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

Mechanisms of bunyavirus morphogenesis and egress

Unlike many segmented negative-sense RNA viruses, most members of the Bunyavirales bud at Golgi membranes, as opposed to the plasma membrane. Central players in this assembly process are the envelope glycoproteins, Gn and Gc, which upon translation undergo proteolytic processing, glycosylation and trafficking to the Golgi, where they interact with ribonucleoprotein genome segments and bud into Golgi-derived compartments. The processes involved in genome packaging during virion assembly can lead to the generation of reassorted viruses, if a cell is co-infected with two different bunyaviruses, due to mismatching of viral genome segment packaging. This can lead to viruses with high pathogenic potential, as demonstrated by the emergence of Schmallenberg virus. This review focuses on the assembly pathways of tri-segmented bunyaviruses, highlighting some areas in need of further research to understand these important pathogens with zoonotic potential.

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

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

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

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

Rice stripe tenuivirus NSvc2 glycoproteins targeted to the golgi body by the N-terminal transmembrane domain and adjacent cytosolic 24 amino acids via the COP I- and COP II-dependent secretion pathway

The NSvc2 glycoproteins encoded by Rice stripe tenuivirus (RSV) share many characteristics common to the glycoproteins found among Bunyaviridae. Within this viral family, glycoproteins targeting to the Golgi apparatus play a pivotal role in the maturation of the enveloped spherical particles. RSV particles, however, adopt a long filamentous morphology. Recently, RSV NSvc2 glycoproteins were shown to localize exclusively to the ER in Sf9 insect cells. Here, we demonstrate that the amino-terminal NSvc2 (NSvc2-N) targets to the Golgi apparatus in Nicotiana benthamiana cells, whereas the carboxyl-terminal NSvc2 (NSvc2-C) accumulates in the endoplasmic reticulum (ER). Upon coexpression, NSvc2-N redirects NSvc2-C from the ER to the Golgi bodies. The NSvc2 glycoproteins move together with the Golgi stacks along the ER/actin network. The targeting of the NSvc2 glycoproteins to the Golgi bodies was strictly dependent on functional anterograde traffic out of the ER to the Golgi bodies or on a retrograde transport route from the Golgi apparatus. The analysis of truncated and chimeric NSvc2 proteins demonstrates that the Golgi targeting signal comprises amino acids 269 to 315 of NSvc2-N, encompassing the transmembrane domain and 24 adjacent amino acids in the cytosolic tail. Our findings demonstrate for the first time that the glycoproteins from an unenveloped Tenuivirus could target Golgi bodies in plant cells.

IMPORTANCE

NSvc2 glycoprotein encoded by unenveloped Rice stripe tenuivirus (RSV) share many characteristics in common with glycoprotein found among Bunyaviridae in which all members have membrane-enveloped sphere particle. Recently, RSV NSvc2 glycoproteins were shown to localize exclusively to the ER in Sf9 insect cells. In this study, we demonstrated that the RSV glycoproteins could target Golgi bodies in plant cells. The targeting of NSvc2 glycoproteins to the Golgi bodies was dependent on active COP II or COP I. The Golgi targeting signal was mapped to the 23-amino-acid transmembrane domain and the adjacent 24 amino acids of the cytosolic tail of the NSvc2-N. In light of the evidence from viruses in Bunyaviridae that targeting Golgi bodies is important for the viral particle assembly and vector transmission, we propose that targeting of RSV glycoproteins into Golgi bodies in plant cells represents a physiologically relevant mechanism in the maturation of RSV particle complex for insect vector transmission.

Reverse genetics technology for Rift Valley fever virus: current and future applications for the development of therapeutics and vaccines

The advent of reverse genetics technology has revolutionized the study of RNA viruses, making it possible to manipulate their genomes and evaluate the effects of these changes on their biology and pathogenesis. The fundamental insights gleaned from reverse genetics-based studies over the last several years provide a new momentum for the development of designed therapies for the control and prevention of these viral pathogens. This review summarizes the successes and stumbling blocks in the development of reverse genetics technologies for Rift Valley fever virus and their application to the further dissection of its pathogenesis and the design of new therapeutics and safe and effective vaccines.

