Uukuniemi virus maturation: accumulation of virus particles and viral antigens in the Golgi complex

Assembly and Cellular Exit of Coronaviruses: Hijacking an Unconventional Secretory Pathway from the Pre-Golgi Intermediate Compartment via the Golgi Ribbon to the Extracellular Space

Coronaviruses (CoVs) assemble by budding into the lumen of the intermediate compartment (IC) at the endoplasmic reticulum (ER)-Golgi interface. However, why CoVs have chosen the IC as their intracellular site of assembly and how progeny viruses are delivered from this compartment to the extracellular space has remained unclear. Here we address these enigmatic late events of the CoV life cycle in light of recently described properties of the IC. Of particular interest are the emerging spatial and functional connections between IC elements and recycling endosomes (REs), defined by the GTPases Rab1 and Rab11, respectively. The establishment of IC-RE links at the cell periphery, around the centrosome and evidently also at the noncompact zones of the Golgi ribbon indicates that—besides traditional ER-Golgi communication—the IC also promotes a secretory process that bypasses the Golgi stacks, but involves its direct connection with the endocytic recycling system. The initial confinement of CoVs to the lumen of IC-derived large transport carriers and their preferential absence from Golgi stacks is consistent with the idea that they exit cells following such an unconventional route. In fact, CoVs may share this pathway with other intracellularly budding viruses, lipoproteins, procollagen, and/or protein aggregates experimentally introduced into the IC lumen.

A Broad-Spectrum Antiviral Peptide Blocks Infection of Viruses by Binding to Phosphatidylserine in the Viral Envelope

The ongoing threat of viral infections and the emergence of antiviral drug resistance warrants a ceaseless search for new antiviral compounds. Broadly-inhibiting compounds that act on elements shared by many viruses are promising antiviral candidates. Here, we identify a peptide derived from the cowpox virus protein CPXV012 as a broad-spectrum antiviral peptide. We found that CPXV012 peptide hampers infection by a multitude of clinically and economically important enveloped viruses, including poxviruses, herpes simplex virus-1, hepatitis B virus, HIV-1, and Rift Valley fever virus. Infections with non-enveloped viruses such as Coxsackie B3 virus and adenovirus are not affected. The results furthermore suggest that viral particles are neutralized by direct interactions with CPXV012 peptide and that this cationic peptide may specifically bind to and disrupt membranes composed of the anionic phospholipid phosphatidylserine, an important component of many viral membranes. The combined results strongly suggest that CPXV012 peptide inhibits virus infections by direct interactions with phosphatidylserine in the viral envelope. These results reiterate the potential of cationic peptides as broadly-acting virus inhibitors.

A bite so sweet: the glycobiology interface of tick-host-pathogen interactions

Vector-borne diseases constitute 17% of all infectious diseases in the world; among the blood-feeding arthropods, ticks transmit the highest number of pathogens. Understanding the interactions between the tick vector, the mammalian host and the pathogens circulating between them is the basis for the successful development of vaccines against ticks or the tick-transmitted pathogens as well as for the development of specific treatments against tick-borne infections. A lot of effort has been put into transcriptomic and proteomic analyses; however, the protein-carbohydrate interactions and the overall glycobiology of ticks and tick-borne pathogens has not been given the importance or priority deserved. Novel (bio)analytical techniques and their availability have immensely increased the possibilities in glycobiology research and thus novel information in the glycobiology of ticks and tick-borne pathogens is being generated at a faster pace each year. This review brings a comprehensive summary of the knowledge on both the glycosylated proteins and the glycan-binding proteins of the ticks as well as the tick-transmitted pathogens, with emphasis on the interactions allowing the infection of both the ticks and the hosts by various bacteria and tick-borne encephalitis virus.

Bunyaviridae RdRps: structure, motifs, and RNA synthesis machinery

Bunyaviridae family is the largest and most diverse family of RNA viruses. It has more than 350 members divided into five genera: Orthobunyavirus, Phlebovirus, Nairovirus, Hantavirus, and Tospovirus. They are present in the five continents, causing recurrent epidemics, epizootics, and considerable agricultural loss. The genome of bunyaviruses is divided into three segments of negative single-stranded RNA according to their relative size: L (Large), M (Medium) and S (Small) segment. Bunyaviridae RNA-dependent RNA polymerase (RdRp) is encoded by the L segment, and is in charge of the replication and transcription of the viral RNA in the cytoplasm of the infected cell. Viral RdRps share a characteristic right hand-like structure with three subdomains: finger, palm, and thumb subdomains that define the formation of the catalytic cavity. In addition to the N-terminal endonuclease domain, eight conserved motifs (A-H) have been identified in the RdRp of Bunyaviridae. In this review, we have summarized the recent insights from the structural and functional studies of RdRp to understand the roles of different motifs shared by RdRps, the mechanism of viral RNA replication, genome segment packaging by the nucleoprotein, cap-snatching, mRNA transcription, and other RNA mechanisms of bunyaviruses.

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.

