Distinct ligands for the same G-protein-coupled receptor (GPCR) activate intracellular signalling partners to varying extents, but the molecular mechanisms that drive these differences remain elusive. Here, hypothesizing that such differences in signalling efficacy might be captured structurally in intermediate states under non-equilibrium conditions, we use a time-resolved cryo-electron-microscopy approach to visualize the GTP-induced activation of the Gαiβγ heterotrimer by the μ-opioid receptor bound to three ligands that show partial, full or super agonism on the receptor1. We resolve ensembles of conformational states along the G-protein activation pathway, including an intermediate state that enables us to visualize receptor dynamics as a function of bound ligand. The results reveal ligand-dependent differences in state occupancy and conformational stability, with higher ligand efficacy correlating with increased dynamics of the receptor’s transmembrane helices 5 and 6. Furthermore, we identify key differences between Gi and Gs in the mechanism of GTP-induced activation, which are likely to underlie the distinct activation kinetics of these G-protein types. Corroborated by molecular-dynamics simulations and single-molecule fluorescence assays, our findings provide a dynamic structural landscape of GPCR–G-protein interactions for ligands of various efficacies, and suggest that partial agonists produce a ‘kinetic trap’ during G-protein activation.
