Viral avoidance and exploitation of the ubiquitin system

The nucleocapsid protein facilitates p53 ubiquitination-dependent proteasomal degradation via recruiting host ubiquitin ligase COP1 in PEDV infection

Porcine epidemic diarrhea virus (PEDV) is a highly contagious enteric pathogen of the coronavirus family and caused severe economic losses to the global swine industry. Previous studies have established that p53 is a host restriction factor for PEDV infection, and p53 degradation occurs in PEDV-infected cells. However, the underlying molecular mechanisms through which PEDV viral proteins regulate p53 degradation remain unclear. In this study, we found that PEDV infection or expression of the nucleocapsid protein downregulates p53 through a post-translational mechanism: increasing the ubiquitination of p53 and preventing its nuclear translocation. We also show that the PEDV N protein functions by recruiting the E3 ubiquitin ligase COP1 and suppressing COP1 self-ubiquitination and protein degradation, thereby augmenting COP1-mediated degradation of p53. Additionally, COP1 knockdown compromises N-mediated p53 degradation. Functional mapping using truncation analysis showed that the N-terminal domains of N protein were responsible for interacting with COP1 and critical for COP1 stability and p53 degradation. The results presented here suggest the COP1-dependent mechanism for PEDV N protein to abolish p53 activity. This study significantly increases our understanding of PEDV in antagonizing the host antiviral factor p53 and will help initiate novel antiviral strategies against PEDV.

In the moonlight: non-catalytic functions of ubiquitin and ubiquitin-like proteases

Proteases that cleave ubiquitin or ubiquitin-like proteins (UBLs) are critical players in maintaining the homeostasis of the organism. Concordantly, their dysregulation has been directly linked to various diseases, including cancer, neurodegeneration, developmental aberrations, cardiac disorders and inflammation. Given their potential as novel therapeutic targets, it is essential to fully understand their mechanisms of action. Traditionally, observed effects resulting from deficiencies in deubiquitinases (DUBs) and UBL proteases have often been attributed to the misregulation of substrate modification by ubiquitin or UBLs. Therefore, much research has focused on understanding the catalytic activities of these proteins. However, this view has overlooked the possibility that DUBs and UBL proteases might also have significant non-catalytic functions, which are more prevalent than previously believed and urgently require further investigation. Moreover, multiple examples have shown that either selective loss of only the protease activity or complete absence of these proteins can have different functional and physiological consequences. Furthermore, DUBs and UBL proteases have been shown to often contain domains or binding motifs that not only modulate their catalytic activity but can also mediate entirely different functions. This review aims to shed light on the non-catalytic, moonlighting functions of DUBs and UBL proteases, which extend beyond the hydrolysis of ubiquitin and UBL chains and are just beginning to emerge.

Cellular targets and lysine selectivity of the HERC5 ISG15 ligase

ISG15 is a type I interferon-induced ubiquitin-like modifier that functions in innate immune responses. The major human ISG15 ligase is hHERC5, a ribosome-associated HECT E3 that broadly ISGylates proteins cotranslationally. Here, we characterized the hHERC5-dependent ISGylome and identified over 2,000 modified lysines in over 1,100 proteins in IFN-β-stimulated cells. In parallel, we compared the substrate selectivity hHERC5 to the major mouse ISG15 ligase, mHERC6, and analysis of sequences surrounding ISGylation sites revealed that hHERC5 and mHERC6 have distinct preferences for amino acid sequence context. Several features of the datasets were consistent with ISGylation of ribosome-tethered nascent chains, and mHERC6, like hHERC5, cotranslationally modified nascent polypeptides. The ISGylome datasets presented here represent the largest numbers of protein targets and modification sites attributable to a single Ub/Ubl ligase and the lysine selectivities of the hHERC5 and mHERC6 enzymes may have implications for the activities of HECT domain ligases, generally.

Self-stabilization mechanism encoded by a bacterial toxin facilitates reproductive parasitism

A wide variety of maternally transmitted endosymbionts in insects are associated with reproductive parasitism, whereby they interfere with host reproduction to increase the ratio of infected females and spread within populations.1,2 Recent successes in identifying bacterial factors responsible for reproductive parasitism3,4,5,6,7 as well as further omics approaches8,9,10,11,12 have highlighted the common appearance of deubiquitinase domains, although their biological roles-in particular, how they link to distinct manipulative phenotypes-remain poorly defined. Spiroplasma poulsonii is a helical and motile bacterial endosymbiont of Drosophila,13,14 which selectively kills male progeny with a male-killing toxin Spaid (S. poulsonii androcidin), which encodes an ovarian tumor (OTU) deubiquitinase domain.6 Artificial expression of Spaid in flies reproduces male-killing-associated pathologies that include abnormal apoptosis and neural defects during embryogenesis6,15,16,17,18,19; moreover, it highly accumulates on the dosage-compensated male X chromosome,20 congruent with cellular defects such as the DNA damage/chromatin bridge breakage specifically induced upon that chromosome.6,21,22,23 Here, I show that without the function of OTU, Spaid is polyubiquitinated and degraded through the host ubiquitin-proteasome pathway, leading to the attenuation of male-killing activity as shown previously.6 Furthermore, I find that Spaid utilizes its OTU domain to deubiquitinate itself in an intermolecular manner. Collectively, the deubiquitinase domain of Spaid serves as a self-stabilization mechanism to facilitate male killing in flies, optimizing a molecular strategy of endosymbionts that enables the efficient manipulation of the host at a low energetic cost.

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.

Lysine deserts prevent adventitious ubiquitylation of ubiquitin-proteasome components

In terms of its relative frequency, lysine is a common amino acid in the human proteome. However, by bioinformatics we find hundreds of proteins that contain long and evolutionarily conserved stretches completely devoid of lysine residues. These so-called lysine deserts show a high prevalence in intrinsically disordered proteins with known or predicted functions within the ubiquitin-proteasome system (UPS), including many E3 ubiquitin-protein ligases and UBL domain proteasome substrate shuttles, such as BAG6, RAD23A, UBQLN1 and UBQLN2. We show that introduction of lysine residues into the deserts leads to a striking increase in ubiquitylation of some of these proteins. In case of BAG6, we show that ubiquitylation is catalyzed by the E3 RNF126, while RAD23A is ubiquitylated by E6AP. Despite the elevated ubiquitylation, mutant RAD23A appears stable, but displays a partial loss of function phenotype in fission yeast. In case of UBQLN1 and BAG6, introducing lysine leads to a reduced abundance due to proteasomal degradation of the proteins. For UBQLN1 we show that arginine residues within the lysine depleted region are critical for its ability to form cytosolic speckles/inclusions. We propose that selective pressure to avoid lysine residues may be a common evolutionary mechanism to prevent unwarranted ubiquitylation and/or perhaps other lysine post-translational modifications. This may be particularly relevant for UPS components as they closely and frequently encounter the ubiquitylation machinery and are thus more susceptible to nonspecific ubiquitylation.

Spatiotemporal Dynamics of Coastal Viral Community Structure and Potential Biogeochemical Roles Affected by an Ulva prolifera Green Tide

The world's largest macroalgal green tide, caused by Ulva prolifera, has resulted in serious consequences for coastal waters of the Yellow Sea, China. Although viruses are considered to be one of the key factors in controlling microalgal bloom demise, understanding of the relationship between viral communities and the macroalgal green tide is still poor. Here, a Qingdao coastal virome (QDCV) time-series data set was constructed based on the metagenomic analysis of 17 DNA viromes along three coastal stations of the Yellow Sea, covering different stages of the green tide from Julian days 165 to 271. A total of 40,076 viral contigs were detected and clustered into 28,058 viral operational taxonomic units (vOTUs). About 84% of the vOTUs could not be classified, and 62% separated from vOTUs in other ecosystems. Green tides significantly influenced the spatiotemporal dynamics of the viral community structure, diversity, and potential functions. For the classified vOTUs, the relative abundance of Pelagibacter phages declined with the arrival of the bloom and rebounded after the bloom, while Synechococcus and Roseobacter phages increased, although with a time lag from the peak of their hosts. More than 80% of the vOTUs reached peaks in abundance at different specific stages, and the viral peaks were correlated with specific hosts at different stages of the green tide. Most of the viral auxiliary metabolic genes (AMGs) were associated with carbon and sulfur metabolism and showed spatiotemporal dynamics relating to the degradation of the large amount of organic matter released by the green tide. IMPORTANCE To the best of our knowledge, this study is the first to investigate the responses of viruses to the world's largest macroalgal green tide. It revealed the spatiotemporal dynamics of the unique viral assemblages and auxiliary metabolic genes (AMGs) following the variation and degradation of Ulva prolifera. These findings demonstrate a tight coupling between viral assemblages, and prokaryotic and eukaryotic abundances were influenced by the green tide.

