www.pdb.org) and displayed in the program PyMOLX11Hybrid (DeLano Scientific). The equivalent amino acid residues in rhodopsin for the thermostable mutations in βAR were located in the rhodopsin structure based upon an alignment among the four GPCRs with which we are most familiar, namely rhodopsin, β1 adrenergic receptor, neurotensin receptor and adenosine A.sub.2a receptor[19].

EXAMPLE 2

Mutants of the Adenosine A.SUB.2a .Receptor (A.SUB.2a.R) with Increased Thermostability

(69) 1. 315 site-directed mutants made between residues 2-316 of A.sub.2aR. 2. All of these mutants have been assayed for thermostability using an assay measuring agonist and antagonist binding after the heating step (Ligand(−) format as described in FIG. 12). a. 26 mutants showed improved thermostability when measured with .sup.3H-NECA (agonist): G114 A, G118A, L167A, A184L, R199A, A203L, L208A, Q210A, S213A, E219A, R220A, S223A, T224A, Q226A, K227A, H230A, L241A, P260A, S263A, L267A, L272A, T279A, N284A, Q311A, P313A, K315A. b. 18 mutants showed improved thermostability when assayed with .sup.3H-ZM241385 (antagonist): A54L, V57A, H75A, T88A, G114A, G118A, T119A, K122A, G123A, P149A, E151A, G152A, A203L, A204L, A231L, L235A, V239A. 3. Mutations have been combined to generate mutants in a putative antagonist conformation. Wildtype A.sub.2aR has a Tm of 31° C. with ZM241385 bound. a. Rant17 A54L+K122A+L235A Tm 48° C. (ZM241385 bound) b. Rant19 A54L, T88A, V239A+A204L Tm 47° C. (ZM241385 bound) c. Rant21 A54L, T88A, V239A+K122A Tm 49° C. (ZM241385 bound) 4. Mutations from the agonist screen have been combined, but have led to only a very low level of improvement in Tm of +2° C.

(70) Table (i) in FIG. 29 provides a list of A2aR stabilising mutations.

(71) TABLE-US-00003 TABLE (ii) Stability of best combinations. Tm (° C.) Tm (° C.) − + − + ago- ago- antago- antago- nist nist nist nist Wt 21 29 wt 31 32 Rag 1 26 34 Rant 5 42 46 (A184L/R199A/ (A54L/T88A/ L272A) V239A) Rag 23 22 38 Rant 21 41 49 (Rag 1 + F79A/ (Rant 5 + L208A) K122A) Receptors were solubilised in 1% DDM (no glycerol). A melting profile was obtained by heating the solubilised receptor at different temperatures in absence (apo-state) or presence of ligand (ligand-occupied state). Data shown are representative of at least three independent experiments. S.D. is <1° C.

(72) TABLE-US-00004 TABLE (iii) Summary of results for competition assays of detergent- solubilised wild-type A2aR and thermo-stable mutant Rant 21. K.sub.i (M) Competitor wt Rant 21 XAC 2.3 × 10.sup.−6 2.3 × 10.sup.−6 Theophylline 1.5 × 10.sup.−3 0.9 × 10.sup.−3 NECA 7.0 × 10.sup.−6  >1 × 10.sup.−1 R-PIA 1.6 × 10.sup.−5 3.6 × 10.sup.−3 Values are representative of two independent experiments. Each data point was assayed in triplicate and plotted as mean ± SD. Each solubilised receptor was incubated with ligands for one hour on ice in binding buffer (50 mM Tris pH 7.5 and 0.025% DDM) containing 400 mM NaCl. Binding of [3H]ZM241385 (10 nM) in the absence of unlabeled ligand was set to 100%. Data shown are from two independent experiments with each data point measured in triplicate. Incubation of samples with ligands was for 1 hour on ice with [.sup.3H]ZM241385 at a concentration of 10 nM. K.sub.i values were calculated according to the Cheng and Prusoff equation using the non-linear regression equation of the software Prism, applying a K.sub.D for [.sup.3H]ZM241385 of 12 nM for the wild-type and 15 nM for Rant 21. Rant 21 did not bind NECA sufficiently for an accurate K.sub.i determination (hence indicated as >1 × 10.sup.−1). The affinity of Rant21 for agonist binding is weakened 232 fold for R-PIA and at least by 1900 fold for NECA.

(73) TABLE-US-00005 TABLE (iv) Summary of results for saturation assays of detergent-solubilised wild-type A2aR and thermo-stable mutants. K.sub.D (nM) [.sup.3H]NECA [.sup.3H]ZM241385 Receptor (agonist) (antagonist) wt 32 ± 1 12 ± 3 Rag 1   26 ± 0.4   26 ± 0.5 Rag 23 21 ± 1 62 ± 1 Rant 21 >450 15 ± 3 Values are representative of three independent experiments. Each data point was assayed in triplicate and plotted as mean ± SD. Data were fitted to the Michaelis-Menten equation using the non-linear regression equation of the software Prism.

(74) TABLE-US-00006 TABLE (v) Summary of stability of wild-type and mutant receptors in different detergents. Tm (° C.) Agonist-binding Antagonist-binding wt Rag 23 wt Rant 21 0.01% DDM 27 34 25 39 0.1% DM 23 29 10 28 0.3% NM 22 28 <4 25 0.3% NG † † † 22 0.6% OG <9 16 † 23 0.003% LDAO 28 38 32 42 0.006% FC12 37 39 43 49 Solubilisation of receptors and detergent exchange was performed during the IMAC step. S.D. is <1° C. It was not possible to determine the Tm for some receptor-detergent combinations, because the receptor was too unstable (†).

