A novel cellular protein, VPEF, facilitates vaccinia virus penetration into HeLa cells through fluid phase endocytosis

The vaccinia chondroitin sulfate binding protein drives host membrane curvature to facilitate fusion

Cellular attachment of viruses determines their cell tropism and species specificity. For entry, vaccinia, the prototypic poxvirus, relies on four binding proteins and an eleven-protein entry fusion complex. The contribution of the individual virus binding proteins to virion binding orientation and membrane fusion is unclear. Here, we show that virus binding proteins guide side-on virion binding and promote curvature of the host membrane towards the virus fusion machinery to facilitate fusion. Using a membrane-bleb model system together with super-resolution and electron microscopy we find that side-bound vaccinia virions induce membrane invagination in the presence of low pH. Repression or deletion of individual binding proteins reveals that three of four contribute to binding orientation, amongst which the chondroitin sulfate binding protein, D8, is required for host membrane bending. Consistent with low-pH dependent macropinocytic entry of vaccinia, loss of D8 prevents virion-associated macropinosome membrane bending, disrupts fusion pore formation and infection. Our results show that viral binding proteins are active participants in successful virus membrane fusion and illustrate the importance of virus protein architecture for successful infection.

Veratramine inhibits porcine epidemic diarrhea virus entry through macropinocytosis by suppressing PI3K/Akt pathway

Porcine epidemic diarrhea (PED) is a contagious intestinal disease caused by α-coronavirus porcine epidemic diarrhea virus (PEDV). At present, no effective vaccine is available to prevent the disease. Therefore, research for novel antivirals is important. This study aimed to identify the antiviral mechanism of Veratramine (VAM), which actively inhibits PEDV replication with a 50 % inhibitory concentration (IC50) of ∼5 µM. Upon VAM treatment, both PEDV-nucleocapsid (N) protein level and virus titer decreased significantly. The time-of-addition assay results showed that VAM could inhibit PEDV replication by blocking viral entry. Importantly, VAM could inhibit PEDV-induced phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) activity and further suppress micropinocytosis, which is required for PEDV entry. In addition, PI3K inhibitor LY294002 showed anti-PEDV activity by blocking viral entry as well. Taken together, VAM possessed anti-PEDV properties against the entry stage of PEDV by inhibiting the macropinocytosis pathway by suppressing the PI3K/Akt pathway. VAM could be considered as a lead compound for the development of anti-PEDV drugs and may be used during the viral entry stage of PEDV infection.

Single particle tracking reveals SARS-CoV-2 regulating and utilizing dynamic filopodia for viral invasion

Although severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) entry mechanism has been explored, little is known about how SARS-CoV-2 regulates the subcellular structural remodeling to invade multiple organs and cell types. Here, we unveil how SARS-CoV-2 boosts and utilizes filopodia to enter the target cells by real-time imaging. Using SARS-CoV-2 single virus-like particle (VLP) tracking in live cells and sparse deconvolution algorithm, we uncover that VLPs utilize filopodia to reach the entry site in two patterns, "surfing" and "grabbing", which avoid the virus from randomly searching on the plasma membrane. Moreover, combining mechanical simulation, we elucidate that the formation of virus-induced filopodia and the retraction speed of filopodia depend on cytoskeleton dynamics and friction resistance at the substrate surface caused by loading-virus gravity, respectively. Further, we discover that the entry process of SARS-CoV-2 via filopodia depends on Cdc42 activity and actin-associated proteins fascin, formin, and Arp2/3. Together, our results highlight that the spatial-temporal regulation of actin cytoskeleton by SARS-CoV-2 infection makes filopodia as a highway for virus entry and potentiates it as an antiviral target.

FAM21 is critical for TLR2/CLEC4E-mediated dendritic cell function against Candida albicans

FAM21 (family with sequence similarity 21) is a component of the Wiskott-Aldrich syndrome protein and SCAR homologue (WASH) protein complex that mediates actin polymerization at endosomal membranes to facilitate sorting of cargo-containing vesicles out of endosomes. To study the function of FAM21 in vivo, we generated conditional knockout (cKO) mice in the C57BL/6 background in which FAM21 was specifically knocked out of CD11c-positive dendritic cells. BMDCs from those mice displayed enlarged early endosomes, and altered cell migration and morphology relative to WT cells. FAM21-cKO cells were less competent in phagocytosis and protein antigen presentation in vitro, though peptide antigen presentation was not affected. More importantly, we identified the TLR2/CLEC4E signaling pathway as being down-regulated in FAM21-cKO BMDCs when challenged with its specific ligand Candida albicans Moreover, FAM21-cKO mice were more susceptible to C. albicans infection than WT mice. Reconstitution of WT BMDCs in FAM21-cKO mice rescued them from lethal C. albicans infection. Thus, our study highlights the importance of FAM21 in a host immune response against a significant pathogen.

Widespread Distribution and Evolution of Poxviral Entry-Fusion Complex Proteins in Giant Viruses

Poxviruses are known to encode a set of proteins that form an entry-fusion complex (EFC) to mediate virus entry. However, the diversity, evolution, and origin of these EFC proteins remain poorly understood. Here, we identify the EFC protein homologs in poxviruses and other giant viruses of the phylum Nucleocytoviricota. The 11 EFC genes are present in almost all poxviruses, with the two smallest, G3 and O3, being absent in Entomopoxvirinae and basal lineages of Chordopoxvirinae. Five of the EFC genes are further grouped into two families, A16/G9/J5 and F9/L1, which are widely distributed across other major lineages of Nucleocytoviricota, including metagenome-assembled genomes, but are generally absent in viruses infecting algae or nonamoebozoan heterotrophic protists. The A16/G9/J5 and F9/L1 families cooccur, mostly as single copies, in 93% of the non-Poxviridae giant viruses that have at least one of them. Distribution and phylogenetic patterns suggest that both families originated in the ancestor of Nucleocytoviricota. In addition to the Poxviridae genes, homologs from each of the other Nucleocytoviricota families are largely clustered together, suggesting their ancient presence and vertical inheritance. Despite deep sequence divergences, we observed noticeable conservation of cysteine residues and predicted structures between EFC proteins of Poxviridae and other families. Overall, our study reveals widespread distribution of these EFC protein homologs beyond poxviruses, implies the existence of a conserved membrane fusion mechanism, and sheds light on host range and ancient evolution of Nucleocytoviricota. IMPORTANCE Fusion between virus and host membranes is critical for viruses to release genetic materials and to initiate infection. Whereas most viruses use a single protein for membrane fusion, poxviruses employ a multiprotein entry-fusion complex (EFC). We report that two major families of the EFC proteins are widely distributed within the virus phylum Nucleocytoviricota, which includes poxviruses and other double-stranded (dsDNA) giant viruses that infect animals, amoebozoans, algae, and various microbial eukaryotes. Each of these two protein families is structurally conserved, traces its origin to the root of Nucleocytoviricota, was passed down to the major subclades of Nucleocytoviricota, and is retained in most giant viruses known to infect animals and amoebozoans. The EFC proteins therefore represent a potential mechanism for virus entry in diverse giant viruses. We hypothesize that they may have facilitated the infection of an animal/amoebozoan-like host by the last Nucleocytoviricota common ancestor.

