We investigated the interactions between troponin C and troponin T, and between calmodulin and troponin T, in the presence of Ca2+ and Sr2+. Polyacrylamide-gel electrophoresis under non-denaturing conditions and ultracentrifugation methods was used to examine the interactions. Calmodulin and skeletal troponin C bound to skeletal troponin T only in the presence of bivalent cations, whereas calmodulin and cardiac troponin C bound to cardiac troponin T in both the presence and absence of bivalent cations. Skeletal troponin C bound to the skeletal troponin T-tropomyosin-actin complex only in the presence of bivalent cations, although cardiac troponin C hardly bound to the cardiac troponin T-tropomyosin-actin complex even in the presence of bivalent cations. Calmodulin bound to the skeletal and cardiac troponin T-tropomyosin-actin complexes only in the presence of bivalent cations. Trifluoperazine, a calmodulin antagonist, inhibited the bivalent-cation-dependent interaction between calmodulin and skeletal troponin T. The results suggest that the conformation of cardiac troponin T coupled with the tropomyosin-actin complex differs considerably from that of troponin T alone, and that the interaction between troponin C and troponin T does not play a major role in the troponin regulation of muscle contraction. The bivalent-cation-induced exposure of the hydrophobic region may enable calmodulin to bind to skeletal troponin T.
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