USP7/HAUSP promotes the sequence-specific DNA binding activity of p53

MALAT1: A key regulator in lung cancer pathogenesis and therapeutic targeting

Lung cancer remains a formidable global health burden, necessitating a comprehensive understanding of the underlying molecular mechanisms driving its progression. Recently, lncRNAs have become necessary controllers of various biological functions, including cancer development. MALAT1 has garnered significant attention due to its multifaceted role in lung cancer progression. Lung cancer, among other malignancies, upregulates MALAT1. Its overexpression has been associated with aggressive tumor behavior and poor patient prognosis. MALAT1 promotes cellular proliferation, epithelial-mesenchymal transition (EMT), and angiogenesis in lung cancer, collectively facilitating tumor growth and metastasis. Additionally, MALAT1 enhances cancer cell invasion by interacting with numerous signaling pathways. Furthermore, MALAT1 has been implicated in mediating drug resistance in lung cancer, contributing to the limited efficacy of conventional therapies. Recent advancements in molecular biology and high-throughput sequencing technologies have offered fresh perspectives into the regulatory networks of MALAT1 in lung cancer. It exerts its oncogenic effects by acting as a ceRNA to sponge microRNAs, thereby relieving their inhibitory effects on target genes. Moreover, MALAT1 also influences chromatin remodeling and post-translational modifications to modulate gene expression, further expanding its regulatory capabilities. This review sheds light on the multifaceted roles of MALAT1 in lung cancer progression, underscoring its potential as an innovative therapeutic target and diagnostic biomarker. Targeting MALAT1 alone or combined with existing therapies holds promise to mitigate lung cancer progression and improve patient outcomes.

The equilibrium of tumor suppression: DUBs as active regulators of PTEN

PTEN is among the most commonly lost or mutated tumor suppressor genes in human cancer. PTEN, a bona fide lipid phosphatase that antagonizes the highly oncogenic PI3K-AKT-mTOR pathway, is considered a major dose-dependent tumor suppressor. Although PTEN function can be compromised by genetic mutations in inherited syndromes and cancers, posttranslational modifications of PTEN may also play key roles in the dynamic regulation of its function. Notably, deregulated ubiquitination and deubiquitination lead to detrimental impacts on PTEN levels and subcellular partitioning, promoting tumorigenesis. While PTEN can be targeted by HECT-type E3 ubiquitin ligases for nuclear import and proteasomal degradation, studies have shown that several deubiquitinating enzymes, including HAUSP/USP7, USP10, USP11, USP13, OTUD3 and Ataxin-3, can remove ubiquitin from ubiquitinated PTEN in cancer-specific contexts and thus reverse ubiquitination-mediated PTEN regulation. Researchers continue to reveal the precise molecular mechanisms by which cancer-specific deubiquitinases of PTEN regulate its roles in the pathobiology of cancer, and new methods of pharmacologically for modulating PTEN deubiquitinases are critical areas of investigation for cancer treatment and prevention. Here, we assess the mechanisms and functions of deubiquitination as a recently appreciated mode of PTEN regulation and review the link between deubiquitinases and PTEN reactivation and its implications for therapeutic strategies.

Expression of Ubiquitin-specific protease 7 in oral squamous cell carcinoma promotes tumor cell proliferation and invasion

Oral Squamous Cell Carcinoma (OSCC) is the most common malignant cancer affecting oral cavity. Recent studies have demonstrated that Ubiquitin-specific protease 7 (USP7) was upregulated in several types of cancers. USP7 expression was associated with various proto-oncogenes and tumor suppressor genes. However, USP7 expression level and its functional role in OSCC is unclear. In the current study, we showed that USP7 expression in OSCC tissues was generally upregulated compared to normal adjacent tissues by using IHC. Furthermore, statistical analysis uncovered that USP7 expression was positively correlated with Ki-67, MMP2, VEGF in OSCC tissues. Importantly, high USP7 expression was significantly correlated with lymph node metastasis and histological differentiation in OSCC patients. So, our hypothesis is that USP7 plays a tumor-promoting role in OSCC. Knocking down of USP7 in tumor cells not only suppressed HSC3 cells proliferation, migration and invasion, but also promoted cell apoptosis. Moreover, USP7 siRNA blocked the activation of Akt/ERK signaling pathway. In conclusion, data presented here suggests that USP7 promotes the progression of OSCC. USP7 may be used as a new therapeutic target for OSCC diagnosis and treatment.

