Complete sequences of the glycoproteins and M RNA of Punta Toro phlebovirus compared to those of Rift Valley fever virus

A Look into Bunyavirales Genomes: Functions of Non-Structural (NS) Proteins

In 2016, the Bunyavirales order was established by the International Committee on Taxonomy of Viruses (ICTV) to incorporate the increasing number of related viruses across 13 viral families. While diverse, four of the families (Peribunyaviridae, Nairoviridae, Hantaviridae, and Phenuiviridae) contain known human pathogens and share a similar tri-segmented, negative-sense RNA genomic organization. In addition to the nucleoprotein and envelope glycoproteins encoded by the small and medium segments, respectively, many of the viruses in these families also encode for non-structural (NS) NSs and NSm proteins. The NSs of Phenuiviridae is the most extensively studied as a host interferon antagonist, functioning through a variety of mechanisms seen throughout the other three families. In addition, functions impacting cellular apoptosis, chromatin organization, and transcriptional activities, to name a few, are possessed by NSs across the families. Peribunyaviridae, Nairoviridae, and Phenuiviridae also encode an NSm, although less extensively studied than NSs, that has roles in antagonizing immune responses, promoting viral assembly and infectivity, and even maintenance of infection in host mosquito vectors. Overall, the similar and divergent roles of NS proteins of these human pathogenic Bunyavirales are of particular interest in understanding disease progression, viral pathogenesis, and developing strategies for interventions and treatments.

Structures of phlebovirus glycoprotein Gn and identification of a neutralizing antibody epitope

Severe fever with thrombocytopenia syndrome virus (SFTSV) and Rift Valley fever virus (RVFV) are two arthropod-borne phleboviruses in the Bunyaviridae family, which cause severe illness in humans and animals. Glycoprotein N (Gn) is one of the envelope proteins on the virus surface and is a major antigenic component. Despite its importance for virus entry and fusion, the molecular features of the phleboviruse Gn were unknown. Here, we present the crystal structures of the Gn head domain from both SFTSV and RVFV, which display a similar compact triangular shape overall, while the three subdomains (domains I, II, and III) making up the Gn head display different arrangements. Ten cysteines in the Gn stem region are conserved among phleboviruses, four of which are responsible for Gn dimerization, as revealed in this study, and they are highly conserved for all members in Bunyaviridae Therefore, we propose an anchoring mode on the viral surface. The complex structure of the SFTSV Gn head and human neutralizing antibody MAb 4-5 reveals that helices α6 in subdomain III is the key component for neutralization. Importantly, the structure indicates that domain III is an ideal region recognized by specific neutralizing antibodies, while domain II is probably recognized by broadly neutralizing antibodies. Collectively, Gn is a desirable vaccine target, and our data provide a molecular basis for the rational design of vaccines against the diseases caused by phleboviruses and a model for bunyavirus Gn embedding on the viral surface.

The Role of Phlebovirus Glycoproteins in Viral Entry, Assembly and Release

Bunyaviruses are enveloped viruses with a tripartite RNA genome that can pose a serious threat to animal and human health. Members of the Phlebovirus genus of the family Bunyaviridae are transmitted by mosquitos and ticks to humans and include highly pathogenic agents like Rift Valley fever virus (RVFV) and severe fever with thrombocytopenia syndrome virus (SFTSV) as well as viruses that do not cause disease in humans, like Uukuniemi virus (UUKV). Phleboviruses and other bunyaviruses use their envelope proteins, Gn and Gc, for entry into target cells and for assembly of progeny particles in infected cells. Thus, binding of Gn and Gc to cell surface factors promotes viral attachment and uptake into cells and exposure to endosomal low pH induces Gc-driven fusion of the viral and the vesicle membranes. Moreover, Gn and Gc facilitate virion incorporation of the viral genome via their intracellular domains and Gn and Gc interactions allow the formation of a highly ordered glycoprotein lattice on the virion surface. Studies conducted in the last decade provided important insights into the configuration of phlebovirus Gn and Gc proteins in the viral membrane, the cellular factors used by phleboviruses for entry and the mechanisms employed by phlebovirus Gc proteins for membrane fusion. Here, we will review our knowledge on the glycoprotein biogenesis and the role of Gn and Gc proteins in the phlebovirus replication cycle.

Discovery and characterisation of a new insect-specific bunyavirus from Culex mosquitoes captured in northern Australia

Insect-specific viruses belonging to significant arboviral families have recently been discovered. These viruses appear to be maintained within the insect population without the requirement for replication in a vertebrate host. Mosquitoes collected from Badu Island in the Torres Strait in 2003 were analysed for insect-specific viruses. A novel bunyavirus was isolated in high prevalence from Culex spp. The new virus, provisionally called Badu virus (BADUV), replicated in mosquito cells of both Culex and Aedes origin, but failed to replicate in vertebrate cells. Genomic sequencing revealed that the virus was distinct from sequenced bunyavirus isolates reported to date, but phylogenetically clustered most closely with recently discovered mosquito-borne, insect-specific bunyaviruses in the newly proposed Goukovirus genus. The detection of a functional furin cleavage motif upstream of the two glycoproteins in the M segment-encoded polyprotein suggests that BADUV may employ a unique strategy to process the virion glycoproteins.

Characterization of the Salehabad virus species complex of the genus Phlebovirus (Bunyaviridae)

Genomic and antigenic characterization of the Salehabad virus, a species of the genus Phlebovirus, and four other unclassified phleboviruses (Arbia, Adria, Arumowot and Odrenisrou) demonstrate a serological and genetic relation to one another and are distinct from the eight other recognized species within the genus Phlebovirus. We propose to incorporate these four unclassified viruses as part of the Salehabad species complex within the genus. The known geographical distribution for the members of this species group includes southern Europe, Central Asia and Africa.

