Semliki forest virus mutants with temperature-sensitive transport defect of envelope proteins

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.

Inhibition of the membrane fusion machinery prevents exit from the TGN and proteolytic processing by furin

The Semliki Forest virus (SFV) glycoprotein precursor p62 is processed to the E2 and E3 during the transport from the trans-Golgi network (TGN) to the cell surface. We have studied the regulation of the membrane fusion machinery (Rab/N-ethylmaleimide (NEM)-sensitive fusion protein (NSF)/soluble NSF attachment protein (SNAP)-SNAP receptor) in this processing. Activation of the disassembly of this complex with recombinant NSF stimulated the cleavage of p62 in permeabilized cells. Inactivation of NSF with a mutant alpha-SNAP(L294A) or NEM treatment inhibited processing of p62. Rab GDP dissociation inhibitor inhibited the cleavage. Inactivation of NSF blocks the transport of SFV glycoproteins and vesicular stomatitis virus G-glycoprotein from the TGN membranes to the cell surface. The results support the conclusion that inhibition of membrane fusion arrests p62 in the TGN and prevents its processing by furin.

Limited caspase cleavage of human BAP31

Human BAP31 was cleaved at both of its two identical caspase cleavage sites in two previously reported models of apoptosis. We show here that only the most carboxy-terminal site is cleaved during apoptosis induced in HeLa cells by tunicamycin, tumor necrosis factor and cycloheximide, or staurosporine. Similar results were obtained in HL-60 cells using Fas/APO-1 antibodies, or cycloheximide. This limited cleavage, which is inhibited by several caspase inhibitors, removes eight amino acids from human BAP31 including the KKXX coat protein I binding motif. Ectopic expression of the resulting cleavage product induces redistribution of mannosidase II from the Golgi and prevents endoplasmic reticulum to Golgi transport of virus glycoproteins.

Semliki forest virus budding: assay, mechanisms, and cholesterol requirement

All enveloped viruses must bud through a cellular membrane in order to acquire their lipid bilayer, but little is known about this important stage in virus biogenesis. We have developed a quantitative biochemical assay to monitor the budding of Semliki Forest virus (SFV), an enveloped alphavirus that buds from the plasma membrane in a reaction requiring both viral spike proteins and nucleocapsid. The assay was based on cell surface biotinylation of newly synthesized virus spike proteins and retrieval of biotinylated virions using streptavidin-conjugated magnetic particles. Budding of biotin-tagged SFV was continuous for at least 2 h, independent of microfilaments and microtubules, strongly temperature dependent, and relatively independent of continued exocytic transport. Studies of cell surface spike proteins at early times of infection showed that these spikes did not efficiently bud into virus particles and were rapidly degraded. In contrast, at later times of infection, spike protein degradation was markedly reduced and efficient budding was then observed. The previously described cholesterol requirement in SFV exit was shown to be due to a block in budding in the absence of cholesterol and correlated with the continued degradation of spike proteins at all times of virus infection in sterol-deficient cells.

Virus maturation by budding

Enveloped viruses mature by budding at cellular membranes. It has been generally thought that this process is driven by interactions between the viral transmembrane proteins and the internal virion components (core, capsid, or nucleocapsid). This model was particularly applicable to alphaviruses, which require both spike proteins and a nucleocapsid for budding. However, genetic studies have clearly shown that the retrovirus core protein, i.e., the Gag protein, is able to form enveloped particles by itself. Also, budding of negative-strand RNA viruses (rhabdoviruses, orthomyxoviruses, and paramyxoviruses) seems to be accomplished mainly by internal components, most probably the matrix protein, since the spike proteins are not absolutely required for budding of these viruses either. In contrast, budding of coronavirus particles can occur in the absence of the nucleocapsid and appears to require two membrane proteins only. Biochemical and structural data suggest that the proteins, which play a key role in budding, drive this process by forming a three-dimensional (cage-like) protein lattice at the surface of or within the membrane. Similarly, recent electron microscopic studies revealed that the alphavirus spike proteins are also engaged in extensive lateral interactions, forming a dense protein shell at the outer surface of the viral envelope. On the basis of these data, we propose that the budding of enveloped viruses in general is governed by lateral interactions between peripheral or integral membrane proteins. This new concept also provides answers to the question of how viral and cellular membrane proteins are sorted during budding. In addition, it has implications for the mechanism by which the virion is uncoated during virus entry.

