Fly chromosomes don’t just ride the tips

The motor protein NOD works harder during cell division than researchers thought, Cane et al. show. NOD not only tows chromosomes, it also helps them hitchhike on microtubules.

Polar ejection forces push mitotic chromosomes away from the spindle poles. One source for these forces, researchers think, are motor proteins that drag chromosomes along the spindle microtubules. In vertebrates, the motor protein KID, a member of the kinesin-10 family, hauls chromosomes along microtubules. NOD, the Drosophila homologue of KID, appears to be immobile. However, the molecular motor can link to the ends of microtubules, leading to the model that fly chromosomes are instead pushed away from the poles by the tips of polymerizing microtubules.

Cane et al. found that NOD takes a more active role in Drosophila cells. The researchers noticed that NOD-coated chromatin stretched in two ways as it interacted with microtubules. If a microtubule rammed head-on into chromosomes, the chromatin extended rapidly and then recoiled. The speed at which the chromatin stretched matched the growth rate of microtubules during mitosis, suggesting that this event represents a push from a growing microtubule. But the researchers also observed lingering interactions in which the chromatin appeared to stretch along a microtubule, as if the NOD motors were tugging the chromosome along the filament.

The researchers then replaced NOD’s motor domain with a segment that only attaches to microtubule tips or with a motor domain from another kinesin that readily slides on microtubules. Both altered versions generated polar ejection forces. The work suggests that NOD performs double duty, pulling chromosomes along microtubules as well as enabling them to ride on elongating filaments.