HBV X Protein Induces Degradation of UBXN7, a Novel Negative Regulator of NF-κB Signaling, to Promote HBV Replication

Chronic hepatitis B virus (HBV) infection is a leading cause of hepatocellular carcinoma. However, the function and mechanism of the effect of HBV on host protein ubiquitination remain largely unknown. We aimed at characterizing whether and how HBV promotes self-replication by affecting host protein ubiquitination. In this study, we identified UBXN7, a novel inhibitor for nuclear factor kappa B (NF-κB) signaling, was degraded via interaction with HBV X protein (HBx) to activate NF-κB signaling and autophagy, thereby affecting HBV replication. The expression of UBXN7 was analyzed by Western blot and quantitative reverse transcription polymerase chain reaction in HBV-transfected hepatoma cells and HBV-infected primary human hepatocytes (PHHs). The effects of UBXN7 on HBV replication were analyzed by using in vitro and in vivo assays, including stable isotope labeling by amino acids in cell culture (SILAC) analysis. Changes in HBV replication and the associated molecular mechanisms were analyzed in hepatoma cell lines. SILAC analyses showed that the ubiquitination of UBXN7 was significantly increased in HepG2.2.15 cells compared with control cells. After HBV infection, HBx protein interacted with UBXN7 to promote K48-linked ubiquitination of UBXN7 at K99, leading to UBXN7 degradation. On the other hand, UBXN7 interacted with the ULK domain of IκB kinase β through its ubiquitin-associating domain to facilitate its degradation. This in turn reduced NF-κB signaling, leading to reduced autophagy and consequently decreased HBV replication.

African Swine Fever Virus pI215L Inhibits Type I Interferon Signaling by Targeting Interferon Regulatory Factor 9 for Autophagic Degradation

African swine fever virus (ASFV) is the etiological agent of a highly lethal hemorrhagic disease in domestic pigs and wild boars that has significant economic consequences for the pig industry. The type I interferon (IFN) signaling pathway is a pivotal component of the innate antiviral response, and ASFV has evolved multiple mechanisms to antagonize this pathway and facilitate infection. Here, we reported a novel function of ASFV pI215L in inhibiting type I IFN signaling. Our results showed that ASFV pI215L inhibited IFN-stimulated response element (ISRE) promoter activity and subsequent transcription of IFN-stimulated genes (ISGs) by triggering interferon regulatory factor 9 (IRF9) degradation. Additionally, we found that catalytically inactive pI215L mutations retained the ability to block type I IFN signaling, indicating that this only known viral E2 ubiquitin-conjugating enzyme mediates IFR9 degradation in a ubiquitin-conjugating activity-independent manner. By coimmunoprecipitation, confocal immunofluorescence, and subcellular fractionation approaches, we demonstrated that pI215L interacted with IRF9 and impaired the formation and nuclear translocation of IFN-stimulated gene factor 3 (ISGF3). Moreover, further mechanism studies supported that pI215L induced IRF9 degradation through the autophagy-lysosome pathway in both pI215L-overexpressed and ASFV-infected cells. These findings reveal a new immune evasion strategy evolved by ASFV in which pI215L acts to degrade host IRF9 via the autophagic pathway, thus inhibiting the type I IFN signaling and counteracting the host innate immune response.

IMPORTANCE African swine fever virus (ASFV) causes a highly contagious and lethal disease in pigs and wild boars that is currently present in many countries, severely affecting the global pig industry. Despite extensive research, effective vaccines and antiviral strategies are still lacking, and many fundamental questions regarding the molecular mechanisms underlying host innate immunity escape remain unclear. In this study, we identified ASFV pI215L, the only known viral E2 ubiquitin-conjugating enzyme, which is involved in antagonizing the type I interferon signaling. Mechanistically, pI215L interacted with interferon regulatory factor 9 for autophagic degradation, and this degradation was independent of its ubiquitin-conjugating activity. These results increase the current knowledge regarding ASFV evasion of innate immunity, which may instruct future research on antiviral strategies and dissection of ASFV pathogenesis.

Enzymes in the time of COVID-19: An overview about the effects in the human body, enzyme market, and perspectives for new drugs

The rising pandemic caused by a coronavirus, resulted in a scientific quest to discover some effective treatments against its etiologic agent, the severe acute respiratory syndrome‐coronavirus 2 (SARS‐CoV‐2). This research represented a significant scientific landmark and resulted in many medical advances. However, efforts to understand the viral mechanism of action and how the human body machinery is subverted during the infection are still ongoing. Herein, we contributed to this field with this compilation of the roles of both viral and human enzymes in the context of SARS‐CoV‐2 infection. In this sense, this overview reports that proteases are vital for the infection to take place: from SARS‐CoV‐2 perspective, the main protease (M^pro) and papain‐like protease (PL^pro) are highlighted; from the human body, angiotensin‐converting enzyme‐2, transmembrane serine protease‐2, and cathepsins (CatB/L) are pointed out. In addition, the influence of the virus on other enzymes is reported as the JAK/STAT pathway and the levels of lipase, enzymes from the cholesterol metabolism pathway, amylase, aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, and glyceraldehyde 3‐phosphate dehydrogenase are also be disturbed in SARS‐CoV‐2 infection. Finally, this paper discusses the importance of detailed enzymatic studies for future treatments against SARS‐CoV‐2, and how some issues related to the syndrome treatment can create opportunities in the biotechnological market of enzymes and the development of new drugs.

A bioinformatic approach of targeting SARS-CoV-2 replication by silencing a conserved alternative reserve of the orf8 gene using host miRNAs

The causative agent of the COVID-19 pandemic, the SARS-CoV-2 virus has yielded multiple relevant mutations, many of which have branched into major variants. The Omicron variant has a huge similarity with the original viral strain (first COVID-19 strain from Wuhan). Among different genes, the highly variable orf8 gene is responsible for crucial host interactions and has undergone multiple mutations and indels. The sequence of the orf8 gene of the Omicron variant is, however, identical with the gene sequence of the wild type. orf8 modulates the host immunity making it easier for the virus to conceal itself and remain undetected. Variants seem to be deleting this gene without affecting the viral replication. While analyzing, we came across the conserved orf7a gene in the viral genome which exhibits a partial sequence homology as well as functional similarity with the SARS-CoV-2 orf8. Hence, we have proposed here in our hypothesis that, orf7a might be an alternative reserve of orf8 present in the virus which was compensating for the lost gene. A computational approach was adopted where we screened various miRNAs targeted against the orf8 gene. These miRNAs were then docked onto the orf8 mRNA sequences. The same set of miRNAs was then used to check for their binding affinity with the orf7a reference mRNA. Results showed that miRNAs targeting the orf8 had favorable shape complementarity and successfully docked with the orf7a gene as well. These findings provide a basis for developing new therapeutic approaches where both orf8 and orf7a can be targeted simultaneously.

Upregulated Proteasome Subunits in COVID-19 Patients: A Link with Hypoxemia, Lymphopenia and Inflammation

Severe COVID-19 disease leads to hypoxemia, inflammation and lymphopenia. Viral infection induces cellular stress and causes the activation of the innate immune response. The ubiquitin-proteasome system (UPS) is highly implicated in viral immune response regulation. The main function of the proteasome is protein degradation in its active form, which recognises and binds to ubiquitylated proteins. Some proteasome subunits have been reported to be upregulated under hypoxic and hyperinflammatory conditions. Here, we conducted a prospective cohort study of COVID-19 patients (n = 44) and age-and sex-matched controls (n = 20). In this study, we suggested that hypoxia could induce the overexpression of certain genes encoding for subunits from the α and β core of the 20S proteasome and from regulatory particles (19S and 11S) in COVID-19 patients. Furthermore, the gene expression of proteasome subunits was associated with lymphocyte count reduction and positively correlated with inflammatory molecular and clinical markers. Given the importance of the proteasome in maintaining cellular homeostasis, including the regulation of the apoptotic and pyroptotic pathways, these results provide a potential link between COVID-19 complications and proteasome gene expression.

