Participation of vaccinia virus l2 protein in the formation of crescent membranes and immature virions

Assembly and Evolution of Poxviruses

Poxvirus assembly has been an intriguing area of research for several decades. While advancements in experimental techniques continue to yield fresh insights, many questions are still unresolved. Large genome sizes of up to 380 kbp, asymmetrical structure, an exterior lipid bilayer, and a cytoplasmic life cycle are some notable characteristics of these viruses. Inside the particle are two lateral bodies and a protein wall-bound-biconcave core containing the viral nucleocapsid. The assembly progresses through five major stages-endoplasmic reticulum (ER) membrane alteration and rupture, crescent formation, immature virion formation, genome encapsidation, virion maturation and in a subset of viruses, additional envelopment of the virion prior to its dissemination. Several large dsDNA viruses have been shown to follow a comparable sequence of events. In this chapter, we recapitulate our understanding of the poxvirus morphogenesis process while reviewing the most recent advances in the field. We also briefly discuss how virion assembly aids in our knowledge of the evolutionary links between poxviruses and other Nucleocytoplasmic Large DNA Viruses (NCLDVs).

African Swine Fever Virus Host-Pathogen Interactions

African swine fever virus is a complex double-stranded DNA virus that exhibits tropism for cells of the mononuclear phagocytic system. Virus replication is a multi-step process that involves the nucleus of the host cell as well the formation of large perinuclear sites where progeny virions are assembled prior to transport to, and budding through, the plasma membrane. Like many viruses, African swine fever virus reorganises the cellular architecture to facilitate its replication and has evolved multiple mechanisms to avoid the potential deleterious effects of host cell stress response pathways. However, how viral proteins and virus-induced structures trigger cellular stress pathways and manipulate the subsequent responses is still relatively poorly understood. African swine fever virus alters nuclear substructures, modulates autophagy, apoptosis and the endoplasmic reticulum stress response pathways. The viral genome encodes for at least 150 genes, of which approximately 70 are incorporated into the virion. Many of the non-structural genes have not been fully characterised and likely play a role in host range and modifying immune responses. As the field moves towards approaches that take a broader view of the effect of expression of individual African swine fever genes, we summarise how the different steps in virus replication interact with the host cell and the current state of knowledge on how it modulates the resulting stress responses.

Structure-Function Analysis of Two Interacting Vaccinia Proteins That Are Critical for Viral Morphogenesis: L2 and A30.5

An enduring mystery in poxvirology is the mechanism by which virion morphogenesis is accomplished. A30.5 and L2 are two small regulatory proteins that are essential for this process. Previous studies have shown that vaccinia A30.5 and L2 localize to the ER and interact during infection, but how they facilitate morphogenesis is unknown. To interrogate the relationship between A30.5 and L2, we generated inducible complementing cell lines (CV1-HA-L2; CV1-3xFLAG-A30.5) and deletion viruses (vΔL2; vΔA30.5). Loss of either protein resulted in a block in morphogenesis and a significant (>100-fold) decrease in infectious viral yield. Structure-function analysis of L2 and A30.5, using transient complementation assays, identified key functional regions in both proteins. A clustered charge-to-alanine L2 mutant (L2-RRD) failed to rescue a vΔL2 infection and exhibits a significantly retarded apparent molecular weight in vivo (but not in vitro), suggestive of an aberrant posttranslational modification. Furthermore, an A30.5 mutant with a disrupted putative N-terminal α-helix failed to rescue a vΔA30.5 infection. Using our complementing cell lines, we determined that the stability of A30.5 is dependent on L2 and that wild-type L2 and A30.5 coimmunoprecipitate in the absence of other viral proteins. Further examination of this interaction, using wild-type and mutant forms of L2 or A30.5, revealed that the inability of mutant alleles to rescue the respective deletion viruses is tightly correlated with a failure of L2 to stabilize and interact with A30.5. L2 appears to function as a chaperone-like protein for A30.5, ensuring that they work together as a complex during viral membrane biogenesis. IMPORTANCE Vaccinia virus is a large, enveloped DNA virus that was successfully used as the vaccine against smallpox. Vaccinia continues to be an invaluable biomedical research tool in basic research and in gene therapy vector and vaccine development. Although this virus has been studied extensively, the complex process of virion assembly, termed morphogenesis, still puzzles the field. Our work aims to better understand how two small viral proteins that are essential for viral assembly, L2 and A30.5, function during early morphogenesis. We show that A30.5 requires L2 for stability and that these proteins interact in the absence of other viral proteins. We identify regions in each protein required for their function and show that mutations in these regions disrupt the interaction between L2 and A30.5 and fail to restore virus viability.

Crescent-Shaped Supramolecular Tetrapeptide Nanostructures

Self-assembly of amphiphilic peptide-based building blocks gives rise to a plethora of interesting nanostructures such as ribbons, fibers, and tubes. However, it remains a great challenge to employ peptide self-assembly to directly produce nanostructures with lower symmetry than these highly symmetric motifs. We report here our discovery that persistent and regular crescent nanostructures with a diameter of 28 ± 3 nm formed from a series of tetrapeptides with the general structure AdKSKSEX (Ad = adamantyl group, KS = lysine residue functionalized with an S-aroylthiooxime (SATO) group, E = glutamic acid residue, and X = variable amino acid residue). In the presence of cysteine, the biological signaling gas hydrogen sulfide (H2S) was released from the SATO units of the crescent nanostructures, termed peptide-H2S donor conjugates (PHDCs), reducing levels of reactive oxygen species (ROS) in macrophage cells. Additional in vitro studies showed that the crescent nanostructures alleviated cytotoxicity induced by phorbol 12-myristate-13-acetate more effectively than common H2S donors and a PHDC of a similar chemical structure, AdKSKSE, that formed short nanoworms instead of nanocrescents. Cell internalization studies indicated that nanocrescent-forming PHDCs were more effective in reducing ROS levels in macrophages because they entered into and remained in cells better than nanoworms, highlighting how nanostructure morphology can affect bioactivity in drug delivery.

