RING finger, HC subclass, found in muscle-specific RING finger proteins TRIM63/MuRF-1, TRIM55/MuRF-2 and TRIM54/MuRF-3
This family corresponds to a group of striated muscle-specific tripartite motif (TRIM) proteins, including TRIM63/MuRF-1, TRIM55/MuRF-2, and TRIM54/MuRF-3, which function as E3 ubiquitin ligases in ubiquitin-mediated muscle protein turnover. They are tightly developmentally regulated in skeletal muscle and associate with different cytoskeleton components, such as microtubules, Z-disks and M-bands, as well as with metabolic enzymes and nuclear proteins. They also cooperate with diverse proteins implicated in selective protein degradation by the proteasome and autophagosome, and target proteins of metabolic regulation, sarcomere assembly and transcriptional regulation. Moreover, MURFs display variable fibre-type preferences. TRIM63/MuRF-1 is predominantly fast (type II) fibre-associated in skeletal muscle. TRIM55/MuRF-2 is predominantly slow-fibre associated. TRIM54/MuRF-3 is ubiquitously present. They play an active role in microtubule-mediated sarcomere assembly. MuRFs belong to the C-II subclass of TRIM (tripartite motif) family of proteins that are defined by their N-terminal RBCC (RING, Bbox, and coiled coil) domains, including three consecutive zinc-binding domains, a C3HC4-type RING-HC finger, Bbox2, and a coiled coil region, as well as a COS (carboxyl-terminal subgroup one signature) box, and an acidic residue-rich (AR) domain positioned C-terminal to the RBCC domain. They also harbor a MURF family-specific conserved box (MFC) between its RING-HC finger and Bbox domains.