Structural basis of G protein–coupled receptor–G protein interactions

Crosslinks between the active and inactive M3R and the G-protein G q to which it couples reveal GPCR-G protein movements that can occur upon agonist binding and define basic architectural features of the interface. The interaction of G protein–coupled receptors (GPCRs) with heterotrimeric G proteins represents one of the most fundamental biological processes. However, the molecular architecture of the GPCR–G protein complex remains poorly defined. In the present study, we applied a comprehensive GPCR–G protein α subunit (Gα) chemical cross-linking strategy to map a receptor-Gα interface, both before and after agonist-induced receptor activation. Using the M 3 muscarinic acetylcholine receptor (M3R)-Gα q system as a model system, we examined the ability of ~250 combinations of cysteine-substituted M3R and Gα q proteins to undergo cross-link formation. We identified many specific M3R-Gα q contact sites, in both the inactive and active receptor conformations, allowing us to draw conclusions regarding the basic architecture of the M3R-Gα q interface and the nature of the conformational changes following receptor activation. As heterotrimeric G proteins as well as most GPCRs share a high degree of structural homology, our findings should be of broad general relevance.