Vaccinia Virus Intracellular Movement Is Associated with Microtubules and Independent of Actin Tails

Two mechanisms have been proposed for the intracellular movement of enveloped vaccinia virus virions: rapid actin polymerization and microtubule association. The first mechanism is used by the intracellular pathogens Listeria and Shigella, and the second is used by cellular vesicles transiting from the Golgi network to the plasma membrane. To distinguish between these models, two recombinant vaccinia viruses that express the B5R membrane protein fused to enhanced green fluorescent protein (GFP) were constructed. One had Tyr(112) and Tyr(132) of the A36R membrane protein, which are required for phosphorylation and the nucleation of actin tails, conservatively changed to Phe residues; the other had the A36R open reading frame deleted. Although the Tyr mutant was impaired in Tyr phosphorylation and actin tail formation, digital video and time-lapse confocal microscopy demonstrated that virion movement from the juxtanuclear region to the periphery was saltatory with maximal speeds of >2 microm/s and was inhibited by the microtubule-depolymerizing drug nocodazole. Moreover, this actin tail-independent movement was indistinguishable from that of a control virus with an unmutated A36R gene and closely resembled the movement of vesicles on microtubules. However, in the absence of actin tails, the Tyr mutant did not induce the formation of motile, virus-tipped microvilli and had a reduced ability to spread from cell to cell. The deletion mutant was more severely impaired, suggesting that the A36R protein has additional roles. Optical sections of unpermeabilized, B5R antibody-stained cells that expressed GFP-actin and were infected with wild-type vaccinia virus revealed that all actin tails were associated with virions on the cell surface. We concluded that the intracellular movement of intracellular enveloped virions occurs on microtubules and that the motile actin tails enhance extracellular virus spread to neighboring cells.     

Vaccinia virus utilizes microtubules for movement to the cell surface

Vaccinia virus (VV) egress has been studied using confocal, video, and electron microscopy. Previously, intracellular-enveloped virus (IEV) particles were proposed to induce the polymerization of actin tails, which propel IEV particles to the cell surface. However, data presented support an alternative model in which microtubules transport virions to the cell surface and actin tails form beneath cell-associated enveloped virus (CEV) particles at the cell surface. Thus, VV is unique in using both microtubules and actin filaments for egress. The following data support this proposal. (a) Microscopy detected actin tails at the surface but not the center of cells. (b) VV mutants lacking the A33R, A34R, or A36R proteins are unable to induce actin tail formation but produce CEV and extracellular-enveloped virus. (c) CEV formation is inhibited by nocodazole but not cytochalasin D or 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo(3,4-d)pyrimidine (PP1). (d) IEV particles tagged with the enhanced green fluorescent protein fused to the VV B5R protein moved inside cells at 60 microm/min. This movement was stop-start, was along defined pathways, and was inhibited reversibly by nocodazole. This velocity was 20-fold greater than VV movement on actin tails and consonant with the rate of movement of organelles along microtubules.     

Mapping and functional analysis of interaction sites within the cytoplasmic domains of the vaccinia virus A33R and A36R envelope proteins

Incorporation of the vaccinia virus A36R protein into the outer membrane of intracellular enveloped virions (IEV) is dependent on expression of the A33R protein. Possible interactions of the 200-amino-acid cytoplasmic domain of the A36R protein with itself or with the cytoplasmic domain of the A33R, A34R, B5R, or F12L IEV membrane protein was investigated by using the yeast two-hybrid system. A strong interaction was detected only between the cytoplasmic domains of the A36R and A33R proteins. Upon further analyses, the interaction site was mapped to residues 91 to 111 of the A36R protein. To investigate the role of the A36R:A33R interaction during viral infection, five recombinant vaccinia viruses containing B5R-GFP as a marker were constructed. Four had the full-length A36R gene replaced with various-length C-terminal truncations of A36R, of which two contained residues 91 to 111 and two were missing this region. The fifth recombinant virus had an A33R gene with most of the 40-amino-acid cytoplasmic tail deleted. Residues 91 to 111 of A36R and the cytoplasmic tail of A33R were required for a strong interaction between the two proteins during viral infection and for maximal amounts of A36R protein on IEV. Mutants lacking these regions of A33R or A36R formed IEV that exhibited only short sporadic intracellular movement, displayed no actin tails, and formed small plaques on cell monolayers equivalent to those of an A36R deletion mutant and smaller than those formed by point mutations that specifically abrogate actin tail formation. The A33R interaction site of the A36R protein is highly conserved among orthopoxviruses and may overlap binding sites for cellular proteins needed for microtubular movement and actin tail formation.     

