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Direct visualization of myosin filament symmetry in tarantula striated muscle by electron microscopy
R Padrón1, J R Guerrero, L Alamo
1Laboratorio de Biología Estructural, Instituto Venezolano de Investigaciones Cientificas (IVIC), Caracas.
Abstract:
Chemically demembranated bundles of fibers from tarantula leg muscle were rapidly frozen in the relaxed state and freeze-substituted in the presence of tannic acid. Electron micrographs of thin transverse sections of freeze-substituted specimens frequently showed four clear, regularly organized projections (crossbridges) protruding from the backbones of the myosin filaments and partially wrapping around the filament surface. The rotational power spectra of individual filaments showed a peak at N = 4. Alignment and averaging of the images using correlation methods confirmed the fourfold symmetry and the slewed configuration of the crossbridges on the filament surface. These observations directly reveal essential features of the low-resolution three-dimensional helical reconstruction of negatively stained tarantula filaments calculated previously (R. A. Crowther, R. Padrón, and R. Craig, 1985, J. Mol. Biol. 184, 429-439).
Insights
Tarantula muscle myosin filaments possess four regularly spaced crossbridges, revealed by freeze-substitution electron microscopy. This finding clarifies the structure of these muscle protein filaments.
Area of Science:
- Muscle physiology
- Biophysics
- Structural biology
Background:
- Myosin filaments form the backbone of muscle sarcomeres, crucial for contraction.
- Understanding myosin filament structure is key to elucidating muscle function.
- Previous low-resolution models of tarantula myosin filaments existed.
Purpose of the Study:
- To visualize and characterize the crossbridge structure on tarantula myosin filaments.
- To provide high-resolution structural details supporting previous models.
- To investigate the arrangement of myosin heads in relaxed muscle fibers.
Main Methods:
- Chemically demembranated tarantula leg muscle fibers were used.
- Rapid freezing and freeze-substitution fixation in the presence of tannic acid.
- Transmission electron microscopy of thin transverse sections.
- Analysis of rotational power spectra and image averaging using correlation methods.
Main Results:
- Electron micrographs revealed four regularly organized projections (crossbridges) on myosin filaments.
- Crossbridges were observed protruding from filament backbones and wrapping around the surface.
- Rotational power spectra showed a peak at N=4, indicating fourfold symmetry.
- Image averaging confirmed the fourfold symmetry and slewed configuration of crossbridges.
Conclusions:
- Directly visualized crossbridge structure confirms previous low-resolution models.
- The fourfold symmetry and arrangement of crossbridges are essential features of tarantula myosin filaments.
- This study provides direct structural evidence for myosin filament organization in relaxed muscle.