Inactivation of Genes by Frameshift Mutations Provides Rapid Adaptation of an Attenuated Vaccinia Virus

Unlike RNA viruses, most DNA viruses replicate their genomes with high-fidelity polymerases that rarely make base substitution errors. Nevertheless, experimental evolution studies have revealed rapid acquisition of adaptive mutations during serial passage of attenuated vaccinia virus (VACV). One way in which adaptation can occur is by an accordion mechanism in which the gene copy number increases followed by base substitutions and, finally, contraction of the gene copy number. Here, we show rapid acquisition of multiple adaptive mutations mediated by a gene-inactivating frameshift mechanism during passage of an attenuated VACV. Attenuation had been achieved by exchanging the VACV A8R intermediate transcription factor gene with the myxoma virus ortholog. A total of seven mutations in six different genes occurred in three parallel passages of the attenuated virus. The most frequent mutations were single-nucleotide insertions or deletions within runs of five to seven As or Ts, although a deletion of 11 nucleotides also occurred, leading to frameshifts and premature stop codons. During 10 passage rounds, the attenuated VACV was replaced by the mutant viruses. At the end of the experiment, virtually all remaining viruses had one fixed mutation and one or more additional mutations. Although nucleotide substitutions in the transcription apparatus accounted for two low-frequency mutations, frameshifts in genes encoding protein components of the mature virion, namely, A26L, G6R, and A14.5L, achieved 74% to 98% fixation. The adaptive role of the mutations was confirmed by making recombinant VACV with A26L or G6R or both deleted, which increased virus replication levels and decreased particle/PFU ratios.IMPORTANCE Gene inactivation is considered to be an important driver of orthopoxvirus evolution. Whereas cowpox virus contains intact orthologs of genes present in each orthopoxvirus species, numerous genes are inactivated in all other members of the genus. Inactivation of additional genes can occur upon extensive passaging of orthopoxviruses in cell culture leading to attenuation in vivo, a strategy for making vaccines. Whether inactivation of multiple viral genes enhances replication in the host cells or has a neutral effect is unknown in most cases. Using an experimental evolution protocol involving serial passages of an attenuated vaccinia virus, rapid acquisition of inactivating frameshift mutations occurred. After only 10 passage rounds, the starting attenuated vaccinia virus was displaced by viruses with one fixed mutation and one or more additional mutations. The high frequency of multiple inactivating mutations during experimental evolution simulates their acquisition during normal evolution and extensive virus passaging to make vaccine strains.

Zinc-finger antiviral protein (ZAP) is a restriction factor for replication of modified vaccinia virus Ankara (MVA) in human cells

Modified vaccinia virus Ankara (MVA) is an approved smallpox vaccine and a promising vaccine vector for other pathogens as well as for cancer therapeutics with more than 200 current or completed clinical trials. MVA was derived by passaging the parental Ankara vaccine virus hundreds of times in chick embryo fibroblasts during which it lost the ability to replicate in human and most other mammalian cells. Although this replication deficiency is an important safety feature, the genetic basis of the host restriction is not understood. Here, an unbiased human genome-wide RNAi screen in human A549 cells revealed that the zinc-finger antiviral protein (ZAP), previously shown to inhibit certain RNA viruses, is a host restriction factor for MVA, a DNA virus. Additional studies demonstrated enhanced MVA replication in several human cell lines following knockdown of ZAP. Furthermore, CRISPR-Cas9 knockout of ZAP in human A549 cells increased MVA replication and spread by more than one log but had no effect on a non-attenuated strain of vaccinia virus. The intact viral C16 protein, which had been disrupted in MVA, antagonized ZAP by binding and sequestering the protein in cytoplasmic punctate structures. Studies aimed at exploring the mechanism by which ZAP restricts MVA replication in the absence of C16 showed that knockout of ZAP had no discernible effect on viral DNA or individual mRNA or protein species as determined by droplet digital polymerase chain reaction, deep RNA sequencing and mass spectrometry, respectively. Instead, inactivation of ZAP reduced the number of aberrant, dense, spherical particles that typically form in MVA-infected human cells, suggesting that ZAP has a novel role in interfering with a late step in the assembly of infectious MVA virions in the absence of the C16 protein.

The attenuated vaccine vector known as modified vaccinia virus Ankara (MVA) was derived by extensively passaging the parental strain of vaccinia virus Ankara in chick embryo fibroblasts and is unable to replicate in most mammalian cells. The MVA host range restriction is exceptional in that synthesis of the abundant viral proteins appears unaffected but morphogenesis of virus particles is abortive. Despite the importance of the host range restriction for vaccine safety, the basis for this antiviral effect has remained an enigma. Here we demonstrate that the zinc finger antiviral protein (ZAP), previously shown to be an inhibitor of RNA viruses, is a specific host restriction factor for replication of MVA in human cells. Moreover, the intact vaccinia virus C16 protein, which was disrupted during the attenuation of MVA, sequesters ZAP in cytoplasmic punctae and effectively counteracts the inhibitory effects of ZAP.