Electron cryo-microscopy and single-particle averaging of Rift Valley fever virus: evidence for GN-GC glycoprotein heterodimers

Rift Valley fever virus (RVFV) is a member of the genus Phlebovirus within the family Bunyaviridae. It is a mosquito-borne zoonotic agent that can cause hemorrhagic fever in humans. The enveloped RVFV virions are known to be covered by capsomers of the glycoproteins G(N) and G(C), organized on a T=12 icosahedral lattice. However, the structural units forming the RVFV capsomers have not been determined. Conflicting biochemical results for another phlebovirus (Uukuniemi virus) have indicated the existence of either G(N) and G(C) homodimers or G(N)-G(C) heterodimers in virions. Here, we have studied the structure of RVFV using electron cryo-microscopy combined with three-dimensional reconstruction and single-particle averaging. The reconstruction at 2.2-nm resolution revealed the organization of the glycoprotein shell, the lipid bilayer, and a layer of ribonucleoprotein (RNP). Five- and six-coordinated capsomers are formed by the same basic structural unit. Molecular-mass measurements suggest a G(N)-G(C) heterodimer as the most likely candidate for this structural unit. Both leaflets of the lipid bilayer were discernible, and the glycoprotein transmembrane densities were seen to modulate the curvature of the lipid bilayer. RNP densities were situated directly underneath the transmembrane densities, suggesting an interaction between the glycoprotein cytoplasmic tails and the RNPs. The success of the single-particle averaging approach taken in this study suggests that it is applicable in the study of other phleboviruses, as well, enabling higher-resolution description of these medically important pathogens.

Rift Valley fever virus structural proteins: expression, characterization and assembly of recombinant proteins

Background

Studies on Rift Valley Fever Virus (RVFV) infection process and morphogenesis have been hampered due to the biosafety conditions required to handle this virus, making alternative systems such as recombinant virus-like particles, that may facilitate understanding of these processes are highly desirable. In this report we present the expression and characterization of RVFV structural proteins N, Gn and Gc and demonstrate the efficient generation of RVFV virus-like particles (VLPs) using a baculovirus expression system.

Results

A recombinant baculovirus, expressing nucleocapsid (N) protein of RVFV at high level under the control of the polyhedrin promoter was generated. Gel filtration analysis indicated that expressed N protein could form complex multimers. Further, N protein complex when visualized by electron microscopy (EM) exhibited particulate, nucleocapsid like-particles (NLPs). Subsequently, a single recombinant virus was generated that expressed the RVFV glycoproteins (Gn/Gc) together with the N protein using a dual baculovirus vector. Both the Gn and Gc glycoproteins were detected not only in the cytoplasm but also on the cell surface of infected cells. Moreover, expression of the Gn/Gc in insect cells was able to induce cell-cell fusion after a low pH shift indicating the retention of their functional characteristics. In addition, assembly of these three structural proteins into VLPs was identified by purification of cells' supernatant through potassium tartrate-glycerol gradient centrifugation followed by EM analysis. The purified particles exhibited enveloped structures that were similar to the structures of the wild-type RVFV virion particle. In parallel, a second recombinant virus was constructed that expressed only Gc protein together with N protein. This dual recombinant virus also generated VLPs with clear spiky structures, but appeared to be more pleomorphic than the VLPs with both glycoproteins, suggesting that Gc and probably also Gn interacts with N protein complex independent of each other.

Conclusion

Our results suggest that baculovirus expression system has enormous potential to produce large amount of VLPs that may be used both for fundamental and applied research of RVFV.

The cytoplasmic tails of Uukuniemi Virus (Bunyaviridae) G(N) and G(C) glycoproteins are important for intracellular targeting and the budding of virus-like particles