Interactions between vaccinia virus IEV membrane proteins and their roles in IEV assembly and actin tail formation

The intracellular enveloped form of vaccinia virus (IEV) induces the formation of actin tails that are strikingly similar to those seen in Listeria and Shigella infections. In contrast to the case for Listeria and Shigella, the vaccinia virus protein(s) responsible for directly initiating actin tail formation remains obscure. However, previous studies with recombinant vaccinia virus strains have suggested that the IEV-specific proteins A33R, A34R, A36R, B5R, and F13L play an undefined role in actin tail formation. In this study we have sought to understand how these proteins, all of which are predicted to have small cytoplasmic domains, are involved in IEV assembly and actin tail formation. Our data reveal that while deletion of A34R, B5R, or F13L resulted in a severe reduction in IEV particle assembly, IEVs formed by the DeltaB5R and DeltaF13L deletion strains, but not DeltaA34R, were still able to induce actin tails. The DeltaA36R deletion strain produced normal amounts of IEV particles, although these were unable to induce actin tails. Using several different approaches, we demonstrated that A36R is a type Ib membrane protein with a large, 195-amino-acid cytoplasmic domain exposed on the surface of IEV particles. Finally, coimmunoprecipitation experiments demonstrated that A36R interacts with A33R and A34R but not with B5R and that B5R forms a complex with A34R but not with A33R or A36R. Using extracts from DeltaA34R- and DeltaA36R-infected cells, we found that the interaction of A36R with A33R and that of A34R with B5R are independent of A34R and A36R, respectively. We conclude from our observations that multiple interactions between IEV membrane proteins exist which have important implications for IEV assembly and actin tail formation. Furthermore, these data suggest that while A34R is involved in IEV assembly and organization, A36R is critical for actin tail formation.     

The Envelope Protein Encoded by the A33R Gene Is Required for Formation of Actin-Containing Microvilli and Efficient Cell-to-Cell Spread of Vaccinia Virus

The vaccinia virus (VV) A33R gene encodes a highly conserved 23- to 28-kDa glycoprotein that is specifically incorporated into the viral outer envelope. The protein is expressed early and late after infection, consistent with putative early and late promoter sequences. To determine the role of the protein, two inducible A33R mutants were constructed, one with the late promoter and one with the early and late A33R promoter elements. Decreased A33R expression was associated with small plaques that formed comets in liquid medium. Using both an antibiotic resistance gene and a color marker, an A33R deletion mutant, vA33Δ, was isolated, indicating that the A33R gene is not essential for VV replication. The plaques formed by vA33Δ, however, were tiny, indicating that the A33R protein is necessary for efficient cell-to-cell spread. Rescue of the large-plaque phenotype was achieved by inserting a new copy of the A33R gene into the thymidine kinase locus, confirming the specific genetic basis of the phenotype. Although there was a reduction in intracellular virus formed in cells infected with vA33Δ, the amount of infectious virus in the medium was increased. The virus particles in the medium had the buoyant density of extracellular enveloped viruses (EEV). Additionally, amounts of vA33Δ cell-associated extracellular enveloped viruses (CEV) were found to be normal. Immunogold electron microscopy of cells infected with vA33Δ demonstrated the presence of the expected F13L and B5R proteins in wrapping membranes and EEV; however, fully wrapped vA33Δ intracellular enveloped viruses (IEV) were rare compared to partially wrapped particles. Specialized actin tails that propel IEV particles to the periphery and virus-tipped microvilli (both common in wild-type-infected cells) were absent in cells infected with vA33Δ. This is the first deletion mutant in a VV envelope gene that produces at least normal amounts of fully infectious EEV and CEV and yet has a small-plaque phenotype. These data support a new model for VV spread, emphasizing the importance of virus-tipped actin tails.     

