Abstract
DNA-dependent protein kinase (DNA-PK) is composed of Ku70, Ku86 and catalytic subunit (DNA-PKcs) and acts as a sensor of DNA double-strand breaks (DSBs) during non-homologous end-joining (NHEJ). Here we demonstrated that DNA-PKcs formed nuclear foci rapidly after exposure to ionizing radiation. DNA-PKcs foci were observed just after 5 Gy of X-irradiation. Wortmannin inhibited XRCC4 phosphorylation but not DNA-PKcs foci formation. On the other hand, both DNA-PKcs foci formation and XRCC4 phosphorylation were reduced by Ku86 siRNA. These results suggest that DNA-PKcs foci formation requires Ku proteins and precedes its activation. DNA-PKcs did not response to DNA replication arrest, while NBS1 and histone H2AX, which are essential for homologous recombination (HR), were phosphorylated and formed foci. Also, X-ray-induced DNA-PKcs foci did not colocalize with phosphorylated H2AX. These results further suggest that DNA-PK selectively recognize DSBs repaired by NHEJ independent of HR-related proteins. [J Radiat Res 44:409 (2003)]
