Fluorescence microscopical analysis of the life cycle of vaccinia virus in chick embryo fibroblasts. Virus-cytoskeleton interactions

The role of signalling and the cytoskeleton during Vaccinia Virus egress

Viruses are obligate intracellular parasites that are critically dependent on their hosts to replicate and generate new progeny. To achieve this goal, viruses have evolved numerous elegant strategies to subvert and utilise the different cellular machineries and processes of their unwilling hosts. Moreover, they often accomplish this feat with a surprisingly limited number of proteins. Among the different systems of the cell, the cytoskeleton is often one of the first to be hijacked as it provides a convenient transport system for viruses to reach their site of replication with relative ease. At the latter stages of their replication cycle, the cytoskeleton also provides an efficient means for newly assembled viral progeny to reach the plasma membrane and leave the infected cell. In this review we discuss how Vaccinia virus takes advantage of the microtubule and actin cytoskeletons of its host to promote the spread of infection into neighboring cells. In particular, we highlight how analysis of actin-based motility of Vaccinia has provided unprecedented insights into how a phosphotyrosine-based signalling network is assembled and functions to stimulate Arp2/3 complex-dependent actin polymerization. We also suggest that the formin FHOD1 promotes actin-based motility of the virus by capping the fast growing ends of actin filaments rather than directly promoting filament assembly. We have come a long way since 1976, when electron micrographs of vaccinia-infected cells implicated the actin cytoskeleton in promoting viral spread. Nevertheless, there are still many unanswered questions concerning the role of signalling and the host cytoskeleton in promoting viral spread and pathogenesis.

Viral exploitation of actin: force-generation and scaffolding functions in viral infection

As a fundamental component of the host cellular cytoskeleton, actin is routinely engaged by infecting viruses. Furthermore, viruses from diverse groups, and infecting diverse hosts, have convergently evolved an array of mechanisms for manipulating the actin cytoskeleton for efficacious infection. An ongoing chorus of research now indicates that the actin cytoskeleton is critical for viral replication at many stages of the viral life cycle, including binding, entry, nuclear localization, genomic transcription and reverse transcription, assembly, and egress/dissemination. Specifically, viruses subvert the force-generating and macromolecular scaffolding properties of the actin cytoskeleton to propel viral surfing, internalization, and migration within the cell. Additionally, viruses utilize the actin cytoskeleton to support and organize assembly sites, and eject budding virions for cell-to-cell transmission. It is the purpose of this review to provide an overview of current research, focusing on the various mechanisms and themes of virus-mediated actin modulation described therein.

Arp2/3-mediated actin-based motility: a tail of pathogen abuse

Intracellular pathogens have developed elaborate mechanisms to exploit the different cellular systems of their unwilling hosts to facilitate their entry, replication, and survival. In particular, a diverse range of bacteria and viruses have evolved unique strategies to harness the power of Arp2/3-mediated actin polymerization to enhance their cell-to-cell spread. In this review, we discuss how studying these pathogens has revolutionized our molecular understanding of Arp2/3-dependent actin assembly and revealed key signaling pathways regulating actin assembly in cells. Future analyses of microbe-host interactions are likely to continue uncovering new mechanisms regulating actin assembly and dynamics, as well as unexpected cellular functions for actin. Further, studies with known and newly emerging pathogens will also undoubtedly continue to enhance our understanding of the role of the actin cytoskeleton during pathogenesis and potentially highlight future therapeutic approaches.

The longest micron; transporting poxviruses out of the cell

The large size of poxvirus virions (approximately 250-300 microm) makes them dependent on active transport for intracellular movement during infection. Several recent papers have reported the utilization of the microtubule network by poxviruses during viral egress and their use of conventional kinesin for intracellular transport. This review looks at recent reports of poxvirus intracellular transport for virion egress and their interaction with the microtubule network.

