HIV-1 Vpr protein inhibits telomerase activity via the EDD-DDB1-VPRBP E3 ligase complex

HIV-1 Vpr combats the PU.1-driven antiviral response in primary human macrophages

HIV-1 Vpr promotes efficient spread of HIV-1 from macrophages to T cells by transcriptionally downmodulating restriction factors that target HIV-1 Envelope protein (Env). Here we find that Vpr induces broad transcriptomic changes by targeting PU.1, a transcription factor necessary for expression of host innate immune response genes, including those that target Env. Consistent with this, we find silencing PU.1 in infected macrophages lacking Vpr rescues Env. Vpr downmodulates PU.1 through a proteasomal degradation pathway that depends on physical interactions with PU.1 and DCAF1, a component of the Cul4A E3 ubiquitin ligase. The capacity for Vpr to target PU.1 is highly conserved across primate lentiviruses. In addition to impacting infected cells, we find that Vpr suppresses expression of innate immune response genes in uninfected bystander cells, and that virion-associated Vpr can degrade PU.1. Together, we demonstrate Vpr counteracts PU.1 in macrophages to blunt antiviral immune responses and promote viral spread.

HIV-1 Vpr Functions in Primary CD4+ T Cells

HIV-1 encodes four accesory proteins in addition to its structural and regulatory genes. Uniquely amongst them, Vpr is abundantly present within virions, meaning it is poised to exert various biological effects on the host cell upon delivery. In this way, Vpr contributes towards the establishment of a successful infection, as evidenced by the extent to which HIV-1 depends on this factor to achieve full pathogenicity in vivo. Although HIV infects various cell types in the host organism, CD4+ T cells are preferentially targeted since they are highly permissive towards productive infection, concomitantly bringing about the hallmark immune dysfunction that accompanies HIV-1 spread. The last several decades have seen unprecedented progress in unraveling the activities Vpr possesses in the host cell at the molecular scale, increasingly underscoring the importance of this viral component. Nevertheless, it remains controversial whether some of these advances bear in vivo relevance, since commonly employed cellular models significantly differ from primary T lymphocytes. One prominent example is the "established" ability of Vpr to induce G2 cell cycle arrest, with enigmatic physiological relevance in infected primary T lymphocytes. The objective of this review is to present these discoveries in their biological context to illustrate the mechanisms whereby Vpr supports HIV-1 infection in CD4+ T cells, whilst identifying findings that require validation in physiologically relevant models.

Centrosome amplification and aneuploidy driven by the HIV-1-induced Vpr•VprBP•Plk4 complex in CD4+ T cells

HIV-1 infection elevates the risk of developing various cancers, including T-cell lymphoma. Whether HIV-1-encoded proteins directly contribute to oncogenesis remains unknown. We observe that approximately 1-5% of CD4+ T cells from the blood of people living with HIV-1 exhibit over-duplicated centrioles, suggesting that centrosome amplification underlies the development of HIV-1-associated cancers by driving aneuploidy. Through affinity purification, biochemical, and cellular analyses, we discover that Vpr, an accessory protein of HIV-1, hijacks the centriole duplication machinery and induces centrosome amplification and aneuploidy. Mechanistically, Vpr forms a cooperative ternary complex with an E3 ligase subunit, VprBP, and polo-like kinase 4 (Plk4). Unexpectedly, however, the complex enhances Plk4's functionality by promoting its relocalization to the procentriole assembly and induces centrosome amplification. Loss of either Vpr's C-terminal 17 residues or VprBP acidic region, the two elements required for binding to Plk4 cryptic polo-box, abrogates Vpr's capacity to induce these events. Furthermore, HIV-1 WT, but not its Vpr mutant, induces multiple centrosomes and aneuploidy in human primary CD4+ T cells. We propose that the Vpr•VprBP•Plk4 complex serves as a molecular link that connects HIV-1 infection to oncogenesis and that inhibiting the Vpr C-terminal motif may reduce the occurrence of HIV-1-associated cancers.

HIV-1 Vpr combats the PU.1-driven antiviral response in primary human macrophages

HIV-1 Vpr promotes efficient spread of HIV-1 from macrophages to T cells by transcriptionally downmodulating restriction factors that target HIV-1 Envelope protein (Env). Here we find that Vpr induces broad transcriptomic changes by targeting PU.1, a transcription factor necessary for expression of host innate immune response genes, including those that target Env. Consistent with this, we find silencing PU.1 in infected macrophages lacking Vpr rescues Env. Vpr downmodulates PU.1 through a proteasomal degradation pathway that depends on physical interactions with PU.1 and DCAF1, a component of the Cul4A E3 ubiquitin ligase. The capacity for Vpr to target PU.1 is highly conserved across primate lentiviruses. In addition to impacting infected cells, we find that Vpr suppresses expression of innate immune response genes in uninfected bystander cells, and that virion-associated Vpr can degrade PU.1. Together, we demonstrate Vpr counteracts PU.1 in macrophages to blunt antiviral immune responses and promote viral spread.

UBR5 promotes antiviral immunity by disengaging the transcriptional brake on RIG-I like receptors

The Retinoic acid-Inducible Gene I (RIG-I) like receptors (RLRs) are the major viral RNA sensors essential for the initiation of antiviral immune responses. RLRs are subjected to stringent transcriptional and posttranslational regulations, of which ubiquitination is one of the most important. However, the role of ubiquitination in RLR transcription is unknown. Here, we screen 375 definite ubiquitin ligase knockout cell lines and identify Ubiquitin Protein Ligase E3 Component N-Recognin 5 (UBR5) as a positive regulator of RLR transcription. UBR5 deficiency reduces antiviral immune responses to RNA viruses, while increases viral replication in primary cells and mice. Ubr5 knockout mice are more susceptible to lethal RNA virus infection than wild type littermates. Mechanistically, UBR5 mediates the Lysine 63-linked ubiquitination of Tripartite Motif Protein 28 (TRIM28), an epigenetic repressor of RLRs. This modification prevents intramolecular SUMOylation of TRIM28, thus disengages the TRIM28-imposed brake on RLR transcription. In sum, UBR5 enables rapid upregulation of RLR expression to boost antiviral immune responses by ubiquitinating and de-SUMOylating TRIM28.

