Disruption of interdomain interactions in the glutamate binding pocket affects differentially agonist affinity and efficacy of N-methyl-D-aspartate receptor activation

In ionotropic glutamate receptors, agonist binding occurs in a conserved clam shell-like domain composed of the two lobes D1 and D2. Docking of glutamate into the binding cleft promotes rotation in the hinge region of the two lobes, resulting in closure of the binding pocket, which is thought to represent a prerequisite for channel gating. Here, we disrupted D1D2 interlobe interactions in the NR2A subunit of N-methyl-d-aspartate (NMDA) receptors through systematic mutation of individual residues and studied the influence on the activation kinetics of currents from NR1/NR2 NMDA receptors heterologously expressed in HEK cells. We show that the mutations affect differentially glutamate binding and channel gating, depending on their location within the binding domain, mainly by altering k(off) and k(cl), respectively. Whereas impaired stability of glutamate in its binding site is the only effect of mutations on one side of the ligand binding pocket, close to the hinge region, alterations in gating are the predominant consequence of mutations on the opposite side, at the entrance of the binding pocket. A mutation increasing D1D2 interaction at the entrance of the pocket resulted in an NMDA receptor with an increased open probability as demonstrated by single channel and whole cell kinetic analysis. Thus, the results indicate that agonist-induced binding domain closure is itself a complex process, certain aspects of which are coupled either to binding or to gating. Specifically, we propose that late steps of domain closure, in kinetic terms, represent part of channel gating.