Recent advances in hantavirus molecular biology and disease

Hantaviruses are emerging zoonotic pathogens that belong to the Bunyaviridae family. They have been classified as category A pathogens by CDC (centers for disease control and prevention). Hantaviruses pose a serious threat to human health because their infection causes two highly fatal diseases, hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS). These pathogens are transmitted to humans through aerosolized excreta of their infected rodent hosts. Hantaviruses have a tripartite-segmented negative-sense RNA genome. The three genomic RNA segments, S, M, and L, encode a nucleocapsid protein (N), a precursor glycoprotein that is processed into two envelope glycoproteins (Gn and Gc) and the viral RNA-dependent RNA polymerase (RdRp), respectively. N protein is the major structural component of the virus, its main function is to protect and encapsidate the three genomic RNAs forming three viral ribonucleocapsids. Recent studies have proposed that N in conjunction with RdRp plays important roles in the transcription and replication of viral genome. In addition, N preferentially facilitates the translation of viral mRNA in cells. Glycoproteins, Gn and Gc, play major roles in viral attachment and entry to the host cells, virulence, and assembly and packaging of new virions in infected cells. RdRp functions as RNA replicase and transcriptase to replicate and transcribe the viral RNA and is also thought to have endonuclease activity. Currently, no antiviral therapy or vaccine is available for the treatment of hantavirus-associated diseases. Understanding the molecular details of hantavirus life cycle will help in the identification of targets for antiviral therapeutics and in the design of potential antiviral drug for the treatment of HFRS and HCPS. Due to the alarming fatality of hantavirus diseases, development of an effective vaccine against hantaviruses is a necessity.

DC-SIGN as a receptor for phleboviruses

During natural transmission, bunyaviruses are introduced into the skin through arthropod bites, and dermal dendritic cells (DCs) are the first to encounter incoming viruses. DC-SIGN is a C-type lectin highly expressed on the surface of dermal DCs. We found that several arthropod-borne phleboviruses (Bunyaviridae), including Rift Valley fever and Uukuniemi viruses, exploit DC-SIGN to infect DCs and other DC-SIGN-expressing cells. DC-SIGN binds the virus directly via interactions with high-mannose N-glycans on the viral glycoproteins and is required for virus internalization and infection. In live cells, virus-induced clustering of cell surface DC-SIGN could be visualized. An endocytosis-defective mutant of DC-SIGN was unable to mediate virus uptake, indicating that DC-SIGN is an authentic receptor required for both attachment and endocytosis. After internalization, viruses separated from DC-SIGN and underwent trafficking to late endosomes. Our study provides real-time visualization of virus-receptor interactions on the cell surface and establishes DC-SIGN as a phlebovirus entry receptor.

Recent advances in the molecular and cellular biology of bunyaviruses

The family Bunyaviridae of segmented, negative-stranded RNA viruses includes over 350 members that infect a bewildering variety of animals and plants. Many of these bunyaviruses are the causative agents of serious disease in their respective hosts, and are classified as emerging viruses because of their increased incidence in new populations and geographical locations throughout the world. Emerging bunyaviruses, such as Crimean-Congo hemorrhagic fever virus, tomato spotted wilt virus and Rift Valley fever virus, are currently attracting great interest due to migration of their arthropod vectors, a situation possibly linked to climate change. These and other examples of continued emergence suggest that bunyaviruses will probably continue to pose a sustained global threat to agricultural productivity, animal welfare and human health. The threat of emergence is particularly acute in light of the lack of effective preventative or therapeutic treatments for any of these viruses, making their study an important priority. This review presents recent advances in the understanding of the bunyavirus life cycle, including aspects of their molecular, cellular and structural biology. Whilst special emphasis is placed upon the emerging bunyaviruses, we also describe the extensive body of work involving model bunyaviruses, which have been the subject of major contributions to our overall understanding of this important group of viruses.

Inactivation of hantaviruses by N-ethylmaleimide preserves virion integrity

Thiol groups of cysteine residues are crucial for the infectivity of various enveloped viruses, but their role in the infectivity of viruses of the family Bunyaviridae has thus far not been studied. This report shows that thiol groups are essential to the infectivity of hantaviruses. Alkylation of the thiol functional groups using the membrane-permeable compound N-ethylmaleimide (NEM) and membrane-impermeable compound 5,5′-dithio-bis-(2-nitrobenzoic acid) (DTNB) showed NEM to be a highly effective inactivator of Puumala and Tula hantaviruses. The NEM-inactivated hantavirus maintained the buoyant density of the wild-type virus. Furthermore, the antigenicity of glycoproteins and the cell attachment capacity of virions were retained at NEM concentrations that totally abolished virus infectivity. These results signified preservation of virion integrity following inactivation with NEM, making chemically inactivated virions valuable research antigens. It was demonstrated with biotin-conjugated maleimide, a mechanistic analogue of NEM, that all the structural proteins of hantavirus were sensitive towards thiol alkylation. In contrast to hantaviruses, NEM did not abolish Uukuniemi phlebovirus infectivity to the same extent. This indicates differences in the use of free thiols in virus entry among members of the family Bunyaviridae.

Oligomerization of Uukuniemi virus nucleocapsid protein

Background

Uukuniemi virus (UUKV) belongs to the Phlebovirus genus in the family Bunyaviridae. As a non-pathogenic virus for humans UUKV has served as a safe model bunyavirus in a number of studies addressing fundamental questions such as organization and regulation of viral genes, genome replication, structure and assembly. The present study is focused on the oligomerization of the UUKV nucleocapsid (N) protein, which plays an important role in several steps of virus replication. The aim was to locate the domains involved in the N protein oligomerization and study the process in detail.

