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

A turn-like structure "KKPE" segment mediates the specific binding of viral protein A27 to heparin and heparan sulfate on cell surfaces

Vaccinia viral envelope protein A27 (110 amino acids) specifically interacts with heparin (HP) or heparan sulfate (HS) proteoglycans for cell surface attachment. To examine the binding mechanism, a truncated soluble form of A27 (sA27-aa; residues 21-84 of A27) with Cys(71) and Cys(72) mutated to Ala was used as the parent molecule. sA27-aa consists of two structurally distinct domains, a flexible Arg/Lys-rich heparin-binding site (HBS) (residues 21-32; (21)STKAAKKPEAKR(32)) and a rigid coiled-coil domain (residues 43-84), both essential for the specific binding. As shown by surface plasmon resonance (SPR), the binding affinity of sA27-aa for HP (K(A) = 1.25 x 10(8) m(-1)) was approximately 3 orders of magnitude stronger than that for nonspecific binding, such as to chondroitin sulfate (K(A) = 1.65 x 10(5) m(-1)). Using site-directed mutagenesis of HBS and solution NMR, we identified a "KKPE" segment with a turn-like conformation that mediates specific HP binding. In addition, a double mutant T22K/A25K in which the KKPE segment remained intact showed an extremely high affinity for HP (K(A) = 1.9 x 10(11) m(-1)). Importantly, T22K/A25K retained the binding specificity for HP and HS but not chondroitin sulfate, as shown by in vitro SPR and in vivo cell adhesion and competitive binding assays. Molecular modeling of the HBS was performed by dynamics simulations and provides an explanation of the specific binding mechanism in good agreement with the site-directed mutagenesis and SPR results. We conclude that a turn-like structure introduced by the KKPE segment in vaccinia viral envelope protein A27 is responsible for its specific binding to HP and to HS on cell surfaces.

Disulfide bond formation at the C termini of vaccinia virus A26 and A27 proteins does not require viral redox enzymes and suppresses glycosaminoglycan-mediated cell fusion

Vaccinia virus A26 protein is an envelope protein of the intracellular mature virus (IMV) of vaccinia virus. A mutant A26 protein with a truncation of the 74 C-terminal amino acids was expressed in infected cells but failed to be incorporated into IMV (W. L. Chiu, C. L. Lin, M. H. Yang, D. L. Tzou, and W. Chang, J. Virol 81:2149-2157, 2007). Here, we demonstrate that A27 protein formed a protein complex with the full-length form but not with the truncated form of A26 protein in infected cells as well as in IMV. The formation of the A26-A27 protein complex occurred prior to virion assembly and did not require another A27-binding protein, A17 protein, in the infected cells. A26 protein contains six cysteine residues, and in vitro mutagenesis showed that Cys441 and Cys442 mediated intermolecular disulfide bonds with Cys71 and Cys72 of viral A27 protein, whereas Cys43 and Cys342 mediated intramolecular disulfide bonds. A26 and A27 proteins formed disulfide-linked complexes in transfected 293T cells, showing that the intermolecular disulfide bond formation did not depend on viral redox pathways. Finally, using cell fusion from within and fusion from without, we demonstrate that cell surface glycosaminoglycan is important for virus-cell fusion and that A26 protein, by forming complexes with A27 protein, partially suppresses fusion.

Investigation of orf virus structure and morphogenesis using recombinants expressing FLAG-tagged envelope structural proteins: evidence for wrapped virus particles and egress from infected cells

Orf virus (ORFV) is the type species of the genus Parapoxvirus, but little is known about the structure or morphogenesis of the virus. In contrast, the structure and morphogenesis of vaccinia virus (VACV) has been extensively studied. VACV has two main infectious forms, mature virion (MV) and extracellular virion (EV). The MV is wrapped by two additional membranes derived from the trans-Golgi to produce a wrapped virion (WV), the outermost of which is lost by cellular membrane fusion during viral egress to form the EV. Genome sequencing of ORFV has revealed that it has homologues of almost all of the VACV structural genes. Notable exceptions are A36R, K2L, A56R and B5R, which are associated with WV and EV envelopes. This study investigated the morphogenesis and structure of ORFV by fusing FLAG peptide to the structural proteins 10 kDa, F1L and ORF-110 to form recombinant viruses. 10 kDa and F1L are homologues of VACV A27L and H3L MV membrane proteins, whilst ORF-110 is homologous to VACV A34R, an EV membrane protein. Immunogold labelling of FLAG proteins on virus particles isolated from lysed cells showed that FLAG-F1L and FLAG-10 kDa were displayed on the surface of infectious particles, whereas ORF-110-FLAG could not be detected. Western blot analysis of solubilized recombinant ORF-110-FLAG particles revealed that ORF-110-FLAG was abundant and undergoes post-translational modification indicative of endoplasmic reticulum trafficking. Fluorescent microscopy confirmed the prediction that ORF-110-FLAG localized to the Golgi in virus-infected cells. Finally, immunogold labelling of EVs showed that ORF-110-FLAG became exposed on the surface of EV-like particles as a result of egress from the cell.

Engineering the vaccinia virus L1 protein for increased neutralizing antibody response after DNA immunization

Background

The licensed smallpox vaccine, comprised of infectious vaccinia virus, has associated adverse effects, particularly for immunocompromised individuals. Therefore, safer DNA and protein vaccines are being investigated. The L1 protein, a component of the mature virion membrane that is conserved in all sequenced poxviruses, is required for vaccinia virus entry into host cells and is a target for neutralizing antibody. When expressed by vaccinia virus, the unglycosylated, myristoylated L1 protein attaches to the viral membrane via a C-terminal transmembrane anchor without traversing the secretory pathway. The purpose of the present study was to investigate modifications of the gene expressing the L1 protein that would increase immunogenicity in mice when delivered by a gene gun.

Results

The L1 gene was codon modified for optimal expression in mammalian cells and potential N-glycosylation sites removed. Addition of a signal sequence to the N-terminus of L1 increased cell surface expression as shown by confocal microscopy and flow cytometry of transfected cells. Removal of the transmembrane domain led to secretion of L1 into the medium. Induction of binding and neutralizing antibodies in mice was enhanced by gene gun delivery of L1 containing the signal sequence with or without the transmembrane domain. Each L1 construct partially protected mice against weight loss caused by intranasal administration of vaccinia virus.

Conclusion

Modifications of the vaccinia virus L1 gene including codon optimization and addition of a signal sequence with or without deletion of the transmembrane domain can enhance the neutralizing antibody response of a DNA vaccine.

