Structure of active β-arrestin1 bound to a G protein-coupled receptor phosphopeptide

The functions of G-protein coupled receptors (GPCRs) are primarily mediated and modulated by three families of proteins: the heterotrimeric G proteins, the G-protein coupled receptor kinases (GRKs), and the arrestins(1). G proteins mediate activation of second messenger-generating enzymes and other effectors, GRKs phosphorylate activated receptors(2), and arrestins subsequently bind phosphorylated receptors and cause receptor desensitization(3). Arrestins activated by interaction with phosphorylated receptors can also mediate G protein-independent signaling by serving as adaptors to link receptors to numerous signaling pathways(4). Despite their central role in regulation and signaling of GPCRs, a structural understanding of β-arrestin activation and interaction with GPCRs is still lacking. Here, we report the crystal structure of β-arrestin1 in complex with a fully phosphorylated 29 amino acid carboxy-terminal peptide derived from the V(2) vasopressin receptor (V(2)Rpp). This peptide has previously been shown to functionally and conformationally activate β-arrestin1(5). To capture this active conformation, we utilized a conformationally-selective synthetic antibody fragment (Fab30) that recognizes the phosphopeptide-activated state of β-arrestin1. The structure of the β-arrestin1:V(2)Rpp:Fab30 complex shows striking conformational differences in β-arrestin1 compared to its inactive conformation. These include rotation of the amino and carboxy-terminal domains relative to each other, and a major reorientation of the “lariat loop” implicated in maintaining the inactive state of β-arrestin1. These results reveal, for the first time at high resolution, a receptor-interacting interface on β-arrestin, and they suggest a potentially general molecular mechanism for activation of these multifunctional signaling and regulatory proteins.