Bimodal regulation of the PRC2 complex by USP7 underlies tumorigenesis

Although overexpression of EZH2, a catalytic subunit of the polycomb repressive complex 2 (PRC2), is an eminent feature of various cancers, the regulation of its abundance and function remains insufficiently understood. We report here that the PRC2 complex is physically associated with ubiquitin-specific protease USP7 in cancer cells where USP7 acts to deubiquitinate and stabilize EZH2. Interestingly, we found that USP7-catalyzed H2BK120ub1 deubiquitination is a prerequisite for chromatin loading of PRC2 thus H3K27 trimethylation, and this process is not affected by H2AK119 ubiquitination catalyzed by PRC1. Genome-wide analysis of the transcriptional targets of the USP7/PRC2 complex identified a cohort of genes including FOXO1 that are involved in cell growth and proliferation. We demonstrated that the USP7/PRC2 complex drives cancer cell proliferation and tumorigenesis in vitro and in vivo. We showed that the expression of both USP7 and EZH2 elevates during tumor progression, corresponding to a diminished FOXO1 expression, and the level of the expression of USP7 and EZH2 strongly correlates with histological grades and prognosis of tumor patients. These results reveal a dual role for USP7 in the regulation of the abundance and function of EZH2, supporting the pursuit of USP7 as a therapeutic target for cancer intervention.

Molecular dynamics simulation reveals the possible druggable hot-spots of USP7

The plasticity in Ubiquitin Specific Proteases (USP7) inducing conformational changes at important areas has highlighted an intricate mechanism, by which USP7 is regulated. Given the importance of USP7 in oncogenic pathways and immune-oncology, identification of USP7 inhibitors has attracted considerable interest. Despite substantial efforts, the discovery of deubiquitinases (DUBs) inhibitors, knowledge of their binding site and understanding the possible mechanism of action has proven particularly challenging. We disclose the most likely binding site of P5091 (a potent USP7 inhibitor), which reveal a cryptic allosteric site through extensive computational studies in an inhibitor dependent and independent manner. Overall, these findings demonstrate the tractability and druggability of USP7. Through a series of molecular dynamics simulations and detailed quantitative analysis, a dynamically stable allosteric binding site near catalytic center of the inactive state of USP7 (site partially absent in active state), along with two newly identified sites have been revealed, which opens the avenue for rational structure-guided inhibitor designing in USP7 specific-manner.

Targeting ubiquitin specific protease 7 in cancer: A deubiquitinase with great prospects

Deubiquitinase (DUB)-mediated cleavage of ubiquitin chain balances ubiquitination and deubiquitination for determining protein fate. USP7 is one of the best characterized DUBs and functionally important. Numerous proteins have been identified as potential substrates and binding partners of USP7; those play crucial roles in diverse array of cellular and biological processes including tumour suppression, cell cycle, DNA repair, chromatin remodelling, and epigenetic regulation. This review aims at summarizing the current knowledge of this wide association of USP7 with many cellular processes that enlightens the possibility of abnormal USP7 activity in promoting oncogenesis and the importance of identification of specific inhibitors.

Molecular Understanding of USP7 Substrate Recognition and C-Terminal Activation

The deubiquitinating enzyme USP7 has a pivotal role in regulating the stability of proteins involved in fundamental cellular processes of normal biology and disease. Despite the importance of USP7, the mechanisms underlying substrate recognition and catalytic activation are poorly understood. Here we present structural, biochemical, and biophysical analyses elucidating the molecular mechanism by which the C-terminal 19 amino acids of USP7 (residues 1084-1102) enhance the ubiquitin cleavage activity of the deubiquitinase (DUB) domain. Our data demonstrate that the C-terminal peptide binds the activation cleft in the catalytic domain and stabilizes the catalytically competent conformation of USP7. Additional structures of longer fragments of USP7, as well as solution studies, provide insight into full-length USP7, the role of the UBL domains, and demonstrate that both substrate recognition and deubiquitinase activity are highly regulated by the catalytic and noncatalytic domains of USP7, a feature that could be essential for the proper function of multi-domain DUBs.

