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

Lehman, William and Hatch, Victoria and Korman, Vicci and Rosol, Michael and Thomas, Lorie and Maytum, Robin and Geeves, Michael A. and van Eyk, Jennifer E. and Tobacman, Larry S. and Craig, Roger (2000) Tropomyosin and actin isoforms modulate the localization of tropomyosin strands on actin filaments. Journal of Molecular Biology, 302 (3). pp. 593-606. ISSN 0022-2836. (The full text of this publication is not currently available from this repository. You may be able to access a copy if URLs are provided)

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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 nonmuscle 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.

Item Type: Article
Uncontrolled keywords: actin; electron microscopy; muscle regulation; tropomyosin; troponin
Subjects: Q Science
R Medicine
Divisions: Faculties > Science Technology and Medical Studies > School of Biosciences
Depositing User: O.O. Odanye
Date Deposited: 02 Apr 2009 12:02
Last Modified: 23 May 2014 15:34
Resource URI: (The current URI for this page, for reference purposes)
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