The NHEJ pathway is initiated in response to formation of a DNA double-strand break (DSB) by a DNA-damaging agent such as ionizing radiation. Step 2: The Ku70/80 heterodimer to the ends of the DSB. Step 3: The catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs) is recruited to form the DNA-PK holoenzyme. Step 4. Two molecules of DNA-PK may come together in a synaptic complex. Step 5. XRCC4/DNA ligase IV is recruited. Step 6. Other factors may be required for processing of the DNA ends prior to re joining of the DNA ends. Step 7: Protein factors must be removed from the DNA prior to rejoining. Step 8. DNA ends are ligated and the DNA is repaired. Mg-ATP and the protein kinase activity of DNA-PKcs are required for NHEJ but the precise step at which it is required is not known with certainty. DNA-PKcs, Ku70, Ku80 and XRCC4 become phosphorylated in vitro when incubated with DNA-PK. DNA-PKcs phosphorylation sites are important for NHEJ. Inositol hexaphosphate (IP6) stimulates end joining in vitro and binds to Ku. It is not known where in the NHEJ pathway that it acts.
S.P. Jackson (2002) Carcinogenesis, 23, 687; S.P. Lees-Miller (2003) DNA Repair, Invited review, submitted Sept 2003