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1.  High-Throughput Screening AlphaScreen Assay for Identification of Small-Molecule Inhibitors of Ubiquitin E3 Ligase SCFSkp2-Cks1 
Journal of biomolecular screening  2013;18(8):910-920.
Decreased levels of cell cycle inhibitor p27Kip1 due to excessive degradation occur in a variety of aggressive human tumors. Since reduced p27Kip1 expression has been associated with a poor prognosis in many human cancers and resistance to certain antitumor therapies, elevation of p27Kip1 expression could improve prognosis and prevent excessive cell proliferation. SCFSkp2 is one of the major ubiquitin E3 ligases responsible for degradation of p27Kip1. Ubiquitination of p27Kip1 also requires a small adaptor protein, Cks1, which facilitates substrate recruitment by bridging the interaction between Skp2 and p27Kip1. It has been shown previously that a direct interaction between Cks1 and Skp2 is required for p27Kip1 degradation. Accordingly, perturbation of the Skp2-Cks1 interaction may represent an attractive target for pharmacological intervention. Here we describe a high-throughput AlphaScreen assay for discovering small-molecule inhibitors of the Skp2-Cks1 protein-protein interaction in vitro. Two compounds (NSC689857 and NSC681152) were identified and validated through a structure-activity relationship analysis. Both compounds were also shown to inhibit p27Kip1 ubiquitination in vitro. These studies demonstrate that disruption of the Skp2-Cks1 interaction provides a viable strategy to prevent p27Kip1 ubiquitination and may potentially be useful for the control of excessive degradation of this cell cycle inhibitor in tumor cells.
PMCID: PMC4168015  PMID: 23589337
E3 ligase; inhibitor; Skp2; Cks1; p27kip1; ubiquitin; proteolysis
2.  Novel Inhibitors of Rad6 Ubiquitin Conjugating Enzyme: Design, Synthesis, Identification, and Functional Characterization 
Molecular cancer therapeutics  2013;12(4):10.1158/1535-7163.MCT-12-0793.
Protein ubiquitination is important for cell signaling, DNA repair, and proteasomal degradation, and it is not surprising that alterations in ubiquitination occur frequently in cancer. Ubiquitin-conjugating enzymes (E2) mediate ubiquitination by selective interactions with ubiquitin-activating (E1) and ubiquitin ligase (E3) enzymes, and thus selective E2 small molecule inhibitor (SMI) will provide specificity unattainable with proteasome inhibitors. Here we describe synthesis and functional characterization of the first SMIs of human E2 Rad6B, a fundamental component of translesion synthesis DNA repair. A pharmacophore model for consensus E2 ubiquitin-binding sites was generated for virtual screening to identify E2 inhibitor candidates. Twelve triazine (TZ) analogs screened in silico by molecular docking to the Rad6B X-ray structure were verified by their effect on Rad6B ubiquitination of histone H2A. TZs #8 and 9 docked to the Rad6B catalytic site with highest complementarity. TZs #1, 2, 8, and 9 inhibited Rad6B-ubiquitin thioester formation and subsequent ubiquitin transfer to histone H2A. SMI #9 inhibition of Rad6 was selective as BCA2 ubiquitination by E2 UbcH5 was unaffected by SMI #9. SMI #9 more potently inhibited proliferation, colony formation, and migration than SMI #8, and induced MDA-MB-231 breast cancer cell G2–M arrest and apoptosis. Ubiquitination assays using Rad6 immunoprecipitated from SMI #8- or 9-treated cells confirmed inhibition of endogenous Rad6 activity. Consistent with our previous data showing Rad6B-mediated polyubiquitination stabilizes β-catenin, MDAMB-231 treatment with SMIs #8 or 9 decreased β-catenin protein levels. Together these results describe identification of the first Rad6 SMIs.
PMCID: PMC3840907  PMID: 23339190
3.  Mechanism of ubiquitin ligation and lysine prioritization by a HECT E3 
eLife  2013;2:e00828.
Ubiquitination by HECT E3 enzymes regulates myriad processes, including tumor suppression, transcription, protein trafficking, and degradation. HECT E3s use a two-step mechanism to ligate ubiquitin to target proteins. The first step is guided by interactions between the catalytic HECT domain and the E2∼ubiquitin intermediate, which promote formation of a transient, thioester-bonded HECT∼ubiquitin intermediate. Here we report that the second step of ligation is mediated by a distinct catalytic architecture established by both the HECT E3 and its covalently linked ubiquitin. The structure of a chemically trapped proxy for an E3∼ubiquitin-substrate intermediate reveals three-way interactions between ubiquitin and the bilobal HECT domain orienting the E3∼ubiquitin thioester bond for ligation, and restricting the location of the substrate-binding domain to prioritize target lysines for ubiquitination. The data allow visualization of an E2-to-E3-to-substrate ubiquitin transfer cascade, and show how HECT-specific ubiquitin interactions driving multiple reactions are repurposed by a major E3 conformational change to promote ligation.
eLife digest
Ubiquitin is a small protein that can be covalently linked to other, ‘target’, proteins in a cell to influence their behavior. Ubiquitin can be linked to its targets either as single copies or as polyubiquitin chains in which several ubiquitin molecules are bound end-on-end to each other, with one end of the chain attached to the target protein. A multi-step cascade involving enzymes known as E1, E2, and E3 adds ubiquitin to its targets. These enzymes function in a manner like runners in a relay, with ubiquitin a baton that is passed from E1 to E2 to E3 to the target.
The E3 enzyme is a ligase that catalyzes the formation of a new chemical bond between a ubiquitin and its target. There are approximately 600 different E3 enzymes in human cells that regulate a wide variety of target proteins. A major class of E3 enzymes, called HECT E3s, attaches ubiquitin to its targets in a unique two-step mechanism: the E2 enzymes covalently link a ubiquitin to a HECT E3 to form a complex that subsequently transfers the ubiquitin to its target protein. The ubiquitin is typically added to a particular amino acid, lysine, on the target protein, but the details of how HECT E3s execute this transfer are not well understood. To address this issue, Kamadurai et al. investigate how Rsp5, a HECT E3 ligase in yeast, attaches ubiquitin to a target protein called Sna3.
All HECT E3s have a domain—the HECT domain—that catalyzes the transfer of ubiquitin to its target protein. This domain consists of two sub-structures: the C-lobe, which can receive ubiquitin from E2 and then itself become linked to ubiquitin, and the N-lobe. These lobes were previously thought to adopt various orientations relative to each other to deliver ubiquitin to sites on different target proteins (including to multiple lysines on a single target protein). Unexpectedly, Kamadurai et al. find that in order to transfer the ubiquitin to Sna3, Rsp5 adopts a discrete HECT domain architecture that creates an active site in which parts of the C-lobe and the N-lobe, which are normally separated, are brought together with a ubiquitin molecule. This architecture also provides a mechanism that dictates which substrate lysines can be ubiquitinated based on how accessible they are to this active site.
The same regions of Rsp5 transfer ubiquitin to targets other than Sna3, suggesting that a uniform mechanism—which Kamadurai et al. show is conserved in two related human HECT E3 ligases—might transfer ubiquitin to all its targets. These studies therefore represent a significant step toward understanding how a major class of E3 enzymes modulates the functions of their targets.
PMCID: PMC3738095  PMID: 23936628
ubiquitin; HECT; E3 ligase; E2 conjugating enzyme; NEDD4; Rsp5; S. cerevisiae
4.  Ras Binding Triggers Ubiquitination of the Ras Exchange Factor Ras-GRF2 
Molecular and Cellular Biology  2001;21(6):2107-2117.
