Slow and fast grouping of cargo velocities in axonal transport due to single versus multi-motor transport.

We have recently been exploring the idea that axonal transport velocity is "track and motor limited." That is, microtubule length as well as microtubule-associated obstructions interact with the number of motors attached to a specific cargo to determine average cargo velocities. We assert that "slow" and "fast" transport as they are commonly referred to in the literature are really single- versus multi-motor transport along interrupted and obstructed track. To this end, we have recently developed a cargo-level motor model that appears to readily reproduce fast and slow transport simply by altering the number of motors. In the work presented here, we explore the ramifications of this model across a wide range of cargo sizes and motor-motor interactions. We find that categorization of cargo transport into "slow" and "fast" might be a natural consequence of track and motor limited transport as cargo load versus average velocity distribution produced by this model are clearly bi-modal with a curved (roughly square-root) relationship between number of motors and cargo load being the best at reproducing experimental data.