Comparison of the effects of tropomyosin and troponin-tropomyosin on the binding of myosin subfragment 1 to actin.

The effect of tropomyosin alone on the binding of myosin subfragment 1 (S-1) to actin was compared to that of troponin-tropomyosin, with and without Ca2+. Over a range of ionic strength and with different ligands (adenyl-5'-yl imidodiphosphate, pyrophosphate, and adenosine 5'-diphosphate), tropomyosin confers slight cooperativity to the binding of S-1 to actin. In the presence of Ca2+, troponin does not affect the cooperative action of tropomyosin alone. In addition, troponin-tropomyosin and tropomyosin alone are also identical in their ability to strengthen the binding of S-1-ligand to actin 3-fold and the binding of S-1 alone to actin 7-fold, at high levels of saturation of the actin with S-1. Although troponin does not significantly affect the cooperative action of tropomyosin alone in the presence of Ca2+, it does markedly enhance the cooperativity in the binding of S-1 to actin in the absence of Ca2+.