Functional role of Ca(2+)-binding site IV of scallop troponin C.

Scallop troponin C (TnC) binds only one Ca(2+)/mol and the single Ca(2+)-binding site has been suggested to be site IV on the basis of the primary structure [K. Nishita, H. Tanaka, and T. Ojima (1994) J. Biol. Chem. 269, 3464-3468; T. Ojima, H. Tanaka, and K. Nishita (1994) Arch. Biochem. Biophys. 311, 272-276]. In the present study, the functional role of Ca(2+)-binding site IV of akazara scallop (Chlamys nipponensis akazara) TnC in Ca(2+)-regulation was investigated using a site-directed mutant with an inactivated site IV (TnC-ZEQ), N- and C-terminal half molecule mutants (TnC(N) and TnC(C)), and wild-type TnC (TnC(W)). Equilibrium dialysis using (45)Ca(2+) demonstrated that TnC(W) and TnC(C) bind 0.6-0.8 mol of Ca(2+)/mol, but that TnC-ZEQ and TnC(N) bind virtually no Ca(2+). The UV difference spectra of TnC(W) and TnC(C) showed bands at around 280-290 nm due to the perturbation of Tyr and Trp upon Ca(2+)-binding, while TnC-ZEQ and TnC(N) did not show these bands. In addition, TnC(W) and TnC(C) showed retardation of elution from Sephacryl S-200 upon the addition of 1 mM CaCl(2), unlike TnC-ZEQ and TnC(N). These results indicate that Ca(2+) binds only to site IV and that Ca(2+)-binding causes structural changes in both the whole TnC molecule and the C-terminal half molecule. In addition, TnC(W), TnC-ZEQ, and TnC(C), but not TnC(N), were shown to form soluble complexes with scallop TnI at physiological ionic strength. On the other hand, the Mg-ATPase activity of reconstituted rabbit actomyosin in the presence of scallop tropomyosin was inhibited by scallop TnI and recovered by the addition of an equimolar amount of TnC(W), TnC-ZEQ, or TnC(C), but not TnC(N). These results imply that the site responsible for the association with TnI is located in the C-terminal half domain of TnC. Ternary complex constructed from scallop TnT, TnI, and TnC(W) conferred Ca(2+)-sensitivity to the Mg-ATPase of rabbit actomyosin to the same extent as native troponin, but the TnC(N)-TnT-TnI and TnC-ZEQ-TnT-TnI complexes conferred no Ca(2+)-sensitivity, while the TnC(C)-TnT-TnI complex conferred weak Ca(2+)-sensitivity. Thus, the major functions of scallop TnC, such as Ca(2+)-binding and interaction with TnI, are located in the C-terminal domain, however, the full Ca(2+)-regulatory function requires the presence of the N-terminal domain.