E367 is essential for Cl⁻/HCO₃⁻ exchange by Slc26a3. (A and B) Individual traces and (C) the mean ± SEM of Slc26a3-mediated Cl⁻/HCO₃⁻ exchange measured in oocytes expressing Slc26a3, Slc26a3(E367A), and Slc26a3(E367K). Note that the E367K mutation eliminates the exchange, whereas the E367A mutation markedly inhibited the exchange while changing the stoichiometry of the residual Cl⁻ and HCO₃⁻ fluxes from 2:1 to 1:1.