Identification of β2 microglobulin, the product of B2M gene, as a Host Factor for Vaccinia Virus Infection by Genome-Wide CRISPR genetic screens

Genome-wide genetic screens are powerful tools to identify genes that act as host factors of viruses. We have applied this technique to analyze the infection of HeLa cells by Vaccinia virus, in an attempt to find genes necessary for infection. Infection of cell populations harboring single gene inactivations resulted in no surviving cells, suggesting that no single gene knock-out was able to provide complete resistance to Vaccinia virus and thus allow cells to survive infection. In the absence of an absolute infection blockage, we explored if some gene inactivations could provide partial protection leading to a reduced probability of infection. Multiple experiments using modified screening procedures involving replication restricted viruses led to the identification of multiple genes whose inactivation potentially increase resistance to infection and therefore cell survival. As expected, significant gene hits were related to proteins known to act in virus entry, such as ITGB1 and AXL as well as genes belonging to their downstream related pathways. Additionally, we consistently found β2-microglobulin, encoded by the B2M gene, among the screening top hits, a novel finding that was further explored. Inactivation of B2M resulted in 54% and 91% reduced VV infection efficiency in HeLa and HAP1 cell lines respectively. In the absence of B2M, while virus binding to the cells was unaffected, virus internalization and early gene expression were significantly diminished. These results point to β2-microglobulin as a relevant factor in the Vaccinia virus entry process.

Poxviruses package viral redox proteins in lateral bodies and modulate the host oxidative response

All poxviruses contain a set of proteinaceous structures termed lateral bodies (LB) that deliver viral effector proteins into the host cytosol during virus entry. To date, the spatial proteotype of LBs remains unknown. Using the prototypic poxvirus, vaccinia virus (VACV), we employed a quantitative comparative mass spectrometry strategy to determine the poxvirus LB proteome. We identified a large population of candidate cellular proteins, the majority being mitochondrial, and 15 candidate viral LB proteins. Strikingly, one-third of these are VACV redox proteins whose LB residency could be confirmed using super-resolution microscopy. We show that VACV infection exerts an anti-oxidative effect on host cells and that artificial induction of oxidative stress impacts early and late gene expression as well as virion production. Using targeted repression and/or deletion viruses we found that deletion of individual LB-redox proteins was insufficient for host redox modulation suggesting there may be functional redundancy. In addition to defining the spatial proteotype of VACV LBs, these findings implicate poxvirus redox proteins as potential modulators of host oxidative anti-viral responses and provide a solid starting point for future investigations into the role of LB resident proteins in host immunomodulation.

Myosin motors on the pathway of viral infections

Molecular motors are microscopic machines that use energy from adenosine triphosphate (ATP) hydrolysis to generate movement. While kinesins and dynein are molecular motors associated with microtubule tracks, myosins bind to and move on actin filaments. Mammalian cells express several myosin motors. They power cellular processes such as endo- and exocytosis, intracellular trafficking, transcription, migration, and cytokinesis. As viruses navigate through cells, they may take advantage or be hindered by host components and machinery, including the cytoskeleton. This review delves into myosins' cell roles and compares them to their reported functions in viral infections. In most cases, the previously described myosin functions align with their reported role in viral infections, although not in all cases. This opens the possibility that knowledge obtained from studying myosins in viral infections might shed light on new physiological roles for myosins in cells. However, given the high number of myosins expressed and the variety of viruses investigated in the different studies, it is challenging to infer whether the interactions found are specific to a single virus or can be applied to other viruses with the same characteristics. We conclude that the participation of myosins in viral cycles is still a largely unexplored area, especially concerning unconventional myosins.

The emerging roles of retromer and sorting nexins in the life cycle of viruses

Retromer and sorting nexins (SNXs) transport cargoes from endosomes to the trans-Golgi network or plasma membrane. Recent studies have unveiled the emerging roles for retromer and SNXs in the life cycle of viruses, including members of Coronaviridae, Flaviviridae and Retroviridae. Key components of retromer/SNXs, such as Vps35, Vps26, SNX5 and SNX27, can affect multiple steps of the viral life cycle, including facilitating the entry of viruses into cells, participating in viral replication, and promoting the assembly of virions. Here we present a comprehensive updated review on the interplay between retromer/SNXs and virus, which will shed mechanistic insights into controlling virus infection.

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Retromer/SNXs could regulate viral infection directly or indirectly.

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Retromer/SNXs plays an important role for SARS-CoV-2 infection.

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HPV entry is mediated by retromer/SNXs.

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Retromer is required for HCV replication.

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Retromer affects the late step of HIV replication.

FAM21C Promotes Hepatocellular Carcinoma Invasion and Metastasis by Driving Actin Cytoskeleton Remodeling via Inhibiting Capping Ability of CAPZA1

Hepatocellular carcinoma (HCC) is characterized by a high incidence of metastasis. The dynamic remodeling of the actin cytoskeleton plays an important role in the invasion and migration of HCC cells. In previous studies, we found that CAPZA1, a capping protein, can promote EMT of HCC cells by regulating the remodeling of the actin filament (F-actin) cytoskeleton, thus promoting the invasion and migration of HCC cells. In this study, we found that FAM21C may have a regulatory effect on CAPZA1, and we conducted an in-depth study on its potential regulatory mechanism. First, we found that FAM21C is highly expressed in HCC tissues and its high expression could promote the malignant progression of HCC. Meanwhile, the high expression of FAM21C promoted the invasion and migration of HCC cells in vitro and in vivo. Further, FAM21C interacted with CAPZA1, and their binding inhibited the capping capacity of CAPZA1, thus promoting the invasion and migration of HCC cells. This effect of FAM21C was abolished by mutating the CP-interacting (CPI) domain, the CAPZA1 binding site on FAM21C. In conclusion, high expression of FAM21C in HCC tissues can promote malignant progression of HCC and its potential mechanism involves FAM21C inhibition of CAPZA1 capping capacity by binding to CAPZA1, which drives F-actin cytoskeleton remodeling, and thus promotes invasion and migration of HCC cells.

Membrane Rafts: Portals for Viral Entry

Membrane rafts are dynamic, small (10-200 nm) domains enriched with cholesterol and sphingolipids that compartmentalize cellular processes. Rafts participate in roles essential to the lifecycle of different viral families including virus entry, assembly and/or budding events. Rafts seem to participate in virus attachment and recruitment to the cell surface, as well as the endocytic and non-endocytic mechanisms some viruses use to enter host cells. In this review, we will introduce the specific role of rafts in viral entry and define cellular factors implied in the choice of one entry pathway over the others. Finally, we will summarize the most relevant information about raft participation in the entry process of enveloped and non-enveloped viruses.

Hepatitis B Virus Entry into Cells

Hepatitis B virus (HBV), an enveloped partially double-stranded DNA virus, is a widespread human pathogen responsible for more than 250 million chronic infections worldwide. Current therapeutic strategies cannot eradicate HBV due to the persistence of the viral genome in a special DNA structure (covalently closed circular DNA, cccDNA). The identification of sodium taurocholate co-transporting polypeptide (NTCP) as an entry receptor for both HBV and its satellite virus hepatitis delta virus (HDV) has led to great advances in our understanding of the life cycle of HBV, including the early steps of infection in particular. However, the mechanisms of HBV internalization and the host factors involved in this uptake remain unclear. Improvements in our understanding of HBV entry would facilitate the design of new therapeutic approaches targeting this stage and preventing the de novo infection of naïve hepatocytes. In this review, we provide an overview of current knowledge about the process of HBV internalization into cells.

