Covalent Fragment Screening and Optimization Identifies the Chloroacetohydrazide Scaffold as Inhibitors for Ubiquitin C-terminal Hydrolase L1.

Dysregulation of the ubiquitin-proteasome systems is a hallmark of various disease states including neurodegenerative diseases and cancer. Ubiquitin C-terminal hydrolase L1 (UCHL1), a deubiquitinating enzyme, is expressed primarily in the central nervous system under normal physiological conditions, however, is considered an oncogene in various cancers, including melanoma, lung, breast, and lymphoma. Thus, UCHL1 inhibitors could serve as a viable treatment strategy against these aggressive cancers. Herein, we describe a covalent fragment screen that identified the chloroacetohydrazide scaffold as a covalent UCHL1 inhibitor. Subsequent optimization provided an improved fragment with single-digit micromolar potency against UCHL1 and selectivity over the closely related UCHL3. The molecule demonstrated efficacy in cellular assays of metastasis. Additionally, we report a ligand-bound crystal structure of the most potent molecule in complex with UCHL1, providing insight into the binding mode and information for future optimization.

USP7 as an emerging therapeutic target: A key regulator of protein homeostasis.

Maintaining protein balance within a cell is essential for proper cellular function, and disruptions in the ubiquitin-proteasome pathway, which is responsible for degrading and recycling unnecessary or damaged proteins, can lead to various diseases. Deubiquitinating enzymes play a vital role in regulating protein homeostasis by removing ubiquitin chains from substrate proteins, thereby controlling important cellular processes, such as apoptosis and DNA repair. Among these enzymes, ubiquitin-specific protease 7 (USP7) is of particular interest. USP7 is a cysteine protease consisting of a TRAF region, catalytic region, and C-terminal ubiquitin-like (UBL) region, and it interacts with tumor suppressors, transcription factors, and other key proteins involved in cell cycle regulation and epigenetic control. Moreover, USP7 has been implicated in the pathogenesis and progression of various diseases, including cancer, inflammation, neurodegenerative conditions, and viral infections. Overall, characterizing the functions of USP7 is crucial for understanding the pathophysiology of diverse diseases and devising innovative therapeutic strategies. This article reviews the structure and function of USP7 and its complexes, its association with diseases, and its known inhibitors and thus represents a valuable resource for advancing USP7 inhibitor development and promoting potential future treatment options for a wide range of diseases.

Altered Protein Dynamics and a More Reactive Catalytic Cysteine in a Neurodegeneration-associated UCHL1 Mutant.

A mutant of ubiquitin C-terminal hydrolase L1 (UCHL1) detected in early-onset neurodegenerative patients, UCHL1R178Q, showed higher catalytic activity than wild-type UCHL1 (UCHL1WT). Lying within the active-site pocket, the arginine is part of an interaction network that holds the catalytic histidine in an inactive arrangement. However, the structural basis and mechanism of enzymatic activation upon glutamine substitution was not understood. We combined X-ray crystallography, protein nuclear magnetic resonance (NMR) analysis, enzyme kinetics, covalent inhibition analysis, and biophysical measurements to delineate activating factors in the mutant. While the crystal structure of UCHL1R178Q showed nearly the same arrangement of the catalytic residues and active-site pocket, the mutation caused extensive alteration in the chemical environment and dynamics of more than 30 residues, some as far as 15 Å away from the site of mutation. Significant broadening of backbone amide resonances in the HSQC spectra indicates considerable backbone dynamics changes in several residues, in agreement with solution small-angle X-ray scattering (SAXS) analyses which indicate an overall increase in protein flexibility. Enzyme kinetics show the activation is due to a kcat effect despite a slightly weakened substrate affinity. In line with this, the mutant shows a higher second-order rate constant (kinact/Ki) in a reaction with a substrate-derived irreversible inhibitor, Ub-VME, compared to the wild-type enzyme, an observation indicative of a more reactive catalytic cysteine in the mutant. Together, the observations underscore structural plasticity as a factor contributing to enzyme kinetic behavior which can be modulated through mutational effects.

Phosphorylation of USP20 on Ser334 by IRAK1 promotes IL-1ß-evoked signaling in vascular smooth muscle cells and vascular inflammation.

