β-Arrestin biosensors reveal a rapid, receptor-dependent activation/deactivation cycle

(beta-) Arrestins are important regulators of G-protein-coupled receptors (GPCRs)(1-3). They bind to active, phosphorylated GPCRs and thereby shut off 'classical' signalling to G proteins(3,)4, trigger internalization of GPCRs via interaction with the clathrin machinery(5-7) and mediate signalling via 'non-classical' pathways(1,2). In addition to two visual arrestins that bind to rod and cone photoreceptors (termed arrestin1 and arrestin4), there are only two (non-visual) beta-arrestin proteins (beta-arrestin1 and beta-arrestin2, also termed arrestin2 and arrestin3), which regulate hundreds of different (non-visual) GPCRs. Binding of these proteins to GPCRs usually requires the active form of the receptors plus their phosphorylation by G-protein-coupled receptor kinases (GRKs)(1,3,4). The binding of receptors or their carboxy terminus as well as certain truncations induce active conformations of (beta-) arrestins that have recently been solved by X-ray crystallography(8-10). Here we investigate both the interaction of beta-arrestin with GPCRs, and the beta-arrestin conformational changes in real time and in living human cells, using a series of fluorescence resonance energy transfer (FRET)-based beta-arrestin2 biosensors. We observe receptor-specific patterns of conformational changes in beta-arrestin2 that occur rapidly after the receptor-beta-arrestin2 interaction. After agonist removal, these changes persist for longer than the direct receptor interaction. Our data indicate a rapid, receptor-type-specific, two-step binding and activation process between GPCRs and beta-arrestins. They further indicate that beta-arrestins remain active after dissociation from receptors, allowing them to remain at the cell surface and presumably signal independently. Thus, GPCRs trigger a rapid, receptor-specific activation/deactivation cycle of beta-arrestins, which permits their active signalling.