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

Critical role of complement and viral evasion of complement in acute, persistent, and latent gamma-herpesvirus infection

Several gamma-herpesviruses encode homologs of host regulators of complement activation (RCA) proteins, suggesting that they have evolved immune evasion strategies targeting complement. We evaluated the role of complement factor C3 (C3) and the murine gamma-herpesvirus 68 (gammaHV68) RCA protein in viral pathogenesis. Deletion of the gammaHV68 RCA protein decreased virulence during acute CNS infection, and this attenuation was specifically reversed by deletion of host C3. The gammaHV68 RCA protein was also important for persistent viral replication and virulence in IFNgammaR(-/-) mice. In addition, C3 played a role in regulating latency, but this was not counteracted by the gammaHV68 RCA protein. We conclude that complement is a key host defense against gamma-herpesvirus infection and that gamma-herpesviruses have evolved an immune evasion strategy that is effective against complement-mediated antiviral responses during acute but not latent infection.

Similarities in the induction of post-Golgi vesicles by the vaccinia virus F13L protein and phospholipase D

Intracellular mature vaccinia virions are wrapped by cisternae, derived from virus-modified trans-Golgi or endosomal membranes, and then transported via microtubules to the cell periphery. Two viral proteins, encoded by the F13L and B5R open reading frames, are essential for the membrane-wrapping step. Previous transfection studies indicated that F13L induces the formation of post-Golgi vesicles that incorporate the B5R protein and that this activity depends on an intact F13L phospholipase motif. Here we show that the F13L protein has a general effect on the trafficking of integral membrane proteins from the Golgi apparatus, as both the vaccinia virus A36R protein and the vesicular stomatitis virus G protein also colocalized with the F13L protein in vesicles. In addition, increased expression of cellular phospholipase D, which has a similar phospholipase motif as, but little amino acid sequence identity with, F13L, induced post-Golgi vesicles that contained B5R and A36R proteins. Butanol-1, which prevents the formation of phosphatidic acid by phospholipase D and specifically inhibits phospholipase D-mediated vesicle formation, also inhibited F13L-induced vesicle formation, whereas secondary and tertiary alcohols had no effect. Moreover, inhibition of phospholipase activity by butanol-1 also reduced plaque size and decreased the formation of extracellular vaccinia virus without affecting the yield of intracellular mature virus. Phospholipase D, however, could not complement a vaccinia virus F13L deletion mutant, indicating that F13L has additional virus-specific properties. Taken together, these data support an important role for F13L in inducing the formation of vesicle precursors of the vaccinia virus membrane via phospholipase activity or activation.

Analysis of the monkeypox virus genome

Monkeypox virus (MPV) belongs to the orthopoxvirus genus of the family Poxviridae, is endemic in parts of Africa, and causes a human disease that resembles smallpox. The 196,858-bp MPV genome was analyzed with regard to structural features and open reading frames. Each end of the genome contains an identical but oppositely oriented 6379-bp terminal inverted repetition, which similar to that of other orthopoxviruses, includes a putative telomere resolution sequence and short tandem repeats. Computer-assisted analysis was used to identify 190 open reading frames containing >/=60 amino acid residues. Of these, four were present within the inverted terminal repetition. MPV contained the known essential orthopoxvirus genes but only a subset of the putative immunomodulatory and host range genes. Sequence comparisons confirmed the assignment of MPV as a distinct species of orthopoxvirus that is not a direct ancestor or a direct descendent of variola virus, the causative agent of smallpox.

Replacing the SCR domains of vaccinia virus protein B5R with EGFP causes a reduction in plaque size and actin tail formation but enveloped virions are still transported to the cell surface

A vaccinia virus (VV) recombinant is described in which the outer envelope of extracellular enveloped virus (EEV), cell-associated enveloped virus (CEV) and intracellular enveloped virus (IEV) is labelled with the enhanced green fluorescent protein (EGFP) derived from Aequorea victoria. To construct this virus, EGFP was fused to the VV B5R protein from which the four short consensus repeats (SCRs) of the extracellular domain had been deleted. Cells infected with the recombinant virus expressed a B5R-EGFP fusion protein of 40 kDa that was present on IEV, CEV and EEV, but was absent from IMV. The recombinant virus produced 2- and 3-fold reduced levels of IMV and EEV, respectively. Analysis of infected cells by confocal microscopy showed that actin tail formation by the mutant virus was reduced by 86% compared to wild-type (WT). The virus formed a small plaque compared to WT, consistent with a role for actin tails in promoting cell-to-cell spread of virus. However, the enveloped virions were still transported to the cell surface, confirming that this process is independent of actin tail formation. Lastly, we compared the mutant virus with a recombinant VV in which the B5R SCR domains were deleted and show that, contrary to a previous report, the plaque size of the latter virus was reduced compared to WT. This observation reconciles an inconsistency in the field and confirms that viruses deficient in formation of actin tails form small plaques.

The vaccinia virus F12L protein is associated with intracellular enveloped virus particles and is required for their egress to the cell surface

The vaccinia virus (VV) F12L gene encodes a 65 kDa protein that is expressed late during infection and is important for plaque formation, EEV production and virulence. Here we have used a recombinant virus (vF12LHA) in which the F12L protein is tagged at the C terminus with an epitope recognized by a monoclonal antibody to determine the location of F12L in infected cells and whether it associates with virions. Using confocal and electron microscopy we show that the F12L protein is located on intracellular enveloped virus (IEV) particles, but is absent from immature virions (IV), intracellular mature virus (IMV) and cell-associated enveloped virus (CEV). In addition, F12L shows co-localization with endosomal compartments and microtubules. F12L did not co-localize with virions attached to actin tails, providing further evidence that actin tails are associated with CEV but not IEV particles. In vDeltaF12L-infected cells, virus morphogenesis was arrested after the formation of IEV particles, so that the movement of these virions to the cell surface was inhibited and CEV particles were not found. Previously, virus mutants lacking IEV- or EEV-specific proteins were either unable to make IEV particles (vDeltaF13L and vDeltaB5R), or were unable to form actin tails after formation of CEV particles (vDeltaA36R, vDeltaA33R, vDeltaA34R). The F12L deletion mutant therefore defines a new stage in the morphogenic pathway and the F12L protein is implicated as necessary for microtubule-mediated egress of IEV particles to the cell surface.

