The DNA-Dependent Protein Kinase Catalytic Subunit Is Phosphorylated In Vivo on Threonine 3950, a Highly Conserved Amino Acid in the Protein Kinase Domain

Abstract

The protein kinase activity of the DNA-dependent protein kinase (DNA-PK) is required for the repair of DNA double-strand breaks (DSBs) via the process of nonhomologous end joining (NHEJ). However, to date, the only target shown to be functionally relevant for the enzymatic role of DNA-PK in NHEJ is the large catalytic subunit DNA-PKcs itself. In vitro, autophosphorylation of DNA-PKcs induces kinase inactivation and dissociation of DNA-PKcs from the DNA end-binding component Ku70/Ku80. Phosphorylation within the two previously identified clusters of phosphorylation sites does not mediate inactivation of the assembled complex and only partially regulates kinase disassembly, suggesting that additional autophosphorylation sites may be important for DNA-PK function. Here, we show that DNA-PKcs contains a highly conserved amino acid (threonine 3950) in a region similar to the activation loop or t-loop found in the protein kinase domain of members of the typical eukaryotic protein kinase family. We demonstrate that threonine 3950 is an in vitro autophosphorylation site and that this residue, as well as other previously identified sites in the ABCDE cluster, is phosphorylated in vivo in irradiated cells. Moreover, we show that mutation of threonine 3950 to the phosphomimic aspartic acid abrogates V(D)J recombination and leads to radiation sensitivity. Together, these data suggest that threonine 3950 is a functionally important, DNA damage-inducible phosphorylation site and that phosphorylation of this site regulates the activity of DNA-PKcs.

SUPPLEMENTAL MATERIAL

This work was supported by Public Health Service grant AI048758 (K.M.) and by grant 13639 from the Canadian Institutes of Health Research (S.P.L.-M.). S.P.L.-M. is supported by the Alberta Heritage Foundation for Medical Research and the Canadian Institutes of Health Research and holds the Engineered Air Chair in Cancer Research. W.D.B. was supported by scholarships from the Alberta Heritage Foundation for Medical Research and the Natural Sciences and Engineering Council of Canada.

We thank Graeme Smith (KuDOS Pharmaceuticals) for NU7441 and the MRC Protein Phosphorylation Unit, Dundee, for growing the HEK293 cells.

The laboratories at the Departments of Biochemistry and Molecular Biology and Oncology, University of Calgary, Calgary, Alberta, Canada, and the College of Veterinary Medicine and Department of Pathobiology and Diagnostic Investigation, Michigan State University, East Lansing, contributed equally to this work.