Differential effects on current kinetics by point mutations in the lurcher motif of NR1/NR2A receptors.

The extracellular proton site has emerged as the key site in regulating N-methyl-d-aspartate (NMDA) receptor function. However, the molecular determinants for the proton-sensitive gating of NMDA receptors are still not clearly delineated. The highly conserved lurcher motif plays an important role in determining the proton sensitivity of NMDA receptors. Mutations of several residues in the lurcher motif of either NR1 or NR2A significantly reduce proton sensitivity of recombinant NR1/NR2A receptors. It remains uncertain how these residues play a role in proton inhibition of NMDA receptors. Mutations of these residues could directly reduce the proton affinity. Alternatively, they could alter the proton IC(50) indirectly by increasing channel open probability. In the present study, we recorded the macroscopic NMDA currents in HEK 293 cells with a piezo-based rapid solution exchange system. We show that zinc slows the deactivation of NR1a(A653T)/NR2A receptors and NR1a/NR2A(A651T) receptors. However, NR1a(T648C)/NR2A, NR1a/NR2A(T646C), NR1a(A649C)/NR2A, NR1a/NR2A(A647C), and NR1a(A653T)/NR2A exhibit significantly slower rise time and deactivation time constants under nominally zinc-free conditions. Our data suggest that the channel open probability for these mutant receptors may be significantly increased. The reduction in proton sensitivity by these mutations could be accounted for, at least partially, by the increased channel open probability. In contrast, NR1a/NR2A(A651T) exhibits normal macroscopic currents, suggesting that the reduction of proton sensitivity by this mutation cannot be attributed to any significant change of open probability. Further experiments are needed to determine the exact role of this residue in proton-sensitive gating of NMDA receptors.