Effect of Skeletal Muscle Native Tropomyosin on the Interaction of Amoeba Actin with Heavy Meromyosin

Abstract

ACTIN and myosin, recently identified in protozoa and myxomycetes, are presumably responsible for the motility of these organisms^1–5. Control of this motility is, however, not at all understood. In skeletal muscle, contraction is partly under the control of the protein complex, native tropomyosin which is believed to bind to the actin filaments^6,7. In the presence of Ca²⁺, native tropomyosin allows the interaction of actin and myosin filaments to occur wiiereas in the absence of Ca²⁺ native tropomyosin inhibits this interaction⁶, probably by inhibiting the binding of actin to myosin⁸. No protein like native tropomyosin has yet been identified in a primitive contractile system, but we know that Acanthamoeba actin activates the ATPase of skeletal muscle heavy meromyosin (HMM) and we have used this fact to investigate the effect of skeletal muscle native tropomyosin on the interaction of Acanthamoeba actin with HMM. We have found that native tropomyosin in the presence of Ca²⁺ increases the activation of HMM ATPase by Acanthamoeba actin, while in the absence of Ca²⁺ the native tropomyosin inhibits this activation.