N Protein of Viral Hemorrhagic Septicemia Virus Suppresses STAT1-Mediated MHC Class II Transcription to Impair Antigen Presentation in Sea Perch, Lateolabrax japonicus

Upon virus invasion of the host, APCs process Ags to short peptides for presentation by MHC class II (MHC-II). The recognition of virus-derived peptides in the context of MHC-II by CD4⁺ T cells initiates the adaptive immune response for virus clearance. As a survival instinct, viruses have evolved mechanisms to evade Ag processing and presentation. In this study, we discovered that IFN-γ induced endogenous MHC-II expression by a sea perch brain cell line through the STAT1/IFN regulatory factor 1 (IRF1)/CIITA signaling pathway. Furthermore, viral hemorrhagic septicemia virus infection significantly inhibited the IFN-γ-induced expression of IRF1, CIITA, MHC-II-α, and MHC-II-β genes. By contrast, although STAT1 transcript was upregulated, paradoxically, the STAT1 protein level was attenuated. Moreover, overexpression analysis revealed that viral hemorrhagic septicemia virus N protein blocked the IFN-γ-induced expression of IRF1, CIITA, MHC-II-α, and MHC-II-β genes, but not the STAT1 gene. We also found out that N protein interacted with STAT1 and enhanced the overall ubiquitination level of proteins, including STAT1 in Lateolabrax japonicus brain cells. Enhanced ubiquitination of STAT1 through K48-linked ubiquitination led to its degradation through the ubiquitin-proteasome pathway, thereby inhibiting the biological function of STAT1. Our study suggests that aquatic viruses target Ag presentation in lower vertebrates for immune evasion as do mammalian viruses.

The OH system: A panorama view of the PPV-host interaction

Poxviruses are a family of specialized cytoplasm-parasitic DNA viruses that replicate and assembly in virus factory. In Parapoxvirus (PPV) genus, with the orf virus (ORFV) as a representative species of this genus, their behaviors are significantly different from that of Orthopoxvirus, and the plots of viral practical solutions for evading host immunity are intricate and fascinating, particularly to anti-host and host's antiviral mechanisms. In order to protect the virus factory from immune elimination caused by infection, PPVs attempt to interfere with multiple stress levels of host, mainly by modulating innate immunity response (IIR) and adaptive immunity response (AIR). Given that temporarily constructed by virus infection, ORFV-HOST (OH) system accompanied by viral strategies is carefully managed in the virus factory, thus directing many life-critical events once undergoing the IIR and AIR. Evolutionarily, to reduce the risk of system destruction, ORFV have evolved into a mild-looking mode to avoid overstimulation. Moreover, the current version of development also focus on recognizing and hijacking more than eight antiviral security mechanisms of host cells, such as the 2',5'-oligoadenylate synthetase (OAS)/RNase L and PKR systems, the ubiquitin protease system (UPS), and so on. In summary, this review assessed inescapable pathways as mentioned above, through which viruses compete with their hosts strategically. The OH system provides a panoramic view and a powerful platform for us to study the PPV-Host interaction, as well as the corresponding implications on a great application potential in anti-virus design.

A viral ubiquitination switch attenuates innate immunity and triggers nuclear import of virion DNA and infection

Antiviral defense and virus exclusion from the cell nucleus restrict foreign nucleic acid influx and infection. How the genomes of DNA viruses evade cytosolic pattern recognition and cross the nuclear envelope is incompletely understood. Here, we show that the virion protein V of adenovirus functions as a linchpin between the genome and the capsid, thereby securing particle integrity. Absence of protein V destabilizes cytoplasmic particles and promotes premature genome release, raising cytokine levels through the DNA sensor cGAS. Non-ubiquitinable V yields stable virions, genome misdelivery to the cytoplasm, and increased cytokine levels. In contrast, normal protein V is ubiquitinated at the nuclear pore complex, dissociates from the virion depending on the E3 ubiquitin ligase Mib1 and the proteasome, and allows genome delivery into the nucleus for infection. Our data uncover previously unknown cellular and viral mechanisms of viral DNA nuclear import in pathogenesis, vaccination, gene therapy, and synthetic biology.

Rotaviral nonstructural protein 5 (NSP5) promotes proteasomal degradation of up-frameshift protein 1 (UPF1), a principal mediator of nonsense-mediated mRNA decay (NMD) pathway, to facilitate infection

Nonsense-mediated mRNA decay (NMD), a cellular RNA quality system, has been shown to be an ancestral form of cellular antiviral response that can restrict viral infection by targeting viral RNA for degradation or other various mechanisms. In support to this hypothesis, emerging evidences unraveled that viruses have evolved numerous mechanisms to circumvent or modulate the NMD pathway to ensure unhindered replication within the host cell. In this study, we investigated the potential interplay between the cellular NMD pathway and rotavirus (RV). Our data suggested that rotavirus infection resulted in global inhibition of NMD pathway by downregulating the expression of UPF1 in a strain independent manner. UPF1 expression was found to be regulated at the post-transcriptional level by ubiquitin-proteasome mediated degradation pathway. Subsequent studies revealed rotaviral non-structural protein 5 (NSP5) associates with UPF1 and promotes its cullin-dependent proteasome mediated degradation. Furthermore, ectopic expression of UPF1 during RV infection resulted in reduced expression of viral proteins and viral RNAs leading to diminished production of infective rotavirus particles, suggesting the anti-rotaviral role of UPF1. Finally, the delayed degradation kinetics of transfected rotaviral RNA in UPF1 and UPF2 depleted cells and the association of UPF1 and UPF2 with viral RNAs suggested that NMD targets rotaviral RNAs for degradation. Collectively, the present study demonstrates the antiviral role of NMD pathway during rotavirus infection and also reveals the underlying mechanism by which rotavirus overwhelms NMD pathway to establish successful replication.

Poxvirus Interactions with the Host Ubiquitin System

The ubiquitin system has emerged as a master regulator of many, if not all, cellular functions. With its large repertoire of conjugating and ligating enzymes, the ubiquitin system holds a unique mechanism to provide selectivity and specificity in manipulating protein function. As intracellular parasites viruses have evolved to modulate the cellular environment to facilitate replication and subvert antiviral responses. Poxviruses are a large family of dsDNA viruses with large coding capacity that is used to synthetise proteins and enzymes needed for replication and morphogenesis as well as suppression of host responses. This review summarises our current knowledge on how poxvirus functions rely on the cellular ubiquitin system, and how poxviruses exploit this system to their own advantage, either facilitating uncoating and genome release and replication or rewiring ubiquitin ligases to downregulate critical antiviral factors. Whilst much remains to be known about the intricate interactions established between poxviruses and the host ubiquitin system, our knowledge has revealed crucial viral processes and important restriction factors that open novel avenues for antiviral treatment and provide fundamental insights on the biology of poxviruses and other virus families.

Structural insights into Cullin4-RING ubiquitin ligase remodelling by Vpr from simian immunodeficiency viruses

Viruses have evolved means to manipulate the host's ubiquitin-proteasome system, in order to down-regulate antiviral host factors. The Vpx/Vpr family of lentiviral accessory proteins usurp the substrate receptor DCAF1 of host Cullin4-RING ligases (CRL4), a family of modular ubiquitin ligases involved in DNA replication, DNA repair and cell cycle regulation. CRL4DCAF1 specificity modulation by Vpx and Vpr from certain simian immunodeficiency viruses (SIV) leads to recruitment, poly-ubiquitylation and subsequent proteasomal degradation of the host restriction factor SAMHD1, resulting in enhanced virus replication in differentiated cells. To unravel the mechanism of SIV Vpr-induced SAMHD1 ubiquitylation, we conducted integrative biochemical and structural analyses of the Vpr protein from SIVs infecting Cercopithecus cephus (SIVmus). X-ray crystallography reveals commonalities between SIVmus Vpr and other members of the Vpx/Vpr family with regard to DCAF1 interaction, while cryo-electron microscopy and cross-linking mass spectrometry highlight a divergent molecular mechanism of SAMHD1 recruitment. In addition, these studies demonstrate how SIVmus Vpr exploits the dynamic architecture of the multi-subunit CRL4DCAF1 assembly to optimise SAMHD1 ubiquitylation. Together, the present work provides detailed molecular insight into variability and species-specificity of the evolutionary arms race between host SAMHD1 restriction and lentiviral counteraction through Vpx/Vpr proteins.

