A phosphorylated basic vaccinia virion polypeptide of molecular weight 11,000 is exposed on the surface of mature particles and interacts with actin-containing cytoskeletal elements

Vaccinia Virus Activation and Antagonism of Cytosolic DNA Sensing

Cells express multiple molecules aimed at detecting incoming virus and infection. Recognition of virus infection leads to the production of cytokines, chemokines and restriction factors that limit virus replication and activate an adaptive immune response offering long-term protection. Recognition of cytosolic DNA has become a central immune sensing mechanism involved in infection, autoinflammation, and cancer immunotherapy. Vaccinia virus (VACV) is the prototypic member of the family Poxviridae and the vaccine used to eradicate smallpox. VACV harbors enormous potential as a vaccine vector and several attenuated strains are currently being developed against infectious diseases. In addition, VACV has emerged as a popular oncolytic agent due to its cytotoxic capacity even in hypoxic environments. As a poxvirus, VACV is an unusual virus that replicates its large DNA genome exclusively in the cytoplasm of infected cells. Despite producing large amounts of cytosolic DNA, VACV efficiently suppresses the subsequent innate immune response by deploying an arsenal of proteins with capacity to disable host antiviral signaling, some of which specifically target cytosolic DNA sensing pathways. Some of these strategies are conserved amongst orthopoxviruses, whereas others are seemingly unique to VACV. In this review we provide an overview of the VACV replicative cycle and discuss the recent advances on our understanding of how VACV induces and antagonizes innate immune activation via cytosolic DNA sensing pathways. The implications of these findings in the rational design of vaccines and oncolytics based on VACV are also discussed.

Structure/Function analysis of the vaccinia virus F18 phosphoprotein, an abundant core component required for virion maturation and infectivity

Poxvirus virions, whose outer membrane surrounds two lateral bodies and a core, contain at least 70 different proteins. The F18 phosphoprotein is one of the most abundant core components and is essential for the assembly of mature virions. We report here the results of a structure/function analysis in which the role of conserved cysteine residues, clusters of charged amino acids and clusters of hydrophobic/aromatic amino acids have been assessed. Taking advantage of a recombinant virus in which F18 expression is IPTG (isopropyl-beta-d-thiogalactopyranoside) dependent, we developed a transient complementation assay to evaluate the ability of mutant alleles of F18 to support virion morphogenesis and/or to restore the production of infectious virus. We have also examined protein-protein interactions, comparing the ability of mutant and WT F18 proteins to interact with WT F18 and to interact with the viral A30 protein, another essential core component. We show that F18 associates with an A30-containing multiprotein complex in vivo in a manner that depends upon clusters of hydrophobic/aromatic residues in the N' terminus of the F18 protein but that it is not required for the assembly of this complex. Finally, we confirmed that two PSSP motifs within F18 are the sites of phosphorylation by cellular proline-directed kinases in vitro and in vivo. Mutation of both of these phosphorylation sites has no apparent impact on virion morphogenesis but leads to the assembly of virions with significantly reduced infectivity.

Actin-based motility of intracellular microbial pathogens

A diverse group of intracellular microorganisms, including Listeria monocytogenes, Shigella spp., Rickettsia spp., and vaccinia virus, utilize actin-based motility to move within and spread between mammalian host cells. These organisms have in common a pathogenic life cycle that involves a stage within the cytoplasm of mammalian host cells. Within the cytoplasm of host cells, these organisms activate components of the cellular actin assembly machinery to induce the formation of actin tails on the microbial surface. The assembly of these actin tails provides force that propels the organisms through the cell cytoplasm to the cell periphery or into adjacent cells. Each of these organisms utilizes preexisting mammalian pathways of actin rearrangement to induce its own actin-based motility. Particularly remarkable is that while all of these microbes use the same or overlapping pathways, each intercepts the pathway at a different step. In addition, the microbial molecules involved are each distinctly different from the others. Taken together, these observations suggest that each of these microbes separately and convergently evolved a mechanism to utilize the cellular actin assembly machinery. The current understanding of the molecular mechanisms of microbial actin-based motility is the subject of this review.

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.