Nanoscale polarization of the entry fusion complex of vaccinia virus drives efficient fusion

To achieve efficient binding and subsequent fusion, most enveloped viruses encode between one and five proteins1. For many viruses, the clustering of fusion proteins-and their distribution on virus particles-is crucial for fusion activity2,3. Poxviruses, the most complex mammalian viruses, dedicate 15 proteins to binding and membrane fusion4. However, the spatial organization of these proteins and how this influences fusion activity is unknown. Here, we show that the membrane of vaccinia virus is organized into distinct functional domains that are critical for the efficiency of membrane fusion. Using super-resolution microscopy and single-particle analysis, we found that the fusion machinery of vaccinia virus resides exclusively in clusters at virion tips. Repression of individual components of the fusion complex disrupts fusion-machinery polarization, consistent with the reported loss of fusion activity5. Furthermore, we show that displacement of functional fusion complexes from virion tips disrupts the formation of fusion pores and infection kinetics. Our results demonstrate how the protein architecture of poxviruses directly contributes to the efficiency of membrane fusion, and suggest that nanoscale organization may be an intrinsic property of these viruses to assure successful infection.

Vaccinia viral A26 protein is a fusion suppressor of mature virus and triggers membrane fusion through conformational change at low pH

Vaccinia mature virus requires A26 envelope protein to mediate acid-dependent endocytosis into HeLa cells in which we hypothesized that A26 protein functions as an acid-sensitive membrane fusion suppressor. Here, we provide evidence showing that N-terminal domain (aa1-75) of A26 protein is an acid-sensitive region that regulates membrane fusion. Crystal structure of A26 protein revealed that His48 and His53 are in close contact with Lys47, Arg57, His314 and Arg312, suggesting that at low pH these His-cation pairs could initiate conformational changes through protonation of His48 and His53 and subsequent electrostatic repulsion. All the A26 mutant mature viruses that interrupted His-cation pair interactions of His48 and His 53 indeed have lost virion infectivity. Isolation of revertant viruses revealed that second site mutations caused frame shifts and premature termination of A26 protein such that reverent viruses regained cell entry through plasma membrane fusion. Together, we conclude that viral A26 protein functions as an acid-sensitive fusion suppressor during vaccinia mature virus endocytosis.

Vaccinia virus is a complex large DNA virus with a large number of viral membrane proteins to facilitate cell entry. Although it is well established that vaccinia mature virus uses endocytosis to enter cells, it remains unclear how it triggers membrane fusion in the acidic environment of endosomes. Recently, we hypothesized that A26 protein in vaccinia mature virus functions as an acid-sensitive membrane fusion suppressor, which suggests a novel viral regulation not present in other enveloped viruses. We postulated that conformational changes of A26 protein at low pH result in de-repression of viral fusion complex activity to trigger viral and endosomal membrane fusion. Here, we provide structural, biochemical and biological evidence demonstrating that vaccinia A26 protein does indeed function as an acid-sensitive fusion suppressor protein to regulate vaccinia mature virus membrane fusion during endocytosis. Our data reveal an important and unique “checkpoint” for vaccinia mature virus endocytosis that has not been described for other viruses. Furthermore, by isolating adaptive vaccinia mutants that escaped endocytic blockage, we discovered that mutations within the A26L gene serve as an effective strategy for switching the viral infection route from endocytosis to plasma membrane fusion, expanding viral host range.

Kinetics of Mimivirus Infection Stages Quantified Using Image Flow Cytometry

Due to the heterogeneity of viruses and their hosts, a comprehensive view of viral infection is best achieved by analyzing large populations of infected cells. However, information regarding variation in infected cell populations is lost in bulk measurements. Motivated by an interest in the temporal progression of events in virally infected cells, we used image flow cytometry (IFC) to monitor changes in Acanthamoeba polyphaga cells infected with Mimivirus. This first use of IFC to study viral infection required the development of methods to preserve morphological features of adherent amoeba cells prior to detachment and analysis in suspension. It also required the identification of IFC parameters that best report on key events in the Mimivirus infection cycle. The optimized IFC protocol enabled the simultaneous monitoring of diverse processes including generation of viral factories, transport, and fusion of replication centers within the cell, accumulation of viral progeny, and changes in cell morphology for tens of thousands of cells. After obtaining the time windows for these processes, we used IFC to evaluate the effects of perturbations such as oxidative stress and cytoskeletal disruptors on viral infection. Accurate dose‐response curves could be generated, and we found that mild oxidative stress delayed multiple stages of virus production, but eventually infection processes occurred with approximately the same amplitudes. We also found that functional actin cytoskeleton is required for fusion of viral replication centers and later for the production of viral progeny. Through this report, we demonstrate that IFC offers a quantitative, high‐throughput, and highly robust approach to study viral infection cycles and virus–host interactions. © The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry.

Endosomal receptor trafficking: Retromer and beyond

The tubular endolysosomal network is a quality control system that ensures the proper delivery of internalized receptors to specific subcellular destinations in order to maintain cellular homeostasis. Although retromer was originally described in yeast as a regulator of endosome-to-Golgi receptor recycling, mammalian retromer has emerged as a central player in endosome-to-plasma membrane recycling of a variety of receptors. Over the past decade, information regarding the mechanism by which retromer facilitates receptor trafficking has emerged, as has the identification of numerous retromer-associated molecules including the WASH complex, sorting nexins (SNXs) and TBC1d5. Moreover, the recent demonstration that several SNXs can directly interact with retromer cargo to facilitate endosome-to-Golgi retrieval has provided new insight into how these receptors are trafficked in cells. The mechanism by which SNX17 cargoes are recycled out of the endosomal system was demonstrated to involve a retromer-like complex termed the retriever, which is recruited to WASH positive endosomes through an interaction with the COMMD/CCDC22/CCDC93 (CCC) complex. Lastly, the mechanisms by which bacterial and viral pathogens highjack this complex sorting machinery in order to escape the endolysosomal system or remain hidden within the cells are beginning to emerge. In this review, we will highlight recent studies that have begun to unravel the intricacies by which the retromer and associated molecules contribute to receptor trafficking and how deregulation at this sorting domain can contribute to disease or facilitate pathogen infection.

A comparative review of viral entry and attachment during large and giant dsDNA virus infections

Viruses enter host cells via several mechanisms, including endocytosis, macropinocytosis, and phagocytosis. They can also fuse at the plasma membrane and can spread within the host via cell-to-cell fusion or syncytia. The mechanism used by a given viral strain depends on its external topology and proteome and the type of cell being entered. This comparative review discusses the cellular attachment receptors and entry pathways of dsDNA viruses belonging to the families Adenoviridae, Baculoviridae, Herpesviridae and nucleocytoplasmic large DNA viruses (NCLDVs) belonging to the families Ascoviridae, Asfarviridae, Iridoviridae, Phycodnaviridae, and Poxviridae, and giant viruses belonging to the families Mimiviridae and Marseilleviridae as well as the proposed families Pandoraviridae and Pithoviridae. Although these viruses have several common features (e.g., topology, replication and protein sequence similarities) they utilize different entry pathways to infect wide-range of hosts, including humans, other mammals, invertebrates, fish, protozoa and algae. Similarities and differences between the entry methods used by these virus families are highlighted, with particular emphasis on viral topology and proteins that mediate viral attachment and entry. Cell types that are frequently used to study viral entry are also reviewed, along with other factors that affect virus-host cell interactions.

