Poly(ADP-ribose) polymerase I (PARP1) is a primary DNA damage sensor whose (ADP-ribose) polymerase activity is acutely regulated by interaction with DNA breaks. Upon activation at sites of DNA damage, PARP1 modifies itself and other proteins by covalent addition of long branched polymers of ADP-ribose, which in turn recruit downstream DNA repair and chromatin remodelling factors. PARP1 recognizes DNA damage through its N-terminal DNA-binding domain (DBD), which consists of a tandem repeat of an unusual zinc-finger (ZnF) domain. We have now determined the crystal structure of the human PARP1-DBD bound to a DNA break. Along with functional analysis of PARP1 recruitment to sites of DNA damage in vivo, the structure reveals a dimeric assembly whereby ZnF1 and ZnF2 domains from separate PARP1 molecules form a strand-break recognition module that helps activate PARP1 by facilitating its dimerization and consequent trans-automodification.Short-patch repair of DNA single-strand breaks is initiated by poly(ADP-ribose) polymerase-1 (PARP1) -a multi-domain enzyme activated by binding of its N-terminal DNA-binding domain (DBD) to DNA breaks [1][2][3][4][5] . Activated PARP1 utilises NAD + to Correspondence to: Andreas G. Ladurner; Laurence H. Pearl; Antony W. Oliver. AUTHOR CONTRIBUTIONS A.A.E.A. purified the protein, crystallized the complex and collected the X-ray diffraction data; G.T. designed and constructed the PARP1-EGFP constructs and performed the laser DNA damage experiments; M.K. performed the FRAP experiments; P.O.H. engineered the knockdown PARP1 cell line and the wild-type imaging reporter constructs, and performed the in vitro complementation assays; M.H. and R.A.-B. engineered and purified mutant PARP1 constructs; A.G.L. designed the study and analysed the data; L.H.P designed the study, analysed the data and wrote the paper; A.W.O. made the baculovirus constructs, designed the purification protocol, and solved and refined the crystal structure. All authors discussed the results and commented on the manuscript. We have now determined the crystal structure of the DNA-binding domain of PARP1 (PARP1-DBD) encompassing the first two zinc-finger (ZnF) domains, bound to a DNA break. In contrast with structural analysis of the separated domains 22 , we show that DNA binding by both zinc-finger domains is essential to damage recruitment in vivo, and that ZnF1 and ZnF2 domains from separate PARP1 molecules act as a functional unit to generate a dimeric binding module that specifically recognizes the single-strand / double-strand transition at a recessed DNA break. Mutational analysis in vitro and in cells demonstrates the functional requirement for zinc-finger dimerisation and reveals a mechanism for bringing two PARP1 molecules into close proximity at a DNA break as a prerequisite for transmodification. RESULTS Structure of the PARP1-DBD -DNA ComplexAn N-terminal segment of human PARP1 (residues 5-202) was expressed in insect cells and purified by column chromatography. Screening with a range of DNA molecules...

Non-homologous end joining (NHEJ) is critical for the maintenance of genetic integrity and DNA double-strand break (DSB) repair. NHEJ is regulated by a series of interactions between core components of the pathway, including Ku heterodimer, XLF/Cernunnos, and XRCC4/ DNA Ligase 4 (Lig4). However, the mechanisms by which these proteins assemble into functional protein-DNA complexes are not fully understood. Here, we show that the von Willebrand (vWA) domain of Ku80 fulfills a critical role in this process by recruiting Aprataxin-and-PNK-Like Factor (APLF) into Ku-DNA complexes. APLF, in turn, functions as a scaffold protein and promotes the recruitment and/or retention of XRCC4-Lig4 and XLF, thereby assembling multi-protein Ku complexes capable of efficient DNA ligation in vitro and in cells. Disruption of the interactions between APLF and either Ku80 or XRCC4-Lig4 disrupts the assembly and activity of Ku complexes, and confers cellular hypersensitivity and reduced rates of chromosomal DSB repair in avian and human cells, respectively. Collectively, these data identify a role for the vWA domain of Ku80 and a molecular mechanism by which DNA ligase proficient complexes are assembled during NHEJ in mammalian cells, and reveal APLF to be a structural component of this critical DSB repair pathway.

The innate immune system is critical in the response to infection by pathogens and it is activated by pattern recognition receptors (PRRs) binding to pathogen associated molecular patterns (PAMPs). During viral infection, the direct recognition of the viral nucleic acids, such as the genomes of DNA viruses, is very important for activation of innate immunity. Recently, DNA-dependent protein kinase (DNA-PK), a heterotrimeric complex consisting of the Ku70/Ku80 heterodimer and the catalytic subunit DNA-PKcs was identified as a cytoplasmic PRR for DNA that is important for the innate immune response to intracellular DNA and DNA virus infection. Here we show that vaccinia virus (VACV) has evolved to inhibit this function of DNA-PK by expression of a highly conserved protein called C16, which was known to contribute to virulence but by an unknown mechanism. Data presented show that C16 binds directly to the Ku heterodimer and thereby inhibits the innate immune response to DNA in fibroblasts, characterised by the decreased production of cytokines and chemokines. Mechanistically, C16 acts by blocking DNA-PK binding to DNA, which correlates with reduced DNA-PK-dependent DNA sensing. The C-terminal region of C16 is sufficient for binding Ku and this activity is conserved in the variola virus (VARV) orthologue of C16. In contrast, deletion of 5 amino acids in this domain is enough to knockout this function from the attenuated vaccine strain modified vaccinia virus Ankara (MVA). In vivo a VACV mutant lacking C16 induced higher levels of cytokines and chemokines early after infection compared to control viruses, confirming the role of this virulence factor in attenuating the innate immune response. Overall this study describes the inhibition of DNA-PK-dependent DNA sensing by a poxvirus protein, adding to the evidence that DNA-PK is a critical component of innate immunity to DNA viruses.

The Ku-binding motif (KBM) is a short peptide module first identified in APLF that we now show is also present in Werner syndrome protein (WRN) and in Modulator of retrovirus infection homologue (MRI). We also identify a related but functionally distinct motif in XLF, WRN, MRI and PAXX, which we denote the XLF-like motif. We show that WRN possesses two KBMs; one at the N terminus next to the exonuclease domain and one at the C terminus next to an XLF-like motif. We reveal that the WRN C-terminal KBM and XLF-like motif function cooperatively to bind Ku complexes and that the N-terminal KBM mediates Ku-dependent stimulation of WRN exonuclease activity. We also show that WRN accelerates DSB repair by a mechanism requiring both KBMs, demonstrating the importance of WRN interaction with Ku. These data define a conserved family of KBMs that function as molecular tethers to recruit and/or stimulate enzymes during NHEJ.