The crystal structure of troponin C in complex with N-terminal fragment of troponin I. The mechanism of how the inhibitory action of troponin I is released by Ca(2+)-binding to troponin C.

Troponin (Tn), the complex of three subunits (TnC, TnI, and TnT), plays a key role in Ca2+ dependent regulation of muscle contraction. To elucidate the interactions between the Tn subunits and the conformation of TnC in the Tn complex, we have determined the crystal structure of TnC in complex with the N-terminal fragment of TnI (TnI1-47). The structure was solved by single isomorphous replacement method in combination with multiple wavelength anomalous dispersion data. The refinement converged to a crystallographic R-factor of 22.2% (R-free = 32.6%). The central, connecting alpha-helix observed in the structure of uncomplexed TnC (TnCfree) is unwound at the center and bent by 90 degrees. As a result, the TnC in the complex has a compact globular shape with direct interactions between the N- and C-lobes, in contrast to the elongated dumb-bell shaped molecule of uncomplexed TnC. The 31-residue long TnI1-47 alpha-helix stretches on the surface of TnC and stabilizes its compact conformation by multiple contacts with both TnC lobes. The amphiphilic C-terminal end of the TnI1-47 alpha-helix is tightly bound in the hydrophobic pocket of the TnC C-lobe through 38 van der Waals interactions. The results indicate the major difference between integrated (TnC) and isolated (calmodulin) Ca2+ receptors. The TnC/TnI1-47 structure suggests the model for a novel regulatory TnI segment bound to TnC and implies the mechanism of how Tn regulates the muscle contraction.