Role of nonconserved charged residues of the AE2 transmembrane domain in regulation of anion exchange by pH

The ubiquitous AE2/SLC4A2 anion exchanger is acutely and independently regulated by intracellular (pH(i)) and extracellular pH (pH(o)), whereas the closely related AE1/SLC4A1 of the red cell and renal intercalated cell is relatively pH-insensitive. We have investigated the contribution of nonconserved charged residues within the C-terminal transmembrane domain (TMD) of AE2 to regulation by pH through mutation to the corresponding AE1 residues. AE2-mediated Cl-/Cl- exchange was measured as 4,4'-di-isothiocyanatostilbene-2,2'-disulfonic acid-sensitive Cl-36(-) efflux from Xenopus oocytes by varying pH(i) at constant pH(o), and by varying pH(o) at near-constant pH(i). All mutations of nonconserved charged residues of the AE2 TMD yielded functional protein, but mutations of some conserved charged residues (R789E, R1056A, R1134C) reduced or abolished function. Individual mutation of AE2 TMD residues R921, F922, P1077, and R1107 exhibited reduced pH(i) sensitivity compared to wt AE2, whereas TMD mutants K1153R, R1155K, R1202L displayed enhanced sensitivity to acidic pH(i). In addition, pH(o) sensitivity was significantly acid- shifted when nonconserved AE2 TMD residues E981, K982, and D1075 were individually converted to the corresponding AE1 residues. These results demonstrate that multiple conserved charged residues are important for basal transport function of AE2 and that certain nonconserved charged residues of the AE2 TMD are essential for wild-type regulation of anion exchange by pH(i) and pH(o).