Functional motifs within the cytoplasmic tails of the two glycoproteins G(N) and G(C) of Uukuniemi virus (UUK) (Bunyaviridae family) were identified with the help of our recently developed virus-like particle (VLP) system for UUK virus (A. K. Overby, V. Popov, E. P. Neve, and R. F. Pettersson, J. Virol. 80:10428-10435, 2006). We previously reported that information necessary for the packaging of ribonucleoproteins into VLPs is located within the G(N) cytoplasmic tail (A. K. Overby, R. F. Pettersson, and E. P. Neve, J. Virol. 81:3198-3205, 2007). The G(N) glycoprotein cytoplasmic tail specifically interacts with the ribonucleoproteins and is critical for genome packaging. In addition, two other regions in the G(N) cytoplasmic tail, encompassing residues 21 to 25 and 46 to 50, were shown to be important for particle generation and release. By the introduction of point mutations within these two regions, we demonstrate that leucines at positions 23 and 24 are crucial for the initiation of VLP budding, while leucine 46, glutamate 47, and leucine 50 are important for efficient exit from the endoplasmic reticulum and subsequent transport to the Golgi complex. We found that budding and particle generation are highly dependent on the intracellular localization of both glycoproteins. The short cytoplasmic tail of UUK G(C) contains a lysine at position -3 from the C terminus that is highly conserved among members of the Phlebovirus, Hantavirus, and Orthobunyavirus genera. Mutating this single amino acid residue in G(C) resulted in the mislocalization of not only G(C) but also G(N) to the plasma membrane, and VLP generation was compromised in cells expressing this mutant. Together, these results demonstrate that the cytoplasmic tails of both G(N) and G(C) contain specific information necessary for efficient virus particle generation.

Mutagenesis of the La Crosse Virus glycoprotein supports a role for Gc (1066-1087) as the fusion peptide

The La Crosse Virus (LACV) M segment encodes two glycoproteins (Gn and Gc), and plays a critical role in the neuropathogenesis of LACV infection as the primary determinant of neuroinvasion. A recent study from our group demonstrated that the region comprising the membrane proximal two-thirds of Gc, amino acids 860-1442, is critical in mediating LACV fusion and entry. Furthermore, computational analysis identified structural similarities between a portion of this region, amino acids 970-1350, and the E1 fusion protein of two alphaviruses: Sindbis virus and Semliki Forrest virus (SFV). Within the region 970-1350, a 22-amino-acid hydrophobic segment (1066-1087) is predicted to correlate structurally with the fusion peptides of class II fusion proteins. We performed site-directed mutagenesis of key amino acids in this 22-amino acid segment and determined the functional consequences of these mutations on fusion and entry. Several mutations within this hydrophobic domain affected glycoprotein expression to some extent, but all mutations either shifted the pH threshold of fusion below that of the wild-type protein, reduced fusion efficiency, or abrogated cell-to-cell fusion and pseudotype entry altogether. These results, coupled with the aforementioned computational modeling, suggest that the LACV Gc functions as a class II fusion protein and support a role for the region Gc 1066-1087 as a fusion peptide.

Presence of broadly reactive and group-specific neutralizing epitopes on newly described isolates of Crimean-Congo hemorrhagic fever virus

Crimean-Congo hemorrhagic fever virus (CCHFV), a member of the genus Nairovirus of the family Bunyaviridae, causes severe disease in humans with high rates of mortality. The virus has a tripartite genome composed of a small (S), a medium (M) and a large (L) RNA segment; the M segment encodes the two viral glycoproteins, G(N) and G(C). Whilst relatively few full-length M segment sequences are available, it is apparent that both G(N) and G(C) may exhibit significant sequence diversity. It is unknown whether considerable antigenic differences exist between divergent CCHFV strains, or whether there are conserved neutralizing epitopes. The M segments derived from viral isolates of a human case of CCHF in South Africa (SPU 41/84), an infected tick (Hyalomma marginatum) in South Africa (SPU 128/81), a human case in Congo (UG 3010), an infected individual in Uzbekistan (U2-2-002) and an infected tick (Hyalomma asiaticum) in China (Hy13) were sequenced fully, and the glycoproteins were expressed. These novel sequences showed high variability in the N-terminal region of G(N) and more modest differences in the remainder of G(N) and in G(C). Phylogenetic analyses placed these newly identified strains in three of the four previously described M segment groups. Studies with a panel of mAbs specific to G(N) and G(C) indicated that there were significant antigenic differences between the M segment groups, although several neutralizing epitopes in both G(N) and G(C) were conserved among all strains examined. Thus, the genetic diversity exhibited by CCHFV strains results in significant antigenic differences that will need to be taken into consideration for vaccine development.

