TTK/hMps1 Participates in the Regulation of DNA Damage Checkpoint Response by Phosphorylating CHK2 on Threonine 68

CHK2/hCds1 plays important roles in the DNA damage-induced cell cycle checkpoint by phosphorylating several important targets, such as Cdc25 and p53. To obtain a better understanding of the CHK2 signaling pathway, we have carried out a yeast two-hybrid screen to search for potential CHK2-interacting proteins. Here, we report the identification of the mitotic checkpoint kinase, TTK/hMps1, as a novel CHK2-interacting protein. TTK/hMps1 directly phosphorylates CHK2 on Thr-68 in vitro. Expression of a TTK kinase-dead mutant, TTK(D647A), interferes with the G(2)/M arrest induced by either ionizing radiation or UV light. Interestingly, induction of CHK2 Thr-68 phosphorylation and of several downstream events, such as cyclin B1 accumulation and Cdc2 Tyr-15 phosphorylation, is also affected. Furthermore, ablation of TTK expression using small interfering RNA results not only in reduced CHK2 Thr-68 phosphorylation, but also in impaired growth arrest. Our results are consistent with a model in which TTK functions upstream from CHK2 in response to DNA damage and suggest possible cross-talk between the spindle assembly checkpoint and the DNA damage checkpoint.