Crystal structure of β-arrestin 2 in complex with an atypical chemokine receptor phosphopeptide reveals an alternative active conformation

{beta}-arrestins ({beta}arrs) critically regulate signaling and trafficking of G protein-coupled receptors (GPCRs), the largest family of drug targets in the human genome, and there are two isoforms of {beta}arrs: {beta}arr1 and {beta}arr2. Most GPCRs interact with both the heterotrimeric G-proteins and {beta}arrs, inducing distinct downstream signal transduction. However, certain chemokine receptors lack functional G-protein coupling, but they can efficiently recruit {beta}arrs upon agonist-stimulation, and they are referred to as atypical chemokine receptors (ACKRs). Receptor phosphorylation is a key determinant for the binding of {beta}arrs, and understanding the intricate details of receptor-{beta}arr interaction is the next frontier in GPCR structural biology. To date, the high-resolution structures of active {beta}arr1 have been revealed, but the activation mechanism of {beta}arr2 by a phosphorylated GPCR remains elusive. Here, we present a 1.95 [A] crystal structure of {beta}arr2 in complex with a phosphopeptide (C7pp) derived from the carboxyl-terminus of ACKR3, also known as CXCR7. The structure of C7pp-bound {beta}arr2 reveals key differences from the previously determined active conformation of {beta}arr1. One of the key differences is that C7pp-bound {beta}arr2 shows a relatively small inter-domain rotation. An antibody-fragment-based conformational sensor and hydrogen/deuterium exchange experiments further corroborate structural features and suggest that the determined structure is an alternative active conformation of {beta}arr2.