Repair of a previously uncharacterized second host-range gene contributes to full replication of modified vaccinia virus Ankara (MVA) in human cells

Modified vaccinia virus Ankara (MVA), a widely used vaccine vector for expression of genes of unrelated pathogens, is safe, immunogenic, and can incorporate large amounts of added DNA. MVA was derived by extensively passaging the chorioallantois vaccinia virus Ankara (CVA) vaccine strain in chicken embryo fibroblasts during which numerous mutations and deletions occurred with loss of replicative ability in most mammalian cells. Restoration of the deleted C12L gene, encoding serine protease inhibitor 1, enhances replication of MVA in human MRC-5 cells but only slightly in other human cells. Here we show that repair of the inactivated C16L/B22R gene of MVA enhances replication in numerous human cell lines. This previously uncharacterized gene is present at both ends of the genome of many orthopoxviruses and is highly conserved in most, including smallpox and monkeypox viruses. The C16L/B22R gene is expressed early in infection from the second methionine of the previously annotated Copenhagen strain open reading frame (ORF) as a 17.4-kDa protein. The C16/B22 and C12 proteins together promote MVA replication in human cells to levels that are comparable to titers in chicken embryo fibroblasts. Both proteins enhance virion assembly, but C16/B22 increases proteolytic processing of core proteins in A549 cells consistent with higher infectious virus titers. Furthermore, human A549 cells expressing codon-optimized C16L/B22R and C12L genes support higher levels of MVA replication than cell lines expressing neither or either alone. Identification of the genes responsible for the host-range defect of MVA may allow more rational engineering of vaccines for efficacy, safety, and propagation.

In-depth analysis of the replication cycle of Orpheovirus

Background

After the isolation of Acanthamoeba polyphaga mimivirus (APMV), the study and search for new giant viruses has been intensified. Most giant viruses are associated with free-living amoebae of the genus Acanthamoeba; however other giant viruses have been isolated in Vermamoeba vermiformis, such as Faustovirus, Kaumoebavirus and Orpheovirus. These studies have considerably expanded our knowledge about the diversity, structure, genomics, and evolution of giant viruses. Until now, there has been only one Orpheovirus isolate, and many aspects of its life cycle remain to be elucidated.

Methods

In this study, we performed an in-depth characterization of the replication cycle and particles of Orpheovirus by transmission and scanning electron microscopy, optical microscopy and IF assays.

Results

We observed, through optical and IF microscopy, morphological changes in V. vermiformis cells during Orpheovirus infection, as well as increased motility at 12 h post infection (h.p.i.). The viral factory formation and viral particle morphogenesis were analysed by transmission electron microscopy, revealing mitochondria and membrane recruitment into and around the electron-lucent viral factories. Membrane traffic inhibitor (Brefeldin A) negatively impacted particle morphogenesis. The first structure observed during particle morphogenesis was crescent-shaped bodies, which extend and are filled by the internal content until the formation of multi-layered mature particles. We also observed the formation of defective particles with different shapes and sizes. Virological assays revealed that viruses are released from the host by exocytosis at 12 h.p.i., which is associated with an increase of particle counts in the supernatant.

Conclusions

The results presented here contribute to a better understanding of the biology, structures and important steps in the replication cycle of Orpheovirus.

SPI-1 is a missing host-range factor required for replication of the attenuated modified vaccinia Ankara (MVA) vaccine vector in human cells

Modified vaccinia virus Ankara (MVA) is the leading poxvirus vector for development of vaccines against diverse infectious diseases. This distinction is based on high expression of proteins and good immunogenicity despite an inability to assemble infectious progeny in human cells, which together promote efficacy and safety. Nevertheless, the basis for the host-range restriction is unknown despite past systematic attempts to identify the relevant missing viral gene(s). The search for host-range factors is exacerbated by the large number of deletions, truncations and mutations that occurred during the long passage history of MVA in chicken embryo fibroblasts. By whole genome sequencing of a panel of recombinant host-range extended (HRE) MVAs generated by marker rescue with 40 kbp segments of vaccinia virus DNA, we identified serine protease inhibitor 1 (SPI-1) as one of several candidate host-range factors present in those viruses that gained the ability to replicate in human cells. Electron microscopy revealed that the interruption of morphogenesis in human cells infected with MVA occurred at a similar stage as that of a vaccinia virus strain WR SPI-1 deletion mutant. Moreover, the introduction of the SPI-1 gene into the MVA genome led to more than a 2-log enhancement of virus spread in human diploid MRC-5 cells, whereas deletion of the gene diminished the spread of HRE viruses by similar extents. Furthermore, MRC-5 cells stably expressing SPI-1 also enhanced replication of MVA. A role for additional host range genes was suggested by the restoration of MVA replication to a lower level relative to HRE viruses, particularly in other human cell lines. Although multiple sequence alignments revealed genetic changes in addition to SPI-1 common to the HRE MVAs, no evidence for their host-range function was found by analysis thus far. Our finding that SPI-1 is host range factor for MVA should simplify use of high throughput RNAi or CRISPR/Cas single gene methods to identify additional viral and human restriction elements.

