On the use of in vivo cargo velocity as a biophysical marker.

Molecular motors move many intracellular cargos along microtubules. Recently, it has been hypothesized that in vivo cargo velocity can be used to determine the number of engaged motors. We use theoretical and experimental approaches to investigate these assertions, and find that this hypothesis is inconsistent with previously described motor behavior, surveyed and re-analyzed in this paper. Studying lipid droplet motion in Drosophila embryos, we compare transport in a mutant, Delta(halo), with that in wild-type embryos. The minus-end moving cargos in the mutant appear to be driven by more motors (based on in vivo stall force observations). Periods of minus-end motion are indeed longer than in wild-type embryos but the corresponding velocities are not higher. We conclude that velocity is not a definitive read-out of the number of motors propelling a cargo.