Synapsis of DNA ends by DNA-dependent protein kinase

The catalytic subunit of DNA-dependent protein kinase (DNA-PKCS) is required for a non-homologous end-joining pathway that repairs DNA double-strand breaks produced by ionizing radiation or V(D)J recombination; however, its role in this pathway has remained obscure. Using a neutravidin pull-down assay. we found that DNA-PKCS mediates formation of a synaptic complex containing two DNA molecules. Furthermore. kinase activity was cooperative with respect to DNA concentration, suggesting that activation of the kinase occurs only after DNA synapsis. Electron microscopy revealed complexes of two DNA ends brought together by two DNA-PKCS molecules. Our results suggest that DNA-PKCS brings DNA ends together and then undergoes activation of its kinase, presumably to regulate subsequent steps for processing and ligation of the ends.