title: Calcium-dependent movement of troponin I between troponin C and actin as revealed by spin-labeling EPR.
abstract: We measured EPR spectra from a spin label on the Cys133 residue of troponin I (TnI) to identify Ca(2+)-induced structural states, based on sensitivity of spin-label mobility to flexibility and tertiary contact of a polypeptide. Spectrum from Tn complexes in the -Ca(2+) state showed that Cys133 was located at a flexible polypeptide segment (rotational correlation time tau=1.9ns) that was free from TnC. Spectra of both Tn complexes alone and those reconstituted into the thin filaments in the +Ca(2+) state showed that Cys133 existed on a stable segment (tau=4.8ns) held by TnC. Spectra of reconstituted thin filaments (-Ca(2+) state) revealed that slow mobility (tau=45ns) was due to tertiary contact of Cys133 with actin, because the same slow mobility was found for TnI-actin and TnI-tropomyosin-actin filaments lacking TnC, T or tropomyosin. We propose that the Cys133 region dissociates from TnC and attaches to the actin surface on the thin filaments, causing muscle relaxation at low Ca(2+) concentrations.
authors.affiliation: Department of Biological Sciences, Graduate School of Science, Osaka University and CREST/JST, Toyonaka, Osaka 560-0043, Japan.
authors.forename: Tomoki
authors.initials: T
authors.lastname: Aihara
authors.forename: Shoji
authors.initials: S
authors.lastname: Ueki
authors.forename: Motoyoshi
authors.initials: M
authors.lastname: Nakamura
authors.forename: Toshiaki
authors.initials: T
authors.lastname: Arata
mesh_terms: D000199:Actins / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002645:Chickens; D003545:Cysteine / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D017433:Protein Structure, Secondary; D011817:Rabbits; D013113:Spin Labels; D013816:Thermodynamics; D014335:Tropomyosin / Q000378:metabolism; D019209:Troponin C / Q000737:chemistry / Q000378:metabolism*; D019210:Troponin I / Q000737:chemistry / Q000378:metabolism*
chemical_list: D000199:Actins; D013113:Spin Labels; D014335:Tropomyosin; D019209:Troponin C; D019210:Troponin I; D003545:Cysteine; D002118:Calcium