Vaccinia virus A21 virion membrane protein is required for cell entry and fusion

Structural and functional analyses of viral H2 protein of the vaccinia virus entry fusion complex

IMPORTANCE

Vaccinia virus infection requires virus-cell membrane fusion to complete entry during endocytosis; however, it contains a large viral fusion protein complex of 11 viral proteins that share no structure or sequence homology to all the known viral fusion proteins, including type I, II, and III fusion proteins. It is thus very challenging to investigate how the vaccinia fusion complex works to trigger membrane fusion with host cells. In this study, we crystallized the ectodomain of vaccinia H2 protein, one component of the viral fusion complex. Furthermore, we performed a series of mutational, biochemical, and molecular analyses and identified two surface loops containing 170LGYSG174 and 125RRGTGDAW132 as the A28-binding region. We also showed that residues in the N-terminal helical region (amino acids 51-90) are also important for H2 function.

Structural and functional analysis of vaccinia viral fusion complex component protein A28 through NMR and molecular dynamic simulations

Host cell entry of vaccinia virus (a poxvirus) proceeds through multiple steps that involve many viral proteins to mediate cell infection. Upon binding to cells, vaccinia virus membrane fuses with host membranes via a viral entry fusion protein complex comprising 11 proteins: A16, A21, A28, F9, G3, G9, H2, J5, L1, L5 and O3. Despite vaccinia virus having two infectious forms, mature and enveloped, that have different membrane layers, both forms require an identical viral entry fusion complex for membrane fusion. Components of the poxvirus entry fusion complex that have been structurally assessed to date share no known homology with all other type I, II and III viral fusion proteins, and the large number of fusion protein components renders it a unique system to investigate poxvirus-mediated membrane fusion. Here, we determined the NMR structure of a truncated version of vaccinia A28 protein. We also expressed a soluble H2 protein and showed that A28 interacts with H2 protein at a 1:1 ratio in vitro. Furthermore, we performed extensive in vitro alanine mutagenesis to identify A28 protein residues that are critical for H2 binding, entry fusion complex formation, and virus-mediated membrane fusion. Finally, we used molecular dynamic simulations to model full-length A28-H2 subcomplex in membranes. In summary, we characterized vaccinia virus A28 protein and determined residues important in its interaction with H2 protein and membrane components. We also provide a structural model of the A28-H2 protein interaction to illustrate how it forms a 1:1 subcomplex on a modeled membrane.

Immunogenic proteins and potential delivery platforms for mpox virus vaccine development: A rapid review

Since May 2022, the mpox virus (MPXV) has spread worldwide and become a potential threat to global public health. Vaccines are important tools for preventing MPXV transmission and infection in the population. However, there are still no available potent and applicable vaccines specifically for MPXV. Herein, we highlight several potential vaccine targets for MPVX and emphasize potent immunogens, such as M1R, E8L, H3L, A29L, A35R, and B6R proteins. These proteins can be integrated into diverse vaccine platforms to elicit powerful B-cell and T-cell responses, thereby providing protective immunity against MPXV infection. Overall, research on the MPXV vaccine targets would provide valuable information for developing timely effective MPXV-specific vaccines.

Monkeypox: epidemiology, pathogenesis, treatment and prevention

Monkeypox is a zoonotic disease that was once endemic in west and central Africa caused by monkeypox virus. However, cases recently have been confirmed in many nonendemic countries outside of Africa. WHO declared the ongoing monkeypox outbreak to be a public health emergency of international concern on July 23, 2022, in the context of the COVID-19 pandemic. The rapidly increasing number of confirmed cases could pose a threat to the international community. Here, we review the epidemiology of monkeypox, monkeypox virus reservoirs, novel transmission patterns, mutations and mechanisms of viral infection, clinical characteristics, laboratory diagnosis and treatment measures. In addition, strategies for the prevention, such as vaccination of smallpox vaccine, is also included. Current epidemiological data indicate that high frequency of human-to-human transmission could lead to further outbreaks, especially among men who have sex with men. The development of antiviral drugs and vaccines against monkeypox virus is urgently needed, despite some therapeutic effects of currently used drugs in the clinic. We provide useful information to improve the understanding of monkeypox virus and give guidance for the government and relative agency to prevent and control the further spread of monkeypox virus.

Insights into the Organization of the Poxvirus Multicomponent Entry-Fusion Complex from Proximity Analyses in Living Infected Cells

Poxviruses are exceptional in having a complex entry-fusion complex (EFC) that is comprised of 11 conserved proteins embedded in the membrane of mature virions. However, the detailed architecture is unknown and only a few bimolecular protein interactions have been demonstrated by coimmunoprecipitation from detergent-treated lysates and by cross-linking. Here, we adapted the tripartite split green fluorescent protein (GFP) complementation system in order to analyze EFC protein contacts within living cells. This system employs a detector fragment called GFP1-9 comprised of nine GFP β-strands. To achieve fluorescence, two additional 20-amino-acid fragments called GFP10 and GFP11 attached to interacting proteins are needed, providing the basis for identification of the latter. We constructed a novel recombinant vaccinia virus (VACV-GFP1-9) expressing GFP1-9 under a viral early/late promoter and plasmids with VACV late promoters regulating each of the EFC proteins with GFP10 or GFP11 attached to their ectodomains. GFP fluorescence was detected by confocal microscopy at sites of virion assembly in cells infected with VACV-GFP1-9 and cotransfected with plasmids expressing one EFC-GFP10 and one EFC-GFP11 interacting protein. Flow cytometry provided a quantitative way to determine the interaction of each EFC-GFP10 protein with every other EFC-GFP11 protein in the context of a normal infection in which all viral proteins are synthesized and assembled. Previous EFC protein interactions were confirmed, and new ones were discovered and corroborated by additional methods. Most remarkable was the finding that the small, hydrophobic O3 protein interacted with each of the other EFC proteins. IMPORTANCE Poxviruses are enveloped viruses with a DNA-containing core that enters cells following fusion of viral and host membranes. This essential step is a target for vaccines and therapeutics. The entry-fusion complex (EFC) of poxviruses is unusually complex and comprised of 11 conserved viral proteins. Determination of the structure of the EFC is a prerequisite for understanding the fusion mechanism. Here, we used a tripartite split green fluorescent protein assay to determine the proximity of individual EFC proteins in living cells. A network connecting components of the EFC was derived.

Loss of the vaccinia virus 35-amino acid hydrophobic O3 protein is partially compensated by mutations in the transmembrane domains of other entry proteins

Eleven highly conserved proteins comprise the poxvirus entry-fusion complex (EFC). We focused on vaccinia virus (VACV) O3, a 35-amino acid, largely hydrophobic component of unknown specific function. Experimental evolution was carried out by blindly passaging a virus that was severely impaired in entry due to deletion of the gene encoding O3. Large plaque variants that arose spontaneously were discerned by round four and their numbers increased thereafter. Genome sequencing of individual cloned viruses revealed mutations in predicted transmembrane domains of three open reading frames encoding proteins with roles in entry. There were frame-shift mutations in consecutive Ts in open reading frames F9L and D8L and a nonsynonymous base substitution in L5R. F9 and L5 are EFC proteins and D8 is involved in VACV cell attachment. The F9L mutation occurred by round four in each of three independant passages, whereas the L5R and D8L mutations were detected only after nearly all of the genomes already had the F9L mutation. Viruses with deletions of O3L and single or double F9L, L5R and D8L mutations were constructed by homologous recombination. In a single round of infection, viruses with adaptive mutations including F9L alone or in combination exhibited statistically significant higher virus titers than the parental O3L deletion mutant or the L5R or D8L mutants, consistent with the order of selection during the passages. Further analyses indicated that the adaptive F9L mutants also had higher infectivities, entered cells more rapidly and increased EFC assembly, which partially compensated for the loss of O3.IMPORTANCE Entry into cells is an essential first step in virus replication and an important target of vaccine- elicited immunity. For enveloped viruses, this step involves the fusion of viral and host membranes to form a pore allowing entry of the genome and associated proteins. Poxviruses are unique in that this function is mediated by an entry-fusion complex (EFC) of eleven transmembrane proteins rather than by one or a few. The large number of proteins has hindered investigation of their individual roles. We focused on O3, a predominantly hydrophobic 35 amino acid component of the vaccinia virus EFC, and found that spontaneous mutations in the transmembrane domains of certain other entry proteins can partially compensate for the absence of O3. The mutants exhibited increased infectivity, entry and assembly or stability of the EFC.