Reversible lysine-63 (K63) polyubiquitination regulates proinflammatory signaling in vascular smooth muscle cells (SMCs) and plays an integral role in atherosclerosis. Ubiquitin-specific peptidase 20 (USP20) reduces NFκB activation triggered by proinflammatory stimuli, and USP20 activity attenuates atherosclerosis in mice. The association of USP20 with its substrates triggers deubiquitinase activity; this association is regulated by phosphorylation of USP20 on Ser334 (mouse) or Ser333 (human). USP20 Ser333 phosphorylation was greater in SMCs of atherosclerotic segments of human arteries as compared with nonatherosclerotic segments. To determine whether USP20 Ser334 phosphorylation regulates proinflammatory signaling, we created USP20-S334A mice using CRISPR/Cas9-mediated gene editing. USP20-S334A mice developed ∼50% less neointimal hyperplasia than congenic WT mice after carotid endothelial denudation. WT carotid SMCs showed substantial phosphorylation of USP20 Ser334, and WT carotids demonstrated greater NFκB activation, VCAM-1 expression, and SMC proliferation than USP20-S334A carotids. Concordantly, USP20-S334A primary SMCs in vitro proliferated and migrated less than WT SMCs in response to IL-1ß. An active site ubiquitin probe bound to USP20-S334A and USP20-WT equivalently, but USP20-S334A associated more avidly with TRAF6 than USP20-WT. IL-1ß induced less K63-linked polyubiquitination of TRAF6 and less downstream NFκB activity in USP20-S334A than in WT SMCs. Using in vitro phosphorylation with purified IRAK1 and siRNA-mediated gene silencing of IRAK1 in SMCs, we identified IRAK1 as a novel kinase for IL-1ß-induced USP20 Ser334 phosphorylation. Our findings reveal novel mechanisms regulating IL-1ß-induced proinflammatory signaling: by phosphorylating USP20 Ser334, IRAK1 diminishes the association of USP20 with TRAF6 and thus augments NFκB activation, SMC inflammation, and neointimal hyperplasia.

Research Progress of BAP1 in Structure, Function, and Cancer.

Cancer is an important chronic non-communicable disease that endangers human health and has become the main cause of death of residents around the world in the 21st century. At present, most of the mature treatment methods stay at the level of cell and tissue, which is difficult to fundamentally solve the problem of cancer. Therefore, explaining the pathogenesis of cancer at the molecular level becomes the answer to the key problem of cancer regulation. BRCA-associated protein 1 (brca1- associated protein 1) is a kind of ubiquitination enzyme encoded by the BAP1 gene and composed of 729 amino acids. As a carcinogenic protein, BAP1 can affect the cancer cell cycle and proliferation capacity, mutation, and deletion. For example, depending on catalytic activity, it participates in the regulation of intracellular function through transcription, epigenetic, and DNA damage repair. This article mainly reviews the basic structure and function of BAP1 in cells, its role in cancer development, and cancer-related mutants.

Basis of the H2AK119 specificity of the Polycomb repressive deubiquitinase.

Repression of gene expression by protein complexes of the Polycomb group is a fundamental mechanism that governs embryonic development and cell-type specification1-3. The Polycomb repressive deubiquitinase (PR-DUB) complex removes the ubiquitin moiety from monoubiquitinated histone H2A K119 (H2AK119ub1) on the nucleosome4, counteracting the ubiquitin E3 ligase activity of Polycomb repressive complex 1 (PRC1)5 to facilitate the correct silencing of genes by Polycomb proteins and safeguard active genes from inadvertent silencing by PRC1 (refs. 6-9). The intricate biological function of PR-DUB requires accurate targeting of H2AK119ub1, but PR-DUB can deubiquitinate monoubiquitinated free histones and peptide substrates indiscriminately; the basis for its exquisite nucleosome-dependent substrate specificity therefore remains unclear. Here we report the cryo-electron microscopy structure of human PR-DUB, composed of BAP1 and ASXL1, in complex with the chromatosome. We find that ASXL1 directs the binding of the positively charged C-terminal extension of BAP1 to nucleosomal DNA and histones H3-H4 near the dyad, an addition to its role in forming the ubiquitin-binding cleft. Furthermore, a conserved loop segment of the catalytic domain of BAP1 is situated near the H2A-H2B acidic patch. This distinct nucleosome-binding mode displaces the C-terminal tail of H2A from the nucleosome surface, and endows PR-DUB with the specificity for H2AK119ub1.

Discovery of a Potent and Selective Degrader for USP7.

