Ca2+ channel selectivity at a single locus for high-affinity Ca2+ interactions

Ca2+ channels display remarkable selectivity and permeability, traditionally attributed to multiple, discrete Ca2+ binding sites lining the pore. Each of the four pore-forming segments of Ca2+ channel alpha 1 subunits contains a glutamate residue that contributes to high-affinity Ca2+ interactions. Replacement of all four P-region glutamates with glutamine or alanine abolished micromolar Ca2+ block of monovalent current without revealing any additional independent high-affinity Ca2+ binding site. Pairwise replacements of the four glutamates excluded the hypothesis that they form two independent high-affinity sites. Systematic alterations of side-chain length, charge, and polarity by glutamate replacement with aspartate, glutamine, or alanine weakened the Ca2+ interaction, with considerable asymmetry from one repeat to another. The P-region glutamate in repeat I was unusual in its sensitivity to aspartate replacement but not glutamine substitution. While all four glutamates cooperate in supporting high-affinity interactions with single Ca2+ ions, they also influence the interaction between multiple divalent cations.