E3 ubiquitin-protein ligase which accepts ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfers the ubiquitin to targeted substrates. Polyubiquitinates POU5F1 by 'Lys-63'-linked conjugation and promotes it to proteasomal degradation; in embryonic stem cells (ESCs) the ubiquitination is proposed to regulate POU5F1 protein level. Ubiquitinates EGR2 and promotes it to proteasomal degradation; in T-cells the ubiquitination inhibits activation-induced cell death. Ubiquitinates SLC11A2; the ubiquitination is enhanced by presence of NDFIP1 and NDFIP2. Ubiquitinates RPB1 and promotes it to proteasomal degradation. 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. Activated by NDFIP1- and NDFIP2-binding. Protein modification; protein ubiquitination. Interacts with POU5F1, RBP1, EGR2 and SLC11A2 (By similarity). Interacts with SCNN1A, SCNN1B, SCNN1G, WBP1, WBP2 and ATN1. Interacts with ERBB4, NDFIP1 AND NDFIP2. Interacts with ARRDC4 (PubMed:23236378). Interacts (via WW domains) with ARRDC1 (via PPxY motifs); ubiquitinates ARRDC1 (PubMed:22315426, PubMed:21191027). Interacts (via WW domains) with ARRDC2 and ARRDC3 (PubMed:21191027). (Microbial infection) Interacts with adenovirus type 2 PIII. Detected in heart, throughout the brain, placenta, lung, liver, muscle, kidney and pancreas. Also detected in spleen and peripheral blood leukocytes. Highly expressed in undifferentiated embryonic stem cells and expression is reduced after embryoid body (EB) formation. Not detectable at day 13 of EB formation; low levels are again detected at day 18 of EB formation. The C2 domain is involved in autoinhibition of the catalytic activity by interacting with the HECT domain. The WW domains mediate interaction with PPxY motif-containing proteins. Autoubiquitinated. Ubiquitinated by the SCF(FBXL15) complex, leading to its degradation by the proteasome. A cysteine residue is required for ubiquitin-thioester formation.