Neuron
Volume 16, Issue 4, April 1996, Pages 869-880
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Article
Identification of a High Affinity Divalent Cation Binding Site near the Entrance of the NMDA Receptor Channel

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Abstract

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 (∼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 ∼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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