Multiple mechanisms for the inhibition of entry and uncoating of superinfecting Semliki Forest virus

Recombinant Semliki Forest viruses (SFV) that express one or none of the viral structural proteins were used to infect cells and to analyze the fate of incoming superinfecting wild-type viruses. It was found that in addition to the previously described block in replication that superinfecting viruses encounter within 15 min of infection, other mechanisms of superinfection inhibition occurred at later times. Over a 6-hr infection period, inhibition was seen in binding of virus to the cell surface, in acid-activated penetration into the cytoplasm, and in uncoating of nucleocapsids. For each of these processes, the inhibitory mechanism was investigated. In summary, we found that infection evoked several independent mechanisms for blocking the entry and uncoating of superinfecting viruses. The results also offered new insights into the normal processes of penetration and uncoating of SFV.

The organization of the endoplasmic reticulum and the intermediate compartment in cultured rat hippocampal neurons

The boundaries of the organelles of the biosynthetic endomembrane system are still controversial. In this paper we take advantage of the unique architectural organization of neurons to investigate the localization of a spectrum of compartment-specific markers with the goal of defining the location of the rough endoplasmic reticulum (ER), smooth ER, intermediate compartment, and the Golgi complex. Markers of the rough ER (signal sequence receptor), Golgi complex (mannosidase II), and the trans Golgi network (TGN38) were essentially restricted to the cell body and the initial segment of one of the cell's dendrites. In contrast the cytochemical reaction product for glucose 6 phosphate, a classical ER marker, in addition to staining ER structures in the cell body also reacted with smooth ER elements that extended into both axons and dendrites. These peripheral smooth ER elements also reacted at the immunofluorescence level for ER marker 3-hydroxy-3-methylglutaryl-coenzyme A reductase, as well as for calnexin and protein disulfide isomerase. We also analyzed the location of rab1, rab2, p58, the KDEL receptor, and beta-subunit of coatomer. These intermediate compartment markers were found predominantly in the cell body but also extended to the proximal parts of the dendrites. Collectively, our data argue that the ER of hippocampal neurons consists of functionally and spatially distinct and separated domains, and they stress the power of the hippocampal neuron system for investigations of the organization of the ER by light microscopy.

Communication of post-Golgi elements with early endocytic pathway: regulation of endoproteolytic cleavage of Semliki Forest virus p62 precursor

A number of cellular proteins and viral spike proteins are cleaved at a basic recognition sequence. To characterize the membrane traffic step at which this proteolysis occurs we have studied the intracellular processing site of Semliki Forest virus (SFV) spike precursor p62 in BHK21 cells. The p62 is endoproteolytically cleaved at a tetrabasic Arg-His-Arg-Arg recognition sequence. Previously, it has been shown that the SFV p62 remains uncleaved when accumulated to the trans-Golgi network (TGN/20 degrees C block site). We show here that exit from the trans-Golgi is required for the cleavage of p62. Proteolytic processing was inhibited in synchronized assays when the 20 degrees C transport block was released in the presence of brefeldin A, energy inhibitors (azide and deoxyglucose; carbonyl cyanide m-chlorophenylhydrazone, CCCP) or an effector of trimeric G proteins, AlFn. Endocytosed antibodies against the SFV spike glycoproteins or antibodies against a peptide corresponding to the enzymatically active motif of furin inhibited cleavage of p62 at a post-TGN location. The results indicate a post-TGN communication step between exocytic and endocytic elements. Kinetic experiments suggested that this communication may involve an early compartment of the endocytic pathway.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

Effect of caffeine and reduced temperature (20 degrees C) on the organization of the pre-Golgi and the Golgi stack membranes

