Protein phosphatases and protein kinases in dictyostelium

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Virginia Polytechnic Institute and State University


In the present work, the chromatographic behavior and partial characterization of 5 protein phosphatases, 6 paranitrophenyl phosphate (pnpp) phosphatases and 2 protein kinases from Dictyostelium are described. .Two of the protein phosphatases are shown to have subunit structures. All of the protein phosphatase activities described were able to dephosphorylate sites on proteins that had previously been phosphorylated by the cAMP-dependent protein kinase. Therefore it may be that these phosphatases function in vivo to antagonize the action of the cAMP-dependent protein kinase. Various pnpp phosphatase activities, varying in size, cation requirements and pH optima are described. Since many pnpp phosphatases also function with protein substrates it is possible that these activities represent protein phosphatases that function with phosphoproteins that have not been tested.

Evidence for the existence of protein kinase C in Dictyostelium is presented. A histone protein kinase was found in Dictyostelium extracts that eluted from DE52 cellulose at the same salt concentration that rabbit brain protein kinase C did. In addition in one experiment clear dependency on calcium and phospholipids is shown.

The purification to homogeneity, and characterization of a low molecular weight autophosphorylating protein kinase, pp20, is described. This protein kinase is a major phosphorylated protein and in addition represents at least 0.03% of the total cellular protein. The autophosphorylation reaction can be inhibited by EDTA but not by EGTA. Following inhibition by EDTA, autophosphorylation can be reactivated by the addition of Mn²⁺, Mg²⁺, or Ca²⁺. Western blotting evidence is presented that shows cross reactivity of pp20 and an 85 KDa protein that may possess casein kinase activity.