Identification of second-site mutations that enhance release and spread of vaccinia virus

The spread of most strains of vaccinia virus in cell monolayers occurs predominantly via extracellular enveloped virions that adhere to the tips of actin-containing microvilli and to a lesser extent via diffusion of released virions. The mechanism by which virions adhere to the cell surface is unknown, although several viral proteins may be involved. The present investigation was initiated with the following premise: spontaneous mutations that increase virus release will be naturally selected by propagating a virus unable to spread by means of actin tails. Starting with an A36R deletion mutant that forms small, round plaques, five independent virus clones with enhanced spread due to the formation of comet or satellite plaques were isolated. The viral membrane glycoprotein genes of the isolates were sequenced; four had mutations causing C-terminal truncations of the A33R protein, and one had a serine replacing proline 189 of the B5R protein. The comet-forming phenotype was specifically reproduced or reversed by homologous recombination using DNA containing the mutated or natural sequence, respectively. Considerably more extracellular enveloped virus was released into the medium by the second-site mutants than by the parental A36R deletion mutant, explaining their selection in tissue culture as well as their comet-forming phenotype. The data suggest that the B5R protein and the C-terminal region of the A33R protein are involved in adherence of cell-associated enveloped virions to cells. In spite of their selective advantage in cultured cells, the second-site mutants were not detectably more virulent than the A36R deletion mutant when administered to mice by the intranasal route.     

The vaccinia virus B5 protein requires A34 for efficient intracellular trafficking from the endoplasmic reticulum to the site of wrapping and incorporation into progeny virions

The glycoproteins encoded by the vaccinia virus A34R and B5R genes are involved in intracellular envelope virus formation and are highly conserved among orthopoxviruses. A recombinant virus that has the A34R gene deleted and the B5R gene replaced with a B5R gene fused to the enhanced green fluorescent protein (B5R-GFP) gene was created (vB5R-GFP/ΔA34R) to investigate the role of A34 during virion morphogenesis. Cells infected with vB5R-GFP/ΔA34R displayed GFP fluorescence throughout the cytoplasm, which differed markedly from that seen in cells infected with a normal B5R-GFP-expressing virus (vB5R-GFP). Immunofluorescence and subcellular fractionation demonstrated that B5-GFP localizes with the endoplasmic reticulum in the absence of A34. Expression of either full-length A34 or a construct consisting of the lumenal and transmembrane domains restored normal trafficking of B5-GFP to the site of wrapping in the juxtanuclear region. Coimmunoprecipitation studies confirmed that B5 and A34 interact through their luminal domains, and further analysis revealed that in the absence of A34, B5 is not efficiently incorporated into virions released from the cell.     

Mutations in the vaccinia virus A33R and B5R envelope proteins that enhance release of extracellular virions and eliminate formation of actin-containing microvilli without preventing tyrosine phosphorylation of the A36R protein

The spread of vaccinia virus in cell cultures is mediated by virions that adhere to the tips of specialized actin-containing microvilli and also by virions that are released into the medium. The use of a small plaque-forming A36R gene deletion mutant to select spontaneous second-site mutants exhibiting enhanced virus release was described previously. Two types of mutations were found: C-terminal truncations of the A33R envelope protein and a single amino acid substitution of the B5R envelope protein. In the present study, we transferred each type of mutation into a wild-type virus background in order to study their effects in vitro and in vivo. The two new mutants conserved the enhanced virus release properties of the original isolates; the A33R mutant produced considerably more extracellular virus than the B5R mutant. The extracellular virus particles contained the truncated A33R protein in one case and the mutated B5R protein in the other. Remarkably, both mutants failed to form actin tails and specialized microvilli, despite the presence of an intact A36R gene. The synthesis of the A36R protein as well as its physical association with the mutated or wild-type A33R protein was demonstrated. Moreover, the A36R protein was tyrosine phosphorylated, a step mediated by a membrane-associated Src kinase that regulates the nucleation of actin polymerization. The presence of large numbers of adherent virions on the cell surface argued against rapid dissociation as having a key role in preventing actin tail formation. Thus, the A33R and B5R proteins may be more directly involved in the formation or stabilization of actin tails than had been previously thought. When mice were inoculated intranasally, the A33R mutant was highly attenuated and the B5R mutant was mildly attenuated compared to wild-type virus. Enhanced virus release, therefore, did not compensate for the loss of actin tails and specialized microvilli.     

Extracellular vaccinia virus formation and cell-to-cell virus transmission are prevented by deletion of the gene encoding the 37,000-Dalton outer envelope protein.

