Bunyamwera virus-induced polypeptide synthesis

Nuclear relocalization of polyadenylate binding protein during rift valley fever virus infection involves expression of the NSs gene

Rift Valley fever virus (RVFV), an ambisense member of the family Bunyaviridae, genus Phlebovirus, is the causative agent of Rift Valley fever, an important zoonotic infection in Africa and the Middle East. Phlebovirus proteins are translated from virally transcribed mRNAs that, like host mRNA, are capped but, unlike host mRNAs, are not polyadenylated. Here, we investigated the role of PABP1 during RVFV infection of HeLa cells. Immunofluorescence studies of infected cells demonstrated a gross relocalization of PABP1 to the nucleus late in infection. Immunofluorescence microscopy studies of nuclear proteins revealed costaining between PABP1 and markers of nuclear speckles. PABP1 relocalization was sharply decreased in cells infected with a strain of RVFV lacking the gene encoding the RVFV nonstructural protein S (NSs). To determine whether PABP1 was required for RVFV infection, we measured the production of nucleocapsid protein (N) in cells transfected with small interfering RNAs (siRNAs) targeting PABP1. We found that the overall percentage of RVFV N-positive cells was not changed by siRNA treatment, indicating that PABP1 was not required for RVFV infection. However, when we analyzed populations of cells producing high versus low levels of PABP1, we found that the percentage of RVFV N-positive cells was decreased in cell populations producing physiologic levels of PABP1 and increased in cells with reduced levels of PABP1. Together, these results suggest that production of the NSs protein during RVFV infection leads to sequestration of PABP1 in the nuclear speckles, creating a state within the cell that favors viral protein production.

[Rift Valley fever virus]

Rift Valley fever virus (RVFV) causes massive mosquito-borne epidemics among humans and decimates ruminants in which the mortality rate is about 1% and 10-30%, respectively. Morbidity in RVFV-infected humans is high largely due to the effects of hemorrhagic fever and encephalitis. This virus is native to sub-Saharan Africa; yet if this virus is introduced into the environment, virus transmission appears to occur whenever sheep and cattle are present with abundant mosquito populations. RVFV is a negative-strand RNA virus which belongs to the family Bunyaviridae, genus Phlebovirus, and contains tripartite-segmented genomes (S, M, and L). S-segment is the ambisense genome, where N and NSs genes are coded in an antiviral-sense and viral sense S-segment, respectively. The inhibition of host mRNA synthesis, which is induced by the binding of NSs protein to RNA polymerase II transcription factor TFIIH, is the primary reason for the host-protein shut-off in RVFV-infected cells. Development of a RVFV reverse genetics system, which has not been accomplished yet, is important for the study of viral replication mechanisms, host virus interaction, viral pathogenicity as well as vaccine evaluation and development.

Nucleotide sequence analysis of the large (L) genomic RNA segment of Bunyamwera virus, the prototype of the family Bunyaviridae

The complete nucleotide sequence of the large (L) genome segment of Bunyamwera virus has been determined from overlapping cDNA clones. The segment is 6875 nucleotides long and has a base composition of 29.8% A, 17.9% C, 15.4% G, and 36.9% U. Eighteen of the terminal 19 nucleotides at the 3' and 5' ends are complementary. In the viral-complementary (+ sense) RNA there is a single long open reading frame (ORF) from AUG at bases 51-53 to a UAG stop codon at bases 6765-6767; this ORF encodes a polypeptide of 2238 amino acids (MW 259,000), corresponding to the L protein which has been mapped to the L RNA segment by analysis of reassortants of Bunyamwera, Batai, and Maguari viruses. The amino-terminal 46 amino acids of the L protein show strong homology (63% identity) with the amino-termini of ORFs predicted from limited sequence analysis of the L segments of La Crosse and snowshoe hare bunyaviruses. Comparison with the polymerase proteins encoded by other negative-strand viruses showed weak homology with part of the influenza virus PB1 protein, but no homology was detected with the other influenza virus polymerase proteins nor with the L proteins of arenaviruses, paramyxoviruses, and rhabdoviruses. At the 5' end of genomic (- sense) RNA there is an AUG-initiated ORF potentially encoding a protein of 14,700; the significance of this ORF is unknown at present.

