Actin filaments copolymerized with both intact and chemically modified actin monomers restored their sliding activity when they were supplemented with tropomyosin extracted from skeletal muscle. In contrast, the ATPase activation of the copolymers was decreased when supplemented with tropomyosin. The results indicate that tropomyosin along with actin monomers may facilitate sliding activity of the entire actin filament but suppress ATPase activation of intact actin monomers themselves. Accordingly, tropomyosin molecules could be viewed as playing a dual role of both mechanical and chemical regulation of actin monomers.