Poxvirus vectors have outstanding properties for development of vaccines against a myriad of infectious agents due to their ability to retain long segments of foreign DNA and high-level gene expression. Safety concerns led to a preference for attenuated poxviruses that lost the ability to produce infectious progeny in human cells. The most widely used poxvirus vector is modified vaccinia virus Ankara (MVA), which exhibits an extreme host-range restriction in most mammalian cells. MVA was attenuated by passaging more than 500 times in chicken embryo fibroblasts during which large deletions and numerous additional genetic changes occurred. Despite ongoing clinical testing of MVA-vectored vaccines, the basis for its host-range restriction remained unknown. Here we show that re-introduction of the SPI-1 gene into MVA or host cells increased virus spread by more than 100-fold in a human diploid cell line, providing an important insight into the mechanism responsible for the host-range restriction. This information could help design improved vectors and develop non-avian cell lines for propagation of candidate MVA vaccines.

Microscopic Analysis of the Tupanvirus Cycle in Vermamoeba vermiformis

Since Acanthamoeba polyphaga mimivirus (APMV) was identified in 2003, several other giant viruses of amoebae have been isolated, highlighting the uniqueness of this group. In this context, the tupanviruses were recently isolated from extreme environments in Brazil, presenting virions with an outstanding tailed structure and genomes containing the most complete set of translation genes of the virosphere. Unlike other giant viruses of amoebae, tupanviruses present a broad host range, being able to replicate not only in Acanthamoeba sp. but also in other amoebae, such as Vermamoeba vermiformis, a widespread, free-living organism. Although the Tupanvirus cycle in A. castellanii has been analyzed, there are no studies concerning the replication of tupanviruses in other host cells. Here, we present an in-depth microscopic study of the replication cycle of Tupanvirus in V. vermiformis. Our results reveal that Tupanvirus can enter V. vermiformis and generate new particles with similar morphology to when infecting A. castellanii cells. Tupanvirus establishes a well-delimited electron-dense viral factory in V. vermiformis, surrounded by lamellar structures, which appears different when compared with different A. castellanii cells. Moreover, viral morphogenesis occurs entirely in the host cytoplasm within the viral factory, from where complete particles, including the capsid and tail, are sprouted. Some of these particles have larger tails, which we named "supertupans." Finally, we observed the formation of defective particles, presenting abnormalities of the tail and/or capsid. Taken together, the data presented here contribute to a better understanding of the biology of tupanviruses in previously unexplored host cells.

Enigmatic origin of the poxvirus membrane from the endoplasmic reticulum shown by 3D imaging of vaccinia virus assembly mutants

The long-standing inability to visualize connections between poxvirus membranes and cellular organelles has led to uncertainty regarding the origin of the viral membrane. Indeed, there has been speculation that viral membranes form de novo in cytoplasmic factories. Another possibility, that the connections are too short-lived to be captured by microscopy during a normal infection, motivated us to identify and characterize virus mutants that are arrested in assembly. Five conserved vaccinia virus proteins, referred to as Viral Membrane Assembly Proteins (VMAPs), that are necessary for formation of immature virions were found. Transmission electron microscopy studies of two VMAP deletion mutants had suggested retention of connections between viral membranes and the endoplasmic reticulum (ER). We now analyzed cells infected with each of the five VMAP deletion mutants by electron tomography, which is necessary to validate membrane continuity, in addition to conventional transmission electron microscopy. In all cases, connections between the ER and viral membranes were demonstrated by 3D reconstructions, supporting a role for the VMAPs in creating and/or stabilizing membrane scissions. Furthermore, coexpression of the viral reticulon-like transmembrane protein A17 and the capsid-like scaffold protein D13 was sufficient to form similar ER-associated viral structures in the absence of other major virion proteins. Determination of the mechanism of ER disruption during a normal VACV infection and the likely participation of both viral and cell proteins in this process may provide important insights into membrane dynamics.

Suppression of Poxvirus Replication by Resveratrol

Poxviruses continue to cause serious diseases even after eradication of the historically deadly infectious human disease, smallpox. Poxviruses are currently being developed as vaccine vectors and cancer therapeutic agents. Resveratrol is a natural polyphenol stilbenoid found in plants that has been shown to inhibit or enhance replication of a number of viruses, but the effect of resveratrol on poxvirus replication is unknown. In the present study, we found that resveratrol dramatically suppressed the replication of vaccinia virus (VACV), the prototypic member of poxviruses, in various cell types. Resveratrol also significantly reduced the replication of monkeypox virus, a zoonotic virus that is endemic in Western and Central Africa and causes human mortality. The inhibitory effect of resveratrol on poxviruses is independent of VACV N1 protein, a potential resveratrol binding target. Further experiments demonstrated that resveratrol had little effect on VACV early gene expression, while it suppressed VACV DNA synthesis, and subsequently post-replicative gene expression.