In the present study we have dissected the transport pathways between the ER and the Golgi complex using a recently introduced (Kuismanen, E., J. Jäntti, V. Mäkiranta, and M. Sariola. 1992. J. Cell Sci. 102:505-513) inhibition of transport by caffeine at 20 degrees C. Recovery of the Golgi complex from brefeldin A (BFA) treatment was inhibited by caffeine at reduced temperature (20 degrees C) suggesting that caffeine inhibits the membrane traffic between the ER and the Golgi complex. Caffeine at 20 degrees C did not inhibit the BFA-induced retrograde movement of the Golgi membranes. Further, incubation of the cells in 10 mM caffeine at 20 degrees C had profound effects on the distribution and the organization of the pre-Golgi and the Golgi stack membranes. Caffeine treatment at 20 degrees C resulted in a selective and reversible translocation of the pre- and cis-Golgi marker protein (p58) to the periphery of the cell. This caffeine-induced effect on the Golgi complex was different from that induced by BFA, since mannosidase II, a Golgi stack marker, remained perinuclearly located and the Golgi stack coat protein, beta-COP, was not detached from Golgi membranes in the presence of 10 mM caffeine at 20 degrees C. Electron microscopic analysis showed that, in the presence of caffeine at 20 degrees C, the morphology of the Golgi stack was altered and accumulation of numerous small vesicles in the Golgi region was observed. The results in the present study suggest that caffeine at reduced temperature (20 degrees C) reveals a functional interface between the pre-Golgi and the Golgi stack.

Spike protein-nucleocapsid interactions drive the budding of alphaviruses

Semliki Forest virus (SFV) particles are released from infected cells by budding of nucleocapsids through plasma membrane regions that are modified by virus spike proteins. The budding process was studied with recombinant SFV genomes which lacked the nucleocapsid protein gene or, alternatively, the spike genes. No subviral particles were released from cells which expressed only the nucleocapsid protein or the spike proteins. Virus release was found to be strictly dependent on the coexpression of the nucleocapsid and the spike proteins. These results provide direct proof for the hypothesis that the alphavirus budding is driven by nucleocapsid-spike interactions. The importance of the viral 42S RNA for virus assembly and budding was investigated by using the heterologous vaccinia virus-T7 expression system for the synthesis of the SFV structural proteins. The results demonstrate that the viral genome is not absolutely required for formation of budding competent nucleocapsids, since small amounts of viruslike particles were assembled in the absence of 42S RNA.

In vitro mutagenesis of a full-length cDNA clone of Semliki Forest virus: the small 6,000-molecular-weight membrane protein modulates virus release

We report on the construction of a full-length cDNA clone of Semliki Forest virus (SFV). By placing the cDNA under the SP6 promoter, infectious RNA can be produced in vitro and used to transfect cells to initiate virus infection. To achieve efficient transfections, a new protocol for electroporation of RNA was developed. This method gave up to 500-fold improvement over the traditional DEAE-dextran transfection procedure. Since virtually 100% of the cells can be transfected by electroporation, this method is a useful tool for detailed biochemical studies of null mutations of SFV that abolish production of infections virus particles. We used the cDNA clone of SFV to study what effects a deletion of the 6,000-molecular-weight membrane protein (6K membrane protein) had on virus replication. The small 6K protein is part of the structural precursor molecule (C-p62-6K-E1) of the virus. Our results conclusively show that the 6K protein is not needed for the heterodimerization of the p62 and E1 spike membrane proteins in the endoplasmic reticulum, nor is it needed for their transport out to the cell surface. The absence of the 6K protein did, however, result in a dramatic reduction in virus release, suggesting that the protein exerts its function late in the assembly pathway, possibly during virus budding.

Biogenesis of type I cytopathic vacuoles in Semliki Forest virus-infected BHK cells

We investigated the biogenesis of type I cytopathic vacuoles (CPVIs) in Semliki Forest virus (SFV)-infected cells by immunofluorescence and electron microscopy. By using the ts1 mutant of SFV at the restrictive temperature to avoid superinfection, we showed that the multiplicity of infection affects the time of appearance but not the number of CPVIs in a cell. Formation of CPVIs did not require incoming virus particles, because they were found in BHK cells transfected with infectious RNA from the SFV prototype strain or ts1 mutant. When the SFV gene for nsP3 was expressed alone in BHK cells, the nsP3 protein was localized to numerous vesiclelike structures and large vacuoles. The nsP3 protein may function as an anchoring protein for the RNA replication complex of SFV.

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.