Anti-nucleocapsid protein immune responses counteract pathogenic effects of Rift Valley fever virus infection in mice

The known virulence factor of Rift Valley fever virus (RVFV), the NSs protein, counteracts the antiviral effects of the type I interferon response. In this study we evaluated the expression of several genes in the liver and spleen involved in innate and adaptive immunity of mice immunized with a RVFV recombinant nucleocapsid protein (recNP) combined with Alhydrogel adjuvant and control animals after challenge with wild type RVFV. Mice immunized with recNP elicited an earlier IFNβ response after challenge compared to non-immunized controls. In the acute phase of liver infection in non-immunized mice there was a massive upregulation of type I and II interferon, accompanied by high viral titers, and the up- and downregulation of several genes involved in the activation of B- and T-cells, indicating that both humoral and cellular immunity is modulated during RVFV infection. Various genes involved in pro-inflammatory responses and with pro-apoptotic effects were strongly upregulated and anti-apoptotic genes were downregulated in liver of non-immunized mice. Expression of many genes involved in B- and T-cell immunity were downregulated in spleen of non-immunized mice but normal in immunized mice. A strong bias towards apoptosis and inflammation in non-immunized mice at an acute stage of liver infection associated with suppression of several genes involved in activation of humoral and cellular immunity in spleen, suggests that RVFV evades the host immune response in more ways than only by inhibition of type I interferon, and that immunopathology of the liver plays a crucial role in RVF disease progression.

Aguacate virus, a new antigenic complex of the genus Phlebovirus (family Bunyaviridae)

Genomic and antigenic characterization of Aguacate virus, a tentative species of the genus Phlebovirus, and three other unclassified viruses, Armero virus, Durania virus and Ixcanal virus, demonstrate a close relationship to one another. They are distinct from the other nine recognized species within the genus Phlebovirus. We propose to designate them as a new (tenth) serogroup or species (Aguacate virus) within the genus. The four viruses were all isolated from phlebotomine sandflies (Lutzomyia sp.) collected in Central and South America. Aguacate virus appears to be a natural reassortant and serves as one more example of the high frequency of reassortment in this genus.

Characterization of the Candiru antigenic complex (Bunyaviridae: Phlebovirus), a highly diverse and reassorting group of viruses affecting humans in tropical America

The genus Phlebovirus of the family Bunyaviridae consists of approximately 70 named viruses, currently assigned to nine serocomplexes (species) based on antigenic similarities. Sixteen other named viruses that show little serologic relationship to the nine recognized groups are also classified as tentative species in the genus. In an effort to develop a more precise classification system for phleboviruses, we are attempting to sequence most of the named viruses in the genus with the goal of clarifying their phylogenetic relationships. In this report, we describe the serologic and phylogenetic relationships of 13 viruses that were found to be members of the Candiru serocomplex; 6 of them cause disease in humans. Analysis of full genome sequences revealed branching inconsistencies that suggest five reassortment events, all involving the M segment, and thus appear to be natural reassortants. This high rate of reassortment illustrates the inaccuracy of a classification system based solely on antigenic relationships.

Characterization of Punta Toro S mRNA species and identification of an inverted complementary sequence in the intergenic region of Punta Toro phlebovirus ambisense S RNA that is involved in mRNA transcription termination

The transcription termination sites for the subgenomic N and NS(S) mRNA species coded by the 1904 nucleotide, ambisense S RNA species of Punta Toro (PT) phlebovirus (Bunyaviridae, Ihara et al., 1984) have been identified by Northern analyses using a series of synthetic oligodeoxynucleotides. For the viral-complementary N mRNA species the oligonucleotides that were used represented viral RNA sequences; for the viral-sense NS(S) mRNA species they represented viral-complementary sequences. The results have been confirmed by employing the same oligodeoxynucleotides to backcopy purified mRNA preparations in the presence of appropriate chain-terminating dideoxynucleotides. The results obtained have allowed the 3' termini of both mRNA species to be mapped to a common region of the viral S RNA between RNA residues 977 and 1017. Based on these results and the presence of short non-viral sequences at the 5' ends of the PT mRNA species (Ihara et al., 1985), the estimated sizes of the PT N and NS(S) mRNA species are of the order of 1000 and 900 nucleotides, respectively. Computer analysis of DNA sequences representing the centrally located intergenic region (i.e., residues 768-1127) revealed a long inverted complementary sequence corresponding to nucleotides 886-1092. The peak of this potential hairpin structure, residue 996, correlates to the region of the genome identified by Northern analyses to be involved in the termination of mRNA transcription. Neither the N nor NS(S) mRNA species appeared to be polyadenylated to the extent that is characteristic of most eukaryotic mRNA species, although from the indicated sequences the viral mRNA species contain 3' proximal regions that are rich in short stretches of adenylic acid residues. This is particularly true for the N mRNA species, permitting its purification by selective oligo(dT) chromatography. In vitro translation of purified N and NSS mRNA species by rabbit reticulocyte lysates resulted in the synthesis of proteins that were identified as the viral N and NS(S) species, respectively, by comparison with proteins obtained from PT virus infected Vero cells.

Identification of a novel C-terminal cleavage of Crimean-Congo hemorrhagic fever virus PreGN that leads to generation of an NSM protein

The structural glycoproteins of Crimean-Congo hemorrhagic fever virus (CCHFV; genus Nairovirus, family Bunyaviridae) are derived through endoproteolytic cleavage of a 1,684-amino-acid M RNA segment-encoded polyprotein. This polyprotein is cotranslationally cleaved into the PreGN and PreGC precursors, which are then cleaved by SKI-1 and a SKI-1-like protease to generate the N termini of GN and GC, respectively. However, the resulting polypeptide defined by the N termini of GN and GC is predicted to be larger (58 kDa) than mature GN (37 kDa). By analogy to the topologically similar M segment-encoded polyproteins of viruses in the Orthobunyavirus genus, the C-terminal region of PreGN that contains four predicted transmembrane domains may also contain a nonstructural protein, NSM. To characterize potential PreGN C-terminal cleavage events, a panel of epitope-tagged PreGN truncation and internal deletion mutants was developed. These constructs allowed for the identification of a C-terminal endoproteolytic cleavage within, or very proximal to, the second predicted transmembrane domain following the GN ectodomain and the subsequent generation of a C-terminal fragment. Pulse-chase experiments showed that PreGN C-terminal cleavage occurred shortly after synthesis of the precursor and prior to generation of the GN glycoprotein. The resulting fragment trafficked to the Golgi compartment, the site of virus assembly. Development of an antiserum specific to the second cytoplasmic loop of PreGN allowed detection of cell-associated NSM proteins derived from transient expression of the complete CCHFV M segment and also in the context of virus infection.