Vaccinia virus motility

Vaccinia virus (VV), the virus smallpox vaccine, replicates in the cytoplasm of infected cells. The intracellular movement of this large virus would be inefficient without specific transport mechanisms; therefore, VV uses microtubules for movement during both entry and egress. In addition, the dissemination of virus from infected cells to adjacent cells is promoted by the polymerization of actin beneath cell surface virions to drive virus particles away from the cell. Last, the roles of different VV particles in virus movement within and between hosts are discussed.

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.

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.

Membrane topology of the vaccinia virus A17L envelope protein

The formation of a lipoprotein membrane within specialized areas of the cytoplasm is the first visible step in poxvirus morphogenesis. The A17L viral protein, an essential nonglycosylated membrane component, was predicted to have four centrally located alpha-helical membrane-spanning domains. The gene was expressed as a 23-kDa protein in a cell-free transcription/translation system containing canine pancreatic microsomes. The N- and C-terminal ends of the membrane-associated protein were susceptible to proteinase digestion, whereas the central region was resistant, consistent with a model in which the first and fourth hydrophobic domains are membrane spanning. This topology was supported by the sizes of the major proteinase-resistant membrane-associated products of genes containing one or more deleted hydrophobic domains and by evidence that the C-terminus was intraluminal and glycosylated on deletion of the second, third, and fourth domains, the third and fourth domains, or just the fourth domain. Moreover, glycosylation also occurred when an N-glycosylation site was introduced into the second hydrophobic domain of the full-length A17L protein. The data indicated a predominant topology in which the N- and C-termini are cytoplasmic, the first and fourth hydrophobic domains span the microsomal membrane, and the second and third hydrophobic domains are intraluminal. This arrangement has important implications for interactions of the A17L protein with other membrane components.

Regulation of vaccinia virus morphogenesis: phosphorylation of the A14L and A17L membrane proteins and C-terminal truncation of the A17L protein are dependent on the F10L kinase

This study focused on three vaccinia virus-encoded proteins that participate in early steps of virion morphogenesis: the A17L and A14L membrane proteins and the F10L protein kinase. We found that (i) the A17L protein was cleaved at or near an AGX consensus motif at amino acid 185, thereby removing its acidic C terminus; (ii) the nontruncated form was associated with immature virions, but only the C-terminal truncated protein was present in mature virions; (iii) the nontruncated form of the A17L protein was phosphorylated on serine, threonine, and tyrosine residues, whereas the truncated form was unphosphorylated; (iv) nontruncated and truncated forms of the A17L protein existed in a complex with the A14L membrane protein; (v) C-terminal cleavage of the A17L protein and phosphorylation of the A17L and A14L proteins failed to occur in cells infected with a F10L kinase mutant at the nonpermissive temperature; and (vi) the F10L kinase was the only viral late protein that was necessary for phosphorylation of the A17L protein, whereas additional proteins were needed for C-terminal cleavage. We suggest that phosphorylation of the A17L and A14L proteins is mediated by the F10L kinase and is required to form the membranes associated with immature virions. Removal of phosphates and the A17L acidic C-terminal peptide occur during the transition to mature virions.

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.

Intracellular pathogens and the actin cytoskeleton

Many pathogens actively exploit the actin cytoskeleton during infection. This exploitation may take place during entry into mammalian cells after engagement of a receptor and/or as series of signaling events culminating in the engulfment of the microorganism. Although actin rearrangements are a common feature of most internalization events (e.g. entry of Listeria, Salmonella, Shigella, Yersinia, Neisseria, and Bartonella), bacterial and other cellular factors involved in entry are specific to each bacterium. Another step during which pathogens harness the actin cytoskeleton takes place in the cytosol, within which some bacteria (Listeria, Shigella, Rickettsia) or viruses (vaccinia virus) are able to move. Movement is coupled to a polarized actin polymerization process, with the formation of characteristic actin tails. Increasing attention has focused on this phenomenon due to its striking similarity to cellular events occurring at the leading edge of locomoting cells. Thus pathogens are convenient systems in which to study actin cytoskeleton rearrangements in response to stimuli at the plasma membrane or inside cells.