La Crosse virus infection of mammalian cells induces mRNA instability

La Crosse virus infection of BHK cells leads to a dramatic shutoff of not only host protein synthesis but also viral protein synthesis later in infection. This shutoff can be accounted for by the loss of the cytoplasmic cellular and viral mRNAs. The induction of mRNA instability requires extensive virus replication, since when cycloheximide is added early in infection the preexisting viral and cellular mRNAs do not decrease upon incubation of the cultures. Pretreatment of the cultures with actinomycin D does not affect the ability of La Crosse virus infection to induce mRNA instability, and examination of the rRNAs shows no evidence of specific degradation due to activation of the interferon-associated latent RNase. The induction of mRNA instability therefore does not appear to operate through an interferon pathway. Viral mRNA synthesis, on the other hand, is not turned off during infection, and the cap-dependent endonuclease involved in viral mRNA initiation may be responsible for the mRNA instability.

Viral RNAs synthesized in cells infected with Germiston Bunyavirus

A rapidly growing strain of Germiston virus was used to study intracellular viral RNA synthesis in BHK cells. The RNAs were separated by electrophoresis into seven bands which fell into three size classes: large (bands L1 and L2), medium (bands M1 and M2), and small (bands S1, S2, and S3). Blot hybridisation established that bands L1, M1, and S1 contained the negative-sense genomic RNAs, while bands L2, M2, S2, and S3 contained positive-sense RNAs complementary to the genomic RNAs within the same size class. After glyoxal treatment the RNAs separated into a large, a medium, and two small bands, indicating that the positive-sense RNAs originally present in bands L2, M2, and S2 are similar in size to their genomic RNAs, while the RNA in S3 is shorter than the small genomic segment. These results suggest that band S2 contains the replicative intermediate RNA and band S3 the messenger RNA of the small genomic segment and also that bands L2 and M2 contain both replicative intermediate and messenger RNAs. Long after virus development had ceased in the infected cells the amounts of RNAs in bands L1, M1, S1, and S2 remained the same, those in bands L2 and M2 were reduced, while only trace amounts of RNAs were observed in band S3, suggesting that the genomic RNAs and the replicative intermediate RNAs form ribonuclease-resistant ribonucleoprotein complexes while the messenger RNAs do not form such complexes. Synthesis of RNA in the infected cells was first evident in bands S3 and M2, after which synthesis was soon observed in all seven bands reaching a maximum rate at the logarithmic phase of growth, suggesting that the pattern of Germiston virus development resembles that of other negative-strand RNA viruses. The presence of defective-interfering particles was indicated by the observation that purified virus preparations contained a minor RNA component originating from the large RNA segment.

Nucleotide sequence of the Bunyamwera virus M RNA segment: conservation of structural features in the Bunyavirus glycoprotein gene product

The complete nucleotide sequence of the Bunyamwera virus M RNA segment was determined from four overlapping cDNA clones and by primer extension. The RNA segment is 4458 bases in length, and encodes a single gene product in the viral complementary RNA. The predicted protein is 1433 amino acids long (mol wt 162,065), contains four potential glycosylation sites, and is relatively cysteine rich. It is presumed that the three proteins G1, G2, and NSM which have been mapped to the M RNA segment are synthesized as a precursor polyprotein which is subsequently proteolytically cleaved. A putative hydrophobic signal sequence at the amino terminus and a hydrophobic anchor sequence at the carboxy terminus of the predicted protein have been identified, in addition to internal regions of hydrophobicity of unknown function. The nucleotide and amino acid sequences of the Bunyamwera virus M segment have been compared with those of the snowshoe hare virus M segment (Y. Eshita and D. H. L. Bishop, Virology 137, 227-240, 1984). Common features include the overall architecture of the RNAs, single cysteine-rich primary gene products, and conservation of hydrophobic domains in the gene products. When aligned the amino acid sequences are 43% homologous, and 66 of 70 cysteine residues can be matched. The evolutionary significance of these findings is 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.

