The present invention relates to mutant G protein coupled receptors (GPCRs) and methods for selecting those with increased stability. In particular, it relates to the selection and preparation of mutant GPCRs which have increased stability under a particular condition compared to their respective parent proteins. Such proteins are more likely to be crystallisable, and hence amenable to structure determination, than the parent proteins. They are also useful for drug discovery and development studies.
Over the past 20 years the rate of determination of membrane protein structures has gradually increased, but most success has been in crystallising membrane proteins from bacteria rather than from eukaryotes . Bacterial membrane proteins have been easier to overexpress using standard techniques in Escherichia coli than eukaryotic membrane proteins [2,3] and the bacterial proteins are sometimes far more stable in detergent, detergent-stability being an essential prerequisite to purification and crystallisation. Genome sequencing projects have also allowed the cloning and expression of many homologues of a specific transporter or ion channel, which also greatly improves the chances of success during crystallisation. However, out of the 120 different membrane protein structures that have been solved to date, there are only seven structures of mammalian integral membrane proteins (http:/blanco.biomol.uci.edu/); five of these membrane proteins were purified from natural sources and are stable in detergent solutions. Apart from the difficulties in overexpressing eukaryotic membrane proteins, they often have poor stability in detergent solutions, which severely restricts the range of crystallisation conditions that can be explored without their immediate denaturation or precipitation. Ideally, membrane proteins should be stable for many days in any given detergent solution, but the detergents that are best suited to growing diffraction-quality crystals tend to be the most destabilising detergents ie those with short aliphatic chains and small or charged head groups. It is also the structures of human membrane proteins that we would like to solve, because these are required to help the development of therapeutic agents by the pharmaceutical industry; often there are substantial differences in the pharmacology of receptors, channels and transporters from different mammals, whilst yeast and bacterial genomes may not include any homologous proteins. There is thus an overwhelming need to develop a generic strategy that will allow the production of detergent-stable eukaryotic integral membrane proteins for crystallisation and structure determination and potentially for other purposes such as drug screening, bioassay and biosensor applications.
Membrane proteins have evolved to be sufficiently stable in the membrane to ensure cell viability, but they have not evolved to be stable in detergent solution, suggesting that membrane proteins could be artificially evolved and detergent-stable mutants isolated . This was subsequently demonstrated for two bacterial proteins, diacylglycerol kinase (DGK) [5,6] and bacteriorhodopsin . Random mutagenesis of DGK identified specific point mutations that increased thermostability and, when combined, the effect was additive so that the optimally stable mutant had a half-life of 35 minutes at 80° C. compared with a half-life of 6 minutes at 55° C. for the native protein. . It was shown that the timer of the detergent-resistant DGK mutant had become stable in SDS and it is thus likely that stabilisation of the oligomeric state played a significant role in thermostabilisation. Although the aim of the mutagenesis was to produce a membrane protein suitable for crystallisation, the structure of DGK has yet to be determined and there have been no reports of successful crystallization. A further study on bacteriorhodopsin by cysteine-scanning mutagenesis along helix B demonstrated that it was not possible to predict which amino acid residues would lead to thermostability upon mutation nor, when studied in the context of the structure, was it clear why thermostabilisation had occurred .
GPCRs constitute a very large family of proteins that control many physiological processes and are the targets of many effective drugs. Thus, they are of considerable pharmacological importance. A list of GPCRs is given in Foord et al (2005) Pharmacol Rev. 57, 279-288, which is incorporated herein by reference. GPCRs are generally unstable when isolated, and despite considerable efforts, it has not been possible to crystallise any except bovine rhodopsin, which naturally is exceptionally stable.
GPCRs are druggable targets, and reference is made particularly to Overington et al (2006) Nature Rev. Drug Discovery 5, 993-996 which indicates that over a quarter of present drugs have a GPCR as a target.
GPCRs are thought to exist in multiple distinct conformations which are associated with different pharmacological classes of ligand such as agonists and antagonists, and to cycle between these conformations in order to function (Kenakin T. (1997) Ann N Y Acad Sci 812, 116-125).
It will be appreciated that the methods of the invention do not include a method as described in D'Antona et al., including binding of [3H]CP55940 to a constitutively inactive mutant human cannabinoid receptor 1 (T210A) in which the Thr residue at position 210 is replaced with an Ala residue.
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.
