Phleboviruses encapsidate their genomes by sequestering RNA bases.

Rift Valley fever and Toscana viruses are human pathogens for which no effective therapeutics exist. These and other phleboviruses have segmented negative-sense RNA genomes that are sequestered by a nucleocapsid protein (N) to form ribonucleoprotein (RNP) complexes of irregular, asymmetric structure, previously uncharacterized at high resolution. N binds nonspecifically to single-stranded RNA with nanomolar affinity. Crystal structures of Rift Valley fever virus N-RNA complexes reconstituted with defined RNAs of different length capture tetrameric, pentameric and hexameric N-RNA multimers. All N-N subunit contacts are mediated by a highly flexible α-helical arm. Arm movement gives rise to the three multimers in the crystal structures and also explains the asymmetric architecture of the RNP. Despite the flexible association of subunits, the crystal structures reveal an invariant, monomeric RNP building block, consisting of the core of one N subunit, the arm of a neighboring N, and four RNA nucleotides with the flanking phosphates. Up to three additional RNA nucleotides bind between subunits. The monomeric building block is matched in size to the repeating unit in viral RNP, as visualized by electron microscopy. N sequesters four RNA bases in a narrow hydrophobic binding slot and has polar contacts only with the sugar-phosphate backbone, which faces the solvent. All RNA bases, whether in the binding slot or in the subunit interface, face the protein in a manner that is incompatible with base pairing or with "reading" by the viral polymerase.

Structure of the Rift Valley fever virus nucleocapsid protein reveals another architecture for RNA encapsidation.

Rift Valley fever virus (RVFV) is a negative-sense RNA virus (genus Phlebovirus, family Bunyaviridae) that infects livestock and humans and is endemic to sub-Saharan Africa. Like all negative-sense viruses, the segmented RNA genome of RVFV is encapsidated by a nucleocapsid protein (N). The 1.93-A crystal structure of RVFV N and electron micrographs of ribonucleoprotein (RNP) reveal an encapsidated genome of substantially different organization than in other negative-sense RNA virus families. The RNP polymer, viewed in electron micrographs of both virus RNP and RNP reconstituted from purified N with a defined RNA, has an extended structure without helical symmetry. N-RNA species of approximately 100-kDa apparent molecular weight and heterogeneous composition were obtained by exhaustive ribonuclease treatment of virus RNP, by recombinant expression of N, and by reconstitution from purified N and an RNA oligomer. RNA-free N, obtained by denaturation and refolding, has a novel all-helical fold that is compact and well ordered at both the N and C termini. Unlike N of other negative-sense RNA viruses, RVFV N has no positively charged surface cleft for RNA binding and no protruding termini or loops to stabilize a defined N-RNA oligomer or RNP helix. A potential protein interaction site was identified in a conserved hydrophobic pocket. The nonhelical appearance of phlebovirus RNP, the heterogeneous approximately 100-kDa N-RNA multimer, and the N fold differ substantially from the RNP and N of other negative-sense RNA virus families and provide valuable insights into the structure of the encapsidated phlebovirus genome.

Efficient cellular release of Rift Valley fever virus requires genomic RNA.

The Rift Valley fever virus is responsible for periodic, explosive epizootics throughout sub-Saharan Africa. The development of therapeutics targeting this virus is difficult due to a limited understanding of the viral replicative cycle. Utilizing a virus-like particle system, we have established roles for each of the viral structural components in assembly, release, and virus infectivity. The envelope glycoprotein, Gn, was discovered to be necessary and sufficient for packaging of the genome, nucleocapsid protein and the RNA-dependent RNA polymerase into virus particles. Additionally, packaging of the genome was found to be necessary for the efficient release of particles, revealing a novel mechanism for the efficient generation of infectious virus. Our results identify possible conserved targets for development of anti-phlebovirus therapies.

Host genetic variation in susceptibility to Punta Toro virus.

Infection of small laboratory animals by Punta Toro virus (PTV), family Bunyaviridae, genus Phlebovirus, is a model for the study of the human pathogen Rift Valley fever virus (RVFV). We have identified inbred mouse strains with significant differences in host response to the Adames strain of PTV. Nine inbred strains of mice representing major branches in the Mus musculus phylogeny were inoculated subcutaneously with a high dose of PTV in survival experiments. Two inbred strains of mice, NZW/LacJ and 129S1/SvImJ, died ~4 days after PTV infection, whereas 7 other strains survived the challenge and showed no clinical signs of disease. Histologically, 129S1/SvImJ mice showed massive hepatocellular necrosis and had additional lesions in lung, brain, and spleen, whereas NZW/LacJ mice had mild piecemeal hepatocellular necrosis. PTV viral loads in the livers of infected mice were determined by reverse transcriptase quantitative PCR. Inbred mice from strains that showed clinical signs and succumbed to PTV infection had higher liver viral loads than did mice of resistant strains. Hybrid F1 mice were generated by crossing susceptible 129S1 and resistant FVB/N mice and tested for susceptibility. The hybrid F1 mice showed significantly higher viral loads in the liver than the resistant parental FVB/N mice, suggesting that susceptibility is dominant. These findings will enable an unbiased genetic approach to identify host genes mediating susceptibility to PTV.

A novel system for identification of inhibitors of rift valley Fever virus replication.

