The rubella virus E2 and E1 spike glycoproteins are targeted to the Golgi complex

A single-point mutation in the rubella virus E1 glycoprotein promotes rescue of recombinant vesicular stomatitis virus

Rubella virus (RuV) is an enveloped plus-sense RNA virus and a member of the Rubivirus genus. RuV infection in pregnant women can lead to miscarriage or an array of severe birth defects known as congenital rubella syndrome. Novel rubiviruses were recently discovered in various mammals, highlighting the spillover potential of other rubiviruses to humans. Many features of the rubivirus infection cycle remain unexplored. To promote the study of rubivirus biology, here, we generated replication-competent recombinant VSV-RuV (rVSV-RuV) encoding the RuV transmembrane glycoproteins E2 and E1. Sequencing of rVSV-RuV showed that the RuV glycoproteins acquired a single-point mutation W448R in the E1 transmembrane domain. The E1 W448R mutation did not detectably alter the intracellular expression, processing, glycosylation, colocalization, or dimerization of the E2 and E1 glycoproteins. Nonetheless, the mutation enhanced the incorporation of RuV E2/E1 into VSV particles, which bud from the plasma membrane rather than the RuV budding site in the Golgi. Neutralization by E1 antibodies, calcium dependence, and cell tropism were comparable between WT-RuV and either rVSV-RuV or RuV containing the E1 W448R mutation. However, the E1 W448R mutation strongly shifted the threshold for the acid pH-triggered virus fusion reaction, from pH 6.2 for the WT RuV to pH 5.5 for the mutant. These results suggest that the increased resistance of the mutant RuV E1 to acidic pH promotes the ability of viral envelope proteins to generate infectious rVSV and provide insights into the regulation of RuV fusion during virus entry and exit.IMPORTANCERubella virus (RuV) infection in pregnant women can cause miscarriage or severe fetal birth defects. While a highly effective vaccine has been developed, RuV cases are still a significant problem in areas with inadequate vaccine coverage. In addition, related viruses have recently been discovered in mammals, such as bats and mice, leading to concerns about potential virus spillover to humans. To facilitate studies of RuV biology, here, we generated and characterized a replication-competent vesicular stomatitis virus encoding the RuV glycoproteins (rVSV-RuV). Sequence analysis of rVSV-RuV identified a single-point mutation in the transmembrane region of the E1 glycoprotein. While the overall properties of rVSV-RuV are similar to those of WT-RuV, the mutation caused a marked shift in the pH dependence of virus membrane fusion. Together, our studies of rVSV-RuV and the identified W448R mutation expand our understanding of rubivirus biology and provide new tools for its study.

Molecular and Structural Insights into the Life Cycle of Rubella Virus

Rubella virus (RUBV), a rubivirus, is an airborne human pathogen that generally causes mild measles-like symptoms in children or adults. However, RUBV infection of pregnant women can result in miscarriage or congenital rubella syndrome (CRS), a collection of long-term birth defects including incomplete organ development and mental retardation. Worldwide vaccination campaigns have significantly reduced the number of RUBV infections, but RUBV continues to be a problem in countries with low vaccination coverage. Further, the recent discovery of pathogenic rubiviruses in other mammals emphasizes the spillover potential of rubella-related viruses to humans. In the last decade, our understanding of RUBV has been significantly increased by virological, biochemical, and structural studies, providing a platform to begin understanding the life cycle of RUBV at the molecular level. This review concentrates on recent work on RUBV, focusing on the virion, its structural components, and its entry, fusion, and assembly mechanisms. Important features of RUBV are compared with those of viruses from other families. We also use comparative genomics, manual curation, and protein homology modeling to highlight distinct features of RUBV that are evolutionarily conserved in the non-human rubiviruses. Since rubella-like viruses may potentially have higher pathogenicity and transmissibility to humans, we also propose a framework for utilizing RUBV as a model to study its more pathogenic cousins.

Inhibition of rubella virus replication by the broad-spectrum drug nitazoxanide in cell culture and in a patient with a primary immune deficiency

Persistent rubella virus (RV) infection has been associated with various pathologies such as congenital rubella syndrome, Fuchs's uveitis, and cutaneous granulomas in patients with primary immune deficiencies (PID). Currently there are no drugs to treat RV infections. Nitazoxanide (NTZ) is an FDA-approved drug for parasitic infections, and has been recently shown to have broad-spectrum antiviral activities. Here we found that empiric 2-month therapy with oral NTZ was associated in the decline/elimination of RV antigen from lesions in a PID patient with RV positive granulomas, while peginterferon treatment had no effect. In addition, we characterized the effects of NTZ on cell culture models of persistent RV infection. NTZ significantly inhibited RV replication in a primary culture of human umbilical vein endothelial cells (HUVEC) and Vero and A549 epithelial cell lines in a dose dependent manner with an average 50% inhibitory concentration of 0.35 μg/ml (1.1 μM). RV strains representing currently circulating genotypes were inhibited to a similar extent. NTZ affected early and late stages of infection by inhibiting synthesis of cellular and RV RNA and interfering with intracellular trafficking of the RV surface glycoproteins, E1 and E2. These results suggest a potential application of NTZ for the treatment of persistent rubella infections, but more studies are required.

Assembly, maturation and three-dimensional helical structure of the teratogenic rubella virus

Viral infections during pregnancy are a significant cause of infant morbidity and mortality. Of these, rubella virus infection is a well-substantiated example that leads to miscarriages or severe fetal defects. However, structural information about the rubella virus has been lacking due to the pleomorphic nature of the virions. Here we report a helical structure of rubella virions using cryo-electron tomography. Sub-tomogram averaging of the surface spikes established the relative positions of the viral glycoproteins, which differed from the earlier icosahedral models of the virus. Tomographic analyses of in vitro assembled nucleocapsids and virions provide a template for viral assembly. Comparisons of immature and mature virions show large rearrangements in the glycoproteins that may be essential for forming the infectious virions. These results present the first known example of a helical membrane-enveloped virus, while also providing a structural basis for its assembly and maturation pathway.

