title: The cardiac-specific N-terminal region of troponin I positions the regulatory domain of troponin C.
abstract: The cardiac isoform of troponin I (cTnI) has a unique 31-residue N-terminal region that binds cardiac troponin C (cTnC) to increase the calcium sensitivity of the sarcomere. The interaction can be abolished by cTnI phosphorylation at Ser22 and Ser23, an important mechanism for regulating cardiac contractility. cTnC contains two EF-hand domains (the N and C domain of cTnC, cNTnC and cCTnC) connected by a flexible linker. Calcium binding to either domain favors an "open" conformation, exposing a large hydrophobic surface that is stabilized by target binding, cTnI[148-158] for cNTnC and cTnI[39-60] for cCTnC. We used multinuclear multidimensional solution NMR spectroscopy to study cTnI[1-73] in complex with cTnC. cTnI[39-60] binds to the hydrophobic face of cCTnC, stabilizing an alpha helix in cTnI[41-67] and a type VIII turn in cTnI[38-41]. In contrast, cTnI[1-37] remains disordered, although cTnI[19-37] is electrostatically tethered to the negatively charged surface of cNTnC (opposite its hydrophobic surface). The interaction does not directly affect the calcium binding affinity of cNTnC. However, it does fix the positioning of cNTnC relative to the rest of the troponin complex, similar to what was previously observed in an X-ray structure [Takeda S, et al. (2003) Nature 424(6944):35-41]. Domain positioning impacts the effective concentration of cTnI[148-158] presented to cNTnC, and this is how cTnI[19-37] indirectly modulates the calcium affinity of cNTnC within the context of the cardiac thin filament. Phosphorylation of cTnI at Ser22/23 disrupts domain positioning, explaining how it impacts many other cardiac regulatory mechanisms, like the Frank-Starling law of the heart.
authors.affiliation: Division of General Internal Medicine, Department of Medicine, and Department of Biochemistry, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, AB, Canada T6G 2H7 phwang1@ualberta.ca.
authors.forename: Peter M
authors.initials: PM
authors.lastname: Hwang
authors.affiliation: Department of Biochemistry, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, AB, Canada T6G 2H7.
authors.forename: Fangze
authors.initials: F
authors.lastname: Cai
authors.affiliation: Department of Biochemistry, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, AB, Canada T6G 2H7.
authors.forename: Sandra E
authors.initials: SE
authors.lastname: Pineda-Sanabria
authors.affiliation: Department of Biochemistry, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, AB, Canada T6G 2H7.
authors.forename: David C
authors.initials: DC
authors.lastname: Corson
authors.affiliation: Department of Biochemistry, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, AB, Canada T6G 2H7.
authors.forename: Brian D
authors.initials: BD
authors.lastname: Sykes
mesh_terms: D001667:Binding, Competitive; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D008958:Models, Molecular; D009154:Mutation; D009206:Myocardium / Q000378:metabolism; D019906:Nuclear Magnetic Resonance, Biomolecular; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D010766:Phosphorylation; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012694:Serine / Q000737:chemistry / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D055672:Static Electricity; D019209:Troponin C / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D019210:Troponin I / Q000737:chemistry* / Q000378:metabolism
chemical_list: D010446:Peptide Fragments; D019209:Troponin C; D019210:Troponin I; D012694:Serine; D002118:Calcium
keywords: dilated cardiomyopathy; fuzzy complex; heart failure; hypertrophic cardiomyopathy; post-translational modification