Effect of calcium ions on the flexibility of reconstituted thin filaments of muscle studied by quasielastic scattering of laser light

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.