Deubiquitinating enzymes as oncotargets

Carcinogenesis is a complex process tightly regulated at multiple levels by post-translational modifications. Epigenetics plays a major role in cancer development, all stable changes to the gene expression process that are not a result of a direct change in the DNA code are described as epigenetics. Epigenetic processes are regulated by post-translational modifications including ubiquitination which can directly affect either histones or transcription factors or may target their co-factors and interacting partners exerting an indirect effect. Deubiquitination of these target proteins is equally important and alterations in this pathway can also lead to cancer development, progression and metastasis. Only the correct, unaltered balance between ubiquitination and deubiquitination ensures healthy cellular homeostasis. In this review we focus on the role of deubiquitinating (DUB) enzymes in various aspects of epigenetics including the regulation of transcription factors, histone modifications, DNA damage repair pathways and cell cycle regulation. We discuss the impact of those processes on tumourigenesis and potential therapeutic applications of DUBs for cancer treatment.

USP7 Enforces Heterochromatinization of p53 Target Promoters by Protecting SUV39H1 from MDM2-Mediated Degradation

The H3K9me3 repressive histone conformation of p53 target promoters is abrogated in response to p53 activation by MDM2-mediated SUV39H1 degradation. Here, we present evidence that the USP7 deubiquitinase protects SUV39H1 from MDM2-mediated ubiquitination in the absence of p53 stimulus. USP7 occupies p53 target promoters in unstressed conditions, a process that is abrogated with p53 activation associated with loss of the H3K9me3 mark on these same promoters. Mechanistically, USP7 forms a trimeric complex with MDM2 and SUV39H1, independent of DNA, and modulates MDM2-dependent SUV39H1 ubiquitination. Furthermore, we show that this protective function of USP7 on SUV39H1 is independent of p53. Finally, USP7 blocking cooperates with p53 in inducing apoptosis by enhancing p53 promoter occupancy and dependent transactivation of target genes. These results uncover a layer of the p53 transcriptional program mediated by USP7, which restrains relaxation of local chromatin conformation at p53 target promoters.

Crystal Structure of USP7 Ubiquitin-like Domains with an ICP0 Peptide Reveals a Novel Mechanism Used by Viral and Cellular Proteins to Target USP7

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.

USP7 is a cellular protein that binds and stabilizes many proteins involved in multiple pathways that regulate oncogenesis and as such is recognized as a potential target for cancer therapy. In addition, USP7 is targeted by several viral proteins in order to promote cell survival and viral infection. One such protein is the ICP0 protein of herpes simplex virus 1, which must bind USP7 in order to manipulate the cell in ways that enable efficient viral infection. Here we use a structural approach to define the mechanism of the USP7-ICP0 peptide interaction, revealing a novel binding site on USP7. We then used this information to identify two cellular proteins, GMPS and UHRF1, that also bind USP7 through this binding site. Therefore we have identified a new mechanism by which both viral and cellular proteins can target USP7. This information will be useful for the development of strategies to block specific protein interactions with USP7.

The ubiquitin ligase RNF220 enhances canonical Wnt signaling through USP7-mediated deubiquitination of β-catenin

Wnt/β-catenin signaling plays critical roles in embryonic development and disease. Here, we identify RNF220, a RING domain E3 ubiquitin ligase, as a new regulator of β-catenin. RNF220 physically interacts with β-catenin, but instead of promoting its ubiquitination and proteasomal degradation, it stabilizes β-catenin and promotes canonical Wnt signaling. Our analysis showed that RNF220 interacts with USP7, a ubiquitin-specific peptidase, which is required for RNF220 to stabilize β-catenin. The RNF220/USP7 complex deubiquitinates β-catenin and enhances canonical Wnt signaling. Interestingly, the stability of RNF220 itself is negatively regulated by Gsk3β, which is a key component of the β-catenin destruction complex and is inhibited upon Wnt stimulation. Accordingly, the RNF220/USP7 complex works as a positive feedback regulator of β-catenin signaling. In colon cancer cells with stimulated Wnt signaling, knockdown of RNF220 or USP7 impairs Wnt signaling and expression of Wnt target genes, suggesting a potentially novel role of RNF220 in Wnt-related tumorigenesis.

