It was found that thin filaments are reconstituted from F-actin, tropomyosin and the tropomyosin binding component of troponin (TN-T) in vitro according to a self-assembly mechanism. Although a gel structure is formed when the three proteins are mixed in the order F-actin, TN-T and tropomyosin, it is in a metastable state and spontaneously transforms to dispersed filaments in an equilibrium state. The rate of the transformation is largely dependent on temperature. When the mixing order of TN-T and tropomyosin is reversed, large amounts of the complex appear immediately in the dispersed filament form. Dependence on the order of mixing is observed only when the molar ratio of TN-T to tropomyosin is about one, i.e. at the physiological ratio. When the molar ratio of TN-T to tropomyosin is either above or below one, a stable gel form or a stable dispersed filament form was, respectively, obtained independently of the mixing order of the three proteins.
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