DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is involved in DNA nonhomologous end joining (NHEJ), which is recruited by Ku70/80 heterodimer to DNA ends and required for double-strand breaks (DSBs) repair. DNA-PKcs phosphorylates a number of protein substrates, including the heat shock protein 90 (HSP90), the transcription factors p53, specificity protein 1 (Sp1), among others. It folds into three well-defined large structural units, consisting of a N-terminal region (arranged in four supersecondary alpha-helical structures, N1-N4), the Circular Cradle (consisting of five supersecondary alpha-helical structures CC1-CC5), and the C-terminal Head (comprising FAT, FRB, kinase, and FATC). The CCs form a curved elliptical ring that serves as a scaffold to maintain the integrity of the whole complex [1-4]. This domain represents a region of the Circular Cradle segment (CC) from DNA-PKcs that covers the complete CC3 and part of CC4. This domain contains the Ku-binding site A and a the highly conserved region (HCR) II [1].

1: DNA-PKcs structure suggests an allosteric mechanism modulating DNA double-strand break repair. Sibanda BL, Chirgadze DY, Ascher DB, Blundell TL; Science. 2017;355:520-524. PMID:28154079

2: Cryo-EM structure of the DNA-PK holoenzyme. Sharif H, Li Y, Dong Y, Dong L, Wang WL, Mao Y, Wu H; Proc Natl Acad Sci U S A. 2017;114:7367-7372. PMID:28652322

3: Cryo-EM structure of human DNA-PK holoenzyme. Yin X, Liu M, Tian Y, Wang J, Xu Y; Cell Res. 2017;27:1341-1350. PMID:28840859

4: Dimers of DNA-PK create a stage for DNA double-strand break repair. Chaplin AK, Hardwick SW, Liang S, Kefala Stavridi A, Hnizda A, Cooper LR, De Oliveira TM, Chirgadze DY, Blundell TL; Nat Struct Mol Biol. 2021;28:13-19. PMID:33077952