Neuron 1996 Aug 01;172:343-52. doi: 10.1016/s0896-6273(00)80165-8.

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

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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 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 NR2-subunit N-site asparagines may account for their unequal role in Ca2+ permeability and Mg2+ block.

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Species referenced: Xenopus laevis
Genes referenced: grin2c nodal1 nodal2