For immunoprecipitation analysis, lysates were diluted 1:5 in E1A buffer (50 mM Hepes pH 7.5, 250 mM NaCl, 0.1% NP-40) supplemented with Complete protease inhibitors and 5 mM NEM. Immunoprecipitation was performed with 2-3 mg of total lysate using a ratio of 3-7 mg antibody/mg of total proteins (see Supplementary Material and Methods in Text S1 for details on antibodies) and protein A-Sepharose beads. For co-immunoprecipitation analysis, cells were lysed in E1A buffer supplemented with Complete protease inhibitors and 5 mM NEM, and successively co-immunoprecipitations were performed with 1.5-5 mg of total lysate and 50 ml of 50% slurry of anti-FLAG-M2-Agarose beads (Sigma).