Lack of Charge Interaction in the Ion Binding Site Determines Anion Selectivity in the Sodium Bicarbonate...

The Na/HCO₃ cotransporter NBCe1 is a member of SLC4A transporters that move HCO₃^- across cell membranes and regulate intracellular pH or transepithelial HCO₃ transport. NBCe1 is highly selective to HCO₃^- and does not transport other anions; the molecular mechanism of anion selectivity is presently unclear. We previously reported that replacing Asp⁵⁵⁵ with a Glu (D555E) in NBCe1 induces increased selectivity to other anions, including Cl^-. This finding is unexpected because all SLC4A transporters contain either Asp or Glu at the corresponding position and maintain a high selectivity to HCO₃^-. In this study, we tested whether the Cl^- transport in D555E is mediated by an interaction between residues in the ion binding site. Human NBCe1 and mutant transporters were expressed in Xenopus oocytes, and their ability to transport Cl^- was assessed by two-electrode voltage clamp. The results show that the Cl^- transport is induced by a charge interaction between Glu⁵⁵⁵ and Lys⁵⁵⁸. The bond length between the two residues is within the distance for a salt bridge, and the ionic strength experiments confirm an interaction. This finding indicates that the HCO₃^- selectivity in NBCe1 is established by avoiding a specific charge interaction in the ion binding site, rather than maintaining such an interaction.