The motors that drive intracellular transport are increasingly
well understood, but the mechanisms that allow cells to
deploy them in a regulated manner remain a mystery. Lipid-droplet
transport in Drosophila embryos provides a unique
model system to unravel these control mechanisms because
it is amenable to genetic, biochemical, and biophysical
analysis.
Droplets move bi-directionally along microtubuies, employing
opposite- polarity motors in quick succession. To investigate
how multiple motors work together, we impaired transport
in one direction genetically and determined the effect
on motion in the opposite direction. Our data suggest
that in the wild type the motors driving transport are
not simply engaged in a tug-of-war, but that their activities
are coordinated.
Despite the constant back-and-forth motion of individual
droplets, the population as a whole displays uni-directional
transport. In a genetic screen for factors that determine
net transport directionality, we isolated mutations in
the regulator klar and in several new loci. The
fact that Klarsicht is also important for nuclear migration
during eye development suggests that principles of motor
regulation discovered in the lipid droplet system are
likely important for intracellular transport in general.
