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 G₂/M phases and that DNA topoisomerase IIα (topoIIα) was a potential target for Plk1 in both interphase and mitosis. Plk1 phosphorylates Ser¹³³⁷ and Ser¹⁵²⁴ of topoIIα. Overexpression of an unphosphorylatable topoIIα mutant led to S phase arrest, suggesting that Plk1-associated phosphorylation first occurs in S phase. Moreover, overexpression of the unphosphorylatable topoIIα mutant activated the ATM/R-dependent DNA damage checkpoint, probably due to reduced catalytic activity of topoIIα, and resulted in accumulation of catenated DNA. Finally, we showed that wild type topoIIα, but not the unphosphorylatable mutant, was able to rescue topoIIα depletion-induced defects in sister chromatid segregation, indicating that Plk1-associated phosphorylation is essential for the functions of topoIIα in mitosis.