Per Os Infectivity Factor 5 Identified as a Substrate of P33 in the Baculoviral Disulfide Bond Formation Pathway

Disulfide bonds are critical for the structure and function of many proteins. Some large DNA viruses encode their own sulfhydryl oxidase for disulfide bond formation. Previous studies have demonstrated that the baculovirus-encoded sulfhydryl oxidase P33 is necessary for progeny virus production, and its enzymatic activity is important for morphogenesis and oral infectivity of baculoviruses. However, the downstream substrates of P33 in the putative redox pathway of baculoviruses are unknown. In this study, we showed that PIF5, one of the per os infectivity factors (PIFs), contained intramolecular disulfide bonds and that the disulfide bond formation was interrupted in the absence of P33. In vivo pulldown and colocalization analyses revealed that PIF5 and P33 interacted with each other during virus infection. Further, in vitro assays validated that the reduced PIF5 proteins could be oxidized by P33. To understand the contribution of disulfide bonds to the function of PIF5, several cysteine-to-serine mutants were constructed, which all interfered with the disulfide bond formation of PIF5 to different extents. All the mutants lost their oral infectivity but had no impact on infectious budding virus (BV) production or virus morphogenesis. Taken together, our results indicated PIF5 as the first identified substrate of P33. Further, the disulfide bonds in PIF5 play an essential role in its function in oral infection.IMPORTANCE Similar to some large DNA viruses that encode their own disulfide bond pathway, baculovirus encodes a viral sulfhydryl oxidase, P33. Enzyme activity of P33 is related to infectious BV production, occlusion-derived virus (ODV) envelopment, occlusion body morphogenesis, and oral infectivity, suggesting that P33 is involved in disulfide bond formation of multiple proteins. A complete disulfide bond formation pathway normally contains a sulfhydryl oxidase, a disulfide-donating enzyme, and one or more substrates. In baculovirus, apart from P33, other components of the putative pathway remain unknown. In this study, we identified PIF5 as the first substrate of P33, which is fundamental for revealing the complete disulfide bond formation pathway in baculovirus. PIF5 is essential for oral infection and is absent from the PIF complex. Our study demonstrated that native disulfide bonds in PIF5 are required for oral infection, which will help us to reveal its mode of action.

Investigating Viruses During the Transformation of Molecular Biology: Part II

My scientific career started at an extraordinary time, shortly after the discoveries of the helical structure of DNA, the central dogma of DNA to RNA to protein, and the genetic code. Part I of this series emphasizes my education and early studies highlighted by the isolation and characterization of numerous vaccinia virus enzymes, determination of the cap structure of messenger RNA, and development of poxviruses as gene expression vectors for use as recombinant vaccines. Here I describe a shift in my research focus to combine molecular biology and genetics for a comprehensive understanding of poxvirus biology. The dominant paradigm during the early years was to select a function, isolate the responsible proteins, and locate the corresponding gene, whereas later the common paradigm was to select a gene, make a mutation, and determine the altered function. Motivations, behind-the-scenes insights, importance of new technologies, and the vital roles of trainees and coworkers are emphasized.

Mutations Near the N Terminus of Vaccinia Virus G9 Protein Overcome Restrictions on Cell Entry and Syncytium Formation Imposed by the A56/K2 Fusion Regulatory Complex

The entry/fusion complex (EFC) consists of 11 conserved proteins embedded in the membrane envelope of mature poxvirus particles. Poxviruses also encode proteins that localize in cell membranes and negatively regulate superinfection and syncytium formation. The vaccinia virus (VACV) A56/K2 fusion regulatory complex associates with the G9/A16 EFC subcomplex, but functional support for the importance of this interaction was lacking. Here, we describe serially passaging VACV in nonpermissive cells expressing A56/K2 as an unbiased approach to isolate and analyze escape mutants. Viruses forming large plaques in A56/K2 cells increased in successive rounds of infection, indicating the occurrence and enrichment of adaptive mutations. Sequencing of genomes of passaged and cloned viruses revealed mutations near the N terminus of the G9 open reading frame but none in A16 or other genes. The most frequent mutation was His to Tyr at amino acid 44; additional escape mutants had a His-to-Arg mutation at amino acid 44 or a duplication of amino acids 26 to 39. An adaptive Tyr-to-Cys substitution at amino acid 42 was discovered using error-prone PCR to generate additional mutations. Myristoylation of G9 was unaffected by the near-N-terminal mutations. The roles of the G9 mutations in enhancing plaque size were validated by homologous recombination. The mutants exhibited enhanced entry and spread in A56/K2 cells and induced syncytia at neutral pH in HeLa cells despite the expression of A56/K2. The data suggest that the mutations perturb the interaction of G9 with A56/K2, although some association was still detected in detergent-treated infected cell lysates.IMPORTANCE The entry of enveloped viruses is achieved by the fusion of viral and cellular membranes, a critical step in infection that determines host range and provides targets for vaccines and therapeutics. Poxviruses encode an exceptionally large number of proteins comprising the entry/fusion complex (EFC), which enables infection of diverse cells. Vaccinia virus (VACV), the prototype member of the poxvirus family, also encodes the fusion regulatory proteins A56 and K2, which are displayed on the plasma membrane and may be beneficial by preventing reinfection and cell-cell fusion. Previous studies showed that A56/K2 interacts with the G9/A16 EFC subcomplex in detergent-treated cell extracts. Functional evidence for the importance of this interaction was obtained by serially passaging wild-type VACV in cells that are nonpermissive because of A56/K2 expression. VACV mutants with amino acid substitutions or duplications near the N terminus of G9 were enriched because of their ability to overcome the block to entry imposed by A56/K2.

