Energy-Coupling of the Transport System of Escherichia coli Dependent on Maltose-Binding Protein

1 Accumulation of substrate via the maltose transport system of Escherichia coli is incompletely inhibited by uncouplers of oxidative phosphorylation. The extent of this inhibition is reduced upon introduction of a mutation in the Mg2+ Ca2+-dependent ATPase. This suggests the lack of importance of the proton motive force for accumulation of substrate. 2 Starved cells of the ATPase mutant show a reduced rate of maltose uptake. This low rate was strongly stimulated by the glycolytic carbon source D-fructose but not by oxidisable energy sources such as d-lactate or ascorbate plus phenazine methosulphate. These findings are also in agreement with the lack of importance of the proton motive force in maltose accumulation. 3 Incubation of wild-type as well as ATPase-less cells with high concentrations of arsenate, that is known to deplete the intracellular level of ATP in E. coli, did not significantly inhibit the uptake of maltose. This suggests that the level of intracellular ATP is not directly the energy source for maltose uptake. 4 The exit of maltose, in both the wild-type and the ATPase-less strains, is inhibited by uncouplers of oxidative phosphorylation. Azide, dinitrophenol, and carbonyl cyanide chlorophenyl-hydrazone inhibited the downhill exit of maltose and also the exchange of maltose; arsenate did not. This suggests a requirement for the proton motive force in transport of maltose in the outward direction, and contrasts from the properties of the entry reaction. These results are consistent with the view that entry and exit of maltose are mediated via different carriers and energized by different modes.