Enhancement of cargo processivity by cooperating molecular motors.

Cellular cargo can be bound to cytoskeletal filaments by one or more active or passive molecular motors. Recent experiments have shown that the presence of auxiliary, nondriving motors results in an enhanced processivity of the cargo, compared to the case of a single active driving motor alone. We model the observed cooperative transport process using a stochastic model that describes the dynamics of two molecular motors, an active one that moves cargo unidirectionally along a filament track, and a passive one that acts as a tether. Analytical expressions obtained from our analysis are fit to experimental data to estimate the microscopic kinetic parameters of our model. Our analysis reveals two qualitatively distinct processivity-enhancing mechanisms: the passive tether can decrease the typical detachment rate of the active motor from the filament track or it can increase the corresponding reattachment rate. Comparing analytical results with experimental data, we can show unambiguously that in the case of kinesin transport on microtubules, a higher average run length arises mainly from the ability of the passive motor to keep the cargo close to the filament, enhancing the reattachment rate of recently detached active kinesin motors. On the other hand, in the case of myosin-driven transport along actin, the passive motor tightly tethers the cargo to the filament, suppressing the detachment rate of the active myosin.