Rubella virus (RV) causes serious fetal defects when contracted during pregnancy. Despite its medical importance, due to the irregular shapes and different sizes of the virions, the RV structure has remained unknown. Using cryo-electron tomography, we have determined the RV structure, which shows a unique, helical outer surface. Subsequent local averaging of the RV surface spikes has established the conformations of its immunogenic glycoproteins. In vitro assembly studies on the virus capsid protein have provided insights into the interactions necessary for virus assembly. Comparisons between mature and immature RV show large conformational changes in the virion structure that are essential for virus maturation. These results help to gain a structural understanding of RV pathogenicity, which may also be relevant to other teratogenic viruses.

[The life cycle of Rubella Virus]

Rubella virus (RV), an infectious agent of rubella, is the sole member of the genus Rubivirus in the family of Togaviridae. RV has a positive-stranded sense RNA as a genome. A natural host of RV is limited to human, and rubella is considered to be a childhood disease in general. When woman is infected with RV during early pregnancy, her fetus may develop severe birth defects known as congenital rubella syndrome. In this review, the RV life cycle from the virus entry to budding is illustrated in comparison with those of member viruses of the genus alphavirus in the same family. The multiple functions of the RV capsid protein are also introduced.

Class II enveloped viruses

A number of viruses transport their genomic material from cell to cell enclosed within a lipid bilayer that is in turn encased within a symmetric protein shell. This review focuses in a group of RNA viruses that have this type of virions. This group includes several of important human pathogenic viruses, such as the hepatitis C virus, dengue virus, chikungunya virus, rubella virus and the bunyaviruses. The best studied are the flaviviruses and the alphaviruses, which have a β-sheet rich class II viral fusion protein used for entry into susceptible cells. We extend here the class II concept to encompass symmetric viruses in which the envelope proteins are derived from a precursor polyprotein containing two transmembrane glycoproteins arranged in tandem. The first glycoprotein acts as chaperone for the folding of the second one, which carries the membrane fusion function. Since the bunyaviruses, included here, are very similar to the class I arenaviruses in other respects, this analysis highlights the patchwork nature of the various viral functional modules acting at different stages of the virus cycle, which appear assembled from genes of different origins.

Rubella virus capsid protein: a small protein with big functions

Virus replication occurs in the midst of a life or death struggle between the virus and the infected host cell. To limit virus replication, host cells can activate a number of antiviral pathways, the most drastic of which is programmed cell death. Whereas large DNA viruses have the luxury of encoding accessory proteins whose main function is to interfere with host cell defences, the genomes of RNA viruses are not large enough to encode proteins of this type. Recent studies have revealed that proteins encoded by RNA viruses often play multiple roles in the battles between viruses and host cells. In this article, we discuss the many functions of the rubella virus capsid protein. This protein has well-defined roles in virus assembly, but recent research suggests that it also functions to modulate virus replication and block host cell defences.

The rubella virus capsid protein inhibits mitochondrial import

The rubella virus (RV) capsid is an RNA-binding protein that functions in nucleocapsid assembly at the Golgi complex, the site of virus budding. In addition to its role in virus assembly, pools of capsid associate with mitochondria, a localization that is not consistent with virus assembly. Here we examined the interaction of capsid with mitochondria and showed that this viral protein inhibits the import and processing of mitochondrial precursor proteins in vitro. Moreover, RV-infected cells were found to contain lower intramitochondrial levels of matrix protein p32. In addition to inhibiting the translocation of substrates into mammalian mitochondria, capsid efficiently blocked import into yeast mitochondria, thereby suggesting that it acts by targeting a highly conserved component of the translocation apparatus. Finally, mutation of a cluster of five arginine residues in the amino terminus of capsid, though not interfering with its binding to mitochondria, abrogated its ability to block protein import into mitochondria. This is the first report of a viral protein that affects the import of proteins into mitochondria.

Determinants of subcellular localization of the rubella virus nonstructural replicase proteins

The rubella virus (RUBV) nonstructural replicase proteins (NSPs), P150 and P90, are proteolytically processed from a P200 precursor. To understand the NSPs' function in the establishment of virus RNA replication complexes (RCs), the NSPs were analyzed in virus-infected cells or cells transfected with NSP-expressing plasmids. In infected cells, P150 was localized in cytoplasmic foci at 24 hpi and in cytoplasmic fibers, unique to RUBV, by 48 hpi. RCs, marked by dsRNA, colocalized with P150-foci, but only occasionally with the endosome/lysosome marker LAMP-2, indicating that RNA synthesis occurs at other sites rather than exclusively in endosomes/lysosomes as was previously thought. An expressed cleavage-deficient form of P200 also localized to cytoplasmic foci, suggesting that the precursor is required for targeting to sites of RC establishment. P150 was found to be the determinant of fiber formation and the NSP membrane-binding domain was mapped to the N-terminus of P150.

Rubella virus capsid protein interacts with poly(a)-binding protein and inhibits translation

During virus assembly, the capsid proteins of RNA viruses bind to genomic RNA to form nucleocapsids. However, it is now evident that capsid proteins have additional functions that are unrelated to nucleocapsid formation. Specifically, their interactions with cellular proteins may influence signaling pathways or other events that affect virus replication. Here we report that the rubella virus (RV) capsid protein binds to poly(A)-binding protein (PABP), a host cell protein that enhances translational efficiency by circularizing mRNAs. Infection of cells with RV resulted in marked increases in the levels of PABP, much of which colocalized with capsid in the cytoplasm. Mapping studies revealed that capsid binds to the C-terminal half of PABP, which interestingly is the region that interacts with other translation regulators, including PABP-interacting protein 1 (Paip1) and Paip2. The addition of capsid to in vitro translation reaction mixtures inhibited protein synthesis in a dose-dependent manner; however, the capsid block was alleviated by excess PABP, indicating that inhibition of translation occurs through a stoichiometric mechanism. To our knowledge, this is the first report of a viral protein that inhibits protein translation by sequestration of PABP. We hypothesize that capsid-dependent inhibition of translation may facilitate the switch from viral translation to packaging RNA into nucleocapsids.

