NMDA receptors with locked glutamate-binding clefts open with high efficacy

Glutamate-gated channels mediate fundamental brain processes, yet the mechanisms by which the neurotransmitter controls channel activation are incompletely understood. Structural studies revealed that the agonist has the critical role of bridging the divide between two flexible extracellular lobes and solidified the view that agonist-induced cleft-closure drives further isomerizations, which eventually open the channel. Within the glutamate receptor family, NMDA-sensitive channels are unique in their requirement that both glycine and glutamate bind to homologous regions on GluN1 and GluN2 subunits, respectively, before the channel can open. To study the gating reaction in separation from agonist binding and dissociation, we characterized the kinetic mechanism of individual NMDA receptors whose ligand-binding clefts were locked shut by disulfide bridges engineered across lobes. We found that locking GluN1 domains had no observable consequences on receptor activity, whereas locking GluN2A domains increased channel activity without reducing the number of resolvable kinetic states. Based on these results, we suggest that glutamate but not glycine activates NMDA receptors with submaximal efficacy. Low glutamate efficacy may represent a mechanism by which the neurotransmitter maintains control over receptor kinetics despite sharing with glycine the task of activation.