Mapping DNA repair interactions using EPR and molecular modelling.

Understanding the process by which DNA breaks are healed via by non-homologous end joining is vital for developing the design and precision of future anti-cancer medicines and the optimization of CRISPR/Cas9-based gene editing. The X-ray repair cross-complementing protein 4 (XRCC4) is a core DNA repair factor known for its importance in this type of DNA repair. As part of the DNA repair process it interacts with other proteins including the core non-homologous end joining factor XRCC4-like factor (XLF), implicated in tethering the DNA ends together promoting DNA-end ligation, the nucleoskeleton protein intermediate filament family orphan 1 (IFFO1), to stabilise the broken ends of the DNA, and the non-homologous end joining ligating enzyme DNA ligase 4 (LIG4), that facilitates the final ligation step in non-homologous end joining. XRCC4 also interacts with chains of small ubiquitin-like modifier (SUMO) units, which experiments suggest block the sites where the other proteins and enzymes interact with XRCC4. In this way SUMO regulates the action of XRCC4 and the non-homologous end joining of DNA.