Proteomic analysis reveals mutually exclusivity of PRC1 paralog family members in PRC1 complexes. (A) Schematic representation of retroviral vectors expressing a bicistronic mRNA resulting in expression of Avi-tagged PCGF4/BMI1 and GFP-BirA. MSCV IRES GFP-BirA was used as a negative control. (B) Anti-BMI1 western blot showing input (I), nonbound (NB), and bound (B) fraction of streptavidin pullout of Avi-BMI1 from K562 cells stably expressing Avi-BMI1. (C) Western analyses showing input (I), nonbound (NB), and bound (B) fraction of BirA, CBX-Avi, Avi-PCGF1, Avi-PCGF2/MEL18, Avi-PCGF4/BMI1, and Avi-RING1A pullouts. Blot was stained using BMI1 and RING1B antibodies. (D) Table showing number of unique peptides of PcG proteins coprecipitated with Avi-PCGF1, Avi-PCGF2, Avi-PCGF4, CBX2-Avi, or Avi-RING1A.