Kinetic characterization and regulation of phosphoenolpyruvate-dependent methyl alpha-D-glucopyranoside transport by Salmonella typhimurium membrane vesicles.

Membrane vesicles from Salmonella typhimurium SB3507 were used to study the kinetics of methyl alpha-D-glucopyranoside (MeGlc) transport by the phosphoenolpyruvate: glycose phosphotransferase system (PTS). During the first minute of phosphoenolpyruvate-dependent MeGlc transport, two distinct rates were observed; an initial rapid rate, V1 (Vmax, 7.4-8.4 nmol X mg-1 X min-1; Km, 8.2-11.2 X 10(-6)M), followed by a second slower rate, V2 (Vmax, 4-4.6 nmol X mg-1 X min-1; Km, 3.4-6.4 X 10(-6) M). The change in rate occurred when the intravesicular MeGlc phosphate concentration was 0.2 mM or less, depending on the external MeGlc concentration. The rate-limiting component in MeGlc transport was found to be enzyme II-BGlc, not phosphoenolpyruvate uptake or the PTS proteins enzyme I, HPr, and IIIGlc. The change from V1 to V2 thus suggests that the PTS is regulated in intact vesicles. However, this regulation was completely relieved by permeabilizing the vesicles with toluene. That is, the toluene-treated vesicles showed only V1 for MeGlc phosphorylation. Evidence was obtained to show that pyruvate and its metabolic products generated by the vesicles exerted no effect on the rate of MeGlc transport. Furthermore, the result from a dual-label experiment excluded exchange transphosphorylation as the mechanism for regulating MeGlc transport by the vesicles. Possible mechanisms for regulation of the PTS are discussed.