Requirement of the N-terminal region of orthobunyavirus nonstructural protein NSm for virus assembly and morphogenesis

The nonstructural protein NSm of Bunyamwera virus (BUNV), the prototype of the Bunyaviridae family, is encoded by the M segment in a polyprotein precursor, along with the virion glycoproteins, in the order Gn-NSm-Gc. As little is known of its function, we examined the intracellular localization, membrane integrality, and topology of NSm and its role in virus replication. We confirmed that NSm is an integral membrane protein and that it localizes in the Golgi complex, together with Gn and Gc. Coimmunoprecipitation assays and yeast two-hybrid analysis demonstrated that NSm was able to interact with other viral proteins. NSm is predicted to contain three hydrophobic (I, III, and V) and two nonhydrophobic (II and IV) domains. The N-terminal nonhydrophobic domain II was found in the lumen of an intracellular compartment. A novel BUNV assembly assay was developed to monitor the formation of infectious virus-like-particles (VLPs). Using this assay, we showed that deletions of either the complete NSm coding region or domains I, II, and V individually seriously compromised VLP production. Consistently, we were unable to rescue viable viruses by reverse genetics from cDNA constructs that contained the same deletions. However, we could generate mutant BUNV with deletions in NSm domains III and IV and also a recombinant virus with the green fluorescent protein open reading frame inserted into NSm domain IV. The mutant viruses displayed differences in their growth properties. Overall, our data showed that the N-terminal region of NSm, which includes domain I and part of domain II, is required for virus assembly and that the C-terminal hydrophobic domain V may function as an internal signal sequence for the Gc glycoprotein.

Interactions between virus proteins and host cell membranes during the viral life cycle

The structure and function of cells are critically dependent on membranes, which not only separate the interior of the cell from its environment but also define the internal compartments. It is therefore not surprising that the major steps of the life cycle of viruses of animals and plants also depend on cellular membranes. Indeed, interactions of viral proteins with host cell membranes are important for viruses to enter into host cells, replicate their genome, and produce progeny particles. To replicate its genome, a virus first needs to cross the plasma membrane. Some viruses can also modify intracellular membranes of host cells to create a compartment in which genome replication will take place. Finally, some viruses acquire an envelope, which is derived either from the plasma membrane or an internal membrane of the host cell. This paper reviews recent findings on the interactions of viral proteins with host cell membranes during the viral life cycle.

Interactions between rubella virus capsid and host protein p32 are important for virus replication

The distribution and morphology of mitochondria are dramatically affected during infection with rubella virus (RV). Expression of the capsid, in the absence of other viral proteins, was found to induce both perinuclear clustering of mitochondria and the formation of electron-dense intermitochondrial plaques, both hallmarks of RV-infected cells. We previously identified p32, a host cell mitochondrial matrix protein, as a capsid-binding protein. Here, we show that two clusters of arginine residues within capsid are required for stable binding to p32. Mutagenic ablation of the p32-binding site in capsid resulted in decreased mitochondrial clustering, indicating that interactions with this cellular protein are required for capsid-dependent reorganization of mitochondria. Recombinant viruses encoding arginine-to-alanine mutations in the p32-binding region of capsid exhibited altered plaque morphology and replicated to lower titers. Further analysis indicated that disruption of stable interactions between capsid and p32 was associated with decreased production of subgenomic RNA and, consequently, infected cells produced significantly lower amounts of viral structural proteins under these conditions. Together, these results suggest that capsid-p32 interactions are important for nonstructural functions of capsid that include regulation of virus RNA replication and reorganization of mitochondria during infection.

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

Background

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

Results

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

Conclusion

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

Expression and trafficking of fluorescent viral membrane proteins in baculovirus-transduced BHK cells

Baculovirus vectors show promise as a novel tool for gene delivery into mammalian cells and gene transfer with wild-type baculovirus has been demonstrated both in vitro and in vivo. To study expression and intracellular trafficking of foreign viral membrane proteins in baculovirus-transduced mammalian cells, the envelope proteins, E1 and E2, of rubella virus (RV) were chosen as a model. The enhanced green fluorescent protein (EGFP) and a red fluorescent protein (RFP) were fused to the C-terminus of E1 and E2, respectively. The proteins were cloned under a cytomegalovirus (CMV) promoter and expressed as fluorescent fusion proteins in baculovirus-transduced baby hamster kidney (BHK) cells. Expression of the chimeric proteins in these cells showed that E1 was retained within the ER and cis-Golgi when expressed alone. In contrast, E2 was efficiently transported to the trans-Golgi network (TGN). However, when expressed together, E1 co-localized with E2 in TGN and to some extent in the lysosomes. The recombinant baculovirus vectors were able to transduce the BHK cells efficiently and the fluorescent fusion constructs allowed easy detection of the trafficking events in the transduced mammalian cells. Consequently, this technique should have wide applications when intracellular analysis of protein synthesis and maturation is under study.

Phosphorylation of rubella virus capsid regulates its RNA binding activity and virus replication

Rubella virus is an enveloped positive-strand RNA virus of the family TOGAVIRIDAE: Virions are composed of three structural proteins: a capsid and two membrane-spanning glycoproteins, E2 and E1. During virus assembly, the capsid interacts with genomic RNA to form nucleocapsids. In the present study, we have investigated the role of capsid phosphorylation in virus replication. We have identified a single serine residue within the RNA binding region that is required for normal phosphorylation of this protein. The importance of capsid phosphorylation in virus replication was demonstrated by the fact that recombinant viruses encoding hypophosphorylated capsids replicated at much lower titers and were less cytopathic than wild-type virus. Nonphosphorylated mutant capsid proteins exhibited higher affinities for viral RNA than wild-type phosphorylated capsids. Capsid protein isolated from wild-type strain virions bound viral RNA more efficiently than cell-associated capsid. However, the RNA-binding activity of cell-associated capsids increased dramatically after treatment with phosphatase, suggesting that the capsid is dephosphorylated during virus assembly. In vitro assays indicate that the capsid may be a substrate for protein phosphatase 1A. As capsid is heavily phosphorylated under conditions where virus assembly does not occur, we propose that phosphorylation serves to negatively regulate binding of viral genomic RNA. This may delay the initiation of nucleocapsid assembly until sufficient amounts of virus glycoproteins accumulate at the budding site and/or prevent nonspecific binding to cellular RNA when levels of genomic RNA are low. It follows that at a late stage in replication, the capsid may undergo dephosphorylation before nucleocapsid assembly occurs.

