Ca2+ Activates Actin-Filament Sliding on Scallop Myosin but Inhibits That on Physarum Myosin1

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Abstract
The actin-activated ATPase activity of Physarum myosin has been shown to be inhibited by μM levels of Ca2+, the mode of which is in contrast to the activating.effect of Ca2+ on scallop myosin (Kohama, K. (1987) Adv. Biophys. 23, 149–182 for a review). To determine if Ca2+ regulates ATP-dependent sliding between actin and the myosins, fluorescent actin-filaments were allowed to move on the myosins fixed to a glass surface. The movement on Physarum and scallop myosins was inhibited and activated, respectively, by Ca2+. For this myosin-linked regulation to occur for Physarum myosin, myosin phosphorylation was shown to be a prerequisite.