Molecular determinants of Ca2+ selectivity and ion permeation in L-type Ca2+ channels

Voltage-gated Ca2+ channels link changes in membrane potential to the delivery of Ca2+, a key second messenger for many cellular responses. Ca2+ channels show selectivity for Ca2+ over more plentiful ions such as Na+ or K+ by virtue of their high-affinity binding of Ca2+ within the pore. It has been suggested that this binding involves four conserved glutamate residues in equivalent positions in the putative pore-lining regions of repeats I-IV in the Ca2+ channel a1 subunit. We have carried out a systematic series of single amino-acid substitutions in each of these positions and find that all four glutamates participate in high-affinity binding of Ca2+ or Cd2+. Each glutamate carboxylate makes a distinct contribution to ion binding, with the carboxylate in repeat III having the strongest effect. Some single glutamate-to-lysine mutations completely abolish micromolar Ca2+ block, indicating that the pore does not possess any high-affinity binding site that acts independently of the four glutamate residues. The prevailing model of Ca2+ permeation must thus be modified to allow binding of two Ca2+ ions in close proximity, within the sphere of influence of the four glutamates. The functional inequality of the glutamates may be advantageous in allowing simultaneous interactions with multiple Ca2+ ions moving single-file within the pore. Competition among Ca2+ ions for individual glutamates, together with repulsive ion-ion electrostatic interaction, may help achieve rapid flux rates through the channel.