The calcium-saturated cTnI/cTnC complex: structure of the inhibitory region of cTnI.

The contiguous inhibitory and regulatory regions of troponin I in the heterotrimeric troponin complex play a critical role in Ca(2+) activation of striated muscle. Knowledge of the structure of this critical region within the complex will enhance efforts toward understanding regulatory mechanisms. Toward this goal, we have used simulated annealing to study the structure of the inhibitory and regulatory regions of cardiac muscle troponin I in the calcium-saturated complex formed between cardiac troponin C and cardiac troponin I. We have incorporated distances determined experimentally by Förster resonance energy transfer in the full-length complex, rather than using peptides derived from cTnI. For these models, we assume a helix-loop-helix conformation for the inhibitory region. We have found several structures that satisfy the experimental constraints fairly well. Although it is not possible to eliminate any of these models at this time, future studies with additional experimental restraints will yield insights on the mechanisms of calcium regulation in cardiac muscle.