Distribution, assembly and mechanism of GluN1/GluN3A excitatory glycine receptors

NMDA receptors play key roles in brain development, plasticity and diseases. While glutamate and glycine co-gated GluN2-containing NMDARs have been extensively characterized, little is known regarding GluN3A-containing NMDARs that form receptors gated by glycine only. Here, combining native purification, mass spectrometry, cryo-EM and electrophysiology, we provide key insights on the molecular logic of GluN3A-NMDARs. We demonstrate that native GluN3A receptors account for a sizeable fraction of total NMDARs, are enriched at extrasynaptic compartments in the adult brain, and assemble specifically as diheteromeric GluN1/GluN3A excitatory glycine receptors (eGlyRs) rather than as triheteromeric GluN1/GluN2/GluN3A receptors. The architecture of eGlyRs reveal splayed and loosely packed extracellular domains, strikingly different from ‘conventional’ GluN1/GluN2 receptors. Through back-and- forth structural and functional validations, we demonstrate how the combined effects of a weak ligand-binding domain (LBD) dimer interface and high intrinsic mobility of the N-terminal domains (NTDs) shape the atypical gating of eGlyRs. These findings illuminate GluN3A-NMDAR physiology and mechanism, with implications for neuronal signaling and pharmacology.