Identification of nonstructural proteins encoded by viruses of the Bunyamwera serogroup (family Bunyaviridae)

The proteins synthesized in BHK cells infected with nine members of the Bunyamwera serogroup (family Bunyaviridae, Bunyavirus genus) were analyzed by polyacrylamide gel electrophoresis. In addition to the virus structural proteins, a number of virus-coded nonstructural proteins were detected. One protein, designated NS1, was shown to be related to the nucleocapsid protein by one-dimensional peptide mapping. A second protein, NS2, was mapped to the M RNA segment by gel electrophoretic analysis of the proteins synthesized in cells infected with reassortants of Batai, Bunyamwera, and Maguari viruses of known genotype. A third protein, NS3, was mapped to the S RNA segment by its pattern of labeling with [35S]cysteine in cells infected with reassortant viruses: the NS3 protein was only labeled when the S RNA segment of Bunyamwera virus was present. The mapping of NS3 was confirmed by in vitro translation of mRNAs which hybridized to recombinant plasmids containing S gene-specific sequences.

Machupo virus polypeptides: identification by immunoprecipitation

The most abundant protein in purified Machupo virions (Corvallo strain) labelled with 14C-Protein hydrolysate is a 64 K polypeptide which is associated with virion RNAs. Another structural polypeptide, 37 K, solubilized by nonionic detergent seems to be a major surface glycoprotein. In addition to this, a 78 K polypeptide and a minor 50 K polypeptide have been detected. In Machupo virus infected cells three virus-specific polypeptides similar in size to those described for structural polypeptides were immunoprecipitated with anti-Machupo virus serum. The most abundant virus-specific polypeptide was nonglycosylated (64 K, NP), and the others were glycosylated polypeptides (78 K and 37 K). The synthesis of NP and 78 K polypeptides was recognized at the beginning of a log phase of virus replication. Pulse-chase experiments as well as experiments with an arginine analogue, canavanine (to block proteolytic processing) suggest that 78 K is a precursor for structural glycoproteins of Machupo virions.

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.

Seven infection-specific polypeptides in BHK cells infected with Bunyamwera virus

Virus-specific polypeptide synthesis was examined in BHK cells and Vero cells infected with Bunyamwera virus. In BHK cells, in addition to the four previously reported virus-coded proteins (L, G1, G2, and N), three other infection-specific proteins were detected. These proteins, of nominal molecular weight 50,000 (p50), 16,000 (p16), and 13,000 (p13), were not labeled in mock-infected cells, were first synthesized between 4 and 8 h after infection, and were relatively prominent among the limited number of proteins generated late in infection. In preparations of purified Bunyamwera virus from BHK cell supernatants, p16 was detected but not p50 or p13. In Vero cells infected with Bunyamwera virus, both p50 and p13 were labeled strongly. Maprik virus, a member of the Mapputta group of arboviruses, is a member of the Bunyavirus genus (S.E. Newton, unpublished data). Maprik virus did not induce the synthesis of p50, p16, or p13; however, two smaller proteins (p17 and p15) which may correspond to p16 and p13 were labeled late in Maprik infection. Our data argue that p16 is a virus-coded component of the Bunyamwera virus particle and that p50 and p13 are virus-coded, nonstructural proteins.

The complete sequence and coding content of snowshoe hare bunyavirus small (S) viral RNA species

The complete sequence of the small (S) viral RNA species of snowshoe hare (SSH) bunyavirus has been determined, principally from a DNA copy of the RNA cloned in the E.coli plasmid pBr322. The viral S RNA (negative sense strand) is 982 nucleotides long (3.3 x 10(5) daltons) with complementary 5' and 3' end sequences. It has a base composition of 30.5%U, 25.8%A, 24.9%C and 18.7%G. In the viral complementary (plus sense) strand there are two overlapping open reading frames initiated by methionine codons. One reading frame codes for a 26.8 x 10(3) dalton protein, the other for a 10.5 x 10(3) dalton protein. The larger gene product is presumably related to the viral nucleoprotein (N) that is coded by the S RNA (Gentsch and Bishop (1978) J. Virol. 28, 417-419). The smaller gene product is probably related to the recently identified S RNA coded nonstructural protein (NSS) induced in virus infected cells (Fuller and Bishop (1982) J. Virol. 41, 643-648).

