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J. Biochem, 1986, Vol. 99, No. 5 1433-1446
© 1986 Japanese Biochemical Society

research-article

The Inhibitory Ca2+-Regulation of the Actin-Activated Mg-ATPase Activity of Myosin from Physarum polycephalum Plasmodia

Kazuhiro KOHAMA1 and John KENDRICK-JONES

Medical Research Council, Laboratory of Molecular Biology Hills Road, Cambridge CB2 2QH, England

1Present address: Department of Pharmacology, Faculty of Medicine, The University of Tokyo, Hongo, Bunkyo ku, Tokyo 113. To whom correspondence should be addressed

Myosin was rapidly prepared from the slime mould, Physarum polycephalum to a high level of homogeneity (>95%), in a high yield (about 10 mg/100 g tissue) and in a phosphorylated state (about 5 mol phosphate/mol of 500,000 Mr myosin).

Actin activated the Mg-ATPase activity of this myosin in the absence of Ca2+ about 30-fold, and this actin-activated ATPase activity was reduced to about 20% of the original activity when the Ca2+ concentration was increased to 50 µM i.e., the actin-myosin-ATP interactions show Ca-inhibition. The Ca2+ concentration giving half-maximum inhibition was 1–3 µM The Ca-inhibition was clearly observed at physiological concentrations of Mg2+ but was obscured at both lower and higher concentrations of Mg2+.

The Ca-inhibitory effect on ATP hydrolysis by actomyosin reconstituted from skeletal actin and Physarum myosin was quick and reversible. Ca-binding measurement showed that myosin bound Ca2+ with half-maximal binding at 2µM Ca2+ and maximum binding of 2 mol per mol myosin, indicating that Ca2+ may inhibit the ATPase activity by binding to myosin.

The involvement of this myosin-linked regulatory system in the Ca2+-control of cytoplasmic streaming is discussed.


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