Functional comparison of mouse slc26a6 anion exchanger with human SLC26A6 polypeptide variants - Differences in anion selectivity, regulation, and electrogenicity

The unusually low 78% amino acid identity between the orthologous human SLC26A6 and mouse slc26a6 polypeptides prompted systematic comparison of their anion transport functions in Xenopus oocytes. Multiple human SLC26A6 variant polypeptides were also functionally compared. Transport was studied as unidirectional fluxes of Cl-36(-), [C-14] oxalate, and [S-35] sulfate; as net fluxes of HCO3- by fluorescence ratio measurement of intracellular pH; as current by two-electrode voltage clamp; and as net Cl- flux by fluorescence intensity measurement of relative changes in extracellular and intracellular [Cl-]. Four human SLC26A6 polypeptide variants each exhibited rates of bidirectional [C-14] oxalate flux, Cl-/HCO3- exchange, and Cl-/OH- exchange nearly equivalent to those of mouse slc26a6. Cl-/HCO3- exchange by both orthologs was cAMP-sensitive, further enhanced by coexpressed wild type cystic fibrosis transmembrane regulator but inhibited by cystic fibrosis transmembrane regulator DeltaF508. However, the very low rates of Cl-36(-) and [S-35] sulfate transport by all active human SLC26A6 isoforms contrasted with the high rates of the mouse ortholog. Human and mouse orthologs also differed in patterns of acute regulation. Studies of human-mouse chimeras revealed cosegregation of the high Cl-36(-) transport phenotype with the transmembrane domain of mouse slc26a6. Mouse slc26a6 and human SLC26A6 each mediated electroneutral Cl-/HCO3- and Cl-/OH- exchange. In contrast, whereas Cl-/oxalate exchange by mouse slc26a6 was electrogenic, that mediated by human SLC26A6 appeared electroneutral. The increased currents observed in oocytes expressing either mouse or human ortholog were pharmacologically distinct from the accompanying monovalent anion exchange activities. The human SLC26A6 polypeptide variants SLC26A6c and SLC26A6d were inactive as transporters of oxalate, sulfate, and chloride. Thus, the orthologous mouse and human SLC26A6 proteins differ in anion selectivity, transport mechanism, and acute regulation, but both mediate electroneutral Cl-/HCO3- exchange.