The tumor suppressor p53 is the most frequently mutated gene in human cancer and more than half of cancers contain p53 mutations. The development of novel and effective therapeutic strategies for p53 mutant cancer therapy is a big challenge and highly desirable. Ubiquitin-specific protease 7 (USP7), also known as HAUSP, is a deubiquitinating enzyme and proposed to stabilize the oncogenic E3 ubiquitin ligase MDM2 that promotes the proteosomal degradation of p53. Herein, we report the design and characterization of U7D-1 as the first selective USP7-degrading Proteolysis Targeting Chimera (PROTAC). U7D-1 showed selective and effective USP7 degradation, and maintained potent cell growth inhibition in p53 mutant cancer cells, with USP7 inhibitor showing no activity. These data clearly demonstrated the practicality and importance of PROTAC as a preliminary chemical tool for investigating USP7 protein functions and a promising method for potential p53 mutant cancer therapy.

Backbone and ILV side-chain NMR resonance assignments of the catalytic domain of human deubiquitinating enzyme USP7.

Ubiquitin specific protease 7 (USP7) is a deubiquitinating enzyme, which removes ubiquitin tag from numerous protein substrates involved in diverse cellular processes such as cell cycle regulation, apoptosis and DNA damage response. USP7 affects stability, interaction network and cellular localization of its cellular and viral substrates by controlling their ubiquitination status. The large 41 kDa catalytic domain of USP7 harbors the active site of the enzyme. Here we present a nearly complete (93%) NMR resonance assignment of isoleucine, leucine and valine (ILV) side-chains of the USP7 catalytic domain along with a refined nearly complete (93%) assignment of its backbone resonances. The reported ILV methyl group assignment will facilitate further NMR investigations of structure, interactions and conformational dynamics of the USP7 enzyme.

Impacts of Cancer-associated Mutations on the Structure-Activity Relationship of BAP1.

BRAC1 associated protein-1 (BAP1) is a major tumor suppressor involved in many cancers. The deubiquitinase (DUB) activity of BAP1 is essential for its nuclear localization, histone remodeling and proteostasis associated with mitochondrial calcium flux. Loss of the DUB activity due to catalytic mutations within the ubiquitin C-terminal hydrolase (UCH) domain of BAP1 (BAP1-UCH) directly contributes to oncogenesis. Nevertheless, it is non-trivial to rationalize how the other high-frequency but non-catalytic mutations within the BAP1-UCH lead to malignancies. Here we used multiplex spectroscopic, thermodynamic and biophysical analyses to investigate the impacts of eleven high-occurrence mutations within BAP1-UCH on the structure, folding and function. Several mutations significantly destabilize BAP1-UCH and increase its aggregation propensity. Hydrogen-deuterium exchange mass spectrometry data revealed allosteric destabilizations caused by mutations distant from the catalytic site. Our findings gave a comprehensive and multiscale account of the molecular basis of how these non-catalytic mutations within BAP1-UCH may be implicated in oncogenesis.

Oxidation of catalytic cysteine of human deubiquitinase BAP1 triggers misfolding and aggregation in addition to functional loss.

Deubiquitinating enzymes (DUBs) form a large protease family involved in a myriad of biological and pathological processes, including ROS sensors. ROS-mediated inhibition of their DUB activities is critical for fine-tuning the stress-activated signaling pathways. Here, we demonstrate that the ubiquitin C-terminal hydrolase (UCH) domain of BAP1 (BAP1-UCH) is highly sensitive to moderate oxidative stress. Oxidation of the catalytic C91 significantly destabilizes BAP1-UCH and increases the population of partially unfolded form, which is prone to aggregation. Unlike other DUBs, the oxidation-induced structural and functional loss of BAP1-UCH cannot be fully reversed by reducing agents. The oligomerization of oxidized BAP1-UCH is attributed to inter-molecular disulfide bond formation. Hydrogen-deuterium mass exchange spectrometry (HDX-MS) reveals increased fluctuations of the central ß-sheet upon oxidation. Our findings suggest that oxidation-mediated functional loss and increased aggregation propensity may contribute to oncogenesis associated with BAP1.

Rational Development and Characterization of a Ubiquitin Variant with Selectivity for Ubiquitin C-Terminal Hydrolase L3.

