Eg5, the vertebrate form of kinesin-5, moves along microtubules either randomly or directionally. In other kinesins, directionality is triggered when the motor binds to its cargo and to the microtubule track. But for Eg5, microtubules are both the track and the cargo, as both ends of the motor protein bind microtubules.
The authors showed that Eg5 moved predominantly randomly when bound to only one microtubule, but switched solely to plus end–directed movement when a second microtubule bound.
Eg5 is one of the main organizers of microtubules in the mitotic spindle, and its ability to slide antiparallel fibers along one another is crucial for pushing opposite poles apart. Alternatively, when encountering parallel fibers, it can “zip them together” if they are skewed, says co-PI Tarun Kapoor. He thinks the ability to move randomly on a single microtubule allows Eg5 to “explore” its environment for another microtubule, “dramatically increasing the efficiency of crosslinking.”