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.

Autographa californica nuclear polyhedrosis virus: subcellular localization and protein trafficking of BV/ODV-E26 to intranuclear membranes and viral envelopes

The Autographa californica nuclear polyhedrosis virus da26 gene codes for an envelope protein of both budded virus (BV) and occlusion derived virus (ODV). Western blot and temporal analysis of infected cell extracts detected a protein of 26 kDa by 4 h postinfection (p.i.). The amount of protein increased by 16 h p.i. and remained at high levels throughout infection. By 36 h p.i. several additional immunoreactive proteins were detected which migrated at approximately 18 kDa and remained through 96 h p.i. Western blot analysis of purified virus envelope and nucleocapsid preparations revealed that both the 26- and 18-kDa proteins are structural proteins of the envelope of BV and ODV. Immunoelectron microscopy performed at a time when only the 26-kDa species of the protein was present confirmed that the protein located to ODV envelope. The protein was named BV/ODV-E26 to designate incorporation into viral progeny, envelope location, and apparent molecular weight. Studies designed to follow localization of BV/ODV-E26 demonstrated that early in infection, the protein was incorporated into cytoplasmic vesicles and by 16 h p.i., BV/ODV-E26 was detected in the nucleus associated with virus-induced intranuclear microvesicles and ODV envelope. Coimmunoprecipitation and yeast two-hybrid assays showed that BV/ODV-E26 and FP25K were capable of interacting with each other to form a complex and coimmunoprecipitation assays indicated that cellular actin was a third component of this complex. Together, these data suggest that FP25K and cellular actin may participate in the regulation, or movement through the cell, of baculovirus proteins and/or virus nucleocapsids.

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.

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.

ENC-1: a novel mammalian kelch-related gene specifically expressed in the nervous system encodes an actin-binding protein

We have identified and characterized a novel murine gene, Ectoderm-Neural Cortex-1 (ENC-1), that is an early and highly specific marker of neural induction in vertebrates. ENC-1, which encodes a kelch family related protein, is expressed during early gastrulation in the prospective neuroectodermal region of the epiblast and later in development throughout the nervous system (NS). ENC-1 expression is highly dynamic and, after neurulation, preferentially defines prospective cortical areas. The only apparent expression of ENC-1 outside the NS is restricted to the rostral-most somitomere of the presomitic mesoderm, at the times corresponding to the epithelialization that precedes somite formation. Cellular expression of epitope-tagged ENC-1 shows extensive co-localization of ENC-1 with the actin cytoskeleton, and immunoprecipitation studies demonstrate a physical association between ENC-1 and actin. ENC-1 functions as an actin-binding protein that may be important in the organization of the actin cytoskeleton during neural fate specification and development of the NS.

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

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

The cytoplasmic and transmembrane domains of the vaccinia virus B5R protein target a chimeric human immunodeficiency virus type 1 glycoprotein to the outer envelope of nascent vaccinia virions

The outer envelope of the extracellular form of vaccinia virus (EEV) is derived from the Golgi membrane and contains at least six viral proteins. Transfection studies indicated that the EEV protein encoded by the B5R gene associates with Golgi membranes when synthesized in the absence of other viral products. A domain swapping strategy was then used to investigate the possibility that the B5R protein contains an EEV targeting signal. We constructed chimeric genes encoding the human immunodeficiency virus (HIV) type 1 glycoprotein with the cytoplasmic and transmembrane domains replaced by the corresponding 42-amino-acid C-terminal segment of the B5R protein. Recombinant vaccinia viruses that stably express a chimeric B5R-HIV protein or a control HIV envelope protein with the original cytoplasmic and transmembrane domains were isolated. Cells infected with recombinant vaccinia viruses that expressed either the unmodified or the chimeric HIV envelope protein formed syncytia with cells expressing the CD4 receptor for HIV. However, biochemical and microscopic studies demonstrated that the HIV envelope proteins with the B5R cytoplasmic and transmembrane domains were preferentially targeted to the EEV. These data are consistent with the presence of EEV localization signals in the cytoplasmic and transmembrane domains of the B5R protein.

