title: Critical role for the second extracellular loop in the binding of both orthosteric and allosteric G protein-coupled receptor ligands.
abstract: The second extracellular (E2) loop of G protein-coupled receptors (GPCRs) plays an essential but poorly understood role in the binding of non-peptidic small molecules. We have utilized both orthosteric ligands and allosteric modulators of the M2 muscarinic acetylcholine receptor, a prototypical Family A GPCR, to probe possible E2 loop binding dynamics. We developed a homology model based on the crystal structure of bovine rhodopsin and predicted novel cysteine substitutions that should dramatically reduce E2 loop flexibility via disulfide bond formation and significantly inhibit the binding of both types of ligands. This prediction was validated experimentally using radioligand binding, dissociation kinetics, and cell-based functional assays. The results argue for a flexible "gatekeeper" role of the E2 loop in the binding of both allosteric and orthosteric GPCR ligands.
authors.affiliation: Drug Discovery Biology Laboratory, Department of Pharmacology, Monash University, Clayton, 3800 Australia.
authors.forename: Vimesh A
authors.initials: VA
authors.lastname: Avlani
authors.forename: Karen J
authors.initials: KJ
authors.lastname: Gregory
authors.forename: Craig J
authors.initials: CJ
authors.lastname: Morton
authors.forename: Michael W
authors.initials: MW
authors.lastname: Parker
authors.forename: Patrick M
authors.initials: PM
authors.lastname: Sexton
authors.forename: Arthur
authors.initials: A
authors.lastname: Christopoulos
mesh_terms: D000494:Allosteric Regulation / Q000502:physiology; D000495:Allosteric Site / Q000502:physiology; D019943:Amino Acid Substitution; D000818:Animals; D002417:Cattle; D003545:Cysteine / Q000737:chemistry / Q000235:genetics; D006801:Humans; D008024:Ligands; D008958:Models, Molecular; D011485:Protein Binding / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000235:genetics; D043585:Receptor, Muscarinic M2 / Q000737:chemistry* / Q000235:genetics; D012243:Rhodopsin / Q000737:chemistry* / Q000235:genetics; D040681:Structural Homology, Protein
chemical_list: D008024:Ligands; D043585:Receptor, Muscarinic M2; D012243:Rhodopsin; D003545:Cysteine