Regulation of AE2-mediated Cl(−) Transport by Intracellular or by Extracellular pH Requires Highly Conserved Amino Acid Residues of the AE2 NH(2)-terminal Cytoplasmic Domain

We reported recently that regulation by intracellular pH (pH(i)) of the murine Cl(−)/HCO(3) (−) exchanger AE2 requires amino acid residues 310–347 of the polypeptide's NH(2)-terminal cytoplasmic domain. We have now identified individual amino acid residues within this region whose integrity is required for regulation of AE2 by pH. (36)Cl(−) efflux from AE2-expressing Xenopus oocytes was monitored during variation of extracellular pH (pH(o)) with unclamped or clamped pH(i), or during variation of pH(i) at constant pH(o). Wild-type AE2–mediated (36)Cl(−) efflux was profoundly inhibited by acid pH(o), with a value of pH(o(50)) = 6.87 ± 0.05, and was stimulated up to 10-fold by the intracellular alkalinization produced by bath removal of the preequilibrated weak acid, butyrate. Systematic hexa-alanine [(A)(6)]bloc substitutions between aa 312–347 identified the greatest acid shift in pH(o(50)) value, ∼0.8 pH units in the mutant (A)(6)342–347, but only a modest acid-shift in the mutant (A)(6)336–341. Two of the six (A)(6) mutants retained normal pH(i) sensitivity of (36)Cl(−) efflux, whereas the (A)(6) mutants 318–323, 336–341, and 342–347 were not stimulated by intracellular alkalinization. We further evaluated the highly conserved region between aa 336–347 by alanine scan and other mutagenesis of single residues. Significant changes in AE2 sensitivity to pH(o) and to pH(i) were found independently and in concert. The E346A mutation acid-shifted the pH(o(50)) value to the same extent whether pH(i) was unclamped or held constant during variation of pH(o). Alanine substitution of the corresponding glutamate residues in the cytoplasmic domains of related AE anion exchanger polypeptides confirmed the general importance of these residues in regulation of anion exchange by pH. Conserved, individual amino acid residues of the AE2 cytoplasmic domain contribute to independent regulation of anion exchange activity by pH(o) as well as pH(i).