Molecular Motors: A Tale of Two Filaments

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.

Myosin V from head to tail

Myosin V (myoV), a processive cargo transporter, has arguably been the most well-studied unconventional myosin of the past decade. Considerable structural information is available for the motor domain, the IQ motifs with bound calmodulin or light chains, and the cargo-binding globular tail, all of which have been crystallized. The repertoire of adapter proteins that link myoV to a particular cargo is becoming better understood, enabling cellular transport processes to be dissected. MyoV is processive, meaning that it takes many steps on actin filaments without dissociating. Its extended lever arm results in long 36-nm steps, making it ideal for single molecule studies of processive movement. In addition, electron microscopy revealed the structure of the inactive, folded conformation of myoV when it is not transporting cargo. This review provides a background on myoV, and highlights recent discoveries that show why myoV will continue to be an active focus of investigation.

Cargo Transport: Two Motors Are Sometimes Better Than One

Molecular motor proteins are crucial for the proper distribution of organelles and vesicles in cells. Much of our current understanding of how motors function stems from studies of single motors moving cargos in vitro. More recently, however, there has been mounting evidence that the cooperation of multiple motors in moving cargos and the regulation of motor-filament affinity could be key mechanisms that cells utilize to regulate cargo transport. Here, we review these recent advances and present a picture of how the different mechanisms of regulating the number of motors moving a cargo could facilitate cellular functions.