ARK2C is an E3 ubiquitin ligase that functions as a critical regulator of motor axon elongation during nervous system development 1. As a RING-domain E3 ligase, ARK2C catalyzes ubiquitin chain assembly through a distinctive mechanism in which its RING domain directly binds free ubiquitin, coordinating with E2-conjugated ubiquitin to stabilize the catalytic complex and enhance ubiquitin transfer efficiency 2. This unique ubiquitin-dependent activation distinguishes ARK2C from many other RING ligases 3. Functionally, ARK2C promotes motor axon extension by enhancing BMP-SMAD signaling; it mediates ubiquitination and degradation of SMAD inhibitors (SMAD6, SMAD7, SKI, and SNON), thereby potentiating phosphorylated SMAD1/5/8 transcriptional responses 1. ARK2C recruits multiple E2 conjugating enzymes with context-specific selectivity, enabling assembly of distinct ubiquitin chain types—both degradative and non-degradative—depending on the E2 partner and substrate 4. Loss of ARK2C in mice causes severe motor innervation defects including reduced motor axon extension and impaired neuromuscular junction formation, resulting in perinatal lethality, underscoring its essential developmental role 1. These findings establish ARK2C as a specialized E3 ligase linking ubiquitin-dependent proteolysis to BMP signaling control in motor neuron development.