Without Ncd (middle), K-fibers don't focus; but without dynein (right), they are not transported toward centrosomes.

Two minus end–directed motors, Ncd (Kinesin-14) and dynein, have complementary capture-and-transport roles that shape the spindle during mitosis, report Goshima et al. on page 229.

Spindle microtubules emanate from the centrosomes or, as long thick ropes called K-fibers, from kinetochores. Previous work showed that two minus end–directed molecular motors—dynein and Ncd—are required to intertwine the two types of microtubules and bring them into the familiar diamond shape. In cells lacking either motor, the microtubules splay apart, failing to gather their minus ends near the centrosomes.

Using video microscopy with GFP-labeled components, FRAP, and RNAi, the team found that the motors have some functional redundancy but also have a preference for jobs. Loss of Ncd results primarily in a loss of K-fiber focusing, whereas loss of dynein compromises the pulling of K-fibers toward centrosomes.

Ncd apparently acts as a dynamic cross-linking protein lashing together individual microtubules into K-fibers. But it also accumulates at the plus ends of the centrosomal microtubules, relying on the plus end tracking protein EB1 for attachment. Clustering at ends probably helps the nonprocessive Ncd to grab onto K-fibers with a robust grip. Then dynein takes over, pulling K-fibers along the centrosomal microtubules toward the centrosome.