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Purified Acanthamoeba myosin binds reversibly to filaments of muscle actin forming a complex which is dissociated by Mg2+ and ATP. Electron microscopy reveals that the Acanthamoeba myosin binds to actin filaments in a regular way which accentuates the helical repeat of the actin filaments but the myosin does not form the distinct arrowhead-shaped complexes typical of other actin-myosin complexes. Actin filaments treated with Acanthamoeba myosin are usually found closely spaced in parallel arrays suggesting that the Acanthamoeba myosin crosslinks the filaments. Under physiological conditions actin activates the Mg2+ ATPase of the Acanthamoeba myosin but this activation requires the presence of another Acanthamoeba protein which we have named the cofactor protein. Cofactor protein has been partially purified and the available data suggest that it is composed of a single polypeptide chain with a molecular weight of 97,000. The Mg2+ ATPase activity that results from the interaction of Acanthamoeba myosin, cofactor, and actin is dependent on the concentration of each of these proteins but it is not affected by the concentration of Ca2+. The cofactor protein, therefore, does not seem to be analagous to the Ca2+-sensitive control proteins found in the muscles of higher animals.
Pollard et al. (Sun,) studied this question.