A gel structure of the F-actin-tropomyosin-troponin complex formed in vitro is spontaneously and irreversibly dissociated to filaments at every temperature. The gel formation and the transformation to dispersed filaments of the complex were investigated mainly by measuring the flow birefringence at various conditions. No loss of calcium sensitivity of the complex was found during the transformation. The rate of transformation was proportional to exp(-ΔH/RT) as a function of temperature, where the activation enthalpy ΔH was nearly equal to 30 kcal/mole. The rate of transformation was dependent on both Ca2+ at a concentration of around 1 μM (the physiological concentration) and pH value. Through these observations, it was concluded that the F-actin-tropomyosin-troponin complex, formed in vitro by mixing the three proteins, is at first in a metastable state (gel form) and then gradually transformed to a stable one (filament form). The simple kinetic scheme of the gel-filament transformation is introduced and discussed on the assumption that there are two states of actin. The filamentous form of the complex obtained here is believed to be the most homogeneous and stable in vitro analogue of the thin filament.
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