Activation of the Bora–Aurora A—PLK1 signaling cascade in late G₂ is critical for the timely regulation of mitotic entry. Although PLK1 plays a key role in cyclin B/CDK1 activation, it has been controversial whether PLK1 inhibition generates a G₂/M block. Activation of PLK1 requires Bora–Aurora A activity, whereas MASTL may indirectly protect PLK1 substrates from dephosphorylation. Therefore, the simultaneous suppression of Aurora A, Bora, MASTL, and PLK1 may potentially generate a more promising G₂/M block than PLK1 inhibition alone.

To effectively inhibit Aurora A and PLK1, two small molecule inhibitors, VX689 and BI2536, have been widely used. Both inhibitors were claimed to possess an extremely high specificity towards their designated target. Nevertheless, this study showed that th...[ Read more ]

Activation of the Bora–Aurora A—PLK1 signaling cascade in late G₂ is critical for the timely regulation of mitotic entry. Although PLK1 plays a key role in cyclin B/CDK1 activation, it has been controversial whether PLK1 inhibition generates a G₂/M block. Activation of PLK1 requires Bora–Aurora A activity, whereas MASTL may indirectly protect PLK1 substrates from dephosphorylation. Therefore, the simultaneous suppression of Aurora A, Bora, MASTL, and PLK1 may potentially generate a more promising G₂/M block than PLK1 inhibition alone.

To effectively inhibit Aurora A and PLK1, two small molecule inhibitors, VX689 and BI2536, have been widely used. Both inhibitors were claimed to possess an extremely high specificity towards their designated target. Nevertheless, this study showed that these inhibitors exhibit off-target effects when compared with knock-out of PLK1 and Aurora A in HeLa cells. These observations demonstrated the power of conditional knock-out systems to address the functional role of Aurora A and PLK1 in mitotic commitment. Indeed, Aurora A, Bora, MASTL, and PLK1 were found to be dispensable for mitotic entry. Synchronized experiment and real-time imaging showed that the concurrent loss of Aurora A, MASTL, and PLK1 activity resulted in a significant delay in mitotic entry, but not a G₂ block. Moreover, Aurora A, Bora, and PLK1 were found to be tightly regulating each other during G₂/M transition and mitosis, contributing to the robust maintenance of Aurora A and PLK1 activity. In summary, conditional gene depletion system provided a reliable and specific tool to resolve the discrepancy in the functional role of Aurora A, Bora, MASTL, and PLK1, which revealed that these proteins are not essential to mitotic entry in HeLa cells.