Site-directed spin labeling electron paramagnetic resonance study of the calcium-induced structural transition in the N-domain of human cardiac troponin C complexed with troponin I

Calcium-induced structural transition in the amino-terminal domain of troponin C (TnC) triggers skeletal and cardiac muscle contraction. The salient feature of this structural transition is the movement of the B and C helices, which is termed the opening of the N-domain. This movement exposes a hydrophobic region, allowing interaction with the regulatory domain of troponin I (TnI) as can be seen in the crystal structure of the troponin ternary complex [Takeda, S., Yamashita, A., Maeda, K., and Maeda, Y. (2003) Nature 424, 35-41]. In contrast to skeletal TnC, Ca2+-binding site I (an EF-hand motif that consists of an A helix-loop-B helix motif) is inactive in cardiac TnC. The question arising from comparisons with skeletal TnC is how both helices move according to Ca2+ binding or interact with TnI in cardiac TnC. In this study, we examined the Ca2+-induced movement of the B and C helices relative to the D helix in a cardiac TnC monomer state and TnC-TnI binary complex by means of site-directed spin labeling electron paramagnetic resonance (EPR). Doubly spin-labeled TnC mutants were prepared, and the spin-spin distances were estimated by analyzing dipolar interactions with the Fourier deconvolution method. An interspin distance of 18.4 Angstrom was estimated for mutants spin labeled at G42C on the B helix and C84 on the D helix in a Mg2+-saturated monomer state. The interspin distance between Q58C on the C helix and C84 on the D helix was estimated to be 18.3 Angstrom under the same conditions. Distance changes were observed by the addition of Call ions and the formation of a complex with TnI. Our data indicated that the C helix moved away from the D helix in a distinct Ca2+-dependent manner, while the B helix did not. A movement of the B helix by interaction with TnI was observed. Both Ca2+ and TnI were also shown to be essential for the full opening of the N-domain in cardiac TnC.