Identification of virus-coded nonstructural polypeptides in bunyavirus-infected cells

Analyses of bunyavirus-infected cell extracts identified at least two virus-induced nonstructural polypeptides. With snowshoe hare (SSH), La Crosse (LAC), and six SSH-LAC reassortant viruses, it was shown that one of these nonstructural polypeptides (NSs, approximate molecular weight, 7.4 X 10(3)) is coded by the SSH small (S)-size viral RNA species. This nonstructural polypeptide was not detected (at least in the same relative abundancies) in LAC virus-infected cells or in cells infected with reassortants having LAC S RNA. For SSH virus, tryptic peptide analyses of either [3H]leucine- or [3H]arginine-labeled NSs indicated that it contains unique sequences not present in the SSH nucleocapsid (N) polypeptide (also coded by the S RNA; J. R. Gentsch and D. H. L. Bishop, J. Virol. 28:417-419, 1978). Analyses of SSH virus-infected cell extracts and extracts of cells infected with SSH-LAC reassortants having SSH medium (M)-size RNA species indicated that a nonstructural polypeptide (NSM; approximate molecular weight, 12 X 10(3)) is coded by the SSH M RNA species. In extracts of LAC virus-infected cells (or cells infected with SSH-LAC reassortants having LAC M RNA), a polypeptide with an electrophoretic mobility slightly faster than that of the SSH NSM polypeptide was observed (approximate molecular weight, 11 X 10(3)); it has been designated LAC NSM. The relationships of the NSM polypeptides to the other M RNA-coded polypeptides (G1 and G2; J. R. Gentsch and D. H. L. Bishop, J. Virol. 30;767-770, 1979) have not been determined. Two additional polypeptides present in both LAC- and SSH-infected cell extracts also appear to be virus induced (one with an approximate molecular weight of 10 X 10(3), p10; the other with an approximate molecular weight of 18 X 10(3), p18). Whether these polypeptides are virus coded has not been determined.

Genetic interactions among viruses of the Bunyamwera complex

Seventy-seven temperature-sensitive (ts) mutants belonging to three antigenically distinct and geographically isolated members of the Bunyamwera complex--Batai virus, Bunyamwera virus, and Maguari virus--have been isolated after 5-fluorouracil treatment. High-frequency recombination was observed, and the mutants of each virus were classified into two groups, which were shown to be equivalent by heterologous recombination experiments. In most combinations heterologous recombination was less efficient than homologous recombination, but all crosses of group I and II mutants yielded viable recombinants. Recombination was an early event. Analysis by polyacrylamide gel electrophoresis of the proteins of the wild-type viruses and recombinant clones obtained from the six possible heterologous combinations of group I and II mutants indicated that recombination occurred by reassortment of genome subunits. Group I appeared to correspond to the genome subunit coding for the N protein, and group II corresponded to the G1/G2 determinant. The G1 (or G2 or both) protein was associated with neutralization specificity and plaque diameter, and the N protein was associated with plaque opacity. Complementation was observed between two nonrecombining mutants of Maguari virus belonging to group I, which may indicate that the N genome subunit codes for an additional protein. There appeared to be no genetic barrier to exchange of genetic material between Batai, Bunyamwera, and Maguari viruses in vitro, and it is concluded that the Bunyamwera complex is potentially a single gene pool if geographical and ecological constraints are discounted.

In vivo transcription and protein synthesis capabilities of bunyaviruses: wild-type snowshoe hare virus and its temperature-sensitive group I, group II, and group I/II mutants

