Biased Signaling of the G-Protein-Coupled Receptor β2AR Is Governed by Conformational Exchange Kinetics

G-protein-coupled receptors (GPCRs) mediate a wide range of human physiological functions by transducing extracellular ligand binding events into intracellular responses. GPCRs can activate parallel, independent signaling pathways mediated by G proteins or beta-arrestins. Whereas balanced'' agonists activate both pathways equally, biased'' agonists dominantly activate one pathway, which is of interest for designing GPCR-targeting drugs because it may mitigate undesirable side effects. Previous studies demonstrated that beta-arrestin activation is associated with transmembrane helix VII (TM VII) of GPCRs. Here, single-molecule fluorescence spectroscopy with the beta(2)-adrenergic receptor (beta(2)AR) in the ligand-free state showed that TM VII spontaneously fluctuates between one inactive and one active-like conformation. The presence of the beta-arrestin-biased agonist isoetharine prolongs the dwell time of TM VII in the active-like conformation compared with the balanced agonist formoterol, suggesting that ligands can induce signaling bias by modulating the kinetics of receptor conformational exchange.