Experimental Evolution To Isolate Vaccinia Virus Adaptive G9 Mutants That Overcome Membrane Fusion Inhibition via the Vaccinia Virus A56/K2 Protein Complex

For cell entry, vaccinia virus requires fusion with the host membrane via a viral fusion complex of 11 proteins, but the mechanism remains unclear. It was shown previously that the viral proteins A56 and K2 are expressed on infected cells to prevent superinfection by extracellular vaccinia virus through binding to two components of the viral fusion complex (G9 and A16), thereby inhibiting membrane fusion. To investigate how the A56/K2 complex inhibits membrane fusion, we performed experimental evolutionary analyses by repeatedly passaging vaccinia virus in HeLa cells overexpressing the A56 and K2 proteins to isolate adaptive mutant viruses. Genome sequencing of adaptive mutants revealed that they had accumulated a unique G9R open reading frame (ORF) mutation, resulting in a single His44Tyr amino acid change. We engineered a recombinant vaccinia virus to express the G9^H44Y mutant protein, and it readily infected HeLa-A56/K2 cells. Moreover, similar to the ΔA56 virus, the G9^H44Y mutant virus on HeLa cells had a cell fusion phenotype, indicating that G9^H44Y-mediated membrane fusion was less prone to inhibition by A56/K2. Coimmunoprecipitation experiments demonstrated that the G9^H44Y protein bound to A56/K2 at neutral pH, suggesting that the H44Y mutation did not eliminate the binding of G9 to A56/K2. Interestingly, upon acid treatment to inactivate A56/K2-mediated fusion inhibition, the G9^H44Y mutant virus induced robust cell-cell fusion at pH 6, unlike the pH 4.7 required for control and revertant vaccinia viruses. Thus, A56/K2 fusion suppression mainly targets the G9 protein. Moreover, the G9^H44Y mutant protein escapes A56/K2-mediated membrane fusion inhibition most likely because it mimics an acid-induced intermediate conformation more prone to membrane fusion.IMPORTANCE It remains unclear how the multiprotein entry fusion complex of vaccinia virus mediates membrane fusion. Moreover, vaccinia virus contains fusion suppressor proteins to prevent the aberrant activation of this multiprotein complex. Here, we used experimental evolution to identify adaptive mutant viruses that overcome membrane fusion inhibition mediated by the A56/K2 protein complex. We show that the H44Y mutation of the G9 protein is sufficient to overcome A56/K2-mediated membrane fusion inhibition. Treatment of virus-infected cells at different pHs indicated that the H44Y mutation lowers the threshold of fusion inhibition by A56/K2. Our study provides evidence that A56/K2 inhibits the viral fusion complex via the latter's G9 subcomponent. Although the G9^H44Y mutant protein still binds to A56/K2 at neutral pH, it is less dependent on low pH for fusion activation, implying that it may adopt a subtle conformational change that mimics a structural intermediate induced by low pH.

The Ectodomain of the Vaccinia Virus Glycoprotein A34 Is Required for Cell Binding by Extracellular Virions and Contains a Large Region Capable of Interaction with Glycoprotein B5

An interaction between the orthopoxvirus glycoproteins A34 and B5 has been reported. The transmembrane and ectodomain of A34 are sufficient for interaction with B5, localization of B5 to the site of intracellular wrapping, and subsequent incorporation into the envelope of released extracellular virions. Several mutagenic approaches were undertaken to better define the B5 interaction domain on A34. A set of C-terminal truncations in A34 identified residues 1 to 80 as sufficient for interaction with B5. Additional truncations identified residues 80 to 130 of A34 as sufficient for interaction with B5. To better understand the function of this region, a set of recombinant viruses expressing A34 with the full, partial, or no B5 interaction site (residues 1 to 130, 1 to 100, and 1 to 70, respectively) was constructed. All the recombinants expressing truncations of A34 incorporated B5 into extracellular virions but had a small-plaque phenotype similar to that of a virus with the A34R gene deleted (vΔA34R). Further characterization indicated that the small-plaque phenotype exhibited by these viruses is due to a combination of abrogated actin tail formation, reduced cell binding, and a defect in polyanion-induced nonfusogenic dissolution. Taken together, these results suggest that residues 80 to 130 of A34 are not necessary for the proper localization and incorporation of B5 into extracellular virions and, furthermore, that the C-terminal residues of A34 are involved in cell binding and dissolution.IMPORTANCE Previous studies have shown that the vaccinia virus glycoproteins A34 and B5 interact, and in the absence of A34, B5 is mislocalized and not incorporated into extracellular virions. Here, using a transient-transfection assay, residues 80 to 130 of the ectodomain of A34 were determined to be sufficient for interaction with B5. Recombinant viruses expressing A34 with a full, partial, or no B5 interaction site were constructed and characterized. All of the A34 truncations interacted with B5 as predicted by the transient-transfection studies but had a small-plaque phenotype. Further analysis revealed that all of the recombinants incorporated detectable levels of B5 into released virions but were defective in cell binding and extracellular virion (EV) dissolution. This study is the first to directly demonstrate that A34 is involved in cell binding and implicate the ectodomain in this role.

The 2.1 Å structure of protein F9 and its comparison to L1, two components of the conserved poxvirus entry-fusion complex

The poxvirus F9 protein is a component of the vaccinia virus entry fusion complex (EFC) which consists of 11 proteins. The EFC forms a unique apparatus among viral fusion proteins and complexes. We solved the atomic structure of the F9 ectodomain at 2.10 Å. A structural comparison to the ectodomain of the EFC protein L1 indicated a similar fold and organization, in which a bundle of five α-helices is packed against two pairs of β-strands. However, instead of the L1 myristoylation site and hydrophobic cavity, F9 possesses a protruding loop between α-helices α3 and α4 starting at Gly90. Gly90 is conserved in all poxviruses except Salmon gill poxvirus (SGPV) and Diachasmimorpha longicaudata entomopoxvirus. Phylogenetic sequence analysis of all Poxviridae F9 and L1 orthologs revealed the SGPV genome to contain the most distantly related F9 and L1 sequences compared to the vaccinia proteins studied here. The structural differences between F9 and L1 suggest functional adaptations during evolution from a common precursor that underlie the present requirement for each protein.

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.

Three Conserved Regions in Baculovirus Sulfhydryl Oxidase P33 Are Critical for Enzymatic Activity and Function

Baculoviruses encode a conserved sulfhydryl oxidase, P33, which is necessary for budded virus (BV) production and multinucleocapsid occlusion-derived virus (ODV) formation. Here, the structural and functional relationship of P33 was revealed by X-ray crystallography, site-directed mutagenesis, and functional analysis. Based on crystallographic characterization and structural analysis, a series of P33 mutants within three conserved regions, i.e., the active site, the dimer interface, and the R127-E183 salt bridge, were constructed. In vitro experiments showed that mutations within the active site and dimer interface severely impaired the sulfhydryl oxidase activity of P33, while the mutations in the salt bridge had a relatively minor influence. Recombinant viruses containing mutated P33 were constructed and assayed in vivo Except for the active-site mutant AXXA, all other mutants produced infectious BVs, although certain mutants had a decreased BV production. The active-site mutant H114A, the dimer interface mutant H227D, and the salt bridge mutant R127A-E183A were further analyzed by electron microscopy and bioassays. The occlusion bodies (OBs) of mutants H114A and R127A-E183A had a ragged surface and contained mostly ODVs with a single nucleocapsid. The OBs of all three mutants contained lower numbers of ODVs and had a significantly reduced oral infectivity in comparison to control virus. Crystallographic analyses further revealed that all three regions may coordinate with one another to achieve optimal function of P33. Taken together, our data revealed that all the three conserved regions are involved in P33 activity and are crucial for virus morphogenesis and peroral infectivity.IMPORTANCE Sulfhydryl oxidase catalyzes disulfide bond formation of substrate proteins. P33, a baculovirus-encoded sulfhydryl oxidase, is different from other cellular and viral sulfhydryl oxidases, bearing unique features in tertiary and quaternary structure organizations. In this study, we found that three conserved regions, i.e., the active site, dimer interface, and the R127-E183 salt bridge, play important roles in the enzymatic activity and function of P33. Previous observations showed that deletion of p33 results in a total loss of budded virus (BV) production and in morphological changes in occlusion-derived virus (ODV). Our study revealed that certain P33 mutants lead to occlusion bodies (OBs) with a ragged surface, decreased embedded ODVs, and reduced oral infectivity. Interestingly, some P33 mutants with impaired ODV/OB still retained BV productivity, indicating that the impacts on BV and on ODV/OB are two distinctly different functions of P33, which are likely to be performed via different substrate proteins.

