Inhibition of Mg ++ATPase activity of actin-activated acanthamoeba myosin by muscle troponin-tropomyosin: Implications for the mechanism of control of amoeba motility and muscle contraction

Troponin-tropomyosin is known to inhibit the Mg ++ATPase activity of muscle actomyosin in the absence, but not in the presence, of Ca ++. In contrast, we have now found that muscle troponin-tropomyosin inhibits the Mg ++ATPase activity of muscle actin-activated Acanthamoeba myosin both in the presence and the absence of Ca ++. Addition of purified tropomyosin and troponins-I, C and T demonstrated that it is troponin-T that acts differently in the two systems which differ only in the source of the myosin. These data suggest that myosin, as well as actin, plays a role in the troponin-tropomyosin control of muscle contraction and make it unlikely that control proteins identical to troponin-tropomyosin function in this amoeba.