Decoding the messaging of the ubiquitin system using chemical and protein probes

Post-translational modification of proteins by ubiquitin is required for nearly all aspects of eukaryotic cell function. The numerous targets of ubiquitylation, and variety of ubiquitin modifications, are often likened to a code, where the ultimate messages are diverse responses to target ubiquitylation. E1, E2, and E3 multiprotein enzymatic assemblies modify specific targets and thus function as messengers. Recent advances in chemical and protein tools have revolutionized our ability to explore the ubiquitin system, through enabling new high-throughput screening methods, matching ubiquitylation enzymes with their cellular targets, revealing intricate allosteric mechanisms regulating ubiquitylating enzymes, facilitating structural revelation of transient assemblies determined by multivalent interactions, and providing new paradigms for inhibiting and redirecting ubiquitylation in vivo as new therapeutics. Here we discuss the development of methods that control, disrupt, and extract the flow of information across the ubiquitin system and have enabled elucidation of the underlying molecular and cellular biology.

How Influenza Virus Uses Host Cell Pathways during Uncoating

Influenza is a zoonotic respiratory disease of major public health interest due to its pandemic potential, and a threat to animals and the human population. The influenza A virus genome consists of eight single-stranded RNA segments sequestered within a protein capsid and a lipid bilayer envelope. During host cell entry, cellular cues contribute to viral conformational changes that promote critical events such as fusion with late endosomes, capsid uncoating and viral genome release into the cytosol. In this focused review, we concisely describe the virus infection cycle and highlight the recent findings of host cell pathways and cytosolic proteins that assist influenza uncoating during host cell entry.

From Recoding to Peptides for MHC Class I Immune Display: Enriching Viral Expression, Virus Vulnerability and Virus Evasion

Many viruses, especially RNA viruses, utilize programmed ribosomal frameshifting and/or stop codon readthrough in their expression, and in the decoding of a few a UGA is dynamically redefined to specify selenocysteine. This recoding can effectively increase viral coding capacity and generate a set ratio of products with the same N-terminal domain(s) but different C-terminal domains. Recoding can also be regulatory or generate a product with the non-universal 21st directly encoded amino acid. Selection for translation speed in the expression of many viruses at the expense of fidelity creates host immune defensive opportunities. In contrast to host opportunism, certain viruses, including some persistent viruses, utilize recoding or adventitious frameshifting as part of their strategy to evade an immune response or specific drugs. Several instances of recoding in small intensively studied viruses escaped detection for many years and their identification resolved dilemmas. The fundamental importance of ribosome ratcheting is consistent with the initial strong view of invariant triplet decoding which however did not foresee the possibility of transitory anticodon:codon dissociation. Deep level dynamics and structural understanding of recoding is underway, and a high level structure relevant to the frameshifting required for expression of the SARS CoV-2 genome has just been determined.

RaFAH: Host prediction for viruses of Bacteria and Archaea based on protein content

Culture-independent approaches have recently shed light on the genomic diversity of viruses of prokaryotes. One fundamental question when trying to understand their ecological roles is: which host do they infect? To tackle this issue we developed a machine-learning approach named Random Forest Assignment of Hosts (RaFAH), that uses scores to 43,644 protein clusters to assign hosts to complete or fragmented genomes of viruses of Archaea and Bacteria. RaFAH displayed performance comparable with that of other methods for virus-host prediction in three different benchmarks encompassing viruses from RefSeq, single amplified genomes, and metagenomes. RaFAH was applied to assembled metagenomic datasets of uncultured viruses from eight different biomes of medical, biotechnological, and environmental relevance. Our analyses led to the identification of 537 sequences of archaeal viruses representing unknown lineages, whose genomes encode novel auxiliary metabolic genes, shedding light on how these viruses interfere with the host molecular machinery. RaFAH is available at https://sourceforge.net/projects/rafah/.

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RaFAH was developed to predict the hosts of viruses of Bacteria and Archaea

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RaFAH displayed comparable or superior performance to other host-prediction tools

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RaFAH performed well across viromes from eight different ecosystems

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RaFAH identified hundreds of genomic sequences as derived from viruses of Archaea

Viruses that infect Bacteria and Archaea are ubiquitous and extremely abundant. Recent advances have led to the discovery of many thousands of complete and partial genomes of these biological entities. Understanding the biology of these viruses and how they influence their ecosystems depends on knowing which hosts they infect. We developed a tool that uses data from complete or fragmented genomes to predict the hosts of viruses using a machine-learning approach. Our tool, RaFAH, displayed performance comparable with or superior to that of other host-prediction tools. In addition, it identified hundreds of sequences as derived from the genomes of viruses of Archaea, which are one of the least characterized fractions of the global virosphere.

Advances in the Development of Antiviral Compounds for Rotavirus Infections

Group A rotaviruses (RVAs) are the major cause of severe acute gastroenteritis (AGE) in children under 5 years of age, annually resulting in nearly 130,000 deaths worldwide. Social conditions in developing countries that contribute to decreased oral rehydration and vaccine efficacy and the lack of approved antiviral drugs position RVA as a global health concern. In this minireview, we present an update in the field of antiviral compounds, mainly in relation to the latest findings in RVA virion structure and the viral replication cycle. In turn, we attempt to provide a perspective on the possible treatments for RVA-associated AGE, with special focus on novel approaches, such as those representing broad-spectrum therapeutic options. In this context, the modulation of host factors, lipid droplets, and the viral polymerase, which is highly conserved among AGE-causing viruses, are analyzed as possible drug targets.

Role of Virally-Encoded Deubiquitinating Enzymes in Regulation of the Virus Life Cycle

The ubiquitin (Ub) proteasome system (UPS) plays a pivotal role in regulation of numerous cellular processes, including innate and adaptive immune responses that are essential for restriction of the virus life cycle in the infected cells. Deubiquitination by the deubiquitinating enzyme, deubiquitinase (DUB), is a reversible molecular process to remove Ub or Ub chains from the target proteins. Deubiquitination is an integral strategy within the UPS in regulating survival and proliferation of the infecting virus and the virus-invaded cells. Many viruses in the infected cells are reported to encode viral DUB, and these vial DUBs actively disrupt cellular Ub-dependent processes to suppress host antiviral immune response, enhancing virus replication and thus proliferation. This review surveys the types of DUBs encoded by different viruses and their molecular processes for how the infecting viruses take advantage of the DUB system to evade the host immune response and expedite their replication.

A swine arterivirus deubiquitinase stabilizes two major envelope proteins and promotes production of viral progeny

Arteriviruses are enveloped positive-strand RNA viruses that assemble and egress using the host cell’s exocytic pathway. In previous studies, we demonstrated that most arteriviruses use a unique -2 ribosomal frameshifting mechanism to produce a C-terminally modified variant of their nonstructural protein 2 (nsp2). Like full-length nsp2, the N-terminal domain of this frameshift product, nsp2TF, contains a papain-like protease (PLP2) that has deubiquitinating (DUB) activity, in addition to its role in proteolytic processing of replicase polyproteins. In cells infected with porcine reproductive and respiratory syndrome virus (PRRSV), nsp2TF localizes to compartments of the exocytic pathway, specifically endoplasmic reticulum-Golgi intermediate compartment (ERGIC) and Golgi complex. Here, we show that nsp2TF interacts with the two major viral envelope proteins, the GP5 glycoprotein and membrane (M) protein, which drive the key process of arterivirus assembly and budding. The PRRSV GP5 and M proteins were found to be poly-ubiquitinated, both in an expression system and in cells infected with an nsp2TF-deficient mutant virus. In contrast, ubiquitinated GP5 and M proteins did not accumulate in cells infected with the wild-type, nsp2TF-expressing virus. Further analysis implicated the DUB activity of the nsp2TF PLP2 domain in deconjugation of ubiquitin from GP5/M proteins, thus antagonizing proteasomal degradation of these key viral structural proteins. Our findings suggest that nsp2TF is targeted to the exocytic pathway to reduce proteasome-driven turnover of GP5/M proteins, thus promoting the formation of GP5-M dimers that are critical for arterivirus assembly.

