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Tropomodulin caps the pointed ends of actin filaments

Academic Article
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Overview

authors

  • Weber, A.
  • Pennise, C. R.
  • Babcock, G. G.
  • Fowler, Velia

publication date

  • December 1994

journal

  • Journal of Cell Biology  Journal

abstract

  • Many proteins have been shown to cap the fast growing (barbed) ends of actin filaments, but none have been shown to block elongation and depolymerization at the slow growing (pointed) filament ends. Tropomodulin is a tropomyosin-binding protein originally isolated from red blood cells that has been localized by immunofluorescence staining to a site at or near the pointed ends of skeletal muscle thin filaments (Fowler, V. M., M. A., Sussman, P. G. Miller, B. E. Flucher, and M. P. Daniels. 1993. J. Cell Biol. 120: 411-420). Our experiments demonstrate that tropomodulin in conjunction with tropomyosin is a pointed end capping protein: it completely blocks both elongation and depolymerization at the pointed ends of tropomyosin-containing actin filaments in concentrations stoichiometric to the concentration of filament ends (Kd < or = 1 nM). In the absence of tropomyosin, tropomodulin acts as a "leaky" cap, partially inhibiting elongation and depolymerization at the pointed filament ends (Kd for inhibition of elongation = 0.1-0.4 microM). Thus, tropomodulin can bind directly to actin at the pointed filament end. Tropomodulin also doubles the critical concentration at the pointed ends of pure actin filaments without affecting either the rate of extent of polymerization at the barbed filament ends, indicating that tropomodulin does not sequester actin monomers. Our experiments provide direct biochemical evidence that tropomodulin binds to both the terminal tropomyosin and actin molecules at the pointed filament end, and is the long sought-after pointed end capping protein. We propose that tropomodulin plays a role in maintaining the narrow length distributions of the stable, tropomyosin-containing actin filaments in striated muscle and in red blood cells.

subject areas

  • Actin Cytoskeleton
  • Actins
  • Animals
  • Carrier Proteins
  • Erythrocytes
  • Gelsolin
  • Microfilament Proteins
  • Muscle, Skeletal
  • Polymers
  • Tropomodulin
  • Tropomyosin
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Identity

PubMed Central ID

  • PMC2120308

International Standard Serial Number (ISSN)

  • 0021-9525

Digital Object Identifier (DOI)

  • 10.1083/jcb.127.6.1627

PubMed ID

  • 7798317
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Additional Document Info

start page

  • 1627

end page

  • 1635

volume

  • 127

issue

  • 6

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