TERT single nucleotide polymorphism rs2736098 but not rs2736100 is associated with telomere length in HIV-infected patients on cART

BACKGROUND

Continuous application of "combination antiretroviral therapy" (cART) has transformed Human immunodeficiency virus (HIV) infection into a manageable chronic disease; however, due to lasting inflammation and cumulative toxicity, progressive pathophysiological changes do occur and potentially lead to accelerated aging, among others, contributing to telomere shortening. The single nucleotide polymorphisms (SNP) rs2736100 and rs2736098 are particularly important for human telomerase (TERT) gene expression. The objective of this study was to evaluate the effects of clinical parameters and single nucleotide polymorphisms in TERT (rs2736100 and rs2736098) on telomere length in HIV-infected patients.

METHODS AND RESULTS

This cross-sectional study included 176 patients diagnosed with HIV infection. Relative telomere length (RTL) was determined by real-time polymerase chain reaction (qPCR), whereas genotyping was performed by polymerase chain reaction, followed by restriction fragment length polymorphism analysis (PCR-RFLP). The mean age of the patients (p = .904), time since HIV diagnosis (p = .220), therapy-related variables such as the cART regimen (0.761), and total cART duration (p = .096) did not significantly affect RTL. TERT rs2736100 genotype showed no association with RTL. However, TERT rs2736098 heterozygotes (GA) had significantly longer telomeres (P = .049) than both homozygotes (GG and AA).

CONCLUSIONS

Our findings support the fact that cellular aging in HIV-infected patients is influenced by the TERT rs2736098 polymorphism.

Identification of viral protein R of human immunodeficiency virus-1 (HIV) and interleukin-6 as risk factors for malignancies in HIV-infected individuals: A cohort study

BACKGROUND

Despite effective antiretroviral therapy, patients with human immunodeficiency virus type-1 (HIV) suffer from a high frequency of malignancies, but related risk factors remain elusive. Here, we focused on blood-circulating viral protein R (Vpr) of HIV, which induces proinflammatory cytokine production and genotoxicity by exogenous functions.

METHODS AND FINDINGS

A total 404 blood samples of HIV patients comprising of 126 patients with malignancies (tumor group) and 278 patients without malignancies (non-tumor group), each of 96 samples was first selected by one-to-one propensity score matching. By a detergent-free enzyme-linked immunosorbent assays (detection limit, 3.9 ng/mL), we detected Vpr at a higher frequency in the matched tumor group (56.3%) than in the matched non-tumor group (39.6%) (P = 0.030), although there was no different distribution of Vpr levels (P = 0.372). We also detected anti-Vpr immunoglobulin (IgG), less frequently in the tumor group compared with the tumor group (22.9% for tumor group vs. 44.8% for non-tumor group, P = 0.002), and the proportion of patients positive for Vpr but negative of anti-Vpr IgG was significantly higher in the tumor group than in the non-tumor group (38.6% vs. 15.6%, respectively, P < 0.001). Additionally, Interleukin-6 (IL-6), the levels of which were high in HIV-1 infected patients (P < 0.001) compared to non-HIV-infected individuals, was significantly higher in advanced cases of tumors (P < 0.001), and IL-6 level was correlated with Vpr in the non-tumor group (P = 0.010). Finally, multivariate logistic regression analysis suggested a positive link of Vpr with tumor occurrence in HIV patients (P = 0.002).

CONCLUSION

Vpr and IL-6 could be risk factors of HIV-1 associated malignancies, and it would be importance to monitor these molecules for well managing people living with HIV-1.

E3 Ubiquitin Ligases in Gammaherpesviruses and HIV: A Review of Virus Adaptation and Exploitation

For productive infection and replication to occur, viruses must control cellular machinery and counteract restriction factors and antiviral proteins. Viruses can accomplish this, in part, via the regulation of cellular gene expression and post-transcriptional and post-translational control. Many viruses co-opt and counteract cellular processes via modulation of the host post-translational modification machinery and encoding or hijacking kinases, SUMO ligases, deubiquitinases, and ubiquitin ligases, in addition to other modifiers. In this review, we focus on three oncoviruses, Epstein-Barr virus (EBV), Kaposi's sarcoma herpesvirus (KSHV), and human immunodeficiency virus (HIV) and their interactions with the ubiquitin-proteasome system via viral-encoded or cellular E3 ubiquitin ligase activity.

Centrosome amplification and aneuploidy driven by the HIV-1-induced Vpr•VprBP•Plk4 complex in CD4+ T cells

HIV-1 infection elevates the risk of developing various cancers, including T-cell lymphoma. Whether HIV-1-encoded proteins directly contribute to oncogenesis remains unknown. We observed that approximately 1-5% of CD4+ T cells from the blood of people living with HIV-1 exhibit over-duplicated centrioles, suggesting that centrosome amplification underlies the development of HIV-1-associated cancers by driving aneuploidy. Through affinity purification, biochemical, and cell biology analyses, we discovered that Vpr, an accessory protein of HIV-1, hijacks the centriole duplication machinery and induces centrosome amplification and aneuploidy. Mechanistically, Vpr formed a cooperative ternary complex with an E3 ligase subunit, VprBP, and polo-like kinase 4 (Plk4). Unexpectedly, however, the complex enhanced Plk4's functionality by promoting its relocalization to the procentriole assembly and induced centrosome amplification. Loss of either Vpr's C-terminal 17 residues or VprBP acidic region, the two elements required for binding to Plk4 cryptic polo-box, abrogated Vpr's capacity to induce all these events. Furthermore, HIV-1 WT, but not its Vpr mutant, induced multiple centrosomes and aneuploidy in primary CD4+ T cells. We propose that the Vpr•VprBP•Plk4 complex serves as a molecular link that connects HIV-1 infection to oncogenesis and that inhibiting the Vpr C-terminal motif may reduce the occurrence of HIV-1-associated cancers.

Structure of the human UBR5 E3 ubiquitin ligase

The human UBR5 is a single polypeptide chain homology to E6AP C terminus (HECT)-type E3 ubiquitin ligase essential for embryonic development in mammals. Dysregulated UBR5 functions like an oncoprotein to promote cancer growth and metastasis. Here, we report that UBR5 assembles into a dimer and a tetramer. Our cryoelectron microscopy (cryo-EM) structures reveal that two crescent-shaped UBR5 monomers assemble head to tail to form the dimer, and two dimers bind face to face to form the cage-like tetramer with all four catalytic HECT domains facing the central cavity. Importantly, the N-terminal region of one subunit and the HECT of the other form an "intermolecular jaw" in the dimer. We show the jaw-lining residues are important for function, suggesting that the intermolecular jaw functions to recruit ubiquitin-loaded E2 to UBR5. Further work is needed to understand how oligomerization regulates UBR5 ligase activity. This work provides a framework for structure-based anticancer drug development and contributes to a growing appreciation of E3 ligase diversity.