There are two types of infectious vaccinia virus particles: intracellular naked virions and extracellular enveloped virions (EEV). To determine the biological role of the enveloped form of vaccinia virus, we produced and characterized a mutant that is defective in EEV formation. The strategy involved replacement by homologous recombination of the gene F13L, encoding a 37,000-Da protein (VP37) that is specific for the outer envelope of EEV, with a selectable antibiotic resistance marker, the Escherichia coli gpt gene. Initial experiments, however, suggested that such a mutation was lethal or prevented plaque formation. By employing a protocol consisting of high-multiplicity passages of intracellular virus from the transfected cells and then limiting dilution cloning, we succeeded in isolating the desired mutant, which was defective in production of plaques and extracellular virus but made normal amounts of intracellular naked virions. Electron microscopic examination indicated that the mutant virus particles, unlike wild type, were neither wrapped with Golgi-derived membranes nor associated with the cell surface. The absence of VP37 did not prevent the transport of the viral hemagglutinin to the plasma membrane but nevertheless abrogated both low-pH- and antibody-mediated cell fusion. These results indicate that VP37 is required for EEV formation and also plays a critical role in the local cell-to-cell transmission of vaccinia virus, perhaps via enveloped virions attached to or released from the cell membrane. By contrast, a mutated virus with a deletion of the K4L open reading frame, which is a homolog of the VP37 gene, was not defective in formation of plaques or EEV.     

The A34R glycoprotein gene is required for induction of specialized actin-containing microvilli and efficient cell-to-cell transmission of vaccinia virus.

The mechanisms allowing vaccinia virus to spread from cell to cell are incompletely understood. The A34R gene of vaccinia virus encodes a glycoprotein that is localized in the outer membranes of extracellular virions. The small-plaque phenotype of an A34R deletion mutant was similar to that of mutants with deletions in other envelope genes that fail to produce extracellular vaccinia virions. Transmission electron microscopy, however, revealed that the A34R mutant produced numerous extracellular particles that were labeled with antibodies to other outer-envelope proteins and with protein A-colloidal gold. Fluorescence and scanning electron microscopy indicated that expression of the A34R protein was necessary for detection of vaccinia virus-induced actin tails, which provide motility to the intracellular enveloped form of vaccinia virus, and of virus-tipped specialized microvilli that project from the cell. The ability of vaccinia virus-infected cells to form syncytia after a brief exposure to a pH below 6, known as fusion from within, failed to occur in the absence of expression of the A34R protein; nevertheless, purified A34R- virions were capable of mediating low-pH-induced fusion from without. The present study provides genetic and microscopic evidence for the involvement of a specific viral protein in the formation or stability of actin-containing microvilli and for a role of these structures in cell-to-cell spread rather than in formation of extracellular virions.     

The vaccinia virus A33R protein provides a chaperone function for viral membrane localization and tyrosine phosphorylation of the A36R protein

The products of the A33R and A36R genes of vaccinia virus are incorporated into the membranes of intracellular enveloped virions (IEV). When extracts of cells that had been infected with vaccinia virus and labeled with H(3)(32)PO(4) were immunoprecipitated with antibodies against the A33R protein, two prominent bands were resolved. The moderately and more intensely labeled bands were identified as phosphorylated A33R and A36R proteins, respectively. The immunoprecipitated complex contained disulfide-bonded dimers of A33R protein that were noncovalently linked to A36R protein. Biochemical analysis indicated that the two proteins were phosphorylated predominantly on serine residues, with lesser amounts on threonines. The A36R protein was also phosphorylated on tyrosine, as determined by specific binding to an anti-phosphotyrosine antibody. Serine phosphorylation and A33R-A36R protein complex formation occurred even when virus assembly was blocked at an early stage with the drug rifampin. Tyrosine phosphorylation was selectively reduced in cells infected with F13L or A34R gene deletion mutants that were impaired in the membrane-wrapping step of IEV formation. In addition, tyrosine phosphorylation was specifically inhibited in cells infected with an A33R deletion mutant that still formed IEV. Immunofluorescence and immunoelectron microscopy indicated that in the absence of the A33R protein, the A36R protein was localized in Golgi membranes but not in IEV. In the absence of the A36R protein, however, the A33R protein still localized to IEV membranes. These studies together with others suggest that the A33R protein guides the A36R protein to the IEV membrane, where it subsequently becomes tyrosine phosphorylated as a signal for actin tail formation.     