Differential Innate Immune Signaling in Macrophages by Wild-Type Vaccinia Mature Virus and a Mutant Virus with a Deletion of the A26 Protein

The Western Reserve (WR) strain of mature vaccinia virus contains an A26 envelope protein that mediates virus binding to cell surface laminin and subsequent endocytic entry into HeLa cells. Removal of the A26 protein from the WR strain mature virus generates a mutant, WRΔA26, that enters HeLa cells through plasma membrane fusion. Here, we infected murine bone marrow-derived macrophages (BMDM) with wild-type strain WR and the WRΔA26 mutant and analyzed viral gene expression and cellular innate immune signaling. In contrast to previous studies, in which both HeLa cells infected with WR and HeLa cells infected with WRΔA26 expressed abundant viral late proteins, we found that WR expressed much less viral late protein than WRΔA26 in BMDM. Microarray analysis of the cellular transcripts in BMDM induced by virus infection revealed that WR preferentially activated type 1 interferon receptor (IFNAR)-dependent signaling but WRΔA26 did not. We consistently detected a higher level of soluble beta interferon secretion and phosphorylation of the STAT1 protein in BMDM infected with WR than in BMDM infected with WRΔA26. When IFNAR-knockout BMDM were infected with WR, late viral protein expression increased, confirming that IFNAR-dependent signaling was differentially induced by WR and, in turn, restricted viral late gene expression. Finally, wild-type C57BL/6 mice were more susceptible to mortality from WRΔA26 infection than to that from WR infection, whereas IFNAR-knockout mice were equally susceptible to WR and WRΔA26 infection, demonstrating that the ability of WRΔA26 to evade IFNAR signaling has an important influence on viral pathogenesis in vivoIMPORTANCE The vaccinia virus A26 protein was previously shown to mediate virus attachment and to regulate viral endocytosis. Here, we show that infection with strain WR induces a robust innate immune response that activates type 1 interferon receptor (IFNAR)-dependent cellular genes in BMDM, whereas infection with the WRΔA26 mutant does not. We further demonstrated that the differential activation of IFNAR-dependent cellular signaling between WR and WRΔA26 not only is important for differential host restriction in BMDM but also is important for viral virulence in vivo Our study reveals a new property of WRΔA26, which is in regulating host antiviral innate immunity in vitro and in vivo.

Poxviruses Utilize Multiple Strategies to Inhibit Apoptosis

Cells have multiple means to induce apoptosis in response to viral infection. Poxviruses must prevent activation of cellular apoptosis to ensure successful replication. These viruses devote a substantial portion of their genome to immune evasion. Many of these immune evasion products expressed during infection antagonize cellular apoptotic pathways. Poxvirus products target multiple points in both the extrinsic and intrinsic apoptotic pathways, thereby mitigating apoptosis during infection. Interestingly, recent evidence indicates that poxviruses also hijack cellular means of eliminating apoptotic bodies as a means to spread cell to cell through a process called apoptotic mimicry. Poxviruses are the causative agent of many human and veterinary diseases. Further, there is substantial interest in developing these viruses as vectors for a variety of uses including vaccine delivery and as oncolytic viruses to treat certain human cancers. Therefore, an understanding of the molecular mechanisms through which poxviruses regulate the cellular apoptotic pathways remains a top research priority. In this review, we consider anti-apoptotic strategies of poxviruses focusing on three relevant poxvirus genera: Orthopoxvirus, Molluscipoxvirus, and Leporipoxvirus. All three genera express multiple products to inhibit both extrinsic and intrinsic apoptotic pathways with many of these products required for virulence.

Glycosaminoglycans-Specific Cell Targeting and Imaging Using Fluorescent Nanodiamonds Coated with Viral Envelope Proteins

Understanding virus-host interactions is crucial for vaccine development. This study investigates such interactions using fluorescent nanodiamonds (FNDs) coated with vaccinia envelope proteins as the model system. To achieve this goal, we noncovalently conjugated 100 nm FNDs with rA27(aa 21-84), a recombinant envelope protein of vaccinia virus, for glycosaminoglycans (GAGs)-specific targeting and imaging of living cells. Another recombinant protein rDA27(aa 33-84) that removes the GAGs-binding sequences was also used for comparison. Three types of A27-coated FNDs were generated, including rA27(aa 21-84)-FND, rDA27(aa 33-84)-FND, and hybrid rA27(aa 21-84)/rDA27(aa 33-84)-FND. The specificity of these viral protein-FND conjugates toward GAGs binding was examined by flow cytometry, fluorescence microscopy, and gel electrophoresis. Results obtained for normal and GAGs-deficient cells showed that the recombinant proteins maintain their GAG-targeting activities even after immobilization on the FND surface. Our studies provide a new nanoparticle-based platform not only to target specific cell types but also to track the fates of these immobilized viral proteins in targeted cells as well as to isolate and enrich GAGs-associated proteins on cell membrane.

Regulated Entry of Hepatitis C Virus into Hepatocytes

Hepatitis C virus (HCV) is a model for the study of virus–host interaction and host cell responses to infection. Virus entry into hepatocytes is the first step in the HCV life cycle, and this process requires multiple receptors working together. The scavenger receptor class B type I (SR-BI) and claudin-1 (CLDN1), together with human cluster of differentiation (CD) 81 and occludin (OCLN), constitute the minimal set of HCV entry receptors. Nevertheless, HCV entry is a complex process involving multiple host signaling pathways that form a systematic regulatory network; this network is centrally controlled by upstream regulators epidermal growth factor receptor (EGFR) and transforming growth factor β receptor (TGFβ-R). Further feedback regulation and cell-to-cell spread of the virus contribute to the chronic maintenance of HCV infection. A comprehensive and accurate disclosure of this critical process should provide insights into the viral entry mechanism, and offer new strategies for treatment regimens and targets for HCV therapeutics.

The Coat Protein and NIa Protease of Two Potyviridae Family Members Independently Confer Superinfection Exclusion

Superinfection exclusion (SIE) is an antagonistic virus-virus interaction whereby initial infection by one virus prevents subsequent infection by closely related viruses. Although SIE has been described in diverse viruses infecting plants, humans, and animals, its mechanisms, including involvement of specific viral determinants, are just beginning to be elucidated. In this study, SIE determinants encoded by two economically important wheat viruses, Wheat streak mosaic virus (WSMV; genus Tritimovirus, family Potyviridae) and Triticum mosaic virus (TriMV; genus Poacevirus, family Potyviridae), were identified in gain-of-function experiments that used heterologous viruses to express individual virus-encoded proteins in wheat. Wheat plants infected with TriMV expressing WSMV P1, HC-Pro, P3, 6K1, CI, 6K2, NIa-VPg, or NIb cistrons permitted efficient superinfection by WSMV expressing green fluorescent protein (WSMV-GFP). In contrast, wheat infected with TriMV expressing WSMV NIa-Pro or coat protein (CP) substantially excluded superinfection by WSMV-GFP, suggesting that both of these cistrons are SIE effectors encoded by WSMV. Importantly, SIE is due to functional WSMV NIa-Pro or CP rather than their encoding RNAs, as altering the coded protein products by minimally changing RNA sequences led to abolishment of SIE. Deletion mutagenesis further revealed that elicitation of SIE by NIa-Pro requires the entire protein while CP requires only a 200-amino-acid (aa) middle fragment (aa 101 to 300) of the 349 aa. Strikingly, reciprocal experiments with WSMV-mediated expression of TriMV proteins showed that TriMV CP, and TriMV NIa-Pro to a lesser extent, likewise excluded superinfection by TriMV-GFP. Collectively, these data demonstrate that WSMV- and TriMV-encoded CP and NIa-Pro proteins are effectors of SIE and that these two proteins trigger SIE independently of each other.