Role of N-linked glycans on bunyamwera virus glycoproteins in intracellular trafficking, protein folding, and virus infectivity

The membrane glycoproteins (Gn and Gc) of Bunyamwera virus (BUN, family Bunyaviridae) contain three potential sites for the attachment of N-linked glycans: one site (N60) on Gn and two (N624 and N1169) on Gc. We determined that all three sites are glycosylated. Digestion of the glycoproteins with endo-beta-N-acetylglucosaminidase H (endo H) or peptide:N-glycosidase F revealed that Gn and Gc differ significantly in their glycan status and that late in infection Gc glycans remain endo H sensitive. The roles of the N-glycans in intracellular trafficking of the glycoproteins to the Golgi, protein folding, and virus replication were investigated by mutational analysis and confocal immunofluorescence. Elimination of the glycan on Gn, by changing N60 to a Q residue, resulted in the protein misfolding and failure of both Gn and Gc proteins to traffic to the Golgi complex. We were unable to rescue a viable virus by reverse genetics from a cDNA containing the N60Q mutation. In contrast, mutant Gc proteins lacking glycans on either N624 or N1169, or both sites, were able to target to the Golgi. Gc proteins containing mutations N624Q and N1169Q acquired endo H resistance. Three viable N glycosylation-site-deficient viruses, lacking glycans on one site or both sites on Gc, were created by reverse genetics. The viability of these recombinant viruses and analysis of growth kinetics indicates that the glycans on Gc are not essential for BUN replication, but they do contribute to the efficiency of virus infection.

Trafficking of viral membrane proteins

Many viruses express membrane proteins. For enveloped viruses in particular, membrane proteins are frequently structural components of the virus that mediate the essential tasks of receptor recognition and membrane fusion. The functional activities of these proteins require that they are sorted correctly in infected cells. These sorting events often depend on the ability of the virus to mimic cellular protein trafficking signals and to interact with the cellular trafficking machinery. Importantly, loss or modification of these signals can influence virus infectivity and pathogenesis.

Cellular localization and antigenic characterization of crimean-congo hemorrhagic fever virus glycoproteins

Crimean-Congo hemorrhagic fever virus (CCHFV), a member of the genus Nairovirus of the family Bunyaviridae, causes severe disease with high rates of mortality in humans. The CCHFV M RNA segment encodes the virus glycoproteins G(N) and G(C). To understand the processing and intracellular localization of the CCHFV glycoproteins as well as their neutralization and protection determinants, we produced and characterized monoclonal antibodies (MAbs) specific for both G(N) and G(C). Using these MAbs, we found that G(N) predominantly colocalized with a Golgi marker when expressed alone or with G(C), while G(C) was transported to the Golgi apparatus only in the presence of G(N). Both proteins remained endo-beta-N-acetylglucosaminidase H sensitive, indicating that the CCHFV glycoproteins are most likely targeted to the cis Golgi apparatus. Golgi targeting information partly resides within the G(N) ectodomain, because a soluble version of G(N) lacking its transmembrane and cytoplasmic domains also localized to the Golgi apparatus. Coexpression of soluble versions of G(N) and G(C) also resulted in localization of soluble G(C) to the Golgi apparatus, indicating that the ectodomains of these proteins are sufficient for the interactions needed for Golgi targeting. Finally, the mucin-like and P35 domains, located at the N terminus of the G(N) precursor protein and removed posttranslationally by endoproteolysis, were required for Golgi targeting of G(N) when it was expressed alone but were dispensable when G(C) was coexpressed. In neutralization assays on SW-13 cells, MAbs to G(C), but not to G(N), prevented CCHFV infection. However, only a subset of G(C) MAbs protected mice in passive-immunization experiments, while some nonneutralizing G(N) MAbs efficiently protected animals from a lethal CCHFV challenge. Thus, neutralization of CCHFV likely depends not only on the properties of the antibody, but on host cell factors as well. In addition, nonneutralizing antibody-dependent mechanisms, such as antibody-dependent cell-mediated cytotoxicity, may be involved in the in vivo protection seen with the MAbs to G(C).

Intracellular localization of Crimean-Congo Hemorrhagic Fever (CCHF) virus glycoproteins

Background

Crimean-Congo Hemorrhagic Fever virus (CCHFV), a member of the genus Nairovirus, family Bunyaviridae, is a tick-borne pathogen causing severe disease in humans. To better understand the CCHFV life cycle and explore potential intervention strategies, we studied the biosynthesis and intracellular targeting of the glycoproteins, which are encoded by the M genome segment.

