A Proposed Mechanism for the Initial Myosin Binding Event on the Cardiac Thin Filament: A Metadynamics Study

Area of Science:

• Muscle physiology
• Biophysics
• Molecular dynamics

Background:

• Tropomyosin movement over actin filaments regulates cardiac muscle cross-bridge cycling by controlling myosin binding sites.
• Tropomyosin exists in three states: blocked (no myosin interaction), closed (weak binding), and open (strong binding).
• Experimental data on how myosin binding influences tropomyosin's position on actin is limited.

Purpose of the Study:

• To investigate the mechanism of myosin binding to the cardiac muscle thin filament.
• To understand how myosin binding influences tropomyosin's positional transitions on actin.
• To identify preferential myosin binding sites and the mechanism of further myosin head recruitment.

Main Methods:

• Utilized molecular dynamics simulations, specifically metadynamics.
• Mimicked the effect of single myosin head binding to the thin filament.
• Calculated the work required to transition tropomyosin segments toward the 'open' position in different regions.

Main Results:

• Found variations in the work required to move tropomyosin, indicating distinct binding preferences.
• Identified the influence of cardiac troponin T in creating these preferential binding sites.
• Elucidated the mechanism by which additional myosin heads are recruited to the thin filament.

Conclusions:

• Myosin head binding to the thin filament is not uniform and is influenced by tropomyosin's interaction with actin and troponin T.
• Cardiac troponin T plays a crucial role in directing myosin binding.
• The study provides a mechanistic understanding of cooperative myosin binding in cardiac muscle contraction.