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].
Gettemans
J
J
Laboratory for Physiological Chemistry, University Gent, Belgium.
De Ville
Y
Y
Vandekerckhove
J
J
Waelkens
E
E
D008841:Actin Cytoskeleton / Q000378:metabolism; D000199:Actins / Q000378:metabolism*; D000470:Alkaloids / Q000494:pharmacology; D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D006495:Heparin, Low-Molecular-Weight / Q000378:metabolism*; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010766:Phosphorylation; D016839:Physarum polycephalum / Q000201:enzymology*; D011494:Protein Kinases / Q000032:analysis / Q000302:isolation & purification / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases; D011817:Rabbits; D019311:Staurosporine; D013379:Substrate Specificity
D000199:Actins; D000470:Alkaloids; D006495:Heparin, Low-Molecular-Weight; D011494:Protein Kinases; C074377:actin kinase; D017346:Protein Serine-Threonine Kinases; D019311:Staurosporine; D002118:Calcium