Filling Knowledge Gaps for Mimivirus Entry, Uncoating, and Morphogenesis

Since the discovery of mimivirus, its unusual structural and genomic features have raised great interest in the study of its biology; however, many aspects concerning its replication cycle remain uncertain. In this study, extensive analyses of electron microscope images, as well as biological assay results, shed light on unclear points concerning the mimivirus replication cycle. We found that treatment with cytochalasin, a phagocytosis inhibitor, negatively impacted the incorporation of mimivirus particles by Acanthamoeba castellanii, causing a negative effect on viral growth in amoeba monolayers. Treatment of amoebas with bafilomicin significantly impacted mimivirus uncoating and replication. In conjunction with microscopic analyses, these data suggest that mimiviruses indeed depend on phagocytosis for entry into amoebas, and particle uncoating (and stargate opening) appears to be dependent on phagosome acidification. In-depth analyses of particle morphogenesis suggest that the mimivirus capsids are assembled from growing lamellar structures. Despite proposals from previous studies that genome acquisition occurs before the acquisition of fibrils, our results clearly demonstrate that the genome and fibrils can be acquired simultaneously. Our data suggest the existence of a specific area surrounding the core of the viral factory where particles acquire the surface fibrils. Furthermore, we reinforce the concept that defective particles can be formed even in the absence of virophages. Our work provides new information about unexplored steps in the life cycle of mimivirus.IMPORTANCE Investigating the viral life cycle is essential to a better understanding of virus biology. The combination of biological assays and microscopic images allows a clear view of the biological features of viruses. Since the discovery of mimivirus, many studies have been conducted to characterize its replication cycle, but many knowledge gaps remain to be filled. In this study, we conducted a new examination of the replication cycle of mimivirus and provide new evidence concerning some stages of the cycle which were previously unclear, mainly entry, uncoating, and morphogenesis. Furthermore, we demonstrate that atypical virion morphologies can occur even in the absence of virophages. Our results, along with previous data, allow us to present an ultimate model for the mimivirus replication cycle.

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.

Vaccinia Virus A6 Is a Two-Domain Protein Requiring a Cognate N-Terminal Domain for Full Viral Membrane Assembly Activity

Poxvirus virion biogenesis is a complex, multistep process, starting with the formation of crescent-shaped viral membranes, followed by their enclosure of the viral core to form spherical immature virions. Crescent formation requires a group of proteins that are highly conserved among poxviruses, including A6 and A11 of vaccinia virus (VACV). To gain a better understanding of the molecular function of A6, we established a HeLa cell line that inducibly expressed VACV-A6, which allowed us to construct VACV mutants with an A6 deletion or mutation. As expected, the A6 deletion mutant of VACV failed to replicate in noncomplementing cell lines with defects in crescent formation and A11 localization. Surprisingly, a VACV mutant that had A6 replaced with a close ortholog from the Yaba-like disease virus YLDV-97 also failed to replicate. This mutant, however, developed crescents and had normal A11 localization despite failing to form immature virions. Limited proteolysis of the recombinant A6 protein identified an N domain and a C domain of approximately 121 and 251 residues, respectively. Various chimeras of VACV-A6 and YLDV-97 were constructed, but only one that precisely combined the N domain of VACV-A6 and the C domain of YLDV-97 supported VACV replication albeit at a reduced efficiency. Our results show that VACV-A6 has a two-domain architecture and functions in both crescent formation and its enclosure to form immature virions. While a cognate N domain is not required for crescent formation, it is required for virion formation, suggesting that interactions of the N domain with cognate viral proteins may be critical for virion assembly.IMPORTANCE Poxviruses are unique among enveloped viruses in that they acquire their primary envelope not through budding from cellular membranes but by forming and extending crescent membranes. The crescents are highly unusual, open-ended membranes, and their origin and biogenesis have perplexed virologists for decades. A group of five viral proteins were recently identified as being essential for crescent formation, including the A6 protein of vaccinia virus. It is thus important to understand the structure and function of A6 in order to solve the long-standing mystery of poxvirus membrane biogenesis. Here, we established an experimental system that allowed the genetic manipulation of the essential A6L gene. By studying A6 mutant viruses, we found that A6 plays an essential role not only in the formation of crescents but also in their subsequent enclosure to form immature virions. We defined the domain architecture of A6 and suggested that one of its two domains cooperates with cognate viral proteins.

Retrograde Transport from Early Endosomes to the trans-Golgi Network Enables Membrane Wrapping and Egress of Vaccinia Virus Virions

The anterograde pathway, from the endoplasmic reticulum through the trans-Golgi network to the cell surface, is utilized by trans-membrane and secretory proteins. The retrograde pathway, which directs traffic in the opposite direction, is used following endocytosis of exogenous molecules and recycling of membrane proteins. Microbes exploit both routes: viruses typically use the anterograde pathway for envelope formation prior to exiting the cell, whereas ricin and Shiga-like toxins and some nonenveloped viruses use the retrograde pathway for cell entry. Mining a human genome-wide RNA interference (RNAi) screen revealed a need for multiple retrograde pathway components for cell-to-cell spread of vaccinia virus. We confirmed and extended these results while discovering that retrograde trafficking was required for virus egress rather than entry. Retro-2, a specific retrograde trafficking inhibitor of protein toxins, potently prevented spread of vaccinia virus as well as monkeypox virus, a human pathogen. Electron and confocal microscopy studies revealed that Retro-2 prevented wrapping of virions with an additional double-membrane envelope that enables microtubular transport, exocytosis, and actin polymerization. The viral B5 and F13 protein components of this membrane, which are required for wrapping, normally colocalize in the trans-Golgi network. However, only B5 traffics through the secretory pathway, suggesting that F13 uses another route to the trans-Golgi network. The retrograde route was demonstrated by finding that F13 was largely confined to early endosomes and failed to colocalize with B5 in the presence of Retro-2. Thus, vaccinia virus makes novel use of the retrograde transport system for formation of the viral wrapping membrane.

