To determine why the duration of mitosis (DM) is less in Taxol than in nocodazole or Eg5 inhibitors we studied the relationship between Taxol concentration, the DM, and the mitotic checkpoint. We found that unlike for other spindle poisons, in Taxol the DM becomes progressively shorter as the concentration surpasses ∼0.5 µM. Studies on RPE1 and PtK2 expressing GFP/cyclin B or YFP/Mad2 revealed that cells ultimately satisfy the checkpoint in Taxol and do so faster at concentrations >0.5 µM. Inhibiting the aurora-B kinase in Taxol-treated RPE1 cells accelerates checkpoint satisfaction by stabilizing syntelic kinetochore attachments and reduces the DM to ∼1.5 h regardless of drug concentration. A similar stabilization of syntelic attachments by Taxol itself appears responsible for accelerated checkpoint satisfaction at concentrations >0.5 µM. Our results provide a novel conceptual framework for how Taxol prolongs mitosis and caution against using it in checkpoint studies. They also offer an explanation for why some cells are more sensitive to lower versus higher Taxol concentrations.

When the spindle assembly checkpoint (SAC) cannot be satisfied, cells exit mitosis via mitotic slippage. In microtubule (MT) poisons, slippage requires cyclin B proteolysis, and it appears to be accelerated in drug concentrations that allow some MT assembly. To determine if MTs accelerate slippage, we followed mitosis in human RPE-1 cells exposed to various spindle poisons. At 37°C, the duration of mitosis in nocodazole, colcemid, or vinblastine concentrations that inhibit MT assembly varied from 20 to 30 h, revealing that different MT poisons differentially depress the cyclin B destruction rate during slippage. The duration of mitosis in Eg5 inhibitors, which induce monopolar spindles without disrupting MT dynamics, was the same as in cells lacking MTs. Thus, in the presence of numerous unattached kinetochores, MTs do not accelerate slippage. Finally, compared with cells lacking MTs, exit from mitosis is accelerated over a range of spindle poison concentrations that allow MT assembly because the SAC becomes satisfied on abnormal spindles and not because slippage is accelerated.