Biochemical studies on the Phlebotomus fever group viruses (Bunyaviridae family)

Three-dimensional organization of Rift Valley fever virus revealed by cryoelectron tomography

Rift Valley fever virus (RVFV) is a member of the Bunyaviridae virus family (genus Phlebovirus) and is considered to be one of the most important pathogens in Africa, causing viral zoonoses in livestock and humans. Here, we report the characterization of the three-dimensional structural organization of RVFV vaccine strain MP-12 by cryoelectron tomography. Vitrified-hydrated virions were found to be spherical, with an average diameter of 100 nm. The virus glycoproteins formed cylindrical hollow spikes that clustered into distinct capsomeres. In contrast to previous assertions that RVFV is pleomorphic, the structure of RVFV MP-12 was found to be highly ordered. The three-dimensional map was resolved to a resolution of 6.1 nm, and capsomeres were observed to be arranged on the virus surface in an icosahedral lattice with clear T=12 quasisymmetry. All icosahedral symmetry axes were visible in self-rotation functions calculated using the Fourier transform of the RVFV MP-12 tomogram. To the best of our knowledge, a triangulation number of 12 had previously been reported only for Uukuniemi virus, a bunyavirus also within the Phlebovirus genus. The results presented in this study demonstrate that RVFV MP-12 possesses T=12 icosahedral symmetry and suggest that other members of the Phlebovirus genus, as well as of the Bunyaviridae family, may adopt icosahedral symmetry. Knowledge of the virus architecture may provide a structural template to develop vaccines and diagnostics, since no effective anti-RVFV treatments are available for human use.

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.

Topological mapping of antigenic sites on the Rift Valley fever virus envelope glycoproteins using monoclonal antibodies

A panel of 17 monoclonal antibodies (MAbs) to the G1 and G2 envelope glycoproteins of Rift Valley fever (RVF) virus were used to analyze the topography and functional properties of the viral antigenic sites. Four heterogeneous antigenic regions which may be interlinked were identified on the G1 protein and four distinct domains on the G2 protein by competitive binding assays. Comparison of the biological activities and epitope specificities of the MAbs against G1 showed that the antigenic domains I, II, and IV were involved in virus neutralization and haemagglutination at different potencies. For both the G1 and G2 proteins, determinants mapping to domain G1 Ia and G2 Ia were associated with very strong neutralization independent of complement (C'), suggesting that they represent biologically important areas. Domain G2 II was involved in haemagglutination and weak C' dependent neutralization while the other two G2 regions had no haemagglutination function and neutralized to a low level only in the presence of C'. Epitopes Ia and IIb on G1 and Ia and IIa on G2 were also associated with protection of mice against virulent RVFV infection, indicating that both envelope glycoproteins play an important role in RVF viral infection and pathogenesis.

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.

The S RNA segment of Sandfly Fever Sicilian virus: evidence for an ambisense genome

The complete nucleotide sequence of the S RNA segment of Sandfly Fever Sicilian (SFS) virus (Phlebovirus, Bunyaviridae) was determined from overlapping cDNA clones and by primer extension. The RNA is 1746 nucleotides in length and has two large open reading frames (ORF), one of which (24.8 kDa) is viral-complementary in sense, and the other (30.4 kDa) is in the viral sense. This ambisense genome arrangement has been seen in another member of the Phlebovirus genus, Punta Toro (PT) virus (T. Ihara, H. Akashi, and D. H. L. Bishop, 1984, Virology 136, 293-306), but not in representatives of either the Bunyavirus or Hantavirus genera of the Bunyaviridae. Comparison of the predicted amino acid sequences for SFS virus with the recognized products of PT S RNA (T. Ihara, Y. Matsuura, and D. H. L. Bishop, 1985, Virology 147, 317-325; H. A. Overton, T. Ihara, and D. H. L. Bishop, 1987, Virology 157, 338-350) indicated that the 24.8-kDa ORF encodes the nucleoprotein (N) of SFS virus, and the 30.4-kDa ORF codes for a nonstructural protein (NSs). Subgenomic messenger RNAs, from which these two proteins are presumably translated, were detected in virus-infected cells.

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.

Punta Toro virus infection of C57BL/6J mice: a model for phlebovirus-induced disease

Punta Toro virus infections of inbred strains of mice have been characterized and evaluated as a model in which to study various aspects of the host response to phlebovirus infections and the requirements for protective immunity. The Adames strain of Punta Toro virus was found to be strongly hepatotropic and lymphotropic and the outcome of infection was largely a function of age. C57BL/6J mice of less than 5 weeks of age uniformly developed fulminant hepatocellular necrosis with mean survival times of 4.2 days. Resistance to lethal infection increased with age such that greater than 95% of 8-week-old mice survived challenge. The kinetics of viremia, antibody production, and hematological changes in 4- and 8-week animals indicated that the survival of the older animals is related to their ability to delay virus replication and the development of hepatic lesions during the initial 48 h of infection and their ability to terminate virus replication and clear virus from the circulation 4 to 5 days after infection. The mechanisms responsible for this resistance were studied using anti-interferon serum, immunosuppression, and passive immunization.

