A model for coupling of H+ and substrate fluxes based on "time-sharing" ofa common binding site

Both prokaryotic and eukaryotic cells contain an array of membrane transpor t systems maintaining the cellular homeostasis. Some of them (primary pumps ) derive energy from redox reactions, ATP hydrolysis, or light absorption, whereas others (ion-coupled transporters) utilize ion electrochemical gradi ents for active transport. Remarkable progress has been made in understandi ng the molecular mechanism of coupling in some of these systems. In many ca ses carboxylic residues are essential for either binding or coupling. Here we suggest a model for the molecular mechanism of coupling in EmrE, an Esch erichia coli 12-kDa multidrug transporter, EmrE confers resistance to a var iety of toxic cations by removing them from the cell interior in exchange f or two protons. EmrE has only one membrane-embedded charged residue, Glu-14 , which is conserved in more than 50 homologous proteins. We have used muta genesis and chemical modification to show that Glu-14 is part of the substr ate-binding site. Its role in proton binding and translocation was shown by a study of the effect of pH on ligand binding, uptake, efflux, and exchang e reactions. The studies suggest that Glu-14 is an essential part of a bind ing site, which is common to substrates and protons. The occupancy of this site by H+ and substrate is mutually exclusive and provides the basis of th e simplest coupling for two fluxes.