Flag m2 beads

For immunoblotting, cells were lysed in radioimmunoprecipitation assay buffer (Thermo Fisher Scientific), Dithiothreitol, and a protease/phosphatase inhibitor cocktail (Cell Signaling Technology). For coimmunoprecipitation assay, cells were lysed in 50 mM Tris, pH 6.8, 0.5% NP-40, 200 mM NaCl, 10% glycerol, 1 mM EDTA, and 1× protease/phosphatase inhibitor cocktail (Cell Signaling Technology). Cell lysates were incubated with Flag-M2 beads for 2 h at room temperature or with V5 antibodies for 2 h at 4°C, then incubated with Protein A Sepharose 4 Fast Flow and Protein G Sepharose 4 Fast Flow beads (GE Healthcare) for 1 h at 4°C. Immunoprecipitates were subjected to Western blotting. The antibodies used were directed against STAT1 (Santa Cruz Biotechnology), STAT2 (Millipore), phospho-Tyr 689 STAT2 (Millipore), ISG15 (Santa Cruz Biotechnology), mAb cl.2.1, a gift of E.J. Borden, Cleveland Clinic, Cleveland, OH), V5 (Invitrogen), and Flag (Sigma-Aldrich). Antibodies to phospho-Tyr 701 STAT1, USP18, β-Actin, IFIT1, and AKT were all from Cell Signaling Technology. The chemiluminescence detection reagent was Pierce ECL Western Blotting Substrate (Thermo Fisher Scientific) or Western Lightning Plus-ECL (PerkinElmer).