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.

Vaccinia virus: a model system for actin-membrane interactions

Our understanding of the interactions between the actin cytoskeleton and cellular membranes at the molecular level is rudimentary. One system that offers an opportunity to examine these interactions in greater detail is provided by vaccinia virus, which induces the nucleation of actin tails from the outer membrane surrounding the virion. To further understand the mechanism of their formation and how they generate motility, we have examined the structure of these actin tails in detail. Actin filaments in vaccinia tails are organized so they splay out at up to 45 degrees from the centre of the tail and are up to 0.74 micron in length, which is considerably longer than those reported in the Listeria system. Actin filaments show unidirectional polarity with their barbed filament ends pointing towards the surface of the virus particle. Rhodamine-actin incorporation experiments show that the first stage of tail assembly involves a polarized recruitment of G-actin, and not pre-formed actin filaments, to the membrane surrounding the virion. Incorporation of actin into the tail only occurs by nucleation from the viral surface, suggesting filament ends in the tail are blocked against further actin addition. As virus particles fuse with the plasma membrane during the extention of projections, actin nucleation sites previously in the viral membrane become localized to the plasma membrane, where they are able to nucleate actin polymerization in a manner analogous to the leading edge of motile cells.

Identification and characterization of a filament-associated protein encoded by Amsacta moorei entomopoxvirus

A novel protein which is expressed at high levels in insect cells infected with Amsacta moorei entomopoxvirus was identified by our laboratory. This viral gene product migrates as a 25/27-kDa doublet when subjected to electrophoresis on sodium dodecyl sulfate-polyacrylamide gels. It is expressed at late times of infection and is present in infected cells but is absent in purified extracellular virions and occlusion bodies. The gene encoding this polypeptide was mapped on the viral genome, and cDNA clones were generated and sequenced. The predicted protein was shown to be phosphorylated and contained an unusual 10-unit proline-glutamic acid repeat element. A polyclonal antiserum was produced against a recombinant form of the protein expressed in Escherichia coli, and a monoclonal antibody which reacted with the proline-glutamic acid motif was also identified. Immunofluorescence and immunoelectron microscopy techniques revealed that this protein is associated with large cytoplasmic fibrils which accumulate in the cytoplasm between 96 and 120 h postinfection. We subsequently called this viral polypeptide filament-associated late protein of entomopoxvirus. The fibrils containing this polypeptide are closely associated with occlusion bodies and may play a role in their morphogenesis and maturation.

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.

iactA of Listeria ivanovii, although distantly related to Listeria monocytogenes actA, restores actin tail formation in an L. monocytogenes actA mutant

A gene homologous to the actA gene of Listeria monocytogenes was cloned from Listeria ivanovii (strain CLIP257) by chromosome walking starting from the ilo gene that encodes the pore-forming toxin ivanolysin. The nucleotide sequence revealed that this gene, named iactA, encodes a protein of 1,044 amino acids (IactA) comprising a central region with seven highly conserved tandem proline-rich repeats of 47 amino acids. Although IactA and ActA share an overall similar structure, these two proteins are only distantly related. Like ActA, IactA migrates aberrantly on sodium dodecyl sulfate gels. When expressed in an L. monocytogenes actA deletion mutant strain, iactA restored actin polymerization.

