Consequences of motor copy number on the intracellular transport of kinesin-1-driven lipid droplets.

SUMMARY The microtubule motor kinesin-1 plays central roles in intracellular transport. It has been widely assumed that many cellular cargos are moved by multiple kinesins and that cargos with more motors move faster andforlongerdistances;concreteevidence,however, is sparse. Here we rigorously test these notions using lipid droplets in Drosophila embryos. We first employ antibody inhibition, genetics, biochemistry, and particle tracking to demonstrate that kinesin-1 mediates plus-enddropletmotion.Wethenmeasurehowvariation in kinesin-1 expression affects the forces driving individual droplets and estimate the number of kinesins actively engaged per droplet. Unlike in vitro, increased motor number results in neither longer travel distances nor higher velocities. Our data suggest that cargos in vivo can simultaneously engage multiple kinesinsandthattransportpropertiesarelargelyunaffected by variation in motor number. Apparently, higher-order regulatory mechanisms rather than motor number per se dominate cargo transport in vivo.