title: Calcium structural transition of human cardiac troponin C in reconstituted muscle fibres as studied by site-directed spin labelling.
abstract: The in situ structure of human cardiac troponin C (hcTnC) has been studied with site-directed, spin labelling, electron paramagnetic resonance (SDSL-EPR). Analysis of the in situ structures of hcTnC is essential for elucidating the molecular mechanism behind its Ca(2+)-sensitive regulation. We prepared two hcTnC mutants (C35S and C84S) containing one native cysteine residue (84 and 35, respectively) for spin labelling. The mutants were labelled with a methane thiosulfonate spin label (MTSSL) and the TnC was reconstituted into permeabilized muscle fibres. The mobility of Cys84-MTSSL changed markedly after addition of Ca2+, while that of the Cys35 residue did not change in the monomer state or in fibres. The rotational correlation time of Cys84-MTSSL decreased from 32ns to 13ns upon Ca(2+)-binding in the monomer state, whereas in fibres the spectrum of Cys84-MTSSL was resolved into mobile (16ns) and immobile (35ns) components and the addition of Ca2+ increased the immobile component. Moreover, the accessibility of Cys84-MTSSL to molecular oxygen increased slightly in the presence of Ca2+. These data suggest that Cys35 remains in the same location regardless of the addition of Ca2+, whereas Cys84 is located at the position that interacts with B and C helices of hcTnC and interacts with troponin I (TnI) at high concentrations of Ca2+. We determined the distances between Cys35 and Cys84 by measuring pulsed electron-electron double resonance spectra. The distances were 26.0 angstroms and 27.2 angstroms in the monomer state and in fibres, respectively, and the addition of Ca2+ decreased the distance to 23.2 angstroms in fibres but only slightly in the monomer state, showing that Ca2+ binding to the N-domain of hcTnC induced a larger structural change in muscle fibres than in the monomer state.
authors.affiliation: Department of Biology, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
authors.forename: Motoyoshi
authors.initials: M
authors.lastname: Nakamura
authors.forename: Shoji
authors.initials: S
authors.lastname: Ueki
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Hara
authors.forename: Toshiaki
authors.initials: T
authors.lastname: Arata
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D003545:Cysteine / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D006801:Humans; D008958:Models, Molecular; D018485:Muscle Fibers, Skeletal / Q000737:chemistry*; D009206:Myocardium / Q000737:chemistry*; D011487:Protein Conformation; D013113:Spin Labels; D019209:Troponin C / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D013113:Spin Labels; D019209:Troponin C; D000251:Adenosine Triphosphatases; D003545:Cysteine; D002118:Calcium