The structure of the amino terminus of tropomyosin is critical for binding to actin in the absence and presence of troponin

R. W. Heald, Sarah Hitchcock

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Analysis of two recombinant variants of chicken striated muscle α-tropomyosin has shown that the structure of the amino terminus is crucial for most aspects of tropomyosin function: affinity to actin, promotion of binding to actin by troponin, and regulation of the actomyosin MgATPase. Initial characterization of variants expressed and isolated from Escherichia coli has been published (Hitchcock-DeGregori, S. E., and Heald, R. W. (1987) J. Biol. Chem. 262, 9730-9735). Fusion tropomyosin contains 80 amino acids of a nonstructural influenza virus protein (NS1) on the amino terminus. Nonfusion tropomyosin is a variant because the amino-terminal methionine is not acetylated (unacetylated tropomyosin). The affinity of tropomyosin labeled at Cys190 with N-[14C]ethylmaleimide for actin was measured by cosedimentation in a Beckman Airfuge. Fusion tropomyosin binds to actin with an affinity slightly greater than that of chicken striated muscle α-tropomyosin (K(app) = 1-2 x 107 versus 0.5-1 x 107 M-1) and more strongly than unacetylated tropomyosin (K(app) = 3 x 105 M-1). Both variants bind cooperatively to actin. Troponin increases the affinity of unacetylated tropomyosin for actin (+Ca2+, K(app) = 6 x 106 M-1; +EGTA, K(app) = 2 x 107 M-1), but the affinity is still lower than that of muscle tropomyosin for actin in the presence of troponin (K(app) >> 108 M-1). Troponin has no effect on the affinity of fusion tropomyosin for actin indicating that binding of troponin T to the overlap region of the adjacent tropomyosin, presumably sterically prevented by the fusion peptide in fusion tropomyosin, is required for troponin to promote the binding of tropomyosin to actin. The role of troponin T in regulation and the mechanism of cooperative binding of tropomyosin to actin have been discussed in relation to this work.

Original languageEnglish (US)
Pages (from-to)5254-5259
Number of pages6
JournalJournal of Biological Chemistry
Issue number11
StatePublished - Jan 1 1988

All Science Journal Classification (ASJC) codes

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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