The protein-tyrosine phosphatase CD45 reaches the cell surface via golgi-dependent and -independent pathways

CD45 is a receptor protein-tyrosine phosphatase essential for T cell development and lymphocyte activation. It is highly glycosylated, with multiple isoforms and glycoforms expressed on the cell surface depending on the cell type and stage of differentiation. Interestingly, we found two pools of newly synthesized CD45 expressed on plasma membrane, one of which arrived by 5 min after synthesis. The remaining pool of CD45 was fully glycosylated and began to arrive at the cell surface at approximately 15 min. The rapidly expressed population of CD45 possessed exclusively endoglycosidase H-sensitive N-linked carbohydrate. Additionally, this rapidly expressed pool of CD45 appeared on the cell surface in a brefeldin A (BFA)-insensitive manner, suggesting that it reached the cell surface independent of the Golgi complex. The remaining CD45 trafficked through the Golgi complex, and transport proceeded via a BFA-sensitive mechanism. These data suggest that CD45 is able to reach the cell surface via two distinct routes. The first is a conventional Golgi-dependent pathway that allows fully processed CD45 to be expressed. The second utilizes an ill defined mechanism that is independent of the Golgi, is BFA-resistant, and allows for the expression of CD45 with immature carbohydrate on the cell surface.

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

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

Cell-permeable ceramides preferentially inhibit coated vesicle formation and exocytosis in Chinese hamster ovary compared with Madin-Darby canine kidney cells by preventing the membrane association of ADP-ribosylation factor

Differential effects of acetyl(C2-) ceramide (N-acetylsphingosine) were studied on coated vesicle formation from Golgi-enriched membranes of Chinese hamster ovary (CHO) and Madin-Darby canine kidney (MDCK) cells. C2-ceramide blocked the translocation of ADP-ribosylation factor-1 (ARF-1) and protein kinase C-alpha (PKC-alpha) to the membranes from CHO cells, but not those of MDCK cells. Consequently, C2-ceramide blocked the stimulation of phospholipase D1 (PLD1) by the cytosol and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) in membranes from CHO cells. Basal specific activity of PLD1 and the concentration of ARF-1 were 3-4 times higher in Golgi-enriched membranes from MDCK cells compared with CHO cells. Moreover, PLD1 activity in MDCK cells was stimulated less by cytosol and GTP[S]. PLD2 was not detectable in the Golgi-enriched membranes. Incubation of intact CHO cells or their Golgi-enriched membranes with C2-ceramide also inhibited COP1 vesicle formation by membranes from CHO, but not MDCK, cells. Specificity was demonstrated, since dihydro-C2-ceramide had no significant effect on ARF-1 translocation, PLD1 activation or vesicle formation in membranes from both cell types. C2-ceramide also decreased the secretion of virus-like particles to a greater extent in CHO compared with MDCK cells, whereas dihydro-C2-ceramide had no significant effect. The results demonstrate a biological effect of C2-ceramide in CHO cells by decreasing ARF-1 and PKC-alpha binding to Golgi-enriched membranes, thereby preventing COP1 vesicle formation.

Rubella virus E2 signal peptide is required for perinuclear localization of capsid protein and virus assembly

The rubella virus (RV) structural proteins capsid, E2, and E1 are synthesized as a polyprotein precursor. The signal peptide that initiates translocation of E2 into the lumen of the endoplasmic reticulum remains attached to the carboxy terminus of the capsid protein after cleavage by signal peptidase. Among togaviruses, this feature is unique to RV. The E2 signal peptide has previously been shown to function as a membrane anchor for the capsid protein. In the present study, we demonstrate that this domain is required for RV glycoprotein-dependent localization of the capsid protein to the juxtanuclear region and subsequent virus assembly at the Golgi complex.

Rubella virus replication and links to teratogenicity

Rubella virus (RV) is the causative agent of the disease known more popularly as German measles. Rubella is predominantly a childhood disease and is endemic throughout the world. Natural infections of rubella occur only in humans and are generally mild. Complications of rubella infection, most commonly polyarthralgia in adult women, do exist; occasionally more serious sequelae occur. However, the primary public health concern of RV infection is its teratogenicity. RV infection of women during the first trimester of pregnancy can induce a spectrum of congenital defects in the newborn, known as congenital rubella syndrome (CRS). The development of vaccines and implementation of vaccination strategies have substantially reduced the incidence of disease and in turn of CRS in developed countries. The pathway whereby RV infection leads to teratogenesis has not been elucidated, but the cytopathology in infected fetal tissues suggests necrosis and/or apoptosis as well as inhibition of cell division of critical precursor cells involved in organogenesis. In cell culture, a number of unusual features of RV replication have been observed, including mitochondrial abnormalities, and disruption of the cytoskeleton; these manifestations are most probably linked and play some role in RV teratogenesis. Further understanding of the mechanism of RV teratogenesis will be brought about by the investigation of RV replication and virus-host interactions.

Rubella virus replication and links to teratogenicity

Rubella virus (RV) is the causative agent of the disease known more popularly as German measles. Rubella is predominantly a childhood disease and is endemic throughout the world. Natural infections of rubella occur only in humans and are generally mild. Complications of rubella infection, most commonly polyarthralgia in adult women, do exist; occasionally more serious sequelae occur. However, the primary public health concern of RV infection is its teratogenicity. RV infection of women during the first trimester of pregnancy can induce a spectrum of congenital defects in the newborn, known as congenital rubella syndrome (CRS). The development of vaccines and implementation of vaccination strategies have substantially reduced the incidence of disease and in turn of CRS in developed countries. The pathway whereby RV infection leads to teratogenesis has not been elucidated, but the cytopathology in infected fetal tissues suggests necrosis and/or apoptosis as well as inhibition of cell division of critical precursor cells involved in organogenesis. In cell culture, a number of unusual features of RV replication have been observed, including mitochondrial abnormalities, and disruption of the cytoskeleton; these manifestations are most probably linked and play some role in RV teratogenesis. Further understanding of the mechanism of RV teratogenesis will be brought about by the investigation of RV replication and virus-host interactions.

Rubella virus capsid protein induces apoptosis in transfected RK13 cells

Rubella virus is an enveloped positive-strand RNA virus that can cause mild to severe birth defects or death in an infected fetus. RV induction of programmed cell death, demonstrated in cell culture, has been implicated in the pathogenesis. The timing of apoptosis, 48 h p.i., suggested that accumulation of RV structural proteins might induce cell death in infected cells. Expression of RV structural proteins, capsid, envelope glycoproteins E1 and E2, in transiently transfected RK13 cells was as potent an inducer of cell death as RV infection. Immunofluorescence microscopy revealed that RV structural protein transfected cells exhibited the condensed nuclei typical of apoptotic cell death. Transfection with the capsid protein construct, but not E2 and E1, resulted in as much cell death as joint expression of all three RV structural proteins. Capsid required a membrane-anchoring domain to induce cell death, but a heterologous polypeptide fused to the capsid membrane anchor did not cause apoptosis. Deletion mutants demonstrated that the apoptosis-inducing activity resides in the N-terminal 170 amino acids of capsid. Though apoptosis-inducing capsid constructs appear to have an ER sub-cellular localization, disruption of the ER calcium storage capacity does not correlate with cell death. Mechanisms consistent with these results are discussed.

