Tropomyosin and actin isoforms modulate the localization of tropomyosin strands on actin filaments

William Lehman, Victoria Hatch, Vicci Korman, Michael Rosol, Lorie Thomas, Robin Maytum, Michael A. Geeves, Jennifer E. Van Eyk, Larry S. Tobacman, Roger Craig

Research output: Contribution to journalArticle


Tropomyosin is present in virtually all eucaryotic cells, where it functions to modulate actin-myosin interaction and to stabilize actin filament structure. In striated muscle, tropomyosin regulates contractility by sterically blocking myosin-binding sites on actin in the relaxed state. On activation, tropomyosin moves away from these sites in two steps, one induced by Ca2+ binding to troponin and a second by the binding of myosin to actin. In smooth muscle and non-muscle cells, where troponin is absent, the precise role and structural dynamics of tropomyosin on actin are poorly understood. Here, the location of tropomyosin on F-actin filaments free of troponin and other actin-binding proteins was determined to better understand the structural basis of its functioning in muscle and non-muscle cells. Using electron microscopy and three-dimensional image reconstruction, the association of a diverse set of wild-type and mutant actin and tropomyosin isoforms, from both muscle and non-muscle sources, was investigated. Tropomyosin position on actin appeared to be defined by two sets of binding interactions and tropomyosin localized on either the inner or the outer domain of actin, depending on the specific actin or tropomyosin isoform examined. Since these equilibrium positions depended on minor amino acid sequence differences among isoforms, we conclude that the energy barrier between thin filament states is small. Our results imply that, in striated muscles, troponin and myosin serve to stabilize tropomyosin in inhibitory and activating states, respectively. In addition, they are consistent with tropomyosin-dependent cooperative switching on and off of actomyosin-based motility. Finally, the locations of tropomyosin that we have determined suggest the possibility of significant competition between tropomyosin and other cellular actin-binding proteins. Based on these results, we present a general framework for tropomyosin modulation of motility and cytoskeletal modelling. (C) 2000 Academic Press.

Original languageEnglish (US)
Pages (from-to)593-606
Number of pages14
JournalJournal of molecular biology
Issue number3
StatePublished - Sep 22 2000


  • Actin
  • Electron microscopy
  • Muscle regulation
  • Tropomyosin
  • Troponin

ASJC Scopus subject areas

  • Structural Biology
  • Molecular Biology

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    Lehman, W., Hatch, V., Korman, V., Rosol, M., Thomas, L., Maytum, R., Geeves, M. A., Van Eyk, J. E., Tobacman, L. S., & Craig, R. (2000). Tropomyosin and actin isoforms modulate the localization of tropomyosin strands on actin filaments. Journal of molecular biology, 302(3), 593-606.