Characterization of soluble enzyme II complexes of the Escherichia coli phosphotransferase system.

Plasmid-encoded His-tagged glucose permease of Escherichia coli, the enzyme IIBCGlc (IIGlc), exists in two physical forms, a membrane-integrated oligomeric form and a soluble monomeric form, which separate from each other on a gel filtration column (peaks 1 and 2, respectively). Western blot analyses using anti-His tag monoclonal antibodies revealed that although IIGlc from the two fractions migrated similarly in sodium dodecyl sulfate gels, the two fractions migrated differently on native gels both before and after Triton X-100 treatment. Peak 1 IIGlc migrated much more slowly than peak 2 IIGlc. Both preparations exhibited both phosphoenolpyruvate-dependent sugar phosphorylation activity and sugar phosphate-dependent sugar transphosphorylation activity. The kinetics of the transphosphorylation reaction catalyzed by the two IIGlc fractions were different: peak 1 activity was subject to substrate inhibition, while peak 2 activity was not. Moreover, the pH optima for the phosphoenolpyruvate-dependent activities differed for the two fractions. The results provide direct evidence that the two forms of IIGlc differ with respect to their physical states and their catalytic activities. These general conclusions appear to be applicable to the His-tagged mannose permease of E. coli. Thus, both phosphoenolpyruvate-dependent phosphotransferase system enzymes exist in soluble and membrane-integrated forms that exhibit dissimilar physical and kinetic properties.