Limited Proteolysis of IIIGlc, a Regulatory Protein of the Phosphoenolpyruvate:glycose Phosphotransferase System, by Membrane-associated Enzymes from Salmonella Typhimurium and Escherichia Coli

In the present studies we report that membrane-associated proteases in Salmonella typhimurium and Escherichia coli catalyze limited proteolysis of IIIGlcSlow. We have previously reported (Meadow, N. D., and Roseman, S. (1982) J. Biol. Chem. 257, 14526-14537) the isolation of two electrophoretically distinguishable forms of IIIGlc, which is a phosphocarrier and regulatory protein of the phosphoenolpyruvate:glycose phosphotransferase system. The two species of IIIGlc were designated IIIGlcFast and IIIGlcSlow; IIIGlcSlow is 7 amino acid residues longer than IIIGlcFast at its NH2 terminus. The majority of the protease activity is located in the outer membrane fraction from both species of bacteria, with the cytoplasmic fraction being devoid of activity. The site of cleavage is at the Lys-Ser bond located at residues 7-8 of IIIGlcSlow. The enzyme is an endopeptidase which liberates the expected heptapeptide (Gly-Leu-Phe-Asp-Lys-Leu-Lys). Both the large fragment of the limited proteolytic reaction, IIIGlcFast, and the small fragment, the heptapeptide, are stable to further proteolysis by membranes for more than 17 h at 37 degrees C. The activity in E. coli membranes has an absolute requirement for divalent metal ion (Mg2+ or Ca2+) and is heat-resistant, whereas the activity in S. typhimurium membranes is stimulated by divalent metal ion and is heat-sensitive. These results suggest significant differences between the two enzymes. The physiological function of the limited proteolysis of IIIGlc is not known.