The erythrocyte anion exchanger AE1 (band 3) serves as an important model for the study of the mechanism of ion transport. Chemical modification of human erythrocyte AE1 has previously suggested that glutamic acid residue 681 lies within the transport pathway and can cross the permeability barrier. This glutamate is conserved in all anion exchangers sequenced to date. We examined the effect on divalent (sulfate) and monovalent (chloride and bicarbonate) anion transport of mutating the corresponding glutamates in mouse AE1 and the closely related anion exchanger, AE2. Substitution of this conserved glutamate with uncharged or basic amino acids had a negligible effect on the maximal rate of sulfate-sulfate exchange in AE-reconstituted proteoliposomes, but largely abolished the steep pH dependence of sulfate transport observed in wild-type AE1 and AE2. In contrast, exchange of monovalent anions was undetectable in cells expressing these mutants. Replacement of the conserved glutamate with aspartate abolished both monovalent and divalent anion transport. These data suggest that the conserved glutamate residue plays a dual role in determining anion selectivity and in proton coupling to sulfate transport. A model explaining the role of the conserved glutamate in promoting ion selectivity and pH regulation is discussed.