Deletion of the Vaccinia Virus I2 Protein Interrupts Virion Morphogenesis, Leading to Retention of the Scaffold Protein and Mislocalization of Membrane-Associated Entry Proteins

The I2L open reading frame of vaccinia virus (VACV) encodes a conserved 72-amino-acid protein with a putative C-terminal transmembrane domain. Previous studies with a tetracycline-inducible mutant demonstrated that I2-deficient virions are defective in cell entry. The purpose of the present study was to determine the step of replication or entry that is affected by loss of the I2 protein. Fluorescence microscopy experiments showed that I2 colocalized with a major membrane protein of immature and mature virions. We generated a cell line that constitutively expressed I2 and allowed construction of the VACV I2L deletion mutant vΔI2. As anticipated, vΔI2 was unable to replicate in cells that did not express I2. Unexpectedly, morphogenesis was interrupted at a stage after immature virion formation, resulting in the accumulation of dense spherical particles instead of brick-shaped mature virions with well-defined core structures. The abnormal particles retained the D13 scaffold protein of immature virions, were severely deficient in the transmembrane proteins that comprise the entry fusion complex (EFC), and had increased amounts of unprocessed membrane and core proteins. Total lysates of cells infected with vΔI2 also had diminished EFC proteins due to instability attributed to their hydrophobicity and failure to be inserted into viral membranes. A similar instability of EFC proteins had previously been found with unrelated mutants blocked earlier in morphogenesis that also accumulated viral membranes retaining the D13 scaffold. We concluded that I2 is required for virion morphogenesis, release of the D13 scaffold, and the association of EFC proteins with viral membranes.IMPORTANCE Poxviruses comprise a large family that infect vertebrates and invertebrates, cause disease in both in humans and in wild and domesticated animals, and are being engineered as vectors for vaccines and cancer therapy. In addition, investigations of poxviruses have provided insights into many aspects of cell biology. The I2 protein is conserved in all poxviruses that infect vertebrates, suggesting an important role. The present study revealed that this protein is essential for vaccinia virus morphogenesis and that its absence results in an accumulation of deformed virus particles retaining the scaffold protein and deficient in surface proteins needed for cell entry.

Antibody Recognition of Immunodominant Vaccinia Virus Envelope Proteins

Vaccinia Virus (VACV) is an enveloped double stranded DNA virus and the active ingredient of the smallpox vaccine. The systematic administration of this vaccine led to the eradication of circulating smallpox (variola virus, VARV) from the human population. As a tribute to its success, global immunization was ended in the late 1970s. The efficacy of the vaccine is attributed to a robust production of protective antibodies against several envelope proteins of VACV, which cross-protect against infection with pathogenic VARV. Since global vaccination was ended, most children and young adults do not possess immunity against smallpox. This is a concern, since smallpox is considered a potential bioweapon. Although the smallpox vaccine is considered the gold standard of all vaccines and the targeted antigens have been widely studied, the epitopes that are targeted by the protective antibodies and their mechanism of binding had been, until recently, poorly characterized. Understanding the precise interaction between the antibodies and their epitopes will be helpful in the design of better vaccines against other diseases. In this review we will discuss the structural basis of recognition of the immunodominant VACV antigens A27, A33, D8, and L1 by protective antibodies and discuss potential implications regarding their protective capacity.

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.

Fine structure of the vaccinia virion determined by controlled degradation and immunolocalization

The vaccinia virion is a membraned, slightly flattened, barrel-shaped particle, with a complex internal structure featuring a biconcave core flanked by lateral bodies. Although the architecture of the purified mature virion has been intensely characterized by electron microscopy, the distribution of the proteins within the virion has been examined primarily using biochemical procedures. Thus, it has been shown that non-ionic and ionic detergents combined or not with a sulfhydryl reagent can be used to disrupt virions and, to a limited degree, separate the constituent proteins in different fractions. Applying a controlled degradation technique to virions adsorbed on EM grids, we were able to immuno-localize viral proteins within the virion particle. Our results show after NP40 and DTT treatment, membrane proteins are removed from the virion surface revealing proteins that are associated with the lateral bodies and the outer layer of the core wall. Combined treatment using high salt and high DTT removed lateral body proteins and exposed proteins of the internal core wall. Cores treated with proteases could be disrupted and the internal components were exposed. Cts8, a mutant in the A3 protein, produces aberrant virus that, when treated with NP-40 and DTT, releases to the exterior the virus DNA associated with other internal core proteins. With these results, we are able to propose a model for the structure the vaccinia virion.

From crescent to mature virion: vaccinia virus assembly and maturation

Vaccinia virus (VACV) has achieved unprecedented success as a live viral vaccine for smallpox which mitigated eradication of the disease. Vaccinia virus has a complex virion morphology and recent advances have been made to answer some of the key outstanding questions, in particular, the origin and biogenesis of the virion membrane, the transformation from immature virion (IV) to mature virus (MV), and the role of several novel genes, which were previously uncharacterized, but have now been shown to be essential for VACV virion formation. This new knowledge will undoubtedly contribute to the rational design of safe, immunogenic vaccine candidates, or effective antivirals in the future. This review endeavors to provide an update on our current knowledge of the VACV maturation processes with a specific focus on the initiation of VACV replication through to the formation of mature virions.

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.

Initial characterization of vaccinia virus B4 suggests a role in virus spread

Currently, little is known about the ankyrin/F-box protein B4. Here, we report that B4R-null viruses exhibited reduced plaque size in tissue culture, and decreased ability to spread, as assessed by multiple-step growth analysis. Electron microscopy indicated that B4R-null viruses still formed mature and extracellular virions; however, there was a slight decrease of virions released into the media following deletion of B4R. Deletion of B4R did not affect the ability of the virus to rearrange actin; however, VACV811, a large vaccinia virus deletion mutant missing 55 open reading frames, had decreased ability to produce actin tails. Using ectromelia virus, a natural mouse pathogen, we demonstrated that virus devoid of EVM154, the B4R homolog, showed decreased spread to organs and was attenuated during infection. This initial characterization suggests that B4 may play a role in virus spread, and that other unidentified mediators of actin tail formation may exist in vaccinia virus.