The influence of viral protein R amino acid substitutions on clinical outcomes in people living with HIV: A systematic review

BACKGROUND

The HIV viral protein R (Vpr) is a multifunction protein involved in the pathophysiology of HIV-1. Recent evidence has suggested that Vpr amino acid substitutions influence the pathophysiology of HIV-1 and clinical outcomes in people living with HIV (PLWH). Several studies have linked Vpr amino acid substitutions to clinical outcomes in PLWH; however, there is no clear consensus as to which amino acids or amino acid substitutions are most important in the pathophysiology and clinical outcomes in PLWH. We, therefore, conducted a systematic review of studies investigating Vpr amino acid substitutions and clinical outcomes in PLWH.

METHODS

PubMed, Scopus and Web of Science databases were searched according to PRISMA guidelines using a search protocol designed specifically for this study.

RESULTS

A total of 22 studies were included for data extraction, comprising 14 cross-sectional and 8 longitudinal studies. Results indicated that Vpr amino acid substitutions were associated with specific clinical outcomes, including disease progressions, neurological outcomes and treatment status. Studies consistently showed that the Vpr substitution 63T was associated with slower disease progression, whereas 77H and 85P were associated with no significant contribution to disease progression.

CONCLUSIONS

Vpr-specific amino acid substitutions may be contributors to clinical outcomes in PLWH, and future studies should consider investigating the Vpr amino acid substitutions highlighted in this review.

A Novel Class of HIV-1 Inhibitors Targeting the Vpr-Induced G2-Arrest in Macrophages by New Yeast- and Cell-Based High-Throughput Screening

The human immunodeficiency virus type 1 (HIV-1) accessory protein, Vpr, arrests the cell cycle of the G2 phase, and this Vpr-mediated G2 arrest is implicated in an efficient HIV-1 spread in monocyte-derived macrophages. Here, we screened new candidates for Vpr-targeting HIV-1 inhibitors by using fission yeast- and mammalian cell-based high-throughput screening. First, fission yeast strains expressing the HIV-1 Vpr protein were generated and then treated for 48 h with 20 μM of a synthetic library, including 140,000 chemical compounds. We identified 268 compounds that recovered the growth of Vpr-overexpressing yeast. The selected compounds were then tested in mammalian cells, and those displaying high cytotoxicity were excluded from further cell cycle analysis and imaging-based screening. A flow cytometry analysis confirmed that seven compounds recovered from the Vpr-induced G2 arrest. The cell toxicity and inhibitory effect of HIV-1 replication in human monocyte-derived macrophages (MDM) were examined, and three independent structural compounds, VTD227, VTD232, and VTD263, were able to inhibit HIV-1 replication in MDM. Furthermore, we showed that VTD227, but not VTD232 and VTD263, can directly bind to Vpr. Our results indicate that three new compounds and their derivatives represent new drugs targeting HIV-1 replication and can be potentially used in clinics to improve the current antiretroviral therapy.

Emerging roles of DYRK2 in cancer

Over the last decade, the CMGC kinase DYRK2 has been reported as a tumor suppressor across various cancers triggering major antitumor and proapoptotic signals in breast, colon, liver, ovary, brain, and lung cancers, with lower DYRK2 expression correlated with poorer prognosis in patients. Contrary to this, various medicinal chemistry studies reported robust antiproliferative properties of DYRK2 inhibitors, whereas unbiased 'omics' and genome-wide association study-based studies identified DYRK2 as a highly overexpressed kinase in various patient tumor samples. A major paradigm shift occurred in the last 4 years when DYRK2 was found to regulate proteostasis in cancer via a two-pronged mechanism. DYRK2 phosphorylated and activated the 26S proteasome to enhance degradation of misfolded/tumor-suppressor proteins while also promoting the nuclear stability and transcriptional activity of its substrate, heat-shock factor 1 triggering protein folding. Together, DYRK2 regulates proteostasis and promotes protumorigenic survival for specific cancers. Indeed, potent and selective small-molecule inhibitors of DYRK2 exhibit in vitro and in vivo anti-tumor activity in triple-negative breast cancer and myeloma models. However, with conflicting and contradictory reports across different cancers, the overarching role of DYRK2 remains enigmatic. Specific cancer (sub)types coupled to spatiotemporal interactions with substrates could decide the procancer or anticancer role of DYRK2. The current review aims to provide a balanced and critical appreciation of the literature to date, highlighting top substrates such as p53, c-Myc, c-Jun, heat-shock factor 1, proteasome, or NOTCH1, to discuss DYRK2 inhibitors available to the scientific community and to shed light on this duality of protumorigenic and antitumorigenic roles of DYRK2.

Determinants of blood telomere length in antiretroviral treatment-naïve HIV-positive participants enrolled in the NEAT 001/ANRS 143 clinical trial

OBJECTIVES

Our aim was to investigate factors associated with baseline blood telomere length in participants enrolled in NEAT 001/ANRS 143, a randomized, open-label trial comparing ritonavir-boosted darunavir (DRV/r) plus raltegravir (RAL) with DRV/r plus tenofovir disoproxil fumarate/emtricitabine (TDF/FTC) in antiretroviral therapy (ART)-naïve HIV-positive adults.

METHODS

A cross-sectional study of 201 randomly selected participants who had stored samples available was carried out. We measured telomere length (i.e. the relative telomere length, calculated as the telomere to single copy gene ratio) at baseline with monochrome quantitative multiplex polymerase chain reaction (PCR). We used multivariable predictive linear regression to calculate mean differences and 95% confidence intervals (CIs) for the association between baseline telomere length and baseline characteristics.

RESULTS

The baseline characteristics of the 201 participants did not differ from those of the 805 participants in the parent trial population: 89% were male, the mean age was 39 years, 83.6% were Caucasian, 93% acquired HIV infection via sexual transmission, the mean estimated time since HIV diagnosis was 2.1 years, the mean HIV-1 RNA load was 4.7 log₁₀ HIV-1 RNA copies/mL, the mean nadir and baseline CD4 counts were 301 and 324 cells/μL, respectively, and the mean CD4:CD8 ratio was 0.4. In the univariate analysis, shorter telomere length was associated with older age (per 10 years) (P < 0.001), HIV-1 RNA ≥ 100 000 copies/mL (P = 0.001), CD4 count < 200 cells/μL (P = 0.037), lower CD4:CD8 ratio (P = 0.018), statin treatment (P = 0.004), and current alcohol consumption (P = 0.035). In the multivariable analysis, older age (P < 0.001) and HIV RNA ≥ 100 000 copies/mL (P = 0.054) were independently associated with shorter telomere length.