Ras is a small GTPase that is activated by upstream guanine nucleotide exchange factors, one of which is Ras-GRF2. GRF2 is a widely expressed protein with several recognizable sequence motifs, including a Ras exchanger motif (REM), a PEST region containing a destruction box (DB), and a Cdc25 domain. The Cdc25 domain possesses guanine nucleotide exchange factor activity and interacts with Ras. Herein we examine if the DB motif in GRF2 results in proteolysis via the ubiquitin pathway. Based on the solved structure of the REM and Cdc25 regions of the Son-of-sevenless (Sos) protein, the REM may stabilize the Cdc25 domain during Ras binding. The DB motif of GRF2 is situated between the REM and the Cdc25 domains, tempting speculation that it may be exposed to ubiquitination machinery upon Ras binding. GRF2 protein levels decrease dramatically upon activation of GRF2, and dominant-negative Ras induces degradation of GRF2, demonstrating that signaling downstream of Ras is not required for the destruction of GRF2 and that binding to Ras is important for degradation. GRF2 is ubiquitinated in vivo, and this can be detected using mass spectrometry. In the presence of proteasome inhibitors, Ras-GRF2 accumulates as a high-molecular-weight conjugate, suggesting that GRF2 is destroyed by the 26S proteasome. Deleting the DB reduces the ubiquitination of GRF2. GRF2 lacking the Cdc25 domain is not ubiquitinated, suggesting that a protein that cannot bind Ras cannot be properly targeted for destruction. Point mutations within the Cdc25 domain that eliminate Ras binding also eliminate ubiquitination, demonstrating that binding to Ras is necessary for ubiquitination of GRF2. We conclude that conformational changes induced by GTPase binding expose the DB and thereby target GRF2 for destruction.
PMCID: PMC86827  PMID: 11238945
5.  E3 ubiquitin ligase-mediated regulation of bone formation and tumorigenesis 
Cell Death & Disease  2013;4(1):e463-.
The ubiquitination–proteasome and degradation system is an essential process that regulates protein homeostasis. This system is involved in the regulation of cell proliferation, differentiation and survival, and dysregulations in this system lead to pathologies including cancers. The ubiquitination system is an enzymatic cascade that mediates the marking of target proteins by an ubiquitin label and thereby directs their degradation through the proteasome pathway. The ubiquitination of proteins occurs through a three-step process involving ubiquitin activation by the E1 enzyme, allowing for the transfer to a ubiquitin-conjugated enzyme E2 and to the targeted protein via ubiquitin-protein ligases (E3), the most abundant group of enzymes involved in ubiquitination. Significant advances have been made in our understanding of the role of E3 ubiquitin ligases in the control of bone turnover and tumorigenesis. These ligases are implicated in the regulation of bone cells through the degradation of receptor tyrosine kinases, signaling molecules and transcription factors. Initial studies showed that the E3 ubiquitin ligase c-Cbl, a multi-domain scaffold protein, regulates bone resorption by interacting with several molecules in osteoclasts. Further studies showed that c-Cbl controls the ubiquitination of signaling molecules in osteoblasts and in turn regulates osteoblast proliferation, differentiation and survival. Recent data indicate that c-Cbl expression is decreased in primary bone tumors, resulting in excessive receptor tyrosine kinase signaling. Consistently, c-Cbl ectopic expression reduces bone tumorigenesis by promoting tyrosine kinase receptor degradation. Here, we review the mechanisms of action of E3 ubiquitin ligases in the regulation of normal and pathologic bone formation, and we discuss how targeting the interactions of c-Cbl with some substrates may be a potential therapeutic strategy to promote osteogenesis and to reduce tumorigenesis.
PMCID: PMC3564004  PMID: 23328670
ubiquitin ligases; proteasome; receptor tyrosine kinases; bone tumors; Cbl proteins; ubiquitination
6.  CD4 Glycoprotein Degradation Induced by Human Immunodeficiency Virus Type 1 Vpu Protein Requires the Function of Proteasomes and the Ubiquitin-Conjugating Pathway 
Journal of Virology  1998;72(3):2280-2288.
The human immunodeficiency virus type 1 (HIV-1) vpu gene encodes a type I anchored integral membrane phosphoprotein with two independent functions. First, it regulates virus release from a post-endoplasmic reticulum (ER) compartment by an ion channel activity mediated by its transmembrane anchor. Second, it induces the selective down regulation of host cell receptor proteins (CD4 and major histocompatibility complex class I molecules) in a process involving its phosphorylated cytoplasmic tail. In the present work, we show that the Vpu-induced proteolysis of nascent CD4 can be completely blocked by peptide aldehydes that act as competitive inhibitors of proteasome function and also by lactacystin, which blocks proteasome activity by covalently binding to the catalytic β subunits of proteasomes. The sensitivity of Vpu-induced CD4 degradation to proteasome inhibitors paralleled the inhibition of proteasome degradation of a model ubiquitinated substrate. Characterization of CD4-associated oligosaccharides indicated that CD4 rescued from Vpu-induced degradation by proteasome inhibitors is exported from the ER to the Golgi complex. This finding suggests that retranslocation of CD4 from the ER to the cytosol may be coupled to its proteasomal degradation. CD4 degradation mediated by Vpu does not require the ER chaperone calnexin and is dependent on an intact ubiquitin-conjugating system. This was demonstrated by inhibition of CD4 degradation (i) in cells expressing a thermally inactivated form of the ubiquitin-activating enzyme E1 or (ii) following expression of a mutant form of ubiquitin (Lys48 mutated to Arg48) known to compromise ubiquitin targeting by interfering with the formation of polyubiquitin complexes. CD4 degradation was also prevented by altering the four Lys residues in its cytosolic domain to Arg, suggesting a role for ubiquitination of one or more of these residues in the process of degradation. The results clearly demonstrate a role for the cytosolic ubiquitin-proteasome pathway in the process of Vpu-induced CD4 degradation. In contrast to other viral proteins (human cytomegalovirus US2 and US11), however, whose translocation of host ER molecules into the cytosol occurs in the presence of proteasome inhibitors, Vpu-targeted CD4 remains in the ER in a transport-competent form when proteasome activity is blocked.
PMCID: PMC109526  PMID: 9499087
7.  Hedgehog-Regulated Ubiquitination Controls Smoothened Trafficking and Cell Surface Expression in Drosophila 
PLoS Biology  2012;10(1):e1001239.
Hedgehog transduces signal by promoting cell surface expression of the seven-transmembrane protein Smoothened (Smo) in Drosophila, but the underlying mechanism remains unknown. Here we demonstrate that Smo is downregulated by ubiquitin-mediated endocytosis and degradation, and that Hh increases Smo cell surface expression by inhibiting its ubiquitination. We find that Smo is ubiquitinated at multiple Lysine residues including those in its autoinhibitory domain (SAID), leading to endocytosis and degradation of Smo by both lysosome- and proteasome-dependent mechanisms. Hh inhibits Smo ubiquitination via PKA/CK1-mediated phosphorylation of SAID, leading to Smo cell surface accumulation. Inactivation of the ubiquitin activating enzyme Uba1 or perturbation of multiple components of the endocytic machinery leads to Smo accumulation and Hh pathway activation. In addition, we find that the non-visual β-arrestin Kurtz (Krz) interacts with Smo and acts in parallel with ubiquitination to downregulate Smo. Finally, we show that Smo ubiquitination is counteracted by the deubiquitinating enzyme UBPY/USP8. Gain and loss of UBPY lead to reciprocal changes in Smo cell surface expression. Taken together, our results suggest that ubiquitination plays a key role in the downregulation of Smo to keep Hh pathway activity off in the absence of the ligand, and that Hh-induced phosphorylation promotes Smo cell surface accumulation by inhibiting its ubiquitination, which contributes to Hh pathway activation.
