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DNA double-strand breaks (DSBs) are a highly cytotoxic form of DNA damage and the incorrect repair of DSBs is linked to carcinogenesis 1 , 2 . The conserved error-prone non-homologous end joining (NHEJ) pathway has a key role in determining the effects of DSB-inducing agents that are used to treat cancer as well as the generation of the diversity in antibodies and T cell receptors 2 , 3 . Here we applied single-particle cryo-electron microscopy to visualize two key DNA–protein complexes that are formed by human NHEJ factors. The Ku70/80 heterodimer (Ku), the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs), DNA ligase IV (LigIV), XRCC4 and XLF form a long-range synaptic complex, in which the DNA ends are held approximately 115 Å apart. Two DNA end-bound subcomplexes comprising Ku and DNA-PKcs are linked by interactions between the DNA-PKcs subunits and a scaffold comprising LigIV, XRCC4, XLF, XRCC4 and LigIV. The relative orientation of the DNA-PKcs molecules suggests a mechanism for autophosphorylation in trans , which leads to the dissociation of DNA-PKcs and the transition into the short-range synaptic complex. Within this complex, the Ku-bound DNA ends are aligned for processing and ligation by the XLF-anchored scaffold, and a single catalytic domain of LigIV is stably associated with a nick between the two Ku molecules, which suggests that the joining of both strands of a DSB involves both LigIV molecules. Double-strand DNA break repair by the non-homologous end joining pathway involves the transition from a complex that bridges the DNA ends to a complex that aligns the DNA for ligation through the dissociation of the kinase subunits of the DNA-PK complexes.

Non-homologous end joining (NHEJ) is a crucial double-strand break repair pathway in mammalian cells, resolving DNA damage from ionizing radiation and during V(D)J recombination. Although DNA Ligase IV is the only mammalian DNA ligase involved in NHEJ, it functions as a single-turnover enzyme in vitro. Recent cryo-electron microscopy (cryo-EM) studies revealed two LigIV molecules at the NHEJ synapse, supporting the hypothesis that two ligase molecules are needed for DSB repair by NHEJ. This study aims to validate this hypothesis by providing evidence for coordinated ligation at optimal distance and polarity preference for catalytic activity by LigIV. We determined that coordination occurs at a 4nt distance, though LigIV's binding is less stable and prone to exchange. There is promising potential of LigIV inhibitors for radiosensitization, although our pan ligase inhibitor derivatives did not yield radiosensitizing candidates. Further understanding of LigIV's role in NHEJ could advance therapeutic strategies to improve cancer treatments.

Non-homologous end joining (NHEJ) is a major pathway of DNA double strand break (DSB) repair, capable of directly joining both damaged strands of DNA through the coordinated activities of repair factors that detect the termini, physically bridge them together, and perform the chemistry necessary to complete repair. NHEJ is capable of repairing a variety of damaged DNA, employing various accessory end-processing factors to resolve chemically blocked ends, trim overhangs, and fill gaps in order to achieve directly ligatable DNA ends. To investigate the molecular mechanisms underlying end-processing, we determined the cryo-EM structure of the NHEJ specific polymerase Pol λ bound to the short-range synaptic complex, uncovering the mode of its recruitment to the complex as well as a putative model for its activity. Furthermore, the coordinated end-processing activities of the short-range (SR) synaptic complex simultaneously bound by both Pol λ and PNKP, another accessory factor, demonstrates the ability of NHEJ to form large, multifunctional repair complexes capable of processing a variety of different DNA end structures to effect repair.