E3 ubiquitin-protein ligase that acts as a regulator of motor axon elongation. Required for efficient motor axon extension in the dorsal forelimb by enhancing the transcriptional responses of the SMAD1/SMAD5/SMAD8 effectors, which are activated downstream of BMP. Acts by mediating ubiquitination and degradation of SMAD inhibitors such as SMAD6, SMAD7, SKI and SNON isoform of SKIL. 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 (PubMed:26656854). Monomer; binding to the ubiquitin-conjugating enzyme E2 does not trigger homodimerization (PubMed:26656854). The RING-type zinc finger mediates the E3 ubiquitin-protein ligase activity and binds directly to free ubiquitin (PubMed:26656854). Non-covalent ubiquitin-binding stabilizes the ubiquitin-conjugating enzyme E2 (donor ubiquitin) in the 'closed' conformation and stimulates ubiquitin transfer (PubMed:26656854). Belongs to the Arkadia family.