Neutralizing monoclonal antibody to the E1 glycoprotein epitope of rubella virus mediates virus arrest in VERO cells

The best-known mechanism of action of antibody-mediated virus neutralization is to impede the entrance of viruses to host cells, as determined by neutralization assays. Antibodies may also inhibit the exit of rubella virus (RV) from infected host cells; in this case, the interaction of the antibodies with their domains must occur on the plasma membrane, because antibodies cannot enter the cells. In the present study, we were able to block temporally the exit of virions from RV-infected cells by the binding of monoclonal antibody (mAb) H3 to their surface. The objective was accomplished in three steps: first, we determined the duration of the viral replication cycle; then we established the kinetics of the presence of the domains defined by our mAbs in the cytoplasm of RV-infected VERO cells; and, finally, we assessed the release of viral particles to the supernatant of infected VERO cells in the presence or absence of mAbs or positive and negative mice sera. RV-specific mice sera and mAb H3, which binds to the amino acid sequence 208-239 of the RV-E1 glycoprotein, were able to delay for 24 hours the release of virions from infected cultures, suggesting that the reaction of mAb H3 with its epitope may arrest any change necessary for the assembly and/or release of virions. In conclusion, the neutralizing domain recognized by mAb induces antibodies that can block the viral replication by several mechanisms of action, such as the obstruction of virus entry into cells and the delay of viral release. All of these mechanisms are intimately involved in the critical virus-host cell interactions that allow self-limitation of the infection.

Rubella virus capsid associates with host cell protein p32 and localizes to mitochondria

Togavirus nucleocapsids have a characteristic icosahedral structure and are composed of multiple copies of a capsid protein complexed with genomic RNA. The assembly of rubella virus nucleocapsids is unique among togaviruses in that the process occurs late in virus assembly and in association with intracellular membranes. The goal of this study was to identify host cell proteins which may be involved in regulating rubella virus nucleocapsid assembly through their interactions with the capsid protein. Capsid was used as bait to screen a CV1 cDNA library using the yeast two-hybrid system. One protein that interacted strongly with capsid was p32, a cellular protein which is known to interact with other viral proteins. The interaction between capsid and p32 was confirmed using a number of different in vitro and in vivo methods, and the site of interaction between these two proteins was shown to be at the mitochondria. Interestingly, overexpression of the rubella virus structural proteins resulted in clustering of the mitochondria in the perinuclear region. The p32-binding site in capsid is a potentially phosphorylated region that overlaps the viral RNA-binding domain of capsid. Our results are consistent with the possibility that the interaction of p32 with capsid plays a role in the regulation of nucleocapsid assembly and/or virus-host interactions.

Role of rubella virus glycoprotein domains in assembly of virus-like particles

Rubella virus is a small enveloped positive-strand RNA virus that assembles on intracellular membranes in a variety of cell types. The virus structural proteins contain all of the information necessary to mediate the assembly of virus-like particles in the Golgi complex. We have recently identified intracellular retention signals within the two viral envelope glycoproteins. E2 contains a Golgi retention signal in its transmembrane domain, whereas a signal for retention in the endoplasmic reticulum has been localized to the transmembrane and cytoplasmic domains of E1 (T. C. Hobman, L. Woodward, and M. G. Farquhar, Mol. Biol. Cell 6:7-20, 1995; T. C. Hobman, H. F. Lemon, and K. Jewell, J. Virol. 71:7670-7680, 1997). In the present study, we have analyzed the role of these retention signals in the assembly of rubella virus-like particles. Deletion or replacement of these domains with analogous regions from other type I membrane glycoproteins resulted in failure of rubella virus-like particles to be secreted from transfected cells. The E1 transmembrane and cytoplasmic domains were not required for targeting of the structural proteins to the Golgi complex and, surprisingly, assembly and budding of virus particles into the lumen of this organelle; however, the resultant particles were not secreted. In contrast, replacement or alteration of the E2 transmembrane or cytoplasmic domain, respectively, abrogated the targeting of the structural proteins to the budding site, and consequently, no virion formation was observed. These results indicate that the transmembrane and cytoplasmic domains of E2 and E1 are required for early and late steps respectively in the viral assembly pathway and that rubella virus morphogenesis is very different from that of the structurally similar alphaviruses.

Effects of mutations in the rubella virus E1 glycoprotein on E1-E2 interaction and membrane fusion activity

Rubella virus (RV) virions contain two glycosylated membrane proteins, E1 and E2, that exist as a heterodimer and form the viral spike complexes on the virion surface. Formation of an E1-E2 heterodimer is required for transport of E1 out of the endoplasmic reticulum lumen to the Golgi apparatus and plasma membrane. To investigate the nature of the E1-E2 interaction, we have introduced mutations in the internal hydrophobic region (residues 81 to 109) of E1. Substitution of serine at Cys82 (mutant C82S) or deletion of this hydrophobic domain (mutant dt) of E1 resulted in a disruption of the E1 conformation that ultimately affected E1-E2 heterodimer formation and cell surface expression of both E1 and E2. Substitution of either aspartic acid at Gly93 (G93D) or glycine at Pro104 (P104G) was found to impair neither E1-E2 heterodimer formation nor the transport of E1 and E2 to the cell surface. Fusion of RV-infected cells is induced by a brief treatment at a pH below 6. 0. To test whether this internal hydrophobic domain is involved in the membrane fusion activity of RV, transformed BHK cell lines expressing either wild-type or mutant spike proteins were exposed to an acidic pH and polykaryon formation was measured. No fusion activity was observed in the C82S, dt, and G93D mutants; however, the wild type and the P104G mutant exhibited fusogenic activities, with greater than 60% and 20 to 40% of the cells being fused, respectively, at pH 4.8. These results suggest that it is likely that the region of E1 between amino acids 81 and 109 is involved in the membrane fusion activity of RV and that it may be important for the interaction of that protein with E2 to form the E1-E2 heterodimer.

