Calcium-dependent Changes in the Flexibility of the Regulatory Domain of Troponin C in the Troponin Complex

With the recent advances in structure determination of the troponin complex, it becomes even more important to understand the dynamics of its components and how they are affected by the presence or absence of Ca2+. We used NMR techniques to study the backbone dynamics of skeletal troponin C (TnC) in the complex. Transverse relaxation-optimized spectroscopy pulse sequences and deuteration of TnC were essential to assign most of the TnC residues in the complex. Backbone amide 15N relaxation times were measured in the presence of Ca2+ or EGTA/Mg2+. T1 relaxation times could not be interpreted precisely, because for a molecule of this size, the longitudinal backbone amide 15N relaxation rate due to chemical shift anisotropy and dipole-dipole interactions becomes too small, and other relaxation mechanisms become relevant. T2 relaxation times were of the expected magnitude for a complex of this size, and most of the variation of T2 times in the presence of Ca2+ could be explained by the anisotropy of the complex, suggesting a relatively rigid molecule. The only exception was EF-hand site III and helix F immediately after, which are more flexible than the rest of the molecule. In the presence of EGTA/Mg2+, relaxation times for residues in the C-domain of TnC are very similar to values in the presence of Ca2+, whereas the N-domain becomes more flexible. Taken together with the high flexibility of the linker between the two domains, we concluded that in the absence of Ca2+, the N-domain of TnC moves independently from the rest of the complex.