Results

A set of experiments concentrating on the N- and C-termini of the protein was performed, first by completely or partially deleting putative N-N-interaction domains and then by introducing point mutations of amino acid residues. Mutagenesis strategy was based on the computer modeling of secondary and tertiary structure of the N protein. The N protein mutants were studied in chemical cross-linking, immunofluorescence, mammalian two-hybrid, minigenome, and virus-like particle-forming assays. The data showed that the oligomerization ability of UUKV-N protein depends on the presence of intact α-helices on both termini of the N protein molecule and that a specific structure in the N-terminal region plays a crucial role in the N-N interaction(s). This structure is formed by two α-helices, rich in amino acid residues with aromatic (W7, F10, W19, F27, F31) or long aliphatic (I14, I24) side chains. Furthermore, some of the N-terminal mutations (e.g. I14A, I24A, F31A) affected the N protein functionality both in mammalian two-hybrid and minigenome assays.

Conclusions

UUKV-N protein has ability to form oligomers in chemical cross-linking and mammalian two-hybrid assays. In mutational analysis, some of the introduced single-point mutations abolished the N protein functionality both in mammalian two-hybrid and minigenome assays, suggesting that especially the N-terminal region of the UUKV-N protein is essential for the N-N interaction.

Molecular biology and pathogenesis of Crimean–Congo hemorrhagic fever virus

Over the last several years, we have experienced an increase of large outbreaks of Crimean–Congo hemorrhagic fever virus in European countries and neighboring areas. This disease poses a great threat to public health owing to its high mortality rate, modes of transmission and geographical distribution. Clinical symptoms of infection commonly include hemorrhage, myalgia and fever. The complexity of the technical and facility requirements, in combination with the sporadic outbreaks and consequent lack of clinical specimens has resulted in very limited research of Crimean–Congo hemorrhagic fever virus. To date, there is no vaccine available and a selective antiviral drug for the treatment of the disease is not expected in the near future. Here, we review the most recent findings on the Crimean–Congo hemorrhagic fever virus molecular biology and pathogenesis, including aspects of virus–host cell interactions.

Requirements for ER-arrest and sequential exit to the golgi of Tomato spotted wilt virus glycoproteins

The envelope glycoproteins Gn and Gc are major determinants in the assembly of Tomato spotted wilt virus (TSWV) particles at the Golgi complex. In this article, the ER-arrest of singly expressed Gc and the transport of both glycoproteins to the Golgi upon coexpression have been analyzed.While preliminary results suggest that the arrest of Gc at the ER (endoplasmic reticulum) did not appear to result from improper folding, transient expression of chimeric Gc, in which the transmembrane domain (TMD) and/or cytoplasmic tail (CT) were swapped for those from Gn, showed that the TMD of Gn was sufficient to allow ER exit and transport to the Golgi. Expression of both glycoproteins in the presence of overexpressed Sar1p specific guanosine nucleotide exchange factor Sec12p, resulted in ER-retention demonstrating that the viral glycoproteins are transported to the Golgi in a COPII (coat protein II)-dependent manner. Inhibition of ER Golgi transport by brefeldin A (BFA) had a similar effect on the localization of Gn. However, inhibition of ER (endoplasmic reticulum) to Golgi transport of coexpressed Gc and Gn by overexpression of Sec12p or by BFA revealed distinct localization patterns, i.e. diffuse ER localization versus concentration at specific spots.

Generation and analysis of recombinant Bunyamwera orthobunyaviruses expressing V5 epitope-tagged L proteins

The L protein of Bunyamwera virus (BUNV; family Bunyaviridae) is an RNA-dependent RNA polymerase, 2238 aa in length, that catalyses transcription and replication of the negative-sense, tripartite RNA genome. To learn more about the molecular interactions of the L protein and to monitor its intracellular distribution we inserted a 14 aa V5 epitope derived from parainfluenza virus type 5, against which high-affinity antibodies are available, into different regions of the protein. Insertion of the epitope at positions 1935 or 2046 resulted in recombinant L proteins that retained functionality in a minireplicon assay. Two viable recombinant viruses, rBUNL4V5 and rBUNL5V5, expressing the tagged L protein were rescued by reverse genetics, and characterized with respect to their plaque size, growth kinetics and protein synthesis profile. The recombinant viruses behaved similarly to wild-type (wt) BUNV in BHK-21 cells, but formed smaller plaques and grew to lower titres in Vero E6 cells compared with wt BUNV. Immunofluorescent staining of infected cells showed the L protein to have a punctate to reticular distribution in the cytoplasm, and cell fractionation studies indicated that the L protein was present in both soluble and microsomal fractions. Co-immunoprecipitation and confocal microscopic assays confirmed an interaction between BUNV L and N proteins. The recombinant viruses expressing tagged L protein will be highly valuable reagents for the detailed dissection of the role of the BUNV L protein in virus replication.