Characterization of an endoplasmic reticulum retention signal in the rubella virus E1 glycoprotein

Rubella virus contains three structural proteins, capsid, E2, and E1. E2 and E1 are type I membrane glycoproteins that form a heterodimer in the endoplasmic reticulum (ER) before they are transported to and retained in the Golgi complex, where virus assembly occurs. The bulk of unassembled E2 and E1 subunits are not transported to the Golgi complex. We have recently shown that E2 contains a Golgi-targeting signal that mediates retention of the E2-E1 complex (T. C. Hobman, L. Woodward, and M. G. Farquhar, Mol. Biol. Cell 6:7-20, 1995). The focus of this study was to determine if E1 glycoprotein also contains intracellular targeting information. We constructed a series of chimeric reporter proteins by fusing domains from E1 to the ectodomains of two other type I membrane proteins which are normally transported to the cell surface, vesicular stomatitis virus G protein (G) and CD8. Fusion of the E1 transmembrane and cytoplasmic regions, but not analogous domains from two control membrane proteins, to the ectodomains of G and CD8 proteins caused the resulting chimeras to be retained in the ER. Association of the ER-retained chimeras with known ER chaperone proteins was not detected. ER localization required both the transmembrane and cytoplasmic regions of E1, since neither of these domains alone was sufficient to retain the reporter proteins. Increasing the length of the E1 cytoplasmic domain by 10 amino acids completely abrogated ER retention. This finding also indicated that the chimeras were not retained as a result of misfolding. In summary, we have identified a new type of ER retention signal that may function to prevent unassembled E1 subunits and/or immature E2-E1 dimers from reaching the Golgi complex, where they could interfere with viral assembly. Accordingly, assembly of E2 and E1 would mask the signal, thereby allowing transport of the heterodimer from the ER.

Characterization of Borna disease virus p56 protein, a surface glycoprotein involved in virus entry

Borna disease virus (BDV) is a nonsegmented negative-stranded (NNS) RNA virus, prototype of a new taxon in the Mononegavirales order. BDV causes neurologic disease manifested by behavioral abnormalities in several animal species, and evidence suggests that it may be a human pathogen. To improve our knowledge about the biology of this novel virus, we have identified and characterized the product of BDV open reading frame IV (BVp56). Based on sequence features, BVp56 encodes a virus surface glycoprotein. Glycoproteins play essential roles in the biology of NNS RNA viruses. Expression of BVp56 resulted in the generation of two polypeptides with molecular masses of about 84 and 43 kDa (GP-84 and GP-43). GP-84 and GP-43 likely correspond to the full-length BVp56 gene and to its C terminus, respectively. Endoglycosidase studies demonstrated that both products were glycosylated and that this process was required for the stabilization of newly synthesized products. Moreover, our results suggested that GP-43 is generated by cleavage of GP-84 by a cellular protease. Subcellular localization studies demonstrated that GP-84 accumulates in the ER, whereas GP-43 reaches the cell surface. Both BVp56 products were found to be associated with infectious virions, and antibodies to BVp56 had neutralizing activity. Our findings suggest that BVp56 exhibits a novel form of processing for an animal NNS RNA virus surface glycoprotein, which might influence the assembly and budding of BDV.

Role of glycoprotein PE2 in formation and maturation of the Sindbis virus spike

Sindbis virus envelope assembly is a multistep process resulting in the maturation of a rigid, highly ordered T=4 icosahedral protein lattice containing 80 spikes composed of trimers of E1-E2 heterodimers. Intramolecular disulfide bonds within E1 stabilize E1-E1 associations required for envelope formation and maintenance of the envelope's structural integrity. The structural integrity of the envelope protein lattice is resistant to reduction by dithiothreitol (DTT), indicating that E1 disulfides which stabilize structural domains become inaccessible to DTT at some point during virus maturation. The development of E1 resistance to DTT occurs prior to the completion of E1 folding and is temporally correlated with spike assembly in the endoplasmic reticulum. From these data we have predicted that in the final stages of spike assembly, E1 intramolecular disulfides, which stabilize the structural integrity of the envelope protein lattice, are buried within the spike and become inaccessible to the reductive activity of DTT. The spike is formed prior to the completion of E1 folding, and we have suggested that PE2 (the precursor to E2) may play a critical role in E1 folding after PE2-E1 oligomer formation has occurred. In this study we have investigated the role of PE2 in E1 folding, oligomer formation, and development of E1 resistance to both protease digestion and reduction by DTT by using a Sindbis virus replicon (SINrep/E1) which allows for the expression of E1 in the presence of truncated PE2. Through pulse-chase analysis of both Sindbis virus- and SINrep/E1-infected cells, we have determined that the folding of E1 into a trypsin-resistant conformation and into its most compact and stable form is not dependent upon association of E1 with PE2. However, E1 association with PE2 is required for oligomer formation, the export of E1 from the endoplasmic reticulum, and E1 acquisition of resistance to DTT.

Involvement of the endoplasmic reticulum in the assembly and envelopment of African swine fever virus

African swine fever (ASF) virus is a large enveloped DNA virus assembled in the cytoplasm of cells. In this study, the membrane compartments involved in the envelopment of ASF virus were investigated. A monoclonal antibody recognizing p73, the major structural protein of ASF virus, was generated to analyze the binding of p73 to membranes during the assembly of the virus. Approximately 50% of the intracellular pool of p73 associated with membranes as a peripheral membrane protein. Binding was rapid and complete within 15 min of synthesis. Subcellular membrane fractionation showed that newly synthesized p73 molecules cosedimented with endoplasmic reticulum (ER) membranes and remained associated with the ER during a 2-h chase. A similar distribution on gradients was recorded for p17, a structural membrane protein of ASF virus. The results suggested that the ER was involved in the assembly of ASF virus. A protease protection assay demonstrated a time-dependent envelopment of the membrane bound, but not cytosolic, pool of p73. Envelopment of p73 took place 1 h after binding to membranes and was completed 1 h before the first detection of p73 in virions secreted from cells. Envelopment was unaffected by brefeldin A and monensin, drugs that block membrane transport between the ER and Golgi. Taken together the results provide evidence for the binding of ASF virus structural proteins to a specific membrane compartment and implicate a role for the ER in the assembly and envelopment of ASF virus.

