DUBs at a Glance - Semantic Scholar

Cell Science at a Glance Rose the Nobel Prize in Chemistry in 2004 (Wilkinson, 2004). This early work spawned a large number of studies that investigated the role of ubiquitin and several other ubiquitin-like proteins (UBLs) as targeting signals in virtually all aspects of cellular protein metabolism (Chen, 2005; Cohn and D’Andrea, 2008; Saksena et al., 2007; Weake and Workman, 2008). The best understood example of ubiquitylation is the marking of proteins for delivery to the 26S proteasome, resulting in their degradation (Chiba and Tanaka, 2004; Guo et al., 2007; Hershko and Ciechanover, 1998; Schwartz and Hochstrasser, 2003; Varshavsky et al., 1989).

DUBs at a glance Keith D. Wilkinson Department of Biochemistry, Emory University School of Medicine, Atlanta, GA 30322, USA e-mail: [email protected] Journal of Cell Science 122, 2325-2329 Published by The Company of Biologists 2009 doi:10.1242/jcs.041046

Journal of Cell Science

Introduction The discovery of protein ubiquitylation three decades ago was the beginning of our understanding of a new mechanism by which proteins are marked for assembly into macromolecular complexes or movement between cellular compartments. The finding that ubiquitylation (the covalent attachment of the small protein ubiquitin to other proteins) targeted proteins for degradation earned Avram Hershko, Aaron Ciechanover and Irwin

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(DUBs) (Amerik and Hochstrasser, 2004; D’Andrea and Pellman, 1998; Wilkinson, 1997). Enzymes that reverse the modification by UBLs are similarly named: desumoylating enzymes remove SUMO (small ubiquitin-related modifier), deneddylating enzymes remove NEDD8 (neural precursor cell expressed, developmentally downregulated 8) and deISGylating enzymes remove ISG15 (interferon-stimulated gene product 15). There are many fine reviews describing the complex enzymatic mechanisms that regulate the conjugation of ubiquitin and UBLs to target proteins; the reader is referred to these for details of the ubiquitylation machinery (Belgareh-Touze et al., 2008; Dye and Schulman, 2007; Hochstrasser, 2007; Starita and Parvin, 2006). Briefly, ubiquitin is first thiolesterified at its C-terminus by the action of the E1 ubiquitin-activating enzyme and

Similar to all regulated targeting pathways, the process of ubiquitylation is reversible. The enzymes that reverse the modification of proteins by ubiquitin are collectively known as deubiquitylating enzymes

DUBs at a Glance Keith D. Wilkinson Substrate-specificity of protease families acting on ubiquitin-like proteins

DUBs both stimulate and inhibit proteolysis

USP Deubiquitylating (isopeptidase)

UCH

Ub PolyUb

Deneddylating

Nedd8

DeISGylating

ISG15

Ub

OTU

JAMM

MJD

Ub PolyUb

PolyUb

PolyUb

ProUb

Nedd8

Nedd8

Ligand Receptor

SENP

USP14 UCH37

A20

Multiple DUBs?

SUMO

RIP1

Substrate

T A T A 2/B 3 1K

Protein

β NE α MO

ProUb

Ub

UCH37 Proteasome 19S regulator

Altered localization, binding, stability or activity

TFassoc. Ub ligases

AMSH?

H2B MLL3

USP3 H3

USP21

IκB NF-κB

CorepressorH2A assoc. Ub ligases

IκB NF-κB

NF-κB α β β α

USP14 PolyUb

Peptides

Base

Ubiquitylated protein

H3 H2B

ATP

βTrCP

K4-Me3

USP22•SAGA

Early endosome

N β E M α O

POH1

Lid

Initiation of transcription

SAGA

L

USP8

CYLD, A20

Substrate

Receptor

E1,E2,E3

Elongation and termination

AMSH?

Cbl

ATP

POH1

DUBs remodel and disassemble polyubiquitin chains

L

AMSH TRAF complex

A20, CYLD

E1,E2,E3

L

L

E1, Ubc13/Uev1A TRAF complex

USP5

Nedd8

Desumoylating

Substrate

Multiple DUBs?