Vaccinia virus exhibits cell-type-dependent entry characteristics

Differing and sometimes conflicting data have been reported regarding several aspects of vaccinia virus (VV) entry. To address this, we used a beta-galactosidase reporter virus to monitor virus entry into multiple cell types under varying conditions. Entry into HeLa, B78H1 and L cells was strongly inhibited by heparin whereas entry into Vero and BSC-1 cells was unaffected. Bafilomycin also exhibited variable and cell-type-specific effects on VV entry. Entry into B78H1 and BSC-1 cells was strongly inhibited by bafilomycin whereas entry into Vero and HeLa cells was only partially inhibited suggesting the co-existence of both pH-dependent and pH-independent VV entry pathways in these cell types. Finally, entry into HeLa, B78H1, L and BSC-1 cells exhibited a lag of 6-9 min whereas this delay was undetectable in Vero cells. Our results suggest that VV exploits multiple cell attachment and entry pathways allowing it to infect a broad range of cells.

Vaccinia virus L1 binds to cell surfaces and blocks virus entry independently of glycosaminoglycans

L1 and A28 are vaccinia virus (VACV) envelope proteins which are essential for cellular entry. However, their specific roles during entry are unknown. We tested whether one or both of these proteins might serve as receptor binding proteins (RBP). We found that a soluble, truncated form of L1, but not A28, bound to cell surfaces independently of glycosaminoglycans (GAGs). Hence, VACV A28 is not likely to be a RBP and functions after attachment during entry. Importantly, soluble L1 inhibited both binding and entry of VACV in GAG-deficient cells, suggesting that soluble L1 blocks entry at the binding step by competing with the virions for non-GAG receptors on cells. In contrast, soluble A27, a VACV protein which attaches to GAGs but is non-essential for virus entry, inhibited binding and entry of VACV in a GAG-dependent manner. To our knowledge, this is the first report of a VACV envelope protein that blocks virus binding and entry independently of GAGs.

The vaccinia virus gene I2L encodes a membrane protein with an essential role in virion entry

The previously unstudied vaccinia virus gene I2L is conserved in all orthopoxviruses. We show here that the 8-kDa I2 protein is expressed at late times of infection, is tightly associated with membranes, and is encapsidated in mature virions. We have generated a recombinant virus in which I2 expression is dependent upon the inclusion of tetracycline in the culture medium. In the absence of I2, the biochemical events of the viral life cycle progress normally, and virion morphogenesis culminates in the production of mature virions. However, these virions show an approximately 400-fold reduction in specific infectivity due to an inability to enter target cells. Several proteins that have been previously identified as components of an essential entry/fusion complex are present at reduced levels in I2-deficient virions, although other membrane proteins, core proteins, and DNA are encapsidated at normal levels. A preliminary structure/function analysis of I2 has been performed using a transient complementation assay: the C-terminal hydrophobic domain is essential for protein stability, and several regions within the N-terminal hydrophilic domain are essential for biological competency. I2 is thus yet another component of the poxvirus virion that is essential for the complex process of entry into target cells.

A novel cellular protein, VPEF, facilitates vaccinia virus penetration into HeLa cells through fluid phase endocytosis

Vaccinia virus is a large DNA virus that infects many cell cultures in vitro and animal species in vivo. Although it has been used widely as a vaccine, its cell entry pathway remains unclear. In this study, we showed that vaccinia virus intracellular mature virions bound to the filopodia of HeLa cells and moved toward the cell body and entered the cell through an endocytic route that required a dynamin-mediated pathway but not a clathrin- or caveola-mediated pathway. Moreover, virus penetration required a novel cellular protein, vaccinia virus penetration factor (VPEF). VPEF was detected on cell surface lipid rafts and on vesicle-like structures in the cytoplasm. Both vaccinia virus and dextran transiently colocalized with VPEF, and, importantly, knockdown of VPEF expression blocked vaccinia virus penetration as well as intracellular transport of dextran, suggesting that VPEF mediates vaccinia virus entry through a fluid uptake endocytosis process in HeLa cells. Intracellular VPEF-containing vesicles did not colocalize with Rab5a or caveolin but partially colocalized with Rab11, supporting the idea that VPEF plays a role in vesicle trafficking and recycling in HeLa cells. In summary, this study characterized the mechanism by which vaccinia virus enters HeLa cells and identified a cellular factor, VPEF, that is exploited by vaccinia virus for cell entry through fluid phase endocytosis.

Vaccinia virus entry, exit, and interaction with differentiated human airway epithelia

Variola virus, the causative agent of smallpox, enters and exits the host via the respiratory route. To better understand the pathogenesis of poxvirus infection and its interaction with respiratory epithelia, we used vaccinia virus and examined its interaction with primary cultures of well-differentiated human airway epithelia. We found that vaccinia virus preferentially infected the epithelia through the basolateral membrane and released viral progeny across the apical membrane. Despite infection and virus production, epithelia retained tight junctions, transepithelial electrical conductance, and a steep transepithelial concentration gradient of virus, indicating integrity of the epithelial barrier. In fact, during the first four days of infection, epithelial height and cell number increased. These morphological changes and maintenance of epithelial integrity required vaccinia virus growth factor, which was released basolaterally, where it activated epidermal growth factor 1 receptors. These data suggest a complex interaction between the virus and differentiated airway epithelia; the virus preferentially enters the cells basolaterally, exits apically, and maintains epithelial integrity by stimulating growth factor receptors.

Vaccinia virus 4c (A26L) protein on intracellular mature virus binds to the extracellular cellular matrix laminin

Vaccinia virus intracellular mature virus (IMV) binds to glycosaminoglycans (GAGs) on cells via three virion proteins, H3L, A27L, and D8L. In this study, we demonstrated that binding of IMV to BSC40 cells was competitively inhibited by soluble laminin but not by fibronectin or collagen V, suggesting that this cell surface extracellular matrix (ECM) protein may play a role in vaccinia virus entry. Moreover, IMV infection of GAG(-) sog9 cells was also inhibited by laminin, demonstrating that virion binding to laminin does not involve a prior interaction with GAGs. Furthermore, comparative envelope protein analyses of wild-type vaccinia virus strain Western Reserve, which binds to laminin, and of a mutant virus, IA27L, which does not, showed that the A26L open reading frame (ORF), encoding an envelope protein, was mutated in IA27L, resulting in A26L being absent from the IMV. Expression of the wild-type A26L ORF in IA27L resulted in laminin binding activity. Moreover, recombinant A26L protein bound to laminin in vitro with a high affinity, providing direct evidence that A26L is the laminin binding protein on IMV. In summary, these results reveal a novel role for the vaccinia viral envelope protein A26L in binding to the ECM protein laminin, an association that is proposed to facilitate IMV entry.

