Rebecca Wattam 06/02/2003 0.83 The methods of cell invasion and viral multiplication within vertebrate cells are described for Rift Valley Fever virus, a member of the family Bunyaviridae and the genus Phlebovirus. wattam@vbi.vt.edu G1and/or G2 Extracellular Ligand binding or carrier It has been proposed that the G1 protein might be the viral attachment protein for vertebrates, whereas G2 is used for arthropod infections. Confirmation of this as a general feature of viruses in the Bunyavirus genus is needed (Schmaljohn, 1996). Neutralizing and hemagglutination inhibiting sites are present on both the G1 and G2 proteins of Rift Valley Fever virus, suggesting that both proteins may be involved in attachment, either directly or due to conformational requirements (Schmaljohn, 1996). Cell membrane receptor Cell membrane Ligand binding or carrier The major glycoproteins are likely involved in virus-cell attachment. However, the specific interactions involved and the nature of cellular receptors for virus infection remain to be elucidated (Elliott et al., 1991). The nature of cell receptors involved in attachment has not been identified for any member of the family (Bishop, 1996). Intracellular virion Phagolysosome Other Electron microscopy of the infection process of Rift Valley Fever virus showed that viral particles appeared to enter cells in phagocytic vacuoles. This observation is consistent with a mode of entry similar to that first described for alphaviruses in which the virus is endocytosed in coated vesicles. These endosomes subsequently become acidified, triggering a fusion of viral membranes and endosomal membranes, which releases the nucleocapsid into the cell cytoplasm. Direct evidence for this process with viruses in the Bunyaviridae has not yet been obtained (Schmaljohn, 1996). Fused virus Phagolysosome Other >Fusion of infected cells at acidic pH has been reported for viruses in the family Bunyaviridae as well as for numerous other enveloped viruses. The pH-dependent fusion is generally believed to related to early events in the infection process, particularly the translocation of RNA and proteins into the cell cytoplasm. Electron microscopy of the infection process of Rift Valley Fever virus showed that viral particles appeared to enter cells in phagocytic vacuoles. This observation is consistent with a mode of entry similar to that first described for alphaviruses in which the virus is endocytosed in coated vesicles. These endosomes subsequently become acidified, triggering a fusion of viral membranes and endosomal membranes, which releases the nucleocapsid into the cell cytoplasm. Direct evidence for this process with viruses in the Bunyaviridae has not yet been obtained (Schmaljohn, 1996). Uncoated virus Cytoplasm Other Entry and uncoating occurs by endocytosis of virions and fusion of the viral membrane with the endosomal membrane to release the three nucleocapsids into the cell cytoplasm (Bishop, 1996). Following uncoating of viral genomes, transcription of the negative-sense viral RNA to complementary mRNA involves an interaction of the virion-associated polymerase with the RNA templates in the individual nucleocapsids (Bishop, 1996). Ribonucleoprotein complex S viral segment Cytoplasm Other The S segment codes for the N protein in the 5’ half of the viral complementary-sense molecule and for the NSs protein in the remaining 5’ half of the genomic-sense RNA, utilizing and ambisense strategy (Lopez et al., 1995). Viral polypeptides are synthesized shortly after infection, suggesting that mRNAs are transcribed and translated rapidly (Schmaljohn, 1996). Complimentary S segment Cytoplasm Other As a consequence of the ambisense expression strategy of the S RNA the synthesis of NSs protein requires prior genome replication (Giorgi 1996). NSs mRNA Cytoplasm Other Transcription of the NSs mRNA takes place on the antigenome only after genome replication has begun (Giorgi 1996). As a consequence of the ambisense expression strategy of the S RNA the synthesis of NSs protein requires prior genome replication (Giorgi 1996). It has been suggested that the NSs gene has evolved during adaptation of Rift Valley Fever virus to the mammalian host and that an important role of the NSs protein was to provide a mechanism to circumvent the interferon (IFN) response of vertebrate cells (Bouloy et al., 2001). There is at least an order of magnitude (in molar terms) more S mRNA species than M mRNA species, which in turn is more abundant than the L mRNA species. The reasons for these differences are unknown (Bishop, 1996). NSs protein Cytoplasm Other The NSs protein is a phosphoprotein of unknown function that is localized in the cytoplasm and the nuclei of infected cells where it forms filamentous structures (Kohl et al., 1999). Recent experiments showed that NSs interacts with itself to form multimers and with viral and cellular structures (Kohl et al., 1999). The NSs phosphoprotein accumulates in large amounts in the nucleus of infected cells, whereas viral replication takes place exclusively in the cytoplasm (Bouloy et al., 2001). NSs protein Nucleus Other The NSs phosphoprotein accumulates in large amounts in the nucleus of infected cells (Bouloy et al., 2001). The NSs protein is of unknown function and is localized in the cytoplasm and the nuclei of infected cells where it forms filamentous structures (Kohl et al., 1999). Recent experiments showed that NSs interacts with itself to form multimers and with viral and cellular structures (Kohl et al., 1999). It has been suggested that the NSs protein is an important virulence factor that prevents alpha/beta interferons from being induced early during the course of Rift Valley Fever virus infection (Bouloy et al., 2001). N mRNA Cytoplasm Other The S segment codes for the N protein in the 5’ half of the viral complementary-sense molecule and for the NSs protein in the remaining 5’ half of the genomic-sense RNA, utilizing and ambisense strategy (Lopez et al., 1995). Expression of RNA molecules derived from the S segment of Rift Valley Fever virus showed that N, but not NSs, RNA molecules interfere with replication of the homologous virus. Results strongly suggest that the N sequence in sense or antisense orientation but not the protein is responsible for this inhibitory effect (Billecocq et al., 1996). There is at least an order of magnitude (in molar terms) more S mRNA species than M mRNA species, which in turn is more abundant than the L mRNA species. The reasons for these differences are unknown (Bishop, 1996). N protein Cytoplasm Other The N protein is the most abundant viral protein found in the virion and in virus-infected cells; it is associated with the genomic RNA to constitute the viral nucleocapsids. Synthesis of N protein can be detected early after infection (2 hour