Structural and regulatory functions of the NH2- and COOH-terminal regions of skeletal muscle troponin I.

Calcium binding to regulatory sites located in the NH2-terminal domain of troponin C (TnC) induces a conformational change that blocks the inhibitory action of troponin I (TnI) and triggers muscle contraction. We used deletion mutants of TnI in conjunction with a series of TnC mutants to understand the structural and functional relationship between different TnI regions and TnC domains. Our results indicate that TnI is organized into structural and regulatory regions which interact in an antiparallel fashion with the corresponding structural and regulatory regions of TnC. Functional studies show that the COOH-terminal region of TnI, when linked to the inhibitory region (TnI103-182) can regulate actomyosin ATPase. A TnI lacking the first 57 amino acids (TnId57) has been shown to have similar properties (Sheng, Z., Pan, B.-S., Miller, T. E., and Potter, J. D. (1992) J. Biol. Chem. 267, 25407-25413). Regulation was not observed with the COOH-terminal region alone (TnI120-182), with the NH2-terminal region alone (TnI1-98), or with the NH2-terminal linked to the inhibitory region (TnI1-116). Binding studies show that the NH2-terminal region of TnI interacts with the COOH-terminal domain of TnC in the presence of Ca2+ or Mg2+ and that the inhibitory plus COOH-terminal region of TnI (TnI103-182) interacts with the NH2-terminal domain of TnC in a Ca(2+)-dependent manner. Based on these results we propose a model for the Ca(2+)-induced conformational change. In our model the NH2-terminal domain of TnI is anchored strongly to the COOH-terminal domain of TnC in the absence and presence of Ca2+ while the inhibitory and COOH-terminal regions of TnI switch between actin-tropomyosin in the absence of Ca2+ to binding sites in both NH2- and COOH-terminal domains of TnC in the presence of Ca2+.