Acanthamoeba Myosin

Abstract

An ATPase that accounts for about 0.3% of the total cell protein has been isolated in 90% purity from Acanthamoeba castellanii. The enzyme has been identified as a myosin-like ATPase by the following criteria. Maximal enzymatic activity occurs in the presence of EDTA and 0.5 m KCl, and is only 10 to 20% as high in the presence of Ca²⁺ and less than 1% in the presence of Mg²⁺. The Mg²⁺ ATPase is activated by actin under physiological conditions and the enzyme binds to actin filaments in the absence but not in the presence of ATP. Acanthamoeba myosin shows some activity toward nucleoside triphosphates other than ATP but does not hydrolyze ADP or AMP. In contrast to myosins from other sources, Acanthamoeba myosin is soluble under physiological conditions, and its partition coefficient on gel filtration suggests a native molecular weight of about 180,000, which is smaller than other myosins. Several different experiments show that the isolated molecule is not formed by proteolytic cleavage of a larger native protein. The purified enzyme consists of three polypeptide chains with molecular weights of 140,000, 16,000, and 14,000. The amino composition is remarkable for the absence of cysteine.