Ligand efficacy modulates conformational dynamics of the µ-opioid receptor

µ-opioid receptor (µOR) activation depends on ligand-specific conformational changes that translate into different intrinsic efficacies at the signal transducer level. Using double electron-electron resonance (DEER) and single-molecule FRET, researchers map how multiple cytoplasmic-face conformations interconvert on different timescales, including a pre-activated state capable of G-protein binding and a fully activated state that reduces GDP affinity in the ternary complex. The study shows that β-arrestin-1 interaction at the µOR core binding site is less specific and occurs with much lower affinity than G_i binding, even as distinct ligand classes vary conformational stabilization. Experiments across nine µOR ligands with distinct pharmacological profiles (including antagonists, G-protein-biased agonists, and super-efficacy agonists) demonstrate how the µOR conformational ensemble is fine-tuned by ligand binding, producing distinctive signaling and signal bias relevant to therapeutic effect and side-effect profiles.