Deubiquitylation of histones regulates transcription and chromatin dynamics

K63-linked ubiquitin used in endocytosis and cargo sorting

K63-specific DUBs in NF-␬B signaling

DUB protein family

Activity

Aurora B

H2A

S10-P

USP16

Lysosome

H3

Multivesicular body 20S protease

2A-DUB•PCAF

KEY

Proteasome

K48-linked polyubiquitin

Reaction


Influence

Monoubiquitin

DUB activity

USP7•GMP synthetase

Silencing and chromatin condensation

DUBs regulate DNA repair and the stability of multiple proteins at cell-cycle checkpoints

UbPlk1

APC/C UbClaspin

UbCyclin E

USP28

APC/C

UbWee1

UbClaspin

Cdk1cyclin B1

SCFFbw7α

pCdk1- pCdk2cyclin B1 cyclin E

Cdk2cyclin E

S-phase progression

G2 block S block

M

USP28 Myc

APC/CpCdh1 G2

USP28

G2/M transition

Proliferation and apoptosis P53

BAP1

USP1/UAF1

Error-free synthesis

?

BRCA1/2 DNA-repair pathways; cancer

CYLD

K63-ubiquitylated TRAFs, RIP1, others

Mutation causes benign tumors and failure to downregulate NF-κB and JNK signaling

DUB1, DUB2

Common cytokine receptor, gamma chain

Overexpression increases substrate half-life, prolongs cytokine response

UCH-L1

?

Mutation causes gracile axonal dystrophy in mice; accumulated in neuronal inclusion bodies in humans

USP1

PCNA, FANCD2

Mutated in Fanconi Anemia; involved in DNA repair

USP2A

Fatty acid synthesase? MDM2?

Involved in prostate cancer; protects from apoptosis

USP4

Rb? Ro52 ubiquitin ligase

Oncoprotein linked to lung cancer and Sjogren’s syndrome

DNA damage

USP6/TRE17

?

Oncogenic when overexpressed from 17p13 translocations

Checkpoint

USP7/HAUSP

p53, MDM2, FOXO

Role in DNA repair and oxidative stress response

UBP14

Proteasome-bound polyubiquitin

Mutated in ataxic mouse; ubiquitin depletion phenotype

USP33/VDU1, USP20/VDU2

pVHL ubiquitin ligase

Regulation of HIF-1α; role in angiogenesis and metastasis

MDM2 USP7

USP7

USP7

Ubprotein

Cell-cycle arrest

Double-strand-break DNA repair

Translesion synthesis RAD5, Ubc13/Uev1

USP3

MDMX/ MDM2

p21CIP/WAF1 Cdk2/ Cyclin E

APC/C FANCD2

RAD18 RAD6

?

ATM, CHK2 pMDMX MDMX

PCNA-mediated DNA repair

Pol η

Polyglutamine repeat expansion causes type 3 Spinal Cerebellar Ataxia

BAP1, USP11

Ubiquitylated APC/CCdc20 USP44 (Active)

p53-mediated arrest

APC/CpCdh1

FANCD2

APC/CCdc20

G1

S

Crosslink repair

Deletion causes prolonged NF-κB responses and inflammation


p31

MAD2 MAD2• APC/CCdc20 (Inactive)

UbMyc

?

Cdc25 phosphatase

Claspin

?

Plk1 Wee1 Claspin p-Wee1 SCFβTrcpp ?

