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. Ubiquitinates ERBB4 isoforms JM-A CYT-1 and JM-B CYT-1, KLF2, KLF5 and TP63 and promotes their proteasomal degradation. Ubiquitinates RNF11 without targeting it for degradation. Ubiquitinates and promotes degradation of TGFBR1; the ubiquitination is enhanced by SMAD7. Ubiquitinates SMAD6 and SMAD7. Ubiquitinates and promotes degradation of SMAD2 in response to TGF-beta signaling, which requires interaction with TGIF. 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. Binds KLF2 AND HIVEP3 (By similarity). Binds SCNN1A, SCNN1B, SCNN1G, WBP1, WBP2, DRPLA and adenovirus type 2 PIII. Interacts with RNF11 (By similarity). Interacts with SPART. Interacts with ERBB4 isoforms JM-B CYT-1 and JM-A CYT-1. Interacts with SMAD1, SMAD2, SMAD3, SMAD5, SMAD6, SMAD7, TGFBR1 AND TGFBR2. Associates with the TGFBR1:TGFBR2 receptor complex in presence of SMAD7. Interacts with SKIL isoform 1. Interacts with TP63 isoform 1 and isoform 2. Interacts with STAMBP and RNF11. Interacts with NDFIP1 and NDFIP2 (Probable); this interaction activates the E3 ubiquitin-protein ligase. Interacts with TGIF. Interacts (via WW domains) with ARRDC1, ARRDC2 and ARRDC3 (PubMed:21191027). (Microbial infection) Interacts with HTLV-1 protein Gag. (Microbial infection) Interacts with ebola virus protein VP40. Additional isoforms seem to exist. Detected in heart, placenta, pancreas, kidney, liver, skeletal muscle, bone marrow, fetal brain, and at much lower levels in adult brain and lung. Isoform 1 and isoform 5 predominate in all tissues tested, except in testis and bone marrow, where isoform 5 is expressed at much higher levels than isoform 1. The WW domains mediate interaction with PPxY motif-containing proteins. Auto-ubiquitinated and ubiquitinated by RNF11.