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

Recent studies provide strong evidence that single myosin class V molecules transport vesicles and organelles processively along F- actin, taking several 36- nm steps, ' hand over hand', for each diffusional encounter. The mechanisms regulating myosin- V's processivity remain unknown. Here, we have used an optical- tweezers- based transducer to measure the effect of load on the mechanical interactions between rabbit skeletal F- actin and a single head of mouse brain myosin- V, which produces its working stroke in two phases. We found that the lifetimes of the first phase of the working stroke changed exponentially and about 10-fold over a range of pushing and pulling forces of +/- 1.5 pN. Stiffness measurements suggest that intramolecular forces could approach 3.6 pN when both heads are bound to F- actin, in which case extrapolation would predict the detachment kinetics of the front head to slow down 50- fold and the kinetics of the rear head to accelerate respectively. This synchronizing effect on the chemo- mechanical cycles of the heads increases the probability of the trail head detaching first and causes a strong increase in the number of forward steps per diffusional encounter over a system with no strain dependence.