Arteriviruses are a rapidly expanding family of positive-stranded RNA viruses, which includes economically important veterinary pathogens like equine arteritis virus (EAV) and two species of porcine reproductive and respiratory syndrome virus (PRRSV-1 and PRRSV-2). In our previous studies, we uncovered an unprecedented arterivirus gene expression mechanism: a highly efficient -2 programmed ribosomal frameshift (PRF) that is controlled by an interaction of viral protein nsp1ß with specific RNA sequences and host poly(C) binding proteins. It is used by PRRSVs, and most other arteriviruses, to efficiently produce a previously unknown nonstructural protein variant, nsp2TF. In this study, we demonstrate that PRRSV nsp2TF interacts with the two major arteriviral envelope proteins, GP5 and M, whose heterodimerization in the secretory pathway is a key step in envelope protein trafficking and virus assembly. Our findings suggest that nsp2TF promotes arterivirus assembly by antagonizing the ubiquitination-dependent proteasomal degradation of GP5 and M proteins. This mechanism is based on the DUB activity of the PLP2 protease domain located within the N-terminal region of nsp2TF. To our knowledge, this is the first study to demonstrate that viruses can express a DUB that functions specifically to counteract the ubiquitination and degradation of key viral structural proteins.

The Functional Classification of ORF8 in SARS-CoV-2 Replication, Immune Evasion, and Viral Pathogenesis Inferred through Phylogenetic Profiling

ORF8 is a highly variable genomic region of SARS-CoV-2. Although non-essential and the precise functions are unknown, it has been suggested that this protein assists in SARS-CoV-2 replication in the early secretory pathway and in immune evasion. We utilized the binding partners of SARS-CoV-2 proteins in human HEK293T cells and performed genome-wide phylogenetic profiling and clustering analyses in 446 eukaryotic species to predict and discover ORF8 binding partners that share associated functional mechanisms based on co-evolution. Results classified 47 ORF8 binding partner proteins into 3 clusters (groups 1-3), which were conserved in vertebrates (group 1), metazoan (group 2), and eukaryotes (group 3). Gene ontology analysis indicated that group 1 had no significant associated biological processes, while groups 2 and 3 were associated with glycoprotein biosynthesis process and ubiquitin-dependent endoplasmic reticulum-associated degradation pathways, respectively. Collectively, our results classified potential genes that might be associated with SARS-CoV-2 viral pathogenesis, specifically related to acute respiratory distress syndrome, and the secretory pathway. Here, we discuss the possible role of ORF8 in viral pathogenesis and in assisting viral replication and immune evasion via secretory pathway, as well as the possible factors associated with the rapid evolution of ORF8.

Comparative Proteomics of Ostreid Herpesvirus 1 and Pacific Oyster Interactions With Two Families Exhibiting Contrasted Susceptibility to Viral Infection

Massive mortality outbreaks affecting Pacific oysters (Crassostrea gigas) spat/juveniles are often associated with the detection of a herpesvirus called ostreid herpesvirus type 1 (OsHV-1). In this work, experimental infection trials of C. gigas spat with OsHV-1 were conducted using two contrasted Pacific oyster families for their susceptibility to viral infection. Live oysters were sampled at 12, 26, and 144 h post infection (hpi) to analyze host-pathogen interactions using comparative proteomics. Shotgun proteomics allowed the detection of seven viral proteins in infected oysters, some of them with potential immunomodulatoy functions. Viral proteins were mainly detected in susceptible oysters sampled at 26 hpi, which correlates with the mortality and viral load observed in this oyster family. Concerning the Pacific oyster proteome, more than 3,000 proteins were identified and contrasted proteomic responses were observed between infected A- and P-oysters, sampled at different post-injection times. Gene ontology (GO) and KEGG pathway enrichment analysis performed on significantly modulated proteins uncover the main immune processes (such as RNA interference, interferon-like pathway, antioxidant defense) which contribute to the defense and resistance of Pacific oysters to viral infection. In the more susceptible Pacific oysters, results suggest that OsHV-1 manipulate the molecular machinery of host immune response, in particular the autophagy system. This immunomodulation may lead to weakening and consecutively triggering death of Pacific oysters. The identification of several highly modulated and defense-related Pacific oyster proteins from the most resistant oysters supports the crucial role played by the innate immune system against OsHV-1 and the viral infection. Our results confirm the implication of proteins involved in an interferon-like pathway for efficient antiviral defenses and suggest that proteins involved in RNA interference process prevent viral replication in C. gigas. Overall, this study shows the interest of multi-omic approaches applied on groups of animals with differing sensitivities and provides novel insight into the interaction between Pacific oyster and OsHV-1 with key proteins involved in viral infection resistance.

Structural Characterization of SARS-CoV-2: Where We Are, and Where We Need to Be

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has rapidly spread in humans in almost every country, causing the disease COVID-19. Since the start of the COVID-19 pandemic, research efforts have been strongly directed towards obtaining a full understanding of the biology of the viral infection, in order to develop a vaccine and therapeutic approaches. In particular, structural studies have allowed to comprehend the molecular basis underlying the role of many of the SARS-CoV-2 proteins, and to make rapid progress towards treatment and preventive therapeutics. Despite the great advances that have been provided by these studies, many knowledge gaps on the biology and molecular basis of SARS-CoV-2 infection still remain. Filling these gaps will be the key to tackle this pandemic, through development of effective treatments and specific vaccination strategies.

The ubiquitin proteasome system is necessary for efficient proliferation of porcine reproductive and respiratory syndrome virus

The ubiquitin-proteasome system (UPS) plays a vital role in cellular protein homeostasis by ensuring protein quality control and maintaining a critical level of important regulatory proteins. Thus, it is not surprising that the functional UPS is manipulated by viruses to assist in viral propagation. Porcine reproductive and respiratory syndrome virus (PRRSV) is an economically significant swine disease that has been devastating the swine industry worldwide. However, the role of UPS in PRRSV infection is unknown. In this study, we found that treatment with the proteasome inhibitor MG132 significantly inhibited PRRSV proliferation in a dose-dependent manner. The anti-PRRSV effect of MG132 was most significant in the middle stage of the PRRSV lifecycle, which is achieved via inhibition of viral attachment and replication. Interestingly, the expression of poly-ubiquitin was drastically decreased and the accumulation of free-ubiquitin was obviously elevated in the middle stage of PRRSV infection. Furthermore, the ectopic expression of ubiquitin in MG132-treated cells partially reversed the inhibitory effect of MG132 on PRRSV proliferation. Taken together, these results suggest that PRRSV manipulates UPS to promote self-proliferation by cheating or taking advantage of the host proteasome, degrading intracellular poly-ubiquitin and increasing the accumulation of free ubiquitin.

Ubiquitination modification: critical regulation of IRF family stability and activity

Interferon regulatory factors (IRFs) play pivotal and critical roles in innate and adaptive immune responses; thus, precise and stringent regulation of the stability and activation of IRFs in physiological processes is necessary. The stability and activities of IRFs are directly or indirectly targeted by endogenous and exogenous proteins in an ubiquitin-dependent manner. However, few reviews have summarized how host E3 ligases/DUBs or viral proteins regulate IRF stability and activity. Additionally, with recent technological developments, details about the ubiquitination of IRFs have been continuously revealed. As knowledge of how these proteins function and interact with IRFs may facilitate a better understanding of the regulation of IRFs in immune responses or other biological processes, we summarized current studies on the direct ubiquitination of IRFs, with an emphasis on how these proteins interact with IRFs and affect their activities, which may provide exciting targets for drug development by regulating the functions of specific E3 ligases.

Viral entry and the ubiquitin-proteasome system

Viruses confiscate cellular components of the ubiquitin-proteasome system (UPS) to facilitate many aspects of the infectious cycle. The 26S proteasome is an ATP-dependent, multisubunit proteolytic machine present in all eukaryotic cells. The proteasome executes the controlled degradation of functional proteins, as well as the hydrolysis of aberrantly folded polypeptides. There is growing evidence for the role of the UPS in viral entry. The UPS assists in several steps of the initiation of infection, including endosomal escape of the entering virion, intracellular transport of incoming nucleocapsids and uncoating of the viral genome. Inhibitors of proteasome activity, including MG132, epoxomicin, lactacystin and bortezomib have been integral to developments in this area. Here, we review the mechanistic details of UPS involvement in the entry process of viruses from a multitude of families. The possibility of proteasome inhibitors as therapeutic antiviral agents is highlighted.

