XLF regulates filament architecture of the XRCC4·ligase IV complex.

DNA ligase IV (LigIV) is critical for nonhomologous end joining (NHEJ), the major DNA double-strand break (DSB) repair pathway in human cells, and LigIV activity is regulated by XRCC4 and XLF (XRCC4-like factor) interactions. Here, we employ small angle X-ray scattering (SAXS) data to characterize three-dimensional arrangements in solution for full-length XRCC4, XRCC4 in complex with LigIV tandem BRCT domains and XLF, plus the XRCC4·XLF·BRCT2 complex. XRCC4 forms tetramers mediated through a head-to-head interface, and the XRCC4 C-terminal coiled-coil region folds back on itself to support this interaction. The interaction between XLF and XRCC4 is also mediated via head-to-head interactions. In the XLF·XRCC4·BRCT complex, alternating repeating units of XLF and XRCC4·BRCT place the BRCT domain on one side of the filament. Collective results identify XRCC4 and XLF filaments suitable to align DNA molecules and function to facilitate LigIV end joining required for DSB repair in vivo.