Skeletal myosin binding protein-C isoforms regulate thin filament activity in a Ca²⁺-dependent manner

Area of Science:

• Muscle physiology
• Molecular biology
• Biophysics

Background:

• Muscle contraction relies on myosin and actin filament interactions.
• Myosin binding protein-C (MyBP-C) isoforms (slow-skeletal, fast-skeletal, cardiac) modulate these interactions.
• Skeletal MyBP-C isoforms are found in cardiac muscle, but their function is unclear.

Purpose of the Study:

• To investigate the functional differences between MyBP-C isoforms.
• To elucidate the regulatory mechanisms of MyBP-C in muscle contraction.
• To characterize the impact of N-terminal MyBP-C fragments on contractile properties.

Main Methods:

• Expression of recombinant N-terminal MyBP-C fragments.
• In vitro motility assays to measure thin filament sliding.
• 3D electron microscopy for structural analysis.
• Assessment of force production in cardiac muscle fibers.

Main Results:

• ssMyBP-C and cMyBP-C activated thin filament sliding at low Ca²⁺.
• Tropomyosin shifts on actin explain activation, with cardiac > slow-skeletal > fast-skeletal.
• fsMyBP-C and cMyBP-C reduced sliding velocity at high Ca²⁺ and increased cardiac muscle force.

Conclusions:

• Cardiac MyBP-C may have dual roles in cardiac muscle due to high Ca²⁺ cycling.
• Skeletal MyBP-C isoforms are likely adapted for specific skeletal muscle functions.
• MyBP-C isoforms differentially regulate muscle contractility based on calcium levels.