Ub ligase and DUB for RIP1

Ataxin-3

Chromosome attachment

APC/CCdh1

Cdc20

Cdh1

A20

Spindle checkpoint

Cdc25-mediated checkpoint arrest Cdc14B

H2AX PCNAbound leision

Brcc36

H2AX

ABRA1 X

P

80

RA

USP1 degradation

Proteolysis USP3

Abbreviations: 2A-DUB, histone H2A deubiquitinase; ABRA1, abraxas protein forming a complex required for doublestrand-break DNA repair; AMSH, associated molecule with the SH3 domain of STAM; APC/C, anaphase-promoting complex/cyclosome [activity requires co-activators Cdc20 (cell division cycle 20) or Cdh1 (Cdc20 homolog 1) whose identity is shown as a superscript]; ATM, ataxia telangiectasia, mutated homolog; BARD1, BRCA1-associated RING domain 1 (heterodimerizes with BRCA1 to form an active ubiquitin ligase); BRCA1, breast cancer 1 tumor suppressor; BRCC36, BRCA1/BRCA2-containing complex subunit 36; βTrCP, β-transducin repeat-containing homolog protein; Cbl, Casitas B-lineage lymphoma; Cdk, cyclin-dependent kinase; Chk2, checkpoint kinase 2; DUB, deubiquitylating enzyme; E1, a ubiquitin-activating enzyme; E2, a ubiquitin-conjugating enzyme; E3, a ubiquitin ligase; FANCD2, product of the Fanconi anemia, complementation group D2 gene; FOXO, forkhead transcription factor O; H2A, histone H2A; H2B, histone H2B; H2AX, histone H2AX; H3, histone H3; HIF-1α, hypoxia-inducible factor 1α; IκB, inhibitor of NF-κB; ISG15, product of the interferon-stimulated gene 15; JAMM, a DUB family of proteins with the Jab1/MPN metalloenzyme domain (Pfam PF01398, EC 3.1.2.15); L, ligand; MAD2, mitotic arrest-deficient 2 protein inhibitor of APC/CCdc20; MDM2, murine double minute 2; MDMX, MDM2-like p53-binding protein; MJD, a DUB family of proteins with the Machado-Joseph Disease protein domain (Pfam PF02099, EC 3.4.22.-); MLL3, myeloid/lymphoid or mixed-lineage leukemia protein 3 complex; Myc, a transcripion

BRCA1/BARD1

Double-strand-break repair

factor that is a homolog of the Myelocytomatosis viral oncogene; Nedd8, neural precursor cell expressed, developmentally downregulated 8; NEMO, NF-κB essential modulator; NF-␬B, nuclear factor-κB; OTU, a DUB family of proteins with the ovarian tumor domain (Pfam PF02338, EC 3.1.2.-); PCAF, p300/CBP-associated factor; PCNA, proliferating cell nuclear antigen; Plk1, polo-like kinase 1; POH1, DUB of the 19S lid complex; Pol η, DNA polymerase eta; ProUb, the pro-protein gene products of ubiquitin-encoding genes; pVHL, von Hippel-Lindau tumor suppressor; RAD, radiation sensitive (genes required for DNA repair functions); RAP80, required for double-strand-break DNA repair; Rb, retinablastoma tumor suppressor; RIP1, receptor-interacting protein 1; SAGA, Spt-Ada-Gcn5-acetyltransferase complex; SCF, a class of ubiquitin ligases consisting of Skp1, a cullin and an F-box protein whose identity is indicated by a superscript; SENP, a family of human SUMO- or Nedd8-specific proteses with the ubiquitin-like protease domain (Pfam PF02902, EC 3.4.22); SUMO, small ubiquitin-related modifier; TAB2/3, TAK1-binding protein 2/3; TAK1, TGFβ-activated kinase 1; TF, transcription factor; TNF, tumor necrosis factor; TRAF, TNF receptor-associated factor; UAF1, USP1-associated factor 1; Ub, ubiquitin; Ubc13/Uev1, heterodmeric E2 ubiquitin-conjugating enzyme that synthesizes K63-linked polyubiquitin chains; UCH, a DUB family of proteins with the ubiquitin C-terminal hydrolase domain (Pfam PF01088, EC,3.4.19.12); USP, a DUB family of proteins with the ubiquitin-specific protease domain (Pfam PF00443, EC 3.1.2.15).

© Journal of Cell Science 2009 (122, pp. 2325-2329)

(See poster insert)


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Journal of Cell Science 122 (14)

then is subsequently transferred to one of several E2 ubiquitin-conjugating enzymes that act as carrier proteins. Finally, ubiquitin is transferred to a lysine residue of the target protein through the combined action of the E2-ubiquitin thiol ester and one of hundreds of E3 ubiquitin ligases. The ubiquitylation signal that is attached can consist of a single ubiquitin, multiple ubiquitins or a polyubiquitin chain in which successive ubiquitin molecules are assembled by the ubiquitylation of ubiquitin itself (Baboshina and Haas, 1996; Chau et al., 1989; Hofmann and Pickart, 1999; Koegl et al., 1999; Tokunaga et al., 2009; Wu-Baer et al., 2003). Because any of the seven lysine residues of ubiquitin, or its amino terminus, can be modified by a subsequent ubiquitin to form a polyubiquitin chain, there is a huge variation in the structure of polyubiquitin signals that can be attached. A polyubiquitin chain can involve linkages to the same lysine residue on each ubiquitin moiety to yield a homogeneous chain, or it can involve linkages to different lysine residues on different ubiquitin moeities, which results in a heterogeneous linear or branched chain. Ubiquitylation is a versatile and dynamic targeting signal. The use of a protein, rather than a small molecule, to modify a target protein confers a large interaction surface that can be recognized by specific receptors. In addition, the many different polymeric forms of ubiquitin allow for structural variation of the signal. Structurally different forms of polyubiquitin are thought to target proteins for different cellular fates. For example, early work showed that K48-, K29-, and K11-linked polyubiquitin chains can target proteins for degradation by the proteasome (Chau et al., 1989; Jin et al., 2008; Koegl et al., 1999); K63-linked polyubiquitin chains participate in DNA repair and signaling kinase complexes (Deng et al., 2000; Spence et al., 1995); monoubiquitin and K63-linked chains are involved in targeting cell-surface proteins for internalization and endosomal sorting (Hicke and Riezman, 1996; Springael et al., 1999); and monoubiquitylation of histones can influence chromatin structure and transcription (Levinger and Varshavsky, 1980). Recent mass spectrometry analysis of ubiquitylated proteins shows that chains with multiple linkages can be attached to a single protein (Bish et al., 2008; Crosas et al., 2006; Kim