The S segment of Punta Toro virus (Bunyaviridae, Phlebovirus) is a major determinant of lethality in the Syrian hamster and codes for a type I interferon antagonist

Two strains of Punta Toro virus (PTV), isolated from febrile humans in Panama, cause a differential pathogenesis in Syrian hamsters, which could be a useful model for understanding the virulence characteristics and differential outcomes in other phleboviral infections such as Rift Valley fever virus. Genetic reassortants produced between the lethal Adames (A/A/A) and nonlethal Balliet (B/B/B) strains were used in this study to investigate viral genetic determinants for pathogenesis and lethality in the hamster model. The S segment was revealed to be a critical genome segment, determining lethality with log(10) 50% lethal doses for each PTV genotype as follows (L/M/S convention): A/A/A, <0.7; B/A/A, <0.7; A/B/A, 1.5; B/B/A, 2.2; B/A/B, 4.7; A/B/B, >4.7; A/A/B, >4.7; B/B/B, >4.7. In addition, the Adames strain inhibits the induction of alpha/beta interferon (IFN-alpha/beta) in vivo and in vitro and inhibits the activation of the IFN-beta promoter. Expression of the PTV Adames NSs protein, encoded by the S RNA segment, inhibited the virus-mediated induction of an IFN-beta promoter-driven reporter gene, suggesting that PTV NSs functions as a type I IFN antagonist. Taken together, these data indicate a mechanism of pathogenesis in which the suppression of the type I IFN response early during PTV infection leads to early and uncontrolled viral replication and, ultimately, hamster death. This study contributes to our understanding of Phlebovirus pathogenesis and identifies potential targets for immune modulation to increase host survival.

Bidirectional infection and release of Rift Valley fever virus in polarized epithelial cells

Rift Valley Fever (RVF) virus is an arbovirus and is responsible for large outbreaks of disease predominantly in sub-Saharan Africa. However, several aspects of RVF virus transmission, such as high viremia, multiple vector species, and broad host range, result in a pathogen with high likelihood of geographic spread. RVF virus infection in humans and livestock is characterized by broad dissemination of RVF virus antigens throughout the body. We sought insight into the high pathogenicity and broad tropism of this virus through a characterization of its interaction with polarized epithelial cells. Our results indicate that infection and release of RVF virus in polarized epithelial cells occurs at both apical and basolateral membranes and hence is bidirectional. Furthermore, our results indicate that RVF virus causes disruptions in both the microfilament and the microtubule networks. These disruptions may provide a mechanism for bidirectional release of RVF virions.

Completion of molecular characterization of Toscana phlebovirus genome: nucleotide sequence, coding strategy of M genomic segment and its amino acid sequence comparison to other phleboviruses

The M RNA segment of Toscana (TOS) phlebovirus was cloned and the complete nucleotide sequence determined. The M RNA segment is 4215 nucleotides in length, and it contains a single major open reading frame (ORF) in the viral-complementary sequence, between nucleotides 18 and 4034, which can encode for a polyprotein of 1339 amino acids (Mr 149 kDa). The viral segment is expressed via a unique mRNA containing 10-14 non-templated nucleotides at the 5' end and it is truncated at the 3' end by about 140 nucleotides in a purine-rich region. In M predicted amino acid sequences, several hydrophobic regions have been identified. They could function as a signal sequence or a transmembrane region for the different proteins. Comparison of the deduced amino acid sequence of M precursor product revealed 38, 36, and 25% identity and 58, 56, and 47% similarity with those of Rift Valley fever (RVF), Punta Toro (PT) and Unkuniemi (UUK) viruses, respectively. Residues conserved among the proteins are mainly located at the COOH-portion of the precursor, while the major divergence is in the NSm coding regions. Based on sequence comparison and similarity of hydropathic pattern of TOS M segment with other phleboviruses the N-termini of TOS GN and GC glycoproteins were placed at residues 297 and 936 of the precursor.

Viruses as model systems in cell biology

This chapter focuses on the contributions that studies with viruses have made to current concepts in cell biology. Among the important advantages that viruses provide in such studies is their structural and genetic simplicity. The chapter describes the methods for growth, assay, and purification of viruses and infection of cells by several viruses that have been widely utilized for studies of cellular processes. Most investigations of virus replication at the cellular level are carried out using animal cells in culture. For the events in individual cells to occur with a high level of synchrony, single cycle growth conditions are used. Cells are infected using a high multiplicity of infectious virus particles in a low volume of medium to enhance the efficiency of virus adsorption to cell surfaces. After the adsorption period, the residual inoculum is removed and replaced with an appropriate culture medium. During further incubation, each individual cell in the culture is at a similar temporal stage in the viral replication process. Therefore, experimental procedures carried out on the entire culture reflect the replicative events occurring within an individual cell. The length of a single cycle of virus growth can range from a few hours to several days, depending on the virus type.

The effect of neutralizing monoclonal antibodies on early events in Rift Valley fever virus infectivity

Monoclonal antibodies (mAb) were used to examine possible stages at which antibody-mediated neutralization of Rift Valley fever virus occurs, and to assess whether binding of antibody is dependent on viral protein structure in order that antibody recognition take place. Analysis of the structural properties of the antigenic determinants revealed that the neutralizing sites are highly conformation-dependent. None of the mAb prevented virus binding, suggesting that the epitopes they define are spatially separate from the site(s) responsible for virus attachment to the cellular receptor. The finding that many of the mAb also did not inhibit virus entry into the cell demonstrated that neutralization of RVFV infectivity by immune antibodies is not dependent on blocking at the early stages in the viral life cycle.

