A positive role for the Ku complex in DNA replication following strand break damage in mammals

Ku70/Ku80 complex is the regulatory subunits of DNA-dependent protein kinase (DNA-PK) and plays an essential role in double strand break (dsb) repair following ionizing radiation (IR). It preferentially interacts with chromosomal breaks and protects DNA ends from nuclease attack. Here we show evidence that cells defective in Ku80 exhibit a significantly slow S-phase progression following DNA damage. IR-induced retardation in S-phase progression in Ku80-/- cells was not due to the lack of DNA-PK kinase activity since both wild-type (WT) cells and DNA-PKcs-deficient cells showed no such symptom. Instead, PCNA dissociated from chromosomes following IR in Ku80-deficient cells, but not in WT or DNA-PKcs-deficient cells. Treatment of HeLa cells with IR induced colocalization of the Ku complex with proliferating cell nuclear antigen (PCNA) on chromosomes. Together, these results suggest that binding of the Ku complex at chromosomal breaks may be necessary to maintain the sliding clamps (PCNA) on chromatin, which would allow cells to resume DNA replication without a major delay following IR.