Roles for the recycling endosome, Rab8, and Rab11 in hantavirus release from epithelial cells

Hantavirus structural proteins are believed to localize to intracellular membranes often identified as Golgi membranes, in virus-infected cells. After virus budding into the Golgi luminal space, virus-containing vesicles are transported to the plasma membrane via trafficking pathways that are not well defined. Using the New World hantavirus, Andes virus, we have investigated the role of various Rab proteins in the release of hantavirus particles from infected cells. Rabs 8 and 11 were found to colocalize with Andes virus proteins in virus infected cells and when expressed from cDNA, implicating the recycling endosome as an organelle important for hantavirus infection. Small interfering RNA-mediated downregulation of Rab11a alone or Rab11a and Rab11b together resulted in a decrease in infectious virus particle secretion from infected cells. Downregulation of Rab8a did not alter infectious virus release but reduction of both isoforms did. These data implicate the recycling endosome and the Rab proteins associated with vesicular transport to or from this intracellular organelle as an important pathway for hantavirus trafficking to the plasma membrane.

Tomato spotted wilt virus glycoproteins induce the formation of endoplasmic reticulum- and Golgi-derived pleomorphic membrane structures in plant cells

Tomato spotted wilt virus (TSWV) particles are spherical and enveloped, an uncommon feature among plant infecting viruses. Previous studies have shown that virus particle formation involves the enwrapment of ribonucleoproteins with viral glycoprotein containing Golgi stacks. In this study, the localization and behaviour of the viral glycoproteins Gn and Gc were analysed, upon transient expression in plant protoplasts. When separately expressed, Gc was solely observed in the endoplasmic reticulum (ER), whereas Gn was found both within the ER and Golgi membranes. Upon co-expression, both glycoproteins were found at ER-export sites and ultimately at the Golgi complex, confirming the ability of Gn to rescue Gc from the ER, possibly due to heterodimerization. Interestingly, both Gc and Gn were shown to induce the deformation of ER and Golgi membranes, respectively, also observed upon co-expression of the two glycoproteins. The behaviour of both glycoproteins within the plant cell and the phenomenon of membrane deformation are discussed in light of the natural process of viral infection.

Role of actin filaments in targeting of Crimean Congo hemorrhagic fever virus nucleocapsid protein to perinuclear regions of mammalian cells

Crimean-Congo hemorrhagic fever virus is the causative agent of a severe disease throughout Africa, Europe, and Asia. Like other members of the genus Nairovirus, the Crimean-Congo hemorrhagic fever virus contains three genomic RNA segments, the small (S), medium (M), and large (L) segments. The S segment encodes the viral nucleocapsid protein (NP), while the M and L segments encode the glycoproteins and the RNA-dependent RNA polymerase, respectively. In this study, the site of expression and assembly of Crimean-Congo hemorrhagic fever virus NP in mammalian cells have been investigated. It was found that the NP is localized in the perinuclear region of infected cells. By using the Semliki forest virus expression system, it was shown that the Crimean-Congo hemorrhagic fever virus NP is targeted to the perinuclear region of cells in the absence of native RNA segments and virally encoded glycoproteins. It was also demonstrated that the Crimean-Congo hemorrhagic fever virus NP was not expressed as a Golgi-membrane associated protein. By using Cytochalasin D, an agent that disrupts actin filaments, it was found that actin filaments are involved in targeting the viral NP to perinuclear regions. We also demonstrated that disruption of actin filaments reduced the assembly of infectious Crimean-Congo hemorrhagic fever virus up to 97%. Furthermore, we showed that the NP of Crimean-Congo hemorrhagic fever virus NP interacts with actin.

Heterogeneous distribution of the unusual phospholipid semilysobisphosphatidic acid through the Golgi complex

To investigate the distribution of lipids through the Golgi complex, we analyzed the envelopes of several viruses that assemble in different subcompartments of the Golgi, as well as subcellular fractions. Our results indicate that each Golgi subcompartment has a distinct phospholipid composition due mainly to differences in the relative amounts of semilysobisphosphatidic acid (SLBPA), sphingomyelin, phosphatidylserine, and phosphatidylinositol. Interestingly, SLBPA is enriched in the adjacent Golgi networks compared with the Golgi stack, and this enrichment varies with cell type. The heterogeneous distribution of SLBPA through the Golgi complex suggests it may play an important role in the structure and/or function of this organelle.

A retention signal necessary and sufficient for Golgi localization maps to the cytoplasmic tail of a Bunyaviridae (Uukuniemi virus) membrane glycoprotein

