A protein kinase of the plasma membrane of Dictyostelium discoideum

D. discoideum amoebae were found to phosphorylate plasma membrane proteins when intact cells were incubated with either [γ-³²P]ATP or [³²P]phosphate. In the first case, the incorporation was largely a consequence of the hydrolysis of [γ-³²P]ATP, cellular uptake of the generated [³²P]phosphate and its subsequent incorporation into ATP. When the contribution of this process to the phosphorylating activity of intact cells was eliminated, an ecto-protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37) activity could be demonstrated. As amoebae progressed through their aggregation program, they showed a decreased ability to phosphorylate their plasma membrane when incubated with [γ-³²P]ATP or [³²P]phosphate. Analysis of ATPase activity, permeability properties and the pattern of proteins phosphorylated by intact cells and isolated plasma membranes lead to the following conclusions: the lower levels of phosphorylation observed with starved cells reflected an altered uptake of [³²P]phosphate by these cells rather than a significant change in the plasma membrane protein kinase activity. Neither the substrates nor the activity of the ecto-protein kinase was dramatically altered during starvation.