Binding of Ku and c-Abl at the kinase homology region of DNA-dependent protein kinase catalytic subunit

The DNA-dependent protein kinase (DNA-PK) con trols the repair of double-stranded DNA breaks in mammalian cells. The protein kinase subunit of DNA-PK (DNA-PKcs) is targeted to DNA breaks by association with the Hu DNA-binding heterodimer. Here we show that a Ku association site is present at the carboxyl terminus of DNA-PKcs (amino acids 3002-3850) near the protein kinase domain. Correspondingly, the nuclear c-Abl tyrosine kinase that associates with DNA-PK also binds to the kinase homology domain. The c-Abl SH3 domain binds to amino acids 3414-3850 of DNA-PKcs. c-Abl phosphorylates C-terminal fragments of DNA-PKcs, particularly amino acids 3414-3850. c-Abl phosphorylation of DNA-PKcs disassociates the DNA-PKcs.Ku complex. Thus, Ku and c-Abl provide opposing functions with regard to DNA-PK activity.