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

Enhancement of infectivity of insect cell-derived La Crosse Virus by human serum

Given the dual life cycle of arboviruses in insect and animal hosts and the importance of serum factors as a first line antiviral defense, we have examined the outcome of interactions between the arbovirus La Crosse Virus (LACV) and human serum. To mimic the life cycle between species, we used LACV derived from insect (I-LACV) and human keratinocyte (HaCaT) cells. Incubation of I-LACV with normal human serum did not result in neutralization, but instead stabilized I-LACV virions and enhanced the amount of infectious virus. Enhanced infectivity was also seen with heat-inactivated serum devoid of complement activity and with serum from a range of animals including mouse, ferret, and non-human primates. Depletion of antibodies from serum resulted in loss of enhancement of infectivity and sucrose gradient sedimentation assays showed IgG co-sedimenting with I-LACV particles. In agreement with our results with I-LACV, HaCaT-derived LACV was not neutralized by complement or antibodies in normal human serum. However, in contrast to I-LACV, HaCaT-derived LACV infectivity was stable when incubated alone and treatment with serum did not enhance infectivity. Our results indicate that LACV derived from insect cells differs substantially from virus derived from human cells, with I-LACV being dependent on serum factors to enhance infectivity. These findings suggest that understanding differential composition of insect versus animal cell-derived LACV may form the foundation for potential new antiviral approaches.

Throw out the Map: Neuropathogenesis of the Globally Expanding California Serogroup of Orthobunyaviruses

The California serogroup (CSG) comprises 18 serologically and genetically related mosquito-borne orthobunyaviruses. Of these viruses, at least seven have been shown to cause neurological disease in humans, including the leading cause of pediatric arboviral encephalitis in the USA, La Crosse virus. Despite the disease burden from these viruses, much is still unknown about the CSG viruses. This review summarizes our current knowledge of the CSG viruses, including human disease and the mechanisms of neuropathogenesis.

Entry of severe fever with thrombocytopenia syndrome virus

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a globe-shaped virus covered by a dense icosahedral array of glycoproteins Gn/Gc that mediate the attachment of the virus to host cells and the fusion of viral and cellular membranes. Several membrane factors are involved in virus entry, including C-type lectins and nonmuscle myosin heavy chain IIA. The post-fusion crystal structure of the Gc protein suggests that it is a class II membrane fusion protein, similar to the E/E1 protein of flaviviruses and alphaviruses. The virus particles are internalized into host cell endosomes through the clathrin-dependent pathway, where the low pH activates the fusion of the virus with the cell membrane. With information from studies on other bunyaviruses, herein we will review our knowledge of the entry process of SFTSV.

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

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

Orthobunyaviruses: recent genetic and structural insights

Orthobunyaviruses, which have small, tripartite, negative-sense RNA genomes and structurally simple virions composed of just four proteins, can have devastating effects on human health and well-being, either by causing disease in humans or by causing disease in livestock and crops. In this Review, I describe the recent genetic and structural advances that have revealed important insights into the composition of orthobunyavirus virions, viral transcription and replication and viral interactions with the host innate immune response. Lastly, I highlight outstanding questions and areas of future research.

Analysis of glycoproteins of viruses in the family Bunyaviridae

The membrane glycoproteins (Gn and Gc) of viruses in the family Bunyaviridae form projections on the virion envelope and are involved in virus entry and eliciting protective immunity. The glycoproteins are modified by N-linked glycosylation and accumulate in the Golgi complex where virions mature and bud. In this chapter, we describe the methods that have been used in our laboratory for the study of the glycoproteins of Bunyamwera virus, the prototype of the family. The protocols cover the expression of viral glycoproteins, examination of intracellular localization by immnunofluorescent confocal microscopy, radiolabeling, immunoprecipitation, and SDS-PAGE analysis of the proteins, and the improved reverse genetic system to rescue recombinant viruses that contain mutations at N-linked glycosylation sites.