Author Summary
The Hedgehog (Hh) family of secreted proteins governs cell growth and patterning in diverse species ranging from Drosophila to human. Hh signals across the cell surface membrane by regulating the subcellular location and conformation of a membrane protein called Smoothened (Smo). In Drosophila, Smo accumulates on the cell surface in response to Hh, whereas in the absence of Hh it is internalized and degraded. The molecular mechanisms that control this intracellular trafficking and degradation of Smo were unknown, but here we show that Smo is modified by attachment of several molecules of a small protein called ubiquitin, which tags it for internalization and degradation within the cell. Hh inhibits this ubiquitination of Smo by inducing another modification, phosphorylation, of its intracellular tail by two types of protein kinase enzymes. This loss of ubiquitination and gain of phosphorylation causes the accumulation of Smo at the cell surface. What's more, we find that another protein called Kurtz interacts with Smo and acts in parallel with the ubiquitination process to promote internalization of Smo, and that the deubiquitinating enzyme UBPY/USP8 counteracts ubiquitination of Smo to promote its cell surface accumulation. Our study demonstrates that reversible ubiquitination plays a key role in regulating Smo trafficking to and from the cell surface and thus it provides novel insights into the mechanism of Hh signaling from the outside to the inside of the cell.
PMCID: PMC3254653  PMID: 22253574
8.  Proteomic snapshot of the EGF-induced ubiquitin network 
In this work, the authors report the first proteome-wide analysis of EGF-regulated ubiquitination, revealing surprisingly pervasive growth factor-induced ubiquitination across a broad range of cellular systems and signaling pathways.
Epidermal growth factor (EGF) triggers a novel ubiquitin (Ub)-based signaling cascade that appears to intersect both housekeeping and regulatory circuitries of cellular physiology.The EGF-regulated Ubiproteome includes scores ubiquitinating and deubiquitinating enzymes, suggesting that the Ub signal might be rapidly transmitted and amplified through the Ub machinery.The EGF-Ubiproteome overlaps significantly with the EGF-phosphotyrosine proteome, pointing to a possible crosstalk between these two signaling mechanisms.The significant number of biological insights uncovered in our study (among which EphA2 as a novel, downstream ubiquitinated target of EGF receptor) illustrates the general relevance of such proteomic screens and calls for further analysis of the dynamics of the Ubiproteome.
Ubiquitination is a process by which one or more ubiquitin (Ub) monomers or chains are covalently attached to target proteins by E3 ligases. Deubiquitinating enzymes (DUBs) revert Ub conjugation, thus ensuring a dynamic equilibrium between pools of ubiquitinated and deubiquitinated proteins (Amerik and Hochstrasser, 2004). Traditionally, ubiquitination has been associated with protein degradation; however, it is now becoming apparent that this post-translation modification is an important signaling mechanism that can modulate the function, localization and protein/protein interaction abilities of targets (Mukhopadhyay and Riezman, 2007; Ravid and Hochstrasser, 2008).
One of the best-characterized signaling pathways involving ubiquitination is the epidermal growth factor (EGF)-induced pathway. Upon EGF stimulation, a variety of proteins are subject to Ub modification. These include the EGF receptor (EGFR), which undergoes both multiple monoubiquitination (Haglund et al, 2003) and K63-linked polyubiquitination (Huang et al, 2006), as well as components of the downstream endocytic machinery, which are modified by monoubiquitination (Polo et al, 2002; Mukhopadhyay and Riezman, 2007). Ubiquitination of the EGFR has been shown to have an impact on receptor internalization, intracellular sorting and metabolic fate (Acconcia et al, 2009). However, little is known about the wider impact of EGF-induced ubiquitination on cellular homeostasis and on the pleiotropic biological functions of the EGFR. In this paper, we attempt to address this issue by characterizing the repertoire of proteins that are ubiquitinated upon EGF stimulation, i.e., the EGF-Ubiproteome.
To achieve this, we employed two different purification procedures (endogenous—based on the purification of proteins modified by endogenous Ub from human cells; tandem affinity purification (TAP)—based on the purification of proteins modified by an ectopically expressed tagged-Ub from mouse cells) with stable isotope labeling with amino acids in cell culture-based MS to obtain both steady-state Ubiproteomes and EGF-induced Ubiproteomes. The steady-state Ubiproteomes consist of 1175 and 582 unambiguously identified proteins for the endogenous and TAP approaches, respectively, which we largely validated. Approximately 15% of the steady-state Ubiproteome was EGF-regulated at 10 min after stimulation; 176 of 1175 in the endogenous approach and 105 of 582 in the TAP approach. Both hyper- and hypoubiquitinated proteins were detected, indicating that EGFR-mediated signaling can modulate the ubiquitin network in both directions. Interestingly, many E2, E3 and DUBs were present in the EGF-Ubiproteome, suggesting that the Ub signal might be rapidly transmitted and amplified through the Ub machinery. Moreover, analysis of Ub-chain topology, performed using mass spectrometry and specific abs, suggested that the K63-linkage was the major Ub-based signal in the EGF-induced pathway.
To obtain a higher-resolution molecular picture of the EGF-regulated Ub network, we performed a network analysis on the non-redundant EGF-Ubiproteome (265 proteins). This analysis revealed that in addition to well-established liaisons with endocytosis-related pathways, the EGF-Ubiproteome intersects many circuitries of intracellular signaling involved in, e.g., DNA damage checkpoint regulation, cell-to-cell adhesion mechanisms and actin remodeling (Figure 5A).
Moreover, the EGF-Ubiproteome was enriched in hubs, proteins that can establish multiple protein/protein interaction and thereby regulate the organization of networks. These results are indicative of a crosstalk between EGFR-activated pathways and other signaling pathways through the Ub-network.
As EGF binding to its receptor also triggers a series of phosphorylation events, we examined whether there was any overlap between our EGF-Ubiproteome and published EGF-induced phosphotyrosine (pY) proteomes (Blagoev et al, 2004; Oyama et al, 2009; Hammond et al, 2010). We observed a significant overlap between ubiquitinated and pY proteins: 23% (61 of 265) of the EGF-Ubiproteome proteins were also tyrosine phosphorylated. Pathway analysis of these 61 Ub/pY-containing proteins revealed a significant enrichment in endocytic and signal-transduction pathways, while ‘hub analysis' revealed that Ub/pY-containing proteins are enriched in highly connected proteins to an even greater extent than Ub-containing proteins alone. These data point to a complex interplay between the Ub and pY networks and suggest that the flow of information from the receptor to downstream signaling molecules is driven by two complementary and interlinked enzymatic cascades: kinases/phosphatases and E3 ligases/DUBs.
Finally, we provided a proof of principle of the biological relevance of our EGF-Ubiproteome. We focused on EphA2, a receptor tyrosine kinase, which is involved in development and is often overexpressed in cancer (Pasquale, 2008). We started from the observation that EphA2 is present in the EGF-Ubiproteome and that proteins of the EGF-Ubiproteome are enriched in the Ephrin receptor signaling pathway(s). We confirmed the MS data by demonstrating that the EphA2 is ubiquitinated upon EGF stimulation. Moreover, EphA2 also undergoes tyrosine phosphorylation, indicating crosstalk between the two receptors. The EGFR kinase domain was essential for these modifications of EphA2, and a partial co-internalization with EGFR upon EGF activation was clearly detectable. Finally, we demonstrated by knockdown of EphA2 in MCF10A cells that this receptor is critically involved in EGFR biological outcomes, such as proliferation and migration (Figure 7).