Sequence and domain organization of scruin, an actin-cross-linking protein in the acrosomal process of Limulus sperm

The acrosomal process of Limulus sperm is an 80-microns long finger of membrane supported by a crystalline bundle of actin filaments. The filaments in this bundle are crosslinked by a 102-kD protein, scruin present in a 1:1 molar ratio with actin. Recent image reconstruction of scruin decorated actin filaments at 13-A resolution shows that scruin is organized into two equally sized domains bound to separate actin subunits in the same filament. We have cloned and sequenced the gene for scruin from a Limulus testes cDNA library. The deduced amino acid sequence of scruin reflects the domain organization of scruin: it consists of a tandem pair of homologous domains joined by a linker region. The domain organization of scruin is confirmed by limited proteolysis of the purified acrosomal process. Three different proteases cleave the native protein in a 5-kD Protease-sensitive region in the middle of the molecule to generate an NH2-terminal 47-kD and a COOH-terminal 56-kD protease-resistant domains. Although the protein sequence of scruin has no homology to any known actin-binding protein, it has similarities to several proteins, including four open reading frames of unknown function in poxviruses, as well as kelch, a Drosophila protein localized to actin-rich ring canals. All proteins that show homologies to scruin are characterized by the presence of an approximately 50-amino acid residue motif that is repeated between two and seven times. Crystallographic studies reveal this motif represents a four beta-stranded fold that is characteristic of the "superbarrel" structural fold found in the sialidase family of proteins. These results suggest that the two domains of scruin seen in EM reconstructions are superbarrel folds, and they present the possibility that other members of this family may also bind actin.

Membrane-bound complement regulatory activity is decreased on vaccinia virus-infected cells

Decay accelerating factor (DAF), membrane cofactor protein (MCP), complement receptor 1 and mouse Crry are cell surface-bound complement regulatory proteins capable of inhibiting C3 convertase activity on cell membranes, and therefore provide a substantial protection from attack by homologous complement activated either by the classical or by the alternative pathway. Decrease in complement regulatory activity might lead to spontaneous complement deposition and subsequent cell injury. MoAb 5I2 can inhibit the complement regulatory activity of molecules on rat cells, resulting in deposition of homologous complement. The antigen recognized by 5I2 MoAb in rats is homologous to mouse Crry. Fifteen to 20 h after infection with vaccinia virus, in vitro cultured KDH-8 rat hepatoma cells show a strong decrease in expression of Crry-like antigen, and proved to be sensitive to complement deposition when 1:5 diluted normal rat serum was added to the culture medium as a source of complement. Addition of complement to the cultured KDH-8 cells infected with a very low dose of vaccinia virus (1 plaque-forming unit (PFU)/1000 cells) substantially reduced spreading of virus infection in the cell culture, while inactivation of complement by heat or zymosan treatment abrogated the protective effect.

Rearrangement of intermediate filament network of BHK-21 cells infected with vaccinia virus

Association between vaccinia virus (VV) structures and intermediate filaments in specific areas of the cytoplasm of infected cells (virus "factories") suggests that VV infection interferes with the cellular architecture by modifying the intermediate filament network. To analyse this question, we examined the array of intermediate filaments of BHK-21 cells infected with VV by laser scanning confocal microscopy using an anti-vimentin mouse monoclonal antibody. We observed a marked reorganization of intermediate filaments around the nucleus of infected cells. Bidimensional analysis of 32PO4-labeled intermediate filament proteins revealed that the acidic isoform of vimentin and two isoforms of desmin have increased phosphorylation levels in infected cells. Our results suggest that the reorganization of intermediate filaments observed during VV infection could be promoted by an increase in the phosphorylation level of the intermediate filament proteins, vimentin and desmin.

