Structural changes induced in Ca2+-regulated myosin filaments by Ca2+ and ATP

Area of Science:

• Muscle physiology
• Biochemistry
• Structural biology

Background:

• Myosin-linked regulation is crucial for muscle contraction.
• Understanding the structural basis of this regulation in scallop striated muscle is essential.

Purpose of the Study:

• To investigate the structural basis of myosin-linked regulation in synthetic scallop striated muscle myosin filaments.
• To determine how ATP and Ca2+ influence myosin filament structure and proteolytic susceptibility.

Main Methods:

• Electron microscopy (negative staining) was used to visualize myosin filament structure.
• Proteolytic susceptibility assays with papain probed the head-rod junction structure.
• Experiments were conducted under various nucleotide and Ca2+ conditions.

Main Results:

• Myosin head-rod junction susceptibility to papain increased significantly under activating (ATP/high Ca2+) or rigor (no ATP) conditions compared to relaxing conditions (ATP/low Ca2+).
• Synthetic filaments exhibited a compact, ordered cross-bridge structure in the relaxed state, which became disordered and projected further from the backbone under activating, rigor, or non-hydrolyzable nucleotide conditions.
• These structural changes were consistent between synthetic and native scallop myosin filaments.

Conclusions:

• Calcium and ATP influence the binding affinity of myosin cross-bridges to the filament backbone or to each other.
• Observed structural changes are intrinsic properties of myosin molecules, not artifacts of negative staining.
• An ordered filament structure is maintained only in the relaxed state, dependent on hydrolyzed ATP and the absence of calcium.