Overall, our results unveil the complex impact of growth factor signaling on Ub-based intracellular networks to levels that extend well beyond what might have been expected and highlight the ‘resource' feature of our EGF-Ubiproteome.
The activity, localization and fate of many cellular proteins are regulated through ubiquitination, a process whereby one or more ubiquitin (Ub) monomers or chains are covalently attached to target proteins. While Ub-conjugated and Ub-associated proteomes have been described, we lack a high-resolution picture of the dynamics of ubiquitination in response to signaling. In this study, we describe the epidermal growth factor (EGF)-regulated Ubiproteome, as obtained by two complementary purification strategies coupled to quantitative proteomics. Our results unveil the complex impact of growth factor signaling on Ub-based intracellular networks to levels that extend well beyond what might have been expected. In addition to endocytic proteins, the EGF-regulated Ubiproteome includes a large number of signaling proteins, ubiquitinating and deubiquitinating enzymes, transporters and proteins involved in translation and transcription. The Ub-based signaling network appears to intersect both housekeeping and regulatory circuitries of cellular physiology. Finally, as proof of principle of the biological relevance of the EGF-Ubiproteome, we demonstrated that EphA2 is a novel, downstream ubiquitinated target of epidermal growth factor receptor (EGFR), critically involved in EGFR biological responses.
PMCID: PMC3049407  PMID: 21245847
EGF; network; proteomics; signaling; ubiquitin
9.  Disease-Associated Mutant Ubiquitin Causes Proteasomal Impairment and Enhances the Toxicity of Protein Aggregates 
PLoS Genetics  2009;5(2):e1000382.
Protein homeostasis is critical for cellular survival and its dysregulation has been implicated in Alzheimer's disease (AD) and other neurodegenerative disorders. Despite the growing appreciation of the pathogenic mechanisms involved in familial forms of AD, much less is known about the sporadic cases. Aggregates found in both familial and sporadic AD often include proteins other than those typically associated with the disease. One such protein is a mutant form of ubiquitin, UBB+1, a frameshift product generated by molecular misreading of a wild-type ubiquitin gene. UBB+1 has been associated with multiple disorders. UBB+1 cannot function as a ubiquitin molecule, and it is itself a substrate for degradation by the ubiquitin/proteasome system (UPS). Accumulation of UBB+1 impairs the proteasome system and enhances toxic protein aggregation, ultimately resulting in cell death. Here, we describe a novel model system to investigate how UBB+1 impairs UPS function and whether it plays a causal role in protein aggregation. We expressed a protein analogous to UBB+1 in yeast (Ubext) and demonstrated that it caused UPS impairment. Blocking ubiquitination of Ubext or weakening its interactions with other ubiquitin-processing proteins reduced the UPS impairment. Expression of Ubext altered the conjugation of wild-type ubiquitin to a UPS substrate. The expression of Ubext markedly enhanced cellular susceptibility to toxic protein aggregates but, surprisingly, did not induce or alter nontoxic protein aggregates in yeast. Taken together, these results suggest that Ubext interacts with more than one protein to elicit impairment of the UPS and affect protein aggregate toxicity. Furthermore, we suggest a model whereby chronic UPS impairment could inflict deleterious consequences on proper protein aggregate sequestration.
Author Summary
The accumulation of cytotoxic protein aggregates occurs in many neurodegenerative diseases. It is difficult to determine if the protein aggregates found in these diseases represent a cause or consequence of the disorder. Degradation pathways, such as the ubiquitin/proteasome system (UPS), remove misfolded proteins that are prone to aggregate. The UPS involves many players that work in concert to target proteins for degradation by the proteasome. A mutant form of ubiquitin has been associated with many diseases, including Alzheimer's disease. We developed a yeast model of the mutant ubiquitin protein in order to investigate its effect on UPS function and protein aggregation. We demonstrate that this mutant ubiquitin causes impairment of the UPS and suggest that it does so by interacting with multiple components of the pathway. Using this model, we evaluated the effects of the mutant ubiquitin on nontoxic protein aggregates and found that they were unaltered by its presence. We demonstrate that the mutant ubiquitin acts as a modifier, which increases cellular susceptibility to the phenotypic effects of deleterious protein aggregates by altering UPS functionality and substrate ubiquitination. Furthermore, the system we developed can be utilized to further understand the complex interplay of proteasomal impairment and protein aggregate toxicity.
PMCID: PMC2633047  PMID: 19214209
10.  DNA Polymerase β as a Novel Target for Chemotherapeutic Intervention of Colorectal Cancer 
PLoS ONE  2011;6(2):e16691.
Chemoprevention presents a major strategy for the medical management of colorectal cancer. Most drugs used for colorectal cancer therapy induce DNA-alkylation damage, which is primarily repaired by the base excision repair (BER) pathway. Thus, blockade of BER pathway is an attractive option to inhibit the spread of colorectal cancer. Using an in silico approach, we performed a structure-based screen by docking small-molecules onto DNA polymerase β (Pol-β) and identified a potent anti-Pol-β compound, NSC-124854. Our goal was to examine whether NSC-124854 could enhance the therapeutic efficacy of DNA-alkylating agent, Temozolomide (TMZ), by blocking BER. First, we determined the specificity of NSC-124854 for Pol-β by examining in vitro activities of APE1, Fen1, DNA ligase I, and Pol-β-directed single nucleotide (SN)- and long-patch (LP)-BER. Second, we investigated the effect of NSC-124854 on the efficacy of TMZ to inhibit the growth of mismatch repair (MMR)-deficient and MMR-proficient colon cancer cell lines using in vitro clonogenic assays. Third, we explored the effect of NSC-124854 on TMZ-induced in vivo tumor growth inhibition of MMR-deficient and MMR-proficient colonic xenografts implanted in female homozygous SCID mice. Our data showed that NSC-124854 has high specificity to Pol-β and blocked Pol-β-directed SN- and LP-BER activities in in vitro reconstituted system. Furthermore, NSC-124854 effectively induced the sensitivity of TMZ to MMR-deficient and MMR-proficient colon cancer cells both in vitro cell culture and in vivo xenograft models. Our findings suggest a potential novel strategy for the development of highly specific structure-based inhibitor for the prevention of colonic tumor progression.
PMCID: PMC3032781  PMID: 21311763
11.  Small ubiquitin-like modifying protein isopeptidase assay based on poliovirus RNA polymerase activity 
Analytical biochemistry  2005;350(2):214-221.
The ubiquitin–proteasome pathway is the major nonlysosomal proteolytic system in eukaryotic cells responsible for regulating the level of many key regulatory molecules within the cells. Modification of cellular proteins by ubiquitin and ubiquitin-like proteins, such as small ubiquitin-like modifying protein (SUMO), plays an essential role in a number of biological schemes, and ubiquitin pathway enzymes have become important therapeutic targets. Ubiquitination is a dynamic reversible process; a multitude of ubiquitin ligases and deubiquitinases (DUBs) are responsible for the wide-ranging influence of this pathway as well as its selectivity. The DUB enzymes serve to maintain adequate pools of free ubiquitin and regulate the ubiquitination status of cellular proteins. Using SUMO fusions, a novel assay system, based on poliovirus RNA-dependent RNA polymerase activity, is described here. The method simplifies the isopeptidase assay and facilitates high-throughput analysis of these enzymes. The principle of the assay is the dependence of the viral polymerase on a free N terminus for activity; accordingly, the polymerase is inactive when fused at its N terminus to SUMO or any other ubiquitin-like protein. The assay is sensitive, reproducible, and adaptable to a high-throughput format for use in screens for inhibitors/activators of clinically relevant SUMO proteases and deubiquitinases.