There is currently a lack of reliable methods and strategies to probe the deubiquitinating enzyme UCHL3. Current small molecules reported for this purpose display reduced potency and selectivity in cellular assays. To bridge this gap and provide an alternative approach to probe UCHL3, our group has carried out the rational design of ubiquitin-variant activity-based probes with selectivity for UCHL3 over the closely related UCHL1 and other DUBs. The approach successfully produced a triple-mutant ubiquitin variant activity-based probe, UbVQ40V/T66K/V70F-PRG, that was ultimately 20,000-fold more selective for UCHL3 over UCHL1 when assessed by rate of inactivation assays. This same variant was shown to selectively form covalent adducts with UCHL3 in MDA-MB-231 breast cancer cells and no reactivity toward other DUBs expressed. Overall, this study demonstrates the feasibility of the approach and also provides insight into how this approach may be applied to other DUB targets.

UCH-L3 structure and function: Insights about a promising drug target.

In the past few years, researchers have shed light on the immense importance of ubiquitin in numerous regulatory pathways. The post-translational addition of mono or poly-ubiquitin molecules namely "ubiquitinoylation" is therefore pivotal to maintain the cell's vitality, maturation, differentiation, and division. Part of conserving homeostasis stems from maintaining the ubiquitin pool in the vicinity of the cell's intracellular environment; this crucial role is played by deubiquitylating enzymes (DUBs) that cleave ubiquitin molecules from target molecules. To date, they are categorized into 7 families with ubiquitin carboxyl c-terminal de-hydrolase family (UCH) as the most common and well-studied. Ubiquitin C-terminal hydrolase L (UCH-L3) is a significant protein in this family as it has been implicated in many molecular and cellular processes with its mRNA identified in a range of body tissues including the brain. It goes without saying that it manifests in maintaining health and when abnormally regulated in disease. As it is an attractive small molecule drug target, scientists have used high throughput screening (HTS) and other drug discovery methods to discover inhibitors for this enzyme for the treatment of cancer and neurodegenerative diseases. In this review we present an overview of UCH-L3 catalytic mechanism, structure, its role in DNA repair and cancer along with the inhibitors discovered so far to halt its activity.

BAP1 forms a trimer with HMGB1 and HDAC1 that modulates gene × environment interaction with asbestos.

Carriers of heterozygous germline BAP1 mutations (BAP1+/-) are affected by the "BAP1 cancer syndrome." Although they can develop almost any cancer type, they are unusually susceptible to asbestos carcinogenesis and mesothelioma. Here we investigate why among all carcinogens, BAP1 mutations cooperate with asbestos. Asbestos carcinogenesis and mesothelioma have been linked to a chronic inflammatory process promoted by the extracellular release of the high-mobility group box 1 protein (HMGB1). We report that BAP1+/- cells secrete increased amounts of HMGB1, and that BAP1+/- carriers have detectable serum levels of acetylated HMGB1 that further increase when they develop mesothelioma. We linked these findings to our discovery that BAP1 forms a trimeric protein complex with HMGB1 and with histone deacetylase 1 (HDAC1) that modulates HMGB1 acetylation and its release. Reduced BAP1 levels caused increased ubiquitylation and degradation of HDAC1, leading to increased acetylation of HMGB1 and its active secretion that in turn promoted mesothelial cell transformation.

USP28: Oncogene or Tumor Suppressor? A Unifying Paradigm for Squamous Cell Carcinoma.

Squamous cell carcinomas are therapeutically challenging tumor entities. Low response rates to radiotherapy and chemotherapy are commonly observed in squamous patients and, accordingly, the mortality rate is relatively high compared to other tumor entities. Recently, targeting USP28 has been emerged as a potential alternative to improve the therapeutic response and clinical outcomes of squamous patients. USP28 is a catalytically active deubiquitinase that governs a plethora of biological processes, including cellular proliferation, DNA damage repair, apoptosis and oncogenesis. In squamous cell carcinoma, USP28 is strongly expressed and stabilizes the essential squamous transcription factor ΔNp63, together with important oncogenic factors, such as NOTCH1, c-MYC and c-JUN. It is presumed that USP28 is an oncoprotein; however, recent data suggest that the deubiquitinase also has an antineoplastic effect regulating important tumor suppressor proteins, such as p53 and CHK2. In this review, we discuss: (1) The emerging role of USP28 in cancer. (2) The complexity and mutational landscape of squamous tumors. (3) The genetic alterations and cellular pathways that determine the function of USP28 in squamous cancer. (4) The development and current state of novel USP28 inhibitors.

Structural and functional characterization of ubiquitin variant inhibitors for the JAMM-family deubiquitinases STAMBP and STAMBPL1.