The vaccinia virus A27L protein is needed for the microtubule-dependent transport of intracellular mature virus particles

The vaccinia virus (VV) A27L gene encodes a 14 kDa protein that is required for the formation of intracellular enveloped virus (IEV) and, consequently, normal sized plaques. Data presented here show that A27L plays an additional role in VV assembly. When cells were infected with the VV WR32-7/Ind 14K, under conditions that repress A27L expression, transport of intracellular mature virus (IMV) from virus factories was inhibited and some IMV was found in aberrant association with virus crescents. In contrast, other VV mutants (vDeltaB5R and vDeltaF13L) that are defective in IEV formation produce IMV particles that are transported out of virus factories. This indicated a specific role for A27L in IMV transport. Induction of A27L expression at 10 h post-infection promoted the dispersal of clustered IMV particles, but only when microtubules were intact. Formation of IEV particles was also impaired when cells were infected with WR32-7/14K, a VV strain expressing a mutated form of the A27L protein; however, this mutation did not inhibit intracellular transport of IMV particles. Collectively, these data define two novel aspects of VV morphogenesis. Firstly, A27L is required for both IMV transport and the process of envelopment that leads to IEV formation. Secondly, movement of IMV particles between the virus factory and the site of IEV formation is microtubule-dependent.

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.

Vaccinia locomotion in host cells: evidence for the universal involvement of actin-based motility sequences ABM-1 and ABM-2

Vaccinia uses actin-based motility for virion movement in host cells, but the specific protein components have yet to be defined. A cardinal feature of Listeria and Shigella actin-based motility is the involvement of vasodilator-stimulated phosphoprotein (VASP). This essential adapter recognizes and binds to actin-based motility 1 (ABM-1) consensus sequences [(D/E)FPPPPX(D/E), X = P or T] contained in Listeria ActA and in the p90 host-cell vinculin fragment generated by Shigella infection. VASP, in turn, provides the ABM-2 sequences [XPPPPP, X = G, P, L, S, A] for binding profilin, an actin-regulatory protein that stimulates actin filament assembly. Immunolocalization using rabbit anti-VASP antibody revealed that VASP concentrates behind motile virions in HeLa cells. Profilin was also present in these actin-rich rocket tails, and microinjection of 10 microM (intracellular) ABM-2 peptide (GPPPPP)3 blocked vaccinia actin-based motility. Vinculin did not colocalize with VASP on motile virions and remained in focal adhesion contacts; however, another ABM-1-containing host protein, zyxin, was concentrated at the rear of motile virions. We also examined time-dependent changes in the location of these cytoskeletal proteins during vaccinia infection. VASP and zyxin were redistributed dramatically several hours before the formation of actin rocket tails, concentrating in the viral factories of the perinuclear cytoplasm. Our findings underscore the universal involvement of ABM-1 and ABM-2 docking sites in actin-based motility of Listeria, Shigella, and now vaccinia.

The myxoma virus EcoRI-O fragment encodes the DNA binding core protein and the major envelope protein of extracellular poxvirus

The nucleotide sequences of the myxoma virus gene homologs encoding the DNA binding core protein (MF17) and the major envelope protein of the extracellular poxvirus particle (MF13) have been localized to the myxoma virus 4 kB EcoRI-O fragment. The EcoRI-O fragment is located approximately 22 kb from the left end of the 163 kb DNA genome and encodes homologs of the F12L, F13L, F15L, F16L, F17R and E1L genes of the Copenhagen strain of vaccinia virus. The inferred amino acid sequences of the myxoma virus EcoRI-O encoded products have been compared to the protein databases to identify related proteins. The myxoma virus open reading frames MF12, MF15, MF16, MF17 and ME1 encode homologs of poxvirus specific proteins while the MF13 envelope protein also shares amino acid similarity with other poxvirus and cellular proteins.