Vaccinia virus G9 protein is an essential component of the poxvirus entry-fusion complex

The vaccinia virus G9R gene (VACWR087) encodes a protein of 340 amino acids with the following structural features that are conserved in all poxviruses: a site for N-terminal myristoylation, 14 cysteines, and a C-terminal transmembrane domain. Previous studies showed that G9 is one of eight proteins associated in a putative entry-fusion complex. Our attempt to isolate a mutant without the G9R gene was unsuccessful, suggesting that it is essential for virus replication. To further investigate its role, we constructed a recombinant vaccinia virus in which G9R is regulated by addition of an inducer. Induced G9 protein was associated with mature infectious virions and could be labeled with a membrane-impermeant biotinylation reagent, indicating surface exposure. Omission of inducer reduced the infectious-virus yield by about 1.5 logs; nevertheless, all stages of virus morphogenesis appeared normal and extracellular virions were present on the cell surface. Purified virions assembled without inducer had a specific infectivity of less than 5% of the normal level and a comparably small amount of G9, whereas their overall polypeptide composition, including other components of the entry-fusion complex, was similar to that of virions made in the presence of inducer or of wild-type virions. G9-deficient virions bound to cells, but penetration of cores into the cytoplasm and early viral RNA synthesis were barely detected, and cell-cell fusion was not triggered by low pH. Of the identified components of the multiprotein complex, G9 is the sixth that has been shown to be required for entry and membrane fusion.

Identification of linear heparin-binding peptides derived from human respiratory syncytial virus fusion glycoprotein that inhibit infectivity

It has been shown previously that the fusion glycoprotein of human respiratory syncytial virus (RSV-F) interacts with cellular heparan sulfate. Synthetic overlapping peptides derived from the F-protein sequence of RSV subtype A (strain A2) were tested for their ability to bind heparin using heparin-agarose affinity chromatography (HAAC). This evaluation identified 15 peptides representing eight linear heparin-binding domains (HBDs) located within F1 and F2 and spanning the protease cleavage activation site. All peptides bound to Vero and A549 cells, and binding was inhibited by soluble heparins and diminished by either enzymatic treatment to remove cell surface glycosaminoglycans or by treatment with sodium chlorate to decrease cellular sulfation. RSV-F HBD peptides were less likely to bind to glycosaminoglycan-deficient CHO-745 cells than parental CHO-K1 cells that express these molecules. Three RSV-F HBD peptides (F16, F26, and F55) inhibited virus infectivity; two of these peptides (F16 and F55) inhibited binding of virus to Vero cells, while the third (F26) did not. These studies provided evidence that two of the linear HBDs mapped by peptides F16 and F55 may mediate one of the first steps in the attachment of virus to cells while the third, F26, inhibited infectivity at a postattachment step, suggesting that interactions with cell surface glycosaminoglycans may play a role in infectivity of some RSV strains.

The envelope G3L protein is essential for entry of vaccinia virus into host cells

The vaccinia virus G3L/WR079 gene encodes a conserved protein with a predicted transmembrane domain. Our proteomic analyses of vaccinia virus revealed that G3L protein is incorporated into intracellular mature virus; however, the function of G3L protein in the vaccinia virus life cycle has not been investigated. In this study, a recombinant vaccinia virus, viG3L, expressing G3L protein under IPTG (isopropyl-beta-d-thiogalactopyranoside) regulation was constructed. Under permissive conditions when G3L protein was expressed, the vaccinia virus life cycle proceeded normally, resulting in plaque formation in BSC40 cells. In contrast, under nonpermissive conditions when G3L protein expression was repressed, no plaques were formed, showing that G3L protein is essential for vaccinia virus growth in cell cultures. In infected cells when G3L protein was not expressed, the formation of intracellular mature virus (IMV) and cell-associated enveloped virus occurred normally, showing that G3L protein is not required for virion morphogenesis. IMV particles containing (G3L(+)) or lacking (G3L(-)) G3L protein were purified and were found to be indistinguishable on microscopic examination. Both G3L(+) and G3L(-) IMV bound to HeLa cells; however, G3L(-) IMV failed to enter the cells, showing that G3L protein is required for IMV penetration into cells. Finally, G3L protein was required for fusion of the infected cells under low-pH treatment. Thus, our results provide direct evidence that G3L is an essential component of the vaccinia virus fusion complex, in addition to the previously reported A28, H2, L5, A21, and A16 proteins.

In a nutshell: structure and assembly of the vaccinia virion

Poxviruses comprise a large family of viruses characterized by a large, linear dsDNA genome, a cytoplasmic site of replication and a complex virion morphology. The most notorious member of the poxvirus family is variola, the causative agent of smallpox. The laboratory prototype virus used for the study of poxviruses is vaccinia, the virus that was used as a live, naturally attenuated vaccine for the eradication of smallpox. Both the morphogenesis and structure of poxvirus virions are unique among viruses. Poxvirus virions apparently lack any of the symmetry features common to other viruses such as helical or icosahedral capsids or nucleocapsids. Instead poxvirus virions appear as "brick shaped" or "ovoid" membrane-bound particles with a complex internal structure featuring a walled, biconcave core flanked by "lateral bodies." The virion assembly pathway involves a remarkable fabrication of membrane-containing crescents and immature virions, which evolve into mature virions in a process that is unparalleled in virology. As a result of significant advances in poxvirus genetics and molecular biology during the past 15 years, we can now positively identify over 70 specific gene products contained in poxvirus virions, and we can describe the effects of mutations in over 50 specific genes on poxvirus assembly. This review summarizes these advances and attempts to assemble them into a comprehensible and thoughtful picture of poxvirus structure and assembly.

Poxvirus entry and membrane fusion

The study of poxvirus entry and membrane fusion has been invigorated by new biochemical and microscopic findings that lead to the following conclusions: (1) the surface of the mature virion (MV), whether isolated from an infected cell or by disruption of the membrane wrapper of an extracellular virion, is comprised of a single lipid membrane embedded with non-glycosylated viral proteins; (2) the MV membrane fuses with the cell membrane, allowing the core to enter the cytoplasm and initiate gene expression; (3) fusion is mediated by a newly recognized group of viral protein components of the MV membrane, which are conserved in all members of the poxvirus family; (4) the latter MV entry/fusion proteins are required for cell to cell spread necessitating the disruption of the membrane wrapper of extracellular virions prior to fusion; and furthermore (5) the same group of MV entry/fusion proteins are required for virus-induced cell-cell fusion. Future research priorities include delineation of the roles of individual entry/fusion proteins and identification of cell receptors.