Constitutive apical secretion of an 80-kD sulfated glycoprotein complex in the polarized epithelial Madin-Darby canine kidney cell line

The biosynthesis, processing, and apical secretion of a group of polypeptides (Kondor-Koch, C., R. Bravo, S. D. Fuller, D. Cutler, and H. Garoff. 1985. Cell. 43:297-306) are studied in MDCK cells using a specific polyclonal antiserum. These polypeptides are synthesized as a precursor protein which has an apparent Mr of 65,000 in its high mannose form. This precursor is converted into a protein with an apparent Mr of 80,000 containing complex carbohydrates and sulfate. After intracellular cleavage of the 80-kD protein, the 35-45-kD subunits are secreted as an 80-kD glycoprotein complex (gp 80) linked together by disulfide bonds. Secretion of the protein complex occurs by a constitutive pathway at the apical surface of the epithelial monolayer. Since the immediate post-translational precursor, the 65-kD protein, is hydrophilic in nature as shown by its partitioning behavior in a phase-separated Triton X-114 solution, gp 80 is segregated into the apical exocytotic pathway as a soluble molecule. The proteolytic maturation of gp 80 is blocked in the presence of chloroquine and its secretion is retarded. The 80-kD precursor is released at the apical cell surface, demonstrating that proteolytic processing is not necessary for the apical secretion of this protein. If N-glycosylation is inhibited by tunicamycin treatment the protein is secreted in equal amounts at both cell surfaces, indicating a role of the carbohydrate moieties in the vectorial transport of this protein.

Efficient transport of Semliki Forest virus glycoproteins through a Golgi complex morphologically altered by Uukuniemi virus glycoproteins

In infected BHK21 cells, the glycoproteins G1 and G2 of a temperature-sensitive mutant (ts12) of Uukuniemi virus (UUK) accumulate at 39 degrees C in the Golgi complex (GC) causing an expansion and vacuolization of this organelle. We have studied whether such an altered Golgi complex can carry out the glycosylation and transport to the plasma membrane (PM) of the Semliki Forest virus (SFV) glycoproteins in double-infected cells. Double-immunofluorescence staining showed that approximately 90% of the cells became infected with both viruses. Almost the same final yield of infectious SFV was obtained from double-infected cells as from cells infected with SFV alone. The rate of transport from the endoplasmic reticulum (ER) via the GC to the plasma membrane of the SFV glycoproteins was analysed by immunofluorescence, surface radioimmunoassay and pulse-chase labeling followed by immunoprecipitation, endoglycosidase H digestion and SDS-PAGE. The results showed that: the SFV glycoproteins were readily transported to the cell surface in double-infected cells, whereas the UUK glycoproteins were retained in the GC; the transport to the PM was retarded by approximately 20 min, due to a delay between the ER and the central Golgi; E1 of SFV appeared at the PM in a sialylated form. These results indicate that the morphologically altered GC had retained its functional integrity to glycosylate and transport plasma membrane glycoproteins.

Uukuniemi virus glycoproteins accumulate in and cause morphological changes of the Golgi complex in the absence of virus maturation

We have studied the transport of the Uukuniemi virus membrane glycoproteins in baby hamster kidney and chick embryo cells by using a temperature-sensitive mutant (ts12). Uukuniemi virus assembles in the Golgi complex, where both glycoproteins G1 and G2 and nucleocapsid protein N accumulate (E. Kuismanen, B. Bång, M. Hurme, and R. F. Pettersson, J. Virol. 51:137-146, 1984). At the restrictive temperature (39 degrees C), the glycoproteins of ts12 were transported to the Golgi complex as in wild-type, virus-infected cells, whereas the nucleocapsid protein failed to accumulate there. Pulse-chase labeling followed by immunoprecipitation and treatment with endo-beta-N-acetylglucosaminidase H showed that G1 synthesized at 39 degrees C in ts12-infected cells had an altered mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggesting a lack of terminal glycosylation. The typical Uukuniemi virus-induced vacuolization and expansion of the Golgi complex could be seen also in ts12-infected cells at 39 degrees C, although no virus particles were formed. This suggests that the morphological changes were induced by the Uukuniemi virus glycoproteins. In wild-type virus- or ts12-infected cells, G1 and G2 could not be chased out from the Golgi complex even after 6 h of treatment with cycloheximide. The glycoproteins were thus retained in the Golgi even under conditions when no virus maturation took place and when nucleocapsids did not accumulate in the Golgi region. Accordingly, the glycoproteins of Uukuniemi virus were found to have properties resembling those of Golgi-specific proteins. This virus model system may be useful in studying the synthesis and transport of membrane proteins that are transported to and retained in the Golgi.

