Slow escape from mitosis

Cells with a defective spindle get delayed—stuck in a mitotic checkpoint—but eventually escape mitosis. In vertebrate cells, say Daniela Brito and Conly Rieder (Wadsworth Center, New York State Department of Health, Albany, NY), that escape is a gradual limp rather than a sudden exit.

The temporary stalling of the cell cycle in a checkpoint gives the cell time to repair damage before continuing. Ted Weinert and Lee Hartwell gave this phenomenon its name, but “they never viewed a checkpoint as leading to a permanent arrest,” says Rieder.

Indeed, even in response to a problem that cannot be fixed, such as high levels of the anti-microtubule drug nocodazole, many cells do leak through the mitotic arrest. In yeast and perhaps flies, the escape occurs via phosphorylation of Cdk1 or induction of a Cdk1 inhibitor. This adaptation pathway turns off the Cdk1/cyclin B complex, whose activity defines mitosis.

A similar abrupt change was expected in vertebrate cells, via Cdk1 phosphorylation, induction of a Cdk1 inhibitor, or rapid degradation of cyclin B. “But it turns out it doesn't do any of these,” says Rieder. Instead, “human cells slowly degrade cyclin B over the course of time.”

This slow, proteasome-dependent degradation of cyclin B was required for nocodazole-treated cells to escape from mitosis. The spindle assembly checkpoint remained intact and active throughout. Thus Brito and Rieder believe that vertebrate cells do not turn on a specific adaptation pathway. Instead there is a constant low level of cyclin B degradation that eventually reduces cyclin B levels below that needed to maintain a mitotic state. In plants the resultant exit may allow a change in ploidy, although its utility in mammals is less clear.

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