Negative dominance studies demonstrate the oligomeric structure of EmrE, a multidrug antiporter from Escherichia coli

EmrE, the smallest known ion-coupled transporter, is an Escherichia coli 12-kDa protein 80% helical and soluble in organic solvents. EmrE is a polyspecific antiporter that exchanges hydrogen ions with aromatic toxic cations such as methyl viologen. Since it is many times smaller than the classical consensus 12 transmembrane segments transporters, it was particularly interesting to determine its oligomeric state. For this purpose, a series of nonfunctional mutants has been generated and characterized to test their effect on the activity of the wild-type protein upon mixing. As opposed to the wild type, these mutants do not confer resistance to methyl viologen, ethidium bromide, or a series of other toxicants. Co-expression of each of the nonfunctional mutants with the wild-type protein results in a reduction in the ability of the functional transporter to confer resistance to several toxicants. To perform mixing experiments in vitro, all the mutants have been purified by extraction with organic solvents, reconstituted in proteoliposomes, and found to be inactive. When co-reconstituted with wild- type protein, they inhibit the activity of the latter in a dose-dependent form up to full inhibition. We assume that this inhibition is due to the formation of mixed oligomers in which the presence of one nonfunctional subunit causes full inactivation. A binomial analysis of the results based on the latter assumptions do not provide statistically significant answers but suggests that the oligomer is composed of three subunits. The results described provide the first in vitro demonstration of the functional oligomeric structure of an ion-coupled transporter.