title: Dynamics of the C-terminal region of TnI in the troponin complex in solution.
abstract: 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.
authors.affiliation: CIHR Group in Structure and Function and Department of Biochemistry, University of Alberta, Edmonton, Alberta T6G 2H7, Canada.
authors.forename: Tharin M A
authors.initials: TM
authors.lastname: Blumenschein
authors.forename: Deborah B
authors.initials: DB
authors.lastname: Stone
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Fletterick
authors.forename: Robert A
authors.initials: RA
authors.lastname: Mendelson
authors.forename: Brian D
authors.initials: BD
authors.lastname: Sykes
mesh_terms: D000199:Actins / Q000737:chemistry / Q000378:metabolism; D000818:Animals; D001703:Biophysics / Q000379:methods; D002118:Calcium / Q000737:chemistry; D002645:Chickens; D018360:Crystallography, X-Ray; D003545:Cysteine / Q000737:chemistry; D004533:Egtazic Acid / Q000737:chemistry; D017353:Gene Deletion; D008274:Magnesium / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D015233:Models, Statistical; D009119:Muscle Contraction; D018482:Muscle, Skeletal / Q000378:metabolism; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D014336:Troponin / Q000737:chemistry*; D019209:Troponin C / Q000737:chemistry; D019210:Troponin I / Q000737:chemistry*
chemical_list: D000199:Actins; D014336:Troponin; D019209:Troponin C; D019210:Troponin I; D004533:Egtazic Acid; D008274:Magnesium; D003545:Cysteine; D002118:Calcium