Ubiquitination is a crucial posttranslational protein modification involved in a myriad of biological pathways. This modification is reversed by deubiquitinases (DUBs) that deconjugate the single ubiquitin (Ub) moiety or poly-Ub chains from substrates. In the past decade, tremendous efforts have been focused on targeting DUBs for drug discovery. However, most chemical compounds with inhibitory activity for DUBs suffer from mild potency and low selectivity. To overcome these obstacles, we developed a phage display-based protein engineering strategy for generating Ub variant (UbV) inhibitors, which was previously successfully applied to the Ub-specific protease (USP) family of cysteine proteases. In this work, we leveraged the UbV platform to selectively target STAMBP, a member of the JAB1/MPN/MOV34 (JAMM) metalloprotease family of DUB enzymes. We identified two UbVs (UbVSP.1 and UbVSP.3) that bind to STAMBP with high affinity but differ in their selectivity for the closely related paralog STAMBPL1. We determined the STAMBPL1-UbVSP.1 complex structure by X-ray crystallography, revealing hotspots of the JAMM-UbV interaction. Finally, we show that UbVSP.1 and UbVSP.3 are potent inhibitors of STAMBP isopeptidase activity, far exceeding the reported small-molecule inhibitor BC-1471. This work demonstrates that UbV technology is suitable to develop molecules as tools to target metalloproteases, which can be used to further understand the cellular function of JAMM family DUBs.

SARS-CoV-2 Papain-Like Protease Potential Inhibitors-In Silico Quantitative Assessment.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) encodes the papain-like protease (PLpro). The protein not only plays an essential role in viral replication but also cleaves ubiquitin and ubiquitin-like interferon-stimulated gene 15 protein (ISG15) from host proteins, making it an important target for developing new antiviral drugs. In this study, we searched for novel, noncovalent potential PLpro inhibitors by employing a multistep in silico screening of a 15 million compound library. The selectivity of the best-scored compounds was evaluated by checking their binding affinity to the human ubiquitin carboxy-terminal hydrolase L1 (UCH-L1), which, as a deubiquitylating enzyme, exhibits structural and functional similarities to the PLpro. As a result, we identified 387 potential, selective PLpro inhibitors, from which we retrieved the 20 best compounds according to their IC50 values toward PLpro estimated by a multiple linear regression model. The selected candidates display potential activity against the protein with IC50 values in the nanomolar range from approximately 159 to 505 nM and mostly adopt a similar binding mode to the known, noncovalent SARS-CoV-2 PLpro inhibitors. We further propose the six most promising compounds for future in vitro evaluation. The results for the top potential PLpro inhibitors are deposited in the database prepared to facilitate research on anti-SARS-CoV-2 drugs.

Exploring the Versatility of the Covalent Thiol-Alkyne Reaction with Substituted Propargyl Warheads: A Deciding Role for the Cysteine Protease.

Terminal unactivated alkynes are nowadays considered the golden standard for cysteine-reactive warheads in activity-based probes (ABPs) targeting cysteine deubiquitinating enzymes (DUBs). In this work, we study the versatility of the thiol-alkyne addition reaction in more depth. Contrary to previous findings with UCHL3, we now show that covalent adduct formation can progress with substituents on the terminal or internal alkyne position. Strikingly, acceptance of alkyne substituents is strictly DUB-specific as this is not conserved among members of the same subfamily. Covalent adduct formation with the catalytic cysteine residue was validated by gel analysis and mass spectrometry of intact ABP-treated USP16CDWT and catalytically inactive mutant USP16CDC205A. Bottom-up mass spectrometric analysis of the covalent adduct with a deuterated propargyl ABP provides mechanistic understanding of the in situ thiol-alkyne reaction, identifying the alkyne rather than an allenic intermediate as the reactive species. Furthermore, kinetic analysis revealed that introduction of (bulky/electron-donating) methyl substituents on the propargyl moiety decreases the rate of covalent adduct formation, thus providing a rational explanation for the commonly lower level of observed covalent adduct compared to unmodified alkynes. Altogether, our work extends the scope of possible propargyl derivatives in cysteine targeting ABPs from unmodified terminal alkynes to internal and substituted alkynes, which we anticipate will have great value in the development of ABPs with improved selectivity profiles.

Kinetic Characterization of ASXL1/2-Mediated Allosteric Regulation of the BAP1 Deubiquitinase.