The product of the vaccinia virus L5R gene is a fourth membrane protein encoded by all poxviruses that is required for cell entry and cell-cell fusion

The L5R gene of vaccinia virus is conserved among all sequenced members of the Poxviridae but has no predicted function or recognized nonpoxvirus homolog. Here we provide the initial characterization of the L5 protein. L5 is expressed following DNA replication with kinetics typical of a viral late protein, contains a single intramolecular disulfide bond formed by the virus-encoded cytoplasmic redox pathway, and is incorporated into intracellular mature virus particles, where it is exposed on the membrane surface. To determine whether L5 is essential for virus replication, we constructed a mutant that synthesizes L5 only in the presence of an inducer. The mutant exhibited a conditional-lethal phenotype, as cell-to-cell virus spread and formation of infectious progeny were dependent on the inducer. Nevertheless, all stages of replication occurred in the absence of inducer and intracellular and extracellular progeny virions appeared morphologically normal. Noninfectious virions lacking L5 could bind to cells, but the cores did not enter the cytoplasm. In addition, virions lacking L5 were unable to mediate low-pH-triggered cell-cell fusion from within or without. The phenotype of the L5R conditional lethal mutant is identical to that of recently described mutants in which expression of the A21, A28, and H2 genes is repressed. Thus, L5 is the fourth component of the poxvirus cell entry/fusion apparatus that is required for entry of both the intracellular and extracellular infectious forms of vaccinia virus.

Vaccinia virus A21 virion membrane protein is required for cell entry and fusion

We provide the initial characterization of the product of the vaccinia virus A21L (VACWR140) gene and demonstrate that it is required for cell entry and low pH-triggered membrane fusion. The A21L open reading frame, which is conserved in all sequenced members of the poxvirus family, encodes a protein of 117 amino acids with an N-terminal hydrophobic domain and four invariant cysteines. Expression of the A21 protein occurred at late times of infection and was dependent on viral DNA replication. The A21 protein contained two intramolecular disulfide bonds, the formation of which required the vaccinia virus-encoded cytoplasmic redox pathway, and was localized on the surface of the lipoprotein membrane of intracellular mature virions. A conditional lethal mutant, in which A21L gene expression was regulated by isopropyl-beta-d-thiogalactopyranoside, was constructed. In the absence of inducer, cell-to-cell spread of virus did not occur, despite the formation of morphologically normal intracellular virions and extracellular virions with actin tails. Purified virions lacking A21 were able to bind to cells, but cores did not penetrate into the cytoplasm and synthesize viral RNA. In addition, virions lacking A21 were unable to mediate low pH-triggered cell-cell fusion. The A21 protein, like the A28 and H2 proteins, is an essential component of the poxvirus entry/fusion apparatus for both intracellular and extracellular virus particles.

The oligomeric structure of vaccinia viral envelope protein A27L is essential for binding to heparin and heparan sulfates on cell surfaces: a structural and functional approach using site-specific mutagenesis

The soluble domain of the self-assembly vaccinia virus envelope protein A27L, sA27L-aa, consists of a flexible extended coil at the N terminus and a rigid hydrophobic coiled-coil region at the C terminus. In the former, a basic strip of 12 residues is responsible for binding to cell-surface heparan sulfates. Although the latter is believed to mediate self-assembly, its biological role is unclear. However, an in vitro bioassay showed that peptides comprising the 12 residue basic region alone failed to interact with heparin, suggesting that the C-terminal coiled-coil region might serve an indispensable role in biological function. To explore this structural and functional relationship, we performed site-specific mutagenesis in an attempt to specifically disrupt the hydrophobic core of the coiled coil. Three single mutants, L47A, L51A, and L54A, and one triple mutant, L47,51,54A, were expressed and purified from Escherichia coli. The physical properties of the mutants were carefully examined by gel-filtration chromatography, CD, and NMR spectroscopy, and the biological activities were assessed by an in vitro SPR bioassay and three in vivo bioassays: binding to cells, blocking virus infection and blocking cell fusion. We showed that the L47A mutant, which is similar to the parental sA27L-aa in forming a hexamer, is biologically active. L51A and L54A mutants form tetramers and are less active. Notably, in the triple mutant, the self-assembly hydrophobic core structure is uncoiled; as a consequence, the tetrameric structure is biologically inactive. Thus, we conclude that the leucine residues, in particular Leu51 and Leu54, sustain the hydrophobic core structure that is essential for the biological function of vaccinia virus envelope protein A27L, binding to cell-surface heparan sulfate.

Vaccinia virus H2 protein is an essential component of a complex involved in virus entry and cell-cell fusion

The vaccinia virus H2R gene (VACWR 100) is conserved in all sequenced members of the poxvirus family and encodes a protein with a predicted transmembrane domain and four invariant cysteines. A recombinant vaccinia virus, in which expression of the H2 protein is stringently regulated, was unable to replicate without inducer. However, under nonpermissive conditions, all stages of virus morphogenesis appeared normal and extracellular virions were detected at the tips of actin tails. Nevertheless, virus did not spread to neighboring cells nor did syncytia form after low-pH treatment. Purified -H2 and +H2 virions from cells infected in the absence or presence of inducer, respectively, were indistinguishable in microscopic appearance and contained the same complement of major proteins, though only +H2 virions were infectious. The -H2 virions bound to cells, but their cores did not penetrate into the cytoplasm. In addition, exogenously added -H2 virions were unable to mediate the formation of syncytia after low-pH treatment. In contrast, virions lacking the A27 (p14) protein, which was previously considered to have an essential role in fusion, penetrated cells and induced extensive syncytia. The properties of H2, however, are very similar to those recently reported for the A28 protein. Moreover, coimmunoprecipitation experiments indicated an interaction between H2 and A28. Therefore, H2 and A28 are the only proteins presently known to be specifically required for vaccinia virus entry and are likely components of a fusion complex.

Entry of the vaccinia virus intracellular mature virion and its interactions with glycosaminoglycans

Vaccinia virus (VACV) produces two distinct enveloped virions, the intracellular mature virus (IMV) and the extracellular enveloped virus (EEV), but the entry mechanism of neither virion is understood. Here, the binding and entry of IMV particles have been investigated. The cell receptors for IMV are unknown, but it was proposed that IMV can bind to glycosaminoglycans (GAGs) on the cell surface and three IMV surface proteins have been implicated in this. In this study, the effect of soluble GAGs on IMV infectivity was reinvestigated and it was demonstrated that GAGs affected IMV infectivity partially in some cells, but not at all in others. Therefore, binding of IMV to GAGs is cell type-specific and not essential for IMV entry. By using electron microscopy, it is demonstrated that IMV from strains Western Reserve and modified virus Ankara enter cells by fusion with the plasma membrane. After an IMV particle bound to the cell, the IMV membrane fused with the plasma membrane and released the virus core into the cytoplasm. IMV surface antigen became incorporated into the plasma membrane and was not left outside the cell, as claimed in previous studies. Continuity between the IMV membrane and the plasma membrane was confirmed by tilt-series analysis to orientate membranes perpendicularly to the beam of the electron microscope. This analysis shows unequivocally that IMV is surrounded by a single lipid membrane and enters by fusion at the cell surface.