Rift Valley fever virus (RVFV) is a human and livestock pathogen endemic to sub-Saharan Africa. We have developed a T7-dependent system for the efficient production of RVFV-like particles (RVF-VLPs) based on the virulent ZH-501 strain of RVFV. The RVF-VLPs are capable of performing a single round of infection, allowing for the study of viral replication, assembly, and infectivity. We demonstrate that these RVF-VLPs are antigenically indistinguishable from authentic RVFV and respond similarly to a wide array of known and previously unknown chemical inhibitors. This system should be useful for screening for small molecule inhibitors of RVFV replication.

Restriction of rift valley Fever virus virulence in mosquito cells.

Arboviruses are maintained in a natural cycle that requires blood-sucking arthropod and vertebrate hosts. Arboviruses are believed to persistently infect their arthropod host without overt pathology and cause acute infection with viremia in their vertebrate host. We have focused on elucidating how a specific arbovirus, Rift Valley fever (RVF) virus, causes cytopathic effect in cells derived from vertebrates and non-cytopathic infection in cells derived from arthropods. We demonstrate that the vertebrate virulence factor, NSs, is functional in arthropod cells but is expressed at significantly lower levels in infected arthropod versus infected vertebrate cells.

Synthesis, proteolytic processing and complex formation of N-terminally nested precursor proteins of the Rift Valley fever virus glycoproteins.

The genomic M RNA segment of Rift Valley fever virus is transcribed to produce a single mRNA with multiple translation initiation sites. The products of translation are an N-terminal nested series of polyproteins. These polyproteins enter the secretory system of the host cell and are proteolytically processed to yield the mature virion glycoproteins, Gn and Gc, and two non-structural glycoproteins. By means of pulse-chase immune precipitation experiments we identify the Gn and Gc precursor molecules and also show that signal peptidase cleavage is required for mature Gn and Gc production. We also demonstrate that a hydrophobic domain at the N-terminus of Gn acts as a signal peptide only in the context of the polyprotein precursors that initiate at the second, fourth or fifth AUGs. In addition, we document that formation of Gn/Gc heteromeric complexes occur rapidly (<5 min) and can occur prior to signal peptidase processing of Gn, suggesting that this complex forms in the endoplasmic reticulum. Interestingly, Gc can form a complex with a glycoprotein that has been considered nonstructural, a discovery that has implications for both the topology and potential packaging of this glycoprotein.

The NSm proteins of Rift Valley fever virus are dispensable for maturation, replication and infection.

Rift Valley fever (RVF) virus belongs to the Bunyaviridae family of segmented negative-strand RNA viruses and causes mosquito-borne disease in sub-Saharan Africa. We report the development of a T7 RNA polymerase-driven plasmid-based genetic system for the virulent Egyptian isolate, ZH501. We have used this system to rescue a virus that has a 387 nucleotide deletion on the genomic M segment that eliminates the coding region for two non-structural proteins known as NSm. This virus, DeltaNSm rZH501, is indistinguishable from the parental ZH501 strain with respect to expression of structural proteins and growth in cultured mammalian cells.

Characterization of the Golgi retention motif of Rift Valley fever virus G(N) glycoprotein.

As Rift Valley fever (RVF) virus, and probably all members of the family Bunyaviridae, matures in the Golgi apparatus, the targeting of the virus glycoproteins to the Golgi apparatus plays a pivotal role in the virus replication cycle. No consensus Golgi localization motif appears to be shared among the glycoproteins of these viruses. The viruses of the family Bunyaviridae synthesize their glycoproteins, G(N) and G(C), as a polyprotein. The Golgi localization signal of RVF virus has been shown to reside within the G(N) protein by use of a plasmid-based transient expression system to synthesize individual G(N) and G(C) proteins. While the distribution of individually expressed G(N) significantly overlaps with cellular Golgi proteins such as beta-COP and GS-28, G(C) expressed in the absence of G(N) localizes to the endoplasmic reticulum. Further analysis of expressed G(N) truncated proteins and green fluorescent protein/G(N) chimeric proteins demonstrated that the RVF virus Golgi localization signal mapped to a 48-amino-acid region of G(N) encompassing the 20-amino-acid transmembrane domain and the adjacent 28 amino acids of the cytosolic tail.

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.

Ngari virus is a Bunyamwera virus reassortant that can be associated with large outbreaks of hemorrhagic fever in Africa.

Two isolates of a virus of the genus Orthobunyavirus (family Bunyaviridae) were obtained from hemorrhagic fever cases during a large disease outbreak in East Africa in 1997 and 1998. Sequence analysis of regions of the three genomic RNA segments of the virus (provisionally referred to as Garissa virus) suggested that it was a genetic reassortant virus with S and L segments derived from Bunyamwera virus but an M segment from an unidentified virus of the genus Orthobunyavirus. While high genetic diversity (52%) was revealed by analysis of virus M segment nucleotide sequences obtained from 21 members of the genus Orthobunyavirus, the Garissa and Ngari virus M segments were almost identical. Surprisingly, the Ngari virus L and S segments showed high sequence identity with those of Bunyamwera virus, showing that Garissa virus is an isolate of Ngari virus, which in turn is a Bunyamwera virus reassortant. Ngari virus should be considered when investigating hemorrhagic fever outbreaks throughout sub-Saharan Africa.