CONCLUSIONS

Both age and HIV RNA viral load correlated with shorter blood telomere length in untreated persons living with HIV. These results suggest that HIV infection and age have synergistic and independent impacts upon immunosenescence.

Cellular Cullin RING Ubiquitin Ligases: Druggable Host Dependency Factors of Cytomegaloviruses

Human cytomegalovirus (HCMV) is a ubiquitous betaherpesvirus that frequently causes morbidity and mortality in individuals with insufficient immunity, such as transplant recipients, AIDS patients, and congenitally infected newborns. Several antiviral drugs are approved to treat HCMV infections. However, resistant HCMV mutants can arise in patients receiving long-term therapy. Additionally, side effects and the risk to cause birth defects limit the use of currently approved antivirals against HCMV. Therefore, the identification of new drug targets is of clinical relevance. Recent work identified DNA-damage binding protein 1 (DDB1) and the family of the cellular cullin (Cul) RING ubiquitin (Ub) ligases (CRLs) as host-derived factors that are relevant for the replication of human and mouse cytomegaloviruses. The first-in-class CRL inhibitory compound Pevonedistat (also called MLN4924) is currently under investigation as an anti-tumor drug in several clinical trials. Cytomegaloviruses exploit CRLs to regulate the abundance of viral proteins, and to induce the proteasomal degradation of host restriction factors involved in innate and intrinsic immunity. Accordingly, pharmacological blockade of CRL activity diminishes viral replication in cell culture. In this review, we summarize the current knowledge concerning the relevance of DDB1 and CRLs during cytomegalovirus replication and discuss chances and drawbacks of CRL inhibitory drugs as potential antiviral treatment against HCMV.

A Mass Spectrometry-Based Profiling of Interactomes of Viral DDB1- and Cullin Ubiquitin Ligase-Binding Proteins Reveals NF-κB Inhibitory Activity of the HIV-2-Encoded Vpx

Viruses and hosts are situated in a molecular arms race. To avoid morbidity and mortality, hosts evolved antiviral restriction factors. These restriction factors exert selection pressure on the viruses and drive viral evolution toward increasingly efficient immune antagonists. Numerous viruses exploit cellular DNA damage-binding protein 1 (DDB1)-containing Cullin RocA ubiquitin ligases (CRLs) to induce the ubiquitination and subsequent proteasomal degradation of antiviral factors expressed by their hosts. To establish a comprehensive understanding of the underlying protein interaction networks, we performed immuno-affinity precipitations for a panel of DDB1-interacting proteins derived from viruses such as mouse cytomegalovirus (MCMV, Murid herpesvirus [MuHV] 1), rat cytomegalovirus Maastricht MuHV2, rat cytomegalovirus English MuHV8, human cytomegalovirus (HCMV), hepatitis B virus (HBV), and human immunodeficiency virus (HIV). Cellular interaction partners were identified and quantified by mass spectrometry (MS) and validated by classical biochemistry. The comparative approach enabled us to separate unspecific interactions from specific binding partners and revealed remarkable differences in the strength of interaction with DDB1. Our analysis confirmed several previously described interactions like the interaction of the MCMV-encoded interferon antagonist pM27 with STAT2. We extended known interactions to paralogous proteins like the interaction of the HBV-encoded HBx with different Spindlin proteins and documented interactions for the first time, which explain functional data like the interaction of the HIV-2-encoded Vpr with Bax. Additionally, several novel interactions were identified, such as the association of the HIV-2-encoded Vpx with the transcription factor RelA (also called p65). For the latter interaction, we documented a functional relevance in antagonizing NF-κB-driven gene expression. The mutation of the DDB1 binding interface of Vpx significantly impaired NF-κB inhibition, indicating that Vpx counteracts NF-κB signaling by a DDB1- and CRL-dependent mechanism. In summary, our findings improve the understanding of how viral pathogens hijack cellular DDB1 and CRLs to ensure efficient replication despite the expression of host restriction factors.

Association of Viral Suppression With Lower AIDS-Defining and Non-AIDS-Defining Cancer Incidence in HIV-Infected Veterans: A Prospective Cohort Study

Background

Viral suppression is a primary marker of HIV treatment success. Persons with HIV are at increased risk for AIDS-defining cancer (ADC) and several types of non-AIDS-defining cancer (NADC), some of which are caused by oncogenic viruses.

Objective

To determine whether viral suppression is associated with decreased cancer risk.

Design

Prospective cohort.

Setting

Department of Veterans Affairs.

Participants

HIV-positive veterans (n = 42 441) and demographically matched uninfected veterans (n = 104 712) from 1999 to 2015.

Measurements

Standardized cancer incidence rates and Poisson regression rate ratios (RRs; HIV-positive vs. uninfected persons) by viral suppression status (unsuppressed: person-time with HIV RNA levels ≥500 copies/mL; early suppression: initial 2 years with HIV RNA levels <500 copies/mL; long-term suppression: person-time after early suppression with HIV RNA levels <500 copies/mL).

Results

Cancer incidence for HIV-positive versus uninfected persons was highest for unsuppressed persons (RR, 2.35 [95% CI, 2.19 to 2.51]), lower among persons with early suppression (RR, 1.99 [CI, 1.87 to 2.12]), and lowest among persons with long-term suppression (RR, 1.52 [CI, 1.44 to 1.61]). This trend was strongest for ADC (unsuppressed: RR, 22.73 [CI, 19.01 to 27.19]; early suppression: RR, 9.48 [CI, 7.78 to 11.55]; long-term suppression: RR, 2.22 [CI, 1.69 to 2.93]), much weaker for NADC caused by viruses (unsuppressed: RR, 3.82 [CI, 3.24 to 4.49]; early suppression: RR, 3.42 [CI, 2.95 to 3.97]; long-term suppression: RR, 3.17 [CI, 2.78 to 3.62]), and absent for NADC not caused by viruses.

Limitation

Lower viral suppression thresholds, duration of long-term suppression, and effects of CD4+ and CD8+ T-cell counts were not thoroughly evaluated.

Conclusion

Antiretroviral therapy resulting in long-term viral suppression may contribute to cancer prevention, to a greater degree for ADC than for NADC. Patients with long-term viral suppression still had excess cancer risk.

Primary Funding Source

National Cancer Institute and National Institute on Alcohol Abuse and Alcoholism of the National Institutes of Health.

