The Cdc14B-Cdh1-Plk1 Axis Controls the G2 DNA-Damage-Response Checkpoint

n response to DNA damage in G2, mammalian cells must avoid entry into mitosis and instead initiate DNA repair. Here, we show that, in response to genotoxic stress in G2, the phosphatase Cdc14B translocates from the nucleolus to the nucleoplasm and induces the activation of the ubiquitin ligase APC/CCdh1, with the consequent degradation of Plk1, a prominent mitotic kinase. This process induces the stabilization of Claspin, an activator of the DNA-damage check- point, and Wee1, an inhibitor of cell-cycle progres- sion, and allows an efficient G2 checkpoint. As a by-product of APC/CCdh1 reactivation in DNA-dam- aged G2 cells, Claspin, which we show to be an APC/ CCdh1 substrate in G1, is targeted for degradation. However, this process is counteracted by the deubi- quitylating enzyme Usp28 to permit Claspin-medi- ated activation of Chk1 in response to DNA damage. These findings define a novel pathway that is crucial for the G2 DNA-damage-response checkpoint.