Results

Following determination of the complete genome sequence of the CCHFV reference strain IbAr10200, we generated expression plasmids for the individual expression of the glycoproteins G_N and G_C, using CMV- and chicken β-actin-driven promoters. The cellular localization of recombinantly expressed CCHFV glycoproteins was compared to authentic glycoproteins expressed during virus infection using indirect immunofluorescence assays, subcellular fractionation/western blot assays and confocal microscopy. To further elucidate potential intracellular targeting/retention signals of the two glycoproteins, GFP-fusion proteins containing different parts of the CCHFV glycoprotein were analyzed for their intracellular targeting. The N-terminal glycoprotein G_N localized to the Golgi complex, a process mediated by retention/targeting signal(s) in the cytoplasmic domain and ectodomain of this protein. In contrast, the C-terminal glycoprotein G_C remained in the endoplasmic reticulum but could be rescued into the Golgi complex by co-expression of G_N.

Conclusion

The data are consistent with the intracellular targeting of most bunyavirus glycoproteins and support the general model for assembly and budding of bunyavirus particles in the Golgi compartment.

Sin Nombre virus glycoprotein trafficking

Sin Nombre virus (SNV) is a major representative of the New World hantaviruses and the most common cause of hantavirus pulmonary syndrome (HPS) with high mortality in North America. Unlike other members of the family Bunyaviridae which mature in the Golgi complex, New World hantaviruses have been previously reported to mature at the cell surface. For family Bunyaviridae viruses, retention of the viral glycoproteins at the Golgi complex is thought to be responsible for their Golgi maturation. In our studies, the majority of SNV glycoproteins, G1 and G2, was localized in the Golgi complex when expressed from a full-length GPC clone or in SNV-infected cells, in agreement with data for other members of the family Bunyaviridae, including the Old World hantaviruses. However, the SNV glycoproteins could also be detected at the cell surface at advanced posttransfection or postinfection time points. G1 expressed in the absence of G2 did not accumulate in the Golgi, but remained predominantly associated with the endoplasmic reticulum (ER). Overexpressed amounts of apparently misfolded G1 were aggregated in a subcellular compartment likely to represent the aggresome. Unexpectedly, an additional major pool of G1 was detected intracellularly in SNV-infected and GPC-expressing transfected cells, by using a SNV G1-specific Fab antibody. This pool of G1 is predominantly localized in late endosomes-lysosomes.

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.

Murine leukemia virus pseudotypes of La Crosse and Hantaan Bunyaviruses: a system for analysis of cell tropism

We have developed a system for the preparation of La Crosse virus (LAC) and Hantaan virus (HTN) pseudotypes using a murine leukemia virus vector. After concentration, the pseudotypes were present in quantities sufficient to analyze cell tropism and neutralization. Cells resistant to LAC could not be infected with the MLV (LAC) pseudotypes, and the pseudotypes were sensitive to neutralizing monoclonal antibodies prepared against LAC glycoproteins, as well as to inhibition by a soluble form of the virus cell-attachment protein, G1. Perhaps because of lower expression of the HTN glycoproteins at the cell surface, MLV (HTN) pseudotypes were present at lower titers. However, they were also sensitive to appropriate neutralizing antibodies. This pseudotype system will be useful for analysis of the entry process of the Bunyaviridae, and for neutralization studies with some Bunyaviruses whose high virulence normally requires specialized containment facilities.

Intracellular transport of the glycoproteins gE and gI of the varicella-zoster virus. gE accelerates the maturation of gI and determines its accumulation in the trans-Golgi network

The varicella-zoster virus (VZV) is the etiological agent of two different human pathologies, chickenpox (varicella) and shingles (zoster). This alphaherpesvirus is believed to acquire its lipidic envelope in the trans-Golgi network (TGN). This is consistent with previous data showing that the most abundant VZV envelope glycoprotein gE accumulates at steady-state in this organelle when expressed from cloned cDNA. In the present study, we have investigated the intracellular trafficking of gI, another VZV envelope glycoprotein. In transfected cells, this protein shows a very slow biosynthetic transport to the cell surface where it accumulates. However, upon co-expression of gE, gI experiences a dramatic increase in its exit rate from the endoplasmic reticulum, it accumulates in a sialyltransferase-positive compartment, presumably the TGN, and cycles between this compartment and the cell surface. This differential behavior results from the ability of gE and gI to form a complex in the early stages of the biosynthetic pathway whose intracellular traffic is exclusively determined by the sorting information in the tail of gE. Thus, gI provides the first example of a molecule localized to the TGN by means of its association with another TGN protein. We also show that, during the early stages of VZV infection, both proteins are also found in the TGN of the host cell. This suggests the existence of an intermediate stage during VZV biogenesis in which the envelope glycoproteins, transiently arrested in the TGN, could promote the envelopment of newly synthesized nucleocapsids into this compartment and, therefore, the assembly of infective viruses.