Antibody-sensitive and antibody-resistant cell-to-cell spread by vaccinia virus: role of the A33R protein in antibody-resistant spread

The roles of vaccinia virus (VV) intracellular mature virus (IMV), intracellular enveloped virus (IEV), cell-associated enveloped virus (CEV) and extracellular enveloped virus (EEV) and their associated proteins in virus spread were investigated. The plaques made by VV mutants lacking individual IEV- or EEV-specific proteins (vDeltaA33R, vDeltaA34R, vDeltaA36R, vDeltaA56R, vDeltaB5R, vDeltaF12L and vDeltaF13L) were compared in the presence of IMV- or EEV-neutralizing antibodies (Ab). Data presented show that for long-range spread, the comet-shaped plaques of VV were caused by the unidirectional spread of EEV probably by convection currents, and for cell-to-cell spread, VV uses a combination of Ab-resistant and Ab-sensitive pathways. Actin tails play a major role in the Ab-resistant pathway, but mutants such as vDeltaA34R and vDeltaA36R that do not make actin tails still spread from cell to cell in the presence of Ab. Most strikingly, the Ab-resistant pathway was abolished when the A33R gene was deleted. This effect was not due to alterations in the efficiency of neutralization of EEV made by this mutant, nor due to a deficiency in IMV wrapping to form IEV, which was indispensable for EEV formation by vDeltaA33R and vDeltaA34R. We suggest a role for A33R in promoting Ab-resistant cell-to-cell spread of virus. The roles of the different virus forms in the VV life-cycle are discussed.

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 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.

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.

Poxvirus homologues of cellular genes

Over the course of time poxviruses have acquired or "captured" numerous homologues of cellular genes and incorporated them into their large DNA genomes. With more poxvirus genome sequencing data becoming available, the number of newly discovered poxviral cellular homologues is constantly increasing. A common feature of these genes is that they are nonessential for virus replication in vitro and they confer selective advantages in dealing with host cell differentiation and immune defense mechanisms in vivo. Poxviral cellular homologues are reviewed in this synopsis considering the specific viral habitats of different poxviruses and the immune defence capabilities of their respective hosts. Possible mechanisms of cellular gene acquisition by poxviruses as suggested by the analysis of mobile genetic elements in large DNA viruses are discussed. The investigation of poxvirus homologues of cellular genes is essential for our understanding of the mechanisms that regulate virus/host interactions on the cellular level and the host response against infection.

A mutational analysis of the vaccinia virus B5R protein

A mutational analysis of the vaccinia virus (VV) B5R protein is presented. This protein is related to the regulators of complement activation (RCA) superfamily, has four short consensus repeats (SCRs) that are typical of this superfamily and is present on extracellular enveloped virus (EEV) particles. Here we have constructed VV mutants in which the cytoplasmic tail (CT) of the B5R protein is progressively truncated, and domains of the B5R protein [the SCR (short consensus repeat) domains, the transmembrane anchor region or the CT] are substituted by corresponding domains from the VV haemagglutinin (HA), another EEV protein. Analysis of these mutant viruses showed that loss of the B5R CT did not affect the formation of intracellular enveloped virus (IEV), actin tails, EEV or virus plaque size. However, if the SCR domains of the B5R protein were replaced by the corresponding region of the HA, the virus plaque size was diminished, the formation of actin tails was decreased severely and the titre of infectious EEV released from cells was reduced approximately 25-fold compared to wild-type virus and 5-fold compared to a virus lacking the entire B5R gene. Thus the linkage of HA to the B5R transmembrane and CT is deleterious for the formation and release of EEV and for cell-to-cell virus spread. In contrast, deletion or substitution of the B5R CT did not affect virus replication, although the amount of cell surface B5R was reduced compared to control.

Visualization of intracellular movement of vaccinia virus virions containing a green fluorescent protein-B5R membrane protein chimera

We produced an infectious vaccinia virus that expressed the B5R envelope glycoprotein fused to the enhanced green fluorescent protein (GFP), allowing us to visualize intracellular virus movement in real time. Previous transfection studies indicated that fusion of GFP to the C-terminal cytoplasmic domain of B5R did not interfere with Golgi localization of the viral protein. To determine whether B5R-GFP was fully functional, we started with a B5R deletion mutant that made small plaques and inserted the B5R-GFP gene into the original B5R locus. The recombinant virus made normal-sized plaques and acquired the ability to form actin tails, indicating reversal of the mutant phenotype. Moreover, immunogold electron microscopy revealed that both intracellular enveloped virions (IEV) and extracellular enveloped virions contained B5R-GFP. By confocal microscopy of live infected cells, we visualized individual fluorescent particles, corresponding to IEV in size and shape, moving from a juxtanuclear location to the periphery of the cell, where they usually collected prior to association with actin tails. The fluorescent particles could be seen emanating from cells at the tips of microvilli. Using a digital camera attached to an inverted fluorescence microscope, we acquired images at 1 frame/s. At this resolution, IEV movement appeared saltatory; in some frames there was no net movement, whereas in others movement exceeded 2 microm/s. Further studies indicated that IEV movement was reversibly arrested by the microtubule-depolymerizing drug nocodazole. This result, together with the direction, speed, and saltatory motion of IEV, was consistent with a role for microtubules in intracellular transport of IEV.

Immunomodulatory proteins of myxoma virus

Poxviruses collectively encode an impressive collection of diverse immunomodulatory proteins. In this review we draw attention to some of the new open reading frames (ORFs) discovered during the sequencing of the myxoma virus DNA genome [Cameron C, Hota-Mitchell S, Chen L, Barrett J, Cao J-X, Macaulay C, Willer D, Evans D, McFadden G (1999) The complete DNA sequence of myxoma virus. Virology 264:298-318] that may function to subvert the host immune system. Most of these predicted functions are speculative but some of the deduced primary amino acid sequences contain intriguing similarities to known cellular and viral proteins in the public domain for which immunomodulatory functions have been assigned.

Antibody neutralization of the extracellular enveloped form of vaccinia virus

The extracellular enveloped form (EEV) of vaccinia virus (VV) is important for the long-range dissemination of the virus inside the host. Early work suggested that both IMV and EEV infectivity could be inhibited by antibodies, although two other studies reported that EEV was resistant to neutralization. Here, we readdressed this question, using four VV-immune antisera and their purified IgG, and showed that EEV infectivity can be inhibited by antibody produced from a live infection in plaque-reduction assays, although EEV is more resistant to neutralization by convalescent antibodies than is IMV. In parallel, indirect immunofluorescent staining and confocal microscopy showed that antibody aggregated EEV and prevented it from binding to cells. Using the IgG and Fab fragments prepared from this antiserum, we tested whether EEV made by VV mutants lacking genes A33R, A34R, A36R, A56R, B5R, F12L, or F13L can be inhibited in plaque-reduction assays. Although vDeltaB5R was slightly more resistant than other mutants, none of these mutants escaped neutralization completely, suggesting that multiple virus proteins are involved in the inhibition. Using an antibody specific to B5R protein and B5R mutants with consecutive short consensus repeat (SCR) domains deleted, the neutralization epitopes on B5R were mapped to within the SCR domain 1.