Synthesis and processing of Semliki forest virus polyprotein in Saccharomyces cerevisiae: a yeast type glycosylation of E1 envelope protein

A cDNA coding for the structural proteins of Semliki Forest virus (SFV) was ligated between the ADC1 promoter and terminator in a yeast expression vector, pAAH5. Synthesis of the SFV-specific proteins in Saccharomyces cerevisiae transformed with this vector was shown by immunoblotting and immunoprecipitation. Detection of the N-terminal and the C-terminal components of the viral polyprotein, capsid protein and E1 envelope protein, respectively, suggested that the entire polyprotein was translated in yeast. The capsid protein was effectively released from the polyprotein as a normal size polypeptide, but the following protein, p62 (E3, E2 precursor) was not detected, suggesting that it was rapidly degraded. Electrophoretic analyses indicated that the final protein, E1, entered the secretory pathway, the signal sequence was cleaved off and the protein became extensively and heterogeneously glycosylated. These data suggest that E1 was transported to the Golgi complex and that yeast-characteristic outer-chain glycans were added to the protein.

Newly synthesized G protein of vesicular stomatitis virus is not transported to the Golgi complex in mitotic cells

Newly synthesized G protein of vesicular stomatitis virus is not transported to the surface of cultured mammalian cells during mitosis (Warren et al., 1983, J. Cell Biol. 97:1623-1628). To determine where intracellular transport is inhibited, we have examined the post-translational modifications of G protein, which are indicators of specific compartments on the transport pathway. G protein in mitotic cells had only endo H-sensitive oligosaccharides containing seven or eight mannose residues, but no terminal glucose, and was not fatty acylated. These modifications were indicative of processing only by enzymes of the endoplasmic reticulum (ER). Quantitative immunocytochemistry was used as an independent method to confirm that transport of G protein out of the ER was inhibited. The density of G protein in the ER cisternae was 2.5 times greater than in infected G1 cells treated similarly. Incubation of infected mitotic cells with cycloheximide, which inhibits protein synthesis without affecting transport, did not result in a decrease in the density of G protein in the ER cisternae, demonstrating that G protein cannot be chased out of the ER. These results suggest that intracellular transport stops at or before the first vesicle-mediated step on the pathway.

Pre- and post-Golgi vacuoles operate in the transport of Semliki Forest virus membrane glycoproteins to the cell surface

The effect of reduced temperature on synchronized transport of SFV membrane proteins from the ER via the Golgi complex to the surface of BHK-21 cells revealed two membrane compartments where transport could be arrested. At 15 degrees C the proteins could leave the ER but failed to enter the Golgi cisternae and accumulated in pre-Golgi vacuolar elements. At 20 degrees C the proteins passed through Golgi stacks but accumulated in trans-Golgi cisternae, vacuoles, and vesicular elements because of a block affecting a distal stage in transport. Both blocks were reversible, allowing study of the synchronous passage of viral membrane proteins through the Golgi complex at high resolution by immunolabeling in electron microscopy. We propose that membrane proteins enter the Golgi stack via tubular extensions of the pre-Golgi vacuolar elements which generate the Golgi cisternae. The proteins pass across the Golgi apparatus following cisternal progression and enter the post-Golgi vacuolar elements to be routed to the cell surface.

Semliki Forest virus: a probe for membrane traffic in the animal cell

The traffic among the cellular compartments is thought to be mediated by membrane vesicles, which bud from one compartment and fuse with the next. Despite the continuous exchange of membrane components among them, the organelles maintain their characteristic protein and lipid compositions such that the traffic remains selective, thus, avoiding intermixing of components. This membrane traffic recycles components from the cell surface to the interior of the cell and back to the cell surface again. The membrane traffic between the ER and the cell surface involves a major sorting problem. Little is known of how the animal cell has solved this problem in molecular terms. One experimental tool in this direction is provided by some enveloped animal viruses, which mature at the cell surface of infected cells. Such viruses include influenza virus, Semliki Forest virus (SFV), Sindbis virus, and vesicular stomatitis virus (VSV). They are extremely simple in makeup and hence are very well characterized. The purpose of this article is to illustrate the use of the enveloped viruses as tools in the study of membrane traffic in the animal cell. This is done in the context of the life cycle of the virus in the host cell. The article will be concerned mainly with Semliki Forest virus (SFV), which is the virus that has been worked upon in the chapter. SFV belongs to the alphaviruses, a genus of the togavirus family.