The rubella virus E2 and E1 spike glycoproteins are targeted to the Golgi complex

Rubella virus (RV) has been reported to bud from intracellular membranes in certain cell types. In this study the intracellular site of targeting of RV envelope E2 and E1 glycoproteins has been investigated in three different cell types (CHO, BHK-21 and Vero cells) transfected with a cDNA encoding the two glycoproteins. By indirect immunofluorescence, E2 and E1 were localized to the Golgi region of all three cell types, and their distribution was disrupted by treatment with BFA or nocodazole. Immunogold labeling demonstrated that E2 and E1 were localized to Golgi cisternae and indicated that the glycoproteins were distributed across the Golgi stack. Analysis of immunoprecipitates obtained from stably transfected CHO cells revealed that E2 and E1 become endo H resistant and undergo sialylation without being transported to the cell surface. Transport of RV glycoproteins to the Golgi complex was relatively slow (t1/2 = 60-90 min). Coprecipitation experiments indicated that E2 and E1 form a heterodimer in the RER. E1 was found to fold much more slowly than E2, suggesting that the delay in transport of the heterodimer to the Golgi may be due to the slow maturation of E1 in the ER. These results indicate that RV glycoproteins behave as integral membrane proteins of the Golgi complex and thus provide a useful model to study targeting and turnover of type I membrane proteins in this organelle.

Analysis of 3' and 5' ends of N and NSs messenger RNAs of Toscana Phlebovirus

The 5' and 3' ends of N and NSs mRNAs, transcribed from the S segment of Toscana Phlebovirus, were analyzed by oligonucleotide primer extension and S1 nuclease mapping procedures. The results showed that both mRNAs acquired, at their 5' end, approximately 9-15 nucleotides not present in the viral template, suggesting an initiation transcription mechanism similar to the one described for influenza virus. Furthermore, the 3' ends of the two mRNAs were located in a sequence motif conserved in the S segment of two other Phleboviruses, the Rift Valley Fever and Sandfly Fever Sicilian viruses. This finding suggests the possible involvement of this sequence in the mechanism of transcription termination.

Dugbe Nairovirus M RNA: nucleotide sequence and coding strategy

The coding assignments of the medium-sized (M) RNA segment of the Dugbe (DUG) virus (Nairovirus, Bunyaviridae) were investigated. The complete nucleotide sequence of 4888 nucleotides (nt) contained one long open reading frame in the viral complementary RNA, extending from an AUG start codon at nt 48-50 to a stop codon at nt 4701-4703 (numbered from the 5' terminus of vcRNA). Comparison of the terminal sequences with the ends of the DUG S segment revealed sequence identity between the first nine nucleotides of both segments. No sequence homologies were found with the M segments of other members of the Bunyaviridae, or with their polypeptide products. Expression of portions of the DUG M open reading frame in Escherichia coli demonstrated the carboxyl terminal region of the M open reading frame codes for the G1 structural glycoprotein, which is the target for neutralising antibodies. Confirmation of this assignment was obtained by sequencing the amino terminus of the G1 protein. Two nonstructural glycoproteins which share epitopes with G1 were identified in virus-infected cells, one of which (85 kDa) is processed over a period of several hours to produce G1. The G2 coding region was located upstream of the G1 sequence. The region between the carboxyl terminus of G2 and the 5' end of the long open reading frame apparently encodes a nonstructural protein of about 70 kDa, which is a precursor of the G2 protein.

Association of the nonstructural protein NSs of Uukuniemi virus with the 40S ribosomal subunit

The small RNA segment (S segment) of Uukuniemi (UUK) virus encodes two proteins, the nucleocapsid protein (N) and a nonstructural protein (NSs), by an ambisense strategy. The function of NSs has not been elucidated for any of the bunyaviruses expressing this protein. We have now expressed the N and NSs proteins in Sf9 insect cells by using the baculovirus expression system. High yields of both proteins were obtained. A monospecific antibody was raised against gel-purified NSs and used to study the synthesis and localization of the protein in UUK virus-infected BHK21 cells. While the N protein was detected as early as 4 h postinfection (p.i.), NSs was identified only after 8 h p.i. Both proteins were still synthesized at high levels at 24 h p.i. The half-life of NSs was about 1.5 h, while that of the N protein was several hours. Sucrose gradient fractionation of [35S]methionine-labeled detergent-solubilized extracts of infected BHK21 cells indicated that NSs was firmly associated with the 40S ribosomal subunit. This association took place shortly after translation and was partially resistant to 1 M NaCl. NSs expressed by using the T7 vaccinia virus expression system, as well as in vitro-translated NSs, was also associated with the 40S subunit. In contrast, in vitro-translated N protein was found on top of the gradient. Immunolocalization of NSs, in UUK virus-infected cells, by using an affinity-purified antibody showed a granular cytoplasmic staining. A very similar pattern was seen for cells expressing NSs from a cDNA copy by using a vaccinia virus expression system. No staining was observed in the nuclei in either case. Furthermore, NSs was found neither in virions nor in nucleocapsids isolated from infected cells. In vivo labeling with 32Pi indicated that NSs is not phosphorylated. The possible function of NSs is discussed in light of these results.

Assembly of G1 and G2 glycoprotein oligomers in Punta Toro virus-infected cells

We have studied the oligomerization of the membrane glycoproteins of Punta Toro virus (PTV), a member of the Phlebovirus genus of the family Bunyaviridae, and the effect of glycosylation on protein stability and transport. By using sucrose gradient centrifugation, the G1 and G2 glycoproteins in PTV-infected or recombinant-transfected cells were found to sediment as dimers after DSP cross-linking, suggesting that the G 1 and G2 proteins are associated as dimers by non-covalent interactions. Pulse-chase and two-dimensional gel analysis indicate that dimerization occurs between newly synthesized G1 and G2 proteins, and that a small fraction of the G2 proteins is assembled into G2 homodimers. The amounts of G1 and G2 proteins were substantially decreased, while the amounts of nucleocapsid protein remained nearly unchanged, when PTV-infected cells were treated with the glycosylation inhibitor tunicamycin, indicating that the G1 and G2 proteins are unstable if glycosylation is prevented.

