Ca2+-induced switching of troponin and tropomyosin on actin filaments as revealed by electron cryo-microscopy1 1Edited by A. Klug

• DOI: 10.1006/jmbi.2001.4598
• Publication Date: 2002-09-18T19:24:54Z
• Authors: Akihiro Narita; Takuo Yasunaga; Takashi Ishikawa; Kota Mayanagi; Takeyuki Wakabayashi
• Keywords: Models, Molecular; 0301 basic medicine; Binding Sites; Fourier Analysis; Protein Conformation; Cryoelectron Microscopy; Static Electricity; Tropomyosin; Myosins; Actins; Troponin; 03 medical and health sciences; X-Ray Diffraction; Image Processing, Computer-Assisted; Animals; Calcium; Rabbits; Muscle, Skeletal; Fluorescent Dyes; Muscle Contraction

Muscle contraction is regulated by the intracellular Ca(2+ )concentration. In vertebrate striated muscle, troponin and tropomyosin on actin filaments comprise a Ca(2+)-sensitive switch that controls contraction. Ca(2+ )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 Ca(2+ )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 approximately 35 A resolution. With Ca(2+), the globular domain of troponin was gourd-shaped and was located over the inner domain of actin. Without Ca(2+), the main body of troponin was shifted by approximately 30 A 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 approximately 35 A supporting the steric blocking model, however it is surprising that the N-terminal half of tropomyosin shifted less than approximately 12 A. Therefore tropomyosin shifted differentially without Ca(2+). With Ca(2+), 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.