The leucine rich region of DNA-PKcs contributes to its innate DNA affinity

DNA-PK is a protein complex that consists of a DNA-binding, regulatory subunit [Ku] and a larger approximately 465 kDa catalytic subunit [DNA-PKcs], a serine/threonine protein kinase. The kinase activity of DNA-PKcs resides between residues 3745 and 4013, a PI3 kinase domain. Another recognized domain within this large protein is a leucine zipper (LZ) motif or perhaps more appropriately designated a leucine rich region (LRR) that spans residues 1503-1602. Whereas, DNA-PK's kinase activity has been shown to be absolutely indispensable for its function in non-homologous end joining (NHEJ), little is known about the functional relevance of the LRR. Here we show that DNA-PKcs with point mutations in the LRR can only partially reverse the radiosensitive phenotype and V(D)J recombination deficits of DNA-PKcs deficient cells. Disruption of the LRR motif affects the ability to purify DNA-PKcs via its binding to DNA-cellulose, but does not affect its interaction with Ku or its catalytic activity. These data suggest that the LRR region of DNA-PKcs may contribute to its intrinsic DNA affinity, and moreover, that intrinsic DNA binding is important for optimal function of DNA-PKcs in repairing double strand breaks in living cells.