Virus-induced cell motility

Many viruses induce profound changes in cell metabolism and function. Here we show that vaccinia virus induces two distinct forms of cell movement. Virus-induced cell migration was demonstrated by an in vitro wound healing assay in which infected cells migrated independently into the wound area while uninfected cells remained relatively static. Time-lapse microscopy showed that the maximal rate of migration occurred between 9 and 12 h postinfection. Virus-induced cell migration was inhibited by preinactivation of viral particles with trioxsalen and UV light or by the addition of cycloheximide but not by addition of cytosine arabinoside or rifampin. The expression of early viral genes is therefore necessary and sufficient to induce cell migration. Following migration, infected cells developed projections up to 160 microm in length which had growth-cone-like structures and were frequently branched. Time-lapse video microscopy showed that these projections were formed by extension and condensation of lamellipodia from the cell body. Formation of extensions was dependent on late gene expression but not the production of intracellular enveloped (IEV) particles. The requirements for virus-induced cell migration and for the formation of extensions therefore differ from each other and are distinct from the polymerization of actin tails on IEV particles. These data show that poxviruses encode genes which control different aspects of cell motility and thus represent a useful model system to study and dissect cell movement.

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 in Listeria, Shigella and Rickettsia 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 microns/min, both intra- and intercellularly. The similarities between the actin-based motility of the vaccinia virus, Listeria, Shigella and Rickettsia 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.

The vaccinia virus F17R protein interacts with actin

We have examined the possible role of the F17R protein in vaccinia virus-induced rearrangements of the host actin cytoskeleton. F17R is localized to vaccinia-induced actin tails late during infection. The recombinant vaccinia strain vRO11k is able to induce actin tails that are indistinguishable from controls in the absence of F17R expression. The association of vaccinia and myxoma virus F17R with the actin cytoskeleton in the absence of additional viral factors suggests a basic region in the N-terminal half of the protein is important for this interaction. A peptide corresponding to this region efficiently bundles actin filaments in vitro, confirming that the protein interacts directly with actin. Our results show F17R is not required for actin tail formation and highlight the difficulty in discriminating functional actin-binding proteins from those that associate by virtue of their basic nature.

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.

Viral manipulations of the actin cytoskeleton

Viruses succeed as intracellular parasites because of their ability to invade cells and appropriate the cellular machinery required during their life cycle. The actin cytoskeleton of the host cell does not escape viral infection unscathed, but is often co-opted by the virus at many different stages of its life cycle to facilitate the infection process.

Actin-based motility of vaccinia virus

The role of the cytoskeleton during viral infection is poorly understood. Here we show, using a combination of mutant and drug studies, that the intracellular enveloped form of vaccinia virus is capable of inducing the formation of actin tails that are strikingly similar to those seen in Listeria, Shigella and Rickettsia infections. Analysis using video microscopy reveals that single viral particles are propelled in vivo on the tip of actin tails, at a speed of 2.8 mumol min-1. On contact with the cell surface, virus particles extend outwards on actin projections at a similar rate, to contact and infect neighboring cells. Given the similarities between the motility of vaccinia virus and bacterial pathogens, we suggest that intracellular pathogens have developed a common mechanism to exploit the actin cytoskeleton as a means to facilitate their direct spread between cells.

Electron spectroscopic imaging of encapsidated DNA in vaccinia virus

We have used electron spectroscopic imaging to locate the phosphorus in vaccinia DNA in situ in unstained, ultrathin sections of virions. The phosphorus of the DNA backbone appeared to form a halo on the core periphery surrounding a phosphorus-impoverished central element. These results constrain models for how DNA could be packaged into mature vaccinia particles.

Viral protein kinases and protein phosphatases

Certain large DNA viruses (e.g. herpesviruses and poxviruses) encode proteins related to cellular protein-serine/threonine kinases, and Hepatitis B virus and vesicular stomatitis virus may encode structurally different protein kinases. Other viruses activate cellular protein kinases, e.g. interferon-induced eukaryotic initiation factor-2 kinase, growth factor-induced kinases and protein kinases that regulate mitosis. Protein phosphatases are encoded by vaccinia virus and bacteriophage lambda and must also play a role in viral infection--as do cellular protein phosphatases. The functions of many of these viral enzymes remain to be determined, but they represent possible new targets for anti-viral therapy.