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 F12L protein is required for actin tail formation, normal plaque size, and virulence

Vaccinia virus gene F12L is shown to encode a 65-kDa protein that is synthesized early and late during infection and that is not modified by glycosylation. Computational sequence comparison revealed that related proteins are encoded by all sequenced chordopoxviruses. A virus deletion mutant lacking the F12L gene (vDeltaF12L) and a revertant virus with the F12L gene reinserted into the deletion mutant (vF12L-rev) were constructed and analyzed. A version of the F12L gene with a C-terminal amino acid tag derived from the influenza virus hemagglutinin and that is recognized by a monoclonal antibody was also inserted into the F12L locus of vDeltaF12L. Loss of the F12L protein reduced the formation of IMV 2-fold, but there was a dramatic (99.5%) reduction in actin tail formation, and the levels of cell-associated enveloped virus and extracellular enveloped virus were reduced 8- to 11-fold and 7-fold, respectively. Consistent with the lack of actin tail formation, vDeltaF12L produced a very small plaque. The vDeltaF12L virus was severely attenuated in vivo, such that a dose of vDeltaF12L 10,000-fold greater than the dose of wild-type virus that induced severe disease was unable to induce disease in mice infected intranasally.

Intracellular localization of vaccinia virus extracellular enveloped virus envelope proteins individually expressed using a Semliki Forest virus replicon

The extracellular enveloped virus (EEV) form of vaccinia virus is bound by an envelope which is acquired by wrapping of intracellular virus particles with cytoplasmic vesicles containing trans-Golgi network markers. Six virus-encoded proteins have been reported as components of the EEV envelope. Of these, four proteins (A33R, A34R, A56R, and B5R) are glycoproteins, one (A36R) is a nonglycosylated transmembrane protein, and one (F13L) is a palmitylated peripheral membrane protein. During infection, these proteins localize to the Golgi complex, where they are incorporated into infectious virus that is then transported and released into the extracellular medium. We have investigated the fates of these proteins after expressing them individually in the absence of vaccinia infection, using a Semliki Forest virus expression system. Significant amounts of proteins A33R and A56R efficiently reached the cell surface, suggesting that they do not contain retention signals for intracellular compartments. In contrast, proteins A34R and F13L were retained intracellularly but showed distributions different from that of the normal infection. Protein A36R was partially retained intracellularly, decorating both the Golgi complex and structures associated with actin fibers. A36R was also transported to the plasma membrane, where it accumulated at the tips of cell projections. Protein B5R was efficiently targeted to the Golgi region. A green fluorescent protein fusion with the last 42 C-terminal amino acids of B5R was sufficient to target the chimeric protein to the Golgi region. However, B5R-deficient vaccinia virus showed a normal localization pattern for other EEV envelope proteins. These results point to the transmembrane or cytosolic domain of B5R protein as one, but not the only, determinant of the retention of EEV proteins in the wrapping compartment.

The vaccinia virus A9L gene encodes a membrane protein required for an early step in virion morphogenesis

The A9L open reading frame of vaccinia virus was predicted to encode a membrane-associated protein. A transcriptional analysis of the A9L gene indicated that it was expressed at late times in vaccinia virus-infected cells. Late expression, as well as virion membrane association, was demonstrated by the construction and use of a recombinant vaccinia virus encoding an A9L protein with a C-terminal epitope tag. Immunoelectron microscopy revealed that the A9L protein was associated with both immature and mature virus particles and was oriented in the membrane with its C terminus exposed on the virion surface. To determine whether the A9L protein functions in viral assembly or infectivity, we made a conditional-lethal inducible recombinant vaccinia virus. In the absence of inducer, A9L expression and virus replication were undetectable. Under nonpermissive conditions, viral late protein synthesis occurred, but maturational proteolytic processing was inhibited, and there was an accumulation of membrane-coated electron-dense bodies, crescents, and immature virus particles, many of which appeared abnormal. We concluded that the product of the A9L gene is a viral membrane-associated protein and functions at an early stage in virion morphogenesis.

Identification and analysis of vaccinia virus palmitylproteins

Vaccinia virus encodes at least eight proteins that incorporate label from tritiated palmitic acid when it is added to infected cell cultures. Three of these palmitylproteins are encoded by the A33R, B5R, and F13L open reading frames and migrate by gel electrophoresis with relative molecular masses of 23-28, 42, and 37 kDa, respectively. In this report we provide evidence that the A22R and A36R open reading frames also encode palmitylproteins with apparent molecular masses of 22 and 50-55 kDa, respectively. Furthermore, the hemagglutinin protein (A56R) from the Copenhagen strain is shown to be palmitylated while the hemagglutinin protein from the WR and IHD-J strains is not. A 94-kDa VV palmitylprotein appears to be a multimeric complex composed of the B5R protein and possibly others. All vaccinia-encoded palmitylproteins are present in the membranous fraction of cells and are specific for the trans-Golgi network membrane-enveloped forms of the virus, suggesting that these proteins play a role in the envelopment and egress of virions or the infectivity of released virus.

Effects of deletion or stringent repression of the H3L envelope gene on vaccinia virus replication

The C-terminal membrane anchor protein encoded by the H3L open reading frame of vaccinia virus is located on the surfaces of intracellular mature virions. To investigate the role of the H3L protein, we constructed deletion (vH3Delta) and inducible (vH3i) null mutants. The H3L protein was not detected in lysates of cells infected with vH3Delta or vH3i in the absence of inducer. Under these conditions, plaques were small and round instead of large and comet shaped, indicative of decreased virus replication or cell-to-cell spread. The mutant phenotype was correlated with reduced yields of infectious intra- and extracellular virus in one-step growth experiments. The defect in vH3i replication could not be attributed to a role of the H3L protein in virus binding, internalization, or any event prior to late gene expression. Electron microscopic examination of cells infected with vH3Delta or vH3i in the absence of inducer revealed that virion assembly was impaired, resulting in a high ratio of immature to mature virus forms with an accumulation of crescent membranes adjacent to granular material and DNA crystalloids. The absence of the H3L protein did not impair the membrane localization of virion surface proteins encoded by the A27L, D8L, and L1R genes. The wrapping of virions and actin tail formation were not specifically blocked, but there was an apparent defect in low-pH-mediated syncytium formation that could be attributed to decreased virus particle production. The phenotypes of the H3L deletion and repression mutants were identical to each other but differed from those produced by null mutations of genes encoding other vaccinia virus membrane components.

