Microtubule-based molecular motors are responsible for the long range transport of many intracellular cargoes. Even though these cargoes are often targeted to a particular destination in the cell, they exhibit bidirectional motion at short time scales. The motors driving this transport in opposite directions are kinesins and cytoplasmic dynein each of which is unidirectional. It is still unclear how the seemingly random bidirectional motion inside the cells is regulated to produce the effective unidirectional transport required by the organism. As shown in the schematic, two main models have been proposed: coordination via a tug-of-war, the dynamics of which depend on the kinetic rates and stall forces of the opposing motors; and coordination via non-motor regulatory proteins.

To elucidate the mechanisms used by the cells to regulate transport we reconstitute in vitro transport of endogenous cargos purified from Drosophila embryos ( see our studies in vivo ). The movie shows a lipid droplet purified from the embryos moving along a microtubule on a glass slide. The lipid droplet is being moved bidirectionally by endogenous molecular motors (do not confuse bidirectional motion with the movie looping!). Working in this controlled in vitro environment provides the flexibility to investigate the effect of regulators on the transport using a combination of biophysical, genetic and biochemical tools.