The solution structure of a cardiac troponin C-troponin I-troponin T complex shows a somewhat compact troponin C interacting with an extended troponin I-troponin T component.

We have investigated the structure of the cTnC-cTnI-cTnT(198-298) calcium-saturated, ternary cardiac troponin complex by small-angle scattering with contrast variation. Shape restoration was also applied to the scattering information resulting from the deuterated cTnC subunit, the unlabeled cTnI-cTnT(198-298) subunits, and the entire complex. The experimental results and modeling indicate that cTnC adopts a partially collapsed conformation, while the cTnI-cTnT(198-298) components have an extended, rod-like structure. Shape restoration applied to the X-ray scattering data and the entire contrast variation series suggest that cTnC and the cTnI-cTnT(198-298) component lie with their long axes roughly parallel to one another with a relatively small surface area for interaction. Our findings indicate that the nature of the interactions between TnC and the TnI-TnT component differs significantly between the cardiac and skeletal isoforms as evidenced by the different degrees of compactness between the cardiac TnC and skeletal TnC in their respective ternary complexes and the fact that the cTnC subunit is not highly intertwined with the other subunits, as observed in the binary complex of the skeletal isoforms [Olah, G. A., and Trewhella, J. (1994) Biochemistry 33, 12800-12806].