The spindle that separates chromosomes during mitosis has a shadowy partner, as Lince-Faria et al. show. The structure serves as a staging area for checkpoint proteins that decide whether the cell can advance through mitosis.
Researchers have long suspected that the microtubular spindle has a companion. This spindle matrix, scientists proposed, houses proteins such as the molecular motors that pull on microtubules and haul the chromosomes apart. However, evidence for such a matrix remains inconclusive.
Lince-Faria et al. tracked a potential spindle matrix protein called Megator. In Drosophila cells, they found, the distribution of Megator mirrors the shape of the spindle. Even after a dose of colchicine causes the microtubules to collapse, Megator remains in the spindle's vicinity, suggesting that something holds it in place.
In turn, Megator holds up mitosis while the spindle matures. Without the protein, cells race through mitosis. Megator prompts the delay using the spindle assembly checkpoint, which halts mitosis if spindle fibers aren't correctly fastened to kinetochores on the chromosomes. As the team discovered, Megator links to Mad2, a component of the checkpoint. And when Megator is missing, less Mad2 gloms onto kinetochores that haven't linked to microtubules, which might make it easier for a cell to rapidly enter mitosis even if the spindle isn't correctly connected. This function appears to be conserved in humans.
The researchers conclude that Megator and Mad2 meet the qualifications for spindle matrix proteins. Further studies should probe what draws Megator to the spindle matrix, they say.