LOSS OF THE CATALYTIC SUBUNIT OF THE DNA-DEPENDENT PROTEIN-KINASE IN DNA DOUBLE-STRAND-BREAK-REPAIR MUTANT MAMMALIAN-CELLS

The DNA-dependent protein kinase (DNA-PK) consists of three polypeptid e components: Ku-70, Ku-80, and an approximate to 350-kDa catalytic su bunit (p350), The gene encoding the Ku-80 subunit is identical to the x-ray-sensitive group 5 complementing gene XRCC5. Expression of the Ku -80 cDNA rescues both DNA double-strand break (DSB) repair and V(D)J r ecombination in group 5 mutant cells. The involvement of Ku-80 in thes e processes suggests that the underlying defect in these mutant cells may be disruption of the DNA-PK holoenzyme. In this report we show tha t the p350 kinase subunit is deleted in cells derived from the severe combined immunodeficiency mouse and in the Chinese hamster ovary cell line V-3, both of which are defective in DSB repair and V(D)J recombin ation. A centromeric fragment of human chromosome 8 that complements t he acid defect also restores p350 protein expression and rescues in vi tro DNA-PK activity, These data suggest the scid gene may encode the p 350 protein or regulate its expression and are consistent with a model whereby DNA-PK is a critical component of the DSB-repair pathway.