Proteolytic fragments of troponin C. Interactions with the other troponin subunits and biological activity.

Fragments of rabbit skeletal muscle Ca2+-binding subunit of troponin (TnC), obtained by cleavage with trypsin, thrombin, and CNBr, were tested for their ability to form binary and ternary complexes with ATPase inhibitory subunit (TnI) and tropomyosin-binding subunit (TnT) and their ability to replace TnC in reversing TnI inhibition of actomyosin ATPase activity. Three regions of TnC were found to be involved in interaction with TnI. Regions near Ca2+-binding sites II and III require Ca2+ for the interaction, while a third region near Ca2+-binding site IV binds TnI whether or not Ca2+ is present. The TnT binding site has been localized in the NH2-terminal half of TnC. Several of the TnC fragments form soluble ternary complexes with TnI and TnT. Fragments that contain amino acid residues 89-100 and at least one pair of Ca2+-binding sites are able to reverse the TnI inhibition of actomyosin ATPase activity, which exhibits the same [Ca2+]1/2 regardless of which of the Ca2+-binding sites are present in the fragment.