Mol. Cell https://doi.org/10.1016/j.molcel.2021.07.005 (2021).
Non-homologous end joining (NHEJ) repairs potentially lethal DNA double-strand breaks (DSBs), and is the major repair pathway in non-replicating human cells. This error-prone pathway, which also mediates immunoglobulin class-switch recombination in B cells, joins broken DNA ends using protein factors to position non-complementary DNA ends for ligation. Repair is initiated when the Ku70/80 heterodimer binds DSB ends and recruits the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to form the DNA-PK complex. This ‘scaffold’ then recruits additional NHEJ factors DNA ligase IV (LigIV), XRCC4 and XLF essential for end ligation. Previous studies have shown that dimer formation of the DNA-PK holoenzyme via the C-terminal end of Ku80 facilitates recruitment of downstream NHEJ factors. Writing in Molecular Cell, Chaplin and colleagues report cryo-EM structures of both monomeric and dimeric forms of the core NHEJ proteins (DNA-PK–LigIV–XLF) that reveal how LigIV, XRCC4 and XLF interact with DNA-PK, and they observe a new XLF-dependent dimer. The distance between the two DNA ends within the new dimer complex is remarkably similar to that reported for the Ku80-mediated DNA dimer, leading the authors to propose that distinct DNA-PK dimers represent alternative forms of the ‘long-range’ synaptic complexes that bring DNA ends into proximity for ligation. Mutations that disrupt XLF-mediated dimerization and those that disrupt Ku80 C-terminal interactions with DNA-PKcs are not additive in VDJ recombination assays, which suggests that these events work in a single repair pathway. We look forward to the identification of potential additional factors that contribute to NHEJ through differential association with the core factor complexes identified in these studies.
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