Abstract

N-Methyl-d-aspartate (NMDA) receptors are modulated by extracellular spermine and protons and are blocked in a voltage-dependent manner by spermine and polyamine derivatives such as N¹-dansyl-spermine (N¹-DnsSpm). The effects of mutations in the first and third transmembrane domains (M1 and M3) and the pore-forming loop (M2) of NMDA receptor subunits were studied. Surprisingly, some mutations in M2 and M3 of the NR1 subunit, including mutations at W608 and N616 in M2, reduced spermine stimulation and proton inhibition. These mutations may have long-range allosteric effects or may change spermine- and pH-dependent gating processes rather than directly affecting the binding sites for these modulators because spermine stimulation and proton inhibition are not voltage dependent and are thought to involve binding sites outside the pore-forming regions of the receptor. A number of mutations in M1–M3, including mutations at tryptophan and tyrosine residues near the extracellular sides of M1 and M3, reduced block by spermine and N¹-DnsSpm. The effects of these mutants on channel block were characterized in detail by using N¹-DnsSpm, which produces block but not stimulation of NMDA receptors. Block by N¹-DnsSpm was studied by using voltage ramps analyzed with the Woodhull model of channel block. Mutations at W563 (in M1) and E621 (immediately after M2) in the NR1a subunit and at Y646 (in M3) and N616 (in the M2 loop) in the NR2B subunit reduced the affinity for N¹-DnsSpm without affecting the voltage dependence of block. These residues may form part of a binding site for N¹-DnsSpm. Mutation of a tryptophan residue at position W607 in the M2 region of NR2B greatly reduced block by N¹-DnsSpm, and N¹-DnsSpm could easily permeate channels containing this mutation. The results suggest that at least parts of the M1 and M3 segments contribute to the pore or vestibule of the NMDA channel and that a tryptophan in M2 (W607 in NR2B) may contribute to the narrow constriction of the pore.