ACTIN KINASE - A PROTEIN-KINASE THAT PHOSPHORYLATES ACTIN OF FRAGMIN-ACTIN COMPLEX

Actin of fragmin-actin complex is phosphorylated by an endogenous kinase from plasmodium of Physarum polycephalum. The phosphorylation abolishes the nucleation and capping activities of fragmin-actin complex. The kinase has been purified and termed actin kinase [Furuhashi, K. & Hatano, S. (1990) J. Cell Biol. 111, 1081-1087]. Enzymatic properties of the purified actin kinase were studied in detail. Actin kinase exhibited the highest activity under conditions physiological for the plasmodium (30 mM KCl, 6 mM MgCl2, pH 7.0). The V(max) and the K(m) of the enzyme for ATP were about 83-mu-mol/min/mg and 25-mu-M, respectively. The K(m) for fragmin-actin complex was 190 nM. The purified actin kinase phosphorylated actin of fragmin-actin complex at a-constant rate regardless of Ca2+ concentration. Similarly, 2-mu-M cAMP, 2-mu-M cGMP, 2-mu-g/ml calmodulin in the presence of Ca2+ or 1 mM GTP showed no effect on the activity of the purified enzyme. Actin kinase did not phosphorylate histone H1, H2B, alpha-casein, or beta-casein, suggesting that actin kinase is a new kind of protein kinase which specifically phosphorylates actin of the fragmin-actin complex.