IMPORTANCE

Efficient cell-to-cell spread of vaccinia virus and other orthopoxviruses depends on the wrapping of infectious particles with a double membrane that enables microtubular transport, exocytosis, and actin polymerization. Interference with wrapping or subsequent steps results in severe attenuation of the virus. Some previous studies had suggested that the wrapping membrane arises from the trans-Golgi network, whereas others suggested an origin from early endosomes. Some nonenveloped viruses use retrograde trafficking for entry into the cell. In contrast, we provided evidence that retrograde transport from early endosomes to the trans-Golgi network is required for the membrane-wrapping step in morphogenesis of vaccinia virus and egress from the cell. The potent in vitro inhibition of this step by the drug Retro-2 suggests that derivatives with enhanced pharmacological properties might serve as useful antipoxviral agents.

Rana grylio virus TK and DUT gene locus could be simultaneously used for foreign gene expression

Ranaviruses (family Iridoviridae, genus Ranavirus) have been recognized as emerging infectious pathogens and caused a great loss to the global biodiversity. Thymidine kinase (TK) and deoxyuridine triphosphatase (dUTPase, DUT, encoded by ORF 67R) are ubiquitous, existing in iridoviruses and other organisms. Previous studies showed that TK and DUT could be individually knocked out without impeding viral replication. In this study, we tried to insert two fluorescence genes into the above loci. We started with Δ67R-RGV, a recently generated recombinant Rana grylio virus (RGV) with the whole DUT replaced by enhanced green fluorescence protein (EGFP) gene. Then, a red fluorescence protein (RFP) gene initiated by RGV immediate-early (IE) ICP18 gene promoter was inserted into TK locus through homologous recombination. A novel recombinant virus, ΔDUT, TK-RGV, was generated by nine successive rounds of plaque isolation using RFP selection. All of the plaques produced by this recombinant virus could emit both green and red fluorescence. Furthermore, one-step and multiple-step growth curves of ΔDUT, TK-RGV were similar to those of wt-RGV and Δ67R-RGV. In conclusion, a novel dual-fluorescence labeled recombinant iridovirus in which DUT and TK gene locus were simultaneously used for foreign gene expression was constructed.

Topology of Endoplasmic Reticulum-Associated Cellular and Viral Proteins Determined with Split-GFP

The split green fluorescent protein (GFP) system was adapted for investigation of the topology of ER-associated proteins. A 215-amino acid fragment of GFP (S1-10) was expressed in the cytoplasm as a free protein or fused to the N-terminus of calnexin and in the ER as an intraluminal protein or fused to the C-terminus of calnexin. A 16-amino acid fragment of GFP (S11) was fused to the N- or C-terminus of the target protein. Fluorescence occurred when both GFP fragments were in the same intracellular compartment. After validation with the cellular proteins PDI and tapasin, we investigated two vaccinia virus proteins (L2 and A30.5) of unknown topology that localize to the ER and are required for assembly of the viral membrane. Our results indicated that the N- and C-termini of L2 faced the cytoplasmic and luminal sides of the ER, respectively. In contrast both the N- and C-termini of A30.5 faced the cytoplasm. The system offers advantages for quickly determining the topology of intracellular proteins: the S11 tag is similar in length to commonly used epitope tags; multiple options are available for detecting fluorescence in live or fixed cells; transfection protocols are adaptable to numerous expression systems and can enable high throughput applications.

Poxvirus membrane biogenesis

Poxviruses differ from most DNA viruses by replicating entirely within the cytoplasm. The first discernible viral structures are crescents and spherical immature virions containing a single lipoprotein membrane bilayer with an external honeycomb lattice. Because this viral membrane displays no obvious continuity with a cellular organelle, a de novo origin was suggested. Nevertheless, transient connections between viral and cellular membranes could be difficult to resolve. Despite the absence of direct evidence, the intermediate compartment (ERGIC) between the endoplasmic reticulum (ER) and Golgi apparatus and the ER itself were considered possible sources of crescent membranes. A break-through in understanding poxvirus membrane biogenesis has come from recent studies of the abortive replication of several vaccinia virus null mutants. Novel images showing continuity between viral crescents and the ER and the accumulation of immature virions in the expanded ER lumen provide the first direct evidence for a cellular origin of this poxvirus membrane.

Structure-function analysis of vaccinia virus H7 protein reveals a novel phosphoinositide binding fold essential for poxvirus replication

Phosphoinositides and phosphoinositide binding proteins play a critical role in membrane and protein trafficking in eukaryotes. Their critical role in replication of cytoplasmic viruses has just begun to be understood. Poxviruses, a family of large cytoplasmic DNA viruses, rely on the intracellular membranes to develop their envelope, and poxvirus morphogenesis requires enzymes from the cellular phosphoinositide metabolic pathway. However, the role of phosphoinositides in poxvirus replication remains unclear, and no poxvirus proteins show any homology to eukaryotic phosphoinositide binding domains. Recently, a group of poxvirus proteins, termed viral membrane assembly proteins (VMAPs), were identified as essential for poxvirus membrane biogenesis. A key component of VMAPs is the H7 protein. Here we report the crystal structure of the H7 protein from vaccinia virus. The H7 structure displays a novel fold comprised of seven α-helices and a highly curved three-stranded antiparallel β-sheet. We identified a phosphoinositide binding site in H7, comprised of basic residues on a surface patch and the flexible C-terminal tail. These residues were found to be essential for viral replication and for binding of H7 to phosphatidylinositol-3-phosphate (PI3P) and phosphatidylinositol-4-phosphate (PI4P). Our studies suggest that phosphoinositide binding by H7 plays an essential role in poxvirus membrane biogenesis.