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

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

A sorting motif localizes the foamy virus glycoprotein to the endoplasmic reticulum

We recently identified an endoplasmic reticulum (ER) retrieval signal-the dilysine motif-in the glycoproteins of all five foamy viruses (FVs) for which sequences were available (P. A. Goepfert, G. Wang, and M. J. Mulligan, Cell 82:543-544, 1995). In the present study, expression of recombinant human FV (HFV) glycoprotein and analyses of oligosaccharide modifications and precursor cleavage indicated that the protein was localized to the ER. HFV glycoproteins encoding seven different dilysine motif mutations were then expressed. The results indicated that disruptions of the dilysine motif resulted in higher levels of forward transport of the HFV glycoprotein from the ER through the Golgi apparatus to the plasma membrane. We conclude that the dilysine motif is responsible for ER sorting of the FV glycoprotein. Signal-mediated ER localization has not previously been described for a retroviral glycoprotein.

Envelope glycoprotein interactions in coronavirus assembly

Coronaviruses are assembled by budding into smooth membranes of the intermediate ER-to-Golgi compartment. We have studied the association of the viral membrane glycoproteins M and S in the formation of the virion envelope. Using coimmunoprecipitation analysis we demonstrated that the M and S proteins of mouse hepatitis virus (MHV) interact specifically forming heteromultimeric complexes in infected cells. These could be detected only when the detergents used for their solubilization from cells or virions were carefully chosen: a combination of nonionic (NP-40) and ionic (deoxycholic acid) detergents proved to be optimal. Pulse-chase experiments revealed that newly made M and S proteins engaged in complex formation with different kinetics. Whereas the M protein appeared in complexes immediately after its synthesis, newly synthesized S protein did so only after a lag phase of > 20 min. Newly made M was incorporated into virus particles faster than S, which suggests that it associates with preexisting S molecules. Using the vaccinia virus T7-driven coexpression of M and S we also demonstrate formation of M/S complexes in the absence of other coronaviral proteins. Pulse-chase labelings and coimmunoprecipitation analyses revealed that M and S associate in pre-Golgi membranes because the unglycosylated form of M appeared in M/S complexes rapidly. Since no association of M and S was detected when protein export from the ER was blocked by brefeldin A, stable complexes most likely arise in the ER-to-Golgi intermediate compartment. Sucrose velocity gradient analysis showed the M/S complexes to be heterogeneous and of higher order, suggesting that they are maintained by homo- and heterotypic interactions. M/S complexes colocalized with alpha-mannosidase II, a resident Golgi protein. They acquired Golgi-specific oligosaccharide modifications but were not detected at the cell surface. Thus, the S protein, which on itself was transported to the plasma membrane, was retained in the Golgi complex by its association with the M protein. Because coronaviruses bud at pre-Golgi membranes, this result implies that the envelope glycoprotein complexes do not determine the site of budding. Yet, the self-association of the MHV envelope glycoproteins into higher order complexes is indicative of its role in the sorting of the viral membrane proteins and in driving the formation of the viral lipoprotein coat in virus assembly.

The two major envelope proteins of equine arteritis virus associate into disulfide-linked heterodimers

In a coimmunoprecipitation assay with monospecific antisera, the two major envelope proteins GL and M of equine arteritis virus were found to occur in heteromeric complexes in virions and infected cells. While the GL protein associated with M rapidly and efficiently, newly synthesized M protein was incorporated into complexes at a slower rate, which implies that it interacts with GL molecules synthesized earlier. Analysis under nonreducing conditions revealed that the GL/M complexes consist of disulfide-linked heterodimeric structures. Pulse-chase experiments showed that virtually all GL monomers ended up in heterodimers, whereas a fraction of the M protein persisted as monomers. The M protein also formed covalently linked homodimers, but only the heterodimers were incorporated into virus particles.

