Assay for assessing conformational stability of membrane protein

1. An assay for assessing the conformational stability of a membrane protein, comprising:

(a) providing a sample comprising a first population and a second population of a membrane protein; wherein the membrane protein in the first population is labelled with a donor label and the membrane protein in the second population is labelled with an acceptor label, or the membrane protein in the first population is labelled with an acceptor label and the membrane protein in the second population is labelled with a donor label, and wherein the populations of membrane protein are provided in a solubilized form,

(b) exposing the first and second populations of the membrane protein provided in the solubilized form to a denaturant or denaturing condition, and

(c) assessing aggregation between membrane proteins of the first and second populations by activating the donor label to permit a distance-dependent interaction with the acceptor label, which interaction produces a detectable signal.

2. The assay of claim 1 , wherein the first population and second population of the membrane protein are present in the sample in a 1:1 ratio.

3. The assay of claim 1 , wherein the donor label is covalently attached to the membrane protein and the acceptor label is covalently attached to the membrane protein.

4. An assay for assessing the conformational stability of a membrane protein, comprising:

(a) providing a sample comprising a membrane protein population, wherein the population of membrane protein is provided in a solubilized form,

(b) exposing the membrane protein population to a denaturant or denaturing condition,

(c) labelling one of the N-terminus or C-terminus of the membrane protein with a donor label and the other of the N-terminus or C-terminus of the membrane protein with an acceptor label, and

(d) assessing aggregation of the membrane proteins in the population by activating the donor label to permit a distance-dependent interaction with the acceptor label, which interaction produces a detectable signal.

5. The assay of claim 1 , wherein the interaction between the donor label and the acceptor label involves the transfer of energy from a donor fluorophore to an acceptor fluorophore.

6. The assay of claim 5 , wherein the donor fluorophore is a lanthanide, optionally wherein the lanthanide is Terbium.

7. The assay of claim 5 , wherein the acceptor fluorophore is EGFP or d2.

8. The assay of claim 1 , wherein the interaction between the donor label and the acceptor label is a chemiluminescent reaction, optionally wherein the interaction between the donor label and the acceptor label involves the generation of singlet oxygen molecules that trigger a chemiluminescent reaction.

9. The assay of claim 1 , wherein the donor label and/or acceptor label is directly attached or indirectly attached to the membrane protein.

10. The assay of claim 1 , wherein the sample comprises one or more detergents selected from the group consisting of DDM, C11-maltoside, C10-maltoside, C9-maltoside, C8-maltoside, C11-glucoside, C10-glucoside, C9-glucoside, C8-glucoside, LDAO, and SDS.

11. The assay of claim 1 , wherein the denaturant or denaturing condition is selected from one or more of heat, a detergent, a chaotropic agent or pH.

12. The assay of claim 1 , wherein the membrane protein is a GPCR.

13. The assay of claim 12 , wherein the sample provided in step (a) comprises a GPCR ligand, the ligand being one that binds to a GPCR when the GPCR is residing in a particular conformation.

14. The assay of claim 13 , wherein the GPCR ligand is from the agonist class of ligands and the particular conformation is an agonist conformation, or the GPCR ligand is from the antagonist class of ligands and the particular conformation is an antagonist conformation.

15. A method for selecting a membrane protein with increased conformational stability, comprising:

(a) comparing the conformational stability of one or more mutants of a parent membrane proteins with the conformational stability of the parent membrane protein according to the assay of claim 1 , and

(b) selecting one or more mutants that have increased conformational stability relative to the parent membrane protein.

16. The method of claim 15 , comprising:

(a) providing one or more mutants of a parent membrane protein;

(b) assessing the conformational stability of the one or more mutants of the parent membrane protein;

(c) assessing the conformational stability of the parent membrane protein; and

(d) selecting one or more mutants of the parent membrane protein that have increased conformational stability compared to the conformational stability of the parent protein.

17. The method of claim 15 , wherein the membrane protein has increased stability to any of heat, a detergent, a chaotropic agent or an extreme of pH.

18. A method for preparing a mutant GPCR, the method comprising:

(a) carrying out the method of claim 15 ,

(b) identifying the position or positions of the mutated amino acid residue or residues in the mutant membrane protein or membrane proteins which has been selected for increased stability, and

(c) synthesising a mutant membrane protein which contains a replacement amino acid at one or more of the positions identified.

19. The assay of claim 1 , wherein the assay is used in drug screening.

20. The assay of claim 4 , wherein the assay is used in drug screening.