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

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

[Rift Valley fever virus]

Rift Valley fever virus (RVFV) causes massive mosquito-borne epidemics among humans and decimates ruminants in which the mortality rate is about 1% and 10-30%, respectively. Morbidity in RVFV-infected humans is high largely due to the effects of hemorrhagic fever and encephalitis. This virus is native to sub-Saharan Africa; yet if this virus is introduced into the environment, virus transmission appears to occur whenever sheep and cattle are present with abundant mosquito populations. RVFV is a negative-strand RNA virus which belongs to the family Bunyaviridae, genus Phlebovirus, and contains tripartite-segmented genomes (S, M, and L). S-segment is the ambisense genome, where N and NSs genes are coded in an antiviral-sense and viral sense S-segment, respectively. The inhibition of host mRNA synthesis, which is induced by the binding of NSs protein to RNA polymerase II transcription factor TFIIH, is the primary reason for the host-protein shut-off in RVFV-infected cells. Development of a RVFV reverse genetics system, which has not been accomplished yet, is important for the study of viral replication mechanisms, host virus interaction, viral pathogenicity as well as vaccine evaluation and development.

Ultrastructural, antigenic and physicochemical characterization of the Mojuí dos Campos (Bunyavirus) isolated from bat in the Brazilian Amazon region

The Mojuí dos Campos virus (MDCV) was isolated from the blood of an unidentified bat (Chiroptera) captured in Mojuí dos Campos, Santarém, State of Pará, Brazil, in 1975 and considerated to be antigenically different from other 102 arboviruses belonging to several antigenic groups isolated in the Amazon region or another region by complement fixation tests. The objective of this work was to develop a morphologic, an antigenic and physicochemical characterization of this virus. MDCV produces cytopathic effect in Vero cells, 24 h post-infection (p.i), and the degree of cellular destruction increases after a few hours. Negative staining electron microscopy of the supernatant of Vero cell cultures showed the presence of coated viral particles with a diameter of around 98 nm. Ultrathin sections of Vero cells, and brain and liver of newborn mice infected with MDCV showed an assembly of the viral particles into the Golgi vesicles. The synthesis kinetics of the proteins for MDCV were similar to that observed for other bunyaviruses, and viral proteins could be detected as early as 6 h p.i. Our results reinforce the original studies which had classified MDCV in the family Bunyaviridae, genus Bunyavirus as an ungrouped virus, and it may represent the prototype of a new serogroup.

Viral acylproteins: greasing the wheels of assembly

Viruses take advantage of the host's protein modification and targeting pathways to modify their own proteins and to ensure that they assume active configurations and locate appropriately for assembly. In many viruses, one recurrent theme in such processes is exploitation of cellular protein acylation pathways for the addition of myristic and palmitic acid to capsid or envelope proteins.

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

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

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

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

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

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

Assembly and polarized release of Punta Toro virus and effects of brefeldin A

Punta Toro virus (PTV), a member of the sandfly fever group of bunyaviruses, is assembled by budding at intracellular membranes of the Golgi complex. We have examined PTV glycoprotein transport, assembly, and release and the effects of brefeldin A (BFA) on these processes. Both the G1 and G2 proteins were transported out of the endoplasmic reticulum (ER) and retained in the Golgi complex in a stable structure, either during PTV infection or when expressed from a vaccinia virus recombinant. BFA treatment causes a rapid and dramatic change in the distribution of the G1 and G2 proteins, from a Golgi pattern to an ER pattern. The G1 and G2 proteins were found to be modified by medial but not trans Golgi network enzymes, in the presence or absence of BFA. We found that BFA blocks PTV release from cells but does not interfere with the intracellular assembly of infectious virions. Further, the BFA block of virus release is fully reversible, with high levels of virus release occurring upon removal of the inhibitor. It was also found that the release of PTV virions is polarized, occurring exclusively from the basolateral surfaces of the polarized Vero C1008 epithelial cell line.

Characterization of baculovirus-expressed Rift Valley fever virus glycoproteins synthesized in insect cells

A cDNA corresponding to the complete coding region of the M RNA of the M12 mutant of Rift Valley fever virus (RVFV) strain ZH548 (K. Takehara, M-K. Min, J.K. Battles, K. Sugiyama, V.C. Emery, J.M. Dalrymple, and D.H.L. Bishop, Virology, 169, 452-457, 1989) has been inserted into the baculovirus transfer vector pAcYM1. By comparison with the parent RVFV, the M RNA of the M12 mutant has a new small open reading frame (ORF1) upstream of the one that initiates the precursor of the viral glycoproteins (ORF2, gene order: NS(M)-G2-G1). A derivative of the M12 cDNA was prepared from which most of the upstream sequences (including a polyT tract and ORF1) were removed. Other cDNA constructs were made from this derivative, constructs in which most of the G1 sequences were also removed, or most of the NS(M) coding sequences, or all of the NS(M) and most of G2 coding sequences. Each RVFV M cDNA construct was inserted into a pAcYM1 transfer vector and recombinant baculoviruses were produced (RVM1-5). The derived viruses were employed to study the expression and properties of the RVFV glycoproteins in Spodoptera frugiperda insect cells. For each recombinant virus evidence was obtained which indicated that the RVFV glycoproteins were produced and processed in the insect cells. Although four of the recombinants gave low expression levels of the RVFV glycoproteins, for the vector that made only the G1 product, the expression level was significantly higher. Immunofluorescence analyses established that the RVFV glycoproteins were present both at intracellular locations and on the surface of the recombinant baculovirus infected insect cells.

