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 (By similarity). Interacts with TNNI3. Forms a ternary complex with RACK1 and PRKCE. Interacts with CKM. Localizes to the M- and Z-lines in skeletal muscle (By similarity). Colocalizes with TNNI3 in myocytes. By hydrogen peroxide. Up-regulated in response to amino acid starvation. The RING-type zinc finger mediates interaction with SUMO2 and localization to the nucleus (By similarity). Also required for the E3 ubiquitin ligase activity. The B box-type zinc finger mediates homodimerization. No visible phenotype under normal conditions. Under amino acid starvation, mice continue to synthesize muscle protein and degrade less muscle protein than wild-type, leading to a decrease in the blood levels of free amino acids. Besides, mutant mice display higher creatine kinase activity under amino acid starvation.