Binding of tropomyosin to copolymers of Acanthamoeba actin and muscle actin.

The binding of tropomyosin to F-actin is strongly dependent on M&+ concentration. With muscle actin, in the presence of 2 mu ATP, binding begins at 4 mM M&+ and is complete at about 4.75 mu Mg+ while, with Acanthamoeba actin, binding is initiated at 6 mu M&+ and reaches saturation at 8.5 InM Mg+. Copolymers of muscle and Acanthamoeba actin, however, behave as unique species of actin, each with a characteristic MS+ dependence and not as a mixture of homopolymers. In all cases, the stoichiometry of binding is one tropomyosin molecule to seven actin monomers. These results suggest that the actin monomers are randomly distributed in the copolymers so that one tropomyosin molecule binds to seven actin monomers of mixed origin and, therefore, neither the muscle nor Acanthamoeba actin can express its individual properties with respect to tropomyosin binding. In 9 mu M$+ and 2 mu ATP, bound tropomyosin inhibits the muscle actin-activated heavy meromyosin ATPase but has no effect on the Acanthamoeba actinactivated heavy meromyosin ATPase. The effect of tropomyosin on the ATPase activation by copolymers of muscle and Acanthamoeba actin was proportional to the ratio of the two kinds of actin just as in mixtures of homopolymers. Similarly, in 5 mM M&+, 80 mu KCl, and 2 mM ATP, bound tropomyosin inhibits the muscle actin-activated heavy meromyosin ATPase while it increases the Acanthamoeba actin-activated heavy meromyosin ATPase both with copolymers and mixtures of homopolymers. These results cannot be attributed to an inability of tropomyosin to inhibit the activation of muscle heavy meromyosin ATPase by Acanthamoeba actin since, in the absence of Ca2+, the addition of troponin together with tropomyosin strongly inhibits the activation of the heavy meromyosin ATPase by Acanthamoeba actin both with homopolymers and copolymers. These data suggest that adjacent actin monomers can interact independently with heavy meromyosin in the presence and absence of tropomyosin and that one tropomyosin molecule can have different effects on adjacent actin monomers in random copolymers.