Visualization of arrestin recruitment by a G-protein-coupled receptor

G-protein-coupled receptors (GPCRs) are critically regulated by beta-arrestins, which not only desensitize G-protein signalling but also initiate a G-protein-independent wave of signalling(1-5). A recent surge of structural data on a number of GPCRs, including the beta(2) adrenergic receptor (beta(2)AR)-G-proteincomplex, has provided novel insights into the structural basis of receptor activation(6-11). However, complementary information has been lacking on the recruitment of beta-arrestins to activated GPCRs, primarily owing to challenges in obtaining stable receptor-beta-arrestin complexes for structural studies. Here we devised a strategy for forming and purifying a functional human beta(2)AR-beta-arrestin-1 complex that allowed us to visualize its architecture by single-particle negative-stain electron microscopy and to characterize the interactions between beta(2)AR and beta-arrestin 1 using hydrogen-deuterium exchange mass spectrometry (HDX-MS) and chemical crosslinking. Electron microscopy two-dimensional averages and three dimensional reconstructions reveal bimodal binding of beta-arrestin 1 to the beta(2)AR, involving two separate sets of interactions, one with the phosphorylated carboxy terminus of the receptor and the other with its seven-transmembrane core. Areas of reduced HDX together with identification of crosslinked residues suggest engagement of the finger loop of beta-arrestin 1 with the seven-transmembrane core of the receptor. In contrast, focal areas of raised HDX levels indicate regions of increased dynamics in both the N and C domains of beta-arrestin 1 when coupled to the beta(2)AR. A molecular model of the beta(2)AR-beta-arrestin signalling complex was made by docking activated beta-arrestin 1 and beta(2)AR crystal structures into the electron microscopymap densities with constraints provided by HDX-MS and crosslinking, allowing us to obtain valuable insights into the overall architecture of a receptor-arrestin complex. The dynamic and structural information presented here provides a framework for better understanding the basis of GPCR regulation by arrestins.