Crystallization and preliminary X-ray diffraction analysis of the human XRCC4–XLF complex

XRCC4 and XLF are key proteins in the repair of DNA double-strand breaks through nonhomologous end-joining. Together, they form a complex that stimulates the ligation of double-strand breaks. Owing to the suggested filamentous nature of this complex, structural studies via X-ray crystallography have proven difficult. Multiple truncations of the XLF and XRCC4 proteins were cocrystallized, but yielded low-resolution diffraction (∼20 Å). However, a combination of microseeding, dehydration and heavy metals improved the diffraction of XRCC4^Δ157–XLF^Δ224 crystals to 3.9 Å resolution. Although molecular replacement alone was unable to produce a solution, when combined with the anomalous signal from tantalum bromide clusters initial phasing was successfully obtained.