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Evidence for a Second Binding/Transport Site for Chloride in Erythrocyte Anion Transporter AE1 Modified at Glutamate 681

Department of Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, Arkansas

Correspondence: Address reprint requests to Michael L. Jennings, Dept. of Physiology and Biophysics, University of Arkansas for Medical Sciences, 4301 W. Markham St., Mail Slot 505, Little Rock, AR 72205. Tel.: 501-296-1438; Fax: 501-686-8167; E-mail: JenningsMichaelL{at}uams.edu.

Transport kinetics have been examined in erythrocyte anion transporter AE1 that has been chemically modified to convert glutamate 681 to an alcohol (E681OH AE1). Outward conductive Cl^– flux in E681OH AE1 is inhibited by removal of extracellular Cl^–; this effect is the opposite of that in native AE1 and is consistent with coupled electrogenic 2:1 Cl^–/Cl^– exchange. A second Cl^– binding/transport site is also suggested by the characteristics of flux in E681OH AE1: bilateral and cis Cl^–, which are normally inhibitory, accelerate flux. These effects would be expected if Cl^– binds to a second transport site on -loaded E681OH AE1, thereby allowing cotransport. Alternatively, the data can be explained without proposing cotransport if the rate-limiting event for exchange is external release, and the binding of external Cl^– accelerates release. With either interpretation, these data indicate that E681OH AE1 has a binding/transport site for Cl^– that is distinct from the main transport site. The effects of graded modification of E681 or inhibition by H₂DIDS are consistent with the idea that the new Cl^– binding site is on the same E681OH-modified subunit of the AE1 dimer as the normal transport site.

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