postinfection) and the protein has a half-life of several hours (Giorgi 1996). The L and N proteins are absolutely required and appear to suffice for transcription (Lopez et al., 1995). In phlebovirus-infected cells the N protein seems to accumulate in the Golgi region during later stages of infection. The Golgi accumulation is thought to be caused by the association of the cytoplasmic ribonucleoprotein with the transmembranal sequences of the viral envelope glycoproteins, which reside in the Golgi complex (Giorgi 1996). The nucleocapsid assembly process for phleboviruses has not yet been elucidated, but it is thought to be controlled at the level of initiation by interaction of the N protein with a specific sequence on its target RNA (Giorgi 1996). N protein Golgi membrane Other The encapsidation signal is recognized, and cRNA is cotranscriptionally complexed with nucleocapsid protein (Schmaljohn, 1996). In phlebovirus-infected cells the N protein seems to accumulate in the Golgi region during later stages of infection. The Golgi accumulation is thought to be caused by the association of the cytoplasmic ribonucleoprotein with the transmembranal sequences of the viral envelope glycoproteins, which reside in the Golgi complex (Giorgi 1996). Ribonucleoprotein structures accumulate on the cytoplasmic face of membranes that have G1 and G2 embedded into them and are exposed on the luminal side (Schmaljohn, 1996). The helical nucleocapsids were found to line up underneath the membrane of distended Golgi vesicles. As G1 and G2 accumulated in the Golgi complex, progressively more nucleocapsids also entered the Golgi region. Little if any N protein was seen associated with the ER or the plasma membrane. Thus, a specific interaction between the nucleocapsids and membranes containing the viral glycoproteins seems to exist only in the Golgi complex. Why no such interaction appear to occur already in the ER, which also contains high amounts of G1 and G2, is not clear, but it may relate to incorrect conformation or organization of the spikes, or to the topology or accessibility of the cytoplasmic tail of one of the glycoproteins that is likely to interact with the nucleocapsids (Pettersson and Melin, 1996). Antigenomic S segment Cytoplasm Genomic S segment At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). Genomic S segment Cytoplasm Other At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). Genomic S segment Golgi membrane Other The encapsidation signal is recognized, and cRNA is cotranscriptionally complexed with nucleocapsid protein (Schmaljohn, 1996). Ribonucleoprotein structures accumulate on the cytoplasmic face of membranes that have G1 and G2 embedded into them and are exposed on the luminal side (Schmaljohn, 1996). Because Rift Valley Fever lacks a matrix protein, the glycoproteins presumably recruit viral ribonucleoprotein complexes and commence budding into the lumen of the Golgi (Gerrard et al, 2002). Electron-dense, ribonucleoprotein structures have been observed immediately beneath the membranes where virus budding occurs. The viral nucleocapsid and spike structures were only seen on the portion of the Golgi vesicle membrane directly involved in the budding process and not on adjacent areas of the same membrane. Nucleocapsids are not found under membranes that did not have spikes, suggesting that an interaction of transmembrane regions of the viral glycoproteins and the nucleocapsids is prerequisite to budding (Bishop, 1996). Ribonucleoprotein complex M viral segment Cytoplasm Other The biology of the family Bunyaviridae is dominated by the M RNA since this sub-unit encodes the genes concerned in many of the most important interactions with the host. Virulence, host range, tissue tropism, transmissibility, neutralization, hemagglutination, and membrane fusion are the principal phenotypic properties that have been attributed to M RNA gene products (Pringle, 1991). Studies of the coding capacity of the M segment have identified and positioned within the ORF four protein products: the two major viral envelope glycoproteins G2 and G1, a glycosylated 78 kDa protein, and a nonglycosylated 14 kDa protein (Kakach et al., 1989). Expression of the full complement of M segment encoded proteins involves independent translational initiation events at both the first and second in-phase ATG codons, giving rise to two primary translation products which are co-translationally processed to yield the four mature proteins (Kakach et al., 1989). Polyprotein precursor mRNA Endoplasmic reticulum Other Viral polypeptides are synthesized shortly after infection, suggesting that mRNAs are transcribed and translated rapidly (Schmaljohn, 1996). All secretory proteins and membrane-bound proteins targeted to various organelles are first synthesized in the rough endoplasmic reticulum and enter the central vacuolar transport pathway. The organelles involved in this transport system include the rough and smooth endoplasmic reticulum, the cis-, medial, and trans-Golgi, the trans-Golgi network, secretory vesicles and granules, the endosomal system, lysosomes, and the plasma membrane (Matsuoka et al., 1991). Indications are that the G1 and G2 proteins are translated from a single mRNA species as a precursor polyprotein and subsequently cleaved into individual polypeptides. It was also suggested that the proteolytic cleavage occurs cotranslationally (Matsuoka et al., 1991). There is at least an order of magnitude (in molar terms) more S mRNA species than M mRNA species, which in turn is more abundant than the L mRNA species. The reasons for these differences are unknown (Bishop, 1996). Polyprotein precursor Endoplasmic reticulum Other All secretory proteins and membrane-bound proteins targeted to various organelles are first synthesized in the rough endoplasmic reticulum and enter the central vacuolar transport pathway. The organelles involved in this transport system include the rough and smooth endoplasmic reticulum, the cis-, medial, and trans-Golgi, the trans-Golgi network, secretory vesicles and granules, the endosomal system, lysosomes, and the plasma membrane (Matsuoka et al., 1991). Two virion glycoproteins, G1 and G2, are encoded by the M genome segment, translated from a single mRNA as a precursor glycoprotein, and cotranslationally cleaved into the final protein products (Chen and Compans, 1991). G1 protein Endoplasmic reticulum Other All secretory proteins and membrane-bound proteins targeted to various organelles are first synthesized in the rough endoplasmic reticulum and enter the central vacuolar transport pathway. The organelles involved in this transport system include the rough and smooth endoplasmic reticulum, the cis-, medial, and trans-Golgi, the trans-Golgi network, secretory vesicles and granules, the endosomal