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 signalling 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 carboxyl-terminal hydrolase 4 (Ubiquitin Specific Protease 4; USP4) is a member of the cysteine protease enzyme family and cloning of the human gene was first described by Gupta et al. (1993). In 1995, USP4 was identified as a proto-oncogene related to USP6, showing a consistently elevated gene expression level in small cell tumours and lung adenocarcinomas suggesting that it may have a possible causative role in neoplasia (Gray et al., 1995). USP4 has been implicated in a number of other processes, including protein quality control in the endoplasmatic reticulum and p53 and Wnt signalling. USP4 has also been reported to inhibit the kinase TAK1 that is ubiquitylated by the AKT regulator TRAF6 (Uras et al., 2012).
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Uras IZ, List T and Nijman SM (2012) Ubiquitin-specific protease 4 inhibits mono-ubiquitination of the master growth factor signalling kinase PDK1. PloS one 7,e31003.