Dynamic Equilibrium of Cardiac Troponin C's Hydrophobic Cleft and Its Modulation by Ca Sensitizers and a Ca Sensitivity Blunting Phosphomimic, cTnT(T204E).

Several studies have suggested that conformational dynamics are important in the regulation of thin filament activation in cardiac troponin C (cTnC); however, little direct evidence has been offered to support these claims. In this study, a dye homodimerization approach is developed and implemented that allows the determination of the dynamic equilibrium between open and closed conformations in cTnC's hydrophobic cleft. Modulation of this equilibrium by Ca, cardiac troponin I (cTnI), cardiac troponin T (cTnT), Ca-sensitizers, and a Ca-desensitizing phosphomimic of cTnT (cTnT(T204E) is characterized. Isolated cTnC contained a small open conformation population in the absence of Ca that increased significantly upon the addition of saturating levels of Ca. This suggests that the Ca-induced activation of thin filament arises from an increase in the probability of hydrophobic cleft opening. The inclusion of cTnI increased the population of open cTnC, and the inclusion of cTnT had the opposite effect. Samples containing Ca-desensitizing cTnT(T204E) showed a slight but insignificant decrease in open conformation probability compared to samples with cardiac troponin T, wild type [cTnT(wt)], while Ca sensitizer treated samples generally increased open conformation probability. These findings show that an equilibrium between the open and closed conformations of cTnC's hydrophobic cleft play a significant role in tuning the Ca sensitivity of the heart.