Watching kinetochores sweat

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...