Dynamics of the C-terminal region of TnI in the troponin complex in solution

Tharin M. A Blumenschein, Deborah B. Stone, Robert J. Fletterick, Robert A. Mendelson, Brian D. Sykes

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The determination of crystal structures of the troponin complex (Takeda et al. 2003. Nature. 424:35–41; Vinogradova et al. 2005. Proc. Natl. Acad. Sci. USA. 102:5038–5043) has advanced knowledge of the regulation of muscle contraction at the molecular level. However, there are domains important for actin binding that are not visualized. We present evidence that the C-terminal region of troponin I (TnI residues 135–182) is flexible in solution and has no stable secondary structure. We use NMR spectroscopy to observe the backbone dynamics of skeletal [2H, 13C, 15N]-TnI in the troponin complex in the presence of Ca2+ or EGTA/Mg2+. Residues in this region give stronger signals than the remainder of TnI, and chemical shift index values indicate little secondary structure, suggesting a very flexible region. This is confirmed by NMR relaxation measurements. Unlike TnC and other regions of TnI in the complex, the C-terminal region of TnI is not affected by Ca2+ binding. Relaxation measurements and reduced spectral density analysis are consistent with the C-terminal region of TnI being a tethered domain connected to the rest of the troponin complex by a flexible linker, residues 137–146, followed by a collapsed region with at most nascent secondary structure.
Original languageEnglish
Pages (from-to)2436-2444
Number of pages9
JournalBiophysical Journal
Issue number7
Publication statusPublished - 29 Oct 2006

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