Abstract

A hallmark feature of N-methyl-d-aspartate (NMDA) receptors is their voltage-dependent block by extracellular Mg²⁺. The structural basis for Mg²⁺ block is not fully understood. Although asparagine residues in the pore-forming M2 regions of NR1 and NR2 subunits influence Mg²⁺ block, it has been speculated that additional residues are likely to be involved. Here, we report the unexpected finding that a tryptophan residue in the M2 region of NR2 subunits controls Mg²⁺ block. An NR2B(W607L) mutation abolished block and greatly increased permeation of extracellular Mg²⁺. A similar effect was seen with a mutation at the equivalent residue in NR2A but not with mutations at the equivalent residue or adjacent residues in NR1. In NR2B, mutations that changed NR2B(W607) to asparagine (W607N) or alanine (W607A) also greatly reduced Mg²⁺ block, whereas mutations that changed W607 to the aromatic residues tyrosine (W607Y) or phenylalanine (W607F) had little or no effect on Mg²⁺ block. Furthermore, the W607L, W607N, and W607A mutants, but not the W607Y and W607F mutants, decreased Ba²⁺ permeability of NMDA channels. Thus, residue NR2B(W607) may be involved in binding of divalent cations, in particular Mg²⁺, through a cation-π interaction with the electron-rich aromatic ring of the tryptophan. We previously suggested that NR2B(W607) may contribute to the narrow constriction of the NMDA channel. A model is now proposed in which the M2 loop of NR2B is folded in such a way that NR2B(W607) is positioned at the narrow constriction, at a level similar to NR2B(N616) and NR1(N616), with these three residues forming a binding site for Mg²⁺.