Oligomerization, transport, and Golgi retention of Punta Toro virus glycoproteins

We have investigated the oligomerization and intracellular transport of the membrane glycoproteins of Punta Toro virus, a member of the Phlebovirus genus of the family Bunyaviridae, which is assembled by budding in the Golgi complex. By using one- or two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis, chemical cross-linking, and sucrose gradient centrifugation, we found that the majority of the G1 and G2 glycoproteins are assembled into noncovalently linked G1-G2 heterodimers. At the same time, a fraction of the G2 protein, possibly produced independently of the G1 protein, is assembled into G2 homodimers. Kinetic analysis indicates that heterodimerization occurs between newly synthesized G1 and G2 within 3 min after protein synthesis, and that the G1 and G2 glycoproteins are associated as dimeric forms both during transport and after accumulation in the Golgi complex. Analysis of a G1-truncated G2 mutant, which is also targeted to the Golgi complex, showed that these molecules also assemble into dimeric forms, which are linked by disulfide bonds. Both the G1-G2 heterodimer and the G2 homodimer were found to be able to exit from the endoplasmic reticulum. Differences in transport kinetics observed for the G1 and G2 proteins may be due to the differences in the transport efficiency between the G1-G2 heterodimer and the G2 homodimer from the endoplasmic reticulum to the Golgi complex. These and previous results (S.-Y. Chen, Y. Matsuoka, and R.W. Compans, Virology 183:351-365, 1991) suggest that Golgi retention of the G2 homodimer occurs by association with the G1-G2 heterodimer, whereas the Golgi targeting of the G1-G2 heterodimer occurs by a specific retention mechanism.

Host-derived 5' ends and overlapping complementary 3' ends of the two mRNAs transcribed from the ambisense S segment of Uukuniemi virus

Two mRNAs, coding for the N and NSS proteins, are transcribed from the small (S) Uukuniemi virus RNA segment by an ambisense strategy (J. F. Simons, U. Hellman, and R. F. Pettersson, J. Virol. 64:247-255, 1990). In this report, we describe the analysis of the 5' and 3' ends of the two mRNAs. Primer extension as well as cloning and sequencing of individual mRNAs showed that the 5' ends of both mRNAs contained nonviral sequences ranging from 7 to 25 residues in length (mean, 12 residues), indicating a cap-snatching mechanism similar to the one originally described for priming of influenza virus mRNA synthesis. In 35% of the cases, the first virion-specified nucleotide (an A residue) was substituted with a G residue. Between the translation termination codons of N and NSS, there is a 74-residue-long noncoding intergenic region (Simons et al., J. Virol. 64:247-255, 1990). Nuclease protection assays using both RNA and DNA hybridization probes showed that the 3' ends of the N and NSS mRNAs overlap each other by about 100 nucleotides. The 3' end of the NSS mRNA extends into the coding sequence of the N mRNA, whereas the N mRNA is terminated just prior to the stop codon of NSS. To our knowledge, this is the first example of overlapping complementary mRNAs in viruses with an ambisense coding strategy. No obvious transcription termination sequence was identified. However, because of a short palindromic sequence in the intergenic region, the 3' ends of both mRNAs (and consequently also the template RNAs) can be folded into an A/U-rich hairpin structure. It remains to be determined whether this structure plays any role in transcription termination.

Golgi complex localization of the Punta Toro virus G2 protein requires its association with the G1 protein

The glycoproteins of bunyaviruses accumulate in membranes of the Golgi complex, where virus maturation occurs by budding. In this study we have constructed a series of full length or truncated mutants of the G2 glycoprotein of Punta Toro virus (PTV), a member of the Phlebovirus genus of the Bunyaviridae, and investigated their transport properties. The results indicate that the hydrophobic domain preceding the G2 glycoprotein can function as a translocational signal peptide, and that the hydrophobic domain near the C-terminus serves as a membrane anchor. A G2 glycoprotein construct with an extra hydrophobic sequence derived from the N-terminal NSM region was stably retained in the ER, and was unable to be transported to the Golgi complex. The full-length G2 glycoprotein, when expressed on its own, was transported out of the ER and expressed on the cell surface, whereas the G1 and G2 proteins when expressed together are retained in the Golgi complex. A truncated anchor-minus form of the G2 glycoprotein was found to be secreted into the culture medium, but was retained in the Golgi complex when coexpressed with the G1 glycoprotein. These results indicate that the G2 membrane glycoprotein is a class I membrane protein which does not contain a signal sufficient for Golgi retention, and suggest that its Golgi localization is a result of association with the G1 glycoprotein.

Assembly and polarized release of Punta Toro virus and effects of brefeldin A

Punta Toro virus (PTV), a member of the sandfly fever group of bunyaviruses, is assembled by budding at intracellular membranes of the Golgi complex. We have examined PTV glycoprotein transport, assembly, and release and the effects of brefeldin A (BFA) on these processes. Both the G1 and G2 proteins were transported out of the endoplasmic reticulum (ER) and retained in the Golgi complex in a stable structure, either during PTV infection or when expressed from a vaccinia virus recombinant. BFA treatment causes a rapid and dramatic change in the distribution of the G1 and G2 proteins, from a Golgi pattern to an ER pattern. The G1 and G2 proteins were found to be modified by medial but not trans Golgi network enzymes, in the presence or absence of BFA. We found that BFA blocks PTV release from cells but does not interfere with the intracellular assembly of infectious virions. Further, the BFA block of virus release is fully reversible, with high levels of virus release occurring upon removal of the inhibitor. It was also found that the release of PTV virions is polarized, occurring exclusively from the basolateral surfaces of the polarized Vero C1008 epithelial cell line.

Characterization of baculovirus-expressed Rift Valley fever virus glycoproteins synthesized in insect cells

A cDNA corresponding to the complete coding region of the M RNA of the M12 mutant of Rift Valley fever virus (RVFV) strain ZH548 (K. Takehara, M-K. Min, J.K. Battles, K. Sugiyama, V.C. Emery, J.M. Dalrymple, and D.H.L. Bishop, Virology, 169, 452-457, 1989) has been inserted into the baculovirus transfer vector pAcYM1. By comparison with the parent RVFV, the M RNA of the M12 mutant has a new small open reading frame (ORF1) upstream of the one that initiates the precursor of the viral glycoproteins (ORF2, gene order: NS(M)-G2-G1). A derivative of the M12 cDNA was prepared from which most of the upstream sequences (including a polyT tract and ORF1) were removed. Other cDNA constructs were made from this derivative, constructs in which most of the G1 sequences were also removed, or most of the NS(M) coding sequences, or all of the NS(M) and most of G2 coding sequences. Each RVFV M cDNA construct was inserted into a pAcYM1 transfer vector and recombinant baculoviruses were produced (RVM1-5). The derived viruses were employed to study the expression and properties of the RVFV glycoproteins in Spodoptera frugiperda insect cells. For each recombinant virus evidence was obtained which indicated that the RVFV glycoproteins were produced and processed in the insect cells. Although four of the recombinants gave low expression levels of the RVFV glycoproteins, for the vector that made only the G1 product, the expression level was significantly higher. Immunofluorescence analyses established that the RVFV glycoproteins were present both at intracellular locations and on the surface of the recombinant baculovirus infected insect cells.

