title: The inhibitory Ca2+-regulation of the actin-activated Mg-ATPase activity of myosin from Physarum polycephalum plasmodia.
abstract: Myosin was rapidly prepared from the slime mould, Physarum polycephalum to a high level of homogeneity (greater than 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 Ca2+ concentration was increased to 50 microM, i.e., the actin-myosin-ATP interactions show Ca-inhibition. The Ca2+ concentration giving half-maximum inhibition was 1-3 microM. 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 2 microM 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.
mesh_terms: D000199:Actins / Q000502:physiology*; D000205:Actomyosin / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D017301:Ca(2+) Mg(2+)-ATPase / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology / Q000502:physiology*; D004789:Enzyme Activation; D006868:Hydrolysis; D008274:Magnesium / Q000494:pharmacology; D009218:Myosins / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D010766:Phosphorylation; D010804:Physarum / Q000201:enzymology*; D011485:Protein Binding
chemical_list: D000199:Actins; D000255:Adenosine Triphosphate; D000205:Actomyosin; D017301:Ca(2+) Mg(2+)-ATPase; D009218:Myosins; D008274:Magnesium; D002118:Calcium