Briefly, cells were resuspended in ice-cold Buffer A (10 mM HEPES pH 7.9, 10 mM KCl, 1.5 mM MgCl2, 0.1 mM EDTA, 0.5 mM DTT, 0.4 % NP-40, 0.5 mM PMSF, complete protease inhibitor cocktail) at 4 °C for 5 min.