IMPORTANCE

Poxvirus viral membrane assembly proteins (VMAPs) were recently shown to be essential for poxvirus membrane biogenesis. One of the key components of VMAPs is the H7 protein. However, no known structural motifs could be identified from its sequence, and there are no homologs of H7 outside the poxvirus family to suggest a biochemical function. We have determined the crystal structure of the vaccinia virus (VACV) H7 protein. The structure displays a novel fold with a distinct and positively charged surface. Our data demonstrate that H7 binds phosphatidylinositol-3-phosphate and phosphatidylinositol-4-phosphate and that the basic surface patch is indeed required for phosphoinositide binding. In addition, mutation of positively charged residues required for lipid binding disrupted VACV replication. Phosphoinositides and phosphoinositide binding proteins play critical roles in membrane and protein trafficking in eukaryotes. Our study demonstrates that VACV H7 displays a novel fold for phosphoinositide binding, which is essential for poxvirus replication.

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.

Viral miniproteins

Many viruses encode short transmembrane proteins that play vital roles in virus replication or virulence. Because many of these proteins are less than 50 amino acids long and not homologous to cellular proteins, their open reading frames were often overlooked during the initial annotation of viral genomes. Some of these proteins oligomerize in membranes and form ion channels. Other miniproteins bind to cellular transmembrane proteins and modulate their activity, whereas still others have an unknown mechanism of action. Based on the underlying principles of transmembrane miniprotein structure, it is possible to build artificial small transmembrane proteins that modulate a variety of biological processes. These findings suggest that short transmembrane proteins provide a versatile mechanism to regulate a wide range of cellular activities, and we speculate that cells also express many similar proteins that have not yet been discovered.

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.

Association of the vaccinia virus A11 protein with the endoplasmic reticulum and crescent precursors of immature virions

The apparent de novo formation of viral membranes within cytoplasmic factories is a mysterious, poorly understood first step in poxvirus morphogenesis. Genetic studies identified several viral proteins essential for membrane formation and the assembly of immature virus particles. Their repression results in abortive replication with the accumulation of dense masses of viroplasm. In the present study, we further characterized one of these proteins, A11, and investigated its association with cellular and viral membranes under normal and abortive replication conditions. We discovered that A11 colocalized in cytoplasmic factories with the endoplasmic reticulum (ER) and L2, another viral protein required for morphogenesis. Confocal microscopy and subcellular fractionation indicated that A11 was not membrane associated in uninfected cells, whereas L2 still colocalized with the ER. Cell-free transcription and translation experiments indicated that both A11 and L2 are tail-anchored proteins that associate posttranslationally with membranes and likely require specific cytoplasmic targeting chaperones. Transmission electron microscopy indicated that A11, like L2, associated with crescent membranes and immature virions during normal infection and with vesicles and tubules near masses of dense viroplasm during abortive infection in the absence of the A17 or A14 protein component of viral membranes. When the synthesis of A11 was repressed, "empty" immature-virion-like structures formed in addition to masses of viroplasm. The immature-virion-like structures were labeled with antibodies to A17 and to the D13 scaffold protein and were closely associated with calnexin-labeled ER. These studies revealed similarities and differences between A11 and L2, both of which may be involved in the recruitment of the ER for virus assembly.

Analysis of the role of vaccinia virus H7 in virion membrane biogenesis with an H7-deletion mutant

Essential vaccinia virus genes are often studied with conditional-lethal inducible mutants. Here, we constructed a deletion mutant lacking the essential H7R gene (the ΔH7 mutant) with an H7-expressing cell line. Compared to an inducible H7 mutant, the ΔH7 mutant showed a defect at an earlier step of virion membrane biogenesis, before the development of short crescent-shaped precursors of the viral envelope. Our studies refine the role of H7 in virion membrane biogenesis and highlight the values of analyzing deletion mutants.

Membrane assembly during the infection cycle of the giant Mimivirus

Although extensively studied, the structure, cellular origin and assembly mechanism of internal membranes during viral infection remain unclear. By combining diverse imaging techniques, including the novel Scanning-Transmission Electron Microscopy tomography, we elucidate the structural stages of membrane biogenesis during the assembly of the giant DNA virus Mimivirus. We show that this elaborate multistage process occurs at a well-defined zone localized at the periphery of large viral factories that are generated in the host cytoplasm. Membrane biogenesis is initiated by fusion of multiple vesicles, ~70 nm in diameter, that apparently derive from the host ER network and enable continuous supply of lipid components to the membrane-assembly zone. The resulting multivesicular bodies subsequently rupture to form large open single-layered membrane sheets from which viral membranes are generated. Membrane generation is accompanied by the assembly of icosahedral viral capsids in a process involving the hypothetical major capsid protein L425 that acts as a scaffolding protein. The assembly model proposed here reveals how multiple Mimivirus progeny can be continuously and efficiently generated and underscores the similarity between the infection cycles of Mimivirus and Vaccinia virus. Moreover, the membrane biogenesis process indicated by our findings provides new insights into the pathways that might mediate assembly of internal viral membranes in general.

