The ability of p53 to activate downstream genes p21(WAF1/cip1) and MDM2, and cell cycle arrest following DNA damage is delayed and attenuated in scidcells deficient in the DNA-dependent protein

scid mouse embryonic fibroblasts are deficient in DNA-dependent protein kin ase activity due to a mutation in the C-terminal domain of the catalytic su bunit (DNA-PKcs). when exposed to ionizing radiation, the increase in level s of p53 was the same as in normal mouse embryonic fibroblasts. However, th e rise in p21(WAF1/cip1) and mdma was found to be delayed and attenuated, w hich ;correlated in time with delayed onset of G(1)/S arrest by now cytomet ric analysis. The p53-dependent G(1) checkpoint was not eliminated: inactiv ation of p53 by the E6 protein in scid cells resulted in the complete loss of detectable G(1)/S arrest after DNA damage, Immunofluorescence analysis o f normal cells revealed p53 to be localized predominantly within the cytopl asm prior to irradiation and then translocate to the nucleus after irradiat ion. In contrast, scid cells show abnormal accumulation of p53 in the nucle us independent of irradiation, which was confirmed by immunoblot analysis o f nuclear lysates. Taken together, these data suggest that loss of DNA-PK a ctivity appears to attenuate the kinetics of p53 to activate downstream gen es, implying that DNA-PK plays a role in post-translational modification of p53, without affecting the increase in levels of p53 in response to DNA da mage.