Expression of Semliki Forest virus proteins from cloned complementary DNA. II. The membrane-spanning glycoprotein E2 is transported to the cell surface without its normal cytoplasmic domain

The E2 protein (422 amino acid residues long) of Semliki Forest virus is a spanning membrane protein which is made in the rough endoplasmic reticulum of the infected cell and transported to the cell surface. The cytoplasmic domain of this protein comprises 31 amino acid residues. We introduced deletions of various sizes into the gene region encoding this part of the protein molecule and analyzed the transport behavior of the mutant proteins. The deletions were made using exonuclease digestions of cloned cDNA encoding the E2 protein. When the mutated DNA molecules, engineered into an expression vector, were introduced into nuclei of baby hamster kidney 21 cells, membrane proteins with cytoplasmic deletions were expressed and routed to the cell surface in the same way as the wild-type protein. This suggests that the cytoplasmic domain of the E2 protein does not carry information that is needed for its transport from the rough endoplasmic reticulum to the cell surface.

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

We studied the maturation of Uukuniemi virus and the localization of the viral surface glycoproteins and nucleocapsid protein in infected cells by electron microscopy, indirect immunofluorescence, and immunoelectron microscopy with specific antisera prepared in rabbits against the two glycoproteins G1 and G2 and the nucleocapsid protein N. Electron microscopy of thin sections from infected cells showed virus particles maturing at smooth-surfaced membranes close to the nucleus. Localization of the G1/G2 and N proteins by indirect immunofluorescence at different stages after infection showed the antigens to be present throughout the cell interior but concentrated in the juxtanuclear region. The G1/G2 antiserum also appeared to stain the nuclear and plasma membranes. Double staining with tetramethylrhodamine isothiocyanate-conjugated wheat germ agglutinin, which preferentially stains the Golgi complex, and fluorescein isothiocyanate-conjugated anti-rabbit immunoglobulin G, which stained the G1/G2 or N proteins, showed that the staining of the juxtanuclear region coincided. Similarly, double staining for thiamine pyrophosphatase, an enzyme activity specific for the Golgi complex, showed the fluorescence and the cytochemical stain to coincide in the juxtanuclear region. Immunoperoxidase electron microscopy of cells permeabilized with saponin revealed that the viral glycoproteins were present in the rough endoplasmic reticulum and the nuclear and Golgi membranes; the latter was heavily stained. With this method, the N protein was localized to the cytoplasm, especially around smooth-surfaced vesicles in the Golgi region. Taken together, the results indicate that Uukuniemi virus and its structural proteins accumulate in the Golgi complex, supporting the idea that this compartment rather than the plasma membrane is the site of virus maturation. This raises the interesting possibility that deficient transport of the glycoproteins to the plasma membrane and hence their accumulation in the Golgi complex determines the site of virus maturation.

Dissection of the Golgi complex. II. Density separation of specific Golgi functions in virally infected cells treated with monensin

In the accompanying paper (Griffiths, G., P. Quinn, and G. Warren, 1983, J. Cell Biol., 96:835-850), we suggested that the Golgi stack could be divided into functionally distinct cis, medial, and trans compartments, each comprising one or two adjacent cisternae. These compartments were identified using Baby hamster kidney (BHK) cells infected with Semliki Forest virus (SFV) and treated with monensin. This drug blocked intracellular transport but not synthesis of the viral membrane proteins that were shown to accumulate in the medial cisternae. In consequence, these cisternae bound nucleocapsids. Here we show that this binding markedly increased the density of the medial cisternae and allowed us to separate them from cis and trans Golgi cisternae. A number of criteria were used to show that the intracellular capsid-binding membranes (ICBMs) observed in vivo were the same as those membranes sedimenting to a higher density in sucrose gradients in vitro, and this separation of cisternae was then used to investigate the distribution, within the Golgi stack, of some specific Golgi functions. After labeling for 2.5 min with [3H]palmitate, most of the fatty acid attached to viral membrane proteins was found in the ICBM fraction. Because the viral membrane proteins appear to move from cis to trans, this suggests that fatty acylation occurs in the cis or medial Golgi cisternae. In contrast, the distribution of alpha 1-2-mannosidase, an enzyme involved in trimming high-mannose oligosaccharides, and of galactosyl transferase, which is involved in the construction of complex oligosaccharides, was not affected by monensin treatment. Together with data in the accompanying paper, this would restrict these two Golgi functions to the trans cisternae. Our data strongly support the view that Golgi functions have specific and discrete locations within the Golgi stack.

