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Molecular motors show an odd sense of teamwork during cargo transport, say Ally et al. In order to get somewhere, they have to pull in opposite directions.
Many types of cargo move through the cell along microtubules, but rather than smoothly gliding to their final destinations, they stutter back and forth due to motor proteins that pull in opposite directions. These bi-directional movements may help cargo navigate through a crowded cytosol. Surprisingly, the opposing motors seem to rely on each other—if one motor is removed, its rival seizes up instead of victoriously hauling its cargo to the microtubule's end.
To investigate how such pairs of motors might communicate, Ally et al. wondered whether any two opposite polarity motors could combine to transport peroxisomes. These organelles are paralyzed in the absence of the plus end–directed motor kinesin-1, even though dynein—the protein that would usually shunt them to microtubule minus ends—is still present. Peroxisome movement in both directions was restored when a different kinesin, Unc104, was artificially targeted to the organelle, suggesting that dynein can pair up with any plus end–directed motor. Similarly, kinesin-1 can partner with minus end motors other than dynein, the researchers found.
Kinesins lacking motor activity couldn't restore peroxisome movements, indicating that transport in one direction is required to activate transport in the other. The mechanical tension produced by one motor might activate its opposing partner, says author Shabeen Ally. Once running, however, the opposing motor activities must be carefully balanced: the slow kinesin Eg5 could replace kinesin-1 and activate dynein, but was powerless to resist the minus end motor's faster movement, causing peroxisomes to incorrectly accumulate in the center of the cell.