Identification of residues lining the translocation pore of human AE1, plasma membrane anion exchange protein.

AE1 is the chloride/bicarbonate anion exchanger of the erythrocyte plasma membrane. We have used scanning cysteine mutagenesis and sulfhydryl-specific chemistry to identify pore-lining residues in the Ser643-Ser690 region of the protein. The Ser643-Ser690 region spans transmembrane segment 8 of AE1 and surrounds Glu681, which may reside at the transmembrane permeability barrier. Glu681 also directly interacts with some anions during anion transport. The introduced cysteine mutants were expressed by transient transfection of HEK293 cells. Anion exchange activity was assessed by measurement of changes of intracellular pH, which follow transmembrane bicarbonate movement mediated by AE1. To identify residues that might form part of an aqueous transmembrane pore, we measured anion exchange activity of each introduced cysteine mutant before and after incubation with the sulfhydryl reagents para-chloromercuribenzene sulfonate and 2-(aminoethyl)methanethiosulfonate hydrobromide. Our data identified transmembrane mutants A666C, S667C, L669C, L673C, L677C, and L680C and intracellular mutants I684C and I688C that could be inhibited by sulfhydryl reagents and may therefore form a part of a transmembrane pore. These residues map to one face of a helical wheel plot. The ability to inhibit two intracellular mutants suggests that transmembrane helix 8 extends at least two helical turns beyond the intracellular membrane surface. The identified hydrophobic pore-lining residues (leucine, isoleucine, and alanine) may limit interactions with substrate anions.