Vaccinia virus infection disrupts microtubule organization and centrosome function

We examined the role of the microtubule cytoskeleton during vaccinia virus infection. We found that newly assembled virus particles accumulate in the vicinity of the microtubule-organizing centre in a microtubule- and dynein-dynactin complex-dependent fashion. Microtubules are required for efficient intracellular mature virus (IMV) formation and are essential for intracellular enveloped virus (IEV) assembly. As infection proceeds, the microtubule cytoskeleton becomes dramatically reorganized in a fashion reminiscent of overexpression of microtubule-associated proteins (MAPs). Consistent with this, we report that the vaccinia proteins A10L and L4R have MAP-like properties and mediate direct binding of viral cores to microtubules in vitro. In addition, vaccinia infection also results in severe reduction of proteins at the centrosome and loss of centrosomal microtubule nucleation efficiency. This represents the first example of viral-induced disruption of centrosome function. Further studies with vaccinia will provide insights into the role of microtubules during viral pathogenesis and regulation of centrosome function.

The vaccinia virus A36R protein is a type Ib membrane protein present on intracellular but not extracellular enveloped virus particles

Vaccinia virus gene A36R encodes a 45-kDa protein that is conserved in orthopoxviruses. A virus lacking the A36R protein formed a small plaque, was unable to induce the polymerization of actin tails, and was avirulent in vivo. Here we present a further characterization of the A36R protein by in vitro transcription and translation and analysis of infected cells by confocal microscopy and immunoelectron microscopy of cryosections using a monoclonal antibody raised against the C-terminal domain of the A36R protein. Translation of the A36R mRNA in vitro produced a protein of the same size whether or not the translation reaction was performed in the presence of canine pancreatic microsomes. However, the polypeptide synthesized in the presence of microsomes was associated integrally with the membrane and was sensitive to digestion by exogenous protease without permeabilization of the membrane with detergent, indicating that the majority of the protein is exposed on the outside of the vesicle. Consistent with this, immunofluorescent analysis of virus-infected cells demonstrated that the C-terminal domain of A36R was not exposed on the cell surface but was detected once the cell membrane was permeabilized. Immunoelectron microscopy of cryosections of infected cells showed that the protein was absent from IMV particles but present on intracellular enveloped virus (IEV) particles, predominantly on the cytosolic face of the IEV outer membrane. Where cell-associated enveloped virus (CEV) particles were attached to the cell surface, the A36R protein was detected only on the cytosolic surface of the plasma membrane where the virus particle remained attached to the cell and not elsewhere on the plasma membrane or on the CEV particle. A36R and actin copurified with EEV particles due to the association of fragments of cellular membranes with the EEV particles. Therefore, A36R represents the first example of a virus-encoded protein that is present on IEV but not CEV particles.

The genome of fowlpox virus

Here we present the genomic sequence, with analysis, of a pathogenic fowlpox virus (FPV). The 288-kbp FPV genome consists of a central coding region bounded by identical 9.5-kbp inverted terminal repeats and contains 260 open reading frames, of which 101 exhibit similarity to genes of known function. Comparison of the FPV genome with those of other chordopoxviruses (ChPVs) revealed 65 conserved gene homologues, encoding proteins involved in transcription and mRNA biogenesis, nucleotide metabolism, DNA replication and repair, protein processing, and virion structure. Comparison of the FPV genome with those of other ChPVs revealed extensive genome colinearity which is interrupted in FPV by a translocation and a major inversion, the presence of multiple and in some cases large gene families, and novel cellular homologues. Large numbers of cellular homologues together with 10 multigene families largely account for the marked size difference between the FPV genome (260 to 309 kbp) and other known ChPV genomes (178 to 191 kbp). Predicted proteins with putative functions involving immune evasion included eight natural killer cell receptors, four CC chemokines, three G-protein-coupled receptors, two beta nerve growth factors, transforming growth factor beta, interleukin-18-binding protein, semaphorin, and five serine proteinase inhibitors (serpins). Other potential FPV host range proteins included homologues of those involved in apoptosis (e.g., Bcl-2 protein), cell growth (e.g., epidermal growth factor domain protein), tissue tropism (e.g., ankyrin repeat-containing gene family, N1R/p28 gene family, and a T10 homologue), and avian host range (e.g., a protein present in both fowl adenovirus and Marek's disease virus). The presence of homologues of genes encoding proteins involved in steroid biogenesis (e.g., hydroxysteroid dehydrogenase), antioxidant functions (e.g., glutathione peroxidase), vesicle trafficking (e.g., two alpha-type soluble NSF attachment proteins), and other, unknown conserved cellular processes (e.g., Hal3 domain protein and GSN1/SUR4) suggests that significant modification of host cell function occurs upon viral infection. The presence of a cyclobutane pyrimidine dimer photolyase homologue in FPV suggests the presence of a photoreactivation DNA repair pathway. This diverse complement of genes with likely host range functions in FPV suggests significant viral adaptation to the avian host.

Golgi network targeting and plasma membrane internalization signals in vaccinia virus B5R envelope protein

The vaccinia virus B5R type I integral membrane protein accumulates in the Golgi network, from where it becomes incorporated into the envelope of extracellular virions. Our objective was to determine the domains of B5R responsible for Golgi membrane targeting in the absence of other viral components. Fusion of an enhanced green fluorescent protein to the C terminus of B5R allowed imaging of the chimeric protein without altering intracellular trafficking and Golgi network localization in transfected cells. Deletion or swapping of B5R domains with corresponding regions of the vesicular stomatitis virus G protein, which is targeted to the plasma membrane, indicated that (i) the N-terminal extracellular domain of B5R had no specific role in Golgi apparatus localization, (ii) the transmembrane domain of B5R was sufficient for exiting the endoplasmic reticulum, and (iii) removal of the cytoplasmic tail impaired Golgi network localization and increased the accumulation of B5R in the plasma membrane. Further experiments demonstrated that the cytoplasmic tail mediated internalization of B5R from the plasma membrane, suggesting a retrieval mechanism. Mutagenesis revealed residues required for Golgi membrane localization and efficient plasma membrane retrieval of the B5R protein: a tyrosine at residue 310 and two adjacent leucines at residues 315 and 316.