We have realised that there are two serious problems associated with trying to crystallise GPCRs, namely their lack of stability in detergent and the fact that they exist in multiple conformations. In order to function GPCRs have evolved to cycle through at least two distinct conformations, the agonist bound form and the antagonist-bound form, and changes between these two conformations can occur spontaneously in the absence of ligand. It is thus likely that any purified receptors populate a mixture of conformations. Just adding ligands to GPCRs during crystallisation trials has not resulted in their structure determination. To improve the likelihood of crystallisation, we therefore selected mutations that improved the stability of the GPCR and, in addition, preferentially locked the receptor in a specific biologically relevant conformation.
We decided to see whether stabilisation of a GPCR in a particular, biologically relevant conformation was possible and whether the effect was sufficiently great that it would significantly improve the chances of obtaining diffraction-quality crystals. In Example 1, the β1-adrenergic receptor (βAR) from turkey erythrocytes  was chosen as a test subject for this study for a number of reasons. The βAR is a G protein-coupled receptor (GPCR) that has well-developed pharmacology with many ligands commercially available and in a radiolabelled form. In addition, overexpression of βAR has been particularly successful using the baculovirus expression system and it can be purified in milligram quantities in a functional form. . In Example 2, a human adenosine receptor was used, and in Example 3, a rat neurotensin receptor was used.
Method for Selecting Mutant GPCRs with Increased Stability
A first aspect of the invention provides a method for selecting a mutant G-protein coupled receptor (GPCR) with increased stability, the method comprising
(a) providing one or more mutants of a parent GPCR,
(b) selecting a ligand, the ligand being one which binds to the parent GPCR when the GPCR is residing in a particular conformation,
(c) determining whether the or each mutant GPCR has increased stability with respect to binding the selected ligand compared to the stability of the parent GPCR with respect to binding that ligand, and
(d) selecting those mutants that have an increased stability compared to the parent GPCR with respect to binding of the selected ligand.
The inventors have appreciated that, in order to improve the likelihood of crystallisation of a GPCR in a biologically relevant form (which is therefore pharmacologically useful), it is desirable not only to increase the stability of the protein, but also for the protein to have this increased stability when in a particular conformation. The conformation is determined by a selected ligand, and is a biologically relevant conformation in particular a pharmacologically relevant conformation. Thus, the method of the invention may be considered to be a method for selecting mutants of a GPCR which have increased stability of a Particular conformation, for example they may have increased conformational thermostability. The method may be used to create stable, conformationally locked GPCRs by mutagenesis. The selected mutant GPCRs are effectively purer forms of the parent molecules in that a much higher proportion of them occupies a particular conformational state. The deliberate selection of a chosen receptor conformation resolved from other conformations by use of a ligand (or ligands) that bind preferentially to this conformation is therefore an important feature of the invention. The method may also be considered to be a method for selecting mutant GPCRs which are more tractable to crystallisation.
Thus the invention includes a method for selecting a mutant G-protein coupled receptor (GPCR) with increased stability, the method comprising
(a) providing one or more mutants of a parent GPCR,
(b) selecting a ligand, the ligand being one which binds to the parent GPCR when the GPCR is residing in a particular conformation,
(c) determining whether the or each mutant GPCR when residing in the particular conformation has increased stability with respect to binding the selected ligand compared to the stability of the parent GPCR when residing in the same particular conformation with respect to binding that ligand, and
(d) selecting those mutants that have an increased stability compared to the parent GPCR with respect to binding of the selected Iigand.
In a review of the druggable genome by Hopkins & Groom (2002) Nature Rev. Drug Discovery 1, 727-730, Table 1 contains a list of protein families many of which are GPCRs. Overington et al (2006) Nature Rev. Drug Discovery 5, 993-996 provides more details of drug targets, and FIG. 1 indicates that more than a quarter of current drugs target GPCRs. There are 52 GPCR targets for orally available drugs out of a total of 186 total targets in this category.
Suitable GPCRs for use in the practice of the invention include, but are not limited to β-adrenergic receptor, adenosine receptor, in particular adenosine A2a receptor, and neurotensin receptor (NTR). Other suitable GP CRs 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.
