Protein extract preparation and Western blot analysis were performed as previously described (27 (link), 31 (link)). Antibodies are listed in Supplementary Table 3. For the nuclear fractionations, 2x107 B cells were incubated for 15 min in 100 µl buffer A (10 mM HEPES pH 7.9: 10 mM KCL; 0.1 mM EDTA; 0.1 mM EGTA; 1 mM DTT; 1x Protease inhibitor (Mini complete, Roche) on ice. Subsequently, 6.25 μl 10% NP-40 (Sigma-Aldrich) was added to each sample and the samples were shaken for 5 min at 4°C on a vortexer. Afterwards, the nuclei were spun down (10.000xg) for 15 min. The supernatant contained the cytoplasmic fraction and was stored at -80°C. The nuclei were washed in 1.5 ml Buffer A. After centrifugation, the pellet was mixed with 40 μl of buffer C (20 mM HEPES pH 7.9; 0.4 M NaCL; 1 mM EDTA; 1 mM EGTA; 1 mM DTT; 1x protease inhibitor (Mini complete, Roche) to lyse the nuclei. Samples were then shaken on a vortexer for 30 min at 4°C. After centrifugation (10.000xg for 15 min) the supernatant, which contained the nuclear fraction, was stored at -80°C.
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