Morphogenesis and release of fowlpox virus

Release of fowlpox virus (FWPV) as extracellular enveloped virus (EEV) appears to proceed both by the budding of intracellular mature virus (IMV) through the plasma membrane and by the fusion of intracellular enveloped virus (IEV) with the plasma membrane. Based on the frequency of budding events compared to wrapping events observed by electron microscopy, FWPV FP9 strain seems to exit chick embryo fibroblast cells predominantly by budding. In contrast to vaccinia virus (VV), the production of FWPV extracellular virus particles is not affected by N(1)-isonicotinoyl-N(2)-3-methyl-4-chlorobenzoylhydrazine (IMCBH). Comparison of the sequence of the VV F13L gene product with its FWPV orthologue showed a mutation, in the fowlpox protein, at the residue involved in IMCBH resistance in a mutant VV. Glucosamine, monensin or brefeldin A did not have any specific effect on FWPV extracellular virus production. Cytochalasin D, which inhibits the formation of actin filaments, reduces the production of extracellular virus particles by inhibiting the release of cell-associated enveloped virus (CEV) particles from the plasma membrane. Involvement of actin filaments in this mechanism is further supported by the co-localization of actin with viral particles close to the plasma membrane in the absence of cytochalasin D. Actin is also co-localized with virus factories.

Analysis of host response modifier ORFs of ectromelia virus, the causative agent of mousepox

From the right-hand end of the ectromelia virus (strain Moscow) genome, 32318 bps have been sequenced, and characterized to include a total of 18 open reading frames (ORFs) and six regions which apparently no longer code for functional proteins. At least six of the ORFs appear to be involved in blocking the inflammatory/immune host response to infection, and therefore probably contribute significantly to the virulence of this virus in its natural host, the mouse. One of these genes encoded an isolog of the poxvirus chemokine binding protein, and was shown to be the most abundant protein secreted from ectromelia virus infected cells. Two regions were found to have significant similarity to poxvirus genes encoding tumor necrosis factor (TNF) binding proteins. Both are distinct from cytokine response modifier (crm)B and crmC but only one is predicted to encode a functional TNF binding protein. A novel similarity between the C-terminal domain of poxvirus TNF binding proteins and several other poxvirus proteins is also presented. The results are discussed in the context of ectromelia virus pathogenesis of mice.

The effects of targeting the vaccinia virus B5R protein to the endoplasmic reticulum on virus morphogenesis and dissemination

The consequence of redirecting the vaccinia virus (VV) B5R protein to the endoplasmic reticulum (ER) has been investigated by the addition of an ER retrieval signal KKSL (K(2)X(2)) to the B5R C-terminus. This mutant B5R gene and a version of the gene with the inactive ER retrieval sequence KKSLAL (K(2)X(4)) were inserted into the thymidine kinase locus of a VV mutant lacking the B5R gene, vDeltaB5R. Similar levels of B5R protein were made by each virus, but the B5R-K(2)X(2) protein remained sensitive to endoglycosidase H and colocalised with protein disulphide isomerase in the ER. In contrast, the B5R-K(2)X(4) protein colocalised with 1, 4-galactosyltransferase in the trans-Golgi network. Electron microscopy revealed that even when the B5R protein was redirected to the ER, intracellular mature virus particles were wrapped by cellular membranes to form intracellular enveloped virus particles, although more incompletely wrapped particles were evident compared with wild type. These intracellular enveloped virus particles were, however, unable to efficiently induce the polymerisation of actin and the plaque size formed by vB5R-K(2)X(2) was small. Nevertheless, the amount and specific infectivity of EEV produced by vB5R-K(2)X(2) were similar to those of wild type, despite the dramatic reduction in the amount of B5R protein present in vB5R-K(2)X(2) EEV.

Vaccinia virus serpin-1 deletion mutant exhibits a host range defect characterized by low levels of intermediate and late mRNAs

Orthopoxviruses encode three serpin homologs-SPI-1, SPI-2 and SPI-3-of which SPI-2 has been well characterized as an inhibitor of ICE-like proteases. A rabbitpox virus SPI-1 deletion mutant exhibited a host range restriction in human lung A549 and pig kidney 15 cell lines that was attributed to apoptosis. Here we report that replication of a vaccinia virus SPI-1 deletion mutant (DeltaSPI-1) was restricted in primary human keratinocytes as well as A549 cells. Although chromatin condensation was detected in some A549 cells, other morphological or biochemical signs of apoptosis including DNA fragmentation, cleavage of poly(ADP-ribose)polymerase or nuclear mitotic apparatus protein, or caspase 3 activation were not found. Moreover, DeltaSPI-1 protected A549 cells from apoptosis induced by tumor necrosis factor, whereas the corresponding DeltaSPI-2 mutant did not. Further studies indicated undiminished amounts of vaccinia virus early mRNA and replicated DNA in the absence of the SPI-1 product. However, there were reduced amounts of viral intermediate and late mRNAs, viral late proteins, cleaved core proteins, and virus particles. These data suggested that apoptosis is not the determining factor in the host range restriction of DeltaSPI-1 and that the SPI-1 gene product is needed to allow efficient expression of intermediate and late genes in A549 cells.

Murine gammaherpesvirus 68 encodes a functional regulator of complement activation

Sequence analysis of the murine gammaherpesvirus 68 (gammaHV68) genome revealed an open reading frame (gene 4) which is homologous to a family of proteins known as the regulators of complement activation (RCA proteins) (H. W. Virgin, P. Latreille, P. Wamsley, K. Hallsworth, K. E. Weck, A. J. Dal Canto, and S. H. Speck, J. Virol. 71:5894-5904, 1997). The predicted gene 4 product has homology to other virally encoded RCA homologs, as well as to the complement-regulatory proteins decay-accelerating factor and membrane cofactor protein. Analyses by Northern blotting and rapid amplification of cDNA ends revealed that gene 4 is transcribed as a 5.2-kb bicistronic transcript of the late kinetic class. Three gammaHV68 RCA protein isoforms (60 to 65 kDa, 50 to 55 kDa, and 40 to 45 kDa) were detected by Western blotting of infected murine NIH 3T12 fibroblast cells. A soluble 40- to 45-kDa isoform was detected in the supernatants of virally infected cells. Flow cytometric analysis revealed that the gammaHV68 RCA protein was expressed on the surfaces of infected cells. Supernatants from virally infected cells contained an activity that inhibited murine complement activation as measured by inhibition of C3 deposition on activated zymosan particles. Recombinant gammaHV68 RCA protein, containing the four conserved short consensus repeats, inhibited murine C3 deposition on zymosan via both classical and alternative pathways and inhibited deposition of human C3 on activated zymosan particles. Expression of this inhibitor of complement activation, both at the cell surface and in the fluid phase, may be important for gammaHV68 pathogenesis via the inhibition of innate and adaptive immunity.

