New Insights into the Mechanism of NO Selectivity in the Human Kidney Chloride Channel ClC-Ka and the CLC Protein Family.

Background: The mechanism of anion selectivity in the human kidney chloride channels ClC-Ka and ClC-Kb is unknown. However, it has been thought to be very similar to that of other channels and antiporters of the CLC protein family, and to rely on anions interacting with a conserved Ser residue (Ser) at the center of three anion binding sites in the permeation pathway S. In both CLC channels and antiporters, mutations of Ser alter the anion selectivity. Structurally, the side chain of Ser of CLC channels and antiporters typically projects into the pore and coordinates the anion bound at S.

Methods: To investigate the role of several residues in anion selectivity of ClC-Ka, we created mutations that resulted in amino acid substitutions in these residues. We also used electrophysiologic techniques to assess the properties of the mutants.

Results: Mutations in ClC-Ka that change Ser to Gly, Pro, or Thr have only minor effects on anion selectivity, whereas the mutations in residues Y425A, F519A, and Y520A increase the NO/Cl permeability ratio, with Y425A having a particularly strong effect.

Conclusion: s ClC-Ka's mechanism of anion selectivity is largely independent of Ser, and it is therefore unique in the CLC protein family. We identified the residue Y425 in ClC-Ka-and the corresponding residue (A417) in the chloride channel ClC-0-as residues that contribute to NO discrimination in these channels. This work provides important and timely insight into the relationship between structure and function for the kidney chloride channels ClC-Ka and ClC-Kb, and for CLC proteins in general.