Poxvirus tropism

Despite the success of the WHO-led smallpox eradication programme a quarter of a century ago, there remains considerable fear that variola virus, or other related pathogenic poxviruses such as monkeypox, could re-emerge and spread disease in the human population. Even today, we are still mostly ignorant about why most poxvirus infections of vertebrate hosts show strict species specificity, or how zoonotic poxvirus infections occur when poxviruses occasionally leap into novel host species. Poxvirus tropism at the cellular level seems to be regulated by intracellular events downstream of virus binding and entry, rather than at the level of specific host receptors as is the case for many other viruses. This review summarizes our current understanding of poxvirus tropism and host range, and discusses the prospects of exploiting host-restricted poxvirus vectors for vaccines, gene therapy or tissue-targeted oncolytic viral therapies for the treatment of human cancers.

Poxvirus host range varies markedly ? some viruses, such as variola and molluscum contagiosum virus (both of which are human-specific), exhibit strict species tropism, whereas others such as cowpox virus are able to infect multiple host species.

Members of four of the eight genera of chordopoxviruses can zoonotically infect man. For example, monkeypox virus can cause severe smallpox-like disease in humans that clinically resembles variola virus.

The species tropism that is exhibited by many poxviruses in terms of causing disease is frequently quite different from the range of cultured cells that can be infected by these viruses.

Specific host-cell receptors do not mediate the distinction between cells that are permissive as opposed to non-permissive for poxvirus infection. Rather, restrictive host cells fail to support the full replication cycle of the infecting poxvirus at a point downstream of binding and entry.

A variety of poxviral host-range genes have been identified that contribute to the control of permissive versus non-permissive infection of cultured mammalian cells. The gene products of these host-range genes regulate the ability of the virus to complete its cytoplasmic replication cycle.

The development of host-restricted vaccines, like modified vaccinia Ankara (MVA), that do not replicate in humans but that retain potent immunogenicity, will provide safer platforms for recombinant vaccines.

Another advance has been the development of poxvirus-based oncolytic vectors that replicate preferentially in human tumour cells.

Heparin binding activity of orf virus F1L protein

The orf virus is the type species of the Parapoxvirus genus and is the causative agent of contagious echtyma, a debilitating skin disease of sheep and goats, which can also affect man. The virus exhibits a restricted host range, even if it has been shown to bind to a wide range of tissues of non-permissive species. This ability is an argument for its potential use as an expression vector. Since most mammalian cell types express heparan sulfate (HS) surface receptors, we assumed that HS could serve as receptors to mediate orf virus binding. In this study, we showed that orf virus is inhibited by the addition of soluble heparin in cell cultures. Affinity chomatography using heparin agarose demonstrated that orf virus F1L is the major heparin binding protein. Furthermore, the recombinant F1L protein was visualised on the cell surface by confocal microscopy, and rabbits immunised with recombinant F1L protein produced virus neutralising antibodies. These results confirm that the F1L immunodominant protein is also involved in virus binding to cells as for the vaccinia homologue H3L protein. Heparin also inhibited the binding of the F1L protein to cells showing that this protein has a role in the early stages of infection.

Vaccinia virus penetration requires cholesterol and results in specific viral envelope proteins associated with lipid rafts

Vaccinia virus infects a wide variety of mammalian cells from different hosts, but the mechanism of virus entry is not clearly defined. The mature intracellular vaccinia virus contains several envelope proteins mediating virion adsorption to cell surface glycosaminoglycans; however, it is not known how the bound virions initiate virion penetration into cells. For this study, we investigated the importance of plasma membrane lipid rafts in the mature intracellular vaccinia virus infection process by using biochemical and fluorescence imaging techniques. A raft-disrupting drug, methyl-beta-cyclodextrin, inhibited vaccinia virus uncoating without affecting virion attachment, indicating that cholesterol-containing lipid rafts are essential for virion penetration into mammalian cells. To provide direct evidence of a virus and lipid raft association, we isolated detergent-insoluble glycolipid-enriched membranes from cells immediately after virus infection and demonstrated that several viral envelope proteins, A14, A17L, and D8L, were present in the cell membrane lipid raft fractions, whereas the envelope H3L protein was not. Such an association did not occur after virions attached to cells at 4 degrees C and was only observed when virion penetration occurred at 37 degrees C. Immunofluorescence microscopy also revealed that cell surface staining of viral envelope proteins was colocalized with GM1, a lipid raft marker on the plasma membrane, consistent with biochemical analyses. Finally, mutant viruses lacking the H3L, D8L, or A27L protein remained associated with lipid rafts, indicating that the initial attachment of vaccinia virions through glycosaminoglycans is not required for lipid raft formation.

Appearance of the bona fide spiral tubule of ORF virus is dependent on an intact 10-kilodalton viral protein

Parapoxviruses can be morphologically distinguished from other poxviruses in conventional negative staining electron microscopy (EM) by their ovoid appearance and the spiral tubule surrounding the virion's surface. However, this technique may introduce artifacts. We have examined Orf virus (ORFV; the prototype species of the Parapoxvirus genus) by cryoelectron microscopy (cryo-EM) and cryo-negative staining EM. From these studies we suggest that the shape and unique spiral tubule are authentic features of the parapoxviruses. We also constructed an ORFV mutant deleted of a gene encoding a 10-kDa protein, which is an orthologue of the vaccinia virus (VACV) 14-kDa fusion protein, and investigated its ultrastructure. This mutant virus multiplied slowly in permissive cells and produced infectious but morphologically aberrant particles. Mutant virions lacked the spiral tubule but displayed short disorganized tubules similar to those observed on the surface of VACV. In addition, thin extensions or loop-like structures were appended to the ORFV mutant particles. We suggest that these appended structures arise from a failure of the mutant virus particles to properly seal and that the sealing activity is dependent on the 10-kDa protein.

Vaccinia virus entry into cells is dependent on a virion surface protein encoded by the A28L gene

The A28L gene of vaccinia virus is conserved in all poxviruses and encodes a protein that is anchored to the surface of infectious intracellular mature virions (IMV) and consequently lies beneath the additional envelope of extracellular virions. A conditional lethal recombinant vaccinia virus, vA28-HAi, with an inducible A28L gene, undergoes a single round of replication in the absence of inducer, producing IMV, as well as extracellular virions with actin tails, but fails to infect neighboring cells. We show here that purified A28-deficient IMV appeared to be indistinguishable from wild-type IMV and were competent to synthesize RNA in vitro. Nevertheless, A28-deficient virions did not induce cytopathic effects, express early genes, or initiate a productive infection. Although A28-deficient IMV bound to the surface of cells, their cores did not penetrate into the cytoplasm. An associated defect in membrane fusion was demonstrated by the failure of low pH to trigger syncytium formation when cells were infected with vA28-HAi in the absence of inducer (fusion from within) or when cells were incubated with a high multiplicity of A28-deficient virions (fusion from without). The correlation between the entry block and the inability of A28-deficient virions to mediate fusion provided compelling evidence for a relationship between these events. Because repression of A28 inhibited cell-to-cell spread, which is mediated by extracellular virions, all forms of vaccinia virus regardless of their outer coat must use a common A28-dependent mechanism of cell penetration. Furthermore, since A28 is conserved, all poxviruses are likely to penetrate cells in a similar way.

