| DNA Ligase IV, or LIG4, is an ATP-dependent DNA ligase from the DNA ligase family that acts as the catalytic core of the non-homologous end-joining pathway, carrying out the final nick-sealing step to repair DNA double-strand breaks caused by ionizing radiation or replication stress. This enzyme contains a DNA-binding domain, a nucleotidyltransferase domain, and an oligonucleotide-binding domain that together dynamically encircle duplex DNA substrates; its active site residues, lysine 305 and lysine 471, form lysyl-AMP intermediates and coordinate phosphoanhydride bonds for 5'-phosphate adenylation, followed by nucleophilic attack from the 3'-hydroxyl group to complete ligation. LIG4's main function is to be recruited in an XRCC4 and XLF-dependent manner to double-strand break ends bound by Ku70/80, where it forms short-range synaptic complexes with a single LIG4-XRCC4 dimer bridging compatible ends for accurate ligation before any end-processing, thereby ensuring high-fidelity non-homologous end joining throughout the cell cycle and preventing the use of error-prone alternative end-joining or microhomology pathways. Its ability to bind DNA ends through both the DNA-binding domain and dynamic weak interactions stabilizes synapsis, supports ligase III as a backup when LIG4 is inactive, and is essential for V(D)J recombination in immune system development. Mutations that reduce LIG4 activity cause LIG4 syndrome, characterized by immunodeficiency, microcephaly, and radiosensitivity due to defective double-strand break rejoining, while oncogenic alterations in LIG4 can drive cancer by promoting genomic instability. |
