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Tropomodulin: a cytoskeletal protein that binds to the end of erythrocyte tropomyosin and inhibits tropomyosin binding to actin

Academic Article
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Overview

authors

  • Fowler, Velia

publication date

  • August 1990

journal

  • Journal of Cell Biology  Journal

abstract

  • Human erythrocytes contain a Mr 43,000 tropomyosin-binding protein that is unrelated to actin and that has been proposed to play a role in modulating the association of tropomyosin with spectrin-actin complexes based on its stoichiometry in the membrane skeleton of one Mr 43,000 monomer per short actin filament (Fowler, V. M. 1987. J. Biol. Chem. 262:12792-12800). Here, we describe an improved procedure to purify milligram quantities to 98% homogeneity and we show that this protein inhibits tropomyosin binding to actin by a novel mechanism. We have named this protein tropomodulin. Unlike other proteins that inhibit tropomyosin-actin interactions, tropomodulin itself does not bind to F-actin. EM of rotary-shadowed tropomodulin-tropomyosin complexes reveal that tropomodulin (14.5 +/- 2.4 nm [SD] in diameter) binds to one of the ends of the rod-like tropomyosin molecules (33 nm long). In agreement with this observation, Dixon plots of inhibition curves demonstrate that tropomodulin is a non-competitive inhibitor of tropomyosin binding to F-actin (Ki = 0.7 microM). Hill plots of the binding of the tropomodulin-tropomyosin complex to actin indicate that binding does not exhibit any positive cooperativity (n = 0.9), in contrast to tropomyosin (n = 1.9), and that the apparent affinity of the complex for actin is reduced 20-fold with respect to that of tropomyosin. These results suggest that binding of tropomodulin to tropomyosin may block the ability of tropomyosin to self-associate in a head-to-tail fashion along the actin filament, thereby weakening its binding to actin. Antibodies to tropomodulin cross-react strongly with striated muscle troponin I (but not with troponin T) as well as with a nontroponin Mr 43,000 polypeptide in muscle and in other nonerythroid cells and tissues, including brain, lens, neutrophils, and endothelial cells. Thus, erythrocyte tropomodulin may be one member of a family of tropomyosin-binding proteins that function to regulate tropomyosin-actin interactions in non-muscle cells and tissues.

subject areas

  • Actins
  • Amino Acids
  • Animals
  • Cytoskeletal Proteins
  • Electrophoresis, Polyacrylamide Gel
  • Erythrocyte Membrane
  • Humans
  • Kinetics
  • Membrane Proteins
  • Microscopy, Electron
  • Muscles
  • Protein Binding
  • Rabbits
  • Tropomyosin
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Identity

PubMed Central ID

  • PMC2116216

International Standard Serial Number (ISSN)

  • 0021-9525

Digital Object Identifier (DOI)

  • 10.1083/jcb.111.2.471

PubMed ID

  • 2380244
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Additional Document Info

start page

  • 471

end page

  • 482

volume

  • 111

issue

  • 2

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