VPTMdb: a viral posttranslational modification database

In viruses, posttranslational modifications (PTMs) are essential for their life cycle. Recognizing viral PTMs is very important for a better understanding of the mechanism of viral infections and finding potential drug targets. However, few studies have investigated the roles of viral PTMs in virus-human interactions using comprehensive viral PTM datasets. To fill this gap, we developed the first comprehensive viral posttranslational modification database (VPTMdb) for collecting systematic information of PTMs in human viruses and infected host cells. The VPTMdb contains 1240 unique viral PTM sites with 8 modification types from 43 viruses (818 experimentally verified PTM sites manually extracted from 150 publications and 422 PTMs extracted from SwissProt) as well as 13 650 infected cells' PTMs extracted from seven global proteomics experiments in six human viruses. The investigation of viral PTM sequences motifs showed that most viral PTMs have the consensus motifs with human proteins in phosphorylation and five cellular kinase families phosphorylate more than 10 viral species. The analysis of protein disordered regions presented that more than 50% glycosylation sites of double-strand DNA viruses are in the disordered regions, whereas single-strand RNA and retroviruses prefer ordered regions. Domain-domain interaction analysis indicating potential roles of viral PTMs play in infections. The findings should make an important contribution to the field of virus-human interaction. Moreover, we created a novel sequence-based classifier named VPTMpre to help users predict viral protein phosphorylation sites. VPTMdb online web server (http://vptmdb.com:8787/VPTMdb/) was implemented for users to download viral PTM data and predict phosphorylation sites of interest.

Turnip yellow mosaic virus protease binds ubiquitin suboptimally to fine-tune its deubiquitinase activity

Single-stranded, positive-sense RNA viruses assemble their replication complexes in infected cells from a multidomain replication polyprotein. This polyprotein usually contains at least one protease, the primary function of which is to process the polyprotein into mature proteins. Such proteases also may have other functions in the replication cycle. For instance, cysteine proteases (PRO) frequently double up as ubiquitin hydrolases (DUB), thus interfering with cellular processes critical for virus replication. We previously reported the crystal structures of such a PRO/DUB from Turnip yellow mosaic virus (TYMV) and of its complex with one of its PRO substrates. Here we report the crystal structure of TYMV PRO/DUB in complex with ubiquitin. We find that PRO/DUB recognizes ubiquitin in an unorthodox way: It interacts with the body of ubiquitin through a split recognition motif engaging both the major and the secondary recognition patches of ubiquitin (Ile⁴⁴ patch and Ile³⁶ patch, respectively, including Leu⁸, which is part of the two patches). However, the contacts are suboptimal on both sides. Introducing a single-point mutation in TYMV PRO/DUB aimed at improving ubiquitin-binding led to a much more active DUB. Comparison with other PRO/DUBs from other viral families, particularly coronaviruses, suggests that low DUB activities of viral PRO/DUBs may generally be fine-tuned features of interaction with host factors.

Tobacco RNA-dependent RNA polymerase 1 affects the expression of defence-related genes in Nicotiana benthamiana upon Tomato leaf curl Gujarat virus infection

MAIN CONCLUSION

RNA-dependent RNA polymerase 1 of Nicotiana tabacum modulates ToLCGV pathogenesis by influencing a number of defence-related genes in N. benthamiana plants. Key means of plants protecting themselves from the invading viruses is through RNA silencing. RNA-dependent RNA polymerase-1 (RDR1) is one of the crucial proteins of the RNA silencing pathway, which is induced after infection by viruses. RDR1 functions in the generation of small interfering RNAs (siRNAs) against the viral genome, thus it is antiviral in nature. Here, we used the transgenic Nicotiana benthamiana plant expressing N. tabacum NtRDR1 and observed reduced susceptibility towards Tomato leaf curl Gujarat virus (ToLCGV) infection compared to the wild-type N. benthamiana plants. To understand the reason for such reduced susceptibility, we prepared high-definition small RNA (sRNA) cDNA libraries from ToLCGV-infected wild-type N. benthamiana and NtRDR1 expressing N. benthamiana lines and carried out next-generation sequencing (NGS). We found that upon ToLCGV infection the majority of siRNAs generated from the host genome were of the 24 nucleotide (nt) class, while viral siRNAs (vsiRNAs) were of the 21-22-nt class, indicating that transcriptional gene silencing (TGS) is the major pathway for silencing of host genes while viral genes are silenced, predominantly, by post transcriptional gene silencing (PTGS) pathways. We estimated the changes in the expression of various defence-related genes, such as Constitutively Photomorphogenic-9 (COP9) signalosome (CSN) complex subunit-7, Pentatricopeptide repeat containing protein (PPRP), Laccase-3, Glutathione peroxidase-1 (GPX-1), Universal stress protein (USP) A-like protein, Heat shock transcription factor B4 (HSTF-B4), Auxin response factor-18 (ARF18), WRKY-6 and Short chain dehydrogenase reductase-3a. The differential expression of these genes might be linked with the enhanced tolerance of NtRDR1 N. benthamiana transgenic plants to ToLCGV. Our study suggests that reduced expression of subunit-7 of CSN complex and WRKY6, and increased expression of USPA-like protein might be linked with the reduced susceptibility of NtRDR1-transgenic N. benthamiana plants to ToLCGV.

A Bunyavirus-Inducible Ubiquitin Ligase Targets RNA Polymerase IV for Degradation during Viral Pathogenesis in Rice

The ubiquitin-proteasome system (UPS) is an important post-translational regulatory mechanism that controls many cellular functions in eukaryotes. Here, we show that stable expression of P3 protein encoded by Rice grassy stunt virus (RGSV), a negative-strand RNA virus in the Bunyavirales, causes developmental abnormities similar to the disease symptoms caused by RGSV, such as dwarfing and excess tillering, in transgenic rice plants. We found that both transgenic expression of P3 and RGSV infection induce ubiquitination and UPS-dependent degradation of rice NUCLEAR RNA POLYMERASE D1a (OsNRPD1a), one of two orthologs of the largest subunit of plant-specific RNA polymerase IV (Pol IV), which is required for RNA-directed DNA methylation (RdDM). Furthermore, we identified a P3-inducible U-box type E3 ubiquitin ligase, designated as P3-inducible protein 1 (P3IP1), which interacts with OsNRPD1a and mediates its ubiquitination and UPS-dependent degradation in vitro and in vivo. Notably, both knockdown of OsNRPD1 and overexpression of P3IP1 in rice plants induced developmental phenotypes similar to RGSV disease symptomss. Taken together, our findings reveal a novel virulence mechanism whereby plant pathogens target host RNA Pol IV for UPS-dependent degradation to induce disease symptoms. Our study also identified an E3 ubiquitin ligase, which targets the RdDM compotent NRPD1 for UPS-mediated degradation in rice.

MARCH8 Ubiquitinates the Hepatitis C Virus Nonstructural 2 Protein and Mediates Viral Envelopment

The mechanisms that regulate envelopment of HCV and other viruses that bud intracellularly and/or lack late-domain motifs are largely unknown. We reported that K63 polyubiquitination of the HCV nonstructural (NS) 2 protein mediates HRS (ESCRT-0 component) binding and envelopment. Nevertheless, the ubiquitin signaling that governs NS2 ubiquitination remained unknown. Here, we map the NS2 interactome with the ubiquitin proteasome system (UPS) via mammalian cell-based screens. NS2 interacts with E3 ligases, deubiquitinases, and ligase regulators, some of which are candidate proviral or antiviral factors. MARCH8, a RING-finger E3 ligase, catalyzes K63-linked NS2 polyubiquitination in vitro and in HCV-infected cells. MARCH8 is required for infection with HCV, dengue, and Zika viruses and specifically mediates HCV envelopment. Our data reveal regulation of HCV envelopment via ubiquitin signaling and both a viral protein substrate and a ubiquitin K63-linkage of the understudied MARCH8, with potential implications for cell biology, virology, and host-targeted antiviral design.

The mechanisms that regulate intracellular viral envelopment are unknown. Kumar et al. report that MARCH8 catalyzes K63-linked polyubiquitination of the HCV nonstructural 2 protein and subsequently ESCRT recruitment and HCV envelopment. MARCH8 is required for infection with other Flaviviridae family members, thereby representing a potential host target for antiviral strategies.