Vaccinia Virus F13L Protein with a Conserved Phospholipase Catalytic Motif Induces Colocalization of the B5R Envelope Glycoprotein in Post-Golgi Vesicles

The wrapping of intracellular mature vaccinia virions by modified trans-Golgi or endosomal cisternae to form intracellular enveloped virions is dependent on at least two viral proteins encoded by the B5R and F13L open reading frames. B5R is a type I integral membrane glycoprotein, whereas F13L is an unglycosylated, palmitylated protein with a motif that is conserved in a superfamily of phospholipid-metabolizing enzymes. Microscopic visualization of the F13L protein was achieved by fusing it to the enhanced green fluorescent protein (GFP). F13L-GFP was functional when expressed by a recombinant vaccinia virus in which it replaced the wild-type F13L gene or by transfection of uninfected cells with a plasmid vector followed by infection with an F13L deletion mutant. In uninfected or infected cells, F13L-GFP was associated with Golgi cisternae and post-Golgi vesicles containing the LAMP 2 late endosomal-lysosomal marker. Association of F13L-GFP with vesicles was dependent on an intact phospholipase catalytic motif and sites of palmitylation. The B5R protein was also associated with LAMP2-containing vesicles when F13L-GFP was coexpressed, but was largely restricted to Golgi cisternae in the absence of F13L-GFP or when the F13L moiety was mutated. We suggest that the F13L protein, like its human phospholipase D homolog, regulates vesicle formation and that this process is involved in intracellular enveloped virion membrane formation.     

An E2–F12 complex is required for intracellular enveloped virus morphogenesis during vaccinia infection

The vaccinia virus protein, F12, has been suggested to play an important role in microtubule-based transport of intracellular enveloped virus (IEV). We found that GFP-F12 is recruited to IEV moving on microtubules but is released from virus particles when they switch to actin-based motility. In the absence of F12, although the majority of IEV remain close to their peri-nuclear site of assembly, a small number of IEV still move with linear trajectories at speeds of 0.85 μm s⁻¹, consistent with microtubule transport. Using a recombinant virus expressing GST-F12, we found that the viral protein E2 interacts directly with F12. In infected cells, GFP-E2 is observed on moving IEV as well as in the Golgi region, but is not associated with actin tails. In the absence of E2L, IEV accumulate in the peri-nuclear region and F12 is not recruited. Conversely, GFP-E2 is not observed on IEV in the absence of F12. Ultra-structural analysis of ΔE2L- and ΔF12L-infected cells reveals that loss of either protein results in defects in membrane wrapping during IEV formation. We suggest that E2 and F12 function as a complex that is necessary for IEV morphogenesis prior to their microtubule-based transport towards the plasma membrane.     

Interaction of vaccinia virus with the actin cytoskeleton

Vaccinia virus infection results in large rearrangements of the host actin cytoskeleton including the formation of actin tails that are strikingly similar to those seen inListeria, Shigella andRickettsia infections. Using actin polymerization as the driving force the intracellular enveloped form of the vaccinia virus (IEV) is propelled on the tip of actin tails at a speed of 2.8 μm/min, both intra- and intercellularly. The similarities between the actin-based motility of the vaccinia virus,Listeria, Shigella andRickettsia suggest that intracellular pathogens have developed a common strategy to exploit the actin cytoskeleton of the host to facilitate their intercellular spread. This review focuses on our current understanding of the interactions between the vaccinia virus,and the actin cytoskeleton. Presented at the1st International Minisymposium on Cellular Microbiology: Cell Biology and Signalization in Host-Pathogen Interactions, Prague, October 6, 1997.     

Vaccinia Virus Protein F12 Associates with Intracellular Enveloped Virions through an Interaction with A36

Vaccinia virus is the prototypical member of the family Poxviridae. Three morphologically distinct forms are produced during infection: intracellular mature virions (IMV), intracellular enveloped virions (IEV), and extracellular enveloped virions (EEV). Two viral proteins, F12 and A36, are found exclusively on IEV but not on IMV and EEV. Analysis of membranes from infected cells showed that F12 was only associated with membranes and is not an integral membrane protein. A yeast two-hybrid assay revealed an interaction between amino acids 351 to 458 of F12 and amino acids 91 to 111 of A36. We generated a recombinant vaccinia virus that expresses an F12, which lacks residues 351 to 458. Characterization of this recombinant revealed a small-plaque phenotype and a subsequent defect in virus release similar to a recombinant virus that had F12L deleted. In addition, F12 lacking residues 351 to 458 was unable to associate with membranes in infected cells. These results suggest that F12 associates with IEV through an interaction with A36 and that this interaction is critical for the function of F12 during viral egress.     