EXAMPLE 3

Mutants of the Neurotensin Receptor (NTR) with Increased Thermostability

(75) 1. 340 site-directed mutants have been made between residues 61-400 of NTR. 2. Initially, all of these mutants were assayed for thermostability using an assay measuring .sup.3H-neurotensin (agonist) binding after the heating step. 24 mutations led to a small but significant increase in thermostability: A356L, H103A, D345A, A86L, A385L, Y349A, C386A, K397A, H393A, 1116A, F358A, S108A, M181A, R392A, D113A, G209A, L205A, L72A, A120L, P399A, Y351A, V268A, T207A, A155L, S362A, F189A, N262A, L109A, W391A, T179A, S182A, M293A, L256A, F147A, D139A, S100A, K176A, L111A, A90L, N270A. 3. Mutants tested for thermostability by heating in the absence of the agonist were re-tested using a slightly different assay where the mutants were heated in the presence of .sup.3H-neurotensin (Ligand(+) format in FIG. 12). Mutants with improved thermostability are: A69L, A73L, A86L, A90L, H103A, V165A, E166A, G215A, V229A, M250A, I253A, A177L, R183A, I260A, T279A, T294A, G306A, L308A, V309A, L310A, V313A, F342A, F358A, V360A, S362A, N370A, S373A, F380A, A385L, P389A, G390A, R395A. 4. There are also mutants that have a significantly enhanced expression level compared to the wildtype receptor and could be used to boost preceptor production levels for crystallisation: A86L, H103A, F358A, S362A, N370A, A385L, G390A. All of these also have increased thermostability.

(76) 5. Preferred combinations are a. Nag7m F358A+A86L+I260A+F342A Tm 51° C. (neurotensin bound) b. Nag7n F358A+H103A+I260A+F342A Tm 51° C. (neurotensin bound) Wildtype NTR has a Tm of 35° C. with neurotensin bound.

EXAMPLE 4

Identification of Structural Motifs in which Stabilising GPCR Mutations Reside

(77) The structure of the β2 adrenergic receptor has been determined (20, 21), which is 59% identical to the turkey β1 receptor, but with a distinctly different pharmacological profile (22, 23). In order to determine the structural motifs in which the stabilising mutations of the turkey β1 receptor reside, we mapped the mutations onto the human β2 structure (21).

(78) The beta adrenergic receptors were first aligned using ClustalW in the MacVector package; thermostabilising mutations in turkey β1 were highlighted along with the corresponding residue in the human β2 sequence. The human β2 model (pdb accession code 2RH1) was visualised in Pymol and the desired amino acids were shown as space filling models by standard procedures known in the art. The structural motifs in which the stabilising mutations were located, were determined by visual inspection.

(79) Table (vi) lists the equivalent positions in the β2 receptor corresponding to the thermostabilising mutations in βAR-m23 and the structural motifs in which they reside.

(80) As seen from Table (vi), the mutations are positioned in a number of distinct localities. Three mutations are in loop regions that are predicted to be accessible to aqueous solvent (loop). Eight mutations are in the transmembrane α-helices and point into the lipid bilayer (lipid); three of these mutations are near the end of the helices and may be considered to be at the hydrophilic boundary layer (lipid boundary). Eight mutations are found at the interfaces between transmembrane α-helices (helix-helix interface), three of which are either within a kinked or distorted region of the helix (kink) and another two mutations occur in one helix but are adjacent to one or more other helices which contain a kink adjacent in space to the mutated residue (opposite kink). These latter mutations could affect the packing of the amino acids within the kinked region, which could result in thermostabilisation. Another mutation is in a substrate binding pocket. (pocket).

(81) TABLE-US-00007 TABLE (vi) Position in the human β2 structure of the amino acid residues equivalent to the thermostabilising mutations found in the turkey β1 receptor and the structural motifs in which they reside. Turkey β1 Human β2 Description Helix 1 I55A I47 3-helix kink interface FIG. 18 Helix 1 G67A A59 lipid boundary Helix 1 R68S K60 lipid boundary FIG. 25 Helix 2 V89L V81 kink FIG. 19 Helix 2 M90V M82 kink FIG. 20 Helix 2 G98A G90 pocket Helix 3 I129V I121 opposite kink FIG. 21 S151E S143 loop Helix 4 V160A V152 lipid Q194A A186 loop Helix 5 L221V V213 lipid Helix 5 Y227A Y219 helix-helix interface FIG. 23 Helix 5 R229Q R221 lipid Helix 5 V230A V222 helix-helix interface Helix 5 A234L A226 helix-helix interface Helix 6 A282L C265 loop FIG. 24 D322A K305 lipid boundary Helix 7 F327A L310 lipid Helix 7 A334L V317 lipid Helix 7 F338M F321 kink FIG. 22

(82) Such structural motifs, by virtue of them containing stabilising mutations, are important in determining protein stability. Therefore, targeting mutations to these motifs will facilitate the generation of stabilised mutant GPCRs. Indeed, there were several instances where more than one mutation mapped to the same structural motif. For example, the Y227A, V230A and A234L mutations in the turkey β1 adrenergic receptor all mapped to the same helical interface, the V89L and M90V mutations mapped to the same helical kink and the F327A and A334L mutations mapped to the same helical surface pointing towards the lipid bilayer (Table (vi)). Thus, when one stabilising mutation has been identified, the determination of the structural motif in which that mutation is located will enable the identification of further stabilising mutations.

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