Calcium-dependent movement of troponin I between troponin C and actin as revealed by spin-labeling EPR |
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Authors: | Aihara Tomoki Ueki Shoji Nakamura Motoyoshi Arata Toshiaki |
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Institution: | Department of Biological Sciences, Graduate School of Science, Osaka University and CREST/JST, Toyonaka, Osaka 560-0043, Japan. |
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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. |
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Keywords: | Muscle regulation Actin Troponin Tropomyosin Spin labeling EPR |
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