The effect of castanospermine on the synthesis of synaptic glycoproteins by rat brain slices

Slices were prepared from rat forebrains and the incorporation of [3H]mannose and [35S]methionine into proteins and glycoproteins determined. The incorporation of methionine continued to increase for up to 8 hours whereas mannose incorporation was maximal between 2 and 4 hours and declined thereafter. Glycopeptides prepared by pronase digestion of [3H]mannose-labeled glycoproteins were digested with endoglucosaminidase H (endo H) and analysed by gel filtration. The major endo H-sensitive oligosaccharide eluted in a position similar to standard Man8GlcNAc. In the presence of castanospermine, which inhibits glucosidase I, the first enzymatic step in the processing of N-linked oligosaccharides, a new endo H-sensitive glycan similar in size to standard Glc3Man9GlcNAc2 accumulated. Synaptic membranes (SMs) were isolated from slices which had been incubated with either [3H]mannose or [35S]methionine in the presence and absence of castanospermine. In the presence of inhibitor the relative incorporation of [3H]mannose into high-mannose glycans of synaptic glycoproteins was increased. The incorporation of newly synthesized, [35S] methionine-labeled, Con A-binding glycoproteins into SMs was not affected by the addition of inhibitor. Many of the glycoproteins synthesized in the presence of castanospermine exhibited a decreased electrophoretic mobility indicative of the presence of altered oligosaccharide chains. The results indicate that changes in oligosaccharide composition produced by castanospermine had little effect on the subsequent transport and incorporation of glycoproteins into synaptic membranes.

Maturation of Japanese encephalitis virus glycoproteins produced by infected mammalian and mosquito cells

The Japanese encephalitis virus (JE) structural glycoprotein (E) and two nonstructural glycoproteins (NS1 and NS1') were processed differently by JE-infected vertebrate and invertebrate cell lines. All three proteins were released slowly (t1/2 greater than 6 hr) from JE-infected monkey cells (Vero cells). Mosquito cell lines released E at a similar rate (t1/2 greater than 8 hr), while NS1 and NS1' were retained in an undegraded form in the cell layer. The proteolytic processing of the three proteins appeared identical in both cell types, but some differences in N-linked glycosylation were observed. E, NS1, and NS1' found within the infected cells of both types contained high-mannose oligosaccharide groups for more than 8 hr after synthesis. Additional sugar residues were added to the single E protein oligosaccharide group prior to release from Vero cells, while sugar residues were trimmed from the E protein oligosaccharide group prior to release from mosquito cells. The forms of NS1 and NS1' found in the culture fluid of infected Vero cells contained one complex and one high-mannose oligosaccharide. All three glycoproteins released from JE-infected Vero cells were associated with extracellular particles, the virion in the case of E and a low density particle in the case of and NS1' exhibited amphipathic properties in Triton X-114 extraction experiments. Taken together, these results suggest that both the structural (E) and nonstructural (NS1 and NS1') glycoproteins were pathway of the infected Vero cells, assembled into particles, and then released into the extracellular fluid.

