CONTROL OF VOLTAGE-INDEPENDENT ZINC INHIBITION OF NMDA RECEPTORS BY THE NR1 SUBUNIT

Zinc inhibits NMDA receptor function through both voltage-dependent an d voltage-independent mechanisms. In this report we have investigated the role that the NR1 subunit plays in voltage-independent Zn2+ inhibi tion. Our data show that inclusion of exon 5 into the NR1 subunit incr eases the IC50 for voltage-independent Zn2+ inhibition from 3-fold to 10-fold when full length exon 22 is also spliced into the mature NR1 t ranscript and the NMDA receptor complex contains the NR2A or NR2B subu nits; exon 5 has little effect on Zn2+ inhibition of receptors that co ntain NR2C and NR2D. Mutagenesis within exon 5 indicates that the same residues that control proton inhibition, including Lys(211), also con trol the effects of exon 5 on Zn2+ inhibition. Amino acid exchanges wi thin the NR1 subunit but outside exon 5 (E181Q, E339Q, E342Q, N616R, N 616Q, D669N, D669E, C744A, and C798A) that are known to decrease the p H sensitivity also decrease the Zn2+ sensitivity, and concentrations o f spermine that relieve tonic proton inhibition also relieve Zn2+ inhi bition, in summary, our results define the subunit composition of Zn2-sensitive NMDA receptors and provide evidence for structural converge nce of three allosteric regulators of receptor function: protons, poly amines, and Zn2+.