E3 ubiquitin-protein ligase that mediates monoubiquitination of 'Lys-119' of histone H2A (H2AK119Ub), thereby playing a central role in histone code and gene regulation. H2AK119Ub gives a specific tag for epigenetic transcriptional repression and participates in X chromosome inactivation of female mammals. May be involved in the initiation of both imprinted and random X inactivation. Essential component of a Polycomb group (PcG) multiprotein PRC1-like complex, a complex class required to maintain the transcriptionally repressive state of many genes, including Hox genes, throughout development. PcG PRC1 complex acts via chromatin remodeling and modification of histones, rendering chromatin heritably changed in its expressibility. E3 ubiquitin-protein ligase activity is enhanced by BMI1/PCGF4. Acts as the main E3 ubiquitin ligase on histone H2A of the PRC1 complex, while RING1 may rather act as a modulator of RNF2/RING2 activity. Plays a role in the transcriptional repression of genes that are required for pluripotency in embryonic stem cells, thereby contributing to differentiation of the ectodermal and endodermal germ layers. Association with the chromosomal DNA is cell-cycle dependent. In resting B- and T-lymphocytes, interaction with AURKB leads to block its activity, thereby maintaining transcription in resting lymphocytes. Also acts as a negative regulator of autophagy by mediating ubiquitination of AMBRA1, leading to its subsequent degradation. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Component of chromatin-associated Polycomb (PcG) complexes. Component of a number of PRC1-like complexes; these complexes contain either the polycomb group ring finger protein PCGF1, or PCGF2, or PCGF3, or BMI1, or PCGF5, or PCGF6. Distinct PRC1-like complexes are composed of a RING1 subunit (RING1B or RING1A), one of the six PCGF proteins (PCGF1, PCGF2, PCGF3, BMI1, PCGF5 or PCGF6), one PHC protein (PHC1, PHC2 or PHC3) and one of the CBX proteins (CBX2, CBX4, CBX6, CBX7 or CBX8) (By similarity). Part of a complex that contains RNF2, UB2D3 and BMI1; within that complex RNF2 and BMI1 form a tight heterodimer, where UB2D3 interacts only with RNF2. The complex composed of RNF2, UB2D3 and BMI1 binds nucleosomes, and has activity only with nucleosomal histone H2A (By similarity). Part of a complex that contains PCGF5, RNF2 and UBE2D3. Part of a complex that contains AUTS2, PCGF5, RNF2, CSNK2B AND RYBP (By similarity). Interacts with CBX6 and CBX8 (By similarity). Interacts with PHC1, PCGF2, RYBP, CBX7, CBX4, CBX2, RNF1/RING1, BMI1 and PHC2. Interaction with RYBP and CBX7 is mutually exclusive; both compete for the same binding site on RNF2 (By similarity). Component of repressive BCOR complex containing a Polycomb group subcomplex at least composed of RYBP, PCGF1, BCOR and RING1 (By similarity). Interacts with CBX2 and PHC1. Interacts with CHTOP. Interacts with AURKB (By similarity). Part of the E2F6.com-1 complex in G0 phase composed of E2F6, MGA, MAX, TFDP1, CBX3, BAT8, EUHMTASE1, RNF1/RING1, RNF2/RING2, MBLR, L3MBTL2 and YAF2 (By similarity). Component of some MLL1/MLL complex, at least composed of the core components KMT2A/MLL1, ASH2L, HCFC1/HCF1, WDR5 and RBBP5, as well as the facultative components BAP18, CHD8, E2F6, HSP70, INO80C, KANSL1, LAS1L, MAX, MCRS1, MGA, MYST1/MOF, PELP1, PHF20, PRP31, RING2, RUVB1/TIP49A, RUVB2/TIP49B, SENP3, TAF1, TAF4, TAF6, TAF7, TAF9 and TEX10. Interacts with RYBP, HIP2 and TFCP2 (By similarity). Interacts with NUPR1 (By similarity). Enriched on inactive X chromosome (Xi) in female trophoblast stem (TS) cells as well as differentiating embryonic stem (ES) cells. The enrichment on Xi is transient during TS and ES cell differentiation. The association with Xi is mitotically stable in non-differentiated TS cells. Monoubiquitinated, by auto-ubiquitination (By similarity). Polyubiquitinated in the presence of UBE2D3 (in vitro) (By similarity).