The amino acid sequences (and the nucleotide sequences of the cDNAs which encode them) of many GPCRs are readily available, for example by reference to GenBank. In particular, Foord et al supra gives the human gene symbols and human, mouse and rat gene IDs from Entrez Gene (http://www.ncbi.nlm.nih.gov/entrez). It should be noted, also, that because the sequence of the human genome is substantially complete, the amino acid sequences of human GPCRs can be deduced therefrom.
Although the GPCR 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 or a bird. It is particularly preferred if the GPCR is derived from rat, mouse, rabbit or dog or non-human primate or mar, or from chicken or turkey. For the avoidance of doubt, we include within the meaning of “derived from” that a cDNA or gene was originally obtained using genetic material from the source, but that the protein may be expressed in any host cell subsequently. Thus, it will be plain that a eukaryotic GPCR (such as an avian or mammalian GPCR) may be expressed in a prokaryotic host cell, such as E. coli, but be considered to be avian- or mammalian-derived, as the case may be.
In some instances, the GPCR may be composed of more than one different subunit. For example, the calcitonin gene-related peptide receptor requires the binding of a single transmembrane helix protein (RAMP1) to acquire its physiological ligand binding characteristics. Effector, accessory, auxiliary or GPCR-interacting proteins which combine with the GPCR to form or modulate a functional complex are well known in the art and include, for example, receptor kinases, G-proteins and arrestins (Bockaert et al (2004) Curr Opinion Drug Discov and Dev 7, 649-657).
The mutants of the parent GPCR may be produced in any suitable way and to provided in any suitable form. Thus, for example, a series of specific mutants of the parent protein may be made in which each amino acid residue in all or a part of the parent protein is independently changed to another amino acid residue. For example, it may be convenient to make mutations in those parts of the protein which are predicted to be membrane spanning. The three-dimensional structure of rhodopsin is known (Li et al (2004) J Mol Biol 343, 1409-1438; Palczewski et al (2000) Science 289, 739-745), and it is possible to model certain GPCRs using this structure. Thus, conveniently, parts of the GPCR to mutate may be based on modelling. Similarly, computer programs are available which model transmembrane regions of GPCRs based on hydrophobicity (Kyle & Dolittle (1982) J. Mol. Biol. 157, 105-132), and use can be made of such models when selecting parts of the protein to mutate. Conventional site-directed mutagenesis may be employed, or polymerase chain reaction-based procedures well known in the art may be used. It is possible, but less desirable, to use ribosome display methods in the selection of the mutant protein.
Typically, each selected amino acid is replaced by Ala (ie Ala-scanning mutagenesis), although it may be replaced by any other amino acid. If the selected amino acid is Ala, it may conveniently be replaced by Leu. Alternatively, the amino acid may be replaced by Gly (ie Gly-scanning mutagenesis), which may allow a closer packing of neighbouring helices that may lock the protein in a particular conformation. If the selected amino acid is Gly, it may conveniently be replaced by Ala.
Although the amino acid used to replace the given amino acid at a particular position is typically a naturally occurring amino acid, typically an “encodeable” amino acid, it may be a non-natural amino acid (in which case the protein is typically made by chemical synthesis or by use of non-natural amino-acyl tRNAs). An “encodeable” amino acid is one which is incorporated into a polypeptide by translation of mRNA. It is also possible to create non-natural amino acids or introduce non-peptide linkages at a given position by covalent chemical modification, for example by post-translational treatment of the protein or semisynthesis. These post-translational modifications may be natural, such as phosphorylation, glycosylation or palmitoylation, or synthetic or biosynthetic.
Alternatively, the mutants may be produced by a random mutagenesis procedure, which may be of the whole protein or of a selected portion thereof. Random mutagenesis procedures are well known in the art.
Conveniently, the mutant GPCR has one replaced amino acid compared to the parent protein (ie it is mutated at one amino acid position). In this way, the contribution to stability of a single amino acid replacement may be assessed. However, the mutant GPCR assayed for stability may have more than one replaced amino acid compared to the parent protein, such as 2 or 3 or 4 or 5 or 6 replacements.
As is discussed in more detail below, combinations of mutations may be made based on the results of the selection method. It has been found that in some specific cases combining mutations in a single mutant protein leads to a further increase in stability. Thus, it will be appreciated that the method of the invention can be used in an iterative way by, for example, carrying it out to identify single mutations which increase stability, combining those mutations in a single mutant GPCRs which is the GPCR then provided in part (a) of the method. Thus, multiply-mutated mutant proteins can be selected using the method.
