DNA-PKcs, Allostery, and DNA Double-Strand Break Repair: Defining the Structure and Setting the Stage.

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is central to the regulation of the DNA damage response and repair through nonhomologous end joining. The structure has proved challenging due to its large size and multiple HEAT repeats. We have recently reported crystals of selenomethionine-labeled DNA-PKcs complexed with native KU80ct194 (KU80 residues 539-732) diffracting to 4.3Å resolution. The novel use of crystals of selenomethionine-labeled protein expressed in HeLa cells has facilitated the use of single anomalous X-ray scattering of this 4128 amino acid, multiple HEAT-repeat structure. The monitoring of the selenomethionines in the anomalous-difference density map has allowed the checking of the amino acid residue registration in the electron density, and the labeling of the Ku-C-terminal moiety with selenomethionine has further allowed its identification in the structure of the complex with DNA-PKcs. The crystal structure defines a stage on which many of the components assemble and regulate the kinase activity through modulating the conformation and allosteric regulation of kinase activity.