Structural changes that occur in scallop myosin filaments upon activation.

Area of Science:

• Muscle physiology
• Biochemistry
• Structural biology

Background:

• Myosin filaments form the thick filaments in muscle, responsible for generating force during contraction.
• The structure of myosin filaments, particularly the arrangement of myosin heads, is thought to regulate muscle activity.
• Calcium ions are known to play a critical role in initiating muscle contraction.

Purpose of the Study:

• To investigate the structural changes in scallop striated muscle myosin filaments upon calcium-induced activation.
• To understand the relationship between myosin filament structure and muscle activation at a molecular level.

Main Methods:

• Isolation of myosin filaments from scallop striated muscle.
• Activation of filaments using calcium-containing solutions.
• Structural analysis using electron microscopy with negative staining.

Main Results:

• Calcium activation caused a loss of the ordered helical arrangement of myosin projections.
• Myosin heads moved outwards, and filament edges became rough, indicating structural disorder.
• The transition from ordered to disordered structure was reversible and occurred at a specific calcium concentration (pCa 5.7).
• Removal of nucleotide or regulatory light chains also disrupted the ordered structure.

Conclusions:

• The ordered helical arrangement of myosin projections in relaxed muscle is maintained by intermolecular interactions involving regulatory light chains.
• Calcium binding to light chains alters these interactions, activating the myosin molecule and propagating this change throughout the thick filament.
• This structural transition is a key step in the calcium-dependent activation of muscle contraction.