A CONSERVED GLUTAMATE IS RESPONSIBLE FOR ION SELECTIVITY AND PH-DEPENDENCE OF THE MAMMALIAN ANION-EXCHANGERS AE1 AND AE2

The erythrocyte anion exchanger AE1 (band 3) serves as an important mo del for the study of the mechanism of ion transport. Chemical modifica tion of human erythrocyte AE1 has previously suggested that glutamic a cid residue 681 lies within the transport pathway and can cross the pe rmeability barrier. This glutamate is conserved in all anion exchanger s 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 ex changer, AE2. Substitution of this conserved glutamate with uncharged or basic amino acids had a negligible effect on the maximal rate of su lfate-sulfate exchange in AE-reconstituted proteoliposomes, but largel y abolished the steep pH dependence of sulfate transport observed in w ild-type AE1 and AE2. In contrast, exchange of monovalent anions was u ndetectable in cells expressing these mutants. Replacement of the cons erved glutamate with aspartate abolished both monovalent and divalent anion transport. These data suggest that the conserved glutamate resid ue plays a dual role in determining anion selectivity and in proton co upling to sulfate transport. A model explaining the role of the conser ved glutamate in promoting ion selectivity and pH regulation is discus sed.