Solution structures of the C-terminal domain of cardiac troponin C free and bound to the N-terminal domain of cardiac troponin I

The N-terminal domain of cardiac troponin I (cTnI) comprising residues 33-8 0 and lacking the cardiac-specific amino terminus forms a stable binary com plex with the C-terminal domain of cardiac troponin C (cTnC) comprising res idues 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 be tween secondary structural elements determined for free and cTnI(33-80)-bou nd cTnC(81-161). We have determined solution structures of Ca2+-saturated c TnC(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, th e 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 moveme nts provide the binding site for the N-terminal domain of cTnI, The putativ e binding site for cTnI(33-80) was determined by mapping amide proton and n itrogen chemical shift changes, induced by the binding of cTnI(33-80), onto the C-terminal cTnC structure. The binding interface for cTnI(33-80), as s uggested from chemical shift changes, involves predominantly hydrophobic in teractions located in the expanded hydrophobic pocket. The largest chemical shift changes were observed in the loop region connecting helices F and G, Inspection of available TnC sequences reveals that these residues are high ly conserved, suggesting a common binding motif for the Ca2+/Mg2+-dependent interaction site in the TnC/TnI complex.