Structural and mechanistic insights into gating and allosteric modulation of GluN1-GluN3A NMDA receptors.

N-methyl-D-aspartate receptors (NMDARs) mediate excitatory signaling essential for synaptic plasticity and memory. Unlike GluN2-containing NMDARs, GluN3-containing receptors are activated solely by glycine and exhibit profound desensitization and paradoxical potentiation by GluN1-selective antagonists, including CGP-78608 (CGP). Although GluN3A-containing NMDARs regulate synapse pruning and excitotoxicity, and are associated with schizophrenia, autism and stroke, their native stoichiometry and gating mechanism are poorly defined. Here, using single-molecule pull-down analysis, we show that native GluN3A-containing receptors are diheteromeric assemblies. Cryogenic-electron microscopy analysis of GluN1-GluN3A receptors in antagonist-bound, preactive, active and desensitized states, augmented by electrophysiology and pharmacology experiments, show how glycine activates the receptor solely via GluN3A-dependent conformational changes, opening the gate with two-fold symmetry, and induces a roughly four-fold symmetric desensitized state. CGP-bound GluN1 restricts GluN3A rotation, promoting glycine-induced activation by blocking desensitization. These findings illuminate how CGP potentiates GluN1-GluN3A receptor activity, place the receptor gating mechanism on a structural foundation and define the molecular basis for pharmacological modulation.