Abstract
A contractile protein closely resembling natural actomyosin (myosin B) of rabbit skeletal muscle was extracted from plasmodia of the slime mold, Physaruln poly-cephalum, by protecting the SH-groups with 8-mercaptoethanol or dithiothreitol. Superprecipitation of the protein induced by Mg2+-ATP at low ionic strength was observed only in the presence of very low concentrations of free Call ions, and the Mg2+-ATPase [EC 3. 6. 1. 3] reaction was activated 2- to 6-fold by 1 μM of free Ca2+ ions.
Crude myosin and actin fractions were separated by centrifuging plasmodium myosin B in the presence of Mg2+-PP1 at high ionic strength. The crude myosin showed both EDTA- and Ca2+-activated ATPase activities. The Mg2+-ATPase activ-ity of crude myosin from plasmodia was markedly activated by the addition of pure F-actin from rabbit skeletal muscle, Addition of the F-actin-regulatory protein com-plex prepared from rabbit skeletal muscle as well as the actin fraction of plasmodium caused the same degree of activation as the addition of pure F-actin only in the presence of very low concentrations of Ca2+ ions, and caused only slight activation in the absence of free Ca2+ ions.