Members of the Bunyaviridae family mature by a budding process in the Golgi complex. The site of maturation is thought to be largely determined by the accumulation of the two spike glycoproteins, G1 and G2, in this organelle. Here we show that the signal for localizing the Uukuniemi virus (a phlebovirus) spike protein complex to the Golgi complex resides in the cytoplasmic tail of G1. We constructed chimeric proteins in which the ectodomain, transmembrane domain (TMD), and cytoplasmic tail (CT) of Uukuniemi virus G1 were exchanged with the corresponding domains of either vesicular stomatitis virus G protein (VSV G), chicken lysozyme, or CD4, all proteins readily transported to the plasma membrane. The chimeras were expressed in HeLa or BHK-21 cells by using either the T7 RNA polymerase-driven vaccinia virus system or the Semliki Forest virus system. The fate of the chimeric proteins was monitored by indirect immunofluorescence, and their localizations were compared by double labeling with markers specific for the Golgi complex. The results showed that the ectodomain and TMD (including the 10 flanking residues on either side of the membrane) of G1 played no apparent role in targeting chimeric proteins to the Golgi complex. Instead, all chimeras containing the CT of G1 were efficiently targeted to the Golgi complex and colocalized with mannosidase II, a Golgi-specific enzyme. Conversely, replacing the CT of G1 with that from VSV G resulted in the efficient transport of the chimeric protein to the cell surface. Progressive deletions of the G1 tail suggested that the Golgi retention signal maps to a region encompassing approximately residues 10 to 50, counting from the proposed border between the TMD and the tail. Both G1 and G2 were found to be acylated, as shown by incorporation of [3H]palmitate into the viral proteins. By mutational analyses of CD4-G1 chimeras, the sites for palmitylation were mapped to two closely spaced cysteine residues in the G1 tail. Changing either or both of these cysteines to alanine had no effect on the targeting of the chimeric protein to the Golgi complex.

Expression and intracellular localization of catechol O-methyltransferase in transfected mammalian cells

The intracellular localization of soluble and membrane-bound isoforms of rat and human catechol O-methyltransferase (COMT) was studied by expressing the recombinant COMT proteins either separately or together in mammalian cell lines (HeLa and COS-7 cells) and in rat primary neurons. The distribution of soluble and membrane-bound COMT enzyme was visualized by immunocytochemistry. For comparison, the localization of native COMT was studied in rat C6 glioma cells by immunoelectron microscopy. Staining of cells expressing membrane-bound COMT with a COMT-specific antiserum revealed an immunofluorescence signal in intracellular reticular structures and in the nuclear membrane. Double-staining of the cells with antisera against proteins specific for the rough endoplasmic reticulum indicated that they colocalized with membrane-bound COMT, suggesting that it resided in the endoplasmic reticulum. Notably, no COMT-specific fluorescence of plasma membranes was detected. The signal in the endoplasmic reticulum was also evident in the cells expressing both recombinant COMT forms. Intracellular native COMT reaction was detected by immunoelectron microscopy in rat C6 glioma cells and an intense cytoplasmic signal was seen in the primary neurons infected with the recombinant Semliki Forest virus. The cells expressing recombinant soluble COMT revealed intense nuclear staining together with diffuse cytoplasmic immunoreactivity, suggesting that a part of soluble COMT is transported to nuclei. Western blotting from rat liver and brain revealed soluble COMT in the nuclei. Enzyme activity measurements from liver cytoplasmic and nuclear fractions suggested that about 5% of the soluble COMT resided in nuclei. The intracellular localization of both COMT forms implies that COMT acts in the cytoplasm and possibly also in the nuclear compartment, and that the physiological substrates of COMT enzymes may have to be internalized before their methylation by COMT.

Ultrastructural studies on the replication and morphogenesis of Nairobi sheep disease virus, a Nairovirus

The Nairovirus Nairobi sheep disease virus (NSDV) affects sheep and goats causing severe hemorrhagic gastroenteritis and high mortality. Replication and morphogenesis of NSDV was determined by electron microscopic examination of ultra-thin sections of 143B and BHK-21 cells at varying times after infection. By 4 h post-infection (p.i.) of 143B cells, virions budding from the luminal side of the bilayer membrane of smooth membrane vesicles were observed. Morphologically mature virus particles were electron-dense, spherical and of uniform size (100 nm diameter) and accumulated in smooth membrane vesicles associated with the Golgi complex. In BHK-21 clone 13 cells, mature virus particles in smooth membrane vesicles were present by 8 h p.i. The morphogenesis of NSDV was restricted to the smooth membrane vesicles of Golgi complex, and budding of virus from other sites was not detected. Extracellular virus particles were observed by 10 h p.i., before expression of cytopathic effects. The cytopathic effects were observed at 24 h p.i. in 143B cells and at 36 h p.i. in BHK-21 cells. The morphology and morphogenesis of NSDV in BHK-21 cells and in 143B cells resembles that of other members of the family Bunyaviridae.

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.

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.

Nuclear and nucleolar targeting signals of Semliki Forest virus nonstructural protein nsP2

Semliki Forest virus nonstructural protein nsP2, when expressed in the absence of other viral proteins, is transported into the nucleus, with a specific enrichment in nucleoli. This implies that information for its nuclear targeting must be in nsP2 itself. To define the nuclear and nucleolar targeting signals of nsP2, a series of in-frame deletions were made in its coding sequence. We have identified one amino-terminal (residues 25-110) and another carboxy-terminal (residues 634-661) deletion, both of which rendered nsP2 cytoplasmic. The carboxy-terminal region had a pentapeptide sequence P 648R RRV, which resembles some of the known nuclear localization signals. When the three arginines were substituted with aspartic acids, the mutant nsP2 was localized in the cytoplasm. Especially arginines 648-649 were shown to be critical. The carboxy-terminal 232 amino acid residues of nsP2 were able to translocate beta-galactosidase to the nucleus as a fusion protein, whereas the amino-terminal 110 residues failed to do so. Studies with different beta-galactosidase-nsP2 fusions showed that the pentapeptide is necessary for nuclear transport but that its activity is strongly affected by the context. The deletion analysis indicated also that the nuclear localization signal was not sufficient for nucleolar targeting of nsP2. The nucleolar targeting sequence of nsP2 was tentatively mapped between residues 470 and 539, and at least residues 470-489 are an essential part of it.

