A methylation-phosphorylation switch determines Plk1 kinase activity and function in DNA damage repair.

Polo-like kinase 1 (Plk1) is a crucial regulator of cell cycle progression; but the mechanism of regulation of Plk1 activity is not well understood. We present evidence that Plk1 activity is controlled by a balanced methylation and phosphorylation switch. The methyltransferase G9a monomethylates Plk1 at Lys209, which antagonizes phosphorylation of T210 to inhibit Plk1 activity. We found that the methyl-deficient Plk1 mutant K209A affects DNA replication, whereas the methyl-mimetic Plk1 mutant K209M prolongs metaphase-to-anaphase duration through the inability of sister chromatids separation. We detected accumulation of Plk1 K209me1 when cells were challenged with DNA damage stresses. Ablation of K209me1 delays the timely removal of RPA2 and RAD51 from DNA damage sites, indicating the critical role of K209me1 in guiding the machinery of DNA damage repair. Thus, our study highlights the importance of a methylation-phosphorylation switch of Plk1 in determining its kinase activity and functioning in DNA damage repair.