Intracellular accumulation of Punta Toro virus glycoproteins expressed from cloned cDNA

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

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

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

Human renin biosynthesis and secretion in normal and ischemic kidneys

The pathway of renin biosynthesis and secretion in normal and ischemic human kidneys has been investigated by pulse-labeling experiments. The results indicate that in normal human kidney, preprorenin is rapidly processed to 47-kDa prorenin. Microradiosequencing showed that this molecule was generated by cleavage between Gly-23 and Leu-24, yielding a 43-amino acid proregion. Analysis of prorenin secreted by the kidney tissue yielded an identical sequence, indicating that prorenin is secreted without any further proteolysis. An examination of the kinetics of processing and secretion suggested that a majority of the newly synthesized prorenin is quickly secreted, while only a small fraction is processed intracellularly to the mature renin. The differences in secretion kinetics between prorenin and mature renin and the selective inhibition of prorenin secretion by monensin suggest that they are secreted independently via two pathways: a constitutive pathway probably from the Golgi or protogranules that rapidly release prorenin and a regulated pathway that secretes mature renin from the mature granules. A comparison of the kinetics of processing between normal and ischemic tissues suggests that renal ischemia leads to an overall increase in the rate of processing of prorenin to mature renin. In addition, prolonged biosynthetic labeling of renin in the ischemic kidney yielded two smaller molecular weight immunoreactive forms suggestive of renin fragments that may be degradative products. These fragments were not detected in normal kidney tissue labeled for similar lengths of time.

Immunoelectron microscopy of Rift Valley fever viral morphogenesis in primary rat hepatocytes

The morphogenesis of the hepatotropic phlebovirus Rift Valley fever virus (RVFV) has been examined by immuno-electron microscopy in primary hepatocyte cultures derived from genetically susceptible and resistant rat strains. RVFV replicates in both cell types with growth kinetics comparable with those seen in other permissive cells. However, in contrast to that has been observed in other cell types, RVFV replication in hepatocytes is associated with maturation at cellular surface membranes in addition to the smooth internal membranes of the Golgi and endoplasmic reticulum. Envelope acquisition at surface membranes occurred primarily on basolateral membranes. The events occurring in RVFV morphogenesis were indistinguishable in hepatocytes from resistant and susceptible animals; however, hepatocytes from susceptible animals produced significantly higher titers of virus.

Variants of vaccinia virus hemagglutinin altered in intracellular transport

Two classes of revertants were isolated from a vaccinia virus mutant whose hemagglutinins (HAs) accumulate on nuclear envelopes and rough endoplasmic reticulums. The HAs of one of the revertants had the same phenotype as the wild type, i.e., rapid and efficient movement to the cell surface. The HAs of the second class had biphasic transport: rapid export to the cell surface as in the wild type and slow movement to the medial cisternae of the Golgi apparatus. Biochemical and nucleotide sequence analyses showed that the HAs of all the mutants examined that have defects in transport from the rough endoplasmic reticulum to the Golgi apparatus have altered cytoplasmic domains and that the HAs of the second class of revertants lack the whole cytoplasmic domain, while the HAs of the first class of revertants have a wild-type cytoplasmic domain.

Variants of vaccinia virus hemagglutinin altered in intracellular transport

Two classes of revertants were isolated from a vaccinia virus mutant whose hemagglutinins (HAs) accumulate on nuclear envelopes and rough endoplasmic reticulums. The HAs of one of the revertants had the same phenotype as the wild type, i.e., rapid and efficient movement to the cell surface. The HAs of the second class had biphasic transport: rapid export to the cell surface as in the wild type and slow movement to the medial cisternae of the Golgi apparatus. Biochemical and nucleotide sequence analyses showed that the HAs of all the mutants examined that have defects in transport from the rough endoplasmic reticulum to the Golgi apparatus have altered cytoplasmic domains and that the HAs of the second class of revertants lack the whole cytoplasmic domain, while the HAs of the first class of revertants have a wild-type cytoplasmic domain.

Intracellular assembly and packaging of hepatitis B surface antigen particles occur in the endoplasmic reticulum

Hepatitis B surface antigen (HBsAg) particles are secreted by Chinese hamster ovary cells that are stably transfected with the S gene of hepatitis B virus. The assembly of HBsAg into cylindrical and spherical particles occurred intracellularly within the endoplasmic reticulum. HBsAg particles accumulated within large dilated areas of the endoplasmic reticulum and remained within these structures for most of the time prior to secretion from the cells. Once the particles were formed, the HBsAg polypeptides did not appear to become associated with subsequent intracellular organelle membranes or the plasma membrane. HBsAg within the dilated structures did not bind wheat germ agglutinin, indicating that its oligosaccharide chains had not yet been processed to the complex form (containing terminal sialic acid-N-acetylglucosamine residues). The oligosaccharide chains of HBsAg are processed to the complex form and can be detected on the HBsAg after secretion, but this event was not detected within cells. In addition, HBsAg was not observed on the cell surface by indirect immunofluorescence or immunoprecipitation, although immunoelectron microscopy revealed some staining at or near the cell surface. These results suggested that HBsAg was either secreted from cells without being incorporated into the plasma membrane, or that the levels of HBsAg in the plasma membrane were below the limits of detection.

