Structural characterization of five functional states of metabotropic glutamate receptor 8

Metabotropic glutamate receptors (mGluRs) are dimeric class C G protein-coupled receptors, which play crucial roles in brain physiology and pathology. Among them, mGlu8 is the least characterized, though it is physiologically important. While recognized to signal via G_i/o proteins, the involvement of β-arrestin is unknown. Here, we found that both mGlu8 agonists and positive allosteric modulators (PAMs) activate G_i signaling, but mainly agonists induce β-arrestin recruitment. We solved five human mGlu8 cryo-electron microscopy (cryo-EM) structures in various states: apo, antagonist-bound, agonist + PAM-bound, agonist + PAM-bound with G_i protein, and agonist-bound with β-arrestin1 states. They revealed a unique PAM-binding pocket at the extracellular side of the TM6/TM7 interface. Agonist and PAM promote active mGlu8 association with one G_i protein asymmetrically (2:1), while two β-arrestin1 can interact symmetrically (2:2) to both subunits of an inactive dimer state to promote constitutive internalization. These findings elucidate how mGlu8 selectively engages transducers, offering insights into its signaling capabilities and selective drug development.