The importance of the carboxyl-terminal domain of cardiac troponin C in Ca2+-sensitive muscle regulation

The interactions between troponin I and troponin C are central to the Ca2+-regulated control of striated muscle. Using isothermal titration microcalorimetry we have studied the binding of human cardiac troponin C (cTnC) and its isolated domains to human cardiac troponin I (cTnI). We provide the first binding data for these proteins while they are free in solution and unmodified by reporter groups. Our data reveal that the C-terminal domain of cTnC is responsible for most of the free energy change upon cTnC cTnI binding. Importantly, the interaction between cTnI and the C-terminal domain of cTnC is 8-fold stronger in the presence of Ca2+ than in the presence of Mg2+, suggesting that the C-terminal domain of cTnC may play a modulatory role in cardiac muscle regulation. Changes in the affinity of cTnI for cTnC and its isolated C-terminal domain in response to ionic strength support this finding, with both following similar trends. At physiological ionic strength the affinity of cTnC for cTnI changed very little in response to Ca2+, although the thermodynamic data show a clear distinction between binding in the presence of Ca2+ and in the presence of Mg2+.