The effects of β-actinin on the interaction between F-actin and heavy meromyosin were studied by quasi-elastic scattering of laser light. The main findings are as follows: 1. 1. Molecular flexing of F-actin in solution has been known to be promoted by bound heavy meromyosin. However, this effect of heavy meromyosin disappeared on the addition of β-actinin (10% by wt to F-actin) in the absence of ATP. 2. 2. When β-actinin was added in the presence of suitable amounts of ATP, the above mentioned effect of β-actinin could not be observed after the ATP was hydrolyzed. However, the effect of β-actinin was observable after application of sheer force to the solution. Subsequent addition of ATP did not eliminate the effect of β-actinin again. 3. 3. Even when β-actinin was added in the presence of ATP, if the concentration of ATP was higher than about 4 mM, the effect of β-actinin was observed. Experiments suggested that the system was not sensitive to the final concentration of ADP, but to the initial concentration of Mg-ATP.
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