The parent GPCR need not be the naturally occurring protein. Conveniently, it may be an engineered version which is capable of expression in a suitable host organism, such as Escherichia coli. For example, as described in Example 1, a convenient engineered version of the turkey β-adrenergic receptor is one which is truncated and lacks residues 1-33 of the amino acid sequence (ie βAR34-424). The parent GPCR may be a truncated form of the naturally occurring protein (truncated at either or both ends), or it may be a fusion, either to the naturally occurring protein or to a fragment thereof. Alternatively or additionally, the parent GPCR, compared to a naturally-occurring GPCR, may be modified in order to improve, for example, solubility, proteolytic stability (eg by truncation, deletion of loops, mutation of glycosylation sites or mutation of reactive amino acid side chains such as cysteine). In any event, the parent GPCR is a protein that is able to bind to the selected ligand which ligand is one which is known to bind the naturally occurring GPCR. Conveniently, the parent GPCR is one which, on addition of an appropriate ligand, can affect any one or more of the downstream activities which are commonly known to be affected by G-protein activation.
However, it will be appreciated that the stability of the mutant is to be compared to a parent in order to be able to assess an increase in stability.
A ligand is selected, the ligand being one which binds to the parent GPCR when residing in a particular conformation. Typically, the ligand will bind to one conformation of the parent GPCR (and may cause the GPCR to adopt this conformation), but does not bind as strongly to another conformation that the GPCR may be able to adopt. Thus, the presence of the ligand may be considered to encourage the GPCR to adopt the particular conformation. Thus, the method may be considered to be a way of selecting mutant GPCRs which are trapped in a conformation of biological relevance (eg ligand bound state), and which are more stable with respect to that conformation.
Preferably the particular conformation in which the GPCR resides in step (c) corresponds to the class of ligand selected in step (b).
Preferably the selected ligand is from the agonist class of ligands and the particular conformation is an agonist conformation, or the selected ligand is from the antagonist class of ligands and the particular conformation is an antagonist conformation.
Preferably the selected ligand is from the agonist class of ligands and the particular conformation in which the GPCR resides in step (c) is the agonist conformation.
Preferably, the selected ligand binding affinity for the mutant receptor should be equal to or greater than that for the wild type receptor; mutants that exhibit significantly reduced binding to the selected ligand are typically rejected.
By “ligand” we include any molecule which binds to the GPCR and which causes the GPCR to reside in a particular conformation. The ligand preferably is one which causes more than half of the GPCR molecules overall to be in a particular conformation.
Many suitable ligands are known.
Typically, the ligand is a full agonist and is able to bind to the GPCR and is capable of eliciting a full (100%) biological response, measured for example by G-protein coupling, downstream signalling events or a physiological output such as vasodilation. Thus, typically, the biological response is GDP/GTP exchange in a G-protein, followed by stimulation of the linked effector pathway. The measurement, typically, is GDP/GTP exchange or a change in the level of the end product of the pathway (eg cGMP or inositol phosphates). The ligand may also be a partial agonist and is able to bind to the GPCR and is capable of eliciting a partial (<100%) biological response.
The ligand may also be an inverse agonist, which is a molecule which binds to a receptor and reduces its basal (ie unstimulated by agonist) activity sometimes even to zero.
The ligand may also be an antagonist, which is a molecule which binds to a receptor and blocks binding of an agonist, so preventing a biological response. Inverse agonists and partial agonists may under certain assay conditions be antagonists.
The above ligands may be orthosteric, by which we include the meaning that they combine with the same site as the endogenous agonist; or they may be allosteric or allotopic, by which we include the meaning that they combine with a site distinct from the orthosteric site. The above ligands may be syntopic, by which we include the meaning that they interact with other ligands) at the same or an overlapping site. They may be reversible or irreversible.
In relation to antagonists, they may be surmountable, by which we include the meaning that the maximum effect of agonist is not reduced by either pre-treatment or simultaneous treatment with antagonist; or they may be insurmountable, by which we include the meaning that the maximum effect of agonist is reduced by either pre-treatment or simultaneous treatment with antagonist; or they may be neutral, by which we include the meaning the antagonist is one without inverse agonist or partial agonist activity Antagonists typically are also inverse agonists.
