The structure of the NMDA receptor channel M2 segment was investigated by probing the extracellular and cytoplasmic faces of cysteine-substituted NR1-NR2C 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 α-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 NR2-subunit N- site asparagines may account for their unequal role in Ca2+ permeability and Mg2+ block.
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