The membrane glycoprotein G1 of Uukuniemi virus contains a signal for localization to the Golgi complex

Members of the Bunyaviridae family acquire their envelopes by budding into the Golgi complex (GC). The accumulation of the membrane glycoproteins G1 and G2 in the GC probably determines the site of maturation. Here we have studied the intracellular transport and targeting to the GC of G1 and G2 of Uukuniemi virus, a member of the Phlebovirus genus, and report on their expression from cloned cDNAs either together or separately by using a T7 RNA polymerase-driven vaccinia virus expression system. When G1 and G2 were expressed together from a full-length cDNA as the p110 precursor, both proteins were localized to the Golgi complex, as evidenced by colocalization with the Golgi marker enzyme mannosidase II. Immunofluorescent staining indicated that G1 expressed alone also localized to the GC. However, pulse-chase experiments showed that G1 remained endoglycosidase H sensitive. G2 expressed alone remained associated with the endoplasmic reticulum (ER). G2 could be rescued from the ER and transported to the GC by coexpression with G1 from separate mRNAs. Coexpression also increased the efficiency of G1 transport to the GC. With none of the constructs could the glycoproteins be observed on the cell surface. These results show that efficient export of G1 and G2 from the ER requires coexpression of both proteins, in conformity with our previous results showing that G1 and G2 form heterodimeric complexes in the ER. Since G1 expressed alone is retained in the GC, we conclude that G1 contains a retention signal for localization to the GC. G2 might thus become associated with the GC indirectly via its interaction with G1.

Targeting of proteins to the Golgi apparatus

The Golgi apparatus maintains a highly organized structure in spite of the intense membrane traffic which flows into and out of this organelle. Resident Golgi proteins must have localization signals to ensure that they are targeted to the correct Golgi compartment and not swept further along the secretory pathway. There are a number of distinct groups of Golgi membrane proteins, including glycosyltransferases, recycling trans-Golgi network proteins, peripheral membrane proteins, receptors and viral glycoproteins. Recent studies indicate that there are a number of different Golgi localization signals and mechanisms for retaining proteins to the Golgi apparatus. This review focuses on the current knowledge in this field.

Viruses as model systems in cell biology

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

Assembly of vaccinia virus: the second wrapping cisterna is derived from the trans Golgi network

During the assembly of vaccinia virus, the intracellular mature virus becomes enwrapped by a cellular cisterna to form the intracellular enveloped virus (IEV), the precursor of the extracellular enveloped virus (EEV). In this study, we have characterized the origin of this wrapping cisterna by electron microscopic immunocytochemistry using lectins, antibodies against endocytic organelles, and recombinant vaccinia viruses expressing proteins which behave as Golgi resident proteins. No labelling for endocytic marker proteins could be detected on the wrapping membrane. However, the wrapping membrane labelled significantly for a trans Golgi network (TGN) marker protein. The recycling pathway from endosomes to the TGN appears to be greatly increased following vaccinia virus infection, since significant amounts of endocytic fluid-phase tracers were found in the lumen of the TGN, Golgi complex, and the wrapping cisternae. Using immunoelectron microscopy, we localized the vaccinia virus membrane proteins VV-p37, VV-p42, VV-p21, and VV-hemagglutinin (VV-HA) in large amounts in the wrapping cisternae, in the outer membranes of the IEV, and in the outermost membrane of the EEV. The bulk of the cellular VV-p37, VV-p21, and VV-p42 were in the TGN, whereas VV-HA was also found in large amounts on the plasma membrane and in endosomes. Collectively, these data argue that the TGN becomes enriched in vaccinia virus membrane proteins that facilitate the wrapping event responsible for the formation of the IEV.

Targeting of viral glycoproteins to the Golgi complex

Certain enveloped viruses are known to assemble on membranes of the Golgi complex. Intracellular budding is facilitated by targeting of the viral glycoproteins to this organelle. It is likely that these viral glycoproteins are retained in the Golgi by the same means as are endogenous Golgi proteins. Consequently, the study of Golgi-specific viral proteins has provided important clues to the nature of Golgi retention signals.

