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Physarum actin is phosphorylated as the actin‐fragmin complex at residues Thr203 and Thr202 by a specific 80 kDa kinase.

The Physarum EGTA‐resistant actin‐fragmin complex, previously named cap 42(a+b), is phosphorylated in the actin subunit by an endogenous kinase [Maruta and Isenberg (1983) J. Biol. Chem., 258, 10151–10158]. This kinase has been purified and characterized. It is an 80 kDa monomeric enzyme, unaffected by known kinase regulators. Staurosporine acts as a potent inhibitor. The actin‐fragmin complex is the preferred substrate. The phosphorylation is inhibited by micromolar Ca2+ concentrations, but only in the presence of additional actin. Polymerized actin (vertebrate muscle and non‐muscle isoforms) and actin complexes with various actin‐binding proteins are poorly phosphorylated. The heterotrimer consisting of two actins and one fragmin, which is formed from cap 42(a+b) and actin in the presence of micromolar concentrations of Ca2+, is also a poor substrate. From the other substrates tested, only histones were significantly phosphorylated, in particular histone H1. In the same manner, casein kinase I could also phosphorylate the actin‐fragmin complex. The major phosphorylation site in actin is Thr203. A second minor site is Thr202. These residues constitute one of the contact sites for DNase I [Kabsch et al. (1990) Nature, 347, 37–44] and are also part of one of the predicted actin‐actin contact sites in the F‐actin model [Holmes et al. (1990) Nature, 347, 44–49].