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Actin-myosin-ATP interaction is supposed to produce the motive force for cytoplasmic streaming in plasmodia of Physarum polycephalum.' We found that the actin-myosin-ATP interaction as measured by superprecipitation of myosin B (natural actomyosin) was inhibited by Ca2+.2) As half-maximal inhibition occurred at ,uM levels of Ca2+, we proposed that Ca2+ controls cytoplasmic streaming by inhibiting the actin-myosin-ATP interaction,2) which is opposite to the role of Ca2+ in the regulation of the actin-myosin-ATP interaction in muscle. Recently, this result has been confirmed by Ogihara et al.3) In our previous studies, the actin-myosin-ATP interaction using actin and myosin purified from the Ca'-sensitive myosin B was apparently insensitive to Ca2+.2) '4) Later, however, we found that Ca2+ at pM levels exerted an inhibitory effect on the skeletal muscle actinactivated ATPase activity of plasmodial myosin5)'6) which was prepared by essentially the same procedures described in the previous study.4) We also noticed that actin-myosin-ATP interaction was influenced by Mgt+.4)-6) Recently, we estimated the intracellular pH and the concentrations of ATP and Mg2+ from studies using 31P-nuclear magnetic resonance of living Physarum plasmodia. 7 > Under these physiological conditions indicated by this study, we re-examined the effect of Ca2+ on the skeletal muscle actin-activated ATPase activity of myosin, paying attention to the question whether myosin or actin is responsible for the effect. Materials and method. Plasmodia of Physarum polycephalum (strain Ng-1 supplied by Prof. S. Hatano, Nagoya University) were grown in the dark on rolled oats.g) The fresh migrating sheets of plasmodia were collected and immediately subjected to myosin B

Myosin confers inhibitory Ca2+-sensitivity on actin-myosin-ATP interaction of Physarum polycephalum plasmodium under physiological conditions.