DNA-DEPENDENT PROTEIN-KINASE CATALYTIC SUBUNIT - A TARGET FOR AN ICE-LIKE PROTEASE IN APOPTOSIS

Radiosensitive cell lines derived from X-ray cross complementing group 5 (XRCC5), SCID mice and a human glioma cell line lack components of the DNA-dependent protein kinase, DNA-PK, suggesting that DNA-PK plays an important role in DNA double-strand break repair. Another enzyme i mplicated in DNA repair, poly(ADP-ribose) polymerase, is cleaved and i nactivated during apoptosis, suggesting that some DNA repair proteins may be selectively targeted for destruction during apoptosis. Here we demonstrate that DNA-PKcs, the catalytic subunit of DNA-PK, is prefere ntially degraded after the exposure of different cell types to a varie ty of agents known to cause apoptosis. However, Ku, the DNA-binding co mponent of the enzyme, remains intact. Degradation of DNA-PKcs was acc ompanied by loss of DNA-PK activity. One cell line resistant to etopos ide-induced apoptosis failed to show degradation of DNA-PKcs. Protease inhibitor data implicated an ICE-like protease in the cleavage of DNA -PKcs, and it was subsequently shown that the cysteine protease CPP32, but not Mch2 alpha, ICE or TX, cleaved purified DNA-PKcs into three f ragments of comparable size with those observed in cells undergoing ap optosis. Cleavage sites in DNA-PKcs, determined by antibody mapping an d microsequencing, were shown to be the same for CPP32 cleavage and fo r cleavage catalyzed by extracts from cells undergoing apoptosis. Thes e observations suggest that DNA-PKcs is a critical target for proteoly sis by an ICE-like protease during apoptosis.