The landscape of somatic mutations in epigenetic regulators across 1,000 paediatric cancer genomes

Studies of paediatric cancers have shown a high frequency of mutation across epigenetic regulators. Here we sequence 633 genes, encoding the majority of known epigenetic regulatory proteins, in over 1,000 paediatric tumours to define the landscape of somatic mutations in epigenetic regulators in paediatric cancer. Our results demonstrate a marked variation in the frequency of gene mutations across 21 different paediatric cancer subtypes, with the highest frequency of mutations detected in high-grade gliomas, T-lineage acute lymphoblastic leukaemia and medulloblastoma, and a paucity of mutations in low-grade glioma and retinoblastoma. The most frequently mutated genes are H3F3A, PHF6, ATRX, KDM6A, SMARCA4, ASXL2, CREBBP, EZH2, MLL2, USP7, ASXL1, NSD2, SETD2, SMC1A and ZMYM3. We identify novel loss-of-function mutations in the ubiquitin-specific processing protease 7 (USP7) in paediatric leukaemia, which result in decreased deubiquitination activity. Collectively, our results help to define the landscape of mutations in epigenetic regulatory genes in paediatric cancer and yield a valuable new database for investigating the role of epigenetic dysregulations in cancer.

The WD40-repeat protein-containing deubiquitinase complex: catalysis, regulation, and potential for therapeutic intervention

Ubiquitination has emerged as an essential signaling mechanism in eukaryotes. Deubiquitinases (DUBs) counteract the activities of the ubiquitination machinery and provide another level of control in cellular ubiquitination. Not surprisingly, DUBs are subjected to stringent regulations. Besides regulation by the noncatalytic domains present in the DUB sequences, DUB-interacting proteins are increasingly realized as essential regulators for DUB activity and function. This review focuses on DUBs that are associated with WD40-repeat proteins. Many human ubiquitin-specific proteases (USPs) were found to interact with WD40-repeat proteins, but little is known as to how this interaction regulates the activity and function of USPs. In recent years, significant progress has been made in understanding a prototypical WD40-repeat protein-containing DUB complex that comprises USP1 and USP1-associated factor 1 (UAF1). It has been shown that UAF1 activates USP1 through a potential active-site modulation, and the complex formation between USP1 and UAF1 is regulated by serine phosphorylation. Recently, human USPs have been recognized as a promising target class for inhibitor discovery. Small molecule inhibitors targeting several human USPs have been reported. USP1 is involved in two major DNA damage response pathways, DNA translesion synthesis and the Fanconi anemia pathway. Inhibiting the USP1/UAF1 deubiquitinase complex represents a new strategy to potentiate cancer cells to DNA-crosslinking agents and to overcome resistance that has plagued clinical cancer chemotherapy. The progress in inhibitor discovery against USPs and the WD40-repeat protein-containing USP complex will be discussed.

Identification of herpesvirus proteins that contribute to G1/S arrest

Lytic infection by herpesviruses induces cell cycle arrest at the G1/S transition. This appears to be a function of multiple herpesvirus proteins, but only a minority of herpesvirus proteins have been examined for cell cycle effects. To gain a more comprehensive understanding of the viral proteins that contribute to G1/S arrest, we screened a library of over 200 proteins from herpes simplex virus type 1, human cytomegalovirus, and Epstein-Barr virus (EBV) for effects on the G1/S interface, using HeLa fluorescent, ubiquitination-based cell cycle indicator (Fucci) cells in which G1/S can be detected colorimetrically. Proteins from each virus were identified that induce accumulation of G1/S cells, predominantly tegument, early, and capsid proteins. The identification of several capsid proteins in this screen suggests that incoming viral capsids may function to modulate cellular processes. The cell cycle effects of selected EBV proteins were further verified and examined for effects on p53 and p21 as regulators of the G1/S transition. Two EBV replication proteins (BORF2 and BMRF1) were found to induce p53 but not p21, while a previously uncharacterized tegument protein (BGLF2) was found to induce p21 protein levels in a p53-independent manner. Proteomic analyses of BGLF2-interacting proteins identified interactions with the NIMA-related protein kinase (NEK9) and GEM-interacting protein (GMIP). Silencing of either NEK9 or GMIP induced p21 without affecting p53 and abrogated the ability of BGLF2 to further induce p21. Collectively, these results suggest multiple viral proteins contribute to G1/S arrest, including BGLF2, which induces p21 levels likely by interfering with the functions of NEK9 and GMIP.

