Structure of the NMDA receptor channel M2 segment inferred from the accessibility of substituted cysteines

The structure of the NMDA receptor channel M2 segment was investigated by probing the extracellular and cytoplasmic faces of cysteine-substituted NR1-NRPC channels with charged sulfhydryl-specific reagents. The pattern of accessible positions suggests that the M2 segment forms a channel-lining loop originating and ending on the cytoplasmic side of the channel, with the ascending limb in an alpha-helical structure and the descending limb in an extended structure. A functionally critical asparagine (N-site) is positioned at the tip of the loop, and a cluster of hydrophilic residues of the descending limb, adjacent to the tip, forms the narrow constriction of the channel. An apparent asymmetric positioning of the NR1- and NRP-subunit N-site asparagines may account for their unequal role in Ca2+ permeability and Mg2+ block.