N-methyl-d-aspartate (NMDA) receptor activation involves a dynamic series of structural rearrangements initiated by glutamate binding to glycine-loaded receptors and culminates with the clearing of the permeation pathway, which allows ionic flux. Along this sequence, three rate-limiting transitions can be quantified with kinetic analyses of single-channel currents, even though the structural determinants of these critical steps are unknown. In inactive receptors, the major permeation barrier resides at the intersection of four M3 transmembrane helices, two from each GluN1 and GluN2 subunits, at the level of the invariant SYTANLAAF sequence, known as the lurcher motif. Because the A7 but not A8 residues in this region display agonist-dependent accessibility to extracellular solutes, they were hypothesized to form the glutamate-sensitive gate. We tested this premise by examining the reaction mechanisms of receptors with substitutions in the lurcher motifs of GluN1 or GluN2A subunits. We found that, consistent with their locations relative to the proposed activation gate, A8Y decreased open-state stability, whereas A7Y dramatically stabilized open states, primarily by preventing gate closure; the equilibrium distribution of A7Y receptors was strongly shifted toward active states and resulted in slower microscopic association and dissociation rate constants for glutamate. In addition, for both A8- and A7-substituted receptors, we noticed patterns of kinetic changes that were specific to GluN1 or GluN2 locations. This may be a first indication that the sequence of discernible kinetic transitions during NMDA receptor activation may reflect subunit-dependent movements of M3 helices. Testing this hypothesis may afford insight into the activation mechanism of NMDA receptors.
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