The mechanism of inducer exclusion. Direct interaction between purified III of the phosphoenolpyruvate:sugar phosphotransferase system and the lactose carrier of Escherichia coli

Abstract

A hypothesis for the regulation of some sugar transport systems by the bacterial phosphoenolpyruvate:sugar transport system postulates an interaction between III of this system and the carrier whose activity is regulated. We have studied this interaction in more detail, employing one of these transport systems, the lactose carrier of Escherichia coli. Purified III of the phosphotransferase system interacted directly with the lactose carrier. The binding of III to lactose carrier required the presence of the non-phosphorylated form of III and substrates of the carrier and exhibited a stoichiometry of 1.2+/- 0.2 mol III/mol lactose carrier. The K(d) of lactose carrier for III was 10 +/- 5 microM. III is apparently unable to interact with a mutant lactose carrier which still binds but does not transport galactosides. The binding of III to the lactose carrier results in a 3.5-fold increase in the apparent affinity of galactosides for the carrier. Significantly, the binding of III to the lactose carrier results in an inhibition of galactoside translocation both in membrane vesicles and liposomes reconstituted with the purified lactose carrier. This inhibition may thus be the basis for the well-documented phenomenon of inducer exclusion.