E3 ubiquitin-protein ligase that acts as a regulator of motor axon elongation (PubMed:23610558). 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 (PubMed:23610558). Acts by mediating ubiquitination and degradation of SMAD inhibitors such as SMAD6, SMAD7, SKI and SNON isoform of SKIL (PubMed:23610558). 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. Monomer; binding to the ubiquitin-conjugating enzyme E2 does not trigger homodimerization. Expressed in neurons of the nervous system. The RING-type zinc finger mediates the E3 ubiquitin-protein ligase activity and binds directly to free ubiquitin. Non-covalent ubiquitin-binding stabilizes the ubiquitin-conjugating enzyme E2 (donor ubiquitin) in the 'closed' conformation and stimulates ubiquitin transfer. Around 10% of mice die at birth. While surviving pups are the same size as their littermates at birth, they fail to thrive and grow, reaching only 50% of the size of their siblings at postnatal day 15. They die before weaning during the first 3 postnatal weeks. Null pups display relaxed forepaws and reduced dorsiflexion. Hind limb defects are also observed. Defects are caused by inefficient growth of motor axons to distant muscles. Belongs to the Arkadia family.