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

We studied the maturation of Uukuniemi virus and the localization of the viral surface glycoproteins and nucleocapsid protein in infected cells by electron microscopy, indirect immunofluorescence, and immunoelectron microscopy with specific antisera prepared in rabbits against the two glycoproteins G1 and G2 and the nucleocapsid protein N. Electron microscopy of thin sections from infected cells showed virus particles maturing at smooth-surfaced membranes close to the nucleus. Localization of the G1/G2 and N proteins by indirect immunofluorescence at different stages after infection showed the antigens to be present throughout the cell interior but concentrated in the juxtanuclear region. The G1/G2 antiserum also appeared to stain the nuclear and plasma membranes. Double staining with tetramethylrhodamine isothiocyanate-conjugated wheat germ agglutinin, which preferentially stains the Golgi complex, and fluorescein isothiocyanate-conjugated anti-rabbit immunoglobulin G, which stained the G1/G2 or N proteins, showed that the staining of the juxtanuclear region coincided. Similarly, double staining for thiamine pyrophosphatase, an enzyme activity specific for the Golgi complex, showed the fluorescence and the cytochemical stain to coincide in the juxtanuclear region. Immunoperoxidase electron microscopy of cells permeabilized with saponin revealed that the viral glycoproteins were present in the rough endoplasmic reticulum and the nuclear and Golgi membranes; the latter was heavily stained. With this method, the N protein was localized to the cytoplasm, especially around smooth-surfaced vesicles in the Golgi region. Taken together, the results indicate that Uukuniemi virus and its structural proteins accumulate in the Golgi complex, supporting the idea that this compartment rather than the plasma membrane is the site of virus maturation. This raises the interesting possibility that deficient transport of the glycoproteins to the plasma membrane and hence their accumulation in the Golgi complex determines the site of virus maturation.

Expression of the structural proteins of Semliki Forest virus from cloned cDNA microinjected into the nucleus of baby hamster kidney cells

The three structural proteins of Semliki Forest virus--i.e., the capsid, p62, and E1 proteins--were expressed in baby hamster kidney cells from cloned DNA transcribed from the virus-specific 4.1-kilobase mRNA. The cDNA was engineered into an expression vector developed by others [Mulligan, R. C. & Berg, P. (1980) Science 209, 1422--1427] downstream from the simian virus 40 early promoter and was introduced into cell nuclei by microneedle injection. Immunofluorescence analysis of injected cells showed that the capsid protein was located in the cell cytoplasm, whereas the membrane proteins were associated with cellular membranes. The p62 protein was shown to be transported from the rough endoplasmic reticulum to the plasma membrane, whereas the E1 protein remained in the rough endoplasmic reticulum.

A membrane glycoprotein that accumulates intracellularly: cellular processing of the large glycoprotein of LaCrosse virus

The intracellular transport and certain posttranslational modifications of the large glycoprotein (G1) of LaCrosse virus (LAC) in BHK cells have been studied. G1 from released LAC virus was characterized by complex oligosaccharides (endo H-resistant) and covalently attached fatty acid. Only a small fraction of total cellular G1 was present on the baby hamster kidney cell surface. Cell-surface G1 contained complex oligosaccharides, while total G1 in infected cells contained largely unprocessed (endo H-sensitive) oligosaccharides. In addition, cell G1 contained significantly less fatty acid than virion-associated G1. Pulse-chase experiments showed that the oligosaccharides of G1 were processed to the complex from much more slowly than the oligosaccharides of the vesicular stomatitis virus (VSV) glycoprotein (G). In addition, transit of LAC G1 to the cell surface and into extracellular virions was two to three fold slower than the transit of VSV G. Thus LAC G1 accumulates intracellularly and is only slowly processed by intracellular processing enzymes. Treatment with monensin caused accumulation in the cell of a form of G1 with partial sensitivity toward endo H, suggesting that monensin may act to inhibit the glycosylation process directly.