HIV-1 Vpr hijacks EDD-DYRK2-DDB1DCAF1 to disrupt centrosome homeostasis

Viruses exploit the host cell machinery for their own profit. To evade innate immune sensing and promote viral replication, HIV type 1 (HIV-1) subverts DNA repair regulatory proteins and induces G2/M arrest. The preintegration complex of HIV-1 is known to traffic along microtubules and accumulate near the microtubule-organizing center. The centrosome is the major microtubule-organizing center in most eukaryotic cells, but precisely how HIV-1 impinges on centrosome biology remains poorly understood. We report here that the HIV-1 accessory protein viral protein R (Vpr) localized to the centrosome through binding to DCAF1, forming a complex with the ubiquitin ligase EDD-DYRK2-DDB1DCAF1 and Cep78, a resident centrosomal protein previously shown to inhibit EDD-DYRK2-DDB1DCAF1 Vpr did not affect ubiquitination of Cep78. Rather, it enhanced ubiquitination of an EDD-DYRK2-DDB1DCAF1 substrate, CP110, leading to its degradation, an effect that could be overcome by Cep78 expression. The down-regulation of CP110 and elongation of centrioles provoked by Vpr were independent of G2/M arrest. Infection of T lymphocytes with HIV-1, but not with HIV-1 lacking Vpr, promoted CP110 degradation and centriole elongation. Elongated centrioles recruited more γ-tubulin to the centrosome, resulting in increased microtubule nucleation. Our results suggest that Vpr is targeted to the centrosome where it hijacks a ubiquitin ligase, disrupting organelle homeostasis, which may contribute to HIV-1 pathogenesis.

Vpr Targets TET2 for Degradation by CRL4VprBP E3 Ligase to Sustain IL-6 Expression and Enhance HIV-1 Replication

HIV-1 expresses several accessory proteins to counteract host anti-viral restriction factors to facilitate viral replication and disease progression. One such protein, Vpr, has been implicated in affecting multiple cellular processes, but its mechanism remains elusive. Here we report that Vpr targets TET2 for polyubiquitylation by the VprBP-DDB1-CUL4-ROC1 E3 ligase and subsequent degradation. Genetic inactivation or Vpr-mediated degradation of TET2 enhances HIV-1 replication and substantially sustains expression of the pro-inflammatory cytokine interleukin-6 (IL-6). This process correlates with reduced recruitment of histone deacetylase 1 and 2 to the IL-6 promoter, thus enhancing its histone H3 acetylation level during resolution phase. Blocking IL-6 signaling reduced the ability of Vpr to enhance HIV-1 replication. We conclude that HIV-1 Vpr degrades TET2 to sustain IL-6 expression to enhance viral replication and disease progression. These results suggest that disrupting the Vpr-TET2-IL6 axis may prove clinically beneficial to reduce both viral replication and inflammation during HIV-1 infection.

An OTU deubiquitinating enzyme in Eimeria tenella interacts with Eimeria tenella virus RDRP

Background

Chicken coccidiosis, a disease caused by seven species of Eimeria (Apicomplexa: Coccidia), inflicts severe economic losses on the poultry industry. Eimeria tenella is the one of the most virulent species pathogenic to chickens. Many parasitic protozoans are parasitised by double-stranded (ds) RNA viruses, and the influence of protozoan viruses on parasitic protozoans has been extensively reported. E. tenella RNA virus 1 (Etv) was identified in E. tenella, and the complete genome sequence of Etv was analysed. Here, we screened Etv-RNA-dependent RNA polymerase (RDRP)-interacting host protein E. tenella ovarian tumour (OTU) protein-like cysteine protease (Et-OTU) using a yeast two-hybrid system with pGBKT7-RDRP plasmid serving as bait. A previous study demonstrated that Et-OTU could regulate the telomerase activity of E. tenella, indicating that Et-OTU affects E. tenella proliferation. However, whether Etv-RDRP affects the molecular biological characteristics of E. tenella by interacting with OTU remains unclear.

Results

We obtained seven positive clones from the initial screen, and six of the seven preys were identified as false-positives. Finally, we identified an RDRP-associated protein predicted to be an E. tenella OTU protein. A α-galactosidase assay showed that the bait vector did not activate the GAL4 reporter gene, indicating no autoactivation activity from the RDRP bait fusion. Pull-down and co-immunoprecipitation assays verified the interaction between Et-OTU and Etv-RDRP both intracellularly and extracellularly. Additionally, Et-OTU was able to deconjugate K48- and K6-linked di-ubiquitin (di-Ub) chains in vitro but not K63-, K11-, K29-, or K33-linked di-Ub chains. The C239A and H351A mutations eliminated the deubiquitinase (DUB) activity of Et-OTU, whereas the D236A mutation did not. Additionally, when combined with RDRP, the DUB activity of Et-OTU towards K48- and K6-linked chains was significantly enhanced.

Conclusion

Etv-RDRP interacts with Et-OTU both intracellularly and extracellularly. Etv-RDRP enhances the hydrolysis of Et-OTU to K6- or K48-linked ubiquitin chains. This study lays the foundation for further research on the relationship between E. tenella and Etv.

Glutamate-rich WD40 repeat containing 1 regulates ribosomal protein L23 levels via the ubiquitin-proteasome system

GRWD1 is a Cdt1-binding protein that promotes MCM loading through its histone chaperone activity. GRWD1 acts as a tumor-promoting factor by downregulating p53 via the RPL11-MDM2-p53 axis. Here, we identified GRWD1-interacting proteins using a proteomics approach and showed that GRWD1 interacts with various proteins involved in transcription, translation, DNA replication and repair, chromatin organization, and ubiquitin-mediated proteolysis. We focused on the ribosomal protein RPL23, which positively regulates nucleolar stress responses through MDM2 binding and inhibition, thereby functioning as a tumor suppressor. Overexpression of GRWD1 decreased RPL23 protein levels and stability; this effect was restored by the proteasome inhibitor MG132. EDD, an E3 ubiquitin ligase that interacts with GRWD1, also downregulated RPL23, and the decrease was further enhanced by co-expression of GRWD1. Conversely, siRNA-mediated GRWD1 knockdown upregulated RPL23. Co-expression of GRWD1 and EDD promoted RPL23 ubiquitination. These data suggest that GRWD1 acts together with EDD to negatively regulate RPL23 via the ubiquitin-proteasome system. GRWD1 reversed the RPL23-mediated inhibition of anchorage-independent growth in cancer cells. Our data suggest that GRWD1-induced RPL23 proteolysis plays a role in p53 downregulation and tumorigenesis.

