E3 ubiquitin-protein ligase that mediates ubiquitination of TFRC, CD86, FAS and MHC class II proteins, such as HLA-DR alpha and beta, and promotes their subsequent endocytosis and sorting to lysosomes via multivesicular bodies. By constitutively ubiquitinating MHC class II proteins in immature dendritic cells, down-regulates their cell surface localization thus sequestering them in the intracellular endosomal system. 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. The RING-CH-type zinc finger domain is required for E3 ligase activity. Has a short half-life. Instability/short half-life permits rapid changes that allow efficient induction of antigen presentation once antigen presenting cells, APCs, receive maturation signals. Small changes in protein levels significantly alter the cell surface display of MHC class II proteins. Shows increase in the cell surface expression or half-life of MHC class II. Null cells have accumulated MHC class II and CD86 at the cell surface and a low antigen-presenting ability for exogenous antigens, in conventional dendritic cells (PubMed:19917682). Null cells have high antigen-presenting ability for exogenous antigens in B-cells (PubMed:17255932).