The cell cycle checkpoint kinase CHK2 mediates DNA damage-induced stabilization of TTK/hMps1

Cell cycle progression is monitored constantly to ensure faithful passage of genetic codes and genome stability. We have demonstrated previously that, upon DNA damage, TTK/hMps1 activates the checkpoint kinase CHK2 by phosphorylating CHK2 at Thr68. However, it remains to be determined whether and how TTK/hMps1 responds to DNA damage. In this report, we present evidence that TTK/hMps1 can be induced by DNA damage in normal human fibroblasts. Interestingly, the induction depends on CHK2 because CHK2-targeting small interfering RNA or a CHK2 inhibitor abolishes the increase. Such induction is mediated through phosphorylation of TTK/hMps1 at Thr288 by CHK2 and requires the CHK2 SQ/TQ cluster domain/forkhead-associated domain. In cells, TTK/hMps1 phosphorylation at Thr288 is induced by DNA damage and forms nuclear foci, which colocalize partially with gamma-H2AX. Reexpression of TTK/hMps1 T288A mutant in TTK/hMps1-knockdown cells causes a defect in G(2)/M arrest, suggesting that phosphorylation at this site participates in the proper checkpoint execution. Our study uncovered a regulatory loop between TTK/hMps1 and CHK2 whereby DNA damage-activated CHK2 may facilitate the stabilization of TTK/hMps1, therefore maintaining the checkpoint control.