Vaccinia virus morphogenesis is interrupted when expression of the gene encoding an 11-kilodalton phosphorylated protein is prevented by the Escherichia coli lac repressor

A conditional lethal vaccinia virus mutant, which constitutively expresses the Escherichia coli lac repressor and has the lac operator controlling the F18R gene (the 18th open reading frame of the HindIII F fragment of the vaccinia virus strain WR genome) encoding an 11-kDa protein, was previously shown to be dependent on the inducer isopropyl-beta-D-thiogalactoside (IPTG) for replication (Y. Zhang and B. Moss, Proc. Natl. Acad. Sci. USA 88:1511-1515, 1991). Further studies indicated that the yield of infectious virus could be regulated by titration with IPTG and that virus production was arrested by IPTG removal at appropriate times. Under nonpermissive conditions, an 11-kDa protein reactive with antiserum raised to a previously described DNA-binding phosphoprotein (S. Y. Kao and W. R. Bauer, Virology 159:399-407, 1987) was not synthesized, indicating that the latter is the product of the F18R gene. In the absence of IPTG, replication of viral DNA and the subsequent resolution of concatemeric DNA molecules appeared normal. Omission of IPTG did not alter the kinetics of early and late viral protein synthesis, although the absence of the 11-kDa polypeptide was noted by labeling infected cells with [35S]methionine or [32P]phosphate. Pulse-chase experiments revealed that proteolytic processing of the major viral structural proteins, P4a and P4b, was inhibited under nonpermissive conditions, suggesting a block in virus maturation. Without addition of IPTG, the failure of virus particle formation was indicated by sucrose gradient centrifugation of infected cell lysates and by the absence of vaccinia virus-mediated pH-dependent cell fusion. Electron microscopic examination of infected cells revealed that immature virus particles, with aberrant internal structures, accumulated when synthesis of the 11-kDa DNA-binding protein was prevented.

Intercellular spread of Shigella flexneri through a monolayer mediated by membranous protrusions and associated with reorganization of the cytoskeletal protein vinculin

The spread of Shigella flexneri in a monolayer of infected Henle and HeLa cells was studied by using immunofluorescence and electron microscopy. Infected cells produced numerous bacterium-containing membranous protrusions up to 18 microns in length that penetrated adjacent cells and were subsequently phagocytosed. Fluorescence staining of actin and vinculin in infected cells with phalloidin and monoclonal antibody to vinculin, respectively, demonstrated that the protrusions containing the bacteria consisted of these cytoskeletal proteins. Actin accumulated predominantly at the poles of bacteria distal to the tip of protrusions and appeared as trails extending back towards the host cell cytoplasm. Vinculin, however, was distributed uniformly around the bacteria and throughout the protrusion. A profound rearrangement of vinculin occurred in Henle and HeLa cells following infection with shigellae: whereas in uninfected cells it was distributed mainly around the cell periphery, in infected cells it concentrated mainly around clusters of bacteria in the cytoplasm. This suggests a possible involvement of the vinculin cytoskeletal protein in the intercellular spread of shigellae during an infection.

A mutation in the gene encoding the vaccinia virus 37,000-M(r) protein confers resistance to an inhibitor of virus envelopment and release

Plaque formation in vaccinia virus is inhibited by the compound N1-isonicotinoyl-N2-3-methyl-4-chlorobenzoylhydrazine (IMCBH). We have isolated a mutant virus that forms wild-type plaques in the presence of the drug. Comparison of wild-type and mutant virus showed that both viruses produced similar amounts of infectious intracellular naked virus in the presence of the drug. In contrast to the mutant, no extracellular enveloped virus was obtained from IMCBH-treated cells infected with wild-type virus. Marker rescue experiments were used to map the mutation conferring IMCBH resistance to the mutant virus. The map position coincided with that of the gene encoding the viral envelope antigen of M(r) 37,000. Sequence analysis of both wild-type and mutant genes showed a single nucleotide change (G to T) in the mutant gene. In the deduced amino acid sequence, the mutation changes the codon for an acidic Asp residue in the wild-type gene to one for a polar noncharged Tyr residue in the mutant.

Inducer-dependent conditional-lethal mutant animal viruses

Regulatory elements of the Escherichia coli lac operon were used to construct an inducer-dependent conditional-lethal mutant animal virus. The gene encoding the repressor protein of the lac operon was integrated into the vaccinia virus genome so that it was expressed constitutively, and the lac operator was inserted next to the promoter of a gene that encodes an 11-kDa virion-associated protein of unknown function. The addition of inducer to the cell culture medium provided permissive conditions for isolation of a conditional-lethal mutant virus. Under nonpermissive conditions, the isolated virus did not form plaques, and the yield was decreased by at least 1000-fold under one-step growth conditions. Transcription of the operator-controlled gene was inducer-dependent and necessary for synthesis of the 11-kDa protein. Application of this mutagenesis strategy to other viruses is discussed.