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.

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

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

Golgi and secreted galactosyltransferase

Galactosyltransferase (GT) belongs to the glycosyltransferases. In several tissues and cell lines, the enzyme is localized by immunocytochemistry to the two to three trans cisternae of the Golgi complex and may thus be considered a specific membrane component of this type of endomembrane. As a consequence, it is the most common Golgi "marker" enzyme in cell fractionation studies. Study of its biosynthesis, membrane orientation, and turnover in several tissues and cultured cell lines has broadened our knowledge about Golgi function itself. The enzyme is oriented towards the lumen of the cisternal space. In this orientation, it catalyzes the transfer of galactose to glycoprotein-bound acetylglucosamine and, in the presence of alpha-lactalbumin, to glucose, as shown in the Golgi complex of mammary gland epithelial cells. The enzymatic properties of GT are well known. The metabolism of GT has been extensively studied in HeLa and human hepatoma cells. The enzyme is synthesized in the rough endoplasmic reticulum (RER) and provided with one N-linked oligosaccharide and palmitate residues. In the Golgi complex, terminal sugars are attached to the N-linked oligosaccharide and extensive O-glycosylation takes place. The half-life of the enzyme is about 20 hr, after which a soluble form appears in the culture medium. Release of GT into the medium is observed in all cell lines studied. This phenomenon is in accordance with the presence of soluble GT in body fluids such as serum, ascites, milk, and saliva. In patients suffering from ovarian and breast cancer, increased levels of GT enzyme activity have been reported. Whether extracellular GT is of biological significance is still a point of discussion.

Effect of monensin on the assembly of Uukuniemi virus in the Golgi complex

The effect of the carboxylic ionophore monensin on the maturation of Uukuniemi virus, a bunyavirus, and the transport of its two membrane glycoproteins, G1 and G2, were studied in chicken embryo fibroblasts and baby hamster kidney cells. Virus maturation, which occurs in the Golgi complex (E. Kuismanen, K. Hedman, J. Saraste, and R. F. Pettersson, Mol. Cell. Biol. 2:1444-1458, 1982; E. Kuismanen, B. Bång, M. Hurme, and R. F. Pettersson, J. Virol. 51:137-146, 1984), was effectively inhibited by the drug (1 or 10 microM) as studied by electron microscopy and by assaying the release of infectious or radiolabeled virus. Immunoelectron microscopy showed that association of viral nucleocapsids with the cytoplasmic surface of glycoprotein-containing Golgi membranes, a prerequisite for virus budding, was unaffected by monensin. In the presence of the drug, the virus glycoproteins assembled into long, tubular structures extending into the lumen of Golgi-derived vacuoles, suggesting that monensin inhibited a terminal step in the assembly of the virus. Intracellular transport and expression of the virus membrane glycoproteins G1 and G2 at the cell surface were not inhibited by monensin as studied by immunocytochemical and radiolabeling techniques. Pulse-chase experiments in the presence of monensin showed that intracellular G1 acquired only partially endo-H-resistant glycans. The sialylation of G1 appearing on the cell surface in the presence of the drug was decreased, whereas sialylation of G2 apparently was inhibited to a lesser extent, as shown by external labeling of the cells with the periodate-boro[3H]hydride method. Thus, monensin exerted a differential effect on the terminal glycosylation of G1 and G2. Unlike several membrane and secretory glycoproteins, both G1 and G2 could enter a functional transport pathway in the presence of monensin and become expressed at the cell surface.