Synthesis of soluble rubella virus spike proteins in two lepidopteran insect cell lines: large scale production of the E1 protein

The two envelope glycoproteins of rubella virus (RV), E1 of 58 kDa and E2 of 42-47 kDa, were individually expressed in lepidopteran Spodoptera frugiperda as well as in Trichoplusia ni insect cells using baculovirus vectors. The authentic signal sequences of E1 and E2 were replaced with the honeybee melittin signal sequence, allowing efficient entrance into the secretory pathway of the insect cell. In addition, the hydrophobic transmembrane anchors at the carboxyl termini of E1 and E2 proteins were removed to enable secretion rather than maintenance in the cellular membranes. Synthesis of the recombinant proteins in the absence and presence of tunicamycin revealed that both E1 and E2 were glycosylated with apparent molecular weights of 52 kDa and 37 kDa, respectively. Recombinant E2 appeared to be partially secreted, whereas E1 was essentially found inside the infected insect cell. The E1 protein was produced in large scale using a 10-1 bioreactor and serum-free medium (SFM). Purification of the recombinant protein product was performed from cytoplasmic extracts by ammonium sulphate precipitation followed by Concanavalin A affinity chromatography. This type of purified recombinant viral glycoproteins may be useful not only in diagnostic medicine or for immunization, but should enable studies designed to solve the structure of the virus particle.

Cell surface expression of a functional rubella virus E1 glycoprotein by addition of a GPI anchor

Rubella virus (RV) envelope glycoproteins E1 and E2 are targeted to the Golgi as heterodimers. While E2 contains a transmembrane Golgi retention signal, E1 is arrested in a pre-Golgi compartment in the absence of E2, and appears to require heterodimerization in order to reach the Golgi. Various forms of E1 with deletions in the ectodomain or lacking the cytoplasmic (CT) and transmembrane (TM) domains, as well as the 29 C-terminal amino acid residues of the ectodomain were also retained intracellularly. We therefore investigated the possibility of targetting E1 to the plasma membrane by addition of a glycosylphosphatidylinositol (GPI) anchor. We found that E1GPI was transported to the cell surface where it retained the hemadsorption activity characteristic of the wild-type E1/E2 heterodimer. Furthermore, coexpression of a mammalian GPI-specific phospholipase D (GPI-PLD) resulted in the release of E1GPI and in constitutive expression of a soluble form of E1. This study thus demonstrates that the GPI anchor has a dominant effect over the E1 pre-Golgi retention signal and that E1 is sufficient for hemadsorption.

The paramyxovirus simian virus 5 hemagglutinin-neuraminidase glycoprotein, but not the fusion glycoprotein, is internalized via coated pits and enters the endocytic pathway

The hemagglutinin-neuraminidase (HN) and fusion (F) glycoproteins of the paramyxovirus simian virus 5 (SV5) are expressed on the surface of virus-infected cells. Although the F protein was found to be expressed stably, the HN protein was internalized from the plasma membrane. HN protein lacks known internalization signals in its cytoplasmic domain that are common to many integral membrane proteins that are internalized via clathrin-coated pits. Thus, the cellular pathway of HN protein internalization was examined. Biochemical analysis indicated that HN was lost from the cell surface with a t1/2 of approximately 45-50 min and turned over with a t1/2 of approximately 2 h. Immunofluorescent analysis showed internalized SV5 HN in vesicle-like structures in a juxtanuclear pattern coincident with the localization of ovalbumin. In contrast the SV5 F glycoprotein and the HN glycoprotein of the highly related parainfluenza virus 3 (hPIV-3) were found only on the cell surface. Immunogold staining of HN on the surface of SV5-infected CV-1 cells and examination using electron microscopy, showed heavy surface labeling that gradually decreased with time. Concomitantly, gold particles were detected in the endosomal system and with increasing time, gold-labeled structures having the morphology of lysosomes were observed. On the plasma membrane approximately 5% of the gold-labeled HN was found in coated pits. The inhibition of the pinching-off of coated pits from the plasma membrane by cytosol acidification significantly reduced HN internalization. Internalized HN was co-localized with gold-conjugated transferrin, a marker for the early endosomal compartments, and with gold-conjugated bovine serum albumin, a marker for late endosomal compartments. Taken together, these data strongly suggest that the HN glycoprotein is internalized via clathrin-coated pits and delivered to the endocytic pathway.

Envelopment of varicella-zoster virus: targeting of viral glycoproteins to the trans-Golgi network

Previous studies suggested that varicella-zoster virus derives its final envelope from the trans-Golgi network (TGN) and that envelope glycoproteins (gps) are transported to the TGN independently of nucleocapsids. We tested the hypothesis that gpI is targeted to the TGN as a result of a signal sequence or patch encoded in its cytosolic domain. cDNAs encoding gpI wild type (wt) and a truncated mutant gpI(trc) lacking transmembrane and cytosolic domains were cloned by using the PCR. Cells transfected with cDNA encoding gpI(wt) or gpI(trc) synthesized and N glycosylated the proteins. gpI(wt) accumulated in the TGN, some reached the plasmalemma, but none was secreted. In contrast, gpI(trc) was retained and probably degraded in the endoplasmic reticulum; none was found on cell surfaces, but some was secreted. The distribution of gpI(trc) was not affected by deletion of potential glycosylation sites. To locate a potential gpI-targeting sequence, cells were transfected with cDNA encoding chimeric proteins in which the ectodomain of a plasmalemmal marker, the interleukin-2 receptor (tac), was fused to different domains of gpI. A chimeric protein in which tac was fused with the transmembrane and cytoplasmic domains of gpI was targeted to the TGN. In contrast, a chimeric protein in which tac was fused only with the gpI transmembrane domain passed through the TGN and concentrated in endosomes. We conclude that gpI is targeted to the TGN as a result of a targeting sequence or patch in its cytosolic domain.

