Structural Diversity and Dynamics of Metabotropic Glutamate Receptor/Beta-Arrestin Coupling

Beta-arrestins (β-arrs) are cytosolic proteins which mediate G protein-coupled receptor (GPCR) desensitization, endocytosis, and signaling. Despite the widespread physiological roles of β-arr coupling, the molecular basis of GPCR/β-arr interaction has been studied primarily in monomeric family A GPCRs. Here we take an integrative biophysical and structural approach to uncover molecular diversity in β-arr coupling to the neuromodulatory metabotropic glutamate receptors (mGluRs), prototypical, dimeric family C GPCRs. We find, using a new single molecule pulldown assay, that mGluRs couple to β-arrs with a 2:1 or 2:2 stoichiometry via a combination of "tail" and "core" interactions. Using single molecule FRET analysis, we also find that β-arr1 stabilizes active conformations of mGluR8. Cryo-EM structures of mGluR8 alone or with either G proteins or β-arr1 reveal transducer-specific mGluR8 active states and, in combination with molecular dynamics simulations, define the positioning of mGluR8-bound β-arr1, supporting a steric mechanism of mGluR desensitization involving interactions with both subunits and the lipid bilayer. Finally, combinatorial mutagenesis enables the identification of a landscape of homo- and hetero-dimeric mGluR/β-arr complexes, including mGluR/β-arr1/β-arr2 megacomplexes, providing a framework for family C GPCR/β-arr coupling and expanding the known range of GPCR/transducer coupling modes.