PURIFICATION AND CHARACTERIZATION OF A SMALL MEMBRANE-ASSOCIATED SUGAR-PHOSPHATE PHOSPHATASE THAT IS ALLOSTERICALLY ACTIVATED BY HPR(SER(P)) OF THE PHOSPHOTRANSFERASE SYSTEM IN LACTOCOCCUS-LACTIS

In the Gram-positive bacterium, Lactococcus lactis, nonmetabolizable cytoplasmic sugar phosphates, accumulated by the phosphoenolpyluvate:sugar phosphotransferase system, are rapidly dephosphorylated and expelled from the cell upon addition of glucose (inducer expulsion). Our recent studies have established that a metabolite-activated, ATP-dependent protein kinase that phosphorylates serine-46 in HPr of the phosphoenolpyruvate:sugar phosphotransferase system activates a sugar phosphate phosphatase, thus initiating the inducer expulsion process. A membrane associated, HPr(Ser(P))-dependent phosphatase has been identified, solubilized from the membrane, separated from other cellular phosphatases, and purified to near homogeneity. It exhibits a low subunit molecular mass (10 kDa) and behaves on gel filtration columns like a monomeric enzyme. It has broad substrate specificity, optimal activity between pH 7.0 and 8.0, is dependent on a divalent cation for activity, and is not inhibited by fluoride. It is stimulated more than 10-fold by HPr(Ser(P)) or a mutant derivative of HPr, S46D HPr, in which the regulatory serine is changed to aspartate, which bears a permanently negative charge as does phosphate. Stimulation is due both to an increase in the maximal velocity (V-max) and a decrease in the Michaelis-Menten kinetic constant (K-m) for sugar phosphate. The enzyme exhibits a K-a for S46D HPr of 15 mu M. Although the enzyme is thermally stable, activation by HPr(Ser(P)) is heat sensitive.