Plk1-dependent phosphorylation regulates functions of DNA topoisomerase IIα in cell cycle progression

Plk1 (Polo-like kinase 1) has been documented as a critical regulator of many mitotic events. However, increasing evidence supports the notion that Plk1 might also have functions outside of mitosis. Using biochemical fractionation and RNA interference approaches, we found that Plk1 was required for both G(1)/S and G(2)/M phases and that DNA topoisomerase II alpha (topoII alpha) was a potential target for Plk1 in both interphase and mitosis. Plk1 phosphorylates Ser(1337) and Ser(1524) of topoII alpha. Overexpression of an unphosphorylatable topoII alpha mutant led to S phase arrest, suggesting that Plk1-associated phosphorylation first occurs in S phase. Moreover, overexpression of the unphosphorylatable topoII alpha mutant activated the ATM/R-dependent DNA damage checkpoint, probably due to reduced catalytic activity of topoII alpha, and resulted in accumulation of catenated DNA. Finally, we showed that wild type topoII alpha, but not the unphosphorylatable mutant, was able to rescue topoII alpha depletion-induced defects in sister chromatid segregation, indicating that Plk1-associated phosphorylation is essential for the functions of topoII alpha in mitosis.