system, lysosomes, and the plasma membrane (Matsuoka et al., 1991). G1 and G2 (and NSm where present) are cotranslationally cleaved from the primary translation product encompassing the single open reading frame in the M RNA. Each membrane protein is preceded by a separate signal sequence for targeting of the nascent chain to, and facilitating its translocation through, the ER membrane (Pettersson and Melin, 1996). G1 protein Golgi Other Following synthesis, folding, glycosylation, and dimerization in the ER, G1 and G2 move to the Golgi, where further transport is arrested (Pettersson and Melin, 1996). Both the carboxy-terminal glycoprotein (Gc, also known as G1) and the amino-terminal glycoprotein (Gn, also known as G2) localize to the Golgi apparatus when expressed together as a polyprotein precursor. However, Gc (G1) does not localize to the Golgi apparatus when expressed in the absence of Gn (G2); it instead localizes to the endoplasmic reticulum. The Gc of all members of the genus Phlebovirus contain lysine-based ER retrieval signals at their extreme carboxy terminus. Therefore, Gc is thought to attain Golgi localization through physical interaction with Gn (Gerrard and Nichol, 2002). G1 protein Golgi membrane Other Bunyavirus glycoproteins accumulate at the membranes of the Golgi apparatus prior to virus assembly, indicating that the glycoproteins may serve to direct other structural components to the site of maturation (Matsuoka et al., 1991). NSm protein Endoplasmic reticulum Unknown Studies of the coding capacity of the M segment have identified and positioned within the ORF four protein products: the two major viral envelope glycoproteins G2 and G1, a glycosylated 78 kDa protein, and a nonglycosylated 14 kDa protein (Kakach et al., 1989). Studies of the 78 kDa and 14 kDa proteins of Rift Valley Fever showed the former to be exclusively localized to the Golgi complex of cells, while the latter was found in the Golgi as well as reticular structures. That the 78 kDa protein is glycosylated, and the 14 kDa is not suggests that the processing and transit of these two products may be distinct (Elliott et al., 1991). G1 and G2 (and NSm where present) are cotranslationally cleaved from the primary translation product encompassing the single open reading frame in the M RNA. Each membrane protein is preceded by a separate signal sequence for targeting of the nascent chain to, and facilitating its translocation through, the ER membrane (Pettersson and Melin, 1996). The fact that no sequence similarities exist with those of other viruses in the family suggests that the 14 kDa (NSm) protein may play a role more specific to phleboviruses. Possibilities for such a specific function include aspects of virus cell tropism and virus-vector transmission (Elliott et al, 1991). NSm protein Golgi Unknown Studies of the 78 kDa and 14 kDa proteins of Rift Valley Fever showed the former to be exclusively localized to the Golgi complex of cells, while the latter was found in the Golgi as well as reticular structures. That the 78 kDa protein is glycosylated, and the 14 kDa is not suggests that the processing and transit of these two products may be distinct (Elliott et al., 1991). The fact that no sequence similarities exist with those of other viruses in the family suggests that the 14 kDa (NSm) protein may play a role more specific to phleboviruses. Possibilities for such a specific function include aspects of virus cell tropism and virus-vector transmission (Elliott et al, 1991). 78 kDa protein Golgi Other Studies of the 78 kDa and 14 kDa proteins of Rift Valley Fever showed the former to be exclusively localized to the Golgi complex of cells, while the latter was found in the Golgi as well as reticular structures. That the 78 kDa protein is glycosylated, and the 14 kDa is not suggests that the processing and transit of these two products may be distinct (Elliott et al., 1991). The function of these two proteins is not known. They do not appear to be necessary for the normal synthesis, processing and transport of the major envelope glycoproteins. The 78 kDa protein, but not the 14 kDa protein, can be found in purified virion preparations (Elliott et al., 1991). G2 protein Endoplasmic reticulum Other All secretory proteins and membrane-bound proteins targeted to various organelles are first synthesized in the rough endoplasmic reticulum and enter the central vacuolar transport pathway. The organelles involved in this transport system include the rough and smooth endoplasmic reticulum, the cis-, medial, and trans-Golgi, the trans-Golgi network, secretory vesicles and granules, the endosomal system, lysosomes, and the plasma membrane (Matsuoka et al., 1991). G1 and G2 (and NSm where present) are cotranslationally cleaved from the primary translation product encompassing the single open reading frame in the M RNA. Each membrane protein is preceded by a separate signal sequence for targeting of the nascent chain to, and facilitating its translocation through, the ER membrane (Pettersson and Melin, 1996). G2 protein Golgi Other Following synthesis, folding, glycosylation, and dimerization in the ER, G1 and G2 move to the Golgi, where further transport is arrested (Pettersson and Melin, 1996). Both the carboxy-terminal glycoprotein (Gc, also known as G1) and the amino-terminal glycoprotein (Gn, also known as G2) localize to the Golgi apparatus when expressed together as a polyprotein precursor. However, Gc (G1) does not localize to the Golgi apparatus when expressed in the absence of Gn (G2); it instead localizes to the endoplasmic reticulum. The Gc of all members of the genus Phlebovirus contain lysine-based ER retrieval signals at their extreme carboxy terminus. Therefore, Gc is thought to attain Golgi localization through physical interaction with Gn (Gerrard and Nichol, 2002). The Rift Valley Fever virus Golgi localization signal mapped to a 48-amino-acid region of Gn(G2) encompassing the 20-amino-acid transmembrane domain and the adjacent 28 amino acids of the cytosolic tail (Gerrard and Nichol, 2002). G2 protein Golgi membrane Other Bunyavirus glycoproteins accumulate at the membranes of the Golgi apparatus prior to virus assembly, indicating that the glycoproteins may serve to direct other structural components to the site of maturation (Matsuoka et al., 1991). Antigenomic M segment Cytoplasm Other At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). Genomic M segment Cytoplasm Other At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). Genomic M segment Golgi membrane Other The encapsidation signal is recognized, and cRNA is cotranscriptionally complexed with nucleocapsid protein (Schmaljohn, 1996). Ribonucleoprotein structures accumulate on the cytoplasmic face of membranes that have G1 and G2 embedded into them and are