IMPORTANCE

Most people are infected with multiple herpesviruses, whose proteins alter the infected cells in several ways. During lytic infection, the viral proteins block cell proliferation just before the cellular DNA replicates. We used a novel screening method to identify proteins from three different herpesviruses that contribute to this block. Several of the proteins we identified had previously unknown functions or were structural components of the virion. Subsets of these proteins from Epstein-Barr virus were studied for their effects on the cell cycle regulatory proteins p53 and p21, thereby identifying two proteins that induce p53 and one that induces p21 (BGLF2). We identified interactions of BGLF2 with two human proteins, both of which regulate p21, suggesting that BGLF2 induces p21 by interfering with the functions of these two host proteins. Our study indicates that multiple herpesvirus proteins contribute to the cell proliferation block, including components of the incoming virions.

Regulation of the transforming growth factor β pathway by reversible ubiquitylation

The transforming growth factor β (TGFβ) signalling pathway plays a central role during embryonic development and in adult tissue homeostasis. It regulates gene transcription through a signalling cascade from cell surface receptors to intracellular SMAD transcription factors and their nuclear cofactors. The extent, duration and potency of signalling in response to TGFβ cytokines are intricately regulated by complex biochemical processes. The corruption of these regulatory processes results in aberrant TGFβ signalling and leads to numerous human diseases, including cancer. Reversible ubiquitylation of pathway components is a key regulatory process that plays a critical role in ensuring a balanced response to TGFβ signals. Many studies have investigated the mechanisms by which various E3 ubiquitin ligases regulate the turnover and activity of TGFβ pathway components by ubiquitylation. Moreover, recent studies have shed new light into their regulation by deubiquitylating enzymes. In this report, we provide an overview of current understanding of the regulation of TGFβ signalling by E3 ubiquitin ligases and deubiquitylases.

Cellular functions of the DUBs

Ubiquitylation is a reversible post-translational modification that has emerged as a key regulator of most complex cellular processes. It may rival phosphorylation in scope and exceed it in complexity. The dynamic nature of ubiquitylation events is important for governing protein stability, maintaining ubiquitin homeostasis and controlling ubiquitin-dependent signalling pathways. The human genome encodes ~80 active deubiquitylating enzymes (DUBs, also referred to as deubiquitinases), which exhibit distinct specificity profiles towards the various ubiquitin chain topologies. As a result of their ability to reverse ubiquitylation, these enzymes control a broad range of key cellular processes. In this Commentary we discuss the cellular functions of DUBs, such as their role in governing membrane traffic and protein quality control. We highlight two key signalling pathways--the Wnt and transforming growth factor β (TGF-β) pathways, for which dynamic ubiquitylation has emerged as a key regulator. We also discuss the roles of DUBs in the nucleus, where they govern transcriptional activity and DNA repair pathways.

STAT3 repressed USP7 expression is crucial for colon cancer development

Interleukin-6 (IL-6) induced STAT3 activation is viewed as crucial for multiple tumor growth and metastasis, including colon cancer. However, the molecular mechanisms remain largely unexplored. Here, we show that expression of ubiquitin-specific protease 7 (USP7), a deubiquitylating enzyme, is decreased in STAT3-positive tumors. IL-6 administration or transfection of a constitutively activated STAT3 in SW480 cells also repressed USP mRNA expression. Using luciferase reporter and ChIP assay, we found that STAT3 bound to the promoter region of USP7 and inhibited its activity through recruiting HDAC1. As a result of the decline of USP7 expression, endogenous P53 protein level was decreased. Thus, our results suggest a previously unknown STAT3-USP7-P53 molecular network controlling colon cancer development.

