E3 ubiquitin-protein ligase that monoubiquitinates H2B to form H2BK143ub1. H2BK143ub1 gives a specific tag for epigenetic transcriptional activation and is also prerequisite for H3K4me and maybe H3K79me. It thereby plays a central role in histone code and gene regulation. Forms a ubiquitin ligase complex in cooperation with the E2 enzyme UBC2/RAD6. 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. May act as a tetramer consisting of two copies of HUB1 and two copies of HUB2. Ubiquitously expressed. Constant throughout the cell cycle. The RING-type zinc finger domain mediates binding to an E2 ubiquitin-conjugating enzyme. Plants have reduced seed dormancy and several pleiotropic phenotypes, including alterations in leaf color, plant architecture and flower morphology. HUB1 and HUB2 are involved in the same processes, but are weakly or not redundant. May be due to an intron retention. Belongs to the BRE1 family.