Role of cell-associated enveloped vaccinia virus in cell-to-cell spread

The roles of intracellular naked (INV), cell-associated enveloped (CEV), and extracellular enveloped (EEV) forms of vaccinia virus in cell-to-cell and longer-range spread were investigated by using two closely related strains of vaccinia virus, WR and IHD-J. We confirmed previous results that WR and IHD-J produced similar amounts of INV and formed similar-size primary plaques but that IHD-J produced 10 to 40 times more EEV and spread to distant cells much more efficiently than did WR. Nevertheless, cells infected with WR and IHD-J had similar amounts of CEV, indicating that wrapping and transport of WR virions were unimpaired. A WR mutant with a deletion in VP37, the major outer envelope protein, formed normal amounts of INV; however, the generation of CEV was blocked and plaque formation was inhibited. These results suggested that CEV is the form of virus that mediates cell-to-cell spread. Marker rescue experiments indicated that the differences in EEV production by WR and IHD-J were not due to sequence differences in VP37. The low amount of WR EEV could be attributed to retention of CEV on the cell membrane. In support of this hypothesis, mild treatment with trypsin released as much or more infectious virus from cells infected with WR as it did with cells infected with IHD-J. Most of the virus released by trypsin sedimented with the buoyant density of EEV. Also, addition of trypsin to cells following inoculation with WR led to a comet-shaped distribution of secondary plaques characteristic of IHD-J. These results demonstrated that the release of CEV from the cell surface was limiting for extracellular virus formation and affirmed the role of EEV in long-range spread.

Sequence analysis, expression, and deletion of a vaccinia virus gene encoding a homolog of profilin, a eukaryotic actin-binding protein

A 4,500-bp BamHI fragment, located within the HindIII A segment of the vaccinia virus genome, was found to contain eight potential coding regions for polypeptides of 78 to 346 amino acids. The open reading frames with 133, 346, and 125 codons were homologous to profilin (an actin-binding protein), 3-beta-hydroxysteroid dehydrogenase, and Cu-Zn superoxide dismutase, respectively. Sequence alignments indicated that the vaccinia virus and mammalian profilins were more closely related to each other than to known profilins of other eukaryotes. The expression and possible role of the profilin homolog in the virus replicative cycle were therefore investigated. Antibody raised to Escherichia coli expressed vaccinia virus profilin was used to demonstrate the synthesis of the 15-kDa polypeptide at late times after vaccinia virus infection of mammalian cells. The protein accumulated in the cytoplasm, but only trace amounts remained associated with highly purified virions. The isolation of vaccinia virus mutants (in strains WR and IHD-J), with nearly the entire profilin gene replaced by the E. coli gpt gene, indicated that the protein is not essential for infectivity. The characteristic vaccinia virus-induced changes in actin fibers, seen by fluorescence microscopy, occurred in cells infected with the mutant. Moreover, the virus-encoded profilin homolog was not required for actin-associated events, including intracellular virus movement to the periphery of the cell, formation of specialized microvilli, or release of mature virions, as shown by electron microscopy and yields of infectious intra- and extracellular virus.

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.

Regulation of histone H5 and H1 zero gene expression under the control of vaccinia virus-specific sequences in interferon-treated chick embryo fibroblasts

The duck histone H5 and human H1 zero were inserted into the thymidine kinase (TK) gene of vaccinia virus and the interferon sensitivity of their expression under the control of the viral TK and P7.5 promoters in chick embryo fibroblasts (CEF) was compared to the interferon sensitivity of vaccinia virus WR specific TK induction. Expression and transport of these histones to the nucleus in CEF infected with the appropriate vaccinia virus recombinants could be detected with antisera raised against chick histone H5. In CEF cultivated for 3 days, interferon treatment that completely inhibited TK synthesis had no or only a marginal inhibitory effect on the expression of the histone genes. Inhibition of the expression of the histones could be detected under conditions of increased interferon sensitivity in aged CEF. The magnitude of inhibition was, however, less pronounced than the inhibition of viral TK synthesis. These data indicate that flanking vaccinia virus DNA regions confer interferon sensitivity to the expression of these histone genes, but that they contain structural information that partially exempts their expression from the inhibitory activity of the interferon-induced regulatory system.

