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. We found that the intracellular cavity of μOR exists in an equilibrium between closed and multiple open conformations with coupled conformational changes on the transmembrane helices 3, 5, 6, and 7, and that the population of each open conformation determines the G‐protein‐ and arrestin‐mediated signaling levels in each ligand‐bound state. These findings provide insight into the biased signaling of GPCRs and will be helpful for development of analgesics that stimulate μOR with reduced tolerance and dependence. An open and closed case: NMR analysis of different ligand‐bound states of the μ‐opioid receptor revealed that the intracellular cavity of the receptor exists in an equilibrium between closed and multiple open conformations, and that the population of each open conformation determines the G‐protein‐ and β‐arrestin‐mediated signaling levels (see picture). These findings provide structural insight into the biased signaling of G‐protein‐coupled receptors.