Systematic discovery of UFM1 receptors reveals a regulatory module in DNA repair directing non-homologous end-joining

Posttranslational modifications with ubiquitin-like modifiers (UBLs) are critical for genome maintenance, yet many remain mechanistically uncharacterised. Here, we identify UFM1 as a key regulator of non-homologous end-joining (NHEJ), a major DNA double-strand break repair pathway. Using a structure-guided chemical biology approach, we develop a photo-crosslinkable UFM1 probe and, in combination with NMR, map non-canonical UFM1-binding interfaces in core NHEJ factors, including the disordered XRCC4 tail. Mechanistically, proximity-dependent proteomics and functional assays identify Ku70 as a crucial UFMylation substrate and reveal a UFM1-dependent axis in which XRCC4 engages UFMylated Ku70 to stabilise NHEJ complex assembly on chromatin. Disruption of this molecular mechanism via UFSP2 depletion or a hypomorphic UBA5 variant in patient-derived cells impairs NHEJ function, linking UFMylation defects to compromised genome integrity processes. Our findings define a complete UFM1 signalling module in DNA repair and establish a generalisable framework for dissecting low-affinity UBL networks with broad functional and disease relevance.