Underlying mechanisms of HIV-1 latency

Similarly to other retroviruses, HIV-1 integrates its genome into the cellular chromosome. Expression of viral genes from the integrated viral DNA could then be regulated by the host genome. If the infected cell suppresses viral gene expression, the virus will undergo latency. The latently infected cells cannot be detected or cleared by the immune system since they do not express viral antigens. These cells remain undetected for several years, even under antiretroviral treatments. The silenced HIV-1 DNA could be reactivated under certain conditions. Despite the efficient use of antiretroviral drugs, HIV-1 latently infected cells remain the major obstacles to a permanent cure. In this review, we discuss the cellular and molecular mechanisms through which HIV-1 establishes latency.

Combination antiretroviral therapy and cancer risk

PURPOSE OF REVIEW

To review the newest research about the effects of combination antiretroviral therapy (cART) on cancer risk.

RECENT FINDINGS

HIV+ persons are at increased risk of cancer. As this risk is higher for malignancies driven by viral and bacterial coinfections, classifying malignancies into infection-related and infection-unrelated has been an emerging trend. Cohorts have detected major reductions in the incidence of Kaposi sarcoma and non-Hodgkin lymphoma (NHL) following cART initiation among immunosuppressed HIV+ persons. However, recent randomized data indicate that cART reduces risk of Kaposi sarcoma and NHL also during early HIV infection before overt immunosuppression occurs. Long-term effects of cART exposure on cancer risk are not well defined; according to basic and epidemiological research, there might be specific associations of each cART class with distinct patterns of cancer risk.

SUMMARY

The relationship between cART exposure and cancer risk is complex and nuanced. It is an intriguing fact that, whether initiated during severe immunosuppression or not, cART reduces risk of Kaposi sarcoma and NHL. Further research should identify mediators of the benefit of immediate cART initiation in reducing cancer risk, understand the relationship between long-term cART exposure and cancer incidence and assess whether adjuvant anti-inflammatory therapies can reduce cancer risk during treated HIV infection.

HIV-1 Vpr reactivates latent HIV-1 provirus by inducing depletion of class I HDACs on chromatin

HIV-1 Vpr is an accessory protein that induces proteasomal degradation of multiple proteins. We recently showed that Vpr targets class I HDACs on chromatin for proteasomal degradation. Here we show that Vpr induces degradation of HDAC1 and HDAC3 in HIV-1 latently infected J-Lat cells. Degradation of HDAC1 and HDAC3 was also observed on the HIV-1 LTR and as a result, markers of active transcription were recruited to the viral promoter and induced viral activation. Knockdown of HDAC1 and HDAC3 activated the latent HIV-1 provirus and complementation with HDAC3 inhibited Vpr-induced HIV-1 reactivation. Viral reactivation and degradation of HDAC1 and HDAC3 was conserved among Vpr proteins of HV-1 group M. Serum Vpr isolated from patients or the release of virion-incorporated Vpr from viral lysates also activated HIV-1 in latently infected cell lines and PBMCs from HIV-1 infected patients. Our results indicate that Vpr counteracts HIV-1 latency by inducing proteasomal degradation of HDAC1 and 3 leading to reactivation of the viral promoter.

HIV-1 Vpr degrades the HLTF DNA translocase in T cells and macrophages

Viruses often interfere with the DNA damage response to better replicate in their hosts. The human immunodeficiency virus 1 (HIV-1) viral protein R (Vpr) protein has been reported to modulate the activity of the DNA repair structure-specific endonuclease subunit (SLX4) complex and to promote cell cycle arrest. Vpr also interferes with the base-excision repair pathway by antagonizing the uracil DNA glycosylase (Ung2) enzyme. Using an unbiased quantitative proteomic screen, we report that Vpr down-regulates helicase-like transcription factor (HLTF), a DNA translocase involved in the repair of damaged replication forks. Vpr subverts the DDB1-cullin4-associated-factor 1 (DCAF1) adaptor of the Cul4A ubiquitin ligase to trigger proteasomal degradation of HLTF. This event takes place rapidly after Vpr delivery to cells, before and independently of Vpr-mediated G2 arrest. HLTF is degraded in lymphocytic cells and macrophages infected with Vpr-expressing HIV-1. Our results reveal a previously unidentified strategy for HIV-1 to antagonize DNA repair in host cells.

RNA Viruses: RNA Roles in Pathogenesis, Coreplication and Viral Load

The review intends to present and recapitulate the current knowledge on the roles and importance of regulatory RNAs, such as microRNAs and small interfering RNAs, RNA binding proteins and enzymes processing RNAs or activated by RNAs, in cells infected by RNA viruses. The review focuses on how non-coding RNAs are involved in RNA virus replication, pathogenesis and host response, especially in retroviruses HIV, with examples of the mechanisms of action, transcriptional regulation, and promotion of increased stability of their targets or their degradation.

HIV-1 Vpr Protein Induces Proteasomal Degradation of Chromatin-associated Class I HDACs to Overcome Latent Infection of Macrophages

Mechanisms underlying HIV-1 latency remain among the most crucial questions that need to be answered to adopt strategies for purging the latent viral reservoirs. Here we show that HIV-1 accessory protein Vpr induces depletion of class I HDACs, including HDAC1, 2, 3, and 8, to overcome latency in macrophages. We found that Vpr binds and depletes chromatin-associated class I HDACs through a VprBP-dependent mechanism, with HDAC3 as the most affected class I HDAC. De novo expression of Vpr in infected macrophages induced depletion of HDAC1 and 3 on the HIV-1 LTR that was associated with hyperacetylation of histones on the HIV-1 LTR. As a result of hyperacetylation of histones on HIV-1 promotor, the virus established an active promotor and this contributed to the acute infection of macrophages. Collectively, HIV-1 Vpr down-regulates class I HDACs on chromatin to counteract latent infections of macrophages.