exposed on the luminal side (Schmaljohn, 1996). Because Rift Valley Fever lacks a matrix protein, the glycoproteins presumably recruit viral ribonucleoprotein complexes and commence budding into the lumen of the Golgi (Gerrard et al, 2002). Electron-dense, ribonucleoprotein structures have been observed immediately beneath the membranes where virus budding occurs. The viral nucleocapsid and spike structures were only seen on the portion of the Golgi vesicle membrane directly involved in the budding process and not on adjacent areas of the same membrane. Nucleocapsids are not found under membranes that did not have spikes, suggesting that an interaction of transmembrane regions of the viral glycoproteins and the nucleocapsids is prerequisite to budding (Bishop, 1996). Ribonucleoprotein complex L viral segment Cytoplasm Other The L segment codes for the L protein (Lopez et al., 1995). All L segments of viruses in the family Bunyaviridae studied to date display conventional negative-sense coding strategies (Schmaljohn, 1996). L mRNA Cytoplasm Other Viral mRNA species are made in the cytoplasm of infected cells (Bishop, 1996). Following uncoating of viral genomes, transcription of the negative-sense viral RNA to complementary mRNA involves an interaction of the virion-associated polymerase with the RNA templates in the individual nucleocapsids (Bishop, 1996). There is at least an order of magnitude (in molar terms) more S mRNA species than M mRNA species, which in turn is more abundant than the L mRNA species. The reasons for these differences are unknown (Bishop, 1996). The L segment codes for the L protein (Lopez et al., 1995). L protein Cytoplasm Enzyme L protein is believed to be a viral transcriptase that is present in viral nucleocapsids together with viral genomes and N protein (Matsuoka et al., 1991). The L and N proteins are absolutely required and appear to suffice for transcription (Lopez et al., 1995). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). For viruses in the family Bunyaviridae, the polymerase protein, either acting alone or in concert with undefined viral or cellular factors, must first function as a cap-dependent endonuclease to generate a primer for transcription of a nonencapsidated transcript of subgenomic length. At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). L protein Golgi membrane Enzyme Ribonucleoprotein structures accumulate on the cytoplasmic face of membranes that have G1 and G2 embedded into them and are exposed on the luminal side (Schmaljohn, 1996). The virion particles of the Bunyaviridae are composed of nucleocapsids containing three different RNA species (L, M, and S) complexed with the nucleocapsid protein (N) and the virion transcriptase/polymerase L. The nucleocapsids are packaged inside a lipid envelope during budding at internal cellular membranes appearing in Golgi vesicles. The lipid envelope contains two viral glycoproteins, G1 and G2. The nucleocapsids are ribonucleoprotein complexes of viral RNA and N protein (with L protein as a minor constituent) (Hewlett and Chiu, 1991). Antigenomic L segment Cytoplasm Other At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). Genomic L segment Cytoplasm Other At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). Genomic L segment Golgi membrane Other The encapsidation signal is recognized, and cRNA is cotranscriptionally complexed with nucleocapsid protein (Schmaljohn, 1996). Ribonucleoprotein structures accumulate on the cytoplasmic face of membranes that have G1 and G2 embedded into them and are exposed on the luminal side (Schmaljohn, 1996). Because Rift Valley Fever lacks a matrix protein, the glycoproteins presumably recruit viral ribonucleoprotein complexes and commence budding into the lumen of the Golgi (Gerrard et al, 2002). Electron-dense, ribonucleoprotein structures have been observed immediately beneath the membranes where virus budding occurs. The viral nucleocapsid and spike structures were only seen on the portion of the Golgi vesicle membrane directly involved in the budding process and not on adjacent areas of the same membrane. Nucleocapsids are not found under membranes that did not have spikes, suggesting that an interaction of transmembrane regions of the viral glycoproteins and the nucleocapsids is prerequisite to budding (Bishop, 1996). Virion Golgi Other One of the earliest notable features found to distinguish members of the family Bunyaviridae from all other negative-strand viruses was that the viral particles are formed intracellularly by a budding process at smooth-surface vesicles in the Golgi area (Schmaljohn, 1996). Although the precise mechanisms of budding of enveloped viruses are not fully understood, it has been suggested that budding involves a transmembrane interaction between membrane glycoproteins and the other components of the virus in the cytoplasm, followed by pinching off from the cell surface. Upon budding, virions acquire their lipid bilayer from the host cell membrane, whereas most host cell membrane proteins are excluded from the viral particles (Matsuoka et al., 1991). The only animal Bunyaviridae member reported to bud at a site other than the Golgi is a strain of the phlebovirus Rift Valley Fever virus that was found to mature both intracellulary (in the Golgi) and at the plasma membrane in primary rat hepatocytes (Pettersson and Melin, 1996). Virion Extracellular Other After the particles bud into the Golgi cisternae, it is believed that they are released in individual small vesicles in a manner analogous to secretory granules of other cell types. The release of virus from infected cells presumable occurs when the cytoplasmic, virus-containing vesicles fuse with the cellular plasma membrane, that is, normal exocytosis (Schmaljohn, 1996). Infection and release of Rift Valley Fever virus in polarized epithelial cells occurs at both apical and basolateral membranes and hence is bi-directional (Gerrard et al., 2002). The phlebovirus Rift Valley Fever is primarily hepatotropic (Schmaljohn, 1996). Although the liver appears to be a major site of replication, Rift Valley Fever virus can be isolated from all organs of an infected animal (Gerrard et al., 2002). Generally, the viruses are cytolytic for their vertebrate host cells in tissue culture, but cause little or no cytopathology in their invertebrate host cells (Bishop, 1996). Rift Valley Fever virus-infected Vero cells displayed reduced host protein synthesis that gradually became more pronounced from 4 to 20 hr after infection (Schmaljohn, 1996). Viral receptors interact with cell membrane receptors, and the virion enters the cell Neutralizing and hemagglutination inhibiting sites are present on both the G1 and G2 proteins of Rift Valley Fever virus, suggesting that both proteins may be involved in