The effect of Ca2+ on the interaction of troponin (TN) and its components with F-actin, tropomyosin, and the F-actin-tropomyosin complex has been studied. The Cabinding component of TN (TN-C) did not bind to F-actin, F-actin-tropomyosin, or tropomyosin regardless of whether Ca2+ was present. A complex of TN-C and of the inhibitory component (TN-I) bound to F-actin only in the presence of tropomyosin and in the absence of Ca2+. TN-I was weakly bound to F-actin and more strongly to F-actin-tropomyosin. Unfractionated troponin bound tightly to tropomyosin and to F-actin-tropomyosin but only weakly to F-actin, and in both cases the binding was not affected by Ca2+. The tropomyosin-binding component of TN (TN-T) and a complex of TN-C and TN-T bound to tropomyosin and to F-actin-tropomyosin but not to F-actin alone. On the basis of these binding studies and the known positions of tropomyosin on the thin actin-containing filaments during contraction and relaxation, a model is proposed which may explain how troponin and Ca2+ regulate muscle contraction.
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