E3 ubiquitin-protein ligase. Plays an important role in regulating the surface level of MUSK on myotubes. Mediates the ubiquitination of MUSK, promoting its endocytosis and lysosomal degradation. Might contribute to terminal myogenic differentiation. 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. Interacts with NLGN1 and EFNB2. Interacts with UBE2D2 and with MUSK via the first PDZ domain (By similarity). Localizes to the postsynaptic region of neuromuscular junctions. The RING-type zinc finger domain is required for E3 ligase activity. Auto-ubiquitinated.