IMPORTANCE

Superinfection exclusion (SIE) is an antagonistic virus-virus interaction that prevents secondary invasions by identical or closely related viruses in the same host cells. Although known to occur in diverse viruses, SIE remains an enigma in terms of key molecular determinants and action mechanisms. In this study, we found that Wheat streak mosaic virus (WSMV) and Triticum mosaic virus (TriMV) encode two independently functioning cistrons that serve as effectors of SIE at the protein but not the RNA level. The coat protein and NIa-Pro encoded by these two viruses, when expressed from a heterologous virus, exerted SIE to the cognate viruses. The identification of virus-encoded effectors of SIE and their transgenic expression could potentially facilitate the development of virus-resistant crop plants. Additionally, functional conservation of SIE in diverse virus groups suggests that a better understanding of the underlying mechanisms of SIE could facilitate the development of novel antiviral therapies against viral diseases.

Vaccinia virus dissemination requires p21-activated kinase 1

The orthopoxvirus vaccinia virus (VACV) interacts with both actin and microtubule cytoskeletons in order to generate and spread progeny virions. Here, we present evidence demonstrating the involvement of PAK1 (p21-activated kinase 1) in the dissemination of VACV. Although PAK1 activation has previously been associated with optimal VACV entry via macropinocytosis, its absence does not affect the production of intracellular mature virions (IMVs) and extracellular enveloped virions (EEVs). Our data demonstrate that low-multiplicity infection of PAK1(-/-) MEFs leads to a reduction in plaque size followed by decreased production of both IMVs and EEVs, strongly suggesting that virus spread was impaired in the absence of PAK1. Confocal and scanning electron microscopy showed a substantial reduction in the amount of VACV-induced actin tails in PAK1(-/-) MEFs, but no significant alteration in the total amount of cell-associated enveloped virions (CEVs). Furthermore, the decreased VACV dissemination in PAK1(-/-) cells was correlated with the absence of phosphorylated ARPC1 (Thr21), a downstream target of PAK1 and a key regulatory subunit of the ARP2/3 complex, which is necessary for the formation of actin tails and viral spread. We conclude that PAK1, besides its role in virus entry, also plays a relevant role in VACV dissemination.

Membrane fusion during poxvirus entry

Poxviruses comprise a large family of enveloped DNA viruses that infect vertebrates and invertebrates. Poxviruses, unlike most DNA viruses, replicate in the cytoplasm and encode enzymes and other proteins that enable entry, gene expression, genome replication, virion assembly and resistance to host defenses. Entry of vaccinia virus, the prototype member of the family, can occur at the plasma membrane or following endocytosis. Whereas many viruses encode one or two proteins for attachment and membrane fusion, vaccinia virus encodes four proteins for attachment and eleven more for membrane fusion and core entry. The entry-fusion proteins are conserved in all poxviruses and form a complex, known as the Entry Fusion Complex (EFC), which is embedded in the membrane of the mature virion. An additional membrane that encloses the mature virion and is discarded prior to entry is present on an extracellular form of the virus. The EFC is held together by multiple interactions that depend on nine of the eleven proteins. The entry process can be divided into attachment, hemifusion and core entry. All eleven EFC proteins are required for core entry and at least eight for hemifusion. To mediate fusion the virus particle is activated by low pH, which removes one or more fusion repressors that interact with EFC components. Additional EFC-interacting fusion repressors insert into cell membranes and prevent secondary infection. The absence of detailed structural information, except for two attachment proteins and one EFC protein, is delaying efforts to determine the fusion mechanism.

Filopodia and Viruses: An Analysis of Membrane Processes in Entry Mechanisms

Filopodia are thin, actin rich bundles protruding from cell plasma membranes, serving physiological purposes, such as probing the environment and facilitating cell-to-cell adhesion. Recent studies have highlighted that actively polymerized filopodial-protrusions are exploited during virus entry, trafficking, spread, and the development of clinical pathology of viral diseases. These observations have caused a surge in investigation of the key determinants of filopodial induction and their influence on cell topography including receptor expression for viral entry. It is now very clear that filopodia can provide unique opportunities for many viruses to invade host cells vertically during primary infection, or horizontally during virus spread from cell-to-cell. These emerging concepts can explain the unprecedented ability of viruses to invade both nearby and long-distant host cells, a feature that may directly contribute to viral tropism. In this review, we summarize the significance of filopodia in viral diseases and discuss future therapeutic possibilities to precisely target filopodial-flyovers to prevent or control infectious diseases.

Cloak and Dagger: Alternative Immune Evasion and Modulation Strategies of Poxviruses

As all viruses rely on cellular factors throughout their replication cycle, to be successful they must evolve strategies to evade and/or manipulate the defence mechanisms employed by the host cell. In addition to their expression of a wide array of host modulatory factors, several recent studies have suggested that poxviruses may have evolved unique mechanisms to shunt or evade host detection. These potential mechanisms include mimicry of apoptotic bodies by mature virions (MVs), the use of viral sub-structures termed lateral bodies for the packaging and delivery of host modulators, and the formation of a second, “cloaked” form of infectious extracellular virus (EVs). Here we discuss these various strategies and how they may facilitate poxvirus immune evasion. Finally we propose a model for the exploitation of the cellular exosome pathway for the formation of EVs.

Intracellular Transport of Vaccinia Virus in HeLa Cells Requires WASH-VPEF/FAM21-Retromer Complexes and Recycling Molecules Rab11 and Rab22

Vaccinia virus, the prototype of the Orthopoxvirus genus in the family Poxviridae, infects a wide range of cell lines and animals. Vaccinia mature virus particles of the WR strain reportedly enter HeLa cells through fluid-phase endocytosis. However, the intracellular trafficking process of the vaccinia mature virus between cellular uptake and membrane fusion remains unknown. We used live imaging of single virus particles with a combination of various cellular vesicle markers, to track fluorescent vaccinia mature virus particle movement in cells. Furthermore, we performed functional interference assays to perturb distinct vesicle trafficking processes in order to delineate the specific route undertaken by vaccinia mature virus prior to membrane fusion and virus core uncoating in cells. Our results showed that vaccinia virus traffics to early endosomes, where recycling endosome markers Rab11 and Rab22 are recruited to participate in subsequent virus trafficking prior to virus core uncoating in the cytoplasm. Furthermore, we identified WASH-VPEF/FAM21-retromer complexes that mediate endosome fission and sorting of virus-containing vesicles prior to virus core uncoating in the cytoplasm.

IMPORTANCE

Vaccinia mature virions of the WR strain enter HeLa cells through fluid phase endocytosis. We previously demonstrated that virus-containing vesicles are internalized into phosphatidylinositol 3-phosphate positive macropinosomes, which are then fused with Rab5-positive early endosomes. However, the subsequent process of sorting the virion-containing vesicles prior to membrane fusion remains unclear. We dissected the intracellular trafficking pathway of vaccinia mature virions in cells up to virus core uncoating in cytoplasm. We show that vaccinia mature virions first travel to early endosomes. Subsequent trafficking events require the important endosome-tethered protein VPEF/FAM21, which recruits WASH and retromer protein complexes to the endosome. There, the complex executes endosomal membrane fission and cargo sorting to the Rab11-positive and Rab22-positive recycling pathway, resulting in membrane fusion and virus core uncoating in the cytoplasm.