attachment, either directly or due to conformational requirements (Schmaljohn, 1996). The nature of cell receptors involved in attachment has not been identified for any member of the family (Bishop, 1996). Electron microscopy of the infection process of Rift Valley Fever virus showed that viral particles appeared to enter cells in phagocytic vacuoles. This observation is consistent with a mode of entry similar to that first described for alphaviruses in which the virus is endocytosed in coated vesicles. These endosomes subsequently become acidified, triggering a fusion of viral membranes and endosomal membranes, which releases the nucleocapsid into the cell cytoplasm. Direct evidence for this process with viruses in the Bunyaviridae has not yet been obtained (Schmaljohn, 1996). Virion fuses with phagocytic vesicle Fusion of infected cells at acidic pH has been reported for viruses in the family Bunyaviridae as well as for numerous other enveloped viruses. The pH-dependent fusion is generally believed to related to early events in the infection process, particularly the translocation of RNA and proteins into the cell cytoplasm. Electron microscopy of the infection process of Rift Valley Fever virus showed that viral particles appeared to enter cells in phagocytic vacuoles. This observation is consistent with a mode of entry similar to that first described for alphaviruses in which the virus is endocytosed in coated vesicles. These endosomes subsequently become acidified, triggering a fusion of viral membranes and endosomal membranes, which releases the nucleocapsid into the cell cytoplasm. Direct evidence for this process with viruses in the Bunyaviridae has not yet been obtained (Schmaljohn, 1996). Virion uncoats Entry and uncoating occurs by endocytosis of virions and fusion of the viral membrane with the endosomal membrane to release the three nucleocapsids into the cell cytoplasm (Bishop, 1996). Following uncoating of viral genomes, transcription of the negative-sense viral RNA to complementary mRNA involves an interaction of the virion-associated polymerase with the RNA templates in the individual nucleocapsids (Bishop, 1996) Ribonucleoprotein segments are released into the cytoplasm The S segment codes for the N protein in the 5’ half of the viral complementary-sense molecule and for the NSs protein in the remaining 5’ half of the genomic-sense RNA, utilizing and ambisense strategy (Lopez et al., 1995). Viral polypeptides are synthesized shortly after infection, suggesting that mRNAs are transcribed and translated rapidly (Schmaljohn, 1996). The biology of the family Bunyaviridae is dominated by the M RNA since this sub-unit encodes the genes concerned in many of the most important interactions with the host. Virulence, host range, tissue tropism, transmissibility, neutralization, hemagglutination, and membrane fusion are the principal phenotypic properties that have been attributed to M RNA gene products (Pringle, 1991). Studies of the coding capacity of the M segment have identified and positioned within the ORF four protein products: the two major viral envelope glycoproteins G2 and G1, a glycosylated 78 kDa protein, and a nonglycosylated 14 kDa protein (Kakach et al., 1989). Expression of the full complement of M segment encoded proteins involves independent translational initiation events at both the first and second in-phase ATG codons, giving rise to two primary translation products which are co-translationally processed to yield the four mature proteins (Kakach et al., 1989). The L segment codes for the L protein (Lopez et al., 1995). All L segments of viruses in the family Bunyaviridae studied to date display conventional negative-sense coding strategies (Schmaljohn, 1996). S segment transcription and replication As a consequence of the ambisense expression strategy of the S RNA the synthesis of NSs protein requires prior genome replication (Giorgi 1996). The S segment codes for the N protein in the 5’ half of the viral complementary-sense molecule and for the NSs protein in the remaining 5’ half of the genomic-sense RNA, utilizing and ambisense strategy (Lopez et al., 1995). Expression of RNA molecules derived from the S segment of Rift Valley Fever virus showed that N, but not NSs, RNA molecules interfere with replication of the homologous virus. Results strongly suggest that the N sequence in sense or antisense orientation but not the protein is responsible for this inhibitory effect (Billecocq et al., 1996). There is at least an order of magnitude (in molar terms) more S mRNA species than M mRNA species, which in turn is more abundant than the L mRNA species. The reasons for these differences are unknown (Bishop, 1996). At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). NSs mRNA transcribed Transcription of the NSs mRNA takes place on the antigenome only after genome replication has begun (Giorgi 1996). As a consequence of the ambisense expression strategy of the S RNA the synthesis of NSs protein requires prior genome replication (Giorgi 1996). It has been suggested that the NSs gene has evolved during adaptation of Rift Valley Fever virus to the mammalian host and that an important role of the NSs protein was to provide a mechanism to circumvent the interferon (IFN) response of vertebrate cells (Bouloy et al., 2001). There is at least an order of magnitude (in molar terms) more S mRNA species than M mRNA species, which in turn is more abundant than the L mRNA species. The reasons for these differences are unknown (Bishop, 1996). NSs protein translated The NSs protein is a phosphoprotein of unknown function that is localized in the cytoplasm and the nuclei of infected cells where it forms filamentous structures (Kohl et al., 1999). Recent experiments showed that NSs interacts with itself to form multimers and with viral and cellular structures (Kohl et al., 1999). The NSs phosphoprotein accumulates in large amounts in the nucleus of infected cells, whereas viral replication takes place exclusively in the cytoplasm (Bouloy et al., 2001). NSs translocates to nucleus The NSs phosphoprotein accumulates in large amounts in the nucleus of infected cells (Bouloy et al., 2001). The NSs protein is of unknown function and is localized in the cytoplasm and the nuclei of infected cells where it forms filamentous structures (Kohl et al., 1999). Recent experiments showed that NSs interacts with itself to form multimers and with viral and cellular structures (Kohl et al., 1999). It has been suggested that the NSs protein is an important virulence factor that prevents alpha/beta interferons from being induced early during the course of Rift Valley Fever virus infection (Bouloy et al., 2001). N protein translation The N protein is the most abundant viral protein found in the virion and in virus-infected cells; it