et al., 2007; Kirkpatrick et al., 2005; Mayor et al., 2005; Xu and Peng, 2008), although the specific pathways in which these more complex polyubiquitin chains are involved remain poorly understood. In this article and its accompanying poster, I summarize our understanding of the metabolic function of DUBs and discuss their roles in regulating several ubiquitindependent processes. Here, I use the term DUBs to refer only to those enzymes that act on ubiquitin. Much less is known about the enzymes that act on UBLs (Hay, 2007; Love et al., 2007; Mikolajczyk et al., 2007; Reverter et al., 2005; Sulea et al., 2006) and they will not be discussed here. Although much of what is known about DUBs was first observed in yeast, the yeast pathways or enzyme names are not emphasized. The poster illustrates the role of over 20 of nearly 100 mammalian DUBs that act on ubiquitin (Nijman et al., 2005), and concentrates on the DUBs about which something is known regarding their physiology or pathology. For pathways where the substrate or the process regulated is known in some detail, specific examples are provided. It should be noted that a role for DUBs has been implied in many other contexts, such as apoptosis, Parkinson’s disease and neuronal-inclusion-body diseases, although in many cases the precise DUB involved has not been identified. Space limitations restrict the inclusion of these aspects in the poster. DUBs are numerous and specific The nearly 100 putative mammalian DUBs are grouped into five different families (Amerik and Hochstrasser, 2004; D’Andrea and Pellman, 1998; Wilkinson, 1997). Four of these families are thiol proteases: the ubiquitin-C-terminal hydrolases (UCHs), ubiquitin-specific proteases (USPs), ovarian-tumor (OTU) domain DUBs and Machado-Joseph domain (MJD) DUBs. The DUBs of a fifth family contain a Jab1/MPN metalloenzyme (JAMM) domain and act as zinc-dependent metalloproteases. The large number of gene families, each with multiple members, suggests that selective pressure to evolve such catalysts has occurred numerous times. In addition, this diversity implies that considerable substrate specificity exists. This assumption is supported by the finding that the mutation, deletion or downregulation

of specific DUBs induces very limited and specific cellular phenotypes and pathologies (Shanmugham and Ovaa, 2008; Singhal et al., 2008). For example, the mutation or deletion of the major neuronal DUB in mammals, UCH-L1 (ubiquitin C-terminal hydrolase L1), causes a localized axonal dystrophy but few other overt effects (Setsuie and Wada, 2007). A benign tumor syndrome of hair follicles known as cylindromatosis is caused by the mutation of CYLD, a USPfamily DUB named after the disease it causes. Although the major defects caused by mutation of CYLD are limited to the nuclear factor-κB (NF-κB) pathway, (Courtois, 2008) this DUB has also been shown to have important roles in cell-cycle regulation (Stegmeier et al., 2007). Interfering with the function of USP1 mainly causes DNA-repair defects (Cohn and D’Andrea, 2008), whereas deleting USP14 in mice results in ataxia, a movement disorder characterized by uncoordinated motions (Crimmins et al., 2006). A significant aspect of specificity is the ability of DUBs to recognize and act on different types of polyubiquitin. The catalytic domain of all DUBs contains a binding site for ubiquitin, and several DUBs bind ubiquitin at submicromolar concentrations. Many other DUBs, however, bind ubiquitin only very weakly (Reyes-Turcu and Wilkinson, 2009). Some DUBs have additional binding sites with affinity for the target protein that is ubiquitylated (Ventii and Wilkinson, 2008); for example, USP7 binds to a peptide sequence present in its substrates p53, MDM2 (murine double minute 2, an oncoprotein) and the Epstein Barr nuclear antigen-1 (Hu et al., 2006). It is clear that differently linked polyubiquitin chains have different structures, and it is thought that some DUBs can distinguish between them. For example, the DUBs CYLD and A20, which are involved in downregulating the NF-κB response, only disassemble K63-linked polyubiquitin chains, the type that is assembled on the signaling components of the NF-κB pathway (Courtois, 2008; Heyninck and Beyaert, 2005). Recent structures of CYLD and A20 suggest that these proteins achieve specific cleavage of K63-linked polyubiquitin chains by recognizing the unique surfaces of ubiquitin that are juxtaposed in this type of polyubiquitin. Similar conclusions are supported by a