Vaccinia Virus Infection Requires Maturation of Macropinosomes

The prototypic poxvirus, vaccinia virus (VACV), occurs in two infectious forms, mature virions (MVs) and extracellular virions (EVs). Both enter HeLa cells by inducing macropinocytic uptake. Using confocal microscopy, live-cell imaging, targeted RNAi screening and perturbants of endosome maturation, we analyzed the properties and maturation pathway of the macropinocytic vacuoles containing VACV MVs in HeLa cells. The vacuoles first acquired markers of early endosomes [Rab5, early endosome antigen 1 and phosphatidylinositol(3)P]. Prior to release of virus cores into the cytoplasm, they contained markers of late endosomes and lysosomes (Rab7a, lysosome-associated membrane protein 1 and sorting nexin 3). RNAi screening of endocytic cell factors emphasized the importance of late compartments for VACV infection. Follow-up perturbation analysis showed that infection required Rab7a and PIKfyve, confirming that VACV is a late-penetrating virus dependent on macropinosome maturation. VACV EV infection was inhibited by depletion of many of the same factors, indicating that both infectious particle forms share the need for late vacuolar conditions for penetration.

The role of signalling and the cytoskeleton during Vaccinia Virus egress

Viruses are obligate intracellular parasites that are critically dependent on their hosts to replicate and generate new progeny. To achieve this goal, viruses have evolved numerous elegant strategies to subvert and utilise the different cellular machineries and processes of their unwilling hosts. Moreover, they often accomplish this feat with a surprisingly limited number of proteins. Among the different systems of the cell, the cytoskeleton is often one of the first to be hijacked as it provides a convenient transport system for viruses to reach their site of replication with relative ease. At the latter stages of their replication cycle, the cytoskeleton also provides an efficient means for newly assembled viral progeny to reach the plasma membrane and leave the infected cell. In this review we discuss how Vaccinia virus takes advantage of the microtubule and actin cytoskeletons of its host to promote the spread of infection into neighboring cells. In particular, we highlight how analysis of actin-based motility of Vaccinia has provided unprecedented insights into how a phosphotyrosine-based signalling network is assembled and functions to stimulate Arp2/3 complex-dependent actin polymerization. We also suggest that the formin FHOD1 promotes actin-based motility of the virus by capping the fast growing ends of actin filaments rather than directly promoting filament assembly. We have come a long way since 1976, when electron micrographs of vaccinia-infected cells implicated the actin cytoskeleton in promoting viral spread. Nevertheless, there are still many unanswered questions concerning the role of signalling and the host cytoskeleton in promoting viral spread and pathogenesis.

A novel mode of poxvirus superinfection exclusion that prevents fusion of the lipid bilayers of viral and cellular membranes

Superinfection exclusion is a widespread phenomenon that prevents secondary infections by closely related viruses. The vaccinia virus A56 and K2 proteins in the cell membrane can prevent superinfection by interacting with the entry-fusion complex of subsequent viruses. Here, we described another form of exclusion that is established earlier in infection and does not require the A56 or K2 protein. Cells infected with one or more infectious virions excluded hundreds of superinfecting vaccinia virus particles. A related orthopoxvirus, but neither a flavivirus nor a rhabdovirus, was also excluded, indicating selectivity. Although superinfecting vaccinia virus bound to cells, infection was inhibited at the membrane fusion step, thereby preventing core entry into the cytoplasm and early gene expression. In contrast, A56/K2 protein-mediated exclusion occurred subsequent to membrane fusion. Induction of resistance to superinfection depended on viral RNA and protein synthesis by the primary virus but did not require DNA replication. Although superinfection resistance correlated with virus-induced changes in the cytoskeleton, studies with mutant vaccinia viruses indicated that the cytoskeletal changes were not necessary for resistance to superinfection. Interferon-inducible transmembrane proteins, which can inhibit membrane fusion in other viral systems, did not prevent vaccinia virus membrane fusion, suggesting that these interferon-inducible proteins are not involved in superinfection exclusion. While the mechanism remains to be determined, the early establishment of superinfection exclusion may provide a "winner-take-all" reward to the first poxvirus particles that successfully initiate infection and prevent the entry and genome reproduction of defective or less fit particles.

IMPORTANCE

The replication of a virus usually follows a defined sequence of events: attachment, entry into the cytoplasm or nucleus, gene expression, genome replication, assembly of infectious particles, and spread to other cells. Although multiple virus particles may enter a cell at the same time, mechanisms exist to prevent infection by subsequent viruses. The latter phenomenon, known as superinfection exclusion, can occur by a variety of mechanisms that are not well understood. We showed that superinfection by vaccinia virus was prevented at the membrane fusion step, which closely followed virion attachment. Thus, neither gene expression nor genome replication of the superinfecting virus occurred. Expression of early proteins by the primary virus was necessary and sufficient to induce the superinfection-resistant state. Superinfection exclusion may be beneficial to vaccinia virus by selecting particles that can infect cells rapidly, excluding defective particles and synchronizing the replication cycle.

Vaccinia viral protein A27 is anchored to the viral membrane via a cooperative interaction with viral membrane protein A17

The vaccinia viral protein A27 in mature viruses specifically interacts with heparan sulfate for cell surface attachment. In addition, A27 associates with the viral membrane protein A17 to anchor to the viral membrane; however, the specific interaction between A27 and A17 remains largely unclear. To uncover the active binding sites and the underlying binding mechanism, we expressed and purified the N-terminal (18-50 residues) and C-terminal (162-203 residues) fragments of A17, which are denoted A17-N and A17-C. Through surface plasmon resonance, the binding affinity of A27/A17-N (KA = 3.40 × 10(8) m(-1)) was determined to be approximately 3 orders of magnitude stronger than that of A27/A17-C (KA = 3.40 × 10(5) m(-1)), indicating that A27 prefers to interact with A17-N rather than A17-C. Despite the disordered nature of A17-N, the A27-A17 interaction is mediated by a specific and cooperative binding mechanism that includes two active binding sites, namely (32)SFMPK(36) (denoted as F1 binding) and (20)LDKDLFTEEQ(29) (F2). Further analysis showed that F1 has stronger binding affinity and is more resistant to acidic conditions than is F2. Furthermore, A27 mutant proteins that retained partial activity to interact with the F1 and F2 sites of the A17 protein were packaged into mature virus particles at a reduced level, demonstrating that the F1/F2 interaction plays a critical role in vivo. Using these results in combination with site-directed mutagenesis data, we established a computer model to explain the specific A27-A17 binding mechanism.

TRAF2 facilitates vaccinia virus replication by promoting rapid virus entry

Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) is a pivotal intracellular mediator of signaling pathways downstream of TNFR1 and -2 with known pro- and antiviral effects. We investigated its role in the replication of the prototype poxvirus vaccinia virus (VACV). Loss of TRAF2 expression, either through small interfering RNA treatment of HeLa cells or through genetic knockout in murine embryonic fibroblasts (MEFs), led to significant reductions in VACV growth following low-multiplicity infection. In single-cycle infections, there was delayed production of both early and late VACV proteins as well as accelerated virus-induced alterations to cell morphology, indicating that TRAF2 influences early stages of virus replication. Consistent with an early role, uncoating assays showed normal virus attachment but delayed virus entry in the absence of TRAF2. Although alterations to c-Jun N-terminal kinase (JNK) signaling were apparent in VACV-infected TRAF2^−/− MEFs, treatment of wild-type cells with a JNK inhibitor did not affect virus entry. Instead, treatment with an inhibitor of endosomal acidification greatly reduced virus entry into TRAF2^−/− MEFs, suggesting that VACV is reliant on the endosomal route of entry in the absence of TRAF2. Thus, TRAF2 is a proviral factor for VACV that plays a role in promoting efficient viral entry, most likely via the plasma membrane.