Direct formation of vaccinia virus membranes from the endoplasmic reticulum in the absence of the newly characterized L2-interacting protein A30.5

Crescents consisting of a single lipoprotein membrane with an external protein scaffold comprise the initial structural elements of poxvirus morphogenesis. Crescents enlarge to form spherical immature virions, which enclose viroplasm consisting of proteins destined to form the cores of mature virions. Previous studies suggest that the L2 protein participates in the recruitment of endoplasmic reticulum (ER)-derived membranes to form immature virions within assembly sites of cytoplasmic factories. Here we show that L2 interacts with the previously uncharacterized 42-amino-acid A30.5 protein. An open reading frame similar in size to the one encoding A30.5 is at the same genome location in representatives of all chordopoxvirus genera. A30.5 has a putative transmembrane domain and colocalized with markers of the endoplasmic reticulum and with L2. By constructing a complementing cell line expressing A30.5, we isolated a deletion mutant virus that exhibits a defect in morphogenesis in normal cells. Large electron-dense cytoplasmic inclusions and clusters of scaffold protein-coated membranes that resemble crescents and immature virions devoid of viroplasm were seen in place of normal structures. Crescent-shaped membranes were continuous with the endoplasmic reticulum membrane and oriented with the convex scaffold protein-coated side facing the lumen, while clusters of completed spherical immature-virion-like forms were trapped within the expanded lumen. Immature-virion-like structures were more abundant in infected RK-13 cells than in BS-C-1 or HeLa cells, in which cytoplasmic inclusions were decorated with scaffold protein-coated membrane arcs. We suggest that the outer surface of the poxvirus virion is derived from the luminal side of the ER membrane.

Analysis of viral membranes formed in cells infected by a vaccinia virus L2-deletion mutant suggests their origin from the endoplasmic reticulum

Assembly of the poxvirus immature virion (IV) membrane is a poorly understood event that occurs within the cytoplasm. At least eight viral proteins participate in formation of the viral membrane. Of these, A14, A17, and D13 are structural components whereas A6, A11, F10, H7, and L2 participate in membrane biogenesis. L2, the object of this study, is conserved in all chordopoxviruses, expressed early in infection, and associated with the endoplasmic reticulum (ER) throughout the cell and at the edges of crescent-shaped IV precursors. Previous studies with an inducible L2 mutant revealed abortive formation of the crescent membrane. However, possible low-level L2 synthesis under nonpermissive conditions led to ambiguity in interpretation. Here, we constructed a cell line that expresses L2, which allowed the creation of an L2-deletion mutant. In noncomplementing cells, replication was aborted prior to formation of mature virions and two types of aberrant structures were recognized. One consisted of short crescents, at the surface of dense masses of viroplasm, which were labeled with antibodies to the A11, A14, A17, and D13 proteins. The other structure consisted of "empty" IV-like membranes, also labeled with antibodies to the viral proteins, which appeared to be derived from adjacent calnexin-containing ER. A subset of 25 proteins examined, exemplified by components of the entry-fusion complex, were greatly diminished in amount. The primary role of L2 may be to recruit ER and modulate its transformation to viral membranes in juxtaposition with the viroplasm, simultaneously preventing the degradation of viral proteins dependent on viral membranes for stability.

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.

Structural and biochemical characterization of the vaccinia virus envelope protein D8 and its recognition by the antibody LA5

Smallpox vaccine is considered a gold standard of vaccines, as it is the only one that has led to the complete eradication of an infectious disease from the human population. B cell responses are critical for the protective immunity induced by the vaccine, yet their targeted epitopes recognized in humans remain poorly described. Here we describe the biochemical and structural characterization of one of the immunodominant vaccinia virus (VACV) antigens, D8, and its binding to the monoclonal antibody LA5, which is capable of neutralizing VACV in the presence of complement. The full-length D8 ectodomain was found to form a tetramer. We determined the crystal structure of the LA5 Fab-monomeric D8 complex at a resolution of 2.1 Å, as well as the unliganded structures of D8 and LA5-Fab at resolutions of 1.42 Å and 1.6 Å, respectively. D8 features a carbonic anhydrase (CAH) fold that has evolved to bind to the glycosaminoglycan (GAG) chondroitin sulfate (CS) on host cells. The central positively charged crevice of D8 was predicted to be the CS binding site by automated docking experiments. Furthermore, sequence alignment of various poxvirus D8 orthologs revealed that this crevice is structurally conserved. The D8 epitope is formed by 23 discontinuous residues that are spread across 80% of the D8 protein sequence. Interestingly, LA5 binds with a high-affinity lock-and-key mechanism above this crevice with an unusually large antibody-antigen interface, burying 2,434 Å(2) of protein surface.

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.

The myristate moiety and amino terminus of vaccinia virus l1 constitute a bipartite functional region needed for entry

Vaccinia virus (VACV) L1 is a myristoylated envelope protein which is required for cell entry and the fusion of infected cells. L1 associates with members of the entry-fusion complex (EFC), but its specific role in entry has not been delineated. We recently demonstrated (Foo CH, et al., Virology 385:368-382, 2009) that soluble L1 binds to cells and blocks entry, suggesting that L1 serves as the receptor-binding protein for entry. Our goal is to identify the structural domains of L1 which are essential for its functions in VACV entry. We hypothesized that the myristate and the conserved residues at the N terminus of L1 are critical for entry. To test our hypothesis, we generated mutants in the N terminus of L1 and used a complementation assay to evaluate their ability to rescue infectivity. We also assessed the myristoylation efficiency of the mutants and their ability to interact with the EFC. We found that the N terminus of L1 constitutes a region that is critical for the infectivity of VACV and for myristoylation. At the same time, the nonmyristoylated mutants were incorporated into mature virions, suggesting that the myristate is not required for the association of L1 with the viral membrane. Although some of the mutants exhibited altered structural conformations, two mutants with impaired infectivity were similar in conformation to wild-type L1. Importantly, these two mutants, with changes at A4 and A5, undergo myristoylation. Overall, our results imply dual differential roles for myristate and the amino acids at the N terminus of L1. We propose a myristoyl switch model to describe how L1 functions.

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.

Antiviral activity of the EB peptide against zoonotic poxviruses

Background

The EB peptide is a 20-mer that was previously shown to have broad spectrum in vitro activity against several unrelated viruses, including highly pathogenic avian influenza, herpes simplex virus type I, and vaccinia, the prototypic orthopoxvirus. To expand on this work, we evaluated EB for in vitro activity against the zoonotic orthopoxviruses cowpox and monkeypox and for in vivo activity in mice against vaccinia and cowpox.

Findings

In yield reduction assays, EB had an EC₅₀ of 26.7 μM against cowpox and 4.4 μM against monkeypox. The EC₅₀ for plaque reduction was 26.3 μM against cowpox and 48.6 μM against monkeypox. A scrambled peptide had no inhibitory activity against either virus. EB inhibited cowpox in vitro by disrupting virus entry, as evidenced by a reduction of the release of virus cores into the cytoplasm. Monkeypox was also inhibited in vitro by EB, but at the attachment stage of infection. EB showed protective activity in mice infected intranasally with vaccinia when co-administered with the virus, but had no effect when administered prophylactically one day prior to infection or therapeutically one day post-infection. EB had no in vivo activity against cowpox in mice.

Conclusions

While EB did demonstrate some in vivo efficacy against vaccinia in mice, the limited conditions under which it was effective against vaccinia and lack of activity against cowpox suggest EB may be more useful for studying orthopoxvirus entry and attachment in vitro than as a therapeutic against orthopoxviruses in vivo.