Functional Analysis of Vaccinia Virus B5R Protein: Essential Role in Virus Envelopment Is Independent of a Large Portion of the Extracellular Domain

Vaccinia virus has two forms of infectious virions: the intracellular mature virus and the extracellular enveloped virus (EEV). EEV is critical for cell-to-cell and long-range spread of the virus. The B5R open reading frame (ORF) encodes a membrane protein that is essential for EEV formation. Deletion of the B5R ORF results in a dramatic reduction of EEV, and as a consequence, the virus produces small plaques in vitro and is highly attenuated in vivo. The extracellular portion of B5R is composed mainly of four domains that are similar to the short consensus repeats (SCRs) present in complement regulatory proteins. To determine the contribution of these putative SCR domains to EEV formation, we constructed recombinant vaccinia viruses that replaced the wild-type B5R gene with a mutated gene encoding a B5R protein lacking the SCRs. The resulting recombinant viruses produced large plaques, indicating efficient cell-to-cell spread in vitro, and gradient centrifugation of supernatants from infected cells confirmed that EEV was formed. In contrast, phalloidin staining of infected cells showed that the virus lacking the SCR domains was deficient in the induction of thick actin bundles. Thus, the highly conserved SCR domains present in the extracellular portion of the B5R protein are dispensable for EEV formation. This indicates that the B5R protein is a key viral protein with multiple functions in the process of virus envelopment and release. In addition, given the similarity of the extracellular domain to complement control proteins, the B5R protein may be involved in viral evasion from host immune responses.     

Role of receptor-mediated endocytosis in the formation of vaccinia virus extracellular enveloped particles

Infectious intracellular mature vaccinia virus particles are wrapped by cisternae, which may arise from trans-Golgi or early endosomal membranes, and are transported along microtubules to the plasma membrane where exocytosis occurs. We used EH21, a dominant-negative form of Eps15 that is an essential component of clathrin-coated pits, to investigate the extent and importance of endocytosis of viral envelope proteins from the cell surface. Several recombinant vaccinia viruses that inducibly or constitutively express an enhanced green fluorescent protein (GFP)-EH21 fusion protein were constructed. Expression of GFP-EH21 blocked uptake of transferrin, a marker for clathrin-mediated endocytosis, as well as association of adaptor protein-2 with clathrin-coated pits. When GFP-EH21 was expressed, there were increased amounts of viral envelope proteins, including A33, A36, B5, and F13, in the plasma membrane, and their internalization was inhibited. Wrapping of virions appeared to be qualitatively unaffected as judged by electron microscopy, a finding consistent with a primary trans-Golgi origin of the cisternae. However, GFP-EH21 expression caused a 50% reduction in released enveloped virions, decreased formation of satellite plaques, and delayed virus spread, indicating an important role for receptor-mediated endocytosis. Due to dynamic interconnection between endocytic and exocytic pathways, viral proteins recovered from the plasma membrane could be used by trans-Golgi or endosomal cisternae to form new viral envelopes. Adherence of enveloped virions to unrecycled viral proteins on the cell surface may also contribute to decreased virus release in the presence of GFP-EH21. In addition to a salvage function, the retrieval of viral proteins from the cell surface may reduce immune recognition.     

Vaccinia Virus Protein Complex F12/E2 Interacts with Kinesin Light Chain Isoform 2 to Engage the Kinesin-1 Motor Complex