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.

Role of heparan sulfate for attachment and entry of tick-borne encephalitis virus

Attachment of the flavivirus tick-borne encephalitis (TBE) virus to different permissive cell lines was investigated by a newly established quantitative assay using fluorescence-labeled virus. Previous work had shown that BHK-21 cell-adapted mutants of TBE virus had acquired potential heparan sulfate (HS) binding sites on the outer surface of protein E. Quantitative analysis of one of these mutants indicated that it attached to HS-expressing cell lines with a 10- to 13-fold higher affinity than wild-type TBE virus strain Neudoerfl. CHO cells deficient in HS synthesis bound less than 5% of the amount of wild-type or mutant virus that could attach to HS-containing CHO cells but were nevertheless found to be highly susceptible to infection with both viruses. Thus, even though HS is a major determinant of TBE virus attachment on HS-expressing cells, our findings suggest the existence of an alternative host cell receptor that is less abundant than HS.

Vaccinia virus J1R protein: a viral membrane protein that is essential for virion morphogenesis

Vaccinia virus, a member of the poxvirus family, contains a conserved J1R open reading frame that encodes a late protein of 17.8 kDa. The 18-kDa J1R protein is associated mainly with the membrane fraction of intracellular mature virus particles. This study examines the biological function of J1R protein in the vaccinia virus life cycle. A recombinant vaccinia virus was constructed to conditionally express J1R protein in an isopropyl-beta-D-galactopyranoside (IPTG)-inducible manner. When J1R is not expressed during vaccinia virus infection, the virus titer is reduced approximately 100-fold. In contrast, J1R protein is not required for viral gene expression, as indicated by protein pulse-labeling. J1R protein is also not required for DNA processing, as the resolution of the concatemer junctions of replicated viral DNA was detected without IPTG. A deficiency of J1R protein caused a severe delay in the processing of p4a and p4b into mature core proteins 4a and 4b, indicating that J1R protein participates in virion morphogenesis. Infected cells grown in the absence of IPTG contained very few intracellular mature virions in the cytoplasm, and enlarged viroplasm structures accumulated with viral crescents attached at the periphery. Abundant intermediate membrane structures of abnormal shapes were observed, and many immature virions were either empty or partially filled, indicating that J1R protein is important for DNA packaging into immature virions. J1R protein also coimmunoprecipited with A45R protein in infected cells. In summary, these results indicate that vaccinia virus J1R is a membrane protein that is required for virus growth and plaque formation. J1R protein interacts with A45R protein and performs an important role during immature virion formation in cultured cells.

Identification and characterization of three immunodominant structural proteins of fowlpox virus

Genes encoding fowlpox virus (FWPV) structural proteins have been identified mainly by sequence homology with those from vaccinia virus (VACV), but little is known about the encoded proteins. Production of monoclonal antibodies (MAbs) against Poxine and HP1-440 (Munich) clone FP9 allowed the identification of three immunodominant FWPV proteins: the 39-kDa core protein (encoded by FPV168, homologous to VACV A4L), a 30- and 35-kDa protein doublet, and an abundant 63-kDa protein. The 30- and 35-kDa proteins are nonglycosylated, antigenically related proteins present in the intracellular mature virus membrane and localizing closely with the viral factories. N-terminal sequencing identified the 35-kDa protein as encoded by FPV140 (the FWPV homolog of VACV H3L). The 63-kDa protein forms covalently linked dimers and oligomers. It remained mainly insoluble upon detergent treatment of purified virus but did not localize closely with the viral factory. N-terminal sequencing was unsuccessful, suggesting N-terminal blocking. CNBr digestion generated a peptide encoded by FPV191, predicted to encode one of two FWPV A-type inclusion (ATI) proteins. The characteristics of the 63-kDa protein were inconsistent with published observations on cowpox or VACV ATI proteins (it appears to be essential). The 63-kDa protein, however, shares characteristics with both VACV p4c virus occlusion and 14-kDa fusion proteins. Gene assignment at the poxvirus ATI locus (between VACV A24R and A28L) is complicated by sequence redundancies and variations, often due to deletions and multiple frameshift mutations. The identity of FPV191 in relation to genes at this locus is discussed.

Structural analysis of the extracellular domain of vaccinia virus envelope protein, A27L, by NMR and CD spectroscopy

This study presents the molecular structure of the extracellular domain of vaccinia virus envelope protein, A27L, determined by NMR and CD spectroscopy. A recombinant protein, eA27L-aa, containing this domain in which cysteines 71 and 72 were replaced with alanine, was constructed to prevent self-assembly due to intermolecular disulfide bonds between these two cysteines. The soluble eA27L-aa protein forms an oligomer resembling that of A27L on vaccinia virions. Heteronuclear correlation NMR spectroscopy was carried out on eA27L-aa in the presence or absence of urea to determine backbone resonance assignments. Chemical shift index (CSI) propensity analysis showed that eA27L-aa has two distinct structural domains, a relatively flexible 22-amino acid random coil in the N-terminal region and a fairly rigid alpha-helix structure in the remainder of the structure. Binding interaction studies using isothermal titration calorimetry suggest that a 12-amino acid lysine/arginine-rich segment in the N-terminal region is responsible for glycosaminoglycan binding. The rigid alpha-helix portion of eA27L-aa is probably involved in the intrinsic self-assembly, and CSI propensity analysis suggests that region N37-E49, with a residual alpha-helix tendency, is probably the self-assembly core. Self-assembly was ascribed to three hydrophobic leucine residues (Leu(41), Leu(45), and Leu(48)) in this segment. The folding mechanism of eA27L-aa was analyzed by CD spectroscopy, which revealed a two-step transition with a Gibbs free energy of 2.5 kcal/mol in the absence of urea. Based on these NMR and CD studies, a residue-specific molecular model of the extracellular domain of A27L is proposed. These studies on the molecular structure of eA27L-aa will help in understanding how vaccinia virus enters cells.

