Neuron 1996 Apr 01;164:869-80. doi: 10.1016/s0896-6273(00)80107-5.

Identification of a high affinity divalent cation binding site near the entrance of the NMDA receptor channel.

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Single channel currents from recombinant N-methyl-D-aspartate (NMDA) receptors having an N-to-Q mutation in M2 reveal a divalent cation binding site that is near the entrance of the pore (approximately 0.2 through the electric field). Ca2+ rapidly binds to this site and readily permeates the channel, while Mg2+ binds more slowly and does not permeate as readily. In wild-type receptors, Mg2+ also blocks the current by occupying a site that is approximately 0.6 through the field. When the more external site is occupied by Ca2+, the conductance of the pore to NA+ is reduced but not abolished, perhaps by an electrostatic blocking mechanism. The site serves to enrich the fraction of NMDA receptor current carried by CA2+.

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