Relation between crossbridge structure and actomyosin ATPase activity in rat heart

Area of Science:

• Cardiac muscle physiology
• Molecular biology
• Protein structure and function

Background:

• Cardiac myofilaments regulate muscle contraction through actin-myosin interactions.
• Myosin binding protein C (C protein) is a key component of thick filaments involved in regulating contraction.
• C protein possesses multiple phosphorylation sites, influencing its function.

Purpose of the Study:

• To investigate the structural impact of C protein phosphorylation on cardiac thick filaments.
• To examine the influence of alpha- and beta-myosin heavy chain (MHC) isoforms on thick filament structure.
• To correlate crossbridge flexibility with actomyosin ATPase activity.

Main Methods:

• Isolation of cardiac thick filaments from rat ventricles expressing alpha- or beta-MHC.
• Electron microscopy and optical diffraction to analyze filament structure.
• Specific phosphorylation of C protein using protein kinase A (PKA).

Main Results:

• In alpha-MHC filaments, PKA-mediated C protein phosphorylation extended crossbridges, altered orientation, increased order, and reduced flexibility.
• Crossbridges in beta-MHC filaments were less ordered and more flexible.
• C protein phosphorylation did not affect crossbridge structure or flexibility in beta-MHC filaments.
• Crossbridge flexibility correlated with actomyosin ATPase hydrolysis rates.

Conclusions:

• Crossbridge flexibility is a critical determinant of crossbridge cycling rates.
• C protein phosphorylation modulates crossbridge position and flexibility, thereby regulating actomyosin ATPase activity.
• This mechanism highlights C protein's role in controlling cardiac contractility.