Vaccinia virus WR gene A5L is required for morphogenesis of mature virions

The vaccinia virus WR A5L open reading frame (corresponding to open reading frame A4L in vaccinia virus Copenhagen) encodes an immunodominant late protein found in the core of the vaccinia virion. To investigate the role of this protein in vaccinia virus replication, we have constructed a recombinant virus, vA5Li, in which the endogenous gene has been deleted and an inducible copy of the A5 gene dependent on isopropyl-beta-D-thiogalactopyranoside (IPTG) for expression has been inserted into the genome. In the absence of inducer, the yield of infectious virus was dramatically reduced. However, DNA synthesis and processing, viral protein expression (except for A5), and early stages in virion formation were indistinguishable from the analogous steps in a normal infection. Electron microscopy revealed that the major vaccinia virus structural form present in cells infected with vA5Li in the absence of inducer was immature virions. Viral particles were purified from vA5Li-infected cells in the presence and absence of inducer. Both particles contained viral DNA and the full complement of viral proteins, except for A5, which was missing from particles prepared in the absence of inducer. The particles prepared in the presence of IPTG were more infectious than those prepared in its absence. The A5 protein appears to be required for the immature virion to form the brick-shaped intracellular mature virion.

Viral complement regulatory proteins

The inactivation of complement provides cells and tissues critical protection from complement-mediated attack and decreases the associated recruitment of other inflammatory mediators. In an attempt to evade the host immune response, viruses have evolved two mechanisms to acquire complement regulatory proteins. They can directly seize the host cell complement regulators onto their outer envelope and/or they can produce their own proteins which are either secreted into the neighboring intercellular space or expressed as membrane-bound proteins on the infected host cell. The following review will concentrate on the viral homologues of the mammalian complement regulatory proteins, specifically those containing complement control protein (CCP) repeats.

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.

Envelope formation is blocked by mutation of a sequence related to the HKD phospholipid metabolism motif in the vaccinia virus F13L protein

The outer envelope of the extracellular form of vaccinia virus is derived from Golgi membranes that have been modified by the insertion of specific viral proteins, of which the major component is the 37-kDa, palmitylated, nonglycosylated product of the F13L gene. The F13L protein contains a variant of the HKD (His-Lys-Asp) motif, which is conserved in numerous enzymes of phospholipid metabolism. Vaccinia virus mutants with a conservative substitution of either the K (K314R) or the D (D319E) residue of the F13L protein formed only tiny plaques similar to those produced by an F13L deletion mutant, were unable to produce extracellular enveloped virions, and failed to mediate low-pH-induced fusion of infected cells. Membrane-wrapped forms of intracellular virus were rarely detected in electron microscopic images of cells infected with either of the mutants. Western blotting and pulse-chase experiments demonstrated that the D319E protein was less stable than either the K314R or wild-type F13L protein. Most striking, however, was the failure of either of the two mutated proteins to concentrate in the Golgi compartment. Palmitylation, oleation, and partitioning of the F13L protein in Triton X-114 detergent were unaffected by the K314R substitution. These results indicated that the F13L protein must retain the K314 and D319 for it to localize in the Golgi compartment and function in membrane envelopment of vaccinia virus.

Functional analysis of vaccinia virus B5R protein: role of the cytoplasmic tail

Vaccinia extracellular enveloped virus (EEV) is important for cell-to-cell and long-range virus spread both in vitro and in vivo. Six genes have been identified that encode protein constituents of the EEV outer membrane, and some of these proteins are critical for EEV formation. The B5R gene encodes an EEV-specific type I membrane protein, and deletion of this gene markedly decreases EEV formation and results in a small plaque phenotype. Data suggest that the transmembrane domain, cytoplasmic tail, or both contain the EEV localization signals that are required for targeting of the B5R protein to EEV and for EEV formation. Here, we report the construction of mutant vaccinia viruses in which the wild-type B5R gene was replaced with a mutated one that encodes a protein with the putative cytoplasmic tail deleted. The mutated protein showed normal intracellular distribution and was properly incorporated into EEV. Vaccinia viruses expressing the B5R protein lacking the cytoplasmic tail formed plaques that were similar in type and size to those formed by wild-type viruses and produced equivalent amounts of infectious EEV. These results indicate that the B5R cytoplasmic tail is not necessary for EEV formation and points to the transmembrane domain as the major determinant for targeting the B5R protein to the outer membrane of EEV and for supporting EEV formation.

The vaccinia virus 14-kilodalton (A27L) fusion protein forms a triple coiled-coil structure and interacts with the 21-kilodalton (A17L) virus membrane protein through a C-terminal alpha-helix

The vaccinia virus 14-kDa protein (encoded by the A27L gene) plays an important role in the biology of the virus, acting in virus-to-cell and cell-to-cell fusions. The protein is located on the surface of the intracellular mature virus form and is essential for both the release of extracellular enveloped virus from the cells and virus spread. Sequence analysis predicts the existence of four regions in this protein: a structureless region from amino acids 1 to 28, a helical region from residues 29 to 37, a triple coiled-coil helical region from residues 44 to 72, and a Leu zipper motif at the C terminus. Circular dichroism spectroscopy, analytical ultracentrifugation, and chemical cross-linking studies of the purified wild-type protein and several mutant forms, lacking one or more of the above regions or with point mutations, support the above-described structural division of the 14-kDa protein. The two contiguous cysteine residues at positions 71 and 72 are not responsible for the formation of 14-kDa protein trimers. The location of hydrophobic residues at the a and d positions on a helical wheel and of charged amino acids in adjacent positions, e and g, suggests that the hydrophobic and ionic interactions in the triple coiled-coil helical region are involved in oligomer formation. This conjecture was supported by the construction of a three-helix bundle model and molecular dynamics. Binding assays with purified proteins expressed in Escherichia coli and cytoplasmic extracts from cells infected with a virus that does not produce the 14-kDa protein during infection (VVindA27L) show that the 21-kDa protein (encoded by the A17L gene) is the specific viral binding partner and identify the putative Leu zipper, the predicted third alpha-helix on the C terminus of the 14-kDa protein, as the region involved in protein binding. These findings were confirmed in vivo, following transfection of animal cells with plasmid vectors expressing mutant forms of the 14-kDa protein and infected with VVindA27L. We find the structural organization of 14kDa to be similar to that of other fusion proteins, such as hemagglutinin of influenza virus and gp41 of human immunodeficiency virus, except for the presence of a protein-anchoring domain instead of a transmembrane domain. Based on our observations, we have established a structural model of the 14-kDa protein.

