Protein phosphorylation in Mycoplasma gallisepticum

Incubation of the soluble fraction derived from Mycoplasma gallisepticum cells with [γ‐ 32 P]ATP results in the phosphorylation of several endogenous proteins. One protein with an apparent molecular mass of 55 kDa was the acceptor of more than 95% of the radioactive phosphate. This protein was also found to be radiolabeled in intact cells grown in the presence of [ 32 P]orthophosphate. Acid hydrolysis of the phosphorylated 55‐kDa protein followed by two‐dimensional electrophoresis revealed that the 32 P‐labelcd material co‐migrated with phosphoserine. The in vitro phosphorylation of the 55‐kDa protein has an optimum pH of 5.5‐6.0 and is not affected by various metabolites of glycolysis, by cAMP or by calmodulin with or without Ca 2+ . The phosphorylation is dependent upon divalent cations, a dependency that is best fulfilled by the simultaneous addition of Ca 2+ and Zn 2+ that act in a specific and cooperative manner. Of a variety of possible exogenous protein acceptors tested, the endogenous protein kinase was capable to phosphorylate only phosvitin. The phosphorylation of the 55‐kDa protein is reversible through the activity of a phosphoprotein phosphatasc present in the soluble fraction of M. gallisepticum . The phosphoprotein phosphatase has an optimum pH of 7.5‐8.0, is inhibited by NaF and stimulated to a large extent by inorganic phosphate and arsenate and to a lesser extent by pyrophosphate ATP and ADP. The possible association of the reversible protein phosphorylation to cell shape and gliding motility of M. gallisepticum are discussed.