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BMC Biochemistry BioMed Central Review Open Access Deubiquitylating enzymes and disease Shweta Singhal1, Matthew C Taylor2 and Rohan T Baker*1 Address: 1Ubiquitin Laboratory, Division of Molecular Bioscience, The John Curtin School of Medical Research, The Australian National University, Canberra, ACT 0200, Australia and 2CSIRO Entomology, GPO Box 1700, Canberra, ACT 2601, Australia Email: Shweta Singhal - [email protected]; Matthew C Taylor - [email protected]; Rohan T Baker* - [email protected] * Corresponding author Published: 21 October 2008 <supplement> <title> <p>Ubiquitin-Proteasome System in Disease Part 2</p> </title> <editor>John Mayer and Rob Layfield</editor> <note>Reviews</note> </supplement> BMC Biochemistry 2008, 9(Suppl 1):S3 doi:10.1186/1471-2091-9-S1-S3 This article is available from: http://www.biomedcentral.com/1471-2091/9/S1/S3 © 2008 Singhal et al; licensee BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Deubiquitylating enzymes (DUBs) can hydrolyze a peptide, amide, ester or thiolester bond at the C-terminus of UBIQ (ubiquitin), including the post-translationally formed branched peptide bonds in mono- or multi-ubiquitylated conjugates. DUBs thus have the potential to regulate any UBIQ- mediated cellular process, the two best characterized being proteolysis and protein trafficking. Mammals contain some 80–90 DUBs in five different subfamilies, only a handful of which have been characterized with respect to the proteins that they interact with and deubiquitylate. Several other DUBs have been implicated in various disease processes in which they are changed by mutation, have altered expression levels, and/or form part of regulatory complexes. Specific examples of DUB involvement in various diseases are presented. While no specific drugs targeting DUBs have yet been described, sufficient functional and structural information has accumulated in some cases to allow their rapid development. Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; http://www.targetedproteinsdb.com). Localization and function In mammals there are some 80–90 DUBs categorized into Introduction five gene families: the ubiquitin C-terminal hydrolases In this chapter, the term 'deubiquitylating enzyme' (DUB) (UCHs); the ubiquitin-specific peptidases (USPs/UBPs); is used to describe any enzyme that can hydrolyze a pep- the ovarian tumor (OTU) domain proteins; the Josephin tide, amide, ester or thiolester bond at the C-terminus of or Machado-Joseph disease (MJD) proteins and the JAMM UBIQ (ubiquitin). DUBs can cleave the linear products of (Jab1/MPN domain-associated metalloisopeptidase) UBIQ gene translation [1]; the post-translationally domain proteins. The first four families are cysteine pepti- formed branched peptide bonds in mono- or multi-ubiq- dases, while the JAMM proteins are zinc metalloisopepti- uitylated conjugates [2]; ubiquitylated remnants resulting dases. These DUB families have been the subjects of recent from proteasome-mediated degradation [3] and other reviews [5-7]. Since linear UBIQ fusion proteins are small amide or ester adducts [3,4]. DUBs thus have the cleaved very rapidly, perhaps co-translationally [1], it is potential to regulate any UBIQ-mediated cellular process, unclear which DUB(s) cleave these precursors in vivo to the two best characterized being proteolysis and protein provide free UBIQ. However, mutating or inhibiting a trafficking/endocytosis. DUB(s) that produces free UBIQ would have pleiotropic effects, in that it would deplete free UBIQ levels and inhibit all UBIQ-dependent functions non-specifically. Page 1 of 8 (page number not for citation purposes) BMC Biochemistry 2008, 9(Suppl 1):S3 http://www.biomedcentral.com/1471-2091/9/S1/S3 DUBs that cleave branched UBIQ conjugates presumably heterologous promoters cause overexpression of normal, make up the bulk of the 80–90 DUBs, and provide sub- full-length UBP6 protein, or whether there have been fur- strate specificity to deubiquitylation. Most DUBs contain ther mutations, deletions, or alternate splicing within a catalytic domain that has sequence similarity within USP6 to produce an altered, oncogenic UBP6 protein. subfamilies and structural similarity across subfamilies [5- 8], and unrelated sequences either N-terminal or C-termi- UBP7/USP7/HAUSP nal (or both) to the catalytic domain. These flanking One well characterized case that illustrates the possible sequences have been shown to mediate substrate binding link between DUBs and disease is the mammalian DUB in a few cases (see DUBs and disease: UBP7/USP7/HAUSP UBP7 (encoded by USP7/HAUSP), a 1102 amino acid and DUBs and