Cell surface proteoglycans are necessary for A27L protein-mediated cell fusion: identification of the N-terminal region of A27L protein as the glycosaminoglycan-binding domain

We previously showed that vaccinia virus infection of BSC40 cells was blocked by soluble heparin, suggesting that cell surface heparan sulfate mediates vaccinia virus binding (C.-S. Chung, J.-C. Hsiao, Y. -S. Chang, and W. Chang, J. Virol. 72:1577-1585, 1998). In this study, we extended our previous work and demonstrated that soluble A27L protein bound to heparan sulfate on cells and interfered with vaccinia virus infection at a postbinding step. In addition, we investigated the structure of A27L protein that provides for its binding to heparan sulfate on cells. A mutant of A27L protein, named D-A27L, devoid of a cluster of 12 amino acids rich in basic residues, was constructed. In contrast to the soluble A27L protein, purified D-A27L protein was inactive in all of our assays, including binding to heparin in vitro, binding to heparan sulfate on cells, and the ability to block virus infection. These data demonstrated that the N-terminal region acts as a glycosaminoglycan (GAG)-binding domain critical for A27L protein binding to cells. Previously A27L protein was thought to be involved in fusion of virus-infected cells induced by acid treatment. When we investigated whether cell surface GAGs also participate in A27L-dependent fusion, our results indicated that soluble A27L protein blocked cell fusion, whereas D-A27L protein did not. Taken together, the results therefore demonstrated that A27L-mediated cell fusion is triggered by its interaction with cell surface GAGs through the N-terminal domain.

Analysis of a vaccinia virus mutant expressing a nonpalmitylated form of p37, a mediator of virion envelopment

Vaccinia virus encodes a 37-kDa palmitylated protein (p37) that is required for envelopment, translocation, and cell-to-cell spread of virions. We have analyzed the biological significance of the palmitate modification by constructing a recombinant vaccinia virus that expresses a nonpalmitylated p37 and comparing its biological activity to that of the wild-type virus. The mutant virus is inefficient at cell-to-cell spread and does not produce or release enveloped virions, although it produces normal amounts of nonenveloped virions. Furthermore, the mutant virus is not able to nucleate actin to propel itself through and out of the cell, a function requiring the indirect participation of p37. The deficiency in protein function appears to result from a lack of appropriate targeting to the membranes of the trans-Golgi network (TGN) which leaves p37 soluble in the cytoplasm. We conclude that the palmitate moiety is necessary for targeting or anchoring p37 to the TGN membrane, where, along with other vaccinia virus-encoded proteins, p37 is involved in the complex process of virion envelopment and release.

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, vA33delta, was isolated, indicating that the A33R gene is not essential for VV replication. The plaques formed by vA33delta, 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 vA33delta, 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 vA33delta cell-associated extracellular enveloped viruses (CEV) were found to be normal. Immunogold electron microscopy of cells infected with vA33delta demonstrated the presence of the expected F13L and B5R proteins in wrapping membranes and EEV; however, fully wrapped vA33delta 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 vA33delta. 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.

Role for the vaccinia virus A36R outer envelope protein in the formation of virus-tipped actin-containing microvilli and cell-to-cell virus spread

A small-plaque-forming vaccinia virus mutant with a deletion in the A36R gene encoding an outer envelope protein (Parkinson and Smith, Virology, 204, 376-390, 1994) was shown to assemble wrapped forms of intra- and extracellular virus particles and to mediate acid-induced polykaryon formation. The intracellular virions, however, did not acquire actin tails and those on the cell surface were not associated with specialized microvilli. This phenotype is similar to that of the A34R (E. J. Wolffe, E. Katz, A. Weisberg, and B. Moss, J. Virol 71, 3904-3915, 1997) and A33R (R. Roper, E. J. Wolffe, A. Weisberg, and B. Moss, J. Virol., in press) deletion mutants. Taken together, these data support a model in which the envelope proteins encoded by the A33R, A34R, and A36R genes are all required for nucleation of actin tails, which facilitate dissemination rather than egress of virus particles.

The genomic sequence analysis of the left and right species-specific terminal region of a cowpox virus strain reveals unique sequences and a cluster of intact ORFs for immunomodulatory and host range proteins

Sequencing and computer analysis of the left (52,283 bp) and right (49,649 bp) variable DNA regions of the cowpox virus strain GRI-90 (CPV-GRI) has revealed 51 and 37 potential open reading frames (ORFs), respectively. Comparison of the structure-function organization of these DNA regions of CPV-GRI with those previously published for corresponding regions of genomes of vaccinia virus, strains Copenhagen (VAC-COP) and Western Reserve (VAC-WR); and variola major virus, strains India-1967 (VAR-IND), Bangladesh-1975 (VAR-BSH); and alastrim variola minor virus, strain Garcia-1966 (VAR-GAR), was performed. Within the left terminal region under study, an extended DNA sequence (14,171 bp), unique to CPV, has been found. Within the right region of the CPV-GRI genome two segments, which are unique to CPV DNA (1579 and 3585 bp) have been found. Numerous differences have been revealed in the genetic structure of CPV-GRI DNA regions, homologous to fragments of the genomes of the above-mentioned orthopoxvirus strains. A cluster of ORFs with structural similarity ot immunomodulatory and host range function of other poxviruses have also been detected. A comparison of the sequences of ORF B, crmA, crmB, crmC, IMP, and CHO hr genes of CPV Brighton strain (CPV-BRI) with the corresponding genes in strain GRI-90 have revealed an identity at the amino acid level ranging from 82 to 96% between the two strains. The findings are significant in light of the recent demonstration of CPV as an important poxvirus model system to probe the precise in vivo role(s) of the unique virally encoded immunomodulatory proteins. Also, the presence of a complete and intact repertoire of immunomodulatory proteins, ring canal proteins family, and host range genes indicates that CPV may have been the most ancient of all studied orthopoxviruses.

The extracellular domain of vaccinia virus protein B5R affects plaque phenotype, extracellular enveloped virus release, and intracellular actin tail formation

Vaccinia virus produces two morphologically distinct forms of infectious virus, termed intracellular mature virus (IMV) and extracellular enveloped virus (EEV). EEV is important for virus dissemination within a host and has different surface proteins which bind to cell receptors different from those used by IMV. Six genes are known to encode EEV-specific proteins. One of these, B5R, encodes a 42-kDa glycoprotein with amino acid similarity to members of the complement control protein superfamily and contains four copies of a 50- to 70-amino-acid repeat called the short consensus repeat (SCR). Deletion of B5R causes a small-plaque phenotype, a 10-fold reduction in EEV formation, and virus attenuation in vivo. In this study, we inserted mutated versions of the B5R gene lacking different combinations of the SCRs into a virus deletion mutant lacking the B5R gene. The resultant viruses each formed small plaques only slightly larger than those of the deletion mutant; however, the virus containing only SCR 1 formed plaques slightly larger than those of viruses with SCRs 1 and 2 or SCRs 1, 2, and 3. All of these viruses produced approximately 50-fold more infectious EEV than wild-type virus and formed comet-shaped plaques under liquid overlay. Despite producing more EEV, the mutant viruses were unable to induce the polymerization of actin on intracellular virus particles. The implications of these results for our understanding of EEV formation, release, and infectivity are discussed.

