E3 ubiquitin ligase. Mediates the ubiquitination and subsequent proteasomal degradation of CKM, GMEB1 and HIBADH. Regulates the proteasomal degradation of muscle proteins under amino acid starvation, where muscle protein is catabolized to provide other organs with amino acids. Inhibits de novo skeletal muscle protein synthesis under amino acid starvation. Regulates proteasomal degradation of cardiac troponin I/TNNI3 and probably of other sarcomeric-associated proteins. May play a role in striated muscle atrophy and hypertrophy by regulating an anti-hypertrophic PKC-mediated signaling pathway. May regulate the organization of myofibrils through TTN in muscle cells. 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. Homodimer. Homooligomer and heterooligomer. Interacts with SUMO2, titin/TTN and GMEB1. Interacts with TRIM54 and probably with TRIM55 and TNNI3. Forms a ternary complex with RACK1 and PRKCE. Interacts with CKM (By similarity). Colocalizes with TNNI3 in myocytes. Localizes to the M- and Z-lines in skeletal muscle (By similarity). Muscle specific. Selectively expressed in heart and skeletal muscle. By interleukin-1, dexamethasone, lipolysaccharide and indinavir. Up-regulated upon muscle denervation, immobilization and unweighting and more generally upon muscle atrophy. Up-regulated upon sepsis. Down-regulated upon aging. The RING-type zinc finger mediates interaction with SUMO2 and localization to the nucleus. Also required for the E3 ubiquitin ligase activity (By similarity). The B box-type zinc finger mediates homodimerization.