Structural snapshots uncover a lock-and-key type conserved activation mechanism of β-arrestins by GPCRs

Agonist-induced phosphorylation of G protein-coupled receptors (GPCRs) is a key determinant for the binding and activation of multifunctional regulatory proteins known as {beta}-arrestins ({beta}arrs). Although the primary sequence and phosphorylation pattern of GPCRs are poorly conserved, the downstream functional responses mediated by {beta}arrs such as receptor desensitization, endocytosis and signaling are broadly applicable across GPCRs. A conserved principle of {beta}arr activation, if any, upon their interaction with different GPCRs harboring divergent phosphorylation patterns remains to be visualized, and it represents a major knowledge gap in our current understanding of GPCR signaling and regulatory paradigms. Here, we present four structural snapshots of activated {beta}arrs, in complex with distinct phosphorylation patterns derived from the carboxyl-terminus of three different GPCRs, determined using cryogenic-electron microscopy (cryo-EM). These structures of activated {beta}arrs elucidate a "lock-and-key" type conserved mechanism of {beta}arr activation wherein a P-X-P-P phosphorylation pattern in GPCRs interacts with a spatially organized K-K-R-R-K-K sequence in the N-domain of {beta}arrs. Interestingly, the P-X-P-P pattern simultaneously engages multiple structural elements in {beta}arrs responsible for maintaining the basal conformation, and thereby, leads to efficient {beta}arr activation. The conserved nature of this lock-and-key mechanism is further illustrated by a comprehensive sequence analysis of the human GPCRome, and demonstrated in cellular context with targeted mutagenesis including "loss-of-function" and "gain-of-function" experiments with respect to {beta}arr activation measured by an intrabody-based conformational sensor. Taken together, our findings uncover previously lacking structural insights, which explain the ability of distinct GPCRs to activate {beta}arrs through a common mechanism, and a key missing link in the conceptual framework of GPCR-{beta}arr interaction and resulting functional outcomes.