PMCID: PMC2094218  PMID: 16356462
Isopeptidases; Protein degradation; N terminus; 3D polymerase
12.  Quantitative Cell-based Protein Degradation Assays to Identify and Classify Drugs That Target the Ubiquitin-Proteasome System* 
The Journal of Biological Chemistry  2011;286(19):16546-16554.
We have generated a set of dual-reporter human cell lines and devised a chase protocol to quantify proteasomal degradation of a ubiquitin fusion degradation (UFD) substrate, a ubiquitin ligase CRL2VHL substrate, and a ubiquitin-independent substrate. Well characterized inhibitors that target different aspects of the ubiquitin-proteasome system can be distinguished by their distinctive patterns of substrate stabilization, enabling assignment of test compounds as inhibitors of the proteasome, ubiquitin chain formation or perception, CRL activity, or the UFD-p97 pathway. We confirmed that degradation of the UFD but not the CRL2VHL or ubiquitin-independent substrates depends on p97 activity. We optimized our suite of assays to establish conditions suitable for high-throughput screening and then validated their performance by screening against 160 cell-permeable protein kinase inhibitors. This screen identified Syk inhibitor III as an irreversible p97/vasolin containing protein inhibitor (IC50 = 1.7 μm) that acts through Cys-522 within the D2 ATPase domain. Our work establishes a high-throughput screening-compatible pipeline for identification and classification of small molecules, cDNAs, or siRNAs that target components of the ubiquitin-proteasome system.
PMCID: PMC3089497  PMID: 21343295
ATPases; High-throughput Screening (HTS); Proteasome; Protein Degradation; Ubiquitin
13.  Terminating protein ubiquitination 
Cell Cycle  2011;10(18):3067-3071.
Ubiquitination is a post-translational modification that generally directs proteins for degradation by the proteasome or by lysosomes. However, ubiquitination has been implicated in many other cellular processes, including transcriptional regulation, DNA repair, regulation of protein-protein interactions and association with ubiquitin-binding scaffolds. Ubiquitination is a dynamic process. Ubiquitin is added to proteins by E3 ubiquitin ligases as a covalent modification to one or multiple lysine residues as well as non-lysine amino acids. Ubiquitin itself contains seven lysines, each of which can also be ubiquitinated, leading to polyubiquitin chains that are best characterized for linkages occurring through K48 and K63. Ubiquitination can also be reversed by the action of deubiquitination enzymes (DUbs). Like E3 ligases, DUbs play diverse and critical roles in cells.1 Ubiquitin is expressed as a fusion protein, as a linear repeat or as a fusion to ribosomal subunits, and DUbs are necessary to liberate free ubiquitin, making them the first enzyme of the ubiquitin cascade. Proteins destined for degradation by the proteasome or by lysosomes are deubiquitinated prior to their degradation, which allows ubiquitin to be recycled by the cell, contributing to the steady-state pool of free ubiquitin. Proteins destined for degradation by lysosomes are also acted upon by both ligases and DUbs. Deubiquitination can also act as a means to prevent protein degradation, and many proteins are thought to undergo rounds of ubiquitination and deubiquitination, ultimately resulting in either the degradation or stabilization of those proteins. Despite years of study, examining the effects of the ubiquitination of proteins remains quite challenging. This is because the methods that are currently being employed to study ubiquitination are limiting. Here, we briefly examine current strategies to study the effects of ubiquitination and describe an additional novel approach that we have developed.
PMCID: PMC3685619  PMID: 21926471
ubiquitin; endosome; ligase; lysosome; degradation
14.  USP13 antagonizes gp78 to maintain functionality of a chaperone in ER-associated degradation 
eLife  2014;3:e01369.
Physiological adaptation to proteotoxic stress in the endoplasmic reticulum (ER) requires retrotranslocation of misfolded proteins into the cytoplasm for ubiquitination and elimination by ER-associated degradation (ERAD). A surprising paradox emerging from recent studies is that ubiquitin ligases (E3s) and deubiquitinases (DUBs), enzymes with opposing activities, can both promote ERAD. Here we demonstrate that the ERAD E3 gp78 can ubiquitinate not only ERAD substrates, but also the machinery protein Ubl4A, a key component of the Bag6 chaperone complex. Remarkably, instead of targeting Ubl4A for degradation, polyubiquitination is associated with irreversible proteolytic processing and inactivation of Bag6. Importantly, we identify USP13 as a gp78-associated DUB that eliminates ubiquitin conjugates from Ubl4A to maintain the functionality of Bag6. Our study reveals an unexpected paradigm in which a DUB prevents undesired ubiquitination to sharpen substrate specificity for an associated ubiquitin ligase partner and to promote ER quality control.
eLife digest
Cells make proteins inside a structure called the endoplasmic reticulum. However, some of these proteins cannot fold into the correct shape, so cells rely on a process called the ERAD pathway to degrade and eliminate these faulty proteins. First, however, the misfolded proteins must be moved from the endoplasmic reticulum to the main body of the cell (the cytosol).
The process by which the misfolded proteins are moved through the membrane that encloses the endoplasmic reticulum is complex, with ‘ERAD machinery proteins’ playing an important role. Among them, a series of enzymes called E3 ligases ‘tag’ the faulty proteins with a small protein called ubiquitin, and a complex called the proteasome then recognizes and degrades those proteins that have been tagged with ubiquitin. However, it is not clear why the E3 ligases that tag the misfolded proteins with ubiquitin don’t also tag the machinery proteins that from complexes with the faulty proteins.
Now Liu et al. have used a combination of biochemical and genetic tools to shed light on this puzzle by studying the interaction of gp78—which is an E3 ligase—and USP13, an enzyme that opposes the actions of the E3 ligases by removing ubiquitin. Liu et al. showed that gp78 can indeed tag certain machinery proteins with ubiquitin, which would stop the removal of misfolded proteins from the endoplasmic reticulum. However, USP13 opposed the action of gp78, thus allowing the removal to continue.
It has been known for some time that enzymes with opposing roles—the addition and removal of ubiquitin—can work together, but the biological significance of this phenomenon was not fully understood. The work of Liu et al. suggests that USP13 makes the elimination of misfolded proteins more efficient by ensuring that gp78 only tags those proteins that are misfolded: it does this by removing ubiquitin from proteins that should not have been tagged. A similar phenomenon is known to occur in genetics during DNA replication, with the enzyme complex that replicates the DNA including an enzyme that performs a proofreading role.
PMCID: PMC3889402  PMID: 24424410
ubiquitin; ERAD; deubiquitinase; ubiquitin ligase; Bag6; gp78; Human
15.  Protacs for Treatment of Cancer 
Pediatric research  2010;67(5):505-508.
Protein degradation is the cell’s mechanism of eliminating misfolded or unwanted proteins. The pathway by which proteins are degraded occurs through the ubiquitin-proteasome system. Ubiquitin is a small 9-kilodalton (kDa) protein that is attached to proteins. A minimum of four ubiquitins is required for proteins to be recognized by the degradation machinery, known as the 26S proteasome. Defects in ubiquitination have been identified in a number of diseases, including cancer, neurodegenerative diseases, and metabolic disorders. We sought to exploit the delicate balance between protein synthesis and degradation to treat cancer by designing a chimeric molecule, known as Protac (Proteolysis Targeting Chimeric molecule). Protacs are heterobifunctional nanomolecules that are approximately 10 nanometers (nm) in size and can recruit proteins that cause cancer to the ubiquitin-proteasome machinery for degradation. In this review, we discuss the development of this novel technology for the treatment of cancer.
