Protein phosphorylation in Mycoplasma gallisepticum

Abstract

Incubation of the soluble fraction derived from Mycoplasma gallisepticum cells with [.gamma.-32P]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 [32P]orthophosphate. Acid hydrolysis of the phosphorylated 55-kDa protein followed by two-dimensional electrophoresis revealed that the 32P-labeled 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 Ca2+. The phosphorylation is dependent upon divalent cations, a dependency that is best fulfilled by the simultaneous addition of Ca2+ and Zn2+ 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 phosphatase 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.