Steady-state fluorescence anisotropy technique was used to determine the binding constant of troponin for IAEDANS-labeled tropomyosin under various conditions. In the absence of actin, Ca does not affect the binding between troponin and tropomyosin. The presence of actin greatly strengthens troponin-tropomyosin binding in the absence of Ca. However, Ca weakens troponin-tropomyosin binding by about 2.5-fold in the reconstituted filament. It is suggested that the Ca-regulated binding may serve as a molecular switch for the troponin molecule to get "on" and "off" the actin-myosin interaction site regulating muscle contraction-relaxation cycles.
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