Characterization of a region of the primary sequence of troponin C involved in calcium ion-dependent interaction with troponin I

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Abstract
The CNBr [cyanogen bromide] digest of troponin C from rabbit fast skeletal muscle was shown to possess many of the functional properties of the whole troponin C molecule. A peptide corresponding to residues 83-134 was isolated, which forms a Ca2+-dependent complex with troponin I and neutralizes the inhibition by troponin I of the Mg2+-stimulated ATPase of desensitized actomyosin. The peptide inhibits the phosphorylation of fast-skeletal-muscle, but not cardiac-muscle, troponin I, by cyclic AMP-dependent protein kinase. In this property it was as effective as whole skeletal-muscle troponin C when compared on a molar basis. Biological activity was also present in other fractions obtained from the CNBr digest. By gel filtration and affinity chromatography of the whole CNBr digest of troponin C, 2 peptides, one of which was identified as representing residues 83-134, were shown to form Ca2+-dependent complexes with troponin I. The significance of these findings for the mechanism of interaction of troponin C and troponin I is discussed.