Identification of a Conformational Equilibrium That Determines the Efficacy and Functional Selectivity of the μ-Opioid Receptor.

G-protein-coupled receptor (GPCR) ligands impart differing degrees of signaling in the G-protein and arrestin pathways, in phenomena called "biased signaling". However, the mechanism underlying the biased signaling of GPCRs is still unclear, although crystal structures of GPCRs bound to the G protein or arrestin are available. In this study, we observed the NMR signals from methionine residues of the μ-opioid receptor (μOR) in the balanced- and biased-ligand-bound states.