Slepchenko et al. report the first measurements of how often cargos jump from one type of fiber to the other.
In the cell's transportation system, microtubules are highways for long-distance journeys, whereas actin fibers are local roads for short trips. Cargos can switch fibers, but researchers haven't been able to observe these transfers because so many filaments crisscross the cytoplasm.
To determine how often switching occurs, Slepchenko et al. combined measurements of organelle movement in fish melanophores, or pigment cells, with mathematical models. The researchers tracked pigment granules traveling toward either the cell center or the periphery on actin and microtubules, including in cells with only one filament type. The team used the data to devise models that describe movement on each kind of cellular road. They then merged the models and applied them to cells with both types of filaments to estimate how often cargos transfer.
On the outward trip, granules were likely to jump from microtubules to actin but rarely did the opposite. As granules moved away from the cell's edge, however, they usually diverted from actin onto microtubules. The researchers concluded that cells dictate which track a cargo takes by changing only the rate of switching from actin to microtubules. This value is 104 times higher for the inbound trip, meaning relatively more cargos make it to microtubules. Cells might draw cargos onto microtubules by boosting the activity of dynein motors, which haul organelles toward the cell center, or by modifying the filaments to make them more available to motors.