Ca2+-Sensitivity of Actomyosin ATPase Purified from Physarum polycephalum1

Abstract

A contractile protein closely resembling natural actomyosin (myosin B) of rabbit skeletal muscle was extracted from plasmodia of the slime mold, Physarum polycephalum, by protecting the SH-groups with β-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 Ca²⁺ 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 activity 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 complex 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.