Kinesin takes peroxisomes out to the cell periphery, whereas dynein brings them back to the interior. No matter which direction ultimately prevails, the peroxisome switches direction many times along the way. These switches might stem from the alternation of active motors or from a tug-of-war with alternating short-term winners. To distinguish between these possibilities, the authors visualized peroxisome movement at high resolution in vivo. The results suggest that either dynein or kinesin, but not both, pulls at any given time.
The high resolution images revealed individual step sizes of 8 nm for each motor, which matches findings from in vitro studies. If opposing motors were pulling simultaneously, “we'd expect to see a bunch of smaller step sizes,” says Selvin. Since that was not seen, Selvin concludes that “when kinesin takes a step, it's probably not dragging dynein.” He guesses that dynein disconnects from the microtubule but stays attached to the peroxisome. And when dynein is working, kinesin returns the favor.
Selvin is currently puzzled by how the coordination is regulated. A small molecule might alternate between the motors, turning on one as it turns off the other. But the speed with which the directional change occurs makes Selvin skeptical of this possibility.
The group also measured peroxisome speed, which indicated that several kinesins or several dyneins often work together to move the cargo more quickly than one motor could by itself. This cooperativity has never been seen by motors pulling beads in vitro. Perhaps something in the peroxisome lipid bilayer is needed for several motors to team up. The authors hope that they might find the needed factor(s) by reconstituting peroxisome movement in vitro.