Immunoblotting

Immunoblotting was performed as described previously (Tatavosian et al., 2015; Zhen et al., 2014). Nuclei were lysed in buffer containing 20 mM Tris-HCl pH 7.4, 2.0% NP-40, 500 mM NaCl, 0.25 mM EDTA, 0.1 mM Na₃VO₄, 0.1 mM PMSF, and protease inhibitors. Proteins were resolved using NuPAGE 4–12% Bis-Tris Gel (NPO322BOX; Life Technologies, Carlsbad, CA) and transferred to 0.45 μm Immobilon-FL PVDF membrane (IPFL00010; EMD Millipore Corporation, Massachusetts, MA). Membranes were probed with anti-Cbx7 (ab21873; Abcam, MA) and anti-HaloTag (G9281; Promega, Sunnyvale, CA). After incubating with HRP-conjugated anti-rabbit antibody (NA934V; GE Healthcare, Pittsburgh, PA), proteins were detected using ECL Plus detection reagents (RPN2106; GE Healthcare, Pittsburgh, PA). Membranes were imaged using a ChemiDoc XRS system (BioRad).