Microtubule length must be tightly controlled for cell functions as diverse as transport, motility, and division. In vitro studies by both groups showed that Kip3p binds all along the length of microtubules, but then motors along to accumulate at their plus ends, where it functions as a depolymerase.
Varga et al. found that Kip3p had a bigger appetite for longer microtubules, as these were devoured more rapidly than short microtubules. The authors suggest that longer microtubules bind more Kip3p and thus accumulate more depolymerase at their plus ends. The ends probably lose some Kip3p as they shorten, and shorter microtubules have less room to pick up more of the motor.
Plus-end accumulation of Kip3p was seen both in vitro and in cells. In the cell, however, the plus end is where growth and shrinkage occur. “The plus end is the place where the action is happening,” says Varga. At this end, he says, there is probably, “a fight between polymerization and destabilizing proteins.” He suggests that microtubules might grow from their plus ends until enough Kip3p accumulates to swing the battle in favor of shortening.