E3 ubiquitin-protein ligase required for mesoderm patterning during embryonic development (By similarity). Acts as an enhancer of the transcriptional responses of the smad2/smad3 effectors, which are activated downstream of BMP. Acts by mediating ubiquitination and degradation of SMAD inhibitors such as smad7, inducing their proteasomal degradation and thereby enhancing the transcriptional activity of TGF-beta and BMP. Specifically binds polysumoylated chains via SUMO interaction motifs (SIMs) and mediates ubiquitination of sumoylated substrates (By similarity). The regulation of the BMP-SMAD signaling is however independent of sumoylation and is not dependent of SUMO interaction motifs (SIMs) (By similarity). 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. Binds free ubiquitin non-covalently via its RING-type zinc finger. Ubiquitin-binding leads to enhance the E3 ubiquitin-protein ligase activity by stabilizing the ubiquitin-conjugating enzyme E2 (donor ubiquitin) in the 'closed' conformation and activating ubiquitin transfer. Protein modification; protein ubiquitination. Monomer. Upon TGF-beta treatment, translocates from nucleus to cytosol. The SUMO interaction motifs (SIMs) mediates the binding to polysumoylated substrate. The RING-type zinc finger mediates the E3 ubiquitin-protein ligase activity and binds directly to free ubiquitin (By similarity). Non-covalent ubiquitin-binding stabilizes the ubiquitin-conjugating enzyme E2 (donor ubiquitin) in the 'closed' conformation and stimulates ubiquitin transfer (By similarity). Belongs to the Arkadia family.