Identification of mutations in the M RNA of a candidate vaccine strain of Rift Valley fever virus

The M RNA species of a candidate vaccine strain of Rift Valley fever virus (RVFV ZH-548M12), derived by consecutive high level mutagenesis using 5-fluorouracil (H. Caplen, C. J. Peters, and D. H. L. Bishop, J. Gen. Virol., 66, 2271-2277, 1985), has been cloned and the cDNA sequenced. The data have been compared to those obtained for the parent virus strain RVFV ZH-548 as well as the previously published data for RVFV ZH-501 (M. S. Collett, A. F. Purchio, K. Keegan, S. Frazier, W. Hays, D. K. Anderson, M. D. Parker, C. Schmaljohn, J. Schmidt, and J. M. Dalrymple, Virology, 144, 228-245, 1985). Some eight nucleotide and three amino acid differences were identified between the M RNAs of ZH-501 and ZH-548. Between the M RNAs of ZH-548 and that of the M12 mutant there were 12 nucleotide and 7 amino acid changes. Unique to the mutant virus is a new AUG codon upstream of that which initiates the open reading frame of the RVFV M gene product (the viral glycoprotein precursor). The significance of this and other differences in the mutant RNA with regard to the derivation and potential attenuation of the candidate vaccine is discussed.

Computer analysis suggests a role for signal sequences in processing polyproteins of enveloped RNA viruses and as a mechanism of viral fusion

We have used a computer program to scan the entire sequence of viral polyproteins for eucaryotic signal sequences. The method is based on that of von Heijne (1). The program calculates a score for each residue in a polyprotein. The score indicates the resemblance of each residue to that at the cleavage site of a typical N-terminal eucaryotic signal sequence. The program correctly predicts the known N-terminal signal sequence cleavage sites of several cellular and viral proteins. The analysis demonstrates that the polyproteins of enveloped RNA viruses--including the alphaviruses, flaviviruses, and bunyaviruses--contain several internal signal-sequence-like regions. The predicted cleavage site in these internal sequences are often known cleavage sites for processing of the polyprotein and are amongst the highest scoring residues with this algorithm. These results indicate a role for the cellular enzyme signal peptidase in the processing of several viral polyproteins. Not all high-scoring residues are sites of cleavage, suggesting a difference between N-terminal and internal signal sequences. This may reflect the secondary structure of the latter. Signal sequences were also found at the N-termini of the fusion proteins of the paramyxoviruses and the retroviruses. This suggests a mechanism of viral fusion analogous to that by which proteins are translocated through the membranes of the endoplasmic reticulum at synthesis.

Intracellular accumulation of Punta Toro virus glycoproteins expressed from cloned cDNA

The Punta Toro virus (PTV) middle size (M) RNA encodes two glycoproteins, G1 and G2, and possibly a nonstructural protein, NSM. A partial cDNA clone of the M segment which contains G1 and G2 glycoprotein coding sequences but lacks most of the NSM sequences was inserted into the genome of vaccinia virus under the control of an early vaccinia promoter. Cells infected with the recombinant virus were found to synthesize two polypeptides with molecular weights of 65,000 (G1) and 55,000 (G2) that reacted specifically with antibody against PTV. Studies using indirect immunofluorescence microscopy revealed that these proteins accumulated intracellularly in the perinuclear region. The results of endoglycosidase H digestion of these glycoproteins suggested that both G1 and G2 glycoproteins were transported from the RER to the Golgi complex. These proteins were not chased out from the Golgi region during a 6-hr incubation in the presence of cycloheximide. Surface immune precipitation and 125I-protein A binding assays also demonstrated that the majority of the G1 and G2 glycoproteins are retained intracellularly. These results indicate that the PTV glycoproteins contain the necessary information for retention in the Golgi apparatus.

RNA genome stability of Toscana virus during serial transovarial transmission in the sandfly Phlebotomus perniciosus

We have carried out a T1 ribonuclease fingerprinting analysis of the RNA genomes of Toscana virus isolates from successive generations of an experimentally virus-infected laboratory colony of Phlebotomus perniciosus sandflies. This analysis detected no virus RNA genome changes during transovarial transmission of the virus over 12 sandfly generations (a period of almost 2 years). These results demonstrate that although RNA viruses can exhibit high rates of mutational change under a variety of conditions, Toscana virus RNA genomes can be maintained in a stable manner during repeated transovarial virus transmission in the natural insect host. The implications of these results for insect RNA virus evolution are discussed.

Nucleotide sequence of the M segment of Germiston virus: comparison of the M gene product of several bunyaviruses

The complete nucleotide sequence of the M RNA segment of Germiston bunyavirus was determined from plasmids containing overlapping M cDNA inserts. The M segment is 4534 nucleotides long and contains a 50-base-long inverted terminal repeat which can form a stable hydrogen-bonded secondary structure with a delta G of -45.8 kcal/mol. The RNA molecule complementary to viral RNA contains a single large open reading frame that encodes a 1437 amino acid-long protein with hydrophobic amino and carboxy terminal regions, which could represent signal and anchor sequences, respectively. It is presumed that this gene product is the polyprotein precursor to glycoproteins G1 and G2 and to the nonstructural polypeptide NSM. The nucleotide and amino acid sequences of the M RNA of Bunyamwera virus (prototype of the serogroup) and snowshow hare and La Crosse viruses (California serogroup) (Lees et al., 1986; Eshita and Bishop, 1984; Grady et al., 1987) were compared to those of Germiston virus. An overall amino acid sequence homology of 44% was found between Germiston and snowshoe hare viruses and of 61% between Germiston and Bunyamwera viruses. Most of the cysteines, three out of seven of the potential glycosylation sites, as well as the N and C terminal hydrophobic domains, are conserved between the four viruses.