Ligands for use in the invention may also be allosteric modulators such as positive allosteric modulators, potentiators, negative allosteric modulators and inhibitors. They may have activity as agonists or inverse agonists in their own right or they may only have activity in the presence of an agonist or inverse agonist in which case they are used in combination with such molecules in order to bind to the GPCR.
Neubig et al (2003) Pharmacol. Rev. 55, 597-606, incorporated herein by reference, describes various classes of ligands.
Preferably, the above-mentioned ligands are small organic or inorganic moieties, but they may be peptides or polypeptides. Typically, when the ligand is a small organic or organic moiety, it has a Mr of from 50 to 2000, such as from 100 to 1000, for example from 100 to 500.
Typically, the ligand binds to the GPCR with a Kd of from mM to pM, such as in the range of from μM (micromolar) to nM. Generally, the ligands with the lowest Kd are preferred.
Small organic molecule ligands are well known in the art, for example see the Examples below. Other small molecule ligands include 5HT which is a full agonist at the 5HT1A receptor; eltoprazine which is a partial agonist at the 5HT1A receptor (see Newman-Tancredi at al (1997) Neurophamacology 36, 451-459); (+)-butaclamol and spiperone are dopamine D2 receptor inverse agonists (see Roberts & Strange (2005) Br. J. Pharmacol. 145, 34-42); and WIN55212-3 is a neutral antagonist of CB2 (Savinainen at al (2005) Br. J. Pharmacol. 145, 636-645).
The ligand may be a peptidomimetic, a nucleic acid, a peptide nucleic acid (PNA) or an aptamer. It may be an ion such as Na+ or Zn2+, a lipid such as oleamide, or a carbohydrate such as heparin.
The ligand may be a polypeptide which binds to the GPCR. Such polypeptides (by which we include oligopeptides) are typically from Mr 500 to Mr 50,000, but may be larger. The polypeptide may be a naturally occurring GPCR-interacting protein or other protein which interacts with the GPCR, or a derivative or fragment thereof, provided that it binds selectively to the GPCR in a particular conformation. GPCR-interacting proteins include those associated with signalling and those associated with trafficking, which often act via PDZ domains in the C terminal portion of the GPCR.
Polypeptides which are known to bind certain GPCRs include any of a G protein, an arrestin, a RGS protein, G protein receptor kinase, a RAMP, a 14-3-3 protein, a NSF, a periplakin, a spinophilin, a GPCR kinase, a receptor tyrosine kinase, an ion channel or subunit thereof, an ankyrin and a Shanks or Homer protein. Other polypeptides include NMDA receptor subunits NR1 or NR2a, calcyon, or a fibronectin domain framework. The polypeptide may be one which binds to an extracellular domain of a GPCR, such as fibulin-1. The polypeptide may be another GPCR, which binds to the selected GPCR in a hetero-oligomer. A review of protein-protein interactions at GPCRs is found in Milligan & White (2001) Trends Pharmacol. Sci. 22, 513-518, or in Bockaert et al (2004) Curr. Opinion Drug Discov. Dev. 7, 649-657 incorporated herein by reference.
The polypeptide ligand may conveniently be an antibody which binds to the GPCR. By the term “antibody” we include naturally-occurring antibodies, monoclonal antibodies and fragments thereof. We also include engineered antibodies and molecules which are antibody-like in their binding characteristics, including single chain Fv (scFv) molecules and domain antibodies (dAbs). Mention is also made of camelid antibodies and engineered camelid antibodies. Such molecules which bind GPCRs are known in the art and in any event can be made using well known technology. Suitable antibodies include ones presently used in radioimmunoassay (RIAs) for GPCRs since they tend to recognise conformational epitopes.
The polypeptide may also be a binding protein based on a modular framework, such as ankyrin repeat proteins, armadillo repeat proteins, leucine rich proteins, tetratriopeptide repeat proteins or Designed Ankyrin Repeat Proteins (DARPins) or proteins based on lipocalin or fibronectin domains or Affilin scaffolds based on either human gamma crystalline or human ubiquitin.
In one embodiment of the invention, the ligand is covalently joined to the GPCR, such as a G-protein or arrestin fusion protein. Some GPCRs (for example thrombin receptor) are cleaved N-terminally by a protease and the new N-terminus binds to the agonist site. Thus, such GPCRs are natural GPCR-ligand fusions.
