Background
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 specific protease 14 (USP14) is a member of the cysteine protease enzyme family and cloning of the gene was first described by Deshpande et al. (1996). USP14 is a proteasome–associated DUB enzyme. Mammalian proteasomes are associated with three DUBs: USP14, UCHL5 (UCH37) and RPN11 (POH1). UCHL5 and USP14 reside on the 19S regulatory particle and remove ubiquitin from the substrate before substrate degradation whereas RPN11's activity is delayed until the proteasome is committed to degrading the substrate (Lee et al., 2010). Ubiquitin-tagged substrates are degraded by the 26S proteasome, which is a multi-subunit complex comprising a proteolytic 20S core particle capped by 19S regulatory particles (Wang et al., 2007). The approval of bortezomib for the treatment of multiple myeloma validated the 20S core particle as an anticancer drug target. Recent experiments have now shown that the 19S regulatory particle is also a potential anticancer drug target (D'Arcy et al., 2011). Alterations in USP14 can lead to proteasome dysfunction and neurological disease. Recent in vitro experiments show that USP14 can also stabilize the expression of over-expressed, disease-associated proteins such as tau and ataxin-3 (Jin et al., 2012). A number of other studies over the years have implicated USP14 in cancer. Ishiwata et al., (2001) first found that the USP14 expression was upregulated in leukemic cells, Shinji et al., (2006) found that the USP14 expression in colorectal cancer is associated with liver and lymph node metastases and Chuensumran, et al., (2011) that USP14 expression is associated with intrahepatic cholangiocarcinoma cell differentiation. A recent review by Wu et al. (2013) also reports that USP14 is a tumour-promoting factor and a promising therapeutic target for non-small cell lung cancer (NSCLC).
The DUB activity of USP14 is known to be activated by proteasomes and alone USP14 has little DUB activity and as such may be used as a control in experiments alongside those performed with proteasome-activated USP14: Such experiments are typically performed using a proteasome preparation where the native DUBs - including USP14 - have been removed and/or inactivated before the proteasome preparation is added back to recombinantly expressed USP14.
References
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