HIV-1 matrix protein p17 resides in cell nuclei in association with genomic RNA

We have shown previously that HIV-1 matrix protein p17 is transported to the nucleus of Jurkat-tat and H9 cells soon after infection. As shown in this combination, gag polyprotein p55 synthesized 48 h after cell infection is cleaved in cytosol rapidly after its synthesis, and nascent p17 enters the nuclei and gradually accumulates there. Uncleaved p55 molecules and intermediate precursors are rapidly transported to the membranes and are also found in nuclei. Mature gag proteins are seen in membranes only after prolonged period of labelling or chase (4 or more hours later). To determine whether the nascent p17 is associated with viral genomic RNA in the nuclei, the cells were fractionated, the viral complexes were immunoprecipitated by monoclonal antibodies (MAbs) against gag proteins, and RNA was extracted and analyzed by slot and blot hybridization. MAb against p17 precipitated all the viral RNA from the nuclei including full-size genomic RNA and essential parts from membranes while MAb against p24 did not precipitate any viral RNA from the nuclei. These data suggest that matrix protein is linked to genomic RNA in the nuclei and raise the possibility that p17 may transfer viral nucleocapsids from the nuclei to plasma membranes, the site of virus assembly.

Cell biology of viruses that assemble along the biosynthetic pathway

In this review we discuss five groups of viruses that bud into, or assemble from, different compartments along the biosynthetic pathway. These are herpes-, rota-, corona-, bunya- and pox-viruses. Our main emphasis will be on the virally-encoded membrane glycoproteins that are responsible for determining the site of virus assembly. In a number of cases these proteins have been well characterized and appear to serve as resident markers of the budding compartments. The assembly and dissemination of these viruses raises many questions of cell biological interest.

Localization to the Golgi complex of Uukuniemi virus glycoproteins G1 and G2 expressed from cloned cDNAs

The membrane glycoproteins G1 and G2 of Uukuniemi virus, a bunyavirus, accumulate in the Golgi complex (GC) during virus infection. These proteins have therefore been considered to be good models for studying the intracellular transport to and retention in the GC. In this study, I have used indirect immunofluorescence to localize in COS cells the Uukuniemi virus glycoproteins G1 and G2 expressed together or separately from cloned cDNAs with use of simian virus 40-based vectors. When expressed together from the full-length cDNA, G1 and G2 were correctly translocated, processed, and targeted to the GC, indicating that the information for GC targeting resides in the proteins. When the proteins were expressed separately, G1 was transported to the GC and retained there. In contrast, G2 could not be detected in the GC but was most probably retained and finally degraded in the endoplasmic reticulum. However, in cells cotransfected with G1 and G2 cDNAs, the proteins could both again be found in the GC. These results suggest that G1 is a responsible for targeting to and retention of the Uukuniemi virus glycoproteins in the GC. G2 would thus accumulate in the GC by virtue of its binding to G1.

The Hantaan virus M-segment glycoproteins G1 and G2 can be expressed independently

The two glycoproteins of Hantaan virus (HTV), G1 and G2, are encoded as a continuous single open reading frame in the M segment of the virion RNA. They are believed to be synthesized contemporaneously via a polypeptide precursor which is then processed to yield two glycoproteins, both of which appear in the Golgi complex of the cell. To study the properties of G1 and G2 as separate entities, we have constructed vaccinia virus recombinants which contain the sequences for each glycoprotein individually. Both glycoproteins made from these recombinants appear normal on sodium dodecyl sulfate-polyacrylamide gels compared with HTV products made in virus-infected cells. Interestingly, in the independently expressed G2 recombinant, a stretch of hydrophobic amino acids preceding the mature G2 N terminus appears to contain the signals necessary for translocation across membranes and proper glycosylation; partial deletion of this hydrophobic sequence results in production of an nonglycosylated form of G2. Thus, both G1 and G2 appear able to be expressed in an authentic fashion quite independently of each other, using their own signal sequences. In addition, it appears that the G1 from vaccinia virus recombinants contains the motif(s) necessary for cellular targeting of the HTV glycoproteins, while G2 from vaccinia virus recombinants remains strongly associated with the endoplasmic reticulum. In contrast, cells doubly infected with G1-vaccinia virus and G2-vaccinia virus recombinants show the G2 in a predominantly perinuclear (Golgi-like) distribution, presumably targeted there through association with G1. A carboxy-terminal deletion of G1 (2-43-Vac), which lacks 82 amino acids proximal to the start of the mature G2, retains a Golgi-like distribution.

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

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

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

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