Glycosaminoglycan Binding Properties of the Myxoma Virus CC-chemokine Inhibitor, M-T1

Poxviruses encode a number of secreted virulence factors that function to mitigate or modulate the host immune response. M-T1 is a secreted 43-kDa glycoprotein produced by the myxoma virus, a poxvirus pathogen of rabbits, that binds CC-chemokines with high affinity, blocks binding to their cognate G-protein coupled receptors, and thereby inhibits chemokine-induced leukocyte chemotaxis. The present study indicates that M-T1, but not the related vaccinia virus 35-kDa CC-chemokine-binding protein, can localize to cell surfaces through an interaction with glycosaminoglycan molecules. In addition to biochemically characterizing the nature of this interaction, we demonstrate that M-T1 can also simultaneously interact with CC-chemokines while bound to heparin, suggesting that the binding sites on M-T1 for chemokines and heparin are distinct. Furthermore, using recombinant baculovirus-expressed M-T1 truncation and internal deletion mutants, we localize the heparin-binding region of M-T1 to the C terminus of the protein, a region that contains a high abundance of basic residues and includes two clusters of basic amino acid residues that resemble Cardin and Weintraub heparin-binding consensus sequences. The ability of M-T1 to simultaneously interact with chemokines and glycosaminoglycans may enable M-T1 to tether to endothelial surfaces or extracellular matrix and capture host chemokines that are expressed close to sites of virus infection.

The fusion domain of HIV gp41 interacts specifically with heparan sulfate on the T-lymphocyte cell surface

Studies of the interaction of the 16 residue fusion peptide domain of human immunodeficiency virus glycoprotein gp41 (gp41(FD)) with T lymphocytes are outlined. Fluorescence measurements of changes in the electrostatic surface and dipole potentials of the plasma membrane following the interaction with gp41(FD) are described. The results show that gp41(FD) interacts with heparan sulfate located on the cell surface. This interaction is blocked by interleukin-8 and abolished by pre-treating the cells with heparitinase. The specificity of the reaction was also assessed by observations that soluble heparan sulfate competes with the cell membrane interaction whereas soluble heparin (at the levels utilized) does not. Following binding to heparan sulfate, the interaction with the membrane seems to take place in a cooperative manner with the formation of gp41(FD) trimers. In simpler phospholipid membranes, however, a trimeric complex does not appear to be the dominant mode of interaction. Finally, by repeating some of these studies within an imaging regime, it appears that the gp41(FD)-T-cell interaction takes place within specific domains on the cell surface to similarly localized heparan sulfate moieties.

Glycosaminoglycan sulfation requirements for respiratory syncytial virus infection

Glycosaminoglycans (GAGs) on the surface of cultured cells are important in the first step of efficient respiratory syncytial virus (RSV) infection. We evaluated the importance of sulfation, the major biosynthetic modification of GAGs, using an improved recombinant green fluorescent protein-expressing RSV (rgRSV) to assay infection. Pretreatment of HEp-2 cells with 50 mM sodium chlorate, a selective inhibitor of sulfation, for 48 h prior to inoculation reduced the efficiency of rgRSV infection to 40%. Infection of a CHO mutant cell line deficient in N-sulfation was three times less efficient than infection of the parental CHO cell line, indicating that N-sulfation is important. In contrast, infection of a cell line deficient in 2-O-sulfation was as efficient as infection of the parental cell line, indicating that 2-O-sulfation is not required for RSV infection. Incubating RSV with the purified soluble heparin, the prototype GAG, before inoculation had previously been shown to neutralize its infectivity. Here we tested chemically modified heparin chains that lack their N-, C6-O-, or C2-O-sulfate groups. Only heparin chains lacking the N-sulfate group lost the ability to neutralize infection, confirming that N-sulfation, but not C6-O- or C2-O-sulfation, is important for RSV infection. Analysis of heparin fragments identified the 10-saccharide chain as the minimum size that can neutralize RSV infectivity. Taken together, these results show that, while sulfate modification is important for the ability of GAGs to mediate RSV infection, only certain sulfate groups are required. This specificity indicates that the role of cell surface GAGs in RSV infection is not based on a simple charge interaction between the virus and sulfate groups but instead involves a specific GAG structural configuration that includes N-sulfate and a minimum of 10 saccharide subunits. These elements, in addition to iduronic acid demonstrated previously (L. K. Hallak, P. L. Collins, W. Knudson, and M. E. Peeples, Virology 271:264-275, 2000), partially define cell surface molecules important for RSV infection of cultured cells.

Iduronic acid-containing glycosaminoglycans on target cells are required for efficient respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is an important human respiratory pathogen, particularly in infants. Glycosaminoglycans (GAGs) have been implicated in the initiation of RSV infection of cultured cells, but it is not clear what type of GAGs and GAG components are involved, whether the important GAGs are on the virus or the cell, or what the magnitude is of their contribution to infection. We constructed and rescued a recombinant green fluorescent protein (GFP)-expressing RSV (rgRSV) and used this virus to develop a sensitive system to assess and quantify infection by flow cytometry. Evaluation of a panel of mutant Chinese hamster ovary cell lines that are genetically deficient in various aspects of GAG synthesis showed that infection was reduced up to 80% depending on the type of GAG deficiency. Enzymatic removal of heparan sulfate and/or chondroitin sulfate from the surface of HEp-2 cells also reduced infection, and the removal of both reduced infection even further. Blocking experiments in which RSV was preincubated with various soluble GAGs revealed the relative blocking order of: heparin > heparan sulfate > chondroitin sulfate B. Iduronic acid is a component common to these GAGs. GAGs that do not contain iduronic acid, namely, chondroitin sulfate A and C and hyaluronic acid, did not inhibit infection. A role for iduronic acid-containing GAGs in RSV infection was confirmed by the ability of basic fibroblast growth factor to block infection, because basic fibroblast growth factor binds to GAGs containing iduronic acid. Pretreatment of cells with protamine sulfate, which binds and blocks GAGs, also reduced infection. In these examples, infection was reduced by pretreatment of the virus with soluble GAGs, pretreatment of the cells with GAG-binding molecules, pretreatment of the cells with GAG-destroying enzymes or in cells genetically deficient in GAGs. These results establish that the GAGs involved in RSV infection are present on the cell rather than on the virus particle. Thus, the presence of cell surface GAGs containing iduronic acid, like heparan sulfate and chondroitin sulfate B, is required for efficient RSV infection in cell culture.

Vaccinia virus envelope H3L protein binds to cell surface heparan sulfate and is important for intracellular mature virion morphogenesis and virus infection in vitro and in vivo

An immunodominant antigen, p35, is expressed on the envelope of intracellular mature virions (IMV) of vaccinia virus. p35 is encoded by the viral late gene H3L, but its role in the virus life cycle is not known. This report demonstrates that soluble H3L protein binds to heparan sulfate on the cell surface and competes with the binding of vaccinia virus, indicating a role for H3L protein in IMV adsorption to mammalian cells. A mutant virus defective in expression of H3L (H3L(-)) was constructed; the mutant virus has a small plaque phenotype and 10-fold lower IMV and extracellular enveloped virion titers than the wild-type virus. Virion morphogenesis is severely blocked and intermediate viral structures such as viral factories and crescents accumulate in cells infected with the H3L(-) mutant virus. IMV from the H3L(-) mutant virus are somewhat altered and less infectious than wild-type virions. However, cells infected by the mutant virus form multinucleated syncytia after low pH treatment, suggesting that H3L protein is not required for cell fusion. Mice inoculated intranasally with wild-type virus show high mortality and severe weight loss, whereas mice infected with H3L(-) mutant virus survive and recover faster, indicating that inactivation of the H3L gene attenuates virus virulence in vivo. In summary, these data indicate that H3L protein mediates vaccinia virus adsorption to cell surface heparan sulfate and is important for vaccinia virus infection in vitro and in vivo. In addition, H3L protein plays a role in virion assembly.

