Kinetochores switch from flux to pacman during mitosis.

Even after decades of study, it is still unclear whether kinetochores or microtubules provide the motive force to pull chromosomes toward opposite poles during cell division. On page 377, Maddox et al. propose that each makes its own contribution. They find that a combination of kinetochore-generated force and poleward flux of the microtubules moves the chromosomes in Xenopus egg extracts.

The authors are the first to distinguish kinetochore microtubules from other microtubules, using high-resolution fluorescent speckle microscopy and labeled kinetochore proteins. Their high-resolution view of kinetochore–microtubule interactions shows that kinetochores exist in either microtubule-polymerizing or -depolymerizing states.

Their data help to explain why metaphase chromosomes oscillate in some types of cells but not others. Like a boat rowing against a current, polymerizing kinetochores resist microtubule flux. But if flux is fast enough, they are still pulled poleward, thus generating tension between sister chromatids. In cells with low flux rates, such as yeast and cultured mammalian cells, the switching of kinetochores between polymerizing and depolymerizing states would cause chromosomes to oscillate. In cells with high flux rates, including Xenopus eggs, the resistive tension from microtubules pulling continuously out of kinetochores promotes polymerization and prevents oscillations. The authors found that, in anaphase, kinetochores switch to depolymerization—rowing with the current—and thus pull chromatids apart faster than the rate of flux. ▪