Abstract
DNA-PK is now considered a pivotal enzyme in the repair of DNA double-strand breaks and there are several lines of evidence indicating that the catalytic activity to phosphorylate protein is essential for the repair function of DNA-PK. Nevertheless, it has been unclear which protein, and for what sake, should be phosphorylated by DNA-PK. Several recent studies have shed light on the importance of the autophosphorylation of DNA-PKcs itself. Then, does DNA-PKcs exist solely to phosphorylate itself?
We have sought to find the true substrate and the physiological significance of phosphorylation by DNA-PK in DNA double-strand break repair. In 2000, we demonstrated that XRCC4 is phosphorylated in response to ionizing radiation in a manner dependent on DNA-PKcs. Now we identified the phosphorylation sites on XRCC4 by DNA-PK, which are really phosphorylated in living cells in response to ionizing radiation and the loss of which lead to elevated radiosensitivity with deficient DNA repair capability. These results in the aggregate would indicate that XRCC4 phosphorylation by DNA-PK is an essential event in DSB repair. Further studies are warranted to clarify what consequence, e.g., change in conformation, property or activity, will be brought by the addition of phosphate group on XRCC4 protein.
The above results not only found a missing link in our understanding of DSB repair mechanism but also may provide us with a new approach for the development of radiosensitizers and the prediction of radiosensitivity and cancer susceptibility, as will be discussed. Furthermore, the phosphorylation sites identified above did not conform to the SQ TQ rule. Thus, the present results would also give us an important lesson in expanding our research on DNA-PK and related kinases ATM/ATR.