Deubiquitinases (DUBs) play important roles and therefore are potential drug targets in various diseases including cancer and neurodegeneration. In this review, we recapitulate structure-function studies of the most studied DUBs including USP7, USP22, CYLD, UCHL1, BAP1, A20, as well as ataxin 3 and connect them to regulatory mechanisms and their growing protein interaction networks. We then describe DUBs that have been associated with endocrine carcinogenesis with a focus on prostate, ovarian, and thyroid cancer, pheochromocytoma, and adrenocortical carcinoma. The goal is enhancing our understanding of the connection between dysregulated DUBs and cancer to permit the design of therapeutics and to establish biomarkers that could be used in diagnosis and prognosis.

Herpes simplex virus-1 immediate-early protein ICP0 activates viral genes during early stages of infection, affects cellular levels of multiple host proteins and is crucial for effective lytic infection. Being a RING-type E3 ligase prone to auto-ubiquitination, ICP0 relies on human deubiquitinating enzyme USP7 for protection against 26S proteasomal mediated degradation. USP7 is involved in apoptosis, epigenetics, cell proliferation and is targeted by several herpesviruses. Several USP7 partners, including ICP0, GMPS, and UHRF1, interact through its C-terminal domain (CTD), which contains five ubiquitin-like (Ubl) structures. Despite the fact that USP7 has emerged as a drug target for cancer therapy, structural details of USP7 regulation and the molecular mechanism of interaction at its CTD have remained elusive. Here, we mapped the binding site between an ICP0 peptide and USP7 and determined the crystal structure of the first three Ubl domains bound to the ICP0 peptide, which showed that ICP0 binds to a loop on Ubl2. Sequences similar to the USP7-binding site in ICP0 were identified in GMPS and UHRF1 and shown to bind USP7-CTD through Ubl2. In addition, co-immunoprecipitation assays in human cells comparing binding to USP7 with and without a Ubl2 mutation, confirmed the importance of the Ubl2 binding pocket for binding ICP0, GMPS and UHRF1. Therefore we have identified a novel mechanism of USP7 recognition that is used by both viral and cellular proteins. Our structural information was used to generate a model of near full-length USP7, showing the relative position of the ICP0/GMPS/UHRF1 binding pocket and the structural basis by which it could regulate enzymatic activity.

Viral interferon regulatory factor 1 (vIRF1), a Kaposi sarcoma herpesvirus protein, destabilizes p53 by inhibiting p53 acetylation and Hdm2 phosphorylation. This leads to increased ubiquitination and degradation of p53 by Hdm2, which cripples the cellular p53-mediated antiviral response. Ubiquitin-specific protease 7 (USP7) deubiquitinates p53 and Hdm2 and regulates their stability. We identified an EGPS consensus sequence in vIRF1, which is identical to that found in Epstein-Barr virus nuclear antigen 1 (EBNA1) that interacts with the N-terminal domain of USP7 (USP7-NTD). GST pulldown assays demonstrated that vIRF1 interacts with USP7-NTD via its EGPS motif. NMR heteronuclear single quantum correlation (HSQC) analysis revealed chemical perturbations after titration of USP7-NTD with vIRF1 44 SPGEGPSGTG 53 peptide. In contrast, these perturbations were reduced with a mutant vIRF1 peptide, 44 SPG-EGPAGTG 53 . Fluorescence polarization analysis indicated that the vIRF1 peptide interacted with USP7-NTD with a K d of 2.0 M. The crystal structure of the USP7-NTD⅐vIRF1 peptide complex revealed an identical mode of binding as that of the EBNA1 peptide to USP7-NTD. We also showed that USP7 interacts with vIRF1 in U2OS cells. Decreased levels of p53, but not Hdm2 or ataxia telangiectasia-mutated (ATM), were seen after expression of vIRF1, but not with a vIRF1 mutant protein. Our results support a new role for vIRF1 through deregulation of the deubiquitinating enzyme USP7 to inhibit p53-mediated antiviral responses. Human herpesviruses (HHVs)2 are double-stranded DNA viruses, classified into ␣, ␤, or ␥ subfamilies (1). To establish latency, HHVs suppress the host immune response to evade the immune system (2). In the latent state, herpesviruses express only a small number of proteins essential for suppressing the host immune system (2). HHVs have evolved various mechanisms for host immune evasion including inhibition of cellular senescence and apoptosis as well as promoting cell proliferation (3). Another strategy to evade host immune surveillance is through expression of viral homologues of genes that are the host's first line of defense against viral infection such as interferons and interferon regulatory factors (IRFs) and therefore sabotage the function and regulation of the cellular proteins (4). Deregulation of cellular proteins involved in growth control by HHVs has led to recognition of some HHVs as underlying agents of cancer (3, 5).Kaposi sarcoma herpesvirus (KSHV), HHV-8, is the causative agent of Kaposi sarcoma, primary effusion lymphoma, and multicentric Castleman disease, which are especially prevalent in immunocompromised patients (4, 6). The KSHV vIRF1 protein is encoded by ORF K9 and is believed to have been acquired through molecular piracy (7). vIRF1 contains two domains, an N-terminal DNA binding domain (DBD) and a C-terminal IRF interaction domain (see Fig. 1A) (8). The vIRF1 DBD has ϳ40% sequence similarity to the DBDs of human IRF3 and IRF7 and contains a helix-turn-helix motif, which is common in I...

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The deubiquitinating enzyme, USP7, regulates the turnover of proteins involved in many diverse cellular processes, including maintenance of genome stability, tumour suppression, epigenetics, DNA replication, cell division, and immune response. Previously defined USP7 substrates, including GMPS, UHRF1, and ICP0, were characterized to interact with the C-terminal domain of USP7 via a KxxxK motif. We identified a common motif in Enhancer of Zeste 2 (EZH2), a histone methyltransferase and the catalytic component of the Polycomb Repressive Complex 2 (PRC2). PRC2 is responsible for methylating Histone 3 Lys27 (H3K27) leading to transcriptional repression of genes involved in differentiation and development. The interaction between USP7 and EZH2 was demonstrated by GST pull-down and co-immunoprecipitation experiments. We co-crystallized Ubl123 of USP7 with an EZH2 peptide containing the predicted interaction site and characterized the structural basis of the interaction, identifying the key residues involved. Mutagenesis studies of these residues demonstrated abolished binding between USP7 and EZH2. The functional relationship between USP7 and EZH2 was investigated using USP7 knock-down and knock-out experiments which corresponded with reduced EZH2 levels in HCT116 carcinoma cells and decreased H3K27Me3 levels in HCT116 USP7 knockout cells respectively. These findings reveal that USP7 regulates both the stability and function of EZH2.

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