Sugar Transport

The histidine-containing protein (HPr) which serves as a phosphoryl carrier in the phosphoenolpyruvate-glycose phosphotransferase system (PTS) has been purified from Escherichio coli.The protein has a molecular weight of about 9500.The amino acid composition of the protein is unusual in that HPr has no detectable tryptophan, tyrosine, or cysteine and thus exhibits a characteristic phenylalanine spectrum.Moreover, HPr does not contain carbohydrate or phosphorus.One phosphoryl group can be incorporated into HPr, and is attached to histidine; HPr contains 2 histidine residues.Characterization of the phosphohistidine isolated from the phosphoprotein, together with the hydrolysis rates of the phosphorylated HPr indicate that the phosphoryl group is linked to N-l of a histidine imidazole ring.During purification, two additional homogeneous proteins (designated HPr-1 and HPr-2) were obtained; they differed from HPr in electrophoretic mobilities and activities in the PTS.These proteins were identical with HPr in amino acid composition; evidence is presented that HPr-1 and HPr-2 are derived from HPr by loss of amide groups during the isolation procedure.Recent studies in this laboratory (1,2) have shown that a number of sugars are phosphorylated by a bacterial phosphotransferase system which transfers the phosphoryl group from phosphoenolpyruvate to the sugar (1, 3); in general, the aldohexoses and their glycosides are phosphorylated at C-6; phosphorylation of fructose takes place at C-l (4).The reaction can be resolved