E3 ubiquitin-protein ligase BRE1B (RNF40)

Component of the RNF20/40 E3 ubiquitin-protein ligase complex that mediates monoubiquitination of'Lys-120' of histone H2B (H2BK120ub1). H2BK120ub1 gives a specific tag for epigenetic transcriptional activation and is also prerequisite for histone H3 'Lys-4' and 'Lys-79' methylation (H3K4me and H3K79me, respectively).

It thereby plays a central role in histone code and gene regulation. The RNF20/40 complex forms a H2B ubiquitin ligase complex in cooperation with the E2 enzyme UBE2A or UBE2B; reports regarding the cooperation with UBE2E1/UBCH are contradictory.