Ca2+-induced switching of troponin and tropomyosin on actin filaments as revealed by electron cryo-microscopy

Akihiro Narita, Takuo Yasunaga, Takashi Ishikawa, Kota Mayanagi, Takeyuki Wakabayashi

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Abstract

Muscle contraction is regulated by the intracellular Ca2+concentration. In vertebrate striated muscle, troponin and tropomyosin on actin filaments comprise a Ca2+-sensitive switch that controls contraction. Ca2+binds to troponin and triggers a series of changes in actin-containing filaments that lead to cyclic interactions with myosin that generate contraction. However, the precise location of troponin relative to actin and tropomyosin and how its structure changes with Ca2+have been not determined. To understand the regulatory mechanism, we visualized the location of troponin by determining the three-dimensional structure of thin filaments from electron cryo-micrographs without imposing helical symmetry to ∼35 Å resolution. With Ca2+the globular domain of troponin was gourd-shaped and was located over the inner domain of actin. Without Ca2+the main body of troponin was shifted by ∼30 Å towards the outer domain and bifurcated, with a horizontal branch (troponin arm) covering the N and C-terminal regions of actin. The C-terminal one-third of tropomyosin shifted towards the outer domain of actin by ∼35 Å supporting the steric blocking model, however it is surprising that the N-terminal half of tropomyosin shifted less than ∼12 Å. Therefore tropomyosin shifted differentially without Ca2+With Ca2+tropomyosin was located entirely over the inner domain thereby allowing greater access of myosin for force generation. The interpretation of three-dimensional maps was facilitated by determining the three-dimensional positions of fluorophores labelled on specific sites of troponin or tropomyosin by applying probabilistic distance geometry to data from fluorescence resonance energy transfer measurements.

Original languageEnglish
Pages (from-to)241-261
Number of pages21
JournalJournal of Molecular Biology
Volume308
Issue number2
DOIs
Publication statusPublished - Apr 27 2001

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All Science Journal Classification (ASJC) codes

  • Structural Biology
  • Molecular Biology

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