co-crystal structure containing the JAMM domain of the DUB AMSH (associated molecule with the SH3 domain of STAM) and K63-linked diubiquitin (Sato et al., 2008). DUBs associate with ubiquitin ligases, scaffold proteins and substrate adaptors In contrast to the specificity of DUBs that is apparent in vivo, assays carried out using artificial substrates in vitro often indicate that DUBs show little specificity. This can, in part, be attributed to the qualitative nature of many assays that do not measure the rates of substrate cleavage, although this factor alone cannot fully explain the apparent lack of specificity. A more likely explanation is that most DUBs contain additional protein interaction domains (which are utilized in vivo but not in in vitro) that direct the binding of DUBs to specific scaffolds or substrate adaptors and thereby confer substrate specificity. Thus, it is thought that in vivo specificity is determined mostly by the colocalization of the DUB and its substrates, and that adaptors are necessary for many DUBs to bind to their substrates (Marfany and Denuc, 2008; Ventii and Wilkinson, 2008). For example, USP1 is known to form a complex with a non-proteolytic subunit, UAF1, and the degradation of UAF1 leads to proteolysis of USP1 and consequent defects in the DNA repair functions that USP1 is involved in regulating (Cohn and D’Andrea, 2008). Similarly, the proteasome-associated DUBs USP14, UCH37 and POH must all be associated with the proteasome for significant DUB activity (Schmidt et al., 2005; Ventii and Wilkinson, 2008). Another surprising observation is that several DUBs have been found to associate with ubiquitin ligases, which suggests that DUBs have a role in regulating ubiquitylation. The proteasome has both ubiquitin ligases and DUBs that associate with it (Crosas et al., 2006), and several DUB-ligase pairs interact directly, including BRCC36-BRCA1, BAP1BRCA1, USP4-Ro52, USP7-MDM2, USP8-GRAIL, USP20-pVHL, USP33pVHL and USP44-APC (Kee and Huibregtse, 2007; Marfany and Denuc, 2008; Ventii and Wilkinson, 2008). One explanation for these associations may be that the associated DUBs counteract the tendency of ubiquitin ligases to autoubiquitylate in the absence of other

substrates. Another purpose that the interaction might serve is to target the DUB for degradation via the ligasecatalyzed ubiquitylation of the associated DUB. In at least some cases, the two interaction partners are indeed transregulated by each other. For example, in the absence of their substrates, the ubiquitin ligases MDM2 and Ro52 (Sjogren’s syndrome associated autoantigen) become autoubiquitylated, and this is reversed by the activity of their associated DUBs, USP7 and USP4, respectively (Clegg et al., 2008; Meulmeester et al., 2005; Wada and Kamitani, 2006). Conversely, USP4 can be ubiquitylated by Ro52 and subsequently degraded. However, another function of these interactions might be to enforce the substrate specificity of ubiquitylation: the action of the DUB might ‘proofread’ ubiquitylation and prevent the assembly of inappropriate ubiquitin linkages. The DUB A20, which contains both a ligase and a DUB domain on the same polypeptide, is the most extreme example of this. Its apparent role is to remodel the polyubiquitin chains that are generated on RIP1 (receptor-interacting protein 1) during tumor necrosis factor (TNF)mediated stimulation of the NF-κB pathway. Removing the K63-linked polyubiquitin downregulates signaling, and assembling a K48-linked chain on RIP1 drives its degradation, further damping signaling (Heyninck and Beyaert, 2005). Pathological conditions related to DUB dysfunction Defects in DUB functions have been implicated in several pathological conditions, most notably cancer, neurological disease and microbial pathogenesis (de Pril et al., 2006; Rytkonen and Holden, 2007; Setsuie and Wada, 2007; Shackelford and Pagano, 2005; Singhal et al., 2008; Stuffers et al., 2008; Yang, 2007). Based on the findings that DUBs have a role in regulating multiple cell-cycle and DNA repair checkpoints, in addition to cytokine-signaling and apoptosis pathways, it is likely that defects in DUB function could contribute to the development of cancer. Notably, mutations in CYLD cause cylindromatosis, and the translocation of the UBP6 coding region downstream of heterologous promoters is an oncogenic event that is found in

