Deconjugating enzymes (DCEs) are proteases that process ubiquitin or ubiquitin-like gene products, reverse the modification of proteins by a single ubiquitin or ubiquitin-like protein (UBL) and remodel polyubiquitin (or poly-UBL) chains on target proteins (Reyes-Turcu et al., 2009). The deubiquitylating - or deubiquitinating - enzymes (DUBs) represent the largest family of DCEs and regulate ubiquitin dependent signaling pathways. The activities of the DUBs include the generation of free ubiquitin from precursor molecules, the recycling of ubiquitin following substrate degradation to maintain cellular ubiquitin homeostasis and the removal of ubiquitin or ubiquitin-like proteins (UBL) modifications through chain editing to rescue proteins from proteasomal degradation or to influence cell signalling events (Komander et al., 2009). There are two main classes of DUB; cysteine proteases and metalloproteases. Ubiquitin specific protease 19 (USP19) is a member of the cysteine protease enzyme family and cloning of the human gene was first described by Nagase et al. (1998). USP19 is a ubiquitin-specific protease anchored via its C-terminal to the endoplasmic-reticulum. USP19 rescues the endoplasmic-reticulum-associated degradation (ERAD) substrates: cystic fibrosis transmembrane conductance regulator (CFTR) and T-cell receptor-α (TCRα) from proteasomal degradation. It also deubiquitylates and prevents proteasomal degradation of RNF123 which in turn stimulates CDKN1B ubiquitin-dependent degradation thereby playing a role in cell proliferation (Hassink et al., 2009). USP19 modulates transcription of major myofibrillar proteins and is involved in decreased protein synthesis in atrophying skeletal muscle. Inhibition of USP19 could be a novel therapeutic mechanism for the prevention and treatment of muscle protein catabolism (Sundaram et al., 2009). During the cell cycle, USP19 regulates p27 levels by deubiquitylating and stabilizing RNF123, an E3 ubiquitin ligase that ubiquitylates p27 and targets it for proteasomal degradation at the G1 phase. Enhanced degradation of p27 in G1 phase is associated with several cancers, and thus USP19 could be a possible target for oncology research (Lu et al., 2009). Cellular inhibitors of apoptosis (c-IAPs) are stabilised by USP19 in a deubiquitylase-independent mechanism, by inhibiting the self-ubiquitin ligase activity of c-IAPs (Mei et al., 2011).
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