A common binding site for substrates and protons in EmrE, an ion-coupled multidrug transporter

EmrE is an Escherichia coli 12-kDa multidrug transporter, which confers res istance to a variety of toxic cations by removing them from the cell interi or in exchange with two protons. EmrE has only one membrane-embedded charge d residue, Glu-14, that is conserved in more than 50 homologous proteins an d it is a simple model system to study the role of carboxylic residues in i on-coupled transporters. We have used mutagenesis and chemical modification to show that Glu-14 is part of the substrate binding site. Its role in pro ton binding and translocation was shown by a study of the effect of pH on l igand binding, uptake, efflux and exchange reactions, We conclude that Glu- 14 is an essential part of a binding site, common to substrates and protons . The occupancy of this site is mutually exclusive and provides the basis o f the simplest coupling of two fluxes. (C) 2000 Federation of European Bioc hemical Societies. Published by Elsevier Science B.V. All rights reserved.