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many mesenchymal tumors. Furthermore, deletion of the gene encoding A20 in mice results in severe inflammation and cachexia (Singhal et al., 2008). The potential role of DUBs in neurological disease is even less well understood. Mutation of USP14 in mice or ataxin-3 in humans causes ataxia (Crimmins et al., 2006; Duenas et al., 2006), whereas the S18Y allele of human UCH-L1 confers protection against sporadic Parkinson’s disease. UCH-L1 is concentrated in a variety of neuronal inclusion bodies in humans, and loss-of-function mutations in this protein cause axonal degeneration in neurons that terminate at the Gracile nucleus, a region of the brainstem that receives dorsal-root fibers conveying sensory innervation of the leg and lower trunk (Setsuie and Wada, 2007). It is possible that interfering with DUB function leads to cellular stress that is not obvious in most tissues but has a major impact in the nervous system, as the death of a small number of neurons can have profound functional consequences. Finally, it is notable that several bacteria (Rytkonen and Holden, 2007) and viruses (Lindner, 2007) have exploited the hostcell ubiquitin pathway by encoding DUBs that play a role in infection and pathogenesis. For example, the SARS coronavirus PLpro processing protease acts on a broad range of ubiquitylated and ISG15-modified host proteins and is required for viral replication (Ratia et al., 2008); the obligate intracellular bacterium, Burkholderia mallei, expresses and secretes a DUB inside infected macrophages (Shanks et al., 2009); and the ChlaDub1 expressed by Chlamydia trachomatis suppresses NF-κB activation (Le Negrate et al., 2008). Presumably these microbial DUBs confer a selective advantage on the pathogen by deubiquitylating host proteins and interfering with their normal cellular functions. The above examples describe pathological conditions that are caused by expression of heterologous DUBs or by mutations of endogenous DUBs, although many other disease states or cellular functions have been shown to be modulated by DUBs. There are only a few DUB mutations that are currently known to cause disease, but it is very likely that more will be recognized in the future. It is also probable that other

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DUBs can modulate the effects of disease. Furthermore, as DUB-dependent processes are integral to many regulatory pathways, it is possible that DUBs will prove to be attractive drug targets in cases where the pathological lesions are caused by other mutations or damage events. Perspectives It is apparent that ubiquitin signals are pervasive, flexible and dynamic. Virtually every cellular process that requires temporally or spatially regulated proteinprotein interactions is affected by ubiquitylation and deubiquitylation. In the past three years, numerous DUBs have been linked to some of the most vital of cellular functions and responses. These enzymes contribute greatly to the dynamic nature of the ubiquitin signal, and act by proofreading and disassembling ubiquitin chains with great specificity. This is achieved through the specificity of DUBs for their target proteins, the type of ubiquitin chain that they recognize and their cellular location. The associations of DUBs with ubiquitin ligases, scaffold proteins, substrates or substrate adaptors are also important factors in conferring this specificity. The metabolic functions of ubiquitylation and deubiquitylation parallel that of phosphorylation. There are estimated to be 500 or more each of kinases and ubiquitin ligases, whereas phosphatases and DUBs number around 100 each. Similar to the many kinases and phosphatases that have been studied for their therapeutic potential, DUBs have a role in numerous physiological and pathological processes. Thus it is obvious that opportunities abound for pharmacological intervention. As the picture is emerging that each DUB has a limited set of substrates, the selective interference of an individual DUB may have highly selective effects on the localization, stability and/or function of specific proteins. Therefore, drugs that inhibit the catalytic activity of specific DUBs, or that interfere with their interactions with other proteins, hold great promise for modulating the ubiquitindependent physiological processes that are involved in human disease. K.D.W. is the recipient of grants GM030308 and GM066355 from the National Institutes of Health. Deposited in PMC for release after 12 months.

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