Intracellular processing of the Newcastle disease virus fusion glycoprotein

The fusion glycoprotein (Fo) of Newcastle disease virus is cleaved at an intracellular site (Nagai et al., Virology 69:523-538, 1976) into F1 and F2. This result was confirmed by comparing the transit time of the fusion protein to the cell surface with the time course of cleavage of Fo. The time required for cleavage of half of the pulse-labeled Fo protein is ca. 40 min faster than the half time of the transit of the fusion protein to the cell surface. To determine the cell compartment in which cleavage occurs, use was made of inhibitors which block glycoprotein migration at specific points and posttranslational modifications known to occur in specific cell membranes. Cleavage of Fo is inhibited by carbonyl cyanide m-chlorophenylhydrazone; thus, cleavage does not occur in the rough endoplasmic reticulum. Monensin blocks the incorporation of Newcastle disease virus glycoproteins into virions and blocks the cleavage of the fusion glycoprotein. However, Fo cannot be radioactively labeled with [3H] fucose, whereas F1 is readily labeled. These results argue that cleavage occurs in the trans Golgi membranes or in a cell compartment occupied by glycoproteins quite soon after their transit through the trans Golgi membranes. The implications of the results presented for the transit times of the fusion protein between subcellular organelles are discussed.

The transfer of myristic and other fatty acids on lipid and viral protein acceptors in cultured cells infected with Semliki Forest and influenza virus

[3H]Myristic and [3H]palmitic acid were compared as tracers for the fatty acylation of cellular lipids and viral glycoproteins in chicken embryo cells infected with fowl plague and Semliki Forest virus (SFV). Both of these substrates are incorporated into glycerolipids to a similar extent, whereas sphingolipids show much higher levels of palmitate than myristate after a 20 h labeling period. Both fatty acid species were found to be subject to metabolic conversions into longer chain fatty acids yielding 11.7% C16:0 from [3H]myristic and 11.8% C18:0 from [3H]palmitic acid. The reverse, a metabolic shortening of the exogenous acyl-chains yielding, for instance, significant levels of myristic acid from palmitic acid was not observed. Out of the various [3H]fatty acids present after in vivo labeling with [3H]myristic acid (C14:0) the elongated acyl-species arising from metabolic conversion (e.g., C16:0; C18:0) are preferred over myristic acid in the acylation of SFV E1 and E2 and of the influenza viral hemagglutinin (HA2). During acylation of exogenous E1 from SFV in vitro incorporation of palmitic acid from palmitoyl CoA exceeds that of myristic acid from myristoyl CoA by a factor of 37. This indicates that specificity for the incorporation of fatty acids into viral membrane proteins occurs at the level of the polypeptide acyltransferase(s).

Posttranslational processing of Uukuniemi virus glycoproteins G1 and G2

Uukuniemi virus, which matures specifically in the Golgi complex, contains two species of envelope glycoproteins, G1 (Mr, 70,000) and G2 (Mr, 65,000). These are translated as a polyprotein, p110, from an mRNA which is complementary to the medium-sized segment of the virion RNAs. By synchronized initiation of protein synthesis and pulse-labeling, it was shown that glycoprotein G1 is amino terminal in precursor protein p110. Apparently, the nonglycosylated forms of these proteins (Mr, 54,000 to 57,000), synthesized in the presence of tunicamycin, comigrate in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, because a similar-sized protein could be isolated by immunoprecipitation with monoclonal antibodies directed against either G1 or G2. The G1 protein, which in the virion contains exclusively endoglycosidase H (endo H)-resistant glycans, was converted to the endo H-resistant form in a half time of about 45 min. The G2 protein, which in the virion has a heterogeneous glycosylation pattern as revealed by endo H digestion, attained this partial endo H resistance only after 90 to 150 min of chase. The transport time of Uukuniemi virus glycoproteins from the endoplasmic reticulum to the Golgi complex was considerably longer than that for alpha and rhabdovirus glycoproteins. Determination of the transport time of G1 and G2 to extracellular virions revealed that G1 is incorporated into mature virions about 10 min faster than G2, suggesting that G1 and G2 are transported with different kinetics to the site of virus maturation.

Uukuniemi virus maturation: immunofluorescence microscopy with monoclonal glycoprotein-specific antibodies