Coexpression of the membrane glycoproteins G1 and G2 of Hantaan virus is required for targeting to the Golgi complex

To study the intracellular transport and targeting to the Golgi complex of the membrane glycoproteins G1 and G2 of Hantaan virus, we have expressed them together and separately using recombinant vaccinia viruses. When expressed from the same recombinant vaccinia virus, G1 and G2 were localized to the Golgi complex as analyzed by both immunofluorescence and subcellular fractionation. However, when the glycoproteins were expressed from separate recombinant viruses, both proteins remained in the endoplasmic reticulum. Using several monoclonal antibodies, it was found that G1 expressed alone did not acquire its correct conformation. Finally, if cells were coinfected with G1- and G2-expressing recombinant viruses, the proteins were again targeted to the Golgi complex. The N-linked glycans remained in all cases largely endoglycosidase-H sensitive. With none of the recombinant viruses were expression of the glycoproteins observed on the cell surface. Neither did chasing in the presence of cycloheximide result in the surface expression of G1 or G2. Our results indicate that for transport out of the endoplasmic reticulum and proper targeting to the Golgi complex, the two glycoproteins have to be coexpressed. The most likely interpretation is that G1 and G2 have to interact with each other in the endoplasmic reticulum in order to become transport competent.

Structure and morphogenesis of Dugbe virus (Bunyaviridae, Nairovirus) studied by immunogold electron microscopy of ultrathin cryosections

We have studied the structure and morphogenesis of Dugbe (DUG) virus (Bunyaviridae, Nairovirus) in cultured porcine kidney (PS) cells and a tick cell line (Ra 243) using immunogold electron microscopy. DUG virus is a tickborne arbovirus, considered to be a low health hazard, that is antigenically and genetically related to Crimean Congo haemorrhagic fever (CCHF) virus (Marriott et al., 1990). We have investigated the maturation and intracellular transport of DUG virus particles as a model for other more pathogenic nairoviruses using monoclonal antibodies for immunogold labelling of ultrathin cryosections and immunofluorescence techniques. The spherical DUG virus particle measures about 90 nm in diameter, with a 5 nm thick membrane covered by 5-7 nm long projections or "spikes". These projections form hollow cylindrical morphological units, about 5 nm in diameter. DUG virus infection caused only a slight cytopathogenic effect in mammalian cells and none in tick cells. DUG virus particles assembled by budding from the Golgi complex, where the DUG virus glycoprotein G1 accumulated in vesicles originating from Golgi cisternae. The nucleocapsid protein N accumulated in scattered foci throughout the cytoplasm, and this appears to be related to the limited maturation of DUG virus particles that occurred. The reduced number of budding virus particles observed in tick cells was correlated with the reduced cytopathology observed.

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

The glycoproteins of bunyaviruses accumulate in membranes of the Golgi complex, where virus maturation occurs by budding. In this study we have constructed a series of full length or truncated mutants of the G2 glycoprotein of Punta Toro virus (PTV), a member of the Phlebovirus genus of the Bunyaviridae, and investigated their transport properties. The results indicate that the hydrophobic domain preceding the G2 glycoprotein can function as a translocational signal peptide, and that the hydrophobic domain near the C-terminus serves as a membrane anchor. A G2 glycoprotein construct with an extra hydrophobic sequence derived from the N-terminal NSM region was stably retained in the ER, and was unable to be transported to the Golgi complex. The full-length G2 glycoprotein, when expressed on its own, was transported out of the ER and expressed on the cell surface, whereas the G1 and G2 proteins when expressed together are retained in the Golgi complex. A truncated anchor-minus form of the G2 glycoprotein was found to be secreted into the culture medium, but was retained in the Golgi complex when coexpressed with the G1 glycoprotein. These results indicate that the G2 membrane glycoprotein is a class I membrane protein which does not contain a signal sufficient for Golgi retention, and suggest that its Golgi localization is a result of association with the G1 glycoprotein.

A single amino acid change in E3 of ts1 mutant inhibits the intracellular transport of SFV envelope protein complex

At 39 degrees the envelope protein complex (E1-p62) of Semliki Forest virus mutant ts1 is arrested in the rough endoplasmic reticulum (RER). When the infected cultures are shifted to 28 degrees, the complex is transported to the cell surface. During the transport p62 is cleaved into E2 under conditions in which no virus budding takes place. We have sequenced the cDNA, which encodes the envelope proteins of ts1. Comparison with the respective wild-type nucleotide sequence shows only one nucleotide change, G----A, causing a replacement of cysteine-58 (TGC) with tyrosine (TAC) in the E3 protein of ts1. A cDNA fragment from the ts1 genome encoding the mutation in E3 was used to replace the respective fragment of prototype SFV in an eukaryotic expression vector. Intracellular arrest of envelope proteins at 39 degrees was seen in transfected BHK21 cells. A shift of the transfected cells to 28 degrees resulted in the appearance of the envelope proteins at the cell surface. We conclude that the single point mutation is solely responsible for the temperature-sensitive transport defect of ts1 envelope glycoproteins.

