Load-dependent kinetics of myosin-V can explain its high processivity

Area of Science:

• Molecular motor mechanics
• Cellular transport mechanisms
• Biophysics of cytoskeletal proteins

Background:

• Myosin V (M5) is a motor protein responsible for processive transport of vesicles and organelles along actin filaments.
• The precise mechanisms governing M5 processivity, particularly the role of load, remain largely unelucidated.
• Understanding M5 function is crucial for comprehending intracellular trafficking dynamics.

Purpose of the Study:

• To investigate the impact of external load on the mechanical interactions of single myosin V heads with F-actin.
• To elucidate the load-dependent kinetics and working stroke phases of myosin V.
• To determine how intramolecular forces influence myosin V's processive movement.

Main Methods:

• Utilized an optical-tweezers-based transducer to precisely measure forces and displacements.
• Examined the mechanical interactions between rabbit skeletal F-actin and a single head of mouse brain myosin V.
• Analyzed the two-phase working stroke of myosin V under varying loads (+/- 1.5 pN).

Main Results:

• The lifetime of the first phase of the myosin V working stroke exhibited an exponential dependence on load, changing ~10-fold.
• Stiffness measurements indicated potential intramolecular forces up to 3.6 pN when both myosin heads bind F-actin.
• Calculations suggested load-induced alterations in head detachment kinetics, favoring the rear head detaching first.

Conclusions:

• Load significantly modulates the chemo-mechanical cycles of myosin V's heads, impacting its processivity.
• Intramolecular forces and load-dependent kinetics contribute to myosin V's efficient hand-over-hand movement along actin.
• This study provides critical insights into the mechanical regulation of myosin V-based transport.