A newly developing technique, quasielastic scattering of laser light, was applied to the dynamic study of F-actin interacting with other muscle proteins, heavy meromyosin, S-1 (i.e. a single head of myosin) and tropomyosin. The following three systems were studied: (1) the interaction between F-actin and heavy meromyosin or S-1 in the absence of ATP; (2) the interaction between F-actin and tropomyosin at various salt concentrations; and (3) the interaction between heavy meromyosin and the complex of F-actin and tropomyosin in the absence of ATP. F-actin was most flexible when the mole ratio of heavy meromyosin to F-actin monomer was 1:6. At this ratio, one heavy meromyosin molecule binds to about half a pitch of one strand of the actin filament. On the other hand, flexibility of F-actin was not changed on the addition of S-l. The addition of tropomyosin to F-actin solutions made F-actin stiffer. Tropomyosin appears to bind directly to F-actin up to a weight ratio of tropomyosin to F-actin of 1:6. This direct binding of tropomyosin affected the interaction between F-actin and heavy meromyosin. On the addition of heavy meromyosin to the complex solution of F-actin and tropomyosin, the complex was most flexible when the molar ratio of heavy meromyosin to F-actin was between 1:3 and 1:2. F-actin is not a rigid rod; rather, it appears to be a partially flexible chain. The flexibility of F-actin may have an important role in the molecular mechanism of muscular contraction.
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