Association of African swine fever virus with the cytoskeleton

The association of African swine fever virus (ASFV) with the cytoskeleton was investigated. Immunofluorescent studies of ASFV infected cells with anti-ASFV serum showed a temporal and spatial development of viral inclusions which moved from a peripheral to a perinuclear location and fused to give a single large perinuclear factory. The migration and fusion of viral inclusions was inhibited by colchicine suggesting a function for microtubules in assembly site organization not previously described. Accumulation of virions outside the inclusions and inhibition of viral release was also observed in colchicine treated cells. Viral antigens and structural elements were retained on the cytoskeleton fraction of Triton X-100 extracted cells. Reorganization of cytoskeletal elements around the assembly sites was demonstrated by transmission electronmicroscopy and by immunofluorescent studies using monoclonal antibodies against actin, tubulin and vimentin. Intermediate filaments accumulated around the viral factories, microtubules were greatly decreased in number and microfilaments were reorganized in association with the plasma membrane. Bundles of 15 nm tubules of unknown origin were also observed around the assembly sites. The distribution of viral proteins in soluble, cytoskeleton and detergent insoluble nuclear fractions was studied by pulse-chase experiments with [35S]methionine. SDS-PAGE analysis showed the presence in the cytoskeletal and nuclear fractions of 150, 72, 38, 28, 19 and 15 kDa virus structural proteins which increased after a 5 h chase. Our results indicate a close association of ASFV replication with the cytoskeleton similar to events described during FV3 replication but which differ from those occurring in poxvirus-infected cells.

Interaction of frog virus 3 with the cytomatrix. III. Role of microfilaments in virus release

The role of microfilaments in the release of frog virus 3 (FV3) from the plasma membrane was studied. Scanning electron microscopic study of FV3-infected baby hamster kidney (BHK) cells showed that late in infection (15 hr), numerous microvillus-like projections containing virions and microfilaments occur on the cell surface. Two microfilament-disrupting drugs, cytochalasin B and cytochalasin D, inhibited both the formation of microvillus-like projections and virus release. In the drug-treated cells, the virions accumulated in large numbers beneath the plasma membrane (transmission electron microscopy), suggesting that both drugs affected the release of the virus at the level of plasma membrane rather than the traverse of the virus to the plasma membrane. Two-dimensional gel analysis of actin from FV3-infected and uninfected cells revealed the following. There was no difference in the synthesis of actin in infected versus uninfected cells. However, the actin of infected cells is more acidic than its counterpart in uninfected cells. Temporally, the change in actin preceded the formation of microvilli-like projections involved in virus release. The change in actin is virus induced and is linked to virus maturation since a ts mutant of FV3 (ts9467), which is deficient in virus production at the restrictive temperature (30 degrees), did not modify actin. The mutant, at the permissive temperature (25 degrees), produced virions and altered the actin. Together, the above results attribute an active role for microfilaments in virus release.

Electron microscopic study on vaccinia virus release

FL cells infected with the IHD-W strain of vaccinia virus were studied by scanning and transmission electron microscopy. A large number of naked virus particles were found to accumulate beneath the host cell plasma membrane and to protrude from the cell surface. It was seen in some cases that naked viral particles were released by budding not only from the cell surface but also from the surface of cytoplasmic packets which were seen along the cell periphery.

Interaction of frog virus-3 with the cytoskeleton. I. Altered organization of microtubules, intermediate filaments, and microfilaments

The progressive cytoskeletal alterations of frog virus 3-infected baby hamster kidney (BHK) and fathead minnow (FHM) cells were studied by immunofluorescence and electron microscopy. The virus assembly sites, which contain viral genomes and viral proteins, were detected in the cytoplasm at 4 h (FHM) or 6 h (BHK) and mature virions appeared 2 h later. When infected cells were treated with Triton X-100, the assembly sites were found in association with the cytoskeleton. In infected cells, the number of microtubules progressively decreased but a few microtubules traversing in the vicinity of the assembly sites remained intact. Early in infection, the intermediate filaments retracted from the cell periphery, delimited the forming assembly sites, and remained there throughout infection. We suggest that intermediate filaments are involved in the formation of assembly sites. In addition, the filaments either by themselves or in conjunction with microtubules may anchor the assembly sites near the nucleus. The microfilament bundles (stress fibers) disappeared with the formation of assembly sites, and late in infection many projections containing microfilaments and virus particles appeared at the cell surface. The observation suggests a role for microfilaments in virus release. Taken together, these results provide the first example of a virus-infected cell in which all three cytoskeletal filaments show profound organizational changes and suggest an active participation of the host cytoskeleton in viral functions.