The in vivo primary and secondary transcription capabilities of wild-type snowshoe hare (SSH) virus and certain of its temperature-sensitive (ts) mutants have been analyzed. The results obtained agree with in vitro studies (Bouloy et al., C.R. Acad. Sci. Paris 280:213-215, 1975; M. Bouloy and C. Hannoun, Virology 69:258-264, 1976; M. Ranki and R. Pettersson, J. Virol. 16:1420-1425, 1975) which have shown that bunyaviruses are negative-stranded RNA viruses with a virion RNA-directed RNA polymerase. The in vivo transcription studies have demonstrated that in the presence of protein synthesis inhibitors (puromycin or cycloheximide) SSH virus can synthesize viral complementary RNA (primary transcription) throughout the infection cycle. The increased levels of viral complementary RNA obtained in the absence of protein synthesis inhibitors (secondary transcription) were not markedly reduced if cells were pretreated with actinomycin D (5 mug/ml), alpha-amanitin (25 mug/ml), or rifampin (100 mug/ml), although progeny virus yields were reduced by up to 80% in the actinomycin D- and rifampin-treated cells. The in vivo transcription capabilities of SSH group I ts mutants at temperatures which were nonpermissive (40 degrees C) for virus replication gave values comparable to those obtained at permissive temperatures (33 degrees C). The SSH group I mutants appear, therefore, to be RNA-positive mutant types. When compared with their transcription capabilities at 33 degrees C, the in vivo transcription abilities of four SSH group II ts mutants (and one double group I/II ts mutant) were found to be more impaired at 40 degrees C than those of the SSH group I ts mutants or wild-type SSH virus at 40 degrees C, although the viral complementary RNA synthetic capabilities of these group II (and group I/II) mutants at 40 degrees C were significantly higher than their primary transcription capabilities (as measured at 33 degrees C in the presence of puromycin or cycloheximide). It was concluded, therefore, that these SSH group II (and double group I/II) ts mutants have an intermediate RNA phenotype. Hybridization studies using (32)P-labeled individual L, M, and S viral RNA species of SSH virus have demonstrated the presence of viral complementary RNA to all three species in extracts of cells infected with SSH ts II-30 and incubated at 33 degrees C (primary and secondary transcription) or 40 degrees C, a nonpermissive temperature for its replication. The results of pulse-labeled in vivo protein analyses indicated that greater quantities of intracellular N protein (coded for by S RNA [J. R. Gentsch and D. H. L. Bishop, J. Virol. 28:417-419, 1978]) than G1 and G2 polypeptides (coded for by M RNA [J. R. Gentsch and D. H. L. Bishop, J. Virol. 30:767-776, 1979]) were present in extracts of cells infected with wild-type SSH virus. In extracts of SSH group I, II, or I/II ts mutant-infected cells incubated at 33 degrees C, N and G1, and for the group II mutant-infected cells, G2, viral polypeptides were detected, whereas in extracts obtained from group I or II mutant virus-infected cells incubated at 40 degrees C, low levels of N and G1 polypeptides were evident.

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

Analyses of the virion polypeptides and genomes of several Phlebotomus fever group viruses, Karimabad, Punta Toro, Chagres, and the sandfly fever Sicilian serotype viruses, have established that they are biochemically similar to the accepted members of the Bunyaviridae family. Like snowshoe hare virus (a member of the California serogroup of the Bunyavirus genus of the Bunyaviridae family), Karimabad, Punta Toro, Chagres, and the sandfly fever Sicilian serotype viruses all have three viral RNA species, designated large (L), medium (M), and small (S). Oligonucleotide fingerprint analyses of Karimabad and Punta Toro virus RNA species indicated that their L, M, and S RNA species are unique. By polyacrylamide gel electrophoresis it was determined for Karimabad virus that the apparent molecular weights of its L, M, and S RNA species are 2.6 X 10(6), 2.2 X 10(6), and 0.8 X 10(6), respectively. For Punta Toro virus, the apparent molecular weights of its L, M, and S RNA species are 2.8 X 10(6), 1.8 X 10(6), and 0.75 X 10(6), respectively. The major internal nucleocapsid (N) protein of Karimabad virus was found to have a molecular weight of 21 X 10(3). A similar polypeptide size class was identified in preparations of sandfly fever Sicilian serotype, Chagres, and Punta Toro viruses. The Karimabad virus glycoproteins formed the external surface projections on virus particles and could be removed from virus preparations by protease treatment. The glycoproteins in an unreduced sample could be resolved into two size classes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. They had apparent molecular weights of 62 X 10(3) and 50 X 10(3) in continuous polyacrylamide gels. When Karimabad virus preparations were reduced with 1% beta-mercaptoethanol, prior to resolution by continuous polyacrylamide gel electrophoresis, all the viral glycoprotein was recovered in a single size class, having an apparent molecular weight of 62 X 10(3). Two or three major virion polypeptides have been identified in preparations of Punta Toro, Chagres, and sandfly fever Sicilian serotype viruses.