Functional Roles of the E3 Ubiquitin Ligase UBR5 in Cancer

The Ubiquitin-Proteasome System (UPS) is an important regulator of cell signaling and proteostasis, which are essential to a variety of cellular processes. The UPS is disrupted in many diseases including cancer, and targeting the UPS for cancer therapy is gaining wide interest. E3 ubiquitin ligases occupy a key position in the hierarchical UPS enzymatic cascade, largely responsible for determining substrate specificity and ubiquitin (Ub) chain topology. The E3 ligase UBR5 (aka EDD1) is emerging as a key regulator of the UPS in cancer and development. UBR5 expression is deregulated in many cancer types and UBR5 is frequently mutated in mantle cell lymphoma. UBR5 is highly conserved in metazoans, has unique structural features, and has been implicated in regulation of DNA damage response, metabolism, transcription, and apoptosis. Hence, UBR5 is a key regulator of cell signaling relevant to broad areas of cancer biology. However, the mechanism by which UBR5 may contribute to tumor initiation and progression remains poorly defined. This review synthesizes emerging insights from genetics, biochemistry, and cell biology to inform our understanding of UBR5 in cancer. These molecular insights indicate a role for UBR5 in integrating/coordinating various cellular signaling pathways. Finally, we discuss outstanding questions in UBR5 biology and highlight the need to systematically characterize substrates, and address limitations in current animal models, to better define the role of UBR5 in cancer.

The MLLE domain of the ubiquitin ligase UBR5 binds to its catalytic domain to regulate substrate binding

E3 ubiquitin ligases catalyze the transfer of ubiquitin from an E2-conjugating enzyme to a substrate. UBR5, homologous to the E6AP C terminus (HECT)-type E3 ligase, mediates the ubiquitination of proteins involved in translation regulation, DNA damage response, and gluconeogenesis. In addition, UBR5 functions in a ligase-independent manner by prompting protein/protein interactions without ubiquitination of the binding partner. Despite recent functional studies, the mechanisms involved in substrate recognition and selective ubiquitination of its binding partners remain elusive. The C terminus of UBR5 harbors the HECT catalytic domain and an adjacent MLLE domain. MLLE domains mediate protein/protein interactions through the binding of a conserved peptide motif, termed PAM2. Here, we characterize the binding properties of the UBR5 MLLE domain to PAM2 peptides from Paip1 and GW182. The crystal structure with a Paip1 PAM2 peptide reveals the network of hydrophobic and ionic interactions that drive binding. In addition, we identify a novel interaction of the MLLE domain with the adjacent HECT domain mediated by a PAM2-like sequence. Our results confirm the role of the MLLE domain of UBR5 in substrate recruitment and suggest a potential role in regulating UBR5 ligase activity.

HIV-1 Vpr Protein Enhances Proteasomal Degradation of MCM10 DNA Replication Factor through the Cul4-DDB1[VprBP] E3 Ubiquitin Ligase to Induce G2/M Cell Cycle Arrest

Human immunodeficiency virus type 1 Vpr is an accessory protein that induces G2/M cell cycle arrest. It is well documented that interaction of Vpr with the Cul4-DDB1[VprBP] E3 ubiquitin ligase is essential for the induction of G2/M arrest. In this study, we show that HIV-1 Vpr indirectly binds MCM10, a eukaryotic DNA replication factor, in a Vpr-binding protein (VprBP) (VprBP)-dependent manner. Binding of Vpr to MCM10 enhanced ubiquitination and proteasomal degradation of MCM10. G2/M-defective mutants of Vpr were not able to deplete MCM10, and we show that Vpr-induced depletion of MCM10 is related to the ability of Vpr to induce G2/M arrest. Our study demonstrates that MCM10 is the natural substrate of the Cul4-DDB1[VprBP] E3 ubiquitin ligase whose degradation is regulated by VprBP, but Vpr enhances the proteasomal degradation of MCM10 by interacting with VprBP.

Degradation of SAMHD1 by Vpx Is Independent of Uncoating

Sterile alpha motif domain and HD domain-containing protein 1 (SAMHD1) restricts human immunodeficiency virus type 1 (HIV-1) replication in myeloid and resting T cells. Lentiviruses such as HIV-2 and some simian immunodeficiency viruses (SIVs) counteract the restriction by encoding Vpx or Vpr, accessory proteins that are packaged in virions and which, upon entry of the virus into the cytoplasm, induce the proteasomal degradation of SAMHD1. As a tool to study these mechanisms, we generated HeLa cell lines that express a fusion protein termed NLS.GFP.SAM595 in which the Vpx binding domain of SAMHD1 is fused to the carboxy terminus of green fluorescent protein (GFP) and a nuclear localization signal is fused to the amino terminus of GFP. Upon incubation of Vpx-containing virions with the cells, the NLS.GFP.SAM595 fusion protein was degraded over several hours and the levels remained low over 5 days as the result of continued targeting of the CRL4 E3 ubiquitin ligase. Degradation of the fusion protein required that it contain a nuclear localization sequence. Fusion to the cytoplasmic protein muNS rendered the protein resistant to Vpx-mediated degradation, confirming that SAMHD1 is targeted in the nucleus. Virions treated with protease inhibitors failed to release Vpx, indicating that Gag processing was required for Vpx release from the virion. Mutations in the capsid protein that altered the kinetics of virus uncoating and the Gag binding drug PF74 had no effect on the Vpx-mediated degradation. These results suggest that Vpx is released from virions without a need for uncoating of the capsid, allowing Vpx to transit to the nucleus rapidly upon entry into the cytoplasm.

IMPORTANCE

SAMHD1 restricts lentiviral replication in myeloid cells and resting T cells. Its importance is highlighted by the fact that viruses such as HIV-2 encode an accessory protein that is packaged in the virion and is dedicated to inducing SAMHD1 degradation. Vpx needs to act rapidly upon infection to allow reverse transcription to proceed. The limited number of Vpx molecules in a virion also needs to clear the cell of SAMHD1 over a prolonged period of time. Using an engineered HeLa cell line that expresses a green fluorescent protein (GFP)-SAMHD1 fusion protein, we showed that the Vpx-dependent degradation occurs without a need for viral capsid uncoating. In addition, the fusion protein was degraded only when it was localized to the nucleus, confirming that SAMHD1 is targeted in the nucleus and thus explaining why Vpx also localizes to the nucleus.

Inflammatory co-morbidities in HIV+ individuals: learning lessons from healthy ageing

Increased life expectancy due to improved efficacy of cART has uncovered an increased risk of age-related morbidities in HIV+ individuals and catalyzed significant research into mechanisms driving these diseases. HIV infection increases the risk of non-communicable diseases common in the aged, including cardiovascular disease, neurocognitive decline, non-AIDS malignancies, osteoporosis, and frailty. These observations suggest that HIV accelerates immunological ageing, and there are many immunological similarities with the aged, including shortened telomeres, accumulation of senescent T cells and altered monocyte phenotype/function. However, the most critical similarity between HIV+ individuals and the elderly, which most likely underpins the heightened risk of non-communicable diseases, is chronic inflammation and associated immune activation. Here, we review the similarities between HIV+ individuals and the aged regarding the pathogenesis of inflammatory diseases, the current evidence for mechanisms driving these processes and discuss current and potential therapeutic strategies for addressing inflammatory co-morbidity in HIV+ infection.

