Protein:Protein Interactions in the Cytoplasmic Membrane Influencing Sugar Transport and Phosphorylation Activities of the E. coli Phosphotransferase System.

The multicomponent phosphoenolpyruvate-dependent sugar-transporting phosphotransferase system (PTS) in Escherichia coli takes up sugar substrates and concomitantly phosphorylates them. We have recently provided evidence that many of the integral membrane PTS permeases interact with the fructose PTS (FruA/FruB) [1]. However, the biochemical and physiological significance of this finding was not known. We have carried out molecular genetic/biochemical/physiological studies that show that interactions of the fructose PTS often enhance, but sometimes inhibit the activities of other PTS transporters many fold, depending on the target PTS system under study. Thus, the glucose, mannose, mannitol and N- acetylglucosamine permeases exhibit enhanced in vivo sugar transport and sometimes in vitro PEP-dependent sugar phosphorylation activities while the galactitol and trehalose systems show inhibited activities. This is observed when the fructose system is induced to high levels and prevented when the fruA/fruB genes are deleted. Overexpression of the fruA and/or fruB genes in the absence of fructose induction during growth also enhances the rates of uptake of other hexoses. The {beta}-galactosidase activities of man, mtl, and gat-lacZ transcriptional fusions and the sugar-specific transphosphorylation activities of these enzyme transporters were not affected either by frustose induction or fruAB overexpression, showing that the rates of synthesis and protein levels in the membrane of the target PTS permeases were not altered. We thus suggest that specific protein-protein interactions within the cytoplasmic membrane regulate transport in vivo (and sometimes the PEP-dependent phosphorylation activities in vitro of PTS permeases) in a physiologically meaningful way that may help to provide a hierarchy of preferred PTS sugars. These observations appear to be applicable in principle to other types of transport systems as well.