IMPORTANCE Tumor necrosis factor receptor-associated factors (TRAFs) are key facilitators of intracellular signaling with roles in innate and adaptive immunity and stress responses. We have discovered that TRAF2 is a proviral factor in vaccinia virus replication in both HeLa cells and mouse embryonic fibroblasts and that its influence is exercised through promotion of efficient virus entry.

Role of the vaccinia virus O3 protein in cell entry can be fulfilled by its Sequence flexible transmembrane domain

The vaccinia virus O3 protein, a component of the entry-fusion complex, is encoded by all chordopoxviruses. We constructed truncation mutants and demonstrated that the transmembrane domain, which comprises two-thirds of this 35 amino acid protein, is necessary and sufficient for interaction with the entry-fusion complex and function in cell entry. Nevertheless, neither single amino acid substitutions nor alanine scanning mutagenesis revealed essential amino acids within the transmembrane domain. Moreover, replication-competent mutant viruses were generated by randomization of 10 amino acids of the transmembrane domain. Of eight unique viruses, two contained only two amino acids in common with wild type and the remainder contained one or none within the randomized sequence. Although these mutant viruses formed normal size plaques, the entry-fusion complex did not co-purify with the mutant O3 proteins suggesting a less stable interaction. Thus, despite low specific sequence requirements, the transmembrane domain is sufficient for function in entry.

The retromer complex - endosomal protein recycling and beyond

The retromer complex is a vital element of the endosomal protein sorting machinery that is conserved across all eukaryotes. Retromer is most closely associated with the endosome-to-Golgi retrieval pathway and is necessary to maintain an active pool of hydrolase receptors in the trans-Golgi network. Recent progress in studies of retromer have identified new retromer-interacting proteins, including the WASH complex and cargo such as the Wntless/MIG-14 protein, which now extends the role of retromer beyond the endosome-to-Golgi pathway and has revealed that retromer is required for aspects of endosome-to-plasma membrane sorting and regulation of signalling events. The interactions between the retromer complex and other macromolecular protein complexes now show how endosomal protein sorting is coordinated with actin assembly and movement along microtubules, and place retromer squarely at the centre of a complex set of protein machinery that governs endosomal protein sorting. Dysregulation of retromer-mediated endosomal protein sorting leads to various pathologies, including neurodegenerative diseases such as Alzheimer disease and spastic paraplegia and the mechanisms underlying these pathologies are starting to be understood. In this Commentary, I will highlight recent advances in the understanding of retromer-mediated endosomal protein sorting and discuss how retromer contributes to a diverse set of physiological processes.

Orthopoxvirus species and strain differences in cell entry

Vaccinia virus (VACV) enters cells by a low pH endosomal route or by direct fusion with the plasma membrane. We previously found differences in entry properties of several VACV strains: entry of WR was enhanced by low pH, reduced by bafilomycin A1 and relatively unaffected by heparin, whereas entry of IHD-J, Copenhagen and Elstree were oppositely affected. Since binding and entry modes may have been selected by specific conditions of in vitro propagation, we now examined the properties of three distinct, recently isolated cowpox viruses and a monkeypox virus as well as additional VACV and cowpox virus strains. The recent isolates were more similar to WR than to other VACV strains, underscoring the biological importance of endosomal entry by orthopoxviruses. Sequence comparisons, gene deletions and gene swapping experiments indicated that viral determinants, other than or in addition to the A26 and A25 "fusion-suppressor" proteins, impact entry properties.

Cooperative vaccinia infection demonstrated at the single-cell level using FluidFM

The mechanisms used by viruses to enter and replicate within host cells are subjects of intense investigation. These studies are ultimately aimed at development of new drugs that interfere with these processes. Virus entry and infection are generally monitored by dispensing bulk virus suspensions on layers of cells without accounting for the fate of each virion. Here, we take advantage of the recently developed FluidFM to deposit single vaccinia virions onto individual cells in a controlled manner. While the majority of virions were blocked prior to early gene expression, infection of individual cells increased in a nondeterministic fashion with respect to the number of viruses placed. Microscopic analyses of several stages of the virus lifecycle indicated that this was the result of cooperativity between virions during early stages of infection. These findings highlight the importance of performing controlled virus infection experiments at the single cell level.

From endocytosis to membrane fusion: emerging roles of dynamin in virus entry

Dynamin, a large guanosine triphosphatase (GTPase), has been implicated in virus entry, but its mechanisms of action are controversial. The entry procedure of most enveloped viruses involves endocytosis and membrane fusion. Dynamin has been suggested to act both as a regulatory GTPase by controlling the early stages of clathrin-mediated endocytosis (CME), which is an important endocytic pathway utilized by many viruses, and as a mechanochemical enzyme that induces membrane fission and pinches endocytic vesicles off from the cellular plasma membrane in later stages in several endocytic pathways, including CME. In addition to its involvement in virus endocytosis, dynamin has also been proposed to participate in membrane fusion between the virus and endosomes following endocytosis. Crystal structures and cryo-electron micrography (cryo-EM) have elucidated the structure of dynamin, which led to development of a mechanochemical model of how dynamin-mediated membrane fission occurs. Based on this, we propose a hypothetical model that explains how dynamin facilitates virus membrane fusion and discuss its roles in virus entry.

Poxvirus cell entry: how many proteins does it take?

For many viruses, one or two proteins enable cell binding, membrane fusion and entry. The large number of proteins employed by poxviruses is unprecedented and may be related to their ability to infect a wide range of cells. There are two main infectious forms of vaccinia virus, the prototype poxvirus: the mature virion (MV), which has a single membrane, and the extracellular enveloped virion (EV), which has an additional outer membrane that is disrupted prior to fusion. Four viral proteins associated with the MV membrane facilitate attachment by binding to glycosaminoglycans or laminin on the cell surface, whereas EV attachment proteins have not yet been identified. Entry can occur at the plasma membrane or in acidified endosomes following macropinocytosis and involves actin dynamics and cell signaling. Regardless of the pathway or whether the MV or EV mediates infection, fusion is dependent on 11 to 12 non-glycosylated, transmembrane proteins ranging in size from 4- to 43-kDa that are associated in a complex. These proteins are conserved in poxviruses making it likely that a common entry mechanism exists. Biochemical studies support a two-step process in which lipid mixing of viral and cellular membranes is followed by pore expansion and core penetration.

