The interaction of equine platelet tropomyosin with skeletal muscle actin

Whereas skeletal muscle tropomyosin binds strongly to muscle F-actin in a buffer containing 30 mM KCl and 1-2 mM free Mg2+, equine platelet tropomyosin only binds stoichiometrically (1 tropomyosin molecule per 6 actin monomers) at higher Mg2+ concentrations (7-8 mM free Mg2+). At low free Mg2+ concentrations (1.5 mM) the binding of the platelet protein is only marginally increased by raising the KCl concentration to an optimal value (0.10-0.20 M). This weaker binding can be attributed to the relatively poor head-to-tail polymerization of platelet tropomyosin and its fewer actin-binding sites. In a buffer containing 30 mM KCl and 3 mM Mg2+, the binding of platelet tropomyosin to F-actin can be induced by the addition of either skeletal muscle myosin subfragment 1 or troponin-I. The binding induced by troponin-I is largely neutralized by the addition of troponin-C both in the presence and absence of Ca2+, but by calmodulin only in the presence of Ca2+.