Hepatitis B Virus HBx Protein Mediates the Degradation of Host Restriction Factors through the Cullin 4 DDB1 E3 Ubiquitin Ligase Complex

The hepatitis B virus (HBV) regulatory HBx protein is required for infection, and its binding to cellular damaged DNA binding protein 1 (DDB1) is critical for this function. DDB1 is an adaptor protein for the cullin 4A Really Interesting New Gene (RING) E3 ubiquitin ligase (CRL4) complex and functions by binding cellular DDB1 cullin associated factor (DCAF) receptor proteins that recruit substrates for ubiquitination and degradation. We compared the proteins found in the CRL4 complex immunoprecipitated from uninfected versus HBV-infected hepatocytes from human liver chimeric mice for insight into mechanisms by which HBV and the cell interact within the CRL4 complex. Consistent with its role as a viral DCAF, HBx was found in the HBV CRL4 complexes. In tissue culture transfection experiments, we showed that HBx expression led to decreased levels of known restriction factor structural maintenance of chromosomes protein 6 (SMC6) and putative restriction factors stromal interaction molecule 1 (STIM1, zinc finger E-box binding homeobox 2 (ZEB2), and proteasome activator subunit 4 (PSME4). Moreover, silencing of these proteins led to increased HBV replication in the HepG2-sodium taurocholate cotransporting polypeptide (NTCP) infection model. We also identified cellular DCAF receptors in CRL4 complexes from humanized mice. Increasing amounts of HBx did not reveal competitive DCAF binding to cullin4 (CUL4)-DDB1 in plasmid-transfected cells. Our results suggest a model in which HBx benefits virus replication by directly or indirectly degrading multiple cellular restriction factors.

Host factor prioritization for pan-viral genetic perturbation screens using random intercept models and network propagation

Genetic perturbation screens using RNA interference (RNAi) have been conducted successfully to identify host factors that are essential for the life cycle of bacteria or viruses. So far, most published studies identified host factors primarily for single pathogens. Furthermore, often only a small subset of genes, e.g., genes encoding kinases, have been targeted. Identification of host factors on a pan-pathogen level, i.e., genes that are crucial for the replication of a diverse group of pathogens has received relatively little attention, despite the fact that such common host factors would be highly relevant, for instance, for devising broad-spectrum anti-pathogenic drugs. Here, we present a novel two-stage procedure for the identification of host factors involved in the replication of different viruses using a combination of random effects models and Markov random walks on a functional interaction network. We first infer candidate genes by jointly analyzing multiple perturbations screens while at the same time adjusting for high variance inherent in these screens. Subsequently the inferred estimates are spread across a network of functional interactions thereby allowing for the analysis of missing genes in the biological studies, smoothing the effect sizes of previously found host factors, and considering a priori pathway information defined over edges of the network. We applied the procedure to RNAi screening data of four different positive-sense single-stranded RNA viruses, Hepatitis C virus, Chikungunya virus, Dengue virus and Severe acute respiratory syndrome coronavirus, and detected novel host factors, including UBC, PLCG1, and DYRK1B, which are predicted to significantly impact the replication cycles of these viruses. We validated the detected host factors experimentally using pharmacological inhibition and an additional siRNA screen and found that some of the predicted host factors indeed influence the replication of these pathogens.

Degradation of proteins by PROTACs and other strategies

Blocking the biological functions of scaffold proteins and aggregated proteins is a challenging goal. PROTAC proteolysis-targeting chimaera (PROTAC) technology may be the solution, considering its ability to selectively degrade target proteins. Recent progress in the PROTAC strategy include identification of the structure of the first ternary eutectic complex, extra-terminal domain-4-PROTAC-Von-Hippel-Lindau (BRD4-PROTAC-VHL), and PROTAC ARV-110 has entered clinical trials for the treatment of prostate cancer in 2019. These discoveries strongly proved the value of the PROTAC strategy. In this perspective, we summarized recent meaningful research of PROTAC, including the types of degradation proteins, preliminary biological data in vitro and in vivo, and new E3 ubiquitin ligases. Importantly, the molecular design, optimization strategy and clinical application of candidate molecules are highlighted in detail. Future perspectives for development of advanced PROTAC in medical fields have also been discussed systematically.

Characterizing the PRRSV nsp2 Deubiquitinase Reveals Dispensability of Cis-Activity for Replication and a Link of nsp2 to Inflammation Induction

The papain-like cysteine protease 2 (PLP2) within the N-terminus of the porcine reproductive and respiratory syndrome virus (PRRSV) nsp2 replicase protein specifies a deubiquitinating enzyme (DUB), but its biochemical properties and the role in infection have remained poorly defined. By using in vitro assays, we found that the purified PLP2 could efficiently cleave K63 and K48 linked polyubiquitin chains Ub3-7 in vitro although displaying a differential activity in converting the respective ubiquitin dimers to monomer. The subsequent mutagenesis analyses revealed that the requirement for PLP2 DUB activity surprisingly resembled that for cis-cleavage activity, as several mutations (e.g., D91R, D85R, etc.) that largely ablated the DUB function also blocked the cis- but not trans-proteolytic cleavage of nsp2/3 polyprotein. Moreover, the analyses identified key mutations that could differentiate DUB from PLP2 cis- and trans-cleavage activities. Further reverse genetics analyses revealed the following findings: (i) mutations that largely blocked the DUB activity were all lethal to the virus, (ii) a point mutation T88G that selectively blocked the cis-cleavage activity of PLP2 did not affect viral viability in cell culture, and (iii) an E90Q mutation that did not affect either of the PLP2 activities led to rescue of WT-like virus but displayed significantly reduced ability to induce TNF-α production. Our findings support the possibility that the PLP2 DUB activity, but not cis-cleavage activity, is essential for PRRSV replication. The data also establish a strong link of nsp2 to pro-inflammatory cytokine induction during infection that operates in a manner independent of PLP2 DUB activity.

An Update on African Swine Fever Virology

Animal diseases constitute a continuing threat to animal health, food safety, national economy, and the environment. Among those, African swine fever (ASF) is one of the most devastating viruses affecting pigs and wild suids due to the lack of vaccine or effective treatment. ASF is endemic in countries in sub-Saharan Africa, but since its introduction to the Caucasus region in 2007, a highly virulent strain of ASF virus (ASFV) has continued to circulate and spread into Eastern Europe and Russia, and most recently into Western Europe, China, and various countries of Southeast Asia. Given the importance of this disease, this review will highlight recent discoveries in basic virology with special focus on proteomic analysis, replication cycle, and some recent data on genes involved in cycle progression and viral–host interactions, such as I215L (E2 ubiquitin-conjugating enzyme), EP402R (CD2v), A104R (histone-like protein), QP509L, and Q706L (RNA helicases) or P1192R (Topoisomerase II). Taking into consideration the large DNA genome of ASFV and its complex interactions with the host, more studies and new approaches are to be taken to understand the basic virus–host interaction for ASFV. Proteomic studies are just paving the way for future research.

Cucurbit Chlorotic Yellows Virus p22 Protein Interacts with Cucumber SKP1LB1 and Its F-Box-Like Motif Is Crucial for Silencing Suppressor Activity

Plants use RNA silencing as a defense against viruses. In response, viruses encode various RNA silencing suppressors to counteract the antiviral silencing. Here, we identified p22 as a silencing suppressor of cucurbit chlorotic yellows crinivirus and showed that p22 interacts with CsSKP1LB1, a Cucumis sativus ortholog of S-phase kinase-associated protein 1 (SKP1). The F-box-like motif of p22 was identified through sequence analysis and found to be necessary for the interaction using a yeast two-hybrid assay. The involvement of the F-box-like motif in p22 silencing suppressor activity was determined. Proteomics analysis of Nicotiana benthamiana leaves expressing p22, and its F-box-like motif deletion mutant showed 228 differentially expressed proteins and five enriched Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways: ABC transporters, sesquiterpenoid and triterpenoid biosynthesis, ubiquitin-mediated proteolysis, riboflavin metabolism, and cysteine and methionine metabolism. Collectively, our results demonstrate the interaction between p22 and CsSKP1LB1 and show that the deletion of F-box-like motif inhibits p22 silencing suppressor activity. The possible pathways regulated by the p22 through the F-box-like motif were identified using proteomics analysis.

African swine fever virus replication events and cell nucleus: New insights and perspectives

African swine fever (ASF) is currently matter for major concerns in global swine industry as it is highly contagious and causes acute fatal haemorrhagic fever in domestic pigs and wild boar. The absence of effective vaccines and treatments pushes ASF control to relay on strict sanitary and stamping out measures with costly socio-economic impacts. The current epidemic scenario of fast spreading throughout Asiatic countries impels further studies on prevention and combat strategies against ASF. Herein we review knowledge on African Swine Fever Virus (ASFV) interactions with the host cell nucleus and on the functional properties of different viral DNA-replication related proteins. This entails, the confirmation of an intranuclear viral DNA replication phase, the characterization of cellular DNA damage responses (DDR), the subnuclear compartments disruption due to viral modulation, and the unravelling of the biological role of several viral proteins (A104R, I215 L, P1192R, QP509 L and Q706 L), so to contribute to underpin rational strategies for vaccine candidates development.