is associated with the genomic RNA to constitute the viral nucleocapsids. Synthesis of N protein can be detected early after infection (2 hour postinfection) and the protein has a half-life of several hours (Giorgi 1996). The L and N proteins are absolutely required and appear to suffice for transcription (Lopez et al., 1995). In phlebovirus-infected cells the N protein seems to accumulate in the Golgi region during later stages of infection. The Golgi accumulation is thought to be caused by the association of the cytoplasmic ribonucleoprotein with the transmembranal sequences of the viral envelope glycoproteins, which reside in the Golgi complex (Giorgi 1996). The nucleocapsid assembly process for phleboviruses has not yet been elucidated, but it is thought to be controlled at the level of initiation by interaction of the N protein with a specific sequence on its target RNA (Giorgi 1996). N protein translocates to Golgi membrane The encapsidation signal is recognized, and cRNA is cotranscriptionally complexed with nucleocapsid protein (Schmaljohn, 1996). In phlebovirus-infected cells the N protein seems to accumulate in the Golgi region during later stages of infection. The Golgi accumulation is thought to be caused by the association of the cytoplasmic ribonucleoprotein with the transmembranal sequences of the viral envelope glycoproteins, which reside in the Golgi complex (Giorgi 1996). Ribonucleoprotein structures accumulate on the cytoplasmic face of membranes that have G1 and G2 embedded into them and are exposed on the luminal side (Schmaljohn, 1996). The helical nucleocapsids were found to line up underneath the membrane of distended Golgi vesicles. As G1 and G2 accumulated in the Golgi complex, progressively more nucleocapsids also entered the Golgi region. Little if any N protein was seen associated with the ER or the plasma membrane. Thus, a specific interaction between the nucleocapsids and membranes containing the viral glycoproteins seems to exist only in the Golgi complex. Why no such interaction appear to occur already in the ER, which also contains high amounts of G1 and G2, is not clear, but it may relate to incorrect conformation or organization of the spikes, or to the topology or accessibility of the cytoplasmic tail of one of the glycoproteins that is likely to interact with the nucleocapsids (Pettersson and Melin, 1996). Replication results in genomic S segment At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). Genomic S segment translocates to Golgi membrane The encapsidation signal is recognized, and cRNA is cotranscriptionally complexed with nucleocapsid protein (Schmaljohn, 1996). Ribonucleoprotein structures accumulate on the cytoplasmic face of membranes that have G1 and G2 embedded into them and are exposed on the luminal side (Schmaljohn, 1996). Because RVF lacks a matrix protein, the glycoproteins presumably recruit viral ribonucleoprotein complexes and commence budding into the lumen of the Golgi (Gerrard et al, 2002). Electron-dense, ribonucleoprotein structures have been observed immediately beneath the membranes where virus budding occurs. The viral nucleocapsid and spike structures were only seen on the portion of the Golgi vesicle membrane directly involved in the budding process and not on adjacent areas of the same membrane. Nucleocapsids are not found under membranes that did not have spikes, suggesting that an interaction of transmembrane regions of the viral glycoproteins and the nucleocapsids is prerequisite to budding (Bishop, 1996). Transcription and replication of M segment Viral polypeptides are synthesized shortly after infection, suggesting that mRNAs are transcribed and translated rapidly (Schmaljohn, 1996). All secretory proteins and membrane-bound proteins targeted to various organelles are first synthesized in the rough endoplasmic reticulum and enter the central vacuolar transport pathway. The organelles involved in this transport system include the rough and smooth endoplasmic reticulum, the cis-, medial, and trans-Golgi, the trans-Golgi network, secretory vesicles and granules, the endosomal system, lysosomes, and the plasma membrane (Matsuoka et al., 1991). Indications are that the G1 and G2 proteins are translated from a single mRNA species as a precursor polyprotein and subsequently cleaved into individual polypeptides. It was also suggested that the proteolytic cleavage occurs cotranslationally (Matsuoka et al., 1991). At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). Polyprotein mRNA translated All secretory proteins and membrane-bound proteins targeted to various organelles are first synthesized in the rough endoplasmic reticulum and enter the central vacuolar transport pathway. The organelles involved in this transport system include the rough and smooth endoplasmic reticulum, the cis-, medial, and trans-Golgi, the trans-Golgi network, secretory vesicles and granules, the endosomal system, lysosomes, and the plasma membrane (Matsuoka et al., 1991). Two virion glycoproteins, G1 and G2, are encoded by the M genome segment, translated from a single mRNA as a precursor glycoprotein, and cotranslationally cleaved into the final protein products (Chen and Compans, 1991). There is at least an order of magnitude (in molar terms) more S mRNA species than M mRNA species, which in turn is more abundant than the L mRNA species. The reasons for these differences are unknown (Bishop, 1996). Polyprotein processing All secretory proteins and membrane-bound proteins targeted to various organelles are first synthesized in the rough endoplasmic reticulum and enter the central vacuolar transport pathway. The organelles involved in this transport system include the rough and smooth endoplasmic reticulum, the cis-, medial, and trans-Golgi, the trans-Golgi network, secretory vesicles and granules, the endosomal system, lysosomes, and the plasma membrane (Matsuoka et al., 1991). G1 and G2 (and NSm where present) are cotranslationally cleaved from the primary translation product encompassing the single open reading frame in the M RNA. Each membrane protein is preceded by a separate signal sequence for targeting of the nascent chain to, and facilitating its translocation through, the ER membrane (Pettersson and Melin, 1996). Studies of the coding capacity of the M segment have identified and positioned within the ORF four protein products: the two major viral envelope glycoproteins G2 and G1, a glycosylated 78 kDa protein, and a nonglycosylated 14 kDa protein (Kakach et al., 1989). Studies of the 78 kDa and 14 kDa proteins of Rift Valley Fever showed the former to be exclusively localized to the Golgi complex of cells, while the latter was found in the Golgi as well as reticular structures. That the 78 kDa protein is glycosylated, and the 14 kDa is not suggests that the processing and transit of these two products may be distinct (Elliott et al., 1991). G1 and G2 (and NSm where present) are cotranslationally cleaved from the primary translation product encompassing the single open reading frame in the M RNA. Each membrane protein is preceded by a separate signal sequence for targeting of the nascent chain to, and facilitating its translocation through, the ER membrane (Pettersson and Melin, 1996). The fact that no sequence similarities exist with those of other viruses in the family suggests that the 14 kDa (NSm) protein may play a role more specific to phleboviruses. Possibilities for such a specific function include aspects of virus cell tropism and virus-vector transmission (Elliott et al, 1991). The 78 kDa protein, but not the 14 kDa protein, can be found in purified virion preparations (Elliott et al., 1991). The function of these two proteins is not known. They do not appear to be necessary for the normal synthesis, processing and transport of the major envelope glycoproteins (Elliott et al., 1991). G1 translocates to Golgi Following synthesis, folding, glycosylation, and dimerization in the ER, G1 and G2 move to the Golgi, where further transport is arrested (Pettersson and Melin, 1996). Both the carboxy-terminal glycoprotein (Gc, also known as G1) and the amino-terminal glycoprotein (Gn, also known as G2) localize to the Golgi apparatus when expressed together as a polyprotein precursor. However, Gc (G1) does not localize to the Golgi apparatus when expressed in the absence of Gn (G2); it instead localizes to the endoplasmic reticulum. The Gc of all members of the genus Phlebovirus contain lysine-based ER retrieval signals at their extreme carboxy terminus. Therefore, Gc is thought to attain Golgi localization through physical interaction with Gn (Gerrard and Nichol, 2002). G1 on Golgi membrane Bunyavirus glycoproteins accumulate at the membranes of the Golgi apparatus prior to virus assembly, indicating that the glycoproteins may serve to direct other structural components to the site of maturation (Matsuoka et al., 1991). NSm translocates to Golgi Studies of the 78 kDa and 14 kDa proteins of Rift Valley Fever showed the former to be exclusively localized to the Golgi complex of cells, while the latter was found in the Golgi as well as reticular structures. That the 78 kDa protein is glycosylated, and the 14 kDa is not suggests that the processing and transit of these two products may be distinct (Elliott et al., 1991). The fact that no sequence similarities exist with those of other viruses in the family suggests that the 14 kDa (NSm) protein may play a role more specific to phleboviruses. Possibilities for such a specific function include aspects of virus cell tropism and virus-vector transmission (Elliott et al, 1991). G2 translocates to Golgi Following synthesis, folding, glycosylation, and dimerization in the ER, G1 and G2 move to the Golgi, where further transport is arrested (Pettersson and Melin, 1996). Both the carboxy-terminal glycoprotein (Gc, also known as G1) and the amino-terminal glycoprotein (Gn, also known as G2) localize to the Golgi apparatus when expressed together as a polyprotein precursor. However, Gc (G1) does not localize to the Golgi apparatus when expressed in the absence of Gn (G2); it instead localizes to the endoplasmic reticulum. The Gc of all members of the genus Phlebovirus contain lysine-based ER retrieval signals at their extreme carboxy terminus. Therefore, Gc is thought to attain Golgi localization through physical interaction with Gn (Gerrard and Nichol, 2002). The Rift Valley Fever virus Golgi localization signal mapped to a 48-amino-acid region of Gn(G2) encompassing the 20-amino-acid transmembrane domain and the adjacent 28 amino acids of the cytosolic tail (Gerrard and Nichol, 2002). G2 on Golgi membrane Bunyavirus glycoproteins accumulate at the membranes of the Golgi apparatus prior to virus assembly, indicating that the glycoproteins may serve to direct other structural components to the site of maturation (Matsuoka et al., 1991). Replication results in genomic M segment At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). Genomic M segment translocates to Golgi membrane The encapsidation signal is recognized, and cRNA is cotranscriptionally complexed with nucleocapsid protein (Schmaljohn, 1996). Ribonucleoprotein structures accumulate on the cytoplasmic face of membranes that have G1 and G2 embedded into them and are exposed on the luminal side (Schmaljohn, 1996). Because Rift Valley Fever lacks a matrix protein, the glycoproteins presumably recruit viral ribonucleoprotein complexes and commence budding into the lumen of the Golgi (Gerrard et al, 2002). Electron-dense, ribonucleoprotein structures have been observed immediately beneath the membranes where virus budding occurs. The viral nucleocapsid and spike structures were only seen on the portion of the Golgi vesicle membrane directly involved in the budding process and not on adjacent areas of the same membrane. Nucleocapsids are not found under membranes that did not have spikes, suggesting that an interaction of transmembrane regions of the viral glycoproteins and the nucleocapsids is prerequisite to budding (Bishop, 1996). Transcription and replication of L segment Viral mRNA species are made in the cytoplasm of infected cells (Bishop, 1996). Following uncoating of viral genomes, transcription of the negative-sense viral RNA to complementary mRNA involves an interaction of the virion-associated polymerase with the RNA templates in the individual nucleocapsids (Bishop, 1996). There is at least an order of magnitude (in molar terms) more S mRNA species than M mRNA species, which in turn is more abundant than the L mRNA species. The reasons for these differences are unknown (Bishop, 1996). The L segment codes for the L protein (Lopez et al., 1995). At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). L mRNA translated L protein is believed to be a viral transcriptase that is present in viral nucleocapsids together with viral genomes and N protein (Matsuoka et al., 1991). The L and N proteins are absolutely required and appear to suffice for transcription (Lopez et al., 1995). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). For viruses in the family Bunyaviridae, the polymerase protein, either acting alone or in concert with undefined viral or cellular factors, must first function as a cap-dependent endonuclease to generate a primer for transcription of a nonencapsidated transcript of subgenomic length. At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). L protein translocates to Golgi membrane Ribonucleoprotein structures accumulate on the cytoplasmic face of membranes that have G1 and G2 embedded into them and are exposed on the luminal side (Schmaljohn, 1996). The virion particles of the Bunyaviridae are composed of nucleocapsids containing three different RNA species (L, M, and S) complexed with the nucleocapsid protein (N) and the virion transcriptase/polymerase L. The nucleocapsids are packaged inside a lipid envelope during budding at internal cellular membranes appearing in Golgi vesicles. The lipid envelope contains two viral glycoproteins, G1 and G2. The nucleocapsids are ribonucleoprotein complexes of viral RNA and N protein (with L protein as a minor constituent) (Hewlett and Chiu, 1991). Replication to form genomic L segment At some point, the polymerase must switch to a process of independently initiating transcription at the precise 3’ end of the template and producing an encapsidated, full-length transcript. The processes involved in making that switch from primary transcription to genome replication have not been defined completely for any member of the family Bunyaviridae. Presumably, some viral or host factor is required to signal a suppression of the transcription termination signal responsible for generation of truncated mRNA and also to prevent the addition of the capped and methylated structures to the 5’ termini of the cRNAs (Schmaljohn, 1996). The L protein of Bunyaviridae is able to synthesize both vRNA and cRNA, suggesting that this protein is involved both in primary transcription and genome replication. In negative strand viruses, the change from primary transcription to replication requires a switch from mRNA synthesis to synthesis of full-length cRNA templates and then vRNA (Schmaljohn, 1996). Replication occurs in the cytoplasm (Pringle, 1996). Genomic L segment translocates to Golgi membrane The encapsidation signal is recognized, and cRNA is cotranscriptionally complexed with nucleocapsid protein (Schmaljohn, 1996). Ribonucleoprotein structures accumulate on the cytoplasmic face of membranes that have G1 and G2 embedded into them and are exposed on the luminal side (Schmaljohn, 1996). Because Rift Valley Fever lacks a matrix protein, the glycoproteins presumably recruit viral ribonucleoprotein complexes and commence budding into the lumen of the Golgi (Gerrard et al, 2002). Electron-dense, ribonucleoprotein structures have been observed immediately beneath the membranes where virus budding occurs. The viral nucleocapsid and spike structures were only seen on the portion of the Golgi vesicle membrane directly involved in the budding process and not on adjacent areas of the same membrane. Nucleocapsids are not found under membranes that did not have spikes, suggesting that an interaction of transmembrane regions of the viral glycoproteins and the nucleocapsids is prerequisite to budding (Bishop, 1996). Virion buds into Golgi One of the earliest notable features found to distinguish members of the family Bunyaviridae from all other negative-strand viruses was that the viral particles are formed intracellularly by a budding process at smooth-surface vesicles in the Golgi area (Schmaljohn, 1996). Although the precise mechanisms of budding of enveloped viruses are not fully understood, it has been suggested that budding involves a transmembrane interaction between membrane glycoproteins and the other components of the virus in the cytoplasm, followed by pinching off from the cell surface. Upon budding, virions acquire their lipid bilayer from the host cell membrane, whereas most host cell membrane proteins are excluded from the viral particles (Matsuoka et al., 1991). The only animal Bunyaviridae member reported to bud at a site other than the Golgi is a strain of the phlebovirus Rift Valley Fever virus that was found to mature both intracellulary (in the Golgi) and at the plasma membrane in primary rat hepatocytes (Pettersson and Melin, 1996). Virion leaves cell After the particles bud into the Golgi cisternae, it is believed that they are released in individual small vesicles in a manner analogous to secretory granules of other cell types. The release of virus from infected cells presumable occurs when the cytoplasmic, virus-containing vesicles fuse with the cellular plasma membrane, that is, normal exocytosis (Schmaljohn, 1996). Infection and release of Rift Valley Fever virus in polarized epithelial cells occurs at both apical and basolateral membranes and hence is bi-directional (Gerrard et al., 2002). The phlebovirus Rift Valley Fever is primarily hepatotropic (Schmaljohn, 1996). Although the liver appears to be a major site of replication, Rift Valley Fever virus can be isolated from all organs of an infected animal (Gerrard et al., 2002). Generally, the viruses are cytolytic for their vertebrate host cells in tissue culture, but cause little or no cytopathology in their invertebrate host cells (Bishop, 1996). Rift Valley Fever virus-infected Vero cells displayed reduced host protein synthesis that gradually became more pronounced from 4 to 20 hr after infection (Schmaljohn, 1996). TEXT TEXT Rift Valley Fever virus molecular pathway The methods of cell invasion and viral multiplication within vertebrate cells are described for Rift Valley Fever virus, a member of the family Bunyaviridae and the genus Phlebovirus. Lopez N, Muller R, Prehaud C, Bouloy M Journal of virology 1995 69 7 3972 3979 Bouloy M, Janzen C, Vialat P, Khun H, Pavlovic J, Huerre M, Haller O Journal of Virology 2001 75 3 1371 1377 Kohl A, di Bartolo V, Bouloy M Virology 1999 263 2 517 525 Rift Valley fever virus M segment: phlebovirus expression strategy and protein glycosylation Virology 1989 170 2 505 510 Gerrard SR, Nichol ST Journal of Virology 2002 76 23 12200 12210 Gerrard SR, Rollin PE, Nichol ST Virology 2002 301 2 226 235 Billecocq A, Vazeille-Falcoz M, Rodhain F, Bouloy M Journal of General Virology 2000 81 Pt 9 2161 2166 Chen SY, Compans RW Journal of Virology 1991 65 11 5902 5909 TEXT TEXT TEXT TEXT TEXT TEXT Hewlett MJ, Chiu W Virion structure Kolakofsky E 1991 79-90 Springer-Verlag Berlin Pettersson RF, Melin L Synthesis, assembly, and intracellular transport of Bunyaviridae membrane proteins Elliot, RM 1996 159-188 Plenum Press Matsuoka Y, Chen SY, Compans RW Bunyavirus protein transport and assembly Kolakofsky E 1991 162-179 Springer-Verlag Berlin Pringle, CR Genetics and genome segment reassortment Elliot, RM 1996 189-226 Plenum Press Bishop, DHL Biology and molecular biology of Bunyaviruses Elliot, RM 1996 19-61 Plenum Press Elliott RM, Schmaljohn CS, Collett MS Bunyaviridae genome structure and gene expression Kolakofsky E 1991 91-141 Springer-Verlag Berlin Giorgi C Molecular biology of phleboviruses Elliot, RM 1996 105-128 Plenum Press Schmaljohn CS Bunyaviridae: The viruses and their replication Fields BN, Knipe DM, Howley, PM 1996 1447-1471 Lippincott-Raven Publishers TEXT TEXT

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