Control of voltage-independent Zinc inhibition of NMDA receptors by the NR1 subunit

Zinc inhibits NMDA receptor function through both voltage-dependent and voltage-independent mechanisms. In this report we have investigated the role that the NR1 subunit plays in voltage-independent Zn²⁺ inhibition. Our data show that inclusion of exon 5 into the NR1 subunit increases 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 transcript and the NMDA receptor complex contains the NR2A or NR2B subunits; exon 5 has little effect on Zn²⁺ inhibition of receptors that contain NR2C and NR2D Mutagenesis within exon 5 indicates that the same residues that control proton inhibition, including Lys²¹¹, also control the effects of exon 5 on Zn²⁺ inhibition. Amino acid exchanges within the NR1 subunit but outside exon 5 (E181Q, E339Q, E342Q, N616R, N616Q, D669N, D669E, C744A, and C798A) that are known to decrease the pH sensitivity also decrease the Zn²⁺ sensitivity, and concentrations of spermine that relieve tonic proton inhibition also relieve Zn²⁺ inhibition. In summary, our results define the subunit composition of Zn²⁺-sensitive NMDA receptors and provide evidence for structural convergence of three allosteric regulators of receptor function: protons, polyamines, and Zn²⁺.