Signaling at the endosome: cryo-EM structure of a GPCR-G protein-beta-arrestin megacomplex.

G protein-coupled receptors (GPCRs) are a large class of cell-surface receptor involved in cellular signaling that are currently the target of over one third of all clinically approved therapeutics. Classically, an agonist-bound, active GPCR couples to and activates G proteins through the receptor intracellular core. To attenuate G protein signaling, the GPCR is phosphorylated at its C-terminal tail and/or relevant intracellular loops, allowing for the recruitment of β-arrestins (βarrs). βarrs then couple to the receptor intracellular core in order to mediate receptor desensitization and internalization. However, our laboratory and others have observed that some GPCRs are capable of continuously signaling through G protein even after internalization. This mode of sustained signaling stands in contrast with our previous understanding of GPCR signaling, and its molecular mechanism is still not well understood. Recently, we have solved the structure of a GPCR-G protein-βarr megacomplex by cryo-electron microscopy. This 'megaplex' structure illustrates the independent and simultaneous coupling of a G protein to the receptor intracellular core, and binding of a βarr to a phosphorylated receptor C-terminal tail, with all three components maintaining their respective canonically active conformations. The structure provides evidence for the ability of a GPCR to activate G protein even while being bound to and internalized by βarr. It also reveals that the binding of G protein and βarr to the same GPCR is not mutually exclusive, and raises a number of future questions to be answered regarding the mechanism of sustained signaling.