Sequential coupling between COPII and COPI vesicle coats in endoplasmic reticulum to Golgi transport

COPI and COPII are vesicle coat complexes whose assembly is regulated by the ARF1 and Sar1 GTPases, respectively. We show that COPI and COPII coat complexes are recruited separately and independently to ER (COPII), pre-Golgi (COPI, COPII), and Golgi (COPI) membranes of mammalian cells. To address their individual roles in ER to Golgi transport, we used stage specific in vitro transport assays to synchronize movement of cargo to and from pre-Golgi intermediates, and GDP- and GTP-restricted forms of Sar1 and ARF1 proteins to control coat recruitment. We find that COPII is solely responsible for export from the ER, is lost rapidly following vesicle budding and mediates a vesicular step required for the build-up of pre-Golgi intermediates composed of clusters of vesicles and small tubular elements. COPI is recruited onto pre-Golgi intermediates where it initiates segregation of the anterograde transported protein vesicular stomatitis virus glycoprotein (VSV-G) from the retrograde transported protein p58, a protein which actively recycles between the ER and pre-Golgi intermediates. We propose that sequential coupling between COPII and COPI coats is essential to coordinate and direct bi-directional vesicular traffic between the ER and pre-Golgi intermediates involved in transport of protein to the Golgi complex.

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

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

Targeting of a heterodimeric membrane protein complex to the Golgi: rubella virus E2 glycoprotein contains a transmembrane Golgi retention signal

Rubella virus (RV) envelope glycoproteins, E2 and E1, form a heterodimeric complex that is targeted to medial/trans-Golgi cisternae. To identify the Golgi targeting signal(s) for the E2/E1 spike complex, we constructed chimeric proteins consisting of domains from RV glycoproteins and vesicular stomatitis virus (VSV) G protein. The location of the chimeric proteins in stably transfected Chinese hamster ovary cells was determined by immunofluorescence, immunoelectron microscopy, and by the extent of processing of their N-linked glycans. A trans-dominant Golgi retention signal was identified within the C-terminal region of E2. When the transmembrane (TM) and cytoplasmic (CT) domains of VSV G were replaced with those of RV E2, the hybrid protein (G-E2TMCT+) was retained in the Golgi. Transport of G-E2TMCT+ to the Golgi was rapid (t1/2 = 10-20 min). The G-E2TMCT+ protein was determined to be distal to or within the medial Golgi based on acquisition of endo H resistance but proximal to the trans-Golgi network since it lacked sialic acid. Deletion analysis revealed that only the TM domain of E2 was required for Golgi targeting. Although the cytoplasmic domain of E2 was not necessary for Golgi retention, it was required for efficient transport of VSV G-RV chimeras out of the endoplasmic reticulum. When assayed in sucrose velocity sedimentations gradients, the Golgi-retained G-E2TMCT+ protein behaved as a dimer. Unlike virtually all other Golgi targeting signals, the E2 TM domain does not contain any polar amino acids. The TM and CT domains of E1 were not required for targeting of E2 and E1 to the Golgi indicating that a heterodimer of two integral membrane proteins can be retained in the Golgi by a single retention signal.

Targeting of proteins to the Golgi apparatus

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

Retention and retrieval in the endoplasmic reticulum and the Golgi apparatus

Resident proteins of the exocytic pathway contain at least two types of information in their primary sequence for determining their subcellular location. The first type of information is found at the carboxyl terminus of soluble proteins of the endoplasmic reticulum (ER) and in the cytoplasmic domain of some ER and Golgi membrane proteins. It acts as a retrieval signal, returning proteins that have left the compartment in which they reside. The second type of information has been found in the membrane-spanning domain of several ER and Golgi proteins and, though the mechanism by which it operates is still unclear, it acts as a retention signal, keeping the protein at a particular location within the organelle. The presence of both a retrieval signal and a retention signal in a trans-Golgi network resident protein suggests that more than one mechanism operates to ensure correct localization of resident proteins along the exocytic pathway.

Expression of soluble forms of rubella virus glycoproteins in mammalian cells

Rubella virus (RV) virions contain two envelope glycoproteins, E1 and E2. Removal of hydrophobic regions in their carboxyl termini by genetic engineering caused them to be secreted rather than maintained in cell membranes of transfected COS cells. Truncated E2 was secreted in the absence of E1, whereas E1 lacking its transmembrane domain required coexpression of E2 for export from the cell. Secreted E2 was found to contain both O-linked and N-linked complex glycans, whereas secreted E1 retained virus neutralization and hemagglutination epitopes, suggesting the possibility of using soluble RV antigens as subunit vaccines and for serodiagnostic purposes. Stable Chinese hamster ovary cell lines secreting RV E1 were constructed for large scale preparation of recombinant E1.

Formation and rearrangement of disulfide bonds during maturation of the Sindbis virus E1 glycoprotein

The rigidly ordered icosahedral lattice of the Sindbis virus envelope is composed of a host-derived membrane bilayer in which the viral glycoproteins E1 and E2 reside. E1-E1 interactions stabilized by intramolecular disulfide bridges play a significant role in maintaining the envelope's structural integrity (R. P. Anthony and D. T. Brown, J. Virol. 65:1187-1194, 1991; R. P. Anthony, A. M. Paredes, and D. T. Brown, Virology 190:330-336, 1992). We have examined the acquisition of disulfide bridges within E1 during its maturation. Prior to exit from the endoplasmic reticulum, E1 folds via at least three intermediates, differing in the number and/or arrangement of their disulfides, into a single, compact form. This E1 species remains stable with respect to its disulfides until late in the secretory pathway, when E1 attains a metastable conformation. At this point, when appropriately triggered, intramolecular thiol-disulfide exchange reactions within E1 can occur, resulting in the generation of alternative E1 species. This metastable nature of mature E1 may have important implications for the mechanism of virus disassembly during the initial stages of the infection process (B. Abell and D. T. Brown, J. Virol. 67:5496-5501, 1993).

Molecular biology of rubella virus

This chapter summarizes the present medical significance of rubella virus. Rubella virus infection is systemic in nature and the accompanying symptoms are generally benign, the most pronounced being a mild rash of short duration. The most common complication of rubella virus infection is transient joint involvement such as polyarthralgia and arthritis. The primary health impact of rubella virus is that it is a teratogenic agent. The vaccination strategy is aimed at elimination of rubella and includes both universal vaccination of infants at 15 months of age with the trivalent measles, mumps, rubella (MMR) vaccine and specific targeting with the rubella vaccine of seronegative women planning pregnancy and seronegative adults who could come in contact with women of childbearing age, although it is recommended that any individual over the age of 12 months without evidence of natural infection or vaccination be vaccinated. Medically, the current challenge posed by rubella virus is to achieve complete vaccination coverage to prevent resurgences.

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.