AbstractMitochondria play important roles in the cellular response to various types of stress, including that triggered by ionizing radiation. We have previously reported that the mitochondrial ribosomal protein death-associated protein 3 (DAP3) regulates the radioresistance of human lung adenocarcinoma (LUAD) cell lines A549 and H1299. However, the underlying mechanism of this regulation remains to be elucidated. To this end, we have herein investigated the role of DAP3 in the cell cycle regulation after irradiation. Notably, the DAP3 knockdown attenuated the radiation-induced increase of the G2/M cell population. Furthermore, western blotting analysis has revealed that the DAP3 knockdown decreased the expression of proteins related to the G2/M arrest, such as those of the phosphorylated cdc2 (Tyr15) and the phosphorylated checkpoint kinase 1 (Ser296), in irradiated A549 cells and H1299 cells. Moreover, by using a chk1 inhibitor, we were able to demonstrate that chk1 is involved in the radiation-induced G2/M arrest in both A549 and H1299 cells. Notably, the chk1 inhibitor was able to enhance the radiosensitivity of H1299 cells, while both chk1 inhibitor-abolished G2 arrest and inhibition of chk2-mediated events such as downregulation of radiation-induced p21 expression were required for enhancing radiosensitivity of A549 cells. Collectively, our findings reveal a novel role of DAP3 to regulate G2/M arrest through pchk1 in irradiated LUAD cells and suggest that chk1-mediated G2/M arrest regulates the radioresistance of H1299 cells, whereas both the chk1-mediated G2/M arrest and the chk2-mediated events contribute to the radioresistance of A549 cells.