BAP1 is an ubiquitin hydrolase whose deubiquitinase activity is mediated by polycomb group-like protein ASXL2. Cancer-related BAP1 mutations/deletions lead to loss-of-function by targeting the catalytic ubiquitin C-terminal hydrolase (UCH) or UCH37-like domain (ULD) domains of BAP1, and the latter disrupts binding to ASXL2, an obligate partner for BAP1 enzymatic activity. However, the biochemical and biophysical properties of domains involved in forming the enzymatically active complex are unknown. Here, we report the molecular dynamics, kinetics, and stoichiometry of these interactions. We demonstrate that interactions between BAP1 and ASXL2 are direct, specific, and stable to biochemical and biophysical manipulations as detected by isothermal titration calorimetry (ITC), GST association, and optical biosensor assays. Association of the ASXL2-AB box greatly stimulates BAP1 activity. A stable ternary complex is formed, comprised of the BAP1-UCH, BAP1-ULD, and ASXL2-AB domains. Stoichiometric analysis revealed that one molecule of the ULD domain directly interacts with one molecule of the AB box. Real-time kinetic analysis of the ULD/AB protein complex to the BAP1-UCH domain, based on surface plasmon resonance, indicated that formation of the ULD/AB complex with the UCH domain is a single-step event with fast association and slow dissociation rates. In vitro experiments validated in cells that the ASXL-AB box directly regulates BAP1 activity. IMPLICATIONS: Collectively, these data elucidate molecular interactions between specific protein domains regulating BAP1 deubiquitinase activity, thus establishing a foundation for small-molecule approaches to reactivate latent wild-type BAP1 catalytic activity in BAP1-mutant cancers.

Cross-over Loop Cysteine C152 Acts as an Antioxidant to Maintain the Folding Stability and Deubiquitinase Activity of UCH-L1 Under Oxidative Stress.

Redox-dependent inactivation of deubiquitinases (DUBs) is a critical factor for attenuating their DUB activity in response to cellular oxidative stress. Ubiquitin C-terminal hydrolase isoform (UCH-L1) is an important DUB that is highly expressed in human neuronal cells and is implicated in a myriad of human diseases such as neurodegenerative diseases and cancer. Increasing evidence suggests an important role of UCH-L1 in redox regulation and the protection of neuronal cells from oxidative stress. In this study, we examined the molecular basis of how UCH-L1 responds to oxidation in a reversible manner. Using H2O2 as a model oxidant, we showed by mass spectrometry that a subset of methionine and cysteine residues, namely (M1, M6, M12, C90, and C152) were more susceptible to oxidation. Spectroscopic analysis showed that oxidation of C90 can lead to profound structural changes in addition to the loss of function. Importantly, we further demonstrated that C152, which is located at the substrate recognition cross-over loop, serves as a reactive oxygen species (ROS) scavenger to protect catalytic C90 from oxidation under moderate oxidative conditions. Hydrogen-deuterium exchange mass spectrometry analysis provided detailed structural mapping of the destabilizing effect of H2O2-mediated oxidation, which resulted in global destabilization far beyond the oxidation sites. These perturbations may be responsible for irreversible aggregation when subject to prolonged oxidative stress.

Disulfiram and 6-Thioguanine synergistically inhibit the enzymatic activities of USP2 and USP21.

Disulfiram is a promising repurposed drug that, combining with radiation and chemotherapy, exhibits effective anticancer activities in several preclinical models. The cellular metabolites of disulfiram have been established, however, the intracellular targets of disulfiram remain largely unexplored. We have previously reported that disulfiram suppresses the coronaviral papain-like proteases through attacking their zinc-finger domains, suggesting an inhibitory function potentially on other proteases with similar catalytic structures. Ubiquitin-specific proteases (USPs) share a highly-conserved zinc-finger subdomain that structurally similar to the papain-like proteases and are attractive anticancer targets as upregulated USPs levels are found in a variety of tumors. Here, we report that disulfiram functions as a competitive inhibitor for both USP2 and USP21, two tumor-related deubiquitinases. In addition, we also observed a synergistic inhibition of USP2 and USP21 by disulfiram and 6-Thioguanine (6TG), a clinical drug for acute myeloid leukemia. Kinetic analyses revealed that both drugs exhibited a slow-binding mechanism, moderate inhibitory parameters, and a synergistically inhibitory effect on USP2 and USP21, suggesting the potential combinatory use of these two drugs for USPs-related tumors. Taken together, our study provides biochemical evidence for repurposing disulfiram and 6TG as a combinatory treatment in clinical applications.