Autophosphorylation of DNA-dependent protein kinase regulates DNA end processing and may also alter double-strand break repair pathway choice

Two highly conserved double-strand break (DSB) repair pathways, homologous recombination (HR) and nonhomologous end joining (NHEJ), function in all eukaryotes. How a cell chooses which pathway to utilize is an area of active research and debate. During NHEJ, the DNA-dependent protein kinase (DNA-PK) functions as a "gatekeeper" regulating DNA end access. Here, we provide evidence that DNA-PK regulates DNA end access via its own autophosphorylation. We demonstrated previously that autophosphorylation within a major cluster of sites likely mediates a conformational change that is critical for DNA end processing. Furthermore, blocking autophosphorylation at these sites inhibits a cell's ability to utilize the other major double-strand break repair pathway, HR. Here, we define a second major cluster of DNA-PK catalytic subunit autophosphorylation sites. Whereas blocking phosphorylation at the first cluster inhibits both end processing and HR, blocking phosphorylation at the second cluster enhances both. We conclude that separate DNA-PK autophosphorylation events may function reciprocally by not only regulating DNA end processing but also affecting DSB repair pathway choice.