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.

Biogenesis of the vaccinia virus membrane: genetic and ultrastructural analysis of the contributions of the A14 and A17 proteins

Vaccinia virus membrane biogenesis requires the A14 and A17 proteins. We show here that both proteins can associate with membranes co- but not posttranslationally, and we perform a structure function analysis of A14 and A17 using inducible recombinants. In the absence of A14, electron-dense virosomes and distinct clusters of small vesicles accumulate; in the absence of A17, small vesicles form a corona around the virosomes. When the proteins are induced at 12 h postinfection (hpi), crescents appear at the periphery of the electron-dense virosomes, with the accumulated vesicles likely contributing to their formation. A variety of mutant alleles of A14 and A17 were tested for their ability to support virion assembly. For A14, biologically important motifs within the N-terminal or central loop region affected crescent maturation and the immature virion (IV)→mature virion (MV) transition. For A17, truncation or mutation of the N terminus of A17 engendered a phenotype consistent with the N terminus of A17 recruiting the D13 scaffold protein to nascent membranes. When N-terminal processing was abrogated, virions attempted to undergo the IV-to-MV transition without removing the D13 scaffold and were therefore noninfectious and structurally aberrant. Finally, we show that A17 is phosphorylated exclusively within the C-terminal tail and that this region is a direct substrate of the viral F10 kinase. In vivo, the biological competency of A17 was reduced by mutations that prevented its serine-threonine phosphorylation and restored by phosphomimetic substitutions. Precleavage of the C terminus or abrogation of its phosphorylation diminished the IV→MV maturation; a block to cleavage spared virion maturation but compromised the yield of infectious virus.

Vaccinia virus virion membrane biogenesis protein A11 associates with viral membranes in a manner that requires the expression of another membrane biogenesis protein, A6

A group of vaccinia virus (VACV) proteins, including A11, L2, and A6, are required for biogenesis of the primary envelope of VACV, specifically, for the acquisition of viral membrane precursors. However, the interconnection among these proteins is unknown and, with the exception of L2, the connection of these proteins with membranes is also unknown. In this study, prompted by the findings that A6 coprecipitated A11 and that the cellular distribution of A11 was dramatically altered by repression of A6 expression, we studied the localization of A11 in cells by using immunofluorescence and cell fractionation analysis. A11 was found to associate with membranes and colocalize with virion membrane proteins in viral replication factories during normal VACV replication. A11 partitioned almost equally between the detergent and aqueous phases upon Triton X-114 phase separation, demonstrating an intrinsic affinity with lipids. However, in the absence of infection or VACV late protein synthesis, A11 did not associate with cellular membranes. Furthermore, when A6 expression was repressed, A11 did not colocalize with any viral membrane proteins or associate with membranes. In contrast, when virion envelope formation was blocked at a later step by repression of A14 expression or by rifampin treatment, A11 colocalized with virion membrane proteins in the factories. Altogether, our data showed that A11 associates with viral membranes during VACV replication, and this association requires A6 expression. This study provides a physical connection between A11 and viral membranes and suggests that A6 regulates A11 membrane association.

Orthopoxvirus targets for the development of new antiviral agents

Investments in the development of new drugs for orthopoxvirus infections have fostered new avenues of research, provided an improved understanding of orthopoxvirus biology and yielded new therapies that are currently progressing through clinical trials. These broad-based efforts have also resulted in the identification of new inhibitors of orthopoxvirus replication that target many different stages of viral replication cycle. This review will discuss progress in the development of new anti-poxvirus drugs and the identification of new molecular targets that can be exploited for the development of new inhibitors. The prototype of the orthopoxvirus group is vaccinia virus and its replication cycle will be discussed in detail noting specific viral functions and their associated gene products that have the potential to serve as new targets for drug development. Progress that has been achieved in recent years should yield new drugs for the treatment of these infections and might also reveal new approaches for antiviral drug development with other viruses.

Vaccinia virus A6 is essential for virion membrane biogenesis and localization of virion membrane proteins to sites of virion assembly

Poxvirus acquires its primary envelope through a process that is distinct from those of other enveloped viruses. The molecular mechanism of this process is poorly understood, but several poxvirus proteins essential for the process have been identified in studies of vaccinia virus (VACV), the prototypical poxvirus. Previously, we identified VACV A6 as an essential factor for virion morphogenesis by studying a temperature-sensitive mutant with a lesion in A6. Here, we further studied A6 by constructing and characterizing an inducible virus (iA6) that could more stringently repress A6 expression. When A6 expression was induced by the inducer isopropyl-β-D-thiogalactoside (IPTG), iA6 replicated normally, and membrane proteins of mature virions (MVs) predominantly localized in viral factories where virions were assembled. However, when A6 expression was repressed, electron microscopy of infected cells showed the accumulation of large viroplasm inclusions containing virion core proteins but no viral membranes. Immunofluorescence and cell fractionation studies showed that the major MV membrane proteins A13, A14, D8, and H3 did not localize to viral factories but instead accumulated in the secretory compartments, including the endoplasmic reticulum. Overall, our results show that A6 is an additional VACV protein that participates in an early step of virion membrane biogenesis. Furthermore, A6 is required for MV membrane protein localization to sites of virion assembly, suggesting that MV membrane proteins or precursors of MV membranes are trafficked to sites of virion assembly through an active, virus-mediated process that requires A6.

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.

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.