Processing and intracellular transport of rubella virus structural proteins in COS cells

Plasmids encoding rubella virus (RV) structural proteins C-E2-E1, E2-E1, E2, and E1 have been constructed in the eukaryotic expression vector pCMV5. The processing and intracellular transport of these proteins have been examined by transient expression of the cDNAs in COS cells. Compared to alphaviruses, processing of RV glycoprotein moieties occurred relatively slowly and the transport of glycoproteins E2 and E1 to the plasma membrane was inefficient. Indirect immunofluorescence revealed that the majority of RV antigen in transfected and infected COS cells was localized to the Golgi region, including the capsid protein. Accumulation of capsid protein in the juxtanuclear region was determined to be RV glycoprotein dependent. Unlike alphaviruses, RV E1 did not require E2 for targeting to the Golgi where it was retained. E2 was however necessary for cell surface expression of E1. This study revealed that the processing and transport of RV structural proteins is quite different from alphaviruses and that the accumulation of antigens in the Golgi region may be significant in light of previous reports which suggest that RV buds from the internal membranes in some cell types.

Efficient synthesis of influenza virus hemagglutinin in mammalian cells with an extrachromosomal Epstein-Barr virus vector

The capability of an Epstein-Barr virus hybrid vector (EBV-CMV), containing the cytomegalovirus (CMV) immediate early enhancer and simian virus 40 promoter, to produce large amounts of authentic mammalian proteins was studied. The cDNA of influenza virus hemagglutinin (HA), a cell surface glycoprotein, was inserted into this vector and the EBV-CMV-HA plasmid was transfected into two human and two monkey cell lines. Southern-blot analysis revealed that the EBV-CMV-HA plasmid was maintained in extrachromosomal state and the recombinant cell clones contained on the average three copies (range 1-24) of the transfected DNA. The recombinant HA polypeptides from different cell clones, selected either randomly or by fluorescence-activated cell sorter, were analysed using immunological techniques. Three of the four cell lines expressed recombinant HA on the cell surface in glycosylated form. The highest production levels, 11.5 micrograms/10(6) cells, were obtained in HeLa cells containing only two copies of EBV-CMV-HA DNA per cell. The protein levels correlated with the mRNA levels in Northern-blot analysis. A corresponding vector, containing the same regulatory signals for HA expression, but lacking the EBV sequences, yielded clones with significantly lower expression levels. The results confirm that the extrachromosomal EBV-CMV vector is very useful in the production of apparently authentic mammalian glycoproteins.

Intracellular accumulation of Punta Toro virus glycoproteins expressed from cloned cDNA

The Punta Toro virus (PTV) middle size (M) RNA encodes two glycoproteins, G1 and G2, and possibly a nonstructural protein, NSM. A partial cDNA clone of the M segment which contains G1 and G2 glycoprotein coding sequences but lacks most of the NSM sequences was inserted into the genome of vaccinia virus under the control of an early vaccinia promoter. Cells infected with the recombinant virus were found to synthesize two polypeptides with molecular weights of 65,000 (G1) and 55,000 (G2) that reacted specifically with antibody against PTV. Studies using indirect immunofluorescence microscopy revealed that these proteins accumulated intracellularly in the perinuclear region. The results of endoglycosidase H digestion of these glycoproteins suggested that both G1 and G2 glycoproteins were transported from the RER to the Golgi complex. These proteins were not chased out from the Golgi region during a 6-hr incubation in the presence of cycloheximide. Surface immune precipitation and 125I-protein A binding assays also demonstrated that the majority of the G1 and G2 glycoproteins are retained intracellularly. These results indicate that the PTV glycoproteins contain the necessary information for retention in the Golgi apparatus.

Rift Valley fever virus M segment: cellular localization of M segment-encoded proteins

The Phlebovirus Rift Valley fever virus (RVFV), like other members of the Bunyaviridae family, matures intracellularly at the smooth-surfaced vesicles in the Golgi region of infected cells. Here we show that in cultured cells the RVFV glycoproteins G2 and G1 accumulate and are retained at this site. To investigate the parameters governing this subcellular localization, we have engineered portions of the cloned RVFV M segment (which encodes a 14- and a 78-kDa protein, in addition to glycoproteins G2 and G1) into vaccinia virus. Immunofluorescent analysis of cells infected with a vaccinia virus recombinant containing the entire open reading frame of the RVFV M segment revealed Golgi localization for glycoproteins G2, G1, the 78-kDa protein, and Golgi as well as some reticular distribution for the 14-kDa protein. These distributions paralleled those seen in authentic RVFV-infected cells. RVFV-vaccinia virus recombinants possessing progressive deletions within the 152 amino acid preglycoprotein sequence of the M segment were analyzed for possible effects on the cellular distribution of G2 and G1. Removal of the first 130 amino acids of the open reading frame amino-terminal to the mature glycoprotein coding sequences, while abolishing production of the 78- and 14-kDa proteins, did not alter the Golgi location of G2 and G1. The data suggest that Golgi-specific signals reside within the G2 and/or G1 glycoprotein sequences. The use of vaccinia virus recombinants provides a genetically manipulable expression system with which to further investigate the sequences involved in the intracellular localization of these Phlebovirus proteins.