Transcriptional repression and RNA silencing act synergistically to demonstrate the function of the eleventh component of the vaccinia virus entry-fusion complex

Poxviruses have an elaborate system for infecting cells comprising several proteins for attachment and a larger number dedicated to membrane fusion and entry. Thus far, 11 proteins have been identified as components of the vaccinia virus (VACV) entry-fusion complex (EFC), and 10 of these proteins have been shown to be required for entry. J5, the remaining functionally uncharacterized component of the complex, is conserved in all poxviruses, has a predicted C-terminal transmembrane domain, and is an N-terminally truncated paralog of two other EFC proteins. To determine the role of J5, we constructed a mutant that inducibly regulates J5 transcription. Although the virus yield was reduced only about 80% without inducer, the inability to isolate a J5 deletion mutant suggested an essential function. To enhance stringency, we employed RNA silencing alone and together with transcriptional repression of the inducible mutant. The yield of infectious virus was reduced 4- to 5-fold by repression, 2-fold by silencing, and 60-fold by the combination of the two. Virus particles made under the latter conditions appeared to contain a full complement of proteins excluding J5 but had very low infectivity. Further studies indicated that after binding to cells, J5-deficient virions had a defect in core entry and an inability to induce syncytium formation. In addition, we confirmed that J5 is associated with the EFC by affinity purification. These data indicate that J5 is a functional component of the EFC and highlights the advantage of combining transcriptional repression and RNA silencing for stringent reduction of gene expression.

The membrane fusion step of vaccinia virus entry is cooperatively mediated by multiple viral proteins and host cell components

For many viruses, one or two proteins allow cell attachment and entry, which occurs through the plasma membrane or following endocytosis at low pH. In contrast, vaccinia virus (VACV) enters cells by both neutral and low pH routes; four proteins mediate cell attachment and twelve that are associated in a membrane complex and conserved in all poxviruses are dedicated to entry. The aim of the present study was to determine the roles of cellular and viral proteins in initial stages of entry, specifically fusion of the membranes of the mature virion and cell. For analysis of the role of cellular components, we used well characterized inhibitors and measured binding of a recombinant VACV virion containing Gaussia luciferase fused to a core protein; viral and cellular membrane lipid mixing with a self-quenching fluorescent probe in the virion membrane; and core entry with a recombinant VACV expressing firefly luciferase and electron microscopy. We determined that inhibitors of tyrosine protein kinases, dynamin GTPase and actin dynamics had little effect on binding of virions to cells but impaired membrane fusion, whereas partial cholesterol depletion and inhibitors of endosomal acidification and membrane blebbing had a severe effect at the later stage of core entry. To determine the role of viral proteins, virions lacking individual membrane components were purified from cells infected with members of a panel of ten conditional-lethal inducible mutants. Each of the entry protein-deficient virions had severely reduced infectivity and except for A28, L1 and L5 greatly impaired membrane fusion. In addition, a potent neutralizing L1 monoclonal antibody blocked entry at a post-membrane lipid-mixing step. Taken together, these results suggested a 2-step entry model and implicated an unprecedented number of viral proteins and cellular components involved in signaling and actin rearrangement for initiation of virus-cell membrane fusion during poxvirus entry.

Poxviruses are large DNA viruses that cause diseases in humans and other animals. To initiate infection, the core of the large, membrane-enveloped particle must penetrate into the cytoplasm where replication occurs. For most enveloped viruses only one or two proteins are needed for attachment and penetration. However, at least sixteen poxvirus proteins are dedicated to entry: four for attachment and twelve for penetration. The latter proteins form the entry fusion complex (EFC) and are conserved in all poxviruses indicating that the entry mechanism has been retained since the origin of the family. The purpose of the present study was to determine the cellular processes and poxviral proteins needed for fusion of the viral and cellular membranes. We found that a variety of inhibitors that interfered with cell signaling and reorganization of the actin cytoskeleton prevented membrane fusion as determined by lipid mixing, whereas others targeted the subsequent stage in entry. In addition, seven viral protein components of the EFC were required for the initial membrane fusion step, whereas three were not. A neutralizing monoclonal antibody to one of the latter also did not interfere with membrane lipid mixing but still prevented core entry supporting a 2-step poxvirus entry model.

Vaccinia virus L2 protein associates with the endoplasmic reticulum near the growing edge of crescent precursors of immature virions and stabilizes a subset of viral membrane proteins

The initial step in poxvirus morphogenesis, the formation of crescent membranes, occurs within cytoplasmic factories. L2 is one of several vaccinia virus proteins known to be necessary for formation of crescents and the only one synthesized early in infection. Virus replication was unaffected when the L2R open reading frame was replaced by L2R containing an N-terminal epitope tag while retaining the original promoter. L2 colocalized with the endoplasmic reticulum (ER) protein calnexin throughout the cytoplasm of infected and transfected cells. Topological studies indicated that the N terminus of L2 is exposed to the cytoplasm with the hydrophobic C terminus anchored in the ER. Using immunogold labeling and electron microscopy, L2 was detected in tubular membranes outside factories and inside factories near crescents and close to the edge or rim of crescents; a similar labeling pattern was found for the ER luminal protein disulfide isomerase (PDI). The phenotype of L2 conditional lethal mutants and the localization of L2 suggest that it participates in elongation of crescents by the addition of ER membrane to the growing edge. Small amounts of L2 and PDI were detected within immature and mature virions, perhaps trapped during assembly. The repression of L2, as well as A11 and A17, two other proteins that are required for viral crescent formation, profoundly decreased the stability of a subset of viral membrane proteins including those comprising the entry-fusion complex. To avoid degradation, these unstable membrane proteins may need to directly insert into the viral membrane or be rapidly shunted there from the ER.

Role of sulfatide in vaccinia virus infection

BACKGROUND INFORMATION

Vaccinia virus (VACV) was used as a surrogate of variola virus (genus Orthopoxvirus), the causative agent of smallpox, to study orthopoxvirus infection. VACV infects cells via attachment and fusion of the viral membrane with the host cell membrane. Glycosphingolipids, expressed in multiple organs, are major components of lipid rafts and have been associated with the infectious route of several pathogens.

RESULTS

We demonstrate that the VACV-WR (VACV Western-Reserve strain) displays no binding to Cer (ceramide) or to Gal-Cer (galactosylceramide), but binds to a natural sulfated derivative of these molecules: the Sulf (sulfatide) 3' sulfogalactosylceramide. The interaction between Sulf and VACV-WR resulted in a time-dependent inhibition of virus infection. Virus cell attachment was the crucial step inhibited by Sulf. Electron microscopy showed that SUVs (small unilamellar vesicles) enriched in Sulf bound to VACV particles. Both the A27 and L5 viral membrane proteins were shown to interact with Sulf, indicating that they could be the major viral ligands for Sulf. Soluble Sulf was successful in preventing mortality, but not morbidity, in a lethal mouse model infection with VACV-WR.

CONCLUSIONS

Together the results suggest that Sulf could play a role as an alternate receptor for VACV-WR and probably other Orthopoxviruses.

Drosophila S2 cells are non-permissive for vaccinia virus DNA replication following entry via low pH-dependent endocytosis and early transcription

Vaccinia virus (VACV), a member of the chordopox subfamily of the Poxviridae, abortively infects insect cells. We have investigated VACV infection of Drosophila S2 cells, which are useful for protein expression and genome-wide RNAi screening. Biochemical and electron microscopic analyses indicated that VACV entry into Drosophila S2 cells depended on the VACV multiprotein entry-fusion complex but appeared to occur exclusively by a low pH-dependent endocytic mechanism, in contrast to both neutral and low pH entry pathways used in mammalian cells. Deep RNA sequencing revealed that the entire VACV early transcriptome, comprising 118 open reading frames, was robustly expressed but neither intermediate nor late mRNAs were made. Nor was viral late protein synthesis or inhibition of host protein synthesis detected by pulse-labeling with radioactive amino acids. Some reduction in viral early proteins was noted by Western blotting. Nevertheless, synthesis of the multitude of early proteins needed for intermediate gene expression was demonstrated by transfection of a plasmid containing a reporter gene regulated by an intermediate promoter. In addition, expression of a reporter gene with a late promoter was achieved by cotransfection of intermediate genes encoding the late transcription factors. The requirement for transfection of DNA templates for intermediate and late gene expression indicated a defect in viral genome replication in VACV-infected S2 cells, which was confirmed by direct analysis. Furthermore, VACV-infected S2 cells did not support the replication of a transfected plasmid, which occurs in mammalian cells and is dependent on all known viral replication proteins, indicating a primary restriction of DNA synthesis.