Monoclonal antibodies directed against Uukuniemi virus glycoproteins G1 and G2 in combination with polyclonal antibodies against the nucleoprotein (N) were used to study the maturation of the virus in Golgi complexes of infected chicken embryo fibroblasts and BHK cells. Of 25 monoclonal antibodies obtained, 10 were shown to be G1 specific and 15 were shown to be G2 specific by immunoblotting and immunoprecipitation. In double-staining experiments, some of the monoclonal antibodies gave similar distributions of fluorescence as compared with the staining obtained from polyclonal rabbit anti-G1-G2 antibodies. Others, however, preferentially stained either the glycoproteins in the Golgi complex or those at the cell surface. This may indicate that the glycoproteins underwent conformational changes during their transport. Uukuniemi virus infection resulted in the vacuolization of the membranes of Golgi complexes where the maturation of the virus was taking place. Double-staining experiments with monoclonal antibodies which preferentially stained the Golgi-associated viral glycoproteins and with anti-N polyclonal rabbit antiserum showed a correlation between the progressive vacuolization of the Golgi complex and the accumulation of viral nucleoprotein in the Golgi region, suggesting that a morphological alteration of the Golgi complex may be a prerequisite for intracellular maturation of the virus. Treatment of Uukuniemi virus-infected cells with tunicamycin, a drug which inhibits N-linked glycosylation, resulted in the accumulation of both glycoproteins at an intracellular location, apparently representing the endoplasmic reticulum. Double-staining experiments showed a parallel accumulation of nucleoprotein at these sites, indicating that local accumulation of glycoproteins is required for nucleoprotein binding to intracellular membranes.

Assembly in vitro of a spanning membrane protein of the endoplasmic reticulum: the E1 glycoprotein of coronavirus mouse hepatitis virus A59

The E1 glycoprotein of coronavirus mouse hepatitis virus A59 was synthesized in vitro by translation of viral mRNA in the presence of dog pancreatic microsomes. Its disposition in the membrane was investigated by digestion with proteases and by selective NH2-terminal labeling. The protein spans the membrane, but only small portions from the NH2 and COOH terminus are exposed respectively in the lumenal and cytoplasmic domains; the bulk of the molecule is apparently buried in the membrane. The protein lacks a cleavable leader sequence and does not acquire its characteristic O-linked oligosaccharides in rough microsomes. It may enter the membrane at any stage during synthesis of the first 150 amino acid residues. These unusual features of the protein might help to explain why it is not transported to the cell surface in vivo but remains in intracellular membranes, causing the virus to bud there.

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.

Intracellular processing of the vesicular stomatitis virus glycoprotein and the Newcastle disease virus hemagglutinin-neuraminidase glycoprotein

The kinetics of intracellular transport of the vesicular stomatitis virus (VSV) glycoprotein (G) and the Newcastle disease virus (NDV) hemagglutinin-neuraminidase (HN) glycoprotein in chicken embryo cells were compared. To assay for the appearance of pulse-labelled glycoprotein at the cell surface, an antibody-binding assay was developed which allowed the precipitation of only those molecules on the outside surfaces of infected cells. Using this assay, it was found that pulse-labelled VSV G protein appeared at the cell surface with a half-time of approximately 27 min, while pulse-labelled NDV HN glycoprotein reached the cell surface with a half-time of approximately 78 min. To determine the transit time of these glycoproteins to trans-Golgi membranes, the kinetics of the acquisition of endoglycosidase H resistance was analyzed. The half-time of the transit of the G protein to the trans-Golgi membranes was found to be approximately 13 min while that of the HN glycoprotein was found to be approximately 60 min. Since the G protein migrates to the trans-Golgi membranes with a half-time of 13 min, and the cell surface with a half-time of 27 min, the half-time for the transit between the trans-Golgi membrane and the plasma membrane must be approximately 14 min. In a similar analysis, the half-time for the transit of the HN glycoprotein from the trans-Golgi membrane to the plasma membrane must be approximately 18 min, a time not significantly different from that of the G protein. Thus the difference in the kinetics of the intracellular transport of these two glycoproteins resides primarily in the transit from the rough endoplasmic reticulum to the trans-Golgi membranes. These results argue against a non-selective mechanism for the transport of plasma membrane glycoproteins to the cell surface.

Replication and morphogenesis of avian coronavirus in Vero cells and their inhibition by monensin

Avian infectious bronchitis virus (IBV) was adapted to Vero cells by serial passage. No significant inhibition of IBV replication was observed when infected Vero cells were treated with alpha-amanitin or actinomycin D. In thin sections of infected cells, assembly of IBV was observed at the rough endoplasmic reticulum (RER), and mature IBV particles were located in dilated cisternae of the RER as well as in smooth cytoplasmic vesicles. In addition to typical IBV particles, enveloped particles containing numerous ribosomes were identified at later times postinfection. Monensin, a sodium ionophore which blocks glycoprotein transport to plasma membranes at the level of the Golgi complex, was found to inhibit the formation of infectious IBV. In thin sections of infected Vero cells treated with the ionophore, IBV particles were located in dilated cytoplasmic vesicles, but fewer particles were found when compared to controls. A similar pattern of virus-specific proteins was detected in control or monensin-treated IBV-infected cells, which included two glycoproteins (170 000 and 24 000 daltons) and a polypeptide of 52 000 daltons. These results suggest that the ionophore inhibits assembly of a virus which matures at intracellular membranes.