Human cytomegalovirus evades antibody-mediated immunity through endoplasmic reticulum-associated degradation of the FcRn receptor

Human cytomegalovirus (HCMV) can persistently infect humans, but how HCMV avoids humoral immunity is not clear. The neonatal Fc receptor (FcRn) controls IgG transport from the mother to the fetus and prolongs IgG half-life. Here we show that US11 inhibits the assembly of FcRn with β₂m and retains FcRn in the endoplasmic reticulum (ER), consequently blocking FcRn trafficking to the endosome. Furthermore, US11 recruits the ubiquitin enzymes Derlin-1, TMEM129 and UbE2J2 to engage FcRn, consequently initiating the dislocation of FcRn from the ER to the cytosol and facilitating its degradation. Importantly, US11 inhibits IgG-FcRn binding, resulting in a reduction of IgG transcytosis across intestinal or placental epithelial cells and IgG degradation in endothelial cells. Hence, these results identify the mechanism by which HCMV infection exploits an ER-associated degradation pathway through US11 to disable FcRn functions. These results have implications for vaccine development and immune surveillance.

Human cytomegalovirus (HCMV) can persist for the life of a host in the face of robust immune responses owing to a wide range of immune evasion strategies. Here Liu and colleagues show that HCMV evades the IgG-mediated response by the endoplasmic reticulum-associated degradation of the neonatal Fc receptor for IgG.

Stabilization of E2-EPF UCP protein is implicated in hepatitis B virus-associated hepatocellular carcinoma progression

Hepatitis B virus (HBV) X protein (HBx) is associated with hepatocarcinogenesis. E2-EPF ubiquitin carrier protein (UCP) catalyzes ubiquitination of itself and von Hippel-Lindau protein (pVHL) for degradation and associates with tumor growth and metastasis. However, it remains unknown whether HBx modulates the enzyme activity of UCP and thereby influences hepatocarcinogenesis. Here, we show that UCP is highly expressed in liver tissues of HBx-transgenic mice, but not non-transgenic mice. UCP was more frequently expressed in HBV-positive liver cancers than in HBV-negative liver cancers. HBx binds to UCP specifically and serotype independently, and forms a ternary complex with UCP and pVHL. HBx inhibits self-ubiquitination of UCP, but enhances UCP-mediated pVHL ubiquitination, resulting in stabilization of hypoxia-inducible factor-1α and -2α. HBx and UCP stabilize each other by mutually inhibiting their ubiquitination. HBx promotes cellular proliferation and metastasis via UCP. Our findings suggest that UCP plays a key role in HBV-related hepatocarcinogenesis.

Cleavage of the Babuvirus Movement Protein B4 into Functional Peptides Capable of Host Factor Conjugation is Required for Virulence

Banana bunchy top virus (BBTV) poses a serious danger to banana crops worldwide. BBTV-encoded protein B4 is a determinant of pathogenicity. However, the relevant molecular mechanisms underlying its effects remain unknown. In this study, we found that a functional peptide could be liberated from protein B4, likely via proteolytic processing. Site-directed mutagenesis indicated that the functional processing of protein B4 is required for its pathogenic effects, including dwarfism and sterility, in plants. The released protein fragment targets host proteins, such as the large subunit of RuBisCO (RbcL) and elongation factor 2 (EF2), involved in protein synthesis. Therefore, the peptide released from B4 (also a precursor) may act as a non-canonical modifier to influence host-pathogen interactions involving BBTV and plants.

E6AP goes viral: the role of E6AP in viral- and non-viral-related cancers

Since its discovery, the E3 ubiquitin ligase E6-associated protein (E6AP) has been studied extensively in two pathological contexts: infection by the human papillomavirus (HPV), and the neurodevelopmental disorder, Angelman syndrome. Vital biological links between E6AP and other viruses, namely hepatitis C virus and encephalomyocarditis virus, have been recently uncovered. Critically, oncogenic E6AP activities have been demonstrated to contribute to cancers of both viral and non-viral origins. HPV-associated cancers serve as the primary example of E6AP involvement in cancers driven by viruses. Studies over the past few years have exposed a role for E6AP in non-viral-related cancers. This has been demonstrated in B-cell lymphoma and prostate cancers, where oncogenic E6AP functions drive these cancers by acting on key tumour suppressors. In this review we discuss the role of E6AP in viral infection, viral propagation and viral-related cancer. We discuss processes affected by oncogenic E6AP, which promote cancers of viral and non-viral aetiology. Overall, recent findings support the role of oncogenic E6AP in disrupting key cellular processes, including tumour suppression and the immune response. E6AP is consequently emerging as an attractive therapeutic target for a number of specific cancers.

The host cell ubiquitin ligase protein CHIP is a potent suppressor of HIV-1 replication

Human immunodeficiency virus-1 (HIV-1) Tat is degraded in the host cell both by proteasomal and lysosomal pathways, but the specific molecules that engage with Tat from these pathways are not known. Because E3 ubiquitin ligases are the primary determinants of substrate specificity within the ubiquitin-dependent proteasomal degradation of proteins, we first sought to identify the E3 ligase associated with Tat degradation. Based on the intrinsic disordered nature of Tat protein, we focused our attention on host cell E3 ubiquitin ligase CHIP (C terminus of HSP70-binding protein). Co-transfection of Tat with a CHIP-expressing plasmid decreased the levels of Tat protein in a dose-dependent manner, without affecting the corresponding mRNA levels. Additionally, the rate of Tat protein degradation as measured by cycloheximide (CHX) chase assay was increased in the presence of CHIP. A CHIP mutant lacking the U-box domain, which is responsible for protein ubiquitination (CHIPΔU-box), was unable to degrade Tat protein. Furthermore, CHIP promoted ubiquitination of Tat by both WT as well as Lys-48-ubiquitin, which has only a single lysine residue at position 48. CHIP transfection in HIV-1 reporter TZM-bl cells resulted in decreased Tat-dependent HIV-1 long-terminal repeat (LTR) promoter transactivation as well as HIV-1 virion production. CHIP knockdown in HEK-293T cells using CRISPR-Cas9 led to higher virion production and enhanced Tat-mediated HIV-1 LTR promoter transactivation, along with stabilization of Tat protein. Together, these results suggest a novel role of host cell E3 ubiquitin ligase protein CHIP in regulating HIV-1 replication through ubiquitin-dependent degradation of its regulatory protein Tat.

A Pathogen and a Non-pathogen Spotted Fever Group Rickettsia Trigger Differential Proteome Signatures in Macrophages

We have previously reported that Rickettsia conorii and Rickettsia montanensis have distinct intracellular fates within THP-1 macrophages, suggesting that the ability to proliferate within macrophages may be a distinguishable factor between pathogenic and non-pathogenic Spotted fever group (SFG) members. To start unraveling the molecular mechanisms underlying the capacity (or not) of SFG Rickettsia to establish their replicative niche in macrophages, we have herein used quantitative proteomics by SWATH-MS to profile the alterations resulted by the challenge of THP-1 macrophages with R. conorii and R. montanensis. We show that the pathogenic, R. conorii, and the non-pathogenic, R. montanensis, member of SFG Rickettsia trigger differential proteomic signatures in macrophage-like cells upon infection. R. conorii specifically induced the accumulation of several enzymes of the tricarboxylic acid cycle, oxidative phosphorylation, fatty acid β-oxidation, and glutaminolysis, as well as of several inner and outer membrane mitochondrial transporters. These results suggest a profound metabolic rewriting of macrophages by R. conorii toward a metabolic signature of an M2-like, anti-inflammatory activation program. Moreover, several subunits forming the proteasome and immunoproteasome are found in lower abundance upon infection with both rickettsial species, which may help bacteria to escape immune surveillance. R. conorii-infection specifically induced the accumulation of several host proteins implicated in protein processing and quality control in ER, suggesting that this pathogenic Rickettsia may be able to increase the ER protein folding capacity. This work reveals novel aspects of macrophage-Rickettsia interactions, expanding our knowledge of how pathogenic rickettsiae explore host cells to their advantage.