Biased agonism of carvedilol in the beta1-adrenergic receptor is governed by conformational exclusion

G protein-coupled receptors (GPCRs) are the therapeutically most important family of receptors in the human genome. The selective activation of specific signaling pathways, termed biased signaling, is at the forefront of GPCR-targeted drug development (1). These pathways are governed by dynamic equilibria among multiple GPCR conformations modulated by orthosteric ligands (2). In this study, we elucidated the molecular mechanism underlying the biased signaling of carvedilol. This widely used beta-blocker is a heavily arrestin-biased agonist and at the same time, a G protein inverse agonist of beta-adrenergic receptors (betaARs) (3). Our solution NMR, cryo-electron microscopy, and biochemical data show that the inactive conformation induced by carvedilol prevents G protein binding but is recognized by arrestin. Arrestin binding to the receptor is independent of the type and presence of orthosteric ligands, with the only requirement that arrestin is activated by a receptor phosphopeptide, which does not need to be covalently linked to the receptor. Thus, arrestin-biased signaling appears governed by conformational exclusion of the G protein in agreement with the promiscuous recognition of GPCRs by arrestin. The provided molecular details reveal the mechanism of selective transducer activation by GPCRs and provide new opportunities for developing safer therapeutics.