Replacement of Troponin Components in Myofibrils1

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Abstract
The Ca2+-sensitive ATPase activity of rabbit skeletal myofibrils was desensitized by treatment with excess troponin T and was found to be activated irrespective of the Ca2+ concentrations. A SDS-gel electrophoretic study showed that both troponin C and troponin I were removed from the myofibrils on treatment with troponin T. The Ca2+- and Sr2+-sensitivities of the ATPase of troponin T-treated myoflbrils reconstituted with troponin C.I were the same as in the intact myofibrils. The Ca2+ -activated ATPase of rabbit skeletal myofibrils was also desensitized on treatment with chicken breast troponin T or its 26K fragment. The SDS-gel electrophoretic study revealed that troponin T, in addition to troponin C and troponin I, was also removed from the myofibrils and, instead, chicken breast troponin T or its 26K fragment was incorporated into the myofibrils. The Ca2+-sensitivity of myofibrils treated with chicken breast troponin T or its 26K fragment was then regained on reconstitution with troponin C.I. These findings indicate that the change in composition of myofibrils on treatment with troponin T or its 26K fragment is due to the selective replacement of the troponin C.I.T complex in the myofibrils as a whole with troponin T or its 26K fragment.

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