INTERACTIONS BETWEEN 2 DIVALENT ION-BINDING SITES IN N-METHYL-D-ASPARTATE RECEPTOR CHANNELS

N-methyl-D-aspartate receptor channels exhibit a high permeability for calcium ions, In this report, we confirm that calcium ions permeate e ffectively through the wild-type channels, and find that their presenc e within the pore blocks the flux of sodium and other ions, Further pr oof for this ionic block comes from the analysis of the epsilon 1(N614 Q) mutation where the high permeability of calcium is unchanged but th e block by calcium ions is increased twofold. In both the wild-type an d mutant channels, calcium ion block is independent of membrane voltag e; therefore, the calcium binding site is outside the voltage gradient through the pore and must be close to the extracellular mouth of the ion conductance pathway, This calcium site is distinct from the magnes ium binding site, which lies 80% into the pore's electrostatic field a nd thus exhibits a marked voltage dependence of binding, The epsilon 1 (N614Q) mutation reduces the affinity of magnesium ion for its binding site but increases the affinity of calcium ion for its binding site, Since a single mutation perturbs two distinct binding sites in opposit e ways, we postulate that binding of divalent ions at the two sites in teract.