Living cells have superhighways of microtubules, crowded with molecular machines that "walk" along them delivering cargo. Sometimes things get a bit crowded, but deliveries arrive on time -- thanks to a unique strategy.
In short, the machines hand off their cargo, one to the next, till it reaches the end of the traffic jam, something like the kid's relay race game of passing a beach ball overhead down the line.
Two recent papers explain how scientists are opening the black box of intracellular transport. In PNAS, "Motor transport of self-assembled cargos in crowded environments," four researchers in Massachusetts sought to explain "how multimotor cargos would navigate a densely crowded filament with many other motors."
Prior theoretical and experimental biophysical model systems of intracellular cargo have assumed fixed teams of motors transporting along bare microtubules or microtubules with fixed obstacles. Here, we investigate a regime of cargos transporting along microtubules crowded with free motors.
Knowing that intracellular transport is vital for survival and that the cell is a crowded place, they were curious about how cells don't appear to jam or dissociate. They reasoned, "motor-driven cargos must be able to circumvent traffic and crowding to efficiently deliver material throughout the cell."
Previous research showed that teams of motors didn't jam, but single motors would sometimes dissociate from the track. But no one had studied "the effects of high-density traffic on cargos with multiple motors." So they attached quantum dots to kinesin motors and set them loose in an artificially high-traffic situation. To their surprise, they found a cooperative system:
Although high densities of kinesin motors hinder forward motion, resulting in a lower velocity, the ability to associate motors appears to enhance the run length and attachment time of the quantum dot, improving overall cargo transport. These results suggest that cargos that can associate new motors as they transport could overcome traffic jams.
They observed pauses and short reversals, but the teams of motors didn't lose their cargo as easily as single motors. One strategy is the lane change: