Cloning, localization and focus formation at DNA damage sites of canine XLF

Understanding the molecular mechanisms of DNA double-strand break (DSB) repair processes, especially nonhomologous DNA-end joining (NHEJ), is critical for developing next-generation radiotherapies and chemotherapeutics for human and animal cancers. The localization, protein-protein interactions and post-translational modifications of core NHEJ factors, such as human Ku70 and Ku80, might play critical roles in controlling NHEJ activity. XRCC4-like factor (XLF) is a core NHEJ factor and plays a key role in the Ku-dependent NHEJ repair process in human cells. Recently, companion animals, such as canines, have been proposed to be a good model for many aspects of cancer research, including the development of chemotherapeutics. However, the localization and regulation of core NHEJ factors in canine cells have not been elucidated. Here, we show that the localization of canine XLF changes dynamically during the cell cycle. EYFP-canine XLF localizes in the nuclei of interphase cells and accumulates immediately at microirradiated DSB sites. The structure of a putative human XLF nuclear localization signal (NLS) and a putative 14-3-3 binding motif are evolutionarily conserved in canine, chimpanzee and mouse XLF. However, the putative β-TRCP-recognizable degron of human XLF is not conserved in canine and mouse. Additionally, some vital human XLF phosphorylation sites, including the ATM major phosphorylation site (S251), are not conserved in canine XLF. Our findings might be useful for the study of the molecular mechanisms of NHEJ in canine cells and for the development of new radiosensitizers that target XLF.