Microarray screening reveals two non-conventional SUMO-binding modules linked to DNA repair by non-homologous end-joining.

SUMOylation is critical for numerous cellular signalling pathways, including the maintenance of genome integrity via the repair of DNA double-strand breaks (DSBs). If misrepaired, DSBs can lead to cancer, neurodegeneration, immunodeficiency and premature ageing. Using systematic human proteome microarray screening combined with widely applicable carbene footprinting, genetic code expansion and high-resolution structural profiling, we define two non-conventional and topology-selective SUMO2-binding regions on XRCC4, a DNA repair protein important for DSB repair by non-homologous end-joining (NHEJ). Mechanistically, the interaction of SUMO2 and XRCC4 is incompatible with XRCC4 binding to three other proteins important for NHEJ-mediated DSB repair. These findings are consistent with SUMO2 forming a redundant NHEJ layer with the potential to regulate different NHEJ complexes at distinct levels including, but not limited to, XRCC4 interactions with XLF, LIG4 and IFFO1. Regulation of NHEJ is not only relevant for carcinogenesis, but also for the design of precision anti-cancer medicines and the optimisation of CRISPR/Cas9-based gene editing. In addition to providing molecular insights into NHEJ, this work uncovers a conserved SUMO-binding module and provides a rich resource on direct SUMO binders exploitable towards uncovering SUMOylation pathways in a wide array of cellular processes.

1H, 13C, 15N backbone resonance assignment for the 1-164 construct of human XRCC4.

DNA double-strand breaks (DSBs) represent the most cytotoxic DNA lesions, as-if mis- or unrepaired-they can cause cell death or lead to genome instability, which in turn can cause cancer. DSBs are repaired by two major pathways termed homologous recombination and non-homologous end-joining (NHEJ). NHEJ is responsible for repairing the vast majority of DSBs arising in human cells. Defects in NHEJ factors are also associated with microcephaly, primordial dwarfism and immune deficiencies. One of the key proteins important for mediating NHEJ is XRCC4. XRCC4 is a dimer, with the dimer interface mediated by an extended coiled-coil. The N-terminal head domain forms a mixed alpha-beta globular structure. Numerous factors interact with the C-terminus of the coiled-coil domain, which is also associated with significant self-association between XRCC4 dimers. A range of construct lengths of human XRCC4 were expressed and purified, and the 1-164 variant had the best NMR properties, as judged by consistent linewidths, and chemical shift dispersion. In this work we report the 1H, 15 N and 13C backbone resonance assignments of human XRCC4 in the solution form of the 1-164 construct. Assignments were obtained by heteronuclear multidimensional NMR spectroscopy. In total, 156 of 161 assignable residues of XRCC4 were assigned to resonances in the TROSY spectrum, with an additional 11 resonances assigned to His-Tag residues. Prediction of solution secondary structure from a chemical shift analysis using the TALOS + webserver is in good agreement with the published X-ray crystal structures of this protein.

SAMHD1 Sheds Moonlight on DNA Double-Strand Break Repair.

SAMHD1 (sterile α motif and histidine (H) aspartate (D) domain-containing protein 1) is known for its antiviral activity of hydrolysing deoxynucleotides required for virus replication. Daddacha et al. identify a hydrolase-independent, moonlighting function of SAMHD1 that facilitates homologous recombination of DNA double-strand breaks (DSBs) by promoting recruitment of C-terminal binding protein interacting protein (CTIP), a DNA-end resection factor, to damaged DNA. These findings could benefit anticancer treatment.