Spatial regulation of astral microtubule dynamics by Kif18B in PtK cells.

The spatial and temporal control of microtubule dynamics is fundamentally important for proper spindle assembly and chromosome segregation. This is achieved, in part, by the multitude of proteins that bind to and regulate spindle microtubules, including kinesin superfamily members, which act as microtubule-destabilizing enzymes. These fall into two general classes: the kinesin-13 proteins, which directly depolymerize microtubules, and the kinesin-8 proteins, which are plus end-directed motors that either destabilize microtubules or cap the microtubule plus ends. Here we analyze the contribution of a PtK kinesin-8 protein, Kif18B, in the control of mitotic microtubule dynamics. Knockdown of Kif18B causes defects in spindle microtubule organization and a dramatic increase in astral microtubules. Kif18B-knockdown cells had defects in chromosome alignment, but there were no defects in chromosome segregation. The long astral microtubules that occur in the absence of Kif18B are limited in length by the cell cortex. Using EB1 tracking, we show that Kif18B activity is spatially controlled, as loss of Kif18B has the most dramatic effect on the lifetimes of astral microtubules that extend toward the cell cortex. Together our studies provide new insight into how diverse kinesins contribute to spatial microtubule organization in the spindle.