The HIV-1 accessory protein Vpr induces the degradation of the anti-HIV-1 agent APOBEC3G through a VprBP-mediated proteasomal pathway

The host anti-HIV-1 factor APOBEC3G (A3G) plays a potential role in restricting HIV-1 replication, although this antagonist can be encountered and disarmed by the Vif protein. In this paper, we report that another HIV-1 accessory protein, viral protein R (Vpr), can interact with A3G and intervene in its antiviral behavior. The interaction of Vpr and A3G was predicted by computer-based screen and confirmed by a co-immunoprecipitation (Co-IP) approach. We found that Vpr could reduce the virion encapsidation of A3G to enhance viral replication. Subsequent experiments showed that Vpr downregulated A3G through Vpr-binding protein (VprBP)-mediated proteasomal degradation, and further confirmed that the reduction of A3G encapsidation associated with Vpr was due to Vpr's degradation-inducing activity. Our findings highlight the versatility of Vpr by unveiling the hostile relationship between Vpr and A3G. In addition, the observation that A3G is targeted to the proteasomal degradation pathway by Vpr in addition to Vif implicates the existence of crosstalk between different HIV-1-host ubiquitin ligase complex systems.

Factors contributing to risk for cancer among HIV-infected individuals, and evidence that earlier combination antiretroviral therapy will alter this risk

PURPOSE OF REVIEW

To critically appraise recent published literature about factors associated with cancer risk likely to be influenced by combination antiretroviral therapy (cART) in HIV-infected individuals, and the potential of earlier cART initiation to reduce this risk.

RECENT FINDINGS

Factors leading to increased risk of non-AIDS-defining malignancies (NADMs) in particular remain poorly understood. Immunodeficiency appears to be key, whereas evidence is emerging that a direct pro-oncogenic effect of HIV, activated inflammatory and coagulation pathways, and cART toxicity may also contribute. By reducing HIV replication, improving immune function, and limiting chronic inflammation, cART initiation at higher CD4 cell counts may, therefore, reduce NADM risk. However, cART only partly normalizes enhanced inflammation and coagulation seen during HIV infection and conflicting laboratory and epidemiological data have been reported as to whether (and how) cART affects NADM risk. Furthermore, secondary analyses of randomized controlled trials comparing early versus delayed cART initiation were inconclusive.

SUMMARY

Continuous epidemiological surveillance is warranted to monitor trends in cancer incidence among HIV-infected individuals and to better understand the impact of earlier cART on NADM risk. The role of adjuvant anti-inflammatory or antithrombotic therapies to reduce cancer risk deserves further investigation.

Aging of the human innate immune system in HIV infection

HIV infection is associated with a chronic inflammatory state arising from multiple factors, including innate immune recognition of HIV, increased microbial translocation, and release of endogenous ligands from damaged cells (such as CD4 T cells). In many respects, this heightened pro-inflammatory environment resembles that associated with aging in the absence of HIV infection, and evidence of dysregulated innate immune responses can be found in not only older HIV-negative adults, but also adults with HIV infection. While the study of innate immune aging in HIV infection is still in its early stages, it seems likely that at least additive, or potentially synergistic effects of aging and HIV infection will be found.

miRNA-1236 inhibits HIV-1 infection of monocytes by repressing translation of cellular factor VprBP

Primary monocytes are refractory to HIV-1 infection and become permissive upon differentiation into monocyte-derived dendritic cells (MDDCs) or macrophages. Multiple mechanisms have been proposed to interpret HIV-1 restriction in monocytes. Human cellular miRNAs can modulate HIV-1 infection by targeting either conserved regions of the HIV-1 genome or host gene transcripts. We have recently reported that the translation of host protein pur-alpha is repressed by abundant cellular miRNAs to inhibit HIV-1 infection in monocytes. Here, we report that the transcript of another cellular factor, VprBP [Vpr (HIV-1)-binding protein], was repressed by cellular miRNA-1236, which contributes to HIV-1 restriction in monocytes. Transfection of miR-1236 inhibitors enhanced translation of VprBP in monocytes and significantly promoted viral infection; exogenous input of synthesized miR-1236 mimics into MDDCs suppressed translation of VprBP, and, accordingly, significantly impaired viral infection. Our data emphasize the role of miRNA in modulating differentiation-dependent susceptibility of the host cell to HIV-1 infection. Understanding the modulation of HIV-1 infection by cellular miRNAs may provide key small RNAs or the identification of new important protein targets regulated by miRNAs for the development of antiviral strategies.

Hyperplastic discs differentially regulates the transcriptional outputs of hedgehog signaling

Hedgehog (Hh) acts as a morphogen to activate the transcription of diverse target genes via its downstream effector Cubitus interruptus (Ci). Currently, it is less understood how Ci recruits co-factors to activate transcription. Here we report that hyperplastic discs (hyd), an E3 ubiquitin ligase, can differentially regulate the transcriptional outputs of Hh signaling. We show that loss of Hyd activity caused upregulation of some, but not all of Hh target genes. Importantly, Hyd does not affect the stability of Ci. Our data suggest that Hyd differentially restrains the transcriptional activity of Ci via selective association with respective promoters.

VprBP (DCAF1): a promiscuous substrate recognition subunit that incorporates into both RING-family CRL4 and HECT-family EDD/UBR5 E3 ubiquitin ligases

The terminal step in the ubiquitin modification system relies on an E3 ubiquitin ligase to facilitate transfer of ubiquitin to a protein substrate. The substrate recognition and ubiquitin transfer activities of the E3 ligase may be mediated by a single polypeptide or may rely on separate subunits. The latter organization is particularly prevalent among members of largest class of E3 ligases, the RING family, although examples of this type of arrangement have also been reported among members of the smaller HECT family of E3 ligases. This review describes recent discoveries that reveal the surprising and distinctive ability of VprBP (DCAF1) to serve as a substrate recognition subunit for a member of both major classes of E3 ligase, the RING-type CRL4 ligase and the HECT-type EDD/UBR5 ligase. The cellular processes normally regulated by VprBP-associated E3 ligases, and their targeting and subversion by viral accessory proteins are also discussed. Taken together, these studies provide important insights and raise interesting new questions regarding the mechanisms that regulate or subvert VprBP function in the context of both the CRL4 and EDD/UBR5 E3 ligases.