CBX4 SUMOylates BMI1 in Polycomb Repressive Complex 1 (PRC1) at lysine-88 (Ismail et al. 2012). SUMOylation of BMI1 is necessary for its accumulation at sites of DNA damage. CBX4 directly binds poly(ADP-ribose) synthesized by PARP1 at sites of damage.
SUMO transferase activity of PRC1 complex [nucleoplasm]