Force-dependent stepping kinetics of myosin-V

Area of Science:

• Biophysics
• Cell Biology
• Molecular Motors

Background:

• Myosin-V is a crucial actin-based motor protein for intracellular transport.
• Understanding myosin-V's behavior under load is key to elucidating its function and inter-head communication.
• Its frequent colocalization with stronger motors like kinesins necessitates studying myosin-V under high forces.

Purpose of the Study:

• To investigate myosin-V's mechanical properties and chemomechanical cycle under controlled external loads.
• To determine how myosin-V behaves under both forward and backward forces, including superstall conditions.
• To compare myosin-V's force response to that of other motor proteins like kinesins.

Main Methods:

• Utilized optical tweezers with long-range force feedback for precise force control.
• Studied myosin-V motion under a wide range of applied forces (5 pN forward to 1.5 pN backward).
• Analyzed myosin-V's step size, run length, and chemomechanical cycle transitions in response to force.

Main Results:

• Mean step size remained constant at ~36 nm across tested forces.
• Identified two force-dependent transitions in the chemomechanical cycle.
• Myosin-V's run length was largely independent of force, unlike kinesin motors.
• Observed continuous backward stepping at 5 pN superstall force, indicating power stroke reversal is possible.

Conclusions:

• Myosin-V exhibits remarkable force tolerance and a unique response to load.
• Force-dependent transitions suggest distinct rate-limiting steps in its cycle, including leading head diffusion.
• Myosin-V's ability to reverse its power stroke under extreme forces has significant implications for intracellular transport dynamics.