Control of conformational equilibria in the human B-2 bradykinin receptor - Modeling of nonpeptidic ligand action and comparison to the rhodopsin structure

A prototypic study of the molecular mechanisms of activation or inactivatio n of peptide hormone G protein-coupled receptors was carried out on the hum an B-2 bradykinin receptor. A detailed pharmacological analysis of receptor mutants possessing either increased constitutive activity or impaired acti vation or ligand recognition allowed us to propose key residues participati ng in intramolecular interaction networks stabilizing receptor inactive or active conformations: Asn(113) and Tyr(115) (TM III), Trp(256) and Phe(259) (TM VI), Tyr(295) (TM VII) which are homologous of the rhodopsin residues Gly(120), Glu(122), Trp(265), Tyr(268), and Lys(296), respectively. An esse ntial experimental finding was the spatial proximity between Asn(113), whic h is the cornerstone of inactive conformations, and Trp(256) which plays a subtle role in controlling the balance between active and inactive conforma tions. Molecular modeling and mutagenesis data showed that Trp(256) and Tyr (295) constitute, together with Gln(288), receptor contact points with orig inal nonpeptidic ligands. It provided an explanation for the ligand inverse agonist behavior on the WT receptor, with underlying restricted motions of TMs III, VI, and VII, and its agonist behavior on the Ala(113) and Phe(256 ) constitutively activated mutants. These data on the B-2 receptor emphasiz e that conformational equilibria are controlled in a coordinated fashion by key residues which are located at strategic positions for several G protei n-coupled receptors. They are discussed in comparison with the recently det ermined rhodopsin crystallographic structure.