Vaccinia virus 15-kilodalton (A14L) protein is essential for assembly and attachment of viral crescents to virosomes

Early stages in vaccinia virus (VV) assembly involve the recruitment of cellular membranes from the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) to virus factories (or virosomes). The key viral factors involved in this process are not yet known. We have previously identified and characterized two viral proteins, of 21 kDa (A17L gene) and 15 kDa (A14L gene), that associate with tubulovesicular elements related to the ERGIC and are localized in viral membranes at all stages of virion assembly. We showed that the 21-kDa protein is not responsible for the recruitment of membranes from the ERGIC to viral factories. However, it appears to be essential for the organization of viral membranes. In this investigation we have generated a VV recombinant, VVindA14L, in which the expression of the A14L gene is inducibly regulated by the Escherichia coli lacI operator-repressor system. Repression of 15-kDa protein synthesis has a dramatic effect on virus yields and severely impairs plaque formation. Compared to wild-type VV, reduced amounts of 15-kDa protein are produced in VVindA14L-infected cells in the presence of IPTG (isopropyl-beta-D-thiogalactoside), and this correlates with a small-plaque phenotype and reduced VVindA14L yields under these conditions. In the absence of the 15-kDa protein, early and late viral protein syntheses proceed normally; however, proteolytic cleavage of the major core precursors is inhibited. Electron microscopic examination of cells infected with VVindA14L under nonpermissive conditions reveals the presence of numerous membranous elements that look like unfinished or disassembled crescents interspersed between electron-dense masses. These abnormal membrane elements are usually well separated from the surfaces of the dense structures. These findings show that the 15-kDa protein is essential for VV morphogenesis and indicate that this polypeptide is necessary both for the correct assembly of viral crescents and for their stable attachment to the surfaces of viral factories.

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.

Host range and cytopathogenicity of the highly attenuated MVA strain of vaccinia virus: propagation and generation of recombinant viruses in a nonhuman mammalian cell line

Modified vaccinia virus Ankara (MVA), attenuated by over 500 passages in primary chick embryo fibroblasts (CEF), is presently being used as a safe expression vector. We compared the host ranges of MVA and the parental Ankara strain in CEF and 15 permanent cell lines. The cells could be grouped into three categories: permissive, semipermissive, and nonpermissive. For MVA, the permissive category consisted of primary CEF, a quail cell line derived from QT6, and the Syrian hamster cell line BHK-21. Only in BHK-21 cells did the virus yield approach that occurring in primary CEF. The semipermissive category included two African green monkey cell lines: BS-C-1 and CV-1. The nonpermissive category for MVA consisted of three human cell lines HeLa, 293, and SW 839; one rhesus monkey cell line FRhK-4; two Chinese hamster cell lines CHO and CHL; one pig cell line PK(15); and three rabbit cell lines RK13, RAB-9, and SIRC. The grouping for MVA with a restored K1L host range gene was similar except for the inclusion of RK13 cells among permissive lines. The grouping for the Ankara strain, however, was quite different with more permissive and semipermissive cell lines. Nevertheless, in cells that were permissive for MVA, the virus replicated to higher levels than Ankara, consistent with both positive and negative growth elements associated with the adaptation of MVA. The cell lines were also characterized according to their susceptibility to MVA-induced cytopathic effects, expression of a late promoter regulated reporter gene by an MVA recombinant, and stage at which virion morphogenesis was blocked. Finally, the permissive BHK-21 cell line was shown to be competent for constructing and propagating recombinant MVA, providing an alternative to primary CEF.

Vaccinia virus immune evasion

Vaccinia virus and other poxviruses express a wide variety of proteins which are non-essential for virus replication in culture but help the virus to evade the host response to infection. Examples include proteins which oppose apoptosis, synthesise steroids, capture chemokines, counteract complement, interfere with interferon and intercept interleukins. This review provides an overview of such proteins, with an emphasis on work from our laboratory, and illustrates how the study of these proteins can increase our understanding of virus pathogenesis, the function of the immune system and how to make safer and more immunogenic poxvirus-based vaccines.

Identification and analysis of three myristylated vaccinia virus late proteins

Previous studies have shown that at least three vaccinia virus (VV) late proteins (with apparent molecular asses of 37, 35, and 25 kDa) label with myristic acid. Time course labeling of VV-infected cells with [3H]myristic acid reveals at least three additional putative myristylproteins, with apparent molecular masses of 92, 17, and 14 kDa. The 25-kDa protein has previously been identified as that encoded by the L1R open reading frame, leaving the identities of the remaining proteins to be determined. Sequence analysis led to the preliminary identification of the 37-, 35-, and 17-kDa proteins as G9R, A16L, and E7R, respectively. Using synthetic oligonucleotides and PCR techniques, each of these open reading frames was amplified by using VV DNA as a template and then cloned individually into expression vectors behind T7 promoters. These plasmid constructs were then transcribed in vitro, and the resulting mRNAs were translated in wheat germ extracts and radiolabeled with either [35S]methionine or [3H]myristic acid. Each wild-type polypeptide was labeled with [35S]methionine or [3H]myristic acid in the translation reactions, while mutants containing an alanine in place of glycine at the N terminus were labeled only with [35S]methionine, not with myristic acid. This result provided strong evidence that the open reading frames had been correctly identified and that each protein is myristylated on a glycine residue adjacent to the initiating methionine. Subcellular fractionations of VV-infected cells suggested that A16L and E7R are soluble, in contrast to L1R, which is a membrane-associated protein.

Attenuation of B5R mutants of rabbitpox virus in vivo is related to impaired growth and not an enhanced host inflammatory response

The rabbitpox virus (RPV) B5R protein, synthesized late in infection, is found as a 45-kDa membrane-associated protein of the envelope of infectious extracellular enveloped virus (EEV) and as a 38-kDa protein secreted from the cell by a process independent of morphogenesis. The protein is not found associated with intracellular mature virus (IMV). Deletion of the gene attenuates the virus (RPV delta B5R) in animals (mice and rabbits), has relatively little effect on formation of IMV, prevents EEV formation in some but not all cells, and leads to a reduced host range. Analysis of the sequence of the protein suggests relatedness to factor H of the complement cascade. Collectively, these observations suggest that attenuation of the virus in vivo could be linked to an inhibition of the inflammatory response, a deficiency in growth, or both. In this report we have analyzed the behavior of RPV delta B5R in infected mice and rabbits and conclude that attenuation of the mutant virus likely results from simple failure to grow within the infected animal and that the inflammatory response probably contributes little to the observed attenuation.