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The N-terminal domain of cardiac troponin I (cTnI) comprising residues 33−80 and lacking the cardiac-specific amino terminus forms a stable binary complex with the C-terminal domain of cardiac troponin C (cTnC) comprising residues 81−161. We have utilized heteronuclear multidimensional NMR to assign the backbone and side-chain resonances of Ca²⁺-saturated cTnC(81−161) both free and bound to cTnI(33−80). No significant differences were observed between secondary structural elements determined for free and cTnI(33−80)-bound cTnC(81−161). We have determined solution structures of Ca²⁺-saturated cTnC(81−161) free and bound to cTnI(33−80). While the tertiary structure of cTnC(81−161) is qualitatively similar to that observed free in solution, the binding of cTnI(33−80) results mainly in an opening of the structure and movement of the loop region between helices F and G. Together, these …