Large-plaque mutants of Sindbis virus show reduced binding to heparan sulfate, heightened viremia, and slower clearance from the circulation

Laboratory strains of Sindbis virus must bind to the negatively charged glycosaminoglycan heparan sulfate in order to efficiently infect cultured cells. During infection of mice, however, we have frequently observed the development of large-plaque viral mutants with a reduced ability to bind to heparan sulfate. Sequencing of these mutants revealed changes of positively charged amino acids in putative heparin-binding domains of the E2 glycoprotein. Recombinant viruses were constructed with these changes as single amino acid substitutions in a strain Toto 1101 background. All exhibited decreased binding to heparan sulfate and had larger plaques than Toto 1101. When injected subcutaneously into neonatal mice, large-plaque viruses produced higher-titer viremia and often caused higher mortality. Because circulating heparin-binding proteins are known to be rapidly sequestered by tissue heparan sulfate, we measured the kinetics of viral clearance following intravenous injection. Much of the parental small-plaque Toto 1101 strain of Sindbis virus was cleared from the circulation by the liver within minutes, in contrast to recombinant large-plaque viruses, which had longer circulating half-lives. These findings indicate that a decreased ability to bind to heparan sulfate allows more efficient viral production in vivo, which may in turn lead to increased mortality. Because Sindbis virus is only one of a growing number of viruses from many families which have been shown to bind to heparan sulfate, these results may be generally applicable to the pathogenesis of such viruses.

Use of chemokine receptors by poxviruses

Chemokine receptors serve as portals of entry for certain intracellular pathogens, most notably human immunodeficiency virus (HIV). Myxoma virus is a member of the poxvirus family that induces a lethal systemic disease in rabbits, but no poxvirus receptor has ever been defined. Rodent fibroblasts (3T3) that cannot be infected with myxoma virus could be made fully permissive for myxoma virus infection by expression of any one of several human chemokine receptors, including CCR1, CCR5, and CXCR4. Conversely, infection of 3T3-CCR5 cells can be inhibited by RANTES, anti-CCR5 polyclonal antibody, or herbimycin A but not by monoclonal antibodies that block HIV-1 infection or by pertussis toxin. These findings suggest that poxviruses, like HIV, are able to use chemokine receptors to infect specific cell subtypes, notably migratory leukocytes, but that their mechanisms of receptor interactions are distinct.

Identification of functional domains in the 14-kilodalton envelope protein (A27L) of vaccinia virus

The mechanism of entry of vaccinia virus (VV) into cells is still a poorly understood process. A 14-kDa protein (encoded by the A27L gene) in the envelope of intracellular mature virus (IMV) has been implicated in virus-cell attachment, virus-cell fusion, and virus release from cells. We have previously described the structural organization of the VV 14-kDa protein, consisting of a triple-stranded coiled-coil region responsible for oligomer formation and a predicted Leu zipper-like third alpha helix with an important role in the interaction with a 21-kDa membrane protein (encoded by the A17L gene) thought to anchor the 14-kDa protein to the envelope of IMV (M.-I. Vázquez, G. Rivas, D. Cregut, L. Serrano, and M. Esteban, J. Virol. 72:10126-10137, 1998). To identify the functional domains important for virus entry and release, we have generated VV recombinants containing a copy of the A27L gene regulated by the lacI operator-repressor system of Escherichia coli (VVIndA27L) in the thymidine kinase locus and a mutant form of the A27L gene in the hemagglutinin locus but expressed constitutively under the control of an early-late VV promoter. Cells infected with a VV recombinant that expresses a mutant 14-kDa form lacking the first 29 amino acids at the N terminus failed to form extracellular enveloped virus (EEV). Fusion-from-without assays with purified virus confirmed that the fusion process was mediated by the 14-kDa protein and the fusion domain to be contained within amino acids 29 to 43 of the N-terminal region. Competitive inhibition of the infection process with soluble heparin and synthetic peptides and in vitro experiments with purified mutant proteins identified the heparin binding domain within amino acids 21 to 33, suggesting that this domain is involved in virus-cell binding via heparan sulfate. Thus, the N terminus of the 14-kDa protein contains a heparin binding domain, a fusion domain, and a domain responsible for interacting with proteins or lipids in the Golgi stacks for EEV formation and virus spread.

Vaccinia virus envelope D8L protein binds to cell surface chondroitin sulfate and mediates the adsorption of intracellular mature virions to cells

We previously showed that an envelope A27L protein of intracellular mature virions (IMV) of vaccinia virus binds to cell surface heparan sulfate during virus infection. In the present study we identified another viral envelope protein, D8L, that binds to chondroitin sulfate on cells. Soluble D8L protein interferes with the adsorption of wild-type vaccinia virions to cells, indicating a role in virus entry. To explore the interaction of cell surface glycosaminoglycans and vaccinia virus, we generated mutant viruses from a control virus, WR32-7/Ind14K (A27L(+) D8L(+)) to be defective in expression of either the A27L or the D8L gene (A27L(+) D8L(-) or A27L(-) D8L(+)) or both (A27L(-) D8L(-)). The A27L(+) D8L(+) and A27L(-) D8L(+) mutants grew well in BSC40 cells, consistent with previous observations. However, the IMV titers of A27L(+) D8L(-) and A27L(-) D8L(-) viruses in BSC40 cells were reduced, reaching only 10% of the level for the control virus. The data suggested an important role for D8L protein in WR32-7/Ind14K virus growth in cell cultures. A27L protein, on the other hand, could not complement the functions of D8L protein. The low titers of the A27L(+) D8L(-) and A27L(-) D8L(-) mutant viruses were not due to defects in the morphogenesis of IMV, and the mutant virions demonstrated a brick shape similar to that of the control virions. Furthermore, the infectivities of the A27L(+) D8L(-) and A27L(-) D8L(-) mutant virions were 6 to 10% of that of the A27L(+) D8L(+) control virus. Virion binding assays revealed that A27L(+) D8L(-) and A27L(-) D8L(-) mutant virions bound less well to BSC40 cells, indicating that binding of viral D8L protein to cell surface chondroitin sulfate could be important for vaccinia virus entry.