PMCID: PMC2881331  PMID: 20075761
16.  Pathogenic Bacteria Target NEDD8-Conjugated Cullins to Hijack Host-Cell Signaling Pathways 
PLoS Pathogens  2010;6(9):e1001128.
The cycle inhibiting factors (Cif), produced by pathogenic bacteria isolated from vertebrates and invertebrates, belong to a family of molecules called cyclomodulins that interfere with the eukaryotic cell cycle. Cif blocks the cell cycle at both the G1/S and G2/M transitions by inducing the stabilization of cyclin-dependent kinase inhibitors p21waf1 and p27kip1. Using yeast two-hybrid screens, we identified the ubiquitin-like protein NEDD8 as a target of Cif. Cif co-compartmentalized with NEDD8 in the host cell nucleus and induced accumulation of NEDD8-conjugated cullins. This accumulation occurred early after cell infection and correlated with that of p21 and p27. Co-immunoprecipitation revealed that Cif interacted with cullin-RING ubiquitin ligase complexes (CRLs) through binding with the neddylated forms of cullins 1, 2, 3, 4A and 4B subunits of CRL. Using an in vitro ubiquitylation assay, we demonstrate that Cif directly inhibits the neddylated CUL1-associated ubiquitin ligase activity. Consistent with this inhibition and the interaction of Cif with several neddylated cullins, we further observed that Cif modulates the cellular half-lives of various CRL targets, which might contribute to the pathogenic potential of diverse bacteria.
Author Summary
Among the arsenal of virulence factors used by bacterial pathogens to infect and manipulate their hosts, cyclomodulins are a growing family of bacterial toxins that interfere with the eukaryotic cell-cycle. Cif is one of these cyclomodulins produced by both mammalian and invertebrate pathogenic bacteria. Cif blocks the host cell cycle by inducing the accumulation of two regulators of cell cycle progression: the cyclin-dependent kinase inhibitors p21 and p27. To decipher the mode of action of Cif, we performed yeast two-hybrid screenings. We show that Cif binds to NEDD8 and induce accumulation of neddylated cullins early after infection. Cullins are scaffold components of cullin-RING ubiquitin ligases (CRLs), which ubiquitinate proteins and target them for degradation by the 26S proteasome. We demonstrate that Cif directly inhibits the ubiquitin ligase activity of these CRLs and consequently the targeting of p21 and p27 for ubiquitin-dependent degradation. Targeting at NEDD8 represents a novel strategy for modulation of host cell functions by bacterial pathogens. By inhibiting the most prominent class of ubiquitin-ligases, Cif controls the stability of a cohort of key regulators and impinge on not only cell cycle progression but also on many cellular and biological processes such as immunity, development, transcription, and cell signaling.
PMCID: PMC2947998  PMID: 20941356
17.  Degradation of Phosphorylated p53 by Viral Protein-ECS E3 Ligase Complex 
PLoS Pathogens  2009;5(7):e1000530.
p53-signaling is modulated by viruses to establish a host cellular environment advantageous for their propagation. The Epstein-Barr virus (EBV) lytic program induces phosphorylation of p53, which prevents interaction with MDM2. Here, we show that induction of EBV lytic program leads to degradation of p53 via an ubiquitin-proteasome pathway independent of MDM2. The BZLF1 protein directly functions as an adaptor component of the ECS (Elongin B/C-Cul2/5-SOCS-box protein) ubiquitin ligase complex targeting p53 for degradation. Intringuingly, C-terminal phosphorylation of p53 resulting from activated DNA damage response by viral lytic replication enhances its binding to BZLF1 protein. Purified BZLF1 protein-associated ECS could be shown to catalyze ubiquitination of phospho-mimetic p53 more efficiently than the wild-type in vitro. The compensation of p53 at middle and late stages of the lytic infection inhibits viral DNA replication and production during lytic infection, suggesting that the degradation of p53 is required for efficient viral propagation. Taken together, these findings demonstrate a role for the BZLF1 protein-associated ECS ligase complex in regulation of p53 phosphorylated by activated DNA damage signaling during viral lytic infection.
Author Summary
Inhibition of p53-mediated transactivation is essential for regulating the cellular environment advantageous for viral infection. Specially, DNA viruses target p53 for inactivation through the ubiquitin-proteasome pathway. The E6 protein of the high-risk human papillomaviruses and the cellular ubiquitin-protein ligase E6AP form a complex which causes ubiquitination and degradation of p53. The adenovirus E1B 55-kDa protein binds to both p53 and E4orf6, and recruits a Cullin-containing complex to direct the ubiquitin-mediated proteolysis of p53. However, in comparison with the effects of the smaller DNA viruses, much less is known regarding the precise mechanisms whereby the Epstein-Barr virus (EBV) inhibits functions of p53. EBV possesses two alternative life cycles, latent and lytic replication. In latent phase, p53 is regulated by MDM2 ubiquitin ligase while after induction of lytic replication p53 is phosphorylated and the level of activated p53 is regulated by a novel system independent of MDM2. This report describes a unique functional role of the BZLF1 protein encoded by EBV in the modulation of activated p53. In this pathway, BZLF1 protein serves as an adaptor molecule for both Cul2- and Cul5-containing E3 ubiquitin ligase complexes to stimulate the ubiquitination and degradation of p53 for inhibiting apoptosis, indicating redundancy in the EBV machinery to downregulate p53 level. Therefore, it would be possible that the complexes regulate not only p53 but also various proteins that interact with BZLF1 protein.
PMCID: PMC2712087  PMID: 19649319
18.  E1 Ubiquitin-Activating Enzyme UBA-1 Plays Multiple Roles throughout C. elegans Development 
PLoS Genetics  2008;4(7):e1000131.
Poly-ubiquitination of target proteins typically marks them for destruction via the proteasome and provides an essential mechanism for the dynamic control of protein levels. The E1 ubiquitin-activating enzyme lies at the apex of the ubiquitination cascade, and its activity is necessary for all subsequent steps in the reaction. We have isolated a temperature-sensitive mutation in the Caenorhabditis elegans uba-1 gene, which encodes the sole E1 enzyme in this organism. Manipulation of UBA-1 activity at different developmental stages reveals a variety of functions for ubiquitination, including novel roles in sperm fertility, control of body size, and sex-specific development. Levels of ubiquitin conjugates are substantially reduced in the mutant, consistent with reduced E1 activity. The uba-1 mutation causes delays in meiotic progression in the early embryo, a process that is known to be regulated by ubiquitin-mediated proteolysis. The uba-1 mutation also demonstrates synthetic lethal interactions with alleles of the anaphase-promoting complex, an E3 ubiquitin ligase. The uba-1 mutation provides a sensitized genetic background for identifying new in vivo functions for downstream components of the ubiquitin enzyme cascade, and it is one of the first conditional mutations reported for the essential E1 enzyme in a metazoan animal model.
Author Summary
Proteins that control an organism's development must first be turned on at the proper time and place, and then turned off when they are no longer needed. One of the “off” signals occurs through the attachment of a small protein, known as ubiquitin, to the target protein, which typically leads to the destruction of the target. Attachment of ubiquitin is controlled by a series of enzymes, the first of which is known as E1. Most organisms have a single gene for the E1 enzyme, and its activity is crucial for the degradation of a wide range of target proteins throughout development. We have identified a temperature-sensitive mutation in the E1 enzyme of the nematode Caenorhabditis elegans. By manipulating the growth temperature, we have determined the various functions of E1 at different stages of development. We find that this enzyme controls embryonic and larval development, sperm fertility, and body size. We also characterized sex-specific roles for E1; males exhibit progressive paralysis and defects in the tail, which is used for mating. In addition to the knowledge gained, this mutation provides a means of identifying both the functions of other ubiquitin enzymes during development as well as the target proteins that are marked for destruction.