Disparate effects of monensin and colchicine on intracellular processing of secretory proteins in cultured rat hepatocytes

We have studied the biosynthesis and intracellular processing of three major secretory proteins, albumin, alpha 1-protease inhibitor and alpha 2u-globulin, in cultured rat hepatocytes. The effect of secretion-blocking agents, monensin, a monovalent ionophore, and the microtubule-affecting agents colchicine and taxol was determined. In the control cells, alpha 1-protease inhibitor, a glycoprotein, was first synthesized as an endoglycosidase-H-sensitive form with Mr 51 000, and then processed to two endoglycosidase-H-resistant forms having Mr 51 000 and 56 000, the latter of which was secreted into the medium. Initially synthesized proalbumin was converted with chase to serum-type albumin, while no pro-type precursor was identified for alpha 2u-globulin. In the cells treated with colchicine or taxol, in which secretion was greatly inhibited, the fully processed alpha 1-protease inhibitor and albumin accumulated and were finally secreted into the medium. In the monensin-treated cells, however, most of the newly synthesized alpha 1-protease inhibitor and albumin were not processed to the final mature forms, resulting in accumulation of two 51 000-Mr forms and proalbumin, respectively. Moreover in treated cells, proalbumin and the endoglycosidase-H-resistant alpha 1-protease inhibitor were finally secreted into the medium. Such an effect was not caused by NH4Cl which also inhibited the secretion and is known to exert the similar effect as monensin on the receptor-mediated endocytosis pathway. Based on these results, the use of monensin may prove valuable for more detailed analysis of intracellular processing of various proteins.

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.

Glycosylation as a criterion for defining subpopulations of fast-transported proteins

The role carbohydrate residues may play in the sorting of newly synthesized fast-transported proteins during the initiation of fast axonal transport has been examined by identifying individual fast-transported glycoproteins that contain either or both fucose and galactose. [3H]Fucose or [3H]galactose was incorporated together with [35S]methionine in vitro in bullfrog dorsal root ganglia. Fast-transported proteins that accumulated proximal to a ligature on the spinal nerve were separated via two-dimensional gel electrophoresis, and 92 gel spots were analyzed quantitatively for the presence of 35S and 3H. Of these spots, 56 (61%) contained either or both fucose and galactose. Glycomoieties were generally associated with families of charged spots whose isoelectric points could be altered with neuraminidase treatment. Single spots tended to be unglycosylated and were unaffected by neuraminidase. The prevalence of glycoproteins was considerably greater in the higher-molecular weight range. Of the 55 spots analyzed with molecular weight greater than approximately 35,000 daltons, 89% were glycosylated, whereas only 19% of the 37 spots with lower molecular weight contained sugar moieties. When considered in light of previous studies in which similar subpopulations have been described, the current findings suggest that the presence or absence of glycomoieties may represent another criterion by which proteins are sorted during the initiation of fast axonal transport.

The confined function model of the Golgi complex: center for ordered processing of biosynthetic products of the rough endoplasmic reticulum

The organized and characteristic elements of the Golgi complex (GC) are the stacked smooth-surfaced cisternae, which are found in the centrosphere of all eukaryotic cells. These cisternae, in conjunction with other associated smooth-surfaced membranes, are responsible for executing net unidirectional intracellular transport (ICT) from the rough endoplasmic reticulum (RER) toward more distally located structures. This chapter focuses on the broad range of accessory activities that occur during transport, the family of “posttranslational modifications.” These events are, in all likelihood, not essential for the “primary” function of the GC yet they are crucial in allowing the cell to tailor its biosynthetic products for its own needs and the needs of the organism as a whole. In addition to modifying products of the rough endoplasmic reticulum, the GC may be involved in processing events because of its participation in other routes of vesicular traffic—for example, centripetal traffic from the cell surface. Various nonequivalent criteria have been used to ascribe processing events to the GC-autoradiography, preparative or analytic subcellular fractionation, interruption by ICT inhibitors, and delay in the impact of cycloheximide.