Preferential binding of IFI16 protein to cruciform structure and superhelical DNA

Interferon (IFN)-inducible HIN-200 proteins play an important role in transcriptional regulation linked to cell cycle control, inflammation, autoimmunity and differentiation. IFI16 has been identified as a target of IFNα and γ and is a member of the HIN-200 protein family. Expression level of IFI16 is often decreased in breast cancers, implicating its role as a tumor suppressor. As a potent transcription factor, IFI16 possesses a transcriptional regulatory region, a PYD/DAPIN/PAAD region which associates with IFN response, DNA-binding domains and binding regions for tumor suppressor proteins BRCA1 and p53. It is also reported that IFI16 protein is capable of binding p53 and cMYC gene promoters. Here, we demonstrate that IFI16 protein binds strongly to negatively superhelical plasmid DNA at a native superhelix density, as evidenced by electrophoretic retardation of supercoiled (sc) DNA in agarose gels. Binding of IFI16 to supercoiled DNA results in the appearance of one or more retarded DNA bands on the gels. After removal of IFI16, the original mobility of the scDNA is recovered. By contrast, IFI16 protein binds very weakly to the same DNA in linear state. Using short oligonucleotide targets, we also detect a strong preference for IFI16 binding to cruciform DNA structure compared to linear DNA topology. Hence, this novel DNA-binding property of IFI16 protein to scDNA and cruciform structures may play critical roles in its tumor suppressor function.

Mechanism of USP7/HAUSP activation by its C-terminal ubiquitin-like domain and allosteric regulation by GMP-synthetase

The ubiquitin-specific protease USP7/HAUSP regulates p53 and MDM2 levels, and cellular localization of FOXO4 and PTEN, and hence is critically important for their role in cellular processes. Here we show how the 64 kDa C-terminal region of USP7 can positively regulate deubiquitinating activity. We present the crystal structure of this USP7/HAUSP ubiquitin-like domain (HUBL) comprised of five ubiquitin-like (Ubl) domains organized in 2-1-2 Ubl units. The last di-Ubl unit, HUBL-45, is sufficient to activate USP7, through binding to a "switching" loop in the catalytic domain, which promotes ubiquitin binding and increases activity 100-fold. This activation can be enhanced allosterically by the metabolic enzyme GMPS. It binds to the first three Ubl domains (HUBL-123) and hyperactivates USP7 by stabilization of the HUBL-45-dependent active state.

Gene expression control by protein deubiquitinases

Protein ubiquitylation is involved in the regulation of virtually all aspects of eukaryotic cell biology, including gene expression. The central function of E3 ubiquitin ligases in target selection is well established. More recently, it has become appreciated that deubiquitylating enzymes (DUBs) are crucial components of ubiquitin-regulated cellular switches. Here, we discuss advances in our understanding of how DUBs regulate chromatin dynamics and gene expression. DUBs are integral components of the transcription machinery, involved in both gene activation and repression. They modulate the ubiquitylation status of histones H2A and H2B, which play pivotal roles in a cascade of molecular events that determine chromatin status. A DUB module in the SAGA coactivator complex is required for gene activation, whereas other DUBs are part of the Polycomb gene-silencing machinery. DUBs also control the level or subcellular compartmentalization of selective transcription factors, including the tumour suppressor p53. Typically, DUB specificity and activity are defined by its partner proteins, enabling remarkably versatile and sophisticated regulation. Recent findings not only underscore the pervasive and pivotal role of DUBs in gene expression control, but also raise paradoxical questions concerning the molecular mechanisms involved.

Interferon-inducible protein 16: insight into the interaction with tumor suppressor p53

IFI16 is a member of the interferon-inducible HIN-200 family of nuclear proteins. It has been implicated in transcriptional regulation by modulating protein-protein interactions with p53 tumor suppressor protein and other transcription factors. However, the mechanisms of interaction remain unknown. Here, we report the crystal structures of both HIN-A and HIN-B domains of IFI16 determined at 2.0 and 2.35 Å resolution, respectively. Each HIN domain comprises a pair of tightly packed OB-fold subdomains that appear to act as a single unit. We show that both HIN domains of IFI16 are capable of enhancing p53-DNA complex formation and transcriptional activation via distinctive means. HIN-A domain binds to the basic C terminus of p53, whereas the HIN-B domain binds to the core DNA-binding region of p53. Both interactions are compatible with the DNA-bound state of p53 and together contribute to the effect of full-length IFI16 on p53-DNA complex formation and transcriptional activation.