disease: UBP33/USP33/VDU1, member of the USP family. The N-terminal domain of UBP20USP20/VDU2) and presumably serve as substrate UBP7 (residues 1–208) contains a TRAF (tumor necrosis binding domains in all DUBs. These flanking sequences, factor receptor-associated factor) domain [15] that binds along with the catalytic core, could also contribute bind- to both the P53 (p53) tumor suppressor protein [16,17] ing and cleavage specificity for different UBIQ-UBIQ link- and the MDM2 ubiquitin ligase, which ubiquitylates P53 ages. Some DUBs function at the proteasome to edit and/ [18]. The remainder of UBP7 comprises a catalytic core or remove UBIQ chains; one example that is linked to dis- (residues 208–560) that cleaves UBIQ, and a C-terminal ease is UBP14 (USP14) (see DUBs and disease: UBP14/ domain (residues 560–1102) of unknown function. Since USP14). UBP7 can deubiquitylate and stabilize P53 in vitro, it was suggested that the role of UBP7 was to stabilize P53 in vivo Since most DUBs have been identified only by means of [16]. More recent work, however, showed that UBP7 can sequence similarity to catalytic motifs, there is little also deubiquitylate and thus stabilize MDM2 (which aut- known functional information on many of these oubiquitylates itself) [19,20]. UBP7 forms a tighter com- enzymes. However, the relatively few examples where plex with MDM2 than with P53 [21], consistent with functional insights have been gained (see DUBs and dis- observations that UBP7's primary role could be deubiq- ease) indicate that DUBs can play crucial regulatory roles uitylating and stabilizing MDM2 (and thus increasing in the ubiquitin proteasome system (UPS), making them ubiquitylation of P53), rather than deubiquitylating and ideal drug target candidates for therapeutic intervention stabilizing P53 [20]. MDM2 is not the only P53 ubiquitin in UPS-related diseases. ligase and additional proteins that play a role in P53 ubiq- uitylation, deubiquitylation and degradation have been DUBs and disease recently reviewed [22]. One further member of this com- UBP6/USP6/TRE-17/TRE-2 plex interplay was recently identified as the protein DAXX UBP6 (encoded by USP6/TRE-17), the first DUB to be (death domain-associated protein). DAXX simultane- identified as an oncogene [9,10], has in recent years been ously binds to MDM2 and UBP7, and mediates the stabi- directly linked to human cancers, primarily aneurysmal lizing effect of UBP7 on MDM2 [23]. In response to DNA bone cysts (ABCs), which are locally aggressive bone damage, DAXX (and UBP7) dissociate from MDM2, tumors. ABCs were previously regarded as non-neoplastic, which correlates with MDM2 self-degradation. but recent cytogenetic studies have identified clonal rear- rangements that often feature chromosome 17p13 – the Pathogenic mutations within UBP7 itself have not yet USP6 or TRE-17 (originally termed TRE-2) locus. There been described. However, a recent study that investigated are five known examples of chromosomal rearrangements UBP7 expression and TP53 gene status in non-small cell that have positioned USP6 downstream of a heterologous lung carcinomas found that, in 93 of the 131 patients gene promoter, in turn forcing inappropriate USP6 examined, either mutant P53 or reduced UBP7 expression expression in a bone/mesenchymal context: Osteomodulin; was observed [24]. A statistically significant association COL1A1 (Collagen 1A1); THRAP3 (TRAP150); CNBP between reduced UBP7 levels and reduced P53 protein (ZNF9) and CDH11 [11-13]. High-level UBP6 expression expression was observed in tumors with wild-type P53, was also detected in four other human cancers originating while a more dramatic association was seen in tumors from mesenchymal neoplastic cells in a bone context (one with mutant P53. The authors concluded that the concur- Ewing's sarcoma, two osteoblastomas and one myofi- rent evaluation of both UBP7 expression and P53 gene broma), but not in 50 other non-ABC tumors, suggesting status was a significant prognostic indicator in adenocar- that UBP6 could have a broader oncogenic role in mesen- cinoma patients [24]. MDM2 expression was not investi- chymal tumors [13]. Recent work has also revealed that gated in this study though it could be another useful the USP6 product regulates actin remodeling and vesicu- prognostic indicator, given that the protein can regulate lar trafficking, and could thus regulate cell motility and P53 levels. invasiveness [14]. In all five cases of UBP6-linked human cancers referred to above, it remains unclear whether the Page 2 of 8 (page number not for citation purposes) BMC Biochemistry 2008, 9(Suppl 1):S3 http://www.biomedcentral.com/1471-2091/9/S1/S3