It will be appreciated that the use of antibodies, or other “universal” binding polypeptides (such as G-proteins which are known to couple with many different GPCRs) may be particularly advantageous in the use of the method on “orphan” GPCRs for which the natural ligand, and small molecule ligands, are not known.
Once the ligand has been selected, it is then determined whether the or each mutant GPCR has increased stability with respect to binding the selected ligand compared to the parent GPCR with respect to binding that ligand. It will be appreciated that this step (c) is one in which it is determined whether the or each mutant GPCR has an increased stability (compared to its parent) for the particular conformation which is determined by the selected ligand. Thus, the mutant GPCR has increased stability with respect to binding the selected ligand as measured by ligand binding or whilst binding the selected ligand. As is discussed below, it is particularly preferred if the increased stability is assessed whilst binding the selected ligand.
The increased stability is conveniently measured by an extended lifetime of the mutant under the imposed conditions which may lead to instability (such as heat, harsh detergent conditions, chaotropic agents and so on). Destabilisation under the imposed condition is typically determined by measuring denaturation or loss of structure. As is discussed below, this may manifest itself by loss of ligand binding ability or loss of secondary or tertiary structure indicators.
As is described with respect to FIG. 12 below (which depicts a particular, preferred embodiment), there are different assay formats which may be used to determine stability of the mutant GPCR.
In one embodiment the mutant GPCR may be brought into contact with a ligand before being subjected to a procedure in which the stability of the mutant is determined (the mutant GPCR and ligand remaining in contact during the test period). Thus, for example, when the method is being used to select for mutant GPCRs which in one conformation bind to a ligand and which have improved thermostablity, the receptor is contacted with the ligand before being heated, and then the amount of ligand bound to the receptor following heating may be used to express thermostability compared to the parent receptor. This provides a measure of the amount of the GPCR which retains ligand binding capacity following exposure to the denaturing conditions (eg heat), which in turn is an indicator of stability.
In an alternative (but less preferred) embodiment, the mutant GPCR is subjected to a procedure in which the stability of the mutant is determined before being contacted with the ligand. Thus, for example, when the method is being used to select for mutant membrane receptors which in one conformation bind to a ligand and which have improved thermostability, the receptor is heated first, before being contacted with the ligand, and then the amount of ligand bound to the receptor may be used to express thermostability. Again, this provides a measure of the amount of the GPCR which retains ligand binding capacity following exposure to the denaturing conditions.
In both embodiments, it will be appreciated that the comparison of stability of the mutant is made by reference to the parent molecule under the same conditions.
It will be appreciated that in both of these embodiments, the mutants that are selected are ones which have increased stability when residing in the particular conformation compared to the parent protein.
The preferred route may be dependent upon the specific GPCR, and will be dependent upon the number of conformations accessible to the protein in the absence of ligand. In the embodiment described in FIG. 12, it is preferred if the ligand is present during the heating step because this increases the probability that the desired conformation is selected.
From the above, it will be appreciated that the invention includes a method for selecting a mutant GPCR with increased thermostability, the method comprising (a) providing one or more mutants of a parent GPCR, (b) selecting an antagonist or an agonist which binds the parent GPCR, (e) determining whether the or each mutant has increased thermostability when in the presence of the said antagonist or agonist by measuring the ability of the mutant GPCR to bind the selected said antagonist or agonist at a particular temperature and after a particular time compared to the parent GPCR and (d) selecting those mutant GPCRs that bind more of the selected said antagonist or agonist at the particular temperature and after the particular time than the parent GPCR under the same conditions. In step (c), a fixed period of time at the particular temperature is typically used in measuring the ability of the mutant GPCR to bind the selected said antagonist or agonist. In step (c), typically a temperature and a time is chosen at which binding of the selected said antagonist or agonist by the parent GPCR is reduced by 50% during the fixed period of time at that temperature (which is indicative that 50% of the receptor is inactivated; “quasi” Tm).
Conveniently, when the ligand is used to assay the GPCR (ie used to determine if it is in a non-denatured state), the ligand is detectably labelled, eg radiolabelled or fluorescently labelled. In another embodiment, ligand binding can be assessed by measuring the amount of unbound ligand using a secondary detection system, for example an antibody or other high affinity binding partner covalently linked to a detectable moiety, for example an enzyme which may be used in a colorimetric assay (such as alkaline phosphatase or horseradish peroxidase). FRET methodology may also be used. It will be appreciated that the ligand used to assay the mutant GPCR in determining its stability need not be the same ligand as selected in step (b) of the method.
