Cleavage and phosphorylation of XRCC4 protein induced by X‐irradiation

Abstract
We report the p35 and p60 forms of XRCC4 protein, appearing in human leukemia MOLT‐4 or U937 cells following X‐irradiation or hyperthermia. p35 appeared in conjunction with the cleavage of DNA‐dependent protein kinase catalytic subunit (DNA‐PKcs) and the fragmentation of internucleosomal DNA, and was suppressed by Ac‐DEVD‐CHO. p35 was also produced in vitro by treating MOLT‐4 cell lysate with recombinant caspases, suggesting that p35 was a caspase‐cleaved fragment of XRCC4 in apoptotic cell death. p60 was sensitive to treatment with phosphatase or wortmannin and was undetectable in M059J cells deficient in DNA‐PKcs. However, p60 was found in ataxia‐telangiectasia cells after irradiation. These results indicated p60 as a phosphorylated form of XRCC4, requiring DNA‐PKcs but not ataxia‐telangiectasia mutated (ATM).