The cross‐linker sulfo‐SDA (sulfosuccinimidyl 4,4′‐azipentanoate; Thermo Scientific) was dissolved in cross‐linking buffer (10 mM HEPES‐NaOH, pH 7.8, 150 mM NaCl, 4 mM MgCl, 0.5 mM TCEP) to 100 mM before use. The labelling step was performed by incubating 23 μg aliquots of the DDB1ΔBPB/E27/DDA1/STAT2 complex at 1 mg/ml with 0.2, 0.3, 0.6 and 0.8 mM sulfo‐SDA, added, respectively, for an hour. The samples were then irradiated with UV light at 365 nm using a Luxigen LZ1 LED emitter (Osram Sylvania Inc.), to form cross links, for 10 s and quenched with 50 mM Tris–HCl pH 7.5 for 20 min. All steps were performed on ice. Reaction products were separated on a Novex Bis‐Tris 4–12% SDS−PAGE gel (Life Technologies) in order to identify suitable cross‐linker concentrations. Samples corresponding to 0.2 and 0.3 mM SDA concentrations were precipitated in 80% acetone at −20°C. All subsequent steps were carried out at room temperature. Protein pellets were resuspended in denaturing buffer (50 mM NH₄HCO₃, 8 M urea), reduced with 2 mM DTT for 30 min and alkylated with 5 mM iodoacetamide for 30 min. Urea concentration was then reduced to 1.5 M by dilution with 50 mM NH₄HCO₃ and samples were digested with trypsin (Thermo Scientific Pierce; Shevchenko et al, 2006 (link)) overnight. The resulting tryptic peptides were extracted, desalted using C18 StageTips (Rappsilber et al, 2003 (link)) and pooled. Eluted peptides were fractionated on a Superdex Peptide 3.2/300 increase column (GE Healthcare) at a flow rate of 10 μl/min using 30% (v/v) acetonitrile and 0.1% (v/v) trifluoroacetic acid as mobile phase. 50 μl fractions were collected and vacuum‐dried.