PMCID: PMC2443343  PMID: 18636104
19.  Cdc48/p97 promotes degradation of aberrant nascent polypeptides bound to the ribosome 
eLife  2013;2:e00308.
Ubiquitin-dependent proteolysis can initiate at ribosomes for myriad reasons including misfolding of a nascent chain or stalling of the ribosome during translation of mRNA. Clearance of a stalled complex is required to recycle the ribosome for future use. Here we show that the ubiquitin (Ub) pathway segregase Cdc48/p97 and its adaptors Ufd1-Npl4 participate in ribosome-associated degradation (RAD) by mediating the clearance of ubiquitinated, tRNA-linked nascent peptides from ribosomes. Through characterization of both endogenously-generated and heterologous model substrates for the RAD pathway, we conclude that budding yeast Cdc48 functions downstream of the Ub ligases Ltn1 and Ubr1 to release nascent proteins from the ribosome so that they can be degraded by the proteasome. Defective RAD could contribute to the pathophysiology of human diseases caused by mutations in p97.
eLife digest
Ribosomes are complex molecular machines that translate the sequence of bases in a messenger RNA (mRNA) transcript into a polypeptide that subsequently folds to form a protein. Each ribosome is composed of two major subunits: the small subunit reads the mRNA transcript, and the large subunit joins amino acids together to form the polypeptide. This process stops when the ribosome encounters a stop codon and releases the completed polypeptide.
It is critical that cells perform some form of quality control on the polypeptides as they are translated to prevent a build up of incomplete, incorrect or toxic proteins in cells. Problems can occur if a ribosome stalls while translating the mRNA transcript, or if the mRNA transcript is defective. For example, most mRNA transcripts contain a stop codon, but some do not, and these non-stop mRNA transcripts result in a non-stop polypeptide that remains tethered to the ribosome. It is important that the cell identifies and removes these faulty polypeptides so as to leave the ribosome free to translate other (non-faulty) mRNA transcripts. A regulatory protein called ubiquitin is responsible for marking and sending proteins that are faulty, or are no longer needed by the cell, to a molecular machine called the proteasome, where they are degraded by a process called proteolysis. In 2010 researchers identified Ltn1 as the enzyme that attaches ubiquitin to non-stop proteins in yeast.
Now, building on this work, Verma et al. identify additional proteins involved in this process. In particular, an ATPase enzyme called Cdc48 (known as p97 or VCP in human cells) and two co-factors—Ufd1 and Npl4—promote release of the ubiquitinated non-stop polypeptides from the ribosomes, thus committing the marked polypeptide to destruction by the proteasome. Verma et al. also show that the Cdc48-Ufd1-Npl4 complex is involved in other aspects of quality control of newly synthesized proteins within cells. Collectively these processes are known as ribosome-associated degradation.
Mutations of the gene that codes for human p97 can cause a number of diseases, including Paget's disease of the bone and frontotemporal dementia, so an improved understanding of ribosome-associated degradation could provide new insights into these diseases.
PMCID: PMC3552423  PMID: 23358411
ubiquitin; ribosome; Cdc48; S. cerevisiae
20.  A Perturbed Ubiquitin Landscape Distinguishes Between Ubiquitin in Trafficking and in Proteolysis* 
Molecular & Cellular Proteomics : MCP  2011;10(5):M111.009753.
Any of seven lysine residues on ubiquitin can serve as the base for chain-extension, resulting in a sizeable spectrum of ubiquitin modifications differing in chain length or linkage type. By optimizing a procedure for rapid lysis, we charted the profile of conjugated cellular ubiquitin directly from whole cell extract. Roughly half of conjugated ubiquitin (even at high molecular weights) was nonextended, consisting of monoubiquitin modifications and chain terminators (endcaps). Of extended ubiquitin, the primary linkages were via Lys48 and Lys63. All other linkages were detected, contributing a relatively small portion that increased at lower molecular weights. In vivo expression of lysineless ubiquitin (K0 Ub) perturbed the ubiquitin landscape leading to elevated levels of conjugated ubiquitin, with a higher mono-to-poly ratio. Affinity purification of these trapped conjugates identified a comprehensive list of close to 900 proteins including novel targets. Many of the proteins enriched by K0 ubiquitination were membrane-associated, or involved in cellular trafficking. Prime among them are components of the ESCRT machinery and adaptors of the Rsp5 E3 ubiquitin ligase. Ubiquitin chains associated with these substrates were enriched for Lys63 linkages over Lys48, indicating that K0 Ub is unevenly distributed throughout the ubiquitinome. Biological assays validated the interference of K0 Ub with protein trafficking and MVB sorting, minimally affecting Lys48-dependent turnover of proteasome substrates. We conclude that despite the shared use of the ubiquitin molecule, the two branches of the ubiquitin machinery—the ubiquitin-proteasome system and the ubiquitin trafficking system—were unevenly perturbed by expression of K0 ubiquitin.
PMCID: PMC3098606  PMID: 21427232
21.  Regulation of Proteolysis by Human Deubiquitinating Enzymes 
Biochimica et biophysica acta  2013;1843(1):10.1016/j.bbamcr.2013.06.027.
The post-translational attachment of one or several ubiquitin molecules to a protein generates a variety of targeting signals that are used in many different ways in the cell. Ubiquitination can alter the activity, localization, protein-protein interactions or stability of the targeted protein. Further, a very large number of proteins are subject to regulation by ubiquitin-dependent processes, meaning that virtually all cellular functions are impacted by these pathways. Nearly a hundred enzymes from five different gene families (the deubiquitinating enzymes or DUBs), reverse this modification by hydrolyzing the (iso)peptide bond tethering ubiquitin to itself or the target protein. Four of these families are thiol proteases and one is a metalloprotease. DUBs of the Ubiquitin C-terminal Hydrolase (UCH) family act on small molecule adducts of ubiquitin, process the ubiquitin proprotein, and trim ubiquitin from the distal end of a polyubiquitin chain. Ubiquitin Specific Proteases (USP) tend to recognize and encounter their substrates by interaction of the variable regions of their sequence with the substrate protein directly, or with scaffolds or substrate adapters in multiprotein complexes. Ovarian Tumor (OTU) domain DUBs show remarkable specificity for different Ub chain linkages and may have evolved to recognize substrates on the basis of those linkages. The Josephin family of DUBs may specialize in distinguishing between polyubiquitin chains of different lengths. Finally, the JAB1/MPN+/MOV34 (JAMM) domain metalloproteases cleave the isopeptide bond near the attachment point of polyubiquitin and substrate, as well as being highly specific for the K63 poly-Ub linkage. These DUBs regulate proteolysis by: directly interacting with and co-regulating E3 ligases; altering the level of substrate ubiquitination; hydrolyzing or remodeling ubiquitinated and poly-ubiquitinated substrates; acting in specific locations in the cell and altering the localization of the target protein; and acting on proteasome bound substrates to facilitate or inhibit proteolysis. Thus, the scope and regulation of the ubiquitin pathway is very similar to that of phosphorylation, with the DUBs serving the same functions as the phosphatase.
PMCID: PMC3833951  PMID: 23845989
Deubiquitinating enzyme; Ubiquitin; Poly-Ubiquitin; Proteolysis; Regulation
22.  Chemical genetics approach to restoring p27Kip1 reveals novel compounds with antiproliferative activity in prostate cancer cells 
BMC Biology  2010;8:153.