Although it is convenient to measure the stability of the parent and mutant GPCR by using the ability to bind a ligand as an indicator of the presence of a non-denatured protein, other methods are known in the art. For example, changes in fluorescence spectra can be a sensitive indicator of unfolding, either by use of intrinsic tryptophan fluorescence or the use of extrinsic fluorescent probes such as 1-anilino-8-napthaleneulfonate (ANS), for example as implemented in the Thermofluor™ method (Mezzasalma at al, J Biomol Screening, 2007, April; 12(3):418-428). Proteolytic stability, deuterium/hydrogen exchange measured by mass spectrometry, blue native gels, capillary zone electrophoresis, circular dichroism (CD) spectra and light scattering may also be used to measure unfolding by loss of signals associated with secondary or tertiary structure. However, all these methods require the protein to be purified in reasonable quantities before they can be used (eg high pmol/nmol quantities), whereas the method described in the Examples makes use of pmol amounts of essentially unpurified GPCR.
In a preferred embodiment, in step (b) two or more ligands of the same class are selected, the presence of each causing the GPCR to reside in the same particular conformation. Thus, in this embodiment, one or more ligands (whether natural or non-natural) of the same class (eg full agonist or partial agonist or antagonist or inverse agonist) may be used. Including multiple ligands of the same class in this process, whether in series or in parallel, minimises the theoretical risk of inadvertently engineering and selecting multiply mutated receptor conformations substantially different to the parent, for example in their binding site, but still able, due to compensatory changes, to bind ligand. The following steps may be used to mitigate this risk:
1. Select a chemically distinct set (eg n=2-5) of ligands, in a common pharmacological class as evidenced by for example a binding or functional or spectroscopic assay. These ligands should be thought to bind to a common spatial region of the receptor, as evidenced for example by competitive binding studies using wild type and/or mutated receptors, and/or by molecular modelling, although they will not necessarily express a common pharmacophore.
2. Make single or multiple receptor mutants intended to increase stability, and assay for tight binding using the full set of ligands. The assays can be parallelised, multiplexed or run in series.
3. Confirm authenticity of stabilised receptor mutant by measurement for example of the binding isotherm for each ligand, and by measurement of the stability shift with ligand (the window should typically be narrowed compared to wild type).
In order to guard against changes in apparent affinity caused by perturbations to the binding site upon mutation, preferably ligands of the same pharmacological class, but different chemical class, should be used to profile the receptor. These should typically show similar shifts in affinity (mutant versus parent, e.g. wild type) in spite of having different molecular recognition properties. Binding experiments should preferably be done using labelled ligand within the same pharmacological class.
Nonetheless it should be recognised that conformational substrates may exist that are specific to chemical classes of ligand within the same pharmacological class, and these may be specifically stabilised in the procedure depending on the chemical class of the selected ligand.
Typically the selected ligand binds to the mutant GPCR with a similar potency to its binding to the parent GPCR. Typically, the Kd values for the particular ligand binding the mutant GPCR and the parent GPCR are within 5-10 fold of each other, such as within 2-3 fold. Typically, the binding of the ligand to the mutant GPCR compared to the parent GPCR would be not more than 5 times weaker and not more than 10 times stronger.
Typically, mutant receptors which have been stabilised in the selected conformation should bind the selected ligand with approximately equal affinity (that is to say typically within 2-3 fold) or greater affinity than does the parent receptor. For agonist-conformation mutants, the mutants typically bind the agonists with the same or higher affinity than the parent GPCR and typically bind antagonists with the same or lower affinity than the parent GPCR. Similarly for antagonist-conformation mutants, the mutants typically bind the antagonists with the same or higher affinity than the parent GPCR and typically bind agonists with the same or lower affinity than the parent GPCR.
Mutants that exhibit a significant reduction (typically greater than 2-3 fold) in affinity for the selecting ligand are typically rejected.
Typically, the rank order of binding of a set of ligands of the same class are comparable, although there may be one or two reversals in the order, or there may be an out-her from the set.
In a further embodiment, two or more ligands that bind simultaneously to the receptor in the same conformation may be used, for example an allosteric modulator and orthosteric agonist.