The use of red cells with fused Semliki Forest virus envelope proteins in antibody determinations by hemolysis in gel

Chicken red blood cells with fused Semliki Forest virus (SFV) proteins on the cell membrane were used in the hemolysis-in-gel (HIG) plates. Optimally the plate contained a 1.5 mm thick gel of 1% agarose with 1% red cells and 1 unit/ml gel of complement. 400 ng of SFV was fused to the red cells in 1 ml of the gel (about 20 virions fused to one red cell). Five microliters of inactivated (56 degrees C, 30 min) serum samples were pipetted into wells in the gel. After 20 h of incubation at 37 degrees C the diameters of the hemolytic zones were directly proportional to the logarithm of the serum dilution. This made it possible to calculate the antibody titers for the samples using an experimental formula T = 10 phi / kappa (T is the titer, phi the diameter in mm and kappa a variable coefficient, which had to be determined for each batch of the plates using a standard serum). Using regression and residual analyses, the formula was shown to fit the experimental results. The fusion-based HIG could be read after as early as 2 h of incubation. The specificity of the test was studied using antisera against Western equine encephalomyelitis, Eastern equine encephalomyelitis, Chikungunya and Uruma viruses, which all gave negative results in the SVF HIG test. Antisera against SFV E1 and E2 proteins were positive, but anti-E3 serum was negative when measured in the SVF HIG test.

Evidence for a separate signal sequence for the carboxy-terminal envelope glycoprotein E1 of Semliki forest virus

When Semliki Forest virus temperature-sensitive mutant ts-3 was grown at the restrictive temperature an aberrant nascent cleavage of the 130,000-dalton structural polyprotein took place relatively frequently. This cleavage yielded an abnormal 86,000-dalton fusion protein (p86) consisting of the amino-terminal capsid protein linked to the amino acid sequences of envelope protein p62 (a precursor of E3 and E2). The other cleavage product was the carboxy-terminal envelope protein E1. p86 was not glycosylated and was sensitive to the action of protease in the microsomal fraction, whereas E1 was glycosylated and protected from proteases, indicating that it had been segregated into the cysternal side of the microsomal vesicles. All attempts to show the E1 protein at the cell surface have failed so far, suggesting that it remains associated with intracellular membranes. When ts-3-infected cells labeled at the restrictive temperature were shifted to the permissive temperature the only labeled protein released with the virus particles was E1, indicating that E1, synthesized at the restrictive temperature, was competent to participate in the virus assembly. These results suggest strongly that there are two separate signal sequences for the envelope proteins of Semliki Forest virus. One follows the capsid protein as shown previously, and the other is for the carboxy-terminal E1. Even if the insertion of the amino-terminal envelope protein (p62) fails due to a cleavage defect, the other signal sequence can operate independently to guide the E1 through the endoplasmic reticulum membrane.

Monensin and FCCP inhibit the intracellular transport of alphavirus membrane glycoproteins

Temperature-sensitive mutants of semliki forest virus (SFV) and sindbis virus (SIN) were used to study the intracellular transport of virus membrane glycoproteins in infected chicken embryo fibroblasts. When antisera against purified glycoproteins and (125)I- labeled protein A from staphylococcus aureus were used only small amounts of virus glycoproteins were detected at the surface of SFV ts-1 and SIN Ts-10 infected cells incubated at the restrictive temperature (39 degrees C). When the mutant-infected cells were shifted to the permissive temperature (28 degrees C), in the presence of cycloheximide, increasing amounts of virus glycoproteins appeared at the cell surface from 20 to 80 min after the shift. Both monensin (10muM) and carbonylcyanide-p- trifluoromethoxyphenylhydrazone (FCCP; 10-20 muM) inhibited the appearance of virus membrane glycoproteins at the cell surface. Vinblastine sulfate (10 mug/ml) inhibited the transport by approximately 50 percent, whereas cytochalasin B (1 mug/ml) had only a marginal effect. Intracellular distribution of virus glycoproteins in the mutant-infected cells was visualized in double-fluorescence studies using lectins as markers for endoplasmic reticulum and Golgi apparatus. At 39 degrees C, the virus membrane glycoproteins were located at the endoplasmic reticulum, whereas after shift to 28 degrees C, a bright juxtanuclear reticular fluorescence was seen in the location of the Golgi apparatus. In the presence of monensin, the virus glycoproteins could migrate to the Golgi apparatus, although transport to the cell surface did not take place. When the shift was carried out in the presence of FCCP, negligible fluorescence was seen in the Golgi apparatus and the glycoproteins apparently remained in the rough endoplasmic reticulum. A rapid inhibition in the accumulation of virus glycoproteins at the cell surface was obtained when FCCP was added during the active transport period, whereas with monensin there was a delay of approximately 10 min. These results suggest a similar intracellular pathway in the maturation of both plasma membrane and secretory glycoproteins.