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. Expressed in antigen presenting cells, APCs, located in lymph nodes and spleen. Also expressed in lung. Expression is high in follicular B-cells, moderate in dendritic cells and low in splenic T-cells. During maturation of dendritic cells, expression is down-regulated and stabilizes MHC class II proteins accumulate at the plasma membrane. By IL10/interleukin-10. 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 (By similarity).