Molecular basis of β-arrestin coupling to the metabotropic glutamate receptor mGlu3.

β-Arrestins (βarrs) mediate the desensitization and internalization of activated G-protein-coupled receptors (GPCRs). The molecular mechanism by which dimeric family C GPCR members recruit arrestins remains elusive. Here we report two structures of metabotropic glutamate receptor subtype 3 (mGlu3) coupled to βarr1, with stoichiometries of 2:1 and 2:2. The L-glutamate-bound mGlu3 dimer adopts an inactive state, with both Venus flytrap domains closed, engaging βarr1 either asymmetrically or symmetrically. The transmembrane domain of the mGlu3 protomer interacts with βarr1 through a binding pocket formed by three intracellular loops and an ordered C-terminal region. Three phosphorylation sites (pS857, pS859 and pT860) on the C-terminal tail of mGlu3 engage the N domain of βarr1. βarr1 stabilizes mGlu3 in an inactive conformation, characterized by a TM3/TM4-TM3/TM4 dimeric interface, previously observed in the negative allosteric modulator-bound structure of mGlu3. Our findings provide important insights into βarr-mediated inactivation of family C GPCRs.