Integrin β1 mediates vaccinia virus entry through activation of PI3K/Akt signaling

Vaccinia virus has a broad range of infectivity in many cell lines and animals. Although it is known that the vaccinia mature virus binds to cell surface glycosaminoglycans and extracellular matrix proteins, whether additional cellular receptors are required for virus entry remains unclear. Our previous studies showed that the vaccinia mature virus enters through lipid rafts, suggesting the involvement of raft-associated cellular proteins. Here we demonstrate that one lipid raft-associated protein, integrin β1, is important for vaccinia mature virus entry into HeLa cells. Vaccinia virus associates with integrin β1 in lipid rafts on the cell surface, and the knockdown of integrin β1 in HeLa cells reduces vaccinia mature virus entry. Additionally, vaccinia mature virus infection is reduced in a mouse cell line, GD25, that is deficient in integrin β1 expression. Vaccinia mature virus infection triggers the activation of phosphatidylinositol 3-kinase (PI3K)/Akt signaling, and the treatment of cells with inhibitors to block P13K activation reduces virus entry in an integrin β1-dependent manner, suggesting that integrin β1-mediates PI3K/Akt activation induced by vaccinia virus and that this signaling pathway is essential for virus endocytosis. The inhibition of integrin β1-mediated cell adhesion results in a reduction of vaccinia virus entry and the disruption of focal adhesion and PI3K/Akt activation. In summary, our results show that the binding of vaccinia mature virus to cells mimics the outside-in activation process of integrin functions to facilitate vaccinia virus entry into HeLa cells.

The lipid raft-associated protein CD98 is required for vaccinia virus endocytosis

Mature vaccinia virus (vaccinia MV) infects a broad range of animals in vivo and cell cultures in vitro; however, the cellular receptors that determine vaccinia MV tropism and entry pathways are poorly characterized. Here, we performed quantitative proteomic analyses of lipid raft-associated proteins upon vaccinia MV entry into HeLa cells. We found that a type II membrane glycoprotein, CD98, is enriched in lipid rafts upon vaccinia MV infection compared to mock-infected HeLa cells. The knockdown of CD98 expression in HeLa cells significantly reduced vaccinia MV entry. Furthermore, CD98 knockout (KO) mouse embryonic fibroblasts (MEFs) also exhibited reduced vaccinia MV infectivity without affecting MV attachment to cells, suggesting a role for CD98 in the postbinding step of virus entry. Further characterization with inhibitors and dominant negative proteins that block different endocytic pathways revealed that vaccinia MV entry into MEFs occurs through a clathrin-independent, caveolin-independent, dynamin-dependent, fluid-phase endocytic pathway, implying that CD98 plays a specific role in the vaccinia MV endocytic pathway. Infections of wild-type and CD98 KO MEF cells with different strains of vaccinia MV provided further evidence that CD98 plays a specific role in MV endocytosis but not in plasma membrane fusion. Finally, different CD98-C69 chimeric proteins were expressed in CD98 KO MEFs, but none were able to reconstitute MV infectivity, suggesting that the overall structure of the CD98 protein is required for vaccinia MV endocytosis.

An overview of the vaccinia virus infectome: a survey of the proteins of the poxvirus-infected cell

We have quantitatively profiled the proteins of vaccinia virus-infected HEK293T cells early and late during vaccinia virus infection. Proteins corresponding to 4,326 accessions were identified, the products of 3,798 genes. One hundred thirty-six of the proteins were vaccinia virus-encoded (∼64% of the known vaccinia virus proteome). The remaining accessions were from the host cell. A total of 3,403 of the 4,326 accessions could be confidently quantitated at the precursor peptide level. Although vaccinia virus gene products spanned the entire abundance dynamic range of the cellular proteome, nearly all of the proteome dynamics observed as a result of infection were manifest in the virus gene products with very little plasticity in the host cell proteome. The vaccinia virus gene products could be grouped into four kinetic classes (i.e., four combinations of pre- and postreplicative expression). These protein kinetic classes reflected, almost entirely, the corresponding gene classes within the recently characterized vaccinia virus transcriptome map. The few cellular gene products that showed notable changes in abundance upon vaccinia virus infection were concentrated largely in just a few functional groups. After all of the quantitated cellular gene products were assigned to Gene Ontology (GO)-specific groups, quantitation values for a number of these GO-specific groups were significantly skewed toward over- or underabundance with respect to the global distribution of quantitation values. Quantitative analysis of host cell functions reflected several known facets of virus infection, along with some novel observations.

Glomulin: a permissivity factor for vaccinia virus infection

In earlier studies we provided evidence that vaccinia virus (VACV) phosphorylation-activation of host cell signaling effectors is critical for subsequent viral replication. In this report, using mass spectrometry-based proteomics, we have identified 387 host cell proteins that co-immunoprecipitate with VACV in infected, permissive PM1.CCR5 human T cells. Among these, glomulin was distinguishable based on its known interaction with a tyrosine kinase receptor, c-Met, its ability to become tyrosine-phosphorylated, and its association with signaling effectors. siRNA knockdown of glomulin expression in PM1.CCR5 T cells reduces VACV infection. Glomulin interacts with the inactive, nonphosphorylated form of c-MET. We demonstrate that treatment of PM1.CCR5 T cells with a c-Met phosphorylation inhibitor leads to a significant reduction in VACV infectivity. Additionally, inhibition of phosphorylation of c-Met abrogates VACV-inducible phosphorylation of Erk 1/2 and IRS-2, signaling effectors identified as critical for VACV infection. These data identify glomulin as a permissivity factor for VACV infection and as a potential therapeutic target for inhibition of VACV infection.

Vaccinia mature virus fusion regulator A26 protein binds to A16 and G9 proteins of the viral entry fusion complex and dissociates from mature virions at low pH

Vaccinia mature virus enters cells through either endocytosis or plasma membrane fusion, depending on virus strain and cell type. Our previous results showed that vaccinia virus mature virions containing viral A26 protein enter HeLa cells preferentially through endocytosis, whereas mature virions lacking A26 protein enter through plasma membrane fusion, leading us to propose that A26 acts as an acid-sensitive fusion suppressor for mature virus (S. J. Chang, Y. X. Chang, R. Izmailyan R, Y. L. Tang, and W. Chang, J. Virol. 84:8422-8432, 2010). In the present study, we investigated the fusion suppression mechanism of A26 protein. We found that A26 protein was coimmunoprecipitated with multiple components of the viral entry-fusion complex (EFC) in infected HeLa cells. Transient expression of viral EFC components in HeLa cells revealed that vaccinia virus A26 protein interacted directly with A16 and G9 but not with G3, L5 and H2 proteins of the EFC components. Consistently, a glutathione S-transferase (GST)-A26 fusion protein, but not GST, pulled down A16 and G9 proteins individually in vitro. Together, our results supported the idea that A26 protein binds to A16 and G9 protein at neutral pH contributing to suppression of vaccinia virus-triggered membrane fusion from without. Since vaccinia virus extracellular envelope proteins A56/K2 were recently shown to bind to the A16/G9 subcomplex to suppress virus-induced fusion from within, our results also highlight an evolutionary convergence in which vaccinia viral fusion suppressor proteins regulate membrane fusion by targeting the A16 and G9 components of the viral EFC complex. Finally, we provide evidence that acid (pH 4.7) treatment induced A26 protein and A26-A27 protein complexes of 70 kDa and 90 kDa to dissociate from mature virions, suggesting that the structure of A26 protein is acid sensitive.

Imaging of vaccinia virus entry into HeLa cells

The recently developed technique of live cell imaging has found numerous applications, including the detection of virus movements in living cells. To monitor virus motility, viruses or cellular proteins are fused with fluorescence markers and then detected by time-lapse fluorescence microscopy. These techniques allow kinetic analyses of individual virus particles in motion during the virus entry process as well as monitoring of dynamic interactions between viruses and cellular structures in real time. The methods presented here describe how to construct a fluorescent recombinant vaccinia virus expressing the core protein A4L fused to mCherry, and how to detect the virus movement on actin-EYFP-expressed HeLa cells.