E3 ubiquitin-protein ligase that acts as a negative regulator of BMP signaling pathway. Mediates ubiquitination and degradation of smad1 and smad5, 2 receptor-regulated SMADs specific for the BMP pathway. Promotes ubiquitination and subsequent proteasomal degradation of TRAF family members and rhoa. May play a role in dendrite formation by melanocytes. 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 from the egg stage to the swimming tadpole, with maximum levels observed in the stages from egg to gastrula. At gastrulation distributed uniformly in embryonic ectoderm and involuting mesoderm, and expression gradually localizes to the nervous system. At early tadpole stages expressed in the CNS, eye, branchial arches, kidney and somites. The C2 domain mediates membrane localization and substrate selection.