Quasielastic scattering of laser light was used to examine the effect of calcium ions on the flexibility of a reconstituted thin filament, namely, an F-actin/tropomyosin/troponin complex of striated muscle. The results showed that the flexibility of a reconstituted thin filament changed reversibly at about 1 μm of free calcium ions (i.e. the physiological concentration). Below 1 μm-Ca2+ a tropomyosin/troponin system suppressed the bending motion of F-actin. This suppression was removed by calcium ions above 1 μm, and the F-actin flexibility became almost the same as that of an F-actin/tropomyosin complex. A quantitative study showed that calcium ions affected the reconstituted thin filament in two ways at the free calcium ion concentrations of about 1 μm and 20 μm, corresponding, respectively, to the two different calcium binding constants of troponin. An over-all conformational change of a reconstituted thin filament is brought about by calcium ions. It is suggested that the effect of calcium ions on troponin can be transferred to F-actin by altering the binding between tropomyosin and several monomer units in F-actin. The possible role of the dynamic nature of F-actin in muscle contraction is discussed.
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