During vaccinia virus morphogenesis, intracellular mature virus (IMV) particles are wrapped by a double lipid bilayer to form triple enveloped virions called intracellular enveloped virus (IEV). IEV are then transported to the cell surface where the outer IEV membrane fuses with the cell membrane to expose a double enveloped virion outside the cell. The F12, E2 and A36 proteins are involved in transport of IEVs to the cell surface. Deletion of the F12L or E2L genes causes a severe inhibition of IEV transport and a tiny plaque size. Deletion of the A36R gene leads to a smaller reduction in plaque size and less severe inhibition of IEV egress. The A36 protein is present in the outer membrane of IEVs, and over-expressed fragments of this protein interact with kinesin light chain (KLC). However, no interaction of F12 or E2 with the kinesin complex has been reported hitherto. Here the F12/E2 complex is shown to associate with kinesin-1 through an interaction of E2 with the C-terminal tail of KLC isoform 2, which varies considerably between different KLC isoforms. siRNA-mediated knockdown of KLC isoform 1 increased IEV transport to the cell surface and virus plaque size, suggesting interaction with KLC isoform 1 is somehow inhibitory of IEV transport. In contrast, knockdown of KLC isoform 2 did not affect IEV egress or plaque formation, indicating redundancy in virion egress pathways. Lastly, the enhancement of plaque size resulting from loss of KLC isoform 1 was abrogated by removal of KLC isoforms 1 and 2 simultaneously. These observations suggest redundancy in the mechanisms used for IEV egress, with involvement of KLC isoforms 1 and 2, and provide evidence of interaction of F12/E2 complex with the kinesin-1 complex.     

Deletion of the vaccinia virus B5R gene encoding a 42-kilodalton membrane glycoprotein inhibits extracellular virus envelope formation and dissemination.

The structure, formation, and function of the virion membranes are among the least well understood aspects of vaccinia virus replication. In this study, we investigated the role of gp42, a glycoprotein component of the extracellular enveloped form of vaccinia virus (EEV) encoded by the B5R gene. The B5R gene was deleted by homologous recombination from vaccinia virus strains IHD-J and WR, which produce high and low levels of EEV, respectively. Isolation of recombinant viruses was facilitated by the insertion into the genome of a cassette containing the Escherichia coli gpt and lacZ genes flanked by the ends of the B5R gene to provide simultaneous antibiotic selection and color screening. Deletion mutant viruses of both strains formed tiny plaques, and those of the IHD-J mutant lacked the characteristic comet shape caused by release of EEV. Nevertheless, similar yields of intracellular infectious virus were obtained whether cells were infected with the B5R deletion mutants or their parental strains. In the case of IHD-J, however, this deletion severely reduced the amount of infectious extracellular virus. Metabolic labeling studies demonstrated that the low extracellular infectivity corresponded with a decrease in EEV particles in the medium. Electron microscopic examination revealed that mature intracellular naked virions (INV) were present in cells infected with mutant virus, but neither membrane-wrapped INV nor significant amounts of plasma membrane-associated virus were observed. Syncytium formation, which occurs in cells infected with wild-type WR and IHD-J virus after brief low-pH treatment, did not occur in cells infected with the B5R deletion mutants. By contrast, syncytium formation induced by antibody to the viral hemagglutinin occurred, suggesting that different mechanisms are involved. When assayed by intracranial injection into weanling mice, both IHD-J and WR mutant viruses were found to be significantly attenuated. These findings demonstrate that the 42-kDa glycoprotein of the EEV is required for efficient membrane enwrapment of INV, externalization of the virus, and transmission and that gp42 contributes to viral virulence in strains producing both low and high levels of EEV.     

The 5′ Cap of Tobacco Mosaic Virus (TMV) Is Required for Virion Attachment to the Actin/Endoplasmic Reticulum Network During Early Infection

Almost nothing is known of the earliest stages of plant virus infections. To address this, we microinjected Cy3 (UTP)‐labelled tobacco mosaic virus (TMV) into living tobacco trichome cells. The Cy3‐virions were infectious, and the viral genome trafficked from cell to cell. However, neither the fluorescent vRNA pool nor the co‐injected green fluorescent protein (GFP) left the injected trichome, indicating that the synthesis of (unlabelled) progeny viral (v)RNA is required to initiate cell‐to‐cell movement, and that virus movement is not accompanied by passive plasmodesmatal gating. Cy3‐vRNA formed granules that became anchored to the motile cortical actin/endoplasmic reticulum (ER) network within minutes of injection. Granule movement on actin/ER was arrested by actin inhibitors indicating actin‐dependent RNA movement. The 5′ methylguanosine cap was shown to be required for vRNA anchoring to the actin/ER. TMV vRNA lacking the 5′ cap failed to form granules and was degraded in the cytoplasm. Removal of the 3′ untranslated region or replicase both inhibited replication but did not prevent granule formation and movement. Dual‐labelled TMV virions in which the vRNA and the coat protein were highlighted with different fluorophores showed that both fluorescent signals were initially located on the same ER‐bound granules, indicating that TMV virions may become attached to the ER prior to uncoating of the viral genome.