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Visualizing myosin’s power stroke in muscle contraction

Mary C. Reedy

Department of Cell Biology, Duke University Medical Center, Durham, NC 27710, 919 684 5674, USA

Freeze-frames of myosin’s power stroke visualized in 3D tomograms of crossbridges in contracting muscle

Edited by Gary Connors from data of Taylor et al. (1999) Cell 99, 421-431.

Power stroke movie showing a longitudinal view of acto-S1 that rotates ~180° about the actin filament axis

The power stroke of myosin crossbridges bound to actin is compiled from 26 S1s rebuilt to fit active crossbridges quick frozen during muscle contraction and samples crossbridges in the 3D tomograms that represented forms from the top to the bottom of the power stroke. The movie begins in the standard longitudinal view used in the tomograms (and other 3D reconstructions) with Z-band towards the bottom, and shows the axial tilt of the crossbridge from ~100° (pre-stroke) to close to ~45° (end).

As the acto-S1 rotates, the movements of the catalytic and LCD domains, particularly in the azimuthal direction, are striking. The catalytic domain moves independently of the LCD at higher angles and shows significant changes in azimuthal position that decrease as the overall angle of the S1 decreases toward the rigor end of the stroke. The LCD shows extensive azimuthal and axial motions. The total stroke distance due to the tilting of the entire crossbridge at the end of the LCD is ~12 nm; the stroke distance due to tilting of the LCD after the catalytic domain stabilizes at the rigor position is ~5 nm.

The color scheme shows the myosin catalytic domain and the heavy chain extending through the LCD in red, the essential light chain in purple and the regulatory light chain in cyan. Actin is green and cyan; tropomyosin is gold.

Power stroke movie in transverse view looking towards the Z-line

A transverse view of the power stroke assembled from 26 rebuilt S1s shows the azimuthal movements of the myosin head. The catalytic domain at the beginning of the stroke is attached further away from tropomyosin (golden ‘hat’) than its position virtually overlapping tropomyosin at the end of the stroke. The color scheme shows the myosin catalytic domain and the heavy chain extending through the LCD in red, the essential light chain in purple and the regulatory light chain in cyan. Actin is green and cyan; tropomyosin is gold.

HST comet movie

The motions of the myosin head during a power stroke in the guise of a whirling dervish. The ‘comet-like’ blur displays all 26 rebuilt S1s superimposed on one actin monomer in longitudinal view and rotates the display 180º around the actin axis. The catalytic domain and the heavy chain through the LCD are red, the essential light chain is purple and the regulatory light chain is cyan. Actin is green and cyan and tropomyosin is yellow.

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