Alkaline-shifted pHo Sensitivity of AE2c1-mediated Anion Exchange Reveals Novel Regulatory Determinants in the AE2 N-terminal Cytoplasmic Domain

The mouse anion exchanger AE2/SLC4A2 Cl–/HCO–3 exchanger is essential to post-weaning life. AE2 polypeptides regulate pHi, chloride concentration, cell volume, and transepithelial ion transport in many tissues. Although the AE2a isoform has been extensively studied, the function and regulation of the other AE2 N-terminal variant mRNAs of mouse (AE2b1, AE2b2, AE2c1, and AE2c2) have not been examined. We now present an extended analysis of AE2 variant mRNA tissue distribution and function. We show in Xenopus oocytes that all AE2 variant polypeptides except AE2c2 mediated Cl– transport are subject to inhibition by acidic pHi and to activation by hypertonicity and NH+4. However, AE2c1 differs from AE2a, AE2b1, and AE2b2 in its alkaline-shifted pHo(50) (7.70 ± 0.11 versus 6.80 ± 0.05), suggesting the presence of a novel AE2a pH-sensitive regulatory site between amino acids 99 and 198. Initial N-terminal deletion mutagenesis restricted this site to the region between amino acids 120 and 150. Further analysis identified AE2a residues 127–129, 130–134, and 145–149 as jointly responsible for the difference in pHo(50) between AE2c1 and the longer AE2a, AE2b1, and AE2b2 polypeptides. Thus, AE2c1 exhibits a unique pHo sensitivity among the murine AE2 variant polypeptides, in addition to a unique tissue distribution. Physiological coexpression of AE2c1 with other AE2 variant polypeptides in the same cell should extend the range over which changing pHo can regulate AE2 transport activity.