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Relaxed and active thin filament structures; a new structural basis for the regulatory mechanism

Research output: Contribution to journalArticle

Original languageEnglish
Pages (from-to)365-371
Number of pages7
JournalJournal of Structural Biology
Issue number3
Early online date1 Feb 2017
DateAccepted/In press - 23 Jan 2017
DateE-pub ahead of print - 1 Feb 2017
DatePublished (current) - Mar 2017


The structures of muscle thin filaments reconstituted using skeletal actin and cardiac troponin and tropomyosin have been determined with and without bound Ca2+ using electron microscopy and reference-free single particle analysis. The resulting density maps have been fitted with atomic models of actin, tropomyosin and troponin showing that: (i) the polarity of the troponin complex is consistent with our 2009 findings, with large shape changes in troponin between the two states; (ii) without Ca2+ the tropomyosin pseudo-repeats all lie at almost equivalent positions in the ‘blocked’ position on actin (over subdomains 1 and 2); (iii) in the active state the tropomyosin pseudo-repeats are all displaced towards subdomains 3 and 4 of actin, but the extent of displacement varies within the regulatory unit depending upon the axial location of the pseudo-repeats with respect to troponin. Individual pseudo-repeats with Ca2+ bound to troponin can be assigned either to the ‘closed’ state, a partly activated conformation, or the ‘M-state’, a fully activated conformation which has previously been thought to occur only when myosin heads bind. These results lead to a modified view of the steric blocking model of thin filament regulation in which cooperative activation is governed by troponin-mediated local interactions of the pseudo-repeats of tropomyosin with actin.

    Research areas

  • Actin, Tropomyosin, Troponin, Thin filament, Regulation

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