Interaction between the G3 and L5 proteins of the vaccinia virus entry-fusion complex

The vaccinia virus entry-fusion complex (EFC) consists of 10 to 12 proteins that are embedded in the viral membrane and individually required for fusion with the cell and entry of the core into the cytoplasm. The architecture of the EFC is unknown except for information regarding two pair-wise interactions: A28 with H2 and A16 with G9. Here we used a technique to destabilize the EFC by repressing the expression of individual components and identified a third pair-wise interaction: G3 with L5. These two proteins remained associated under several different EFC destabilization conditions and in each case were immunopurified together as demonstrated by Western blotting. Further evidence for the specific interaction of G3 and L5 was obtained by mass spectrometry. This interaction also occurred when G3 and L5 were expressed in uninfected cells, indicating that no other viral proteins were required. Thus, the present study extends our knowledge of the protein interactions important for EFC assembly and stability.

Smallpox vaccines: targets of protective immunity

The eradication of smallpox, one of the great triumphs of medicine, was accomplished through the prophylactic administration of live vaccinia virus, a comparatively benign relative of variola virus, the causative agent of smallpox. Nevertheless, recent fears that variola virus may be used as a biological weapon together with the present susceptibility of unimmunized populations have spurred the development of new-generation vaccines that are safer than the original and can be produced by modern methods. Predicting the efficacy of such vaccines in the absence of human smallpox, however, depends on understanding the correlates of protection. This review outlines the biology of poxviruses with particular relevance to vaccine development, describes protein targets of humoral and cellular immunity, compares animal models of orthopoxvirus disease with human smallpox, and considers the status of second- and third-generation smallpox vaccines.

Cytosolic disulfide bond formation in cells infected with large nucleocytoplasmic DNA viruses

Proteins that have evolved to contain stabilizing disulfide bonds generally fold in a membrane-delimited compartment in the cell [i.e., the endoplasmic reticulum (ER) or the mitochondrial intermembrane space (IMS)]. These compartments contain sulfhydryl oxidase enzymes that catalyze the pairing and oxidation of cysteine residues. In contrast, most proteins in a healthy cytosol are maintained in reduced form through surveillance by NADPH-dependent reductases and the lack of sulfhydryl oxidases. Nevertheless, one of the core functionalities that unify the broad and diverse set of nucleocytoplasmic large DNA viruses (NCLDVs) is the ability to catalyze disulfide formation in the cytosol. The substrates of this activity are proteins that contribute to the assembly, structure, and infectivity of the virions. If the last common ancestor of NCLDVs was present during eukaryogenesis as has been proposed, it is interesting to speculate that viral disulfide bond formation pathways may have predated oxidative protein folding in intracellular organelles.

The neutralizing antibody response to the vaccinia virus A28 protein is specifically enhanced by its association with the H2 protein

The vaccinia virus (VACV) entry-fusion complex (EFC) is composed of at least nine membrane proteins. Immunization of mice with individual EFC genes induced corresponding protein-binding antibody but failed to protect against VACV intranasal challenge and only DNA encoding A28 elicited low neutralizing antibody. Because the A28 and H2 proteins interact, we determined the effect of immunizing with both genes simultaneously. This procedure greatly enhanced the amount of antibody that bound intact virions, neutralized infectivity, and provided partial protection against respiratory challenge. Neither injection of A28 and H2 plasmids at different sites or mixing A28 and H2 sera enhanced neutralizing antibody. The neutralizing antibody could be completely removed by binding to the A28 protein alone and the epitope was located in the C-terminal segment. These data suggest that the interaction of H2 with A28 stabilizes the immunogenic form of A28, mimicking an exposed region of the entry-fusion complex on infectious virions.

Congregation of orthopoxvirus virions in cytoplasmic A-type inclusions is mediated by interactions of a bridging protein (A26p) with a matrix protein (ATIp) and a virion membrane-associated protein (A27p)

Some orthopoxviruses, e.g., the cowpox, ectromelia, and raccoonpox viruses, form large, discrete cytoplasmic inclusions within which mature virions (MVs) are embedded by a process called occlusion. These inclusions, which may protect occluded MVs in the environment, are composed of aggregates of the A-type inclusion protein (ATIp), which is truncated in orthopoxviruses such as vaccinia virus (VACV) and variola virus that fail to form inclusions. In addition to an intact ATIp, occlusion requires the A26 protein (A26p). Although VACV contains a functional A26p, determined by complementation of a cowpox virus occlusion-defective mutant, its role in occlusion was unknown. We found that restoration of the full-length ATI gene was sufficient for VACV inclusion formation and the ensuing occlusion of MVs. A26p was present in inclusions even when virion assembly was inhibited, suggesting a direct interaction of A26p with ATIp. Analysis of a panel of ATIp mutants indicated that the C-terminal repeat region was required for inclusion formation and the N-terminal domain for interaction with A26p and occlusion. A26p is tethered to MVs via interaction with the A27 protein (A27p); A27p was not required for association of A26p with ATIp but was necessary for occlusion. In addition, the C-terminal domain of A26p, which mediates A26p-A27p interactions, was necessary but insufficient for occlusion. Taken together, the data suggest a model for occlusion in which A26p has a bridging role between ATIp and A27p, and A27p provides a link to the MV membrane.

Lipid membranes in poxvirus replication

Poxviruses replicate in the cytoplasm, where they acquire multiple lipoprotein membranes. Although a proposal that the initial membrane arises de novo has not been substantiated, there is no accepted explanation for its formation from cellular membranes. A subsequent membrane-wrapping step involving modified trans-Golgi or endosomal cisternae results in a particle with three membranes. These wrapped virions traverse the cytoplasm on microtubules; the outermost membrane is lost during exocytosis, the middle one is lost just prior to cell entry, and the remaining membrane fuses with the cell to allow the virus core to enter the cytoplasm and initiate a new infection.

Poxvirus proteomics and virus-host protein interactions

Studies of the functional proteins encoded by the poxvirus genome provide information about the composition of the virus as well as individual virus-virus protein and virus-host protein interactions, which provides insight into viral pathogenesis and drug discovery. Widely used proteomic techniques to identify and characterize specific protein-protein interactions include yeast two-hybrid studies and coimmunoprecipitations. Recently, various mass spectrometry techniques have been employed to identify viral protein components of larger complexes. These methods, combined with structural studies, can provide new information about the putative functions of viral proteins as well as insights into virus-host interaction dynamics. For viral proteins of unknown function, identification of either viral or host binding partners provides clues about their putative function. In this review, we discuss poxvirus proteomics, including the use of proteomic methodologies to identify viral components and virus-host protein interactions. High-throughput global protein expression studies using protein chip technology as well as new methods for validating putative protein-protein interactions are also discussed.