Vaccinia virus recombinants expressing an 11-kilodalton beta-galactosidase fusion protein incorporate active beta-galactosidase in virus particles

Recombinant plasmids in which vaccinia virus transcriptional regulatory sequences were fused to the Escherichia coli lacZ gene were constructed for insertion of the lacZ gene into the vaccinia virus genome. beta-Galactosidase (beta-gal) was found in some purified recombinant vaccinia virions. By enzyme activity, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and microscopic techniques, the evidence suggested that beta-gal accounted for 5% of the total protein in the virion. These recombinant viruses were constructed so that a portion of the coding sequences of a late vaccinia virus structural polypeptide was fused to the amino terminus of beta-gal to produce the fusion protein. Removal of the coding sequences resulted in the complete loss of beta-gal activity. This demonstrated that a vaccinia virus DNA segment from a late structural gene is responsible for the incorporation of beta-gal into the virion.

Biosynthesis and phosphorylation of vaccinia virus structural protein VP11

One of the major DNA binding proteins contained in vaccinia virus is an 11-kDA species denoted VP11. The biosynthesis of VP11, a late polypeptide, occurs subsequent to the initiation of viral DNA replication. In particular, VP11 synthesis is blocked by cytosine arabinoside, a specific inhibitor of DNA synthesis. We show here that VP11 is specifically phosphorylated subsequent to translation. Phosphorylated VP11 is present both in viral core particles and in the cytoplasm of virus-infected cells. Kinetic analysis reveals that the total amount of phosphorylated VP11 species increases rapidly and remains approximately constant for as long as 17 hr postinfection. Phosphorylation occurs at two different serine residues, progressing from either site singly to the diphosphorylated product. Under steady-state conditions, the phosphorylated derivative constitutes approximately 85% of total VP11 in extracts of vaccinia virus-infected cells. Even though 15% of the VP11 remains unphosphorylated in cell extracts, only phosphorylated VP11 is found in mature viral cores.

Influence of RNA polymerase II upon vaccinia virus-related translation examined by means of alpha-amanitin

Our previous studies employing alpha-amanitin-sensitive H-9 and resistant Ama 102 mutant host cells demonstrated that polymerase II (Pol II), or a drug-sensitive component of the enzyme, is required for replication of vaccinia virus. Evidence was also obtained indicating that transcription from the host genome does not appear to be involved (Silver et al., 1979; Silver and Dales, 1982), suggesting a possible role for Pol II in transcription from the viral genome. This idea is consistent with the present findings, based on immunofluorescence analysis, which revealed that upon infection Pol II antigen is mobilized out of the nucleus into discrete cytoplasmic foci. Effects of treating H-9 rat myoblasts with alpha-amanitin upon vaccinia-specific protein synthesis were also examined. Under the experimental conditions employed, the toxin drastically curtailed in vivo translation into early, late and late-late proteins without altering the spectrum of polypeptides produced. By contrast, treatment with the drug affected, only minimally, the rate of transcription into viral RNA, whether in vivo or from isolated vaccinia factories. The mRNA isolated from infected and treated or untreated cells was translated in a reticulocyte lysate with equal efficiency and general fidelity. This finding suggests that Pol II may be involved in transcription into RNAs related to factors controlling the in vivo translation process. The possible mechanisms for exercising such controls are discussed in relation to factors regulating transcription by host RNA polymerases from a viral DNA genome.

Gene coding for the late 11,000-dalton polypeptide of the Tian Tan strain of vaccinia virus and its 5'-flanking region: nucleotide sequence

A late gene coding for a major structural polypeptide of 11,000 daltons from the Tian Tan strain of vaccinia virus was cloned. A comparison of the sequences from Tian Tan and WR strains of vaccinia virus revealed that there were differences in four nucleotides and one amino acid and that there was little homology between the 5'-flanking sequences of the late and the early genes. However, substantial homology was detected between the 5'-flanking sequences of the late genes.