Identification of the N and NSS proteins coded by the ambisense S RNA of Punta Toro phlebovirus using monospecific antisera raised to baculovirus expressed N and NSS proteins

An essentially complete DNA copy of the ambisense S RNA species of Punta Toro (PT) phlebovirus (T. Ihara, H. Akashi, and D.H.L. Bishop, 1984, Virology 136, 293-306) has been inserted in either orientation into Autographa californica nuclear polyhedrosis baculovirus (AcNPV) in lieu of the 5' coding region of the AcNPV polyhedrin gene (G.E. Smith, M.D. Summers, and M.J. Fraser, 1983, Mol. Cell. Biol. 3, 2156-2165). The two types of recombinant viruses were used to infect Spodoptera frugiperda cells and the expressed PT viral proteins characterized. Recombinant AcNPV having the S DNA in one orientation expressed PT virus N protein in amounts estimated to represent some 50% of the infected cell extracts, whereas recombinants with the S DNA in the other orientation expressed the putative PT virus NSS protein in lower quantities. Antisera that were monospecific with respect to each of the two PT proteins virus were raised in mice using the corresponding S. frugiperda infected cell extracts and were employed to identify N and NSS proteins in PT virus-infected Vero cells.

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.

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

The complete sequence of Punta Toro virus (Phlebovirus, Bunyaviridae) middle size (M), RNA has been determined. The RNA is 4330 nucleotides long (mol wt 1.46 X 10(6), base composition: 26.7% A, 33.6% U, 18.5% G, 21.2% C) and has 3'- and 5'-terminal sequences that, depending on the arrangement, are complementary for some 15 residues. The viral RNA codes in its viral-complementary sequence for a single primary gene product (the viral glycoprotein precursor) that is comprised of 1313 amino acids (146,376 Da) and is abundant in cysteine residues but has few potential asparagine-linked glycosylation sites. The 5'-noncoding region of the Punta Toro M viral-complementary RNA is short (16 nucleotides); the 3'-noncoding sequence is much longer (372 nucleotides). The latter is rich in short stretches of adenylate residues, like the 3'-noncoding regions of the Punta Toro S mRNA species (T. Ihara, H. Akashi, and D. H. L. Bishop, 1984, Virology 136, 293-306). No other large open reading frame has been identified in either the viral, or viral-complementary, M RNA sequences. Limited amino-terminal sequence analyses of the two viral glycoproteins have indicated the gene order and potential cleavage sites in the glycoprotein precursor. The data suggest the existence of a 30 X 10(3)-Da polypeptide (designated NSM) in the glycoprotein precursor that precedes the G1 protein (i.e., gene product order: NSM-G1-G2). Examination of the sequence of the Punta Toro M gene product reveals the presence of multiple hydrophobic sequences including a 19-amino acid, carboxy-proximal, hydrophobic region (G2). This hydrophobic sequence is followed by a 13-amino acid-terminal sequence rich in charged amino acids. The size and constitution of the carboxy-terminal region is consistent with a transmembranal and anchor function for the glycoprotein in the viral envelope. Other regions of the glycoprotein precursor contain sequences of amino acids with a predominantly hydrophobic character (23, 50, and 20 amino acids in length). Their functions are unknown. The amino terminus of the G1 protein is located near the end of the 23-amino acid-long hydrophobic sequence of the presumptive precursor, the hydrophobic 50-amino acid sequence lies within G1, and the amino terminus of G2 is located in the middle of the 20-amino acid-long hydrophobic sequence.(ABSTRACT TRUNCATED AT 400 WORDS)

Distinction between Bunyaviridae genera by surface structure and comparison with Hantaan virus using negative stain electron microscopy

Ultrastructural studies of glutaraldehyde-fixed viruses of the Bunyaviridae were performed by negative-stain electron microscopy. The surface structure of viruses of each genus was compared with that of the other genera and with Hantaan virus, the prototype of a proposed new genus of Bunyaviridae. Viruses of each genus had a surface structure distinct for that genus. In addition, Hantaan virus had a surface structure composed of a grid-like pattern of morphologic subunits not previously described for animal viruses. Careful morphologic studies of suspected Bunyaviridae may be used in considering preliminary generic assignment. This study also supports the assignment of Hantaan-related viruses to a separate generic status within the Bunyaviridae.

Novel coding strategy (ambisense genomic RNA) revealed by sequence analyses of Punta Toro Phlebovirus S RNA

Sequence analyses of Punta Toro viral S RNA species indicate the existence of a novel coding strategy for RNA viruses that involves both viral complementary and viral sense mRNA species. The Punta Toro nucleocapsid protein (N, 26.9 X 10(3) Da) is coded by a discrete viral complementary mRNA species corresponding to the 3' half of the viral S RNA. A second, presumably nonstructural, gene product (NS, 29.1 X 10(3) Da) is coded by a viral sense mRNA species that corresponds to the 5' half of the viral RNA. The ambisense nature of the S RNA is unique by comparison with any other viral RNA and raises questions concerning the family assignment of phleboviruses.

Protein synthesis in Rift Valley fever virus-infected cells

Rift Valley fever virus-induced protein synthesis was examined by polyacrylamide gel electrophoresis and fluorography. Five virus-induced polypeptides were detected, the nucleocapsid protein N, the nucleus-associated nonstructural protein NS1, the glycoproteins G1 and G2, and a protein of molecular weight 80K. The N, G1, G2, and 80K proteins were present in virion preparations. Sequential studies showed that NS1 accumulated in the nucleus as soon as it was formed and readily associated with nuclei partitioned from noninfected cells. The G1 and G2 proteins labelled with [3H]glucosamine and [3H]mannose. NS1 was shown to be the only virus-induced protein which was phosphorylated.