Characterization of a newly identified 35-amino-acid component of the vaccinia virus entry/fusion complex conserved in all chordopoxviruses

The original annotation of the vaccinia virus (VACV) genome was limited to open reading frames (ORFs) of at least 65 amino acids. Here, we characterized a 35-amino-acid ORF (O3L) located between ORFs O2L and I1L. ORFs similar in length to O3L were found at the same genetic locus in all vertebrate poxviruses. Although amino acid identities were low, the presence of a characteristic N-terminal hydrophobic domain strongly suggested that the other poxvirus genes were orthologs. Further studies demonstrated that the O3 protein was expressed at late times after infection and incorporated into the membrane of the mature virion. An O3L deletion mutant was barely viable, producing tiny plaques and a 3-log reduction in infectious progeny. A mutant VACV with a regulated O3L gene had a similar phenotype in the absence of inducer. There was no apparent defect in virus morphogenesis, though O3-deficient virus had low infectivity. The impairment was shown to be at the stage of virus entry, as cores were not detected in the cytoplasm after virus adsorption. Furthermore, O3-deficient virus did not induce fusion of infected cells when triggered by low pH. These characteristics are hallmarks of a group of proteins that form the entry/fusion complex (EFC). Affinity purification experiments demonstrated an association of O3 with EFC proteins. In addition, the assembly or stability of the EFC was impaired when expression of O3 was repressed. Thus, O3 is the newest recognized component of the EFC and the smallest VACV protein shown to have a function.

Appraising the apoptotic mimicry model and the role of phospholipids for poxvirus entry

Entry of vaccinia virus (VACV) into cells occurs by fusion with the plasma membrane and via a low pH-dependent endosomal pathway, presumably involving unidentified cellular receptors. In addition to approximately 25 viral proteins, the membrane of VACV mature virions contains several phospholipids including phosphatidylserine (PS). A recent model posits that PS flags virions as apoptotic debris to activate a common cellular uptake pathway to gain cell entry, perhaps through an interaction with a PS-specific cell surface receptor. To evaluate the apoptotic mimicry model, we reconstituted the membrane of detergent-extracted virions with several different phospholipids. Although the ability of the L-stereoisomer of PS to reconstitute infectivity was confirmed, the nonbiologically relevant D-stereoisomer of PS, and phosphatidylglycerol, which are not normally present in the virion membrane, functioned as well. Regardless of which phospholipid reconstituted infectivity, virus entry was inhibited by a neutralizing monoclonal antibody to a virion surface protein and by the drugs blebbistatin and bafilomycin A1, suggesting that in each case virus uptake was specific and occurred by a similar mechanism involving macropinocytosis and a low-pH endocytic pathway. Lipid-reconstituted and nonreconstituted, membrane-extracted virions were equally capable of binding to cells. However, the physical association of phospholipids with virus particles during membrane reconstitution correlated directly with rescue of particle infectivity and cell entry capability. Our results support a role for PS in poxvirus entry, but demonstrate that other phospholipids, not known to signal uptake of apoptotic debris, can function similarly.

Characterization of Cervidpoxvirus Isolates from Oregon, California, and Eastern Canada

The present report describes the analysis of 4 Deerpox virus isolates from California, Oregon, and Ontario, Canada. All 4 isolates were associated with cutaneous crusting lesions. Examination of selected samples by electron microscopy demonstrated that the viruses were morphologically similar to orthopoxviruses. Phylogenetic analysis of the A21 gene, which is found in all poxviruses, indicated that the 4 isolates form a lineage distinct from other members except for those belonging to the genus Cervidpoxvirus of the subfamily Chordopoxvirinae. Members of the Cervidpoxvirus lineage encode a set of genes not found in other poxviruses. These include homologs of genes encoding interleukin 1 receptor antagonists (IL-1Ra) and C-type lectin-like receptors (CTLR). In the current investigation, genes encoding homologs of IL-1Ra and CTLR were amplified from all the isolates and were found to be closely related to orthologs found in the Cervidpoxvirus genus, which further supports the inclusion of these isolates in the Cervidpoxvirus genus.

Vaccinia virus strain differences in cell attachment and entry

Vaccinia virus (VACV) strain WR can enter cells by a low pH endosomal pathway or direct fusion with the plasma membrane at neutral pH. Here, we compared attachment and entry of five VACV strains in six cell lines and discovered two major patterns. Only WR exhibited pH 5-enhanced rate of entry following neutral pH adsorption to cells, which correlated with sensitivity to bafilomycin A1, an inhibitor of endosomal acidification. Entry of IHD-J, Copenhagen and Elstree strains were neither accelerated by pH 5 treatment nor prevented by bafilomycin A1. Entry of the Wyeth strain, although not augmented by pH 5, was inhibited by bafilomycin A1. WR and Wyeth were both relatively resistant to the negative effects of heparin on entry, whereas the other strains were extremely sensitive due to inhibition of cell binding. The relative sensitivities of individual vaccinia virus strains to heparin correlated inversely with their abilities to bind to and enter glycosaminoglycan-deficient sog9 cells but not other cell lines tested. These results suggested that that IHD-J, Copenhagen and Elstree have a more limited ability than WR and Wyeth to use the low pH endosomal pathway and are more dependent on binding to glycosaminoglycans for cell attachment.

Inhibition of Vaccinia virus entry by a broad spectrum antiviral peptide

Concerns about the possible use of Variola virus, the causative agent of smallpox, as a weapon for bioterrorism have led to renewed efforts to identify new antivirals against orthopoxviruses. We identified a peptide, EB, which inhibited infection by Vaccinia virus with an EC(50) of 15 microM. A control peptide, EBX, identical in composition to EB but differing in sequence, was inactive (EC50>200 microM), indicating sequence specificity. The inhibition was reversed upon removal of the peptide, and EB treatment had no effect on the physical integrity of virus particles as determined by electron microscopy. Viral adsorption was unaffected by the presence of EB, and the addition of EB post-entry had no effect on viral titers or on early gene expression. The addition of EB post-adsorption resulted in the inhibition of beta-galactosidase expression from an early viral promoter with an EC(50) of 45 microM. A significant reduction in virus entry was detected in the presence of the peptide when the number of viral cores released into the cytoplasm was quantified. Electron microscopy indicated that 88% of the virions remained on the surface of cells in the presence of EB, compared to 37% in the control (p<0.001). EB also blocked fusion-from-within, suggesting that virus infection is inhibited at the fusion step. Analysis of EB derivatives suggested that peptide length may be important for the activity of EB. The EB peptide is, to our knowledge, the first known small molecule inhibitor of Vaccinia virus entry.

Vaccinia virus L1 binds to cell surfaces and blocks virus entry independently of glycosaminoglycans

L1 and A28 are vaccinia virus (VACV) envelope proteins which are essential for cellular entry. However, their specific roles during entry are unknown. We tested whether one or both of these proteins might serve as receptor binding proteins (RBP). We found that a soluble, truncated form of L1, but not A28, bound to cell surfaces independently of glycosaminoglycans (GAGs). Hence, VACV A28 is not likely to be a RBP and functions after attachment during entry. Importantly, soluble L1 inhibited both binding and entry of VACV in GAG-deficient cells, suggesting that soluble L1 blocks entry at the binding step by competing with the virions for non-GAG receptors on cells. In contrast, soluble A27, a VACV protein which attaches to GAGs but is non-essential for virus entry, inhibited binding and entry of VACV in a GAG-dependent manner. To our knowledge, this is the first report of a VACV envelope protein that blocks virus binding and entry independently of GAGs.