Structural changes in isometrically contracting insect flight muscle trapped following a mechanical perturbation

Area of Science:

• Muscle physiology
• Biophysics
• Structural biology

Background:

• Muscle contraction relies on myosin cross-bridge interactions with actin filaments.
• Understanding these interactions at a molecular level is key to muscle function.
• Previous studies used length steps to synchronize cross-bridge responses.

Purpose of the Study:

• To investigate the 3-D structure of active myosin cross-bridges in insect flight muscle (IFM) after mechanical perturbations.
• To determine the binding states (weak or strong) of individual cross-bridges to actin.
• To correlate structural changes with mechanical transients during muscle contraction.

Main Methods:

• Applying rapid length steps (0.5% change) to isometrically contracting IFM fibers.
• Rapid freezing of muscle fibers within milliseconds after length perturbation.
• Utilizing electron tomography (ET) to obtain 3-D cross-bridge structures.
• Employing multivariate data analysis and quasi-atomic modeling to classify and analyze cross-bridge structures.

Main Results:

• ~40 distinct classes of cross-bridge structures were identified.
• Strong-binding acto-myosin attachments were localized to specific actin subunits within "target zones".
• A significant decrease in a class of weak-binding cross-bridges, identified as precursors to strong-binding states, was observed after length perturbations.

Conclusions:

• Muscle contraction involves specific binding sites for strong acto-myosin attachments.
• The observed changes in weak-binding cross-bridges support their role as precursors to strong-binding states.
• The findings provide a detailed model for muscle contraction in IFM, potentially applicable to other muscle types.