Rift Valley fever virus M segment: cell-free transcription and translation of virus-complementary RNA

A cell-free system has been used to study gene expression of the M segment RNA of the Phlebovirus Rift Valley fever virus (RVFV). RVFV sequence-containing plasmids were used to synthesize M segment mRNA-like transcripts. These transcripts were then translated in vitro in the absence or presence of microsomal membranes. Cell-free translation of a transcript which closely resembled authentic M segment mRNA (RNA-7) yielded a primary translation product of 133 kilodaltons (kDa), the size expected of a polypeptide encompassing the entire open reading frame (ORF) of the M segment. When translations were conducted in the presence of microsomal membranes, this primary protein was cotranslationally processed to yield the two viral glycoproteins, G1 and G2, as well as proteins of 78, 21, and 14 kDa. With one exception, these in vitro processed polypeptides comigrated with M segment-encoded proteins found in RVFV-infected cell lysates. A polypeptide corresponding to the in vitro 21-kDa protein was not detected in vivo. To investigate translational initiation and processing of the protein products of the M segment, additional transcripts were generated in which varying portions of the amino-terminal "preglycoprotein" region of the M segment ORF were deleted. Translation results indicated that the 78- and 21-kDa proteins were initiated from the first methionine codon of the ORF, and the 14-kDa polypeptide began from the second in-phase ATG. These products and a major portion of the preglycoprotein region sequence were not required for the proper synthesis and processing of the viral glycoproteins in vitro. In light of these results, possible expression strategies used by this Phlebovirus M segment RNA are discussed.

Rift Valley fever virus M segment: use of recombinant vaccinia viruses to study Phlebovirus gene expression

Recombinant vaccinia viruses were constructed and used in conjunction with site-specific antisera to study the coding capacity and detailed expression strategy of the M segment of the Phlebovirus Rift Valley fever virus (RVFV). The M segment could be completely and faithfully expressed in recombinant RVFV-vaccinia virus-infected cells, the gene products apparently being correctly processed and modified in the absence of the RVFV L and S genomic segments. The proteins encoded by the RVFV M segment included, in addition to the viral glycoproteins G2 and G1, two previously uncharacterized polypeptides of 78 and 14 kilodaltons (kDa). By manipulation of RVFV sequences present in the recombinant vaccinia viruses and use of specific antibody reagents, it was found that the 78-kDa protein initiated at the first initiation codon of the open reading frame and encompassed the entire preglycoprotein and glycoprotein G2 coding sequences. The 14-kDa protein appeared to begin from the second in-phase ATG and was composed of only the preglycoprotein sequences. Both viral glycoproteins G2 and G1 could be synthesized and correctly processed in the absence of the 78- and 14-kDa proteins, as well as a large portion of the preglycoprotein sequences. However, the hydrophobic amino acid sequence immediately preceding the mature glycoprotein coding sequences was required for authentic glycoprotein production. The M-segment expression strategy involving aspects of translational initiation and protein processing are discussed. The functional roles of the 78- and 14-kDa proteins remain unclear.

Expression of the Hantaan virus M genome segment by using a vaccinia virus recombinant

A cDNA containing the complete open reading frame of the Hantaan virus (HTN) M genome segment has been cloned into vaccinia virus. This recombinant virus expresses two glycoproteins which are similar to the HTN structural glycoproteins, G1 and G2, in molecular weight, cleavage pattern, and cellular distribution. Both HTN and recombinant vaccinia virus glycoproteins are exclusively associated with the Golgi apparatus of the cell. Despite this intracellular restriction, mice inoculated with the recombinant vaccinia virus raised neutralizing antibodies against HTN. The specificity of virus neutralization appears to reside in the HTN glycoproteins, since a vaccinia virus recombinant expressing the HTN nucleocapsid protein was unable to elicit a neutralizing antibody response.

Complete nucleotide sequence of the M RNA segment of Uukuniemi virus encoding the membrane glycoproteins G1 and G2

We have determined the complete nucleotide sequence of the virion M RNA segment of Uukuniemi virus (Uukuvirus genus, Bunyaviridae) from cloned cDNA. The RNA that encodes the two membrane glycoproteins G1 and G2 is 3231 residues long (mol wt 1.1 X 10(6)). The 5' and 3' ends of the RNA are partially complementary to each other for some 30 bp, enabling the formation of a stable panhandle structure (delta G = -40 kcal/mol) and the circularization of the molecule. The extreme 5' and 3' terminal nucleotides are identical for 10 to 13 residues to those of the M RNA of Punta Toro and Rift Valley fever viruses, two members of the Phlebovirus genus. A single open reading frame comprising 1008 amino acid residues (mol wt 113,588) was found in the mRNA-sense strand between nucleotides 18 and 3042. This probably corresponds to the previously identified 110,000-Da precursor (p110) of G1 and G2. By comparing the partial aminoterminal sequences of purified G1 and G2 with the deduced protein sequence we confirmed that the gene order is NH2-G1-G2-COOH. Both mature G1 and G2 are preceded by a stretch of 17 predominantly hydrophobic amino acids likely to represent the signal sequences. At their COOH-terminal ends, G1 and G2 have a hydrophobic stretch of amino acids, 19 and 27 residues, respectively, that probably anchors the proteins to the lipid bilayer. The sequence indicates that mature G2 is 495 amino acids long (mol wt 54,869), whereas the exact size of G1 is unclear, since the location of the COOH-terminus of G1 is not known. An upper value of 479 amino acids (mol wt 55,181) can, however, be suggested. Both G1 and G2 contain four potential glycosylation sites for Asn-linked glycans and both are unusually rich in cysteines, 6.1% in G1 and 5.4% in G2. Comparison of the amino acid sequence of the M RNA product of Uukuniemi virus with that of Punta Toro and Rift Valley fever viruses showed in both cases a weak homology that was more pronounced for the proteins located at the COOH-terminal end of the precursor. This suggests a distant evolutionary relationship between the Phlebo- and Uukuvirus genera.