Complete nucleotide sequence of the M RNA segment of Uukuniemi virus encoding the membrane glycoproteins G1 and G2

We have determined the complete nucleotide sequence of the virion M RNA segment of Uukuniemi virus (Uukuvirus genus, Bunyaviridae) from cloned cDNA. The RNA that encodes the two membrane glycoproteins G1 and G2 is 3231 residues long (mol wt 1.1 X 10(6)). The 5' and 3' ends of the RNA are partially complementary to each other for some 30 bp, enabling the formation of a stable panhandle structure (delta G = -40 kcal/mol) and the circularization of the molecule. The extreme 5' and 3' terminal nucleotides are identical for 10 to 13 residues to those of the M RNA of Punta Toro and Rift Valley fever viruses, two members of the Phlebovirus genus. A single open reading frame comprising 1008 amino acid residues (mol wt 113,588) was found in the mRNA-sense strand between nucleotides 18 and 3042. This probably corresponds to the previously identified 110,000-Da precursor (p110) of G1 and G2. By comparing the partial aminoterminal sequences of purified G1 and G2 with the deduced protein sequence we confirmed that the gene order is NH2-G1-G2-COOH. Both mature G1 and G2 are preceded by a stretch of 17 predominantly hydrophobic amino acids likely to represent the signal sequences. At their COOH-terminal ends, G1 and G2 have a hydrophobic stretch of amino acids, 19 and 27 residues, respectively, that probably anchors the proteins to the lipid bilayer. The sequence indicates that mature G2 is 495 amino acids long (mol wt 54,869), whereas the exact size of G1 is unclear, since the location of the COOH-terminus of G1 is not known. An upper value of 479 amino acids (mol wt 55,181) can, however, be suggested. Both G1 and G2 contain four potential glycosylation sites for Asn-linked glycans and both are unusually rich in cysteines, 6.1% in G1 and 5.4% in G2. Comparison of the amino acid sequence of the M RNA product of Uukuniemi virus with that of Punta Toro and Rift Valley fever viruses showed in both cases a weak homology that was more pronounced for the proteins located at the COOH-terminal end of the precursor. This suggests a distant evolutionary relationship between the Phlebo- and Uukuvirus genera.

Persistent infection of Aedes albopictus C6/36 cells by Bunyamwera virus

Two cell lines persistently infected with Bunyamwera virus have been established from the C6/36 clone of Aedes albopictus cells. The cells express Bunyamwera virus antigens as detected by immunofluorescence and are resistant to superinfection with Bunyamwera virus and other bunyaviruses, but not Dugbe virus (Nairovirus) nor vesicular stomatitis virus. The virus released from the persistently infected cells developed an altered cloudy or "bull's-eye" plaque morphology with increasing passage level, and a greater temperature sensitivity at 39.5 degrees than standard virus. The persistent virus interfered strongly with the replication of standard Bunyamwera virus in normal C6/36 cells and to a much lesser extent in BHK cells. Interference was not noted with other bunyaviruses or vesicular stomatitis virus. The persistent virus from one cell line, C6/36-PI LO, had a slower migrating nucleocapsid protein on polyacrylamide gels. Analysis of the RNA in persistently infected cells or in persistent virus by Northern blot hybridization with cloned cDNA probes showed that the major viral RNA species was the S segment, while the L and M RNA segments were barely detectable. Our results indicate that Bunyamwera virus can readily establish persistent infections in mosquito cells, and that persistence is accompanied by the generation of viruses with variable genetic and phenotypic characteristics.

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

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

Requirements for transport of HSV-1 glycoproteins to the cell surface membrane of human fibroblasts and Vero cells

The intracellular transport of the HSV-1 glycoproteins gA/gB, gC and gD has been followed by the indirect immunofluorescence technique (IIF). Infected tissue culture cells were stained with monoclonal antibodies made to the individual glycoproteins and with fluorochrome-coupled wheat germ agglutinin reacting specifically with Golgi apparatus of the cells. Staining of either infected, human fibroblasts or of VERO cells at 9 hours p.i. with antibodies to gA/gB showed a prominent ring-like nuclear fluorescence and distinct staining of the Golgi apparatus in the cells. Antibodies to gC and gD stained mainly the Golgi apparatus and areas close to or at the surface of the cells. By immunocytolysis of HSV-1-infected VERO cells the viral glycoproteins were demonstrable at the surface of cells but growth of infected cells in the presence of either TM or monensin inhibited the expression of most of the viral glycoproteins at the cell surface. Blocking of the glycosylation of the viral glycoproteins with tunicamycin (TM) was followed by accumulation of the core of the glycoproteins gA/gB and gD in granular structures close to the nucleus as seen by immunofluorescence microscopy. Antibodies to gC did also stain granules close to the nucleus but in addition the periphery of the cells were stained. Inhibition of intracellular transport from the Golgi apparatus by the carboxylic ionophore monensin was followed by accumulation of all the HSV-1 glycoproteins in vesicles derived from the Golgi apparatus in both human fibroblasts and VERO cells. Our data thus support the hypothesis that the HSV-1 glycoproteins are processed in the Golgi apparatus before the transport to and incorporation into the cell surface membrane of infected cells and into virion envelopes.