The cyclin-dependent kinase (CDK) inhibitor p27Kip1 is downregulated in a majority of human cancers due to ectopic proteolysis by the ubiquitin-proteasome pathway. The expression of p27 is subject to multiple mechanisms of control involving several transcription factors, kinase pathways and at least three different ubiquitin ligases (SCFSKP2, KPC, Pirh2), which regulate p27 transcription, translation, protein stability and subcellular localization. Using a chemical genetics approach, we have asked whether this control network can be modulated by small molecules such that p27 protein expression is restored in cancer cells.
We developed a cell-based assay for measuring the levels of endogenous nuclear p27 in a high throughput screening format employing LNCaP prostate cancer cells engineered to overexpress SKP2. The assay platform was optimized to Z' factors of 0.48 - 0.6 and piloted by screening a total of 7368 chemical compounds. During the course of this work, we discovered two small molecules of previously unknown biological activity, SMIP001 and SMIP004, which increase the nuclear level of p27 at low micromolar concentrations. SMIPs (small molecule inhibitors of p27 depletion) also upregulate p21Cip1, inhibit cellular CDK2 activity, induce G1 delay, inhibit colony formation in soft agar and exhibit preferential cytotoxicity in LNCaP cells relative to normal human fibroblasts. Unlike SMIP001, SMIP004 was found to downregulate SKP2 and to stabilize p27, although neither SMIP is a proteasome inhibitor. Whereas the screening endpoint - nuclear p27 - was robustly modulated by the compounds, SMIP-mediated cell cycle arrest and apoptosis were not strictly dependent on p27 and p21 - a finding that is explained by parallel inhibitory effects of SMIPs on positive cell cycle regulators, including cyclins E and A, and CDK4.
Our data provide proof-of-principle that the screening platform we developed, using endogenous nuclear p27 as an endpoint, presents an effective means of identifying bioactive molecules with cancer selective antiproliferative activity. This approach, when applied to larger and more diverse sets of compounds with refined drug-like properties, bears the potential of revealing both unknown cellular pathways globally impinging on p27 and novel leads for chemotherapeutics targeting a prominent molecular defect of human cancers.
PMCID: PMC3025922  PMID: 21182779
23.  Small molecule inhibitors of ezrin inhibit the invasive phenotype of osteosarcoma cells 
Oncogene  2011;31(3):269-281.
Ezrin is a multifunctional protein that connects the actin cytoskeleton to the extracellular matrix through transmembrane proteins. High ezrin expression is associated with lung metastasis and poor survival in cancer. We screened small molecule libraries for compounds that directly interact with ezrin protein using surface plasmon resonance to identify lead compounds. The secondary functional assays used for lead compound selection included ezrin phosphorylation as measured by immunoprecipitation and in vitro kinase assays, actin binding, chemotaxis, invasion into an endothelial cell monolayer, zebrafish and Xenopus embryonic development, mouse lung organ culture and an in vivo lung metastasis model. Two molecules, NSC305787 and NSC668394, that directly bind to ezrin with low micromolar affinity were selected based on inhibition of ezrin function in multiple assays. They inhibited ezrin phosphorylation, ezrin–actin interaction and ezrin-mediated motility of osteosarcoma (OS) cells in culture. NSC305787 mimicked the ezrin morpholino phenotype, and NSC668394 caused a unique developmental defect consistent with reduced cell motility in zebrafish. Following tail vein injection of OS cells into mice, both molecules inhibited lung metastasis of ezrinsensitive cells, but not ezrin-resistant cells. The small molecule inhibitors NSC305787 and NSC668394 demonstrate a novel targeted therapy that directly inhibits ezrin protein as an approach to prevent tumor metastasis.
PMCID: PMC3513970  PMID: 21706056
ezrin; osteosarcoma; metastasis; small molecule screening; drug discovery
24.  Protein Kinase Cδ Stimulates Proteasome-Dependent Degradation of C/EBPα during Apoptosis Induction of Leukemic Cells 
PLoS ONE  2009;4(8):e6552.
The precise regulation and maintenance of balance between cell proliferation, differentiation and death in metazoan are critical for tissue homeostasis. CCAAT/enhancer-binding protein alpha (C/EBPα) has been implicated as a key regulator of differentiation and proliferation in various cell types. Here we investigated the potential dynamic change and role of C/EBPα protein during apoptosis induction.
Methodology/Principal Findings
Upon onset of apoptosis induced by various kinds of inducers such as NSC606985, etoposide and others, C/EBPα expression presented a profound down-regulation in leukemic cell lines and primary cells via induction of protein degradation and inhibition of transcription, as assessed respectively by cycloheximide inhibition test, real-time quantitative RT-PCR and luciferase reporter assay. Applying chemical inhibition, forced expression of dominant negative mutant and catalytic fragment (CF) of protein kinase Cdelta (PKCδ), which was proteolytically activated during apoptosis induction tested, we showed that the active PKCδ protein contributed to the increased degradation of C/EBPα protein. Three specific proteasome inhibitors antagonized C/EBPα degradation during apoptosis induction. More importantly, ectopic expression of PKCδ-CF stimulated the ubiquitination of C/EBPα protein, while the chemical inhibition of PKCδ action significantly inhibited the enhanced ubiquitination of C/EBPα protein under NSC606985 treatment. Additionally, silencing of C/EBPα expression by small interfering RNAs enhanced, while inducible expression of C/EBPα inhibited NSC606985/etoposide-induced apoptosis in leukemic cells.
These observations indicate that the activation of PKCδ upon apoptosis results in the increased proteasome-dependent degradation of C/EBPα, which partially contributes to PKCδ-mediated apoptosis.
PMCID: PMC2719015  PMID: 19662097
25.  Degradation of the proto-oncogene product c-Fos by the ubiquitin proteolytic system in vivo and in vitro: identification and characterization of the conjugating enzymes. 
Molecular and Cellular Biology  1995;15(12):7106-7116.
The transcription factor c-Fos is a short-lived cellular protein. The levels of the protein fluctuate significantly and abruptly during changing pathophysiological conditions. Thus, it is clear that degradation of the protein plays an important role in its tightly regulated activity. We examined the involvement of the ubiquitin pathway in c-Fos breakdown. Using a mutant cell line, ts20, that harbors a thermolabile ubiquitin-activating enzyme, E1, we demonstrate that impaired function of the ubiquitin system stabilizes c-Fos in vivo. In vitro, we reconstituted a cell-free system and demonstrated that the protein is multiply ubiquitinated. The adducts serve as essential intermediates for degradation by the 26S proteasome. We show that both conjugation and degradation are significantly stimulated by c-Jun, with which c-Fos forms the active heterodimeric transcriptional activator AP-1. Analysis of the enzymatic cascade involved in the conjugation process reveals that the ubiquitin-carrier protein E2-F1 and its human homolog UbcH5, which target the tumor suppressor p53 for degradation, are also involved in c-Fos recognition. The E2 enzyme acts along with a novel species of ubiquitin-protein ligase, E3. This enzyme is distinct from other known E3s, including E3 alpha/UBR1, E3 beta, and E6-AP. We have purified the novel enzyme approximately 350-fold and demonstrated that it is a homodimer with an apparent molecular mass of approximately 280 kDa. It contains a sulfhydryl group that is essential for its activity, presumably for anchoring activated ubiquitin as an intermediate thioester prior to its transfer to the substrate. Taken together, our in vivo and in vitro studies strongly suggest that c-Fos is degraded in the cell by the ubiquitin-proteasome proteolytic pathway in a process that requires a novel recognition enzyme.
PMCID: PMC230966  PMID: 8524278

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