Hantaan virus M RNA: coding strategy, nucleotide sequence, and gene order

The M genome segment of Hantaan virus was molecularly cloned and the nucleotide sequence of cDNA was determined. The virion RNA is 3616 bases long with 3'- and 5'-terminal nucleotide sequences complementary for 18 bases. A single long open reading frame in the viral complementary-sense RNA had the potential to encode 1135 amino acids or a polypeptide of 126,000 Da. Amino-terminal sequences of isolated G1 and G2 envelope glycoproteins were determined, revealing a gene order with respect to message sense RNA of 5'-G1-G2-3'. Mature G1 begins 18 amino acids beyond the first AUG of the open reading frame, preceded by a short, hydrophobic leader sequence. G2 begins at the 649th amino acid of the open reading frame and also follows a hydrophobic sequence. Carboxy termini of G1 and G2 were localized and gene order was verified by immune precipitation of Hantaan proteins with antisera to synthetic peptides generated by using amino acid sequences derived from the cDNA sequence. The antipeptide sera were also reactive by immunoblotting with SDS-denatured G1 and G2. Molecular weights of 64,000 and 53,700 were calculated for the G1 and G2 glycoproteins, respectively, from their predicted amino acid sequences. Five potential asparagine-linked glycosylation sites were contained within the G1 amino acid sequence and two within the G2 sequence. These data are consistent with our previous estimates of the molecular weights and extent of glycosylation of the Hantaan envelope glycoproteins.

Messenger RNA of the M segment RNA of Rift Valley fever virus

The putative messenger RNA (mRNA) of the M segment RNA of the phlebovirus Rift Valley fever virus (RVFV) has been characterized using S1 nuclease mapping and oligonucleotide primer extension procedures. These experiments revealed that the 3' end of the mRNA lacks approximately 112 nucleotides of the M genomic RNA sequences, and that the 5' end of the mRNA possesses all of the sequences present at the 3' end of the M RNA but is further extended beyond the end of the genome by some 12-14 nucleotides of unknown origin. The implications of these data are discussed in relation to the replication and expression strategy of this virus.

Use of bacterial expression cloning to define the amino acid sequences of antigenic determinants on the G2 glycoprotein of Rift Valley fever virus

Four distinct antigenic determinants along the G2 glycoprotein encoded by the M segment RNA of the Phlebovirus Rift Valley fever virus were localized. These epitopes were defined by four monoclonal antibodies, three of which were capable of neutralizing virus infectivity; one was nonneutralizing. Immunoprecipitation by these monoclonal antibodies of either denatured or native antigen characterized the epitopes as having linear or higher order structure. Molecular cloning of G2 glycoprotein-coding sequences into a bacterial expression plasmid utilizing a beta-galactosidase fusion protein system was employed for epitope localization. A nuclease BAL 31 plasmid expression library, in which processive regions of the 3' end of the G2 glycoprotein coding sequences were deleted, allowed for approximation of the carboxy-terminal limit of the antigenic determinants. Further subcloning of limited G2 polypeptide sequences into the bacterial expression vector permitted more refined localization of the epitopes. The characteristics of the immunoreactivity of these small peptide regions (between 11 and 34 amino acids) produced in bacteria as G2-beta-galactosidase fusion proteins were similar to those of the authentic Rift Valley fever virus G2 glycoprotein. These defined antigenic determinants and their importance in virus infectivity are discussed.

Ambisense RNA genomes of arenaviruses and phleboviruses

This chapter reviews the evidence that shows that arenaviruses and members of one genus of the Bunyaviridae (phleboviruses) have some proteins coded in subgenomic, viral-sense mRNA species and other proteins coded in subgenomic, viral-complementary mRNA sequences. This unique feature is discussed in relation to the implications it has on the intracellular infection process and how such a coding arrangement may have evolved. The chapter presents a list of the known members of the arenaviridae, their origins, and the vertebrate hosts from which isolates have been reported. It discusses the structural components, the infection cycle, and genetic attributes of arenaviruses. In order to determine how arenaviruses code for gene products, the S RNA species of Pichinde virus and that of a viscerotropic strain of LCM virus (LCM-WE) have been cloned into DNA and sequenced. The arenavirus S RNA is described as having an ambisense strategy, to denote the fact that both viral and viral-complementary sequences are used to make gene products. The chapter discusses the infection cycle, the structural and genetic properties of bunyaviridae member.

Analyses of the mRNA transcription processes of Punta Toro phlebovirus (Bunyaviridae)

The time course of the syntheses of Punta Toro (PT) phlebovirus (Bunyaviridae) small (S)-size viral RNA (S vRNA), viral complementary RNA (S vcRNA), and messenger RNA (S mRNA) species has been analyzed using single-stranded DNA probes representing the two S-coded gene products. The data obtained support the conclusion that PT S RNA has an ambisense coding strategy (T. Ihara, H. Akashi, and D. H. L. Bishop, Virology 136, 293-306, 1984) with the viral nucleocapsid protein, N, encoded in a viral-complementary, subgenomic, mRNA species and a putative nonstructural protein, NSs, encoded in a viral-sense, subgenomic, second S mRNA species. In the absence of puromycin (or cycloheximide) full-length S vRNA, S vcRNA, and subgenomic N mRNA and putative NSs mRNA species were identified in PT virus-infected cell extracts. In the presence of inhibitors of protein synthesis (puromycin or cycloheximide) newly synthesized N mRNA species were detected, but not full-length S vcRNA, nor S vRNA, nor the S coded NSs mRNA species. The mRNA species recovered from drug-treated cells have been translated in vitro to synthesize viral N protein. Analyses of the 5' ends of the N and NSs mRNA species have shown them to be heterogeneous in sequence and some 11-18 bases longer than the ends of the genomic RNA species, indicating that they represent nonviral primer sequences like those identified for bunyavirus mRNA species (D. H. L. Bishop, M. E. Gay, and Y. Matsuoka, Nucleic Acids Res. 11, 6409-6418, 1983). The presence of such additional sequences on mRNA derived from representatives of two Bunyaviridae genera appears by these analyses to be a more conserved feature than the S RNA coding arrangement of the respective viruses.