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

A phosphorylated vaccinia virus structural polypeptide of an apparent molecular weight of 11,000 (p11K) was isolated by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis and used for antibody induction. After purification by antigen affinity chromatography, the immunoglobulins detected only one target of a rather basic nature in two-dimensional immune blotting procedures of total virion proteins. By use of a combination of biological, biochemical, and microscopic techniques, p11K could be located on the surface of those vaccinia virus particles, with "classical" morphology and a buoyant density of 1.27 g/cm3. Upon immunoprecipitation from radioactively labeled infected cells, p11K appeared to be complexed to two additional virus structural proteins, which could be tentatively identified by their molecular weights as precursors for the two major core constituents. When virus assembly was inhibited by rifampin treatment of infected cells, a great part of p11K, either free or in complexed form, was found associated with actin-containing cytoskeletal elements. The ability of p11K to interact with a not-yet-identified, microfilament-associated cellular protein may be related to previous findings showing that assembled vaccinia particles in situ are found in connection with microfilaments. A possible role for the structures precipitated by p11K-specific antibodies in early stages of particle assembly is discussed.

Animal virus screens for potential teratogens. I. Poxvirus morphogenesis

The growth of poxvirions in cell culture is considered a teratogen screening test, since this virus has a rapid, simple morphogenetic pathway that is dependent upon cell proliferation. Vaccinia WR-infected BSC 40 monolayers were exposed to 42 known teratogens and 9 nonteratogens at dosages from 1 microM to 100 mM. After 24 h of infection, the number of functional virions was determined by plaque assay. Thirty-three of the 42 teratogens inhibited the virus, 3 teratogens stimulated the virus, and 6 teratogens were false-negatives. Eight of the 9 nonteratogens had no effect on virus proliferation at dosages as high as 600 times the lowest reported teratogenic dosage. The number of new virions could be directly related to the concentration of the teratogen in vitro, thus allowing each compound to be characterized by an RD50. The RD50 dosage in milligrams per liter was 98% correlated with the lowest reported teratogenic dose in vivo in milligrams per kilogram. In sum, vaccinia-infected cells have an easily identifiable endpoint, plaque-forming units, which may be an accurate prognosticator of teratogenesis.

Characterization of intracellular and extracellular vaccinia virus variants: N1-isonicotinoyl-N2-3-methyl-4-chlorobenzoylhydrazine interferes with cytoplasmic virus dissemination and release

Infectious vaccinia virus can be purified from whole cells by experimentally induced lysis (intracellular virus) or from supernatant growth medium (extracellular virus). Extracellular virus and intracellular virus differed by buoyant density (1.237 versus 1.272 g/cm3), phospholipid content and composition, and polypeptide pattern. Differences in structural polypeptides on the virus surface could be detected by lactoperoxidase-catalyzed radioiodination or Brij treatment. Characteristic of extracellular virus was an additional polypeptide, with a molecular weight of 37,000 (37K), which represented 5 to 7% of the total particle protein. Antibodies to the 37K protein detected only some of the cell-associated particles late in normal infection. Upon treatment of infected cultures with N1-isonicotinoyl-N2-3-methyl-4-chlorobenzoylhydrazine, a drug which prevents vaccinia virus release, no particle-associated 37K protein could be detected. In all other properties tested so far, except for a slight difference in phospholipid composition, the virus obtained in the presence of the drug resembled the normal intracellular virus. N1-Isonicotinoyl-N2-3-methyl-4-chlorobenzoylhydrazine prevented vesicularization of intracellular viral particles. Lack of vesicularization was accompanied by the absence of particle-associated 37K viral protein and seemed to correlate with an inhibition of virus dissemination to the cell periphery.