E3 ubiquitin-protein ligase that monoubiquitinates H2B to form H2BK143ub1. H2BK143ub1 gives a specific tag for epigenetic transcriptional activation and is a prerequisite for H3 Lys-4 methylation (H3K4me). It thereby plays a central role in histone code and gene regulation (PubMed:26143250, PubMed:26934377). H2B monoubiquitination, mediated by HUB2, modulates transcriptional regulation of anther development, likely by promoting histone H3K4 dimethylation (H3K4me2) in the chromatin of the key tapetum degradation-related genes C4, CP1 and UDT1 (PubMed:26143250). H2B monoubiquitination, mediated by HUB2, modulates transcriptional regulation of genes associated with flowering time and plant yield (PubMed:26934377). 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. Interacts with HUB1. The RING-type zinc finger domain mediates binding to an E2 ubiquitin-conjugating enzyme. Auto-ubiquitinated in vitro. Semi-dwarf plants, early heading, and partial sterility of spikelets due to defects in the anther developmental program and pollen formation. Plant silencing HUB2 exhibit early flowering, increased plant height, and increased panicle and grain length. Belongs to the BRE1 family.