HeLa Cells Override the Mitotic Entry Checkpoint by Highjacking the Survivin-Cdc25B-Cdk1 Pathway

The Survivin protein has roles in repairing incorrect microtubule-kinetochore attachments at prometaphase and the faithful execution of cytokinesis, as part of the C hromosomal P assenger C omplex (CPC). In its absence, mitotic errors often ensue that lead to chromosome missegregation, a cause of aneuploidy, polyploidy and ultimately cancer. Adding to these well-known roles of Survivin, this paper now shows for the first time that this protein is required for cancer cells to enter mitosis, and that, in its absence, HeLa cells accumulate at G2/early prophase, or prior to prometaphase, as claimed before. This early mitotic blockage is demonstrated by the inability of the Survivin-depleted cells to disassemble their nuclear lamina and their low Cdk1 activity. Not surprisingly, the Survivin-ablation arrest can be reproduced by a Survivin double mutant SUR D70A/D71A, which cannot bind to Cdk1. Also, escape from this mutant’s blockage leads to multiple mitotic defects, or mitotic catastrophe , and cell death. Mechanistically, Cdk1 does not localize at the centrosome in the absence of Survivin, pointing at this latter protein contributing to the activation of the mitotic kinase via Cdc25B. Furthermore, absence of Survivin leads to an inactive cytosolic Cdc25B-Cdk1-Cyclin B1 complex, which seems to indicate a role for Survivin in bridging this complex and its centrosomal activator/s. Interestingly, the drop in Cdk1 activity caused by interference with the Survivin function could be rescued when Survivin-depleted HeLa cell lysates were incubated with the recombinant Survivin protein. Also, a role for Survivin in the Cdc25B-mediated activation of Cdk1, and concomitant prophase to prometaphase transition could be confirmed by expression of a gain-of-function Cdc25B mutant, which overrode the G2/prophase blockage caused by Survivin depletion.