Golgi-derived membranes that contain an acylated viral polypeptide are used for vaccinia virus envelopment

A 37,000-dalton polypeptide (p37K) present on purified extracellular vaccinia virus but absent from intracellular virus particles of classical morphology (G. Hiller et al., J. Virol. 39:903-913, 1981; L. G. Payne, J. Virol. 27:28-37, 1978) was further characterized. The polypeptide was only expressed in infected cells after onset of viral DNA replication. Phase partition experiments showed that it is relatively hydrophobic. Although p37K apparently is not a glycoprotein, in vivo radioisotope labeling detected tightly associated palmitic acid. Antibodies to p37K were used to monitor its distribution within infected cells at the light and electron microscopic levels. After synthesis p37K first accumulated in the Golgi region due to a tight membrane association. During progressing infection p37K-carrying membranes were used to form double-walled envelopes around brick-shaped vaccinia particles. Within these specialized vesicles vaccinia particles were moved through the cytoplasm toward the cell's surface, presumably along cellular routes for certain secretory products. Finally, single enveloped viruses were released into the extracellular space by an exocytotic process.

One hundred base pairs of 5' flanking sequence of a vaccinia virus late gene are sufficient to temporally regulate late transcription

A vaccinia virus late gene coding for a major structural polypeptide of 11 kDa was sequenced. Although the 5' flanking gene region is very A+T rich, it shows little homology either to the corresponding region of vaccinia early genes or to consensus sequences characteristic of most eukaryotic genes. Three DNA fragments (100, 200, and 500 base pairs, respectively), derived from the flanking region and including the late gene mRNA start site, were inserted into the coding sequence of the vaccinia virus thymidine kinase (TK) early gene by homologous in vivo recombination. Recombinants were selected on the basis of their TK- phenotype. Cells were infected with the recombinant viruses and RNA was isolated at 1-hr intervals. Transcripts initiating either from the TK early promoter, or from the late gene promoter at its authentic position, or from the translocated late gene promoters within the early gene were detected by nuclease S1 mapping. Early after infection, only transcripts from the TK early promoter were detected. Later in infection, however, transcripts were also initiated from the translocated late promoters. This RNA appeared at the same time and in similar quantities as the RNA from the late promoter at its authentic position. No quantitative differences in promoter efficiency between the 100-, 200-, and 500-base-pair insertions were observed. We conclude that all necessary signals for correct regulation of late-gene expression reside within only 100 base pairs of 5' flanking sequence.

Assembly associated with the cytomatrix

Assembly in vivo has been studied both for endogenous cytoskeletal proteins and for several classes of viruses. Autoradiography of cytoskeletal proteins has shown that many associate with the cytoskeletal framework close to the time and place of synthesis. The cytoskeletal proteins rearrange after association with the cytoskeletal framework. Rearrangement in symmetrical giant cells occurs in a centrifugal and coherent pattern. Many of the cytoskeletal proteins associate cotranslationally, as shown by their puromycin resistance in a cell-free translation system. The assembly of several groups of viruses has been shown to be associated with various components of the cytoskeleton; whether such assembly is cotranslational has not yet been addressed directly.

Mapping of a gene coding for a major late structural polypeptide on the vaccinia virus genome

Cell-free translation of total RNA isolated from vaccinia virus-infected cells late in infection results in a complex mixture of polypeptides. A monospecific antibody directed against one of the major structural proteins of the virus particle immunoprecipitated a single polypeptide with a molecular weight of 11,000 (11K) from this mixture. Immunoprecipitation was therefore used to identify the structural polypeptide among the in vitro translation products of RNA purified by hybridization selection to restriction fragments of the vaccinia virus genome. This allowed us to map the mRNA coding for the 11K polypeptide to the extreme left-hand end of the HindIII E fragment. Detailed transcriptional mapping of this region of the genome by nuclease S1 analysis revealed the presence of a late RNA transcribed from the rightward-reading strand. Its 5' end mapped at ca. 130 base pairs to the left of the HindIII site at the junction between the HindIII F and E fragments. The map position of this RNA coincided precisely with the map position of the late message coding for the 11K polypeptide.