Folding, interaction with GRP78-BiP, assembly, and transport of the human immunodeficiency virus type 1 envelope protein

A detailed kinetic and quantitative analysis of the early and late biosynthetic events undergone by the human immunodeficiency virus type 1 envelope protein expressed by a recombinant vaccinia virus was performed. Early folding events that occurred in the endoplasmic reticulum included disulfide bond formation (t1/2 approximately 10 min), folding of envelope protein into a form competent to bind CD4 (t1/2 approximately 15 min), and specific and transient association and dissociation with GRP78-BiP (t1/2 approximately 25 min). After initial folding, envelope protein monomers formed noncovalently associated dimers with high efficiency (t1/2 approximately 30 min). Studies with brefeldin A, a compound that inhibits endoplasmic reticulum-to-Golgi transport, suggested that assembly occurred in the endoplasmic reticulum while cleavage of gp160 into gp120/gp41 subunits occurred in a post-endoplasmic reticulum compartment. Transport to the Golgi was monitored by modification of N-linked sugars to forms partially resistant to endoglycosidase H. The kinetics of endoglycosidase H resistance were nearly identical to the kinetics of gp160 cleavage (t1/2 approximately 80 min). Cleavage efficiency was strongly cell type dependent, ranging from 13 to 70%. By contrast, approximately 50% of the gp120 generated by the cleavage event was shed (t1/2 approximately 120 min) regardless of the cell type used. The results are discussed in terms of the overall biosynthetic pathway of the envelope protein and provide a framework with which to assess the effects of mutations on structure and function.

Assembly of vaccinia virus: role of the intermediate compartment between the endoplasmic reticulum and the Golgi stacks

Vaccinia virus, the prototype of the Poxviridae, is a large DNA virus which replicates in the cytoplasm of the host cell. The assembly pathway of vaccinia virus displays several unique features, such as the production of two structurally distinct, infectious forms. One of these, termed intracellular naked virus (INV), remains cells associated while the other, termed extracellular enveloped virus (EEV), is released from the cell. In addition, it has long been believed that INVs acquire their lipid envelopes by a unique example of de novo membrane biogenesis. To examine the structure and assembly of vaccinia virus we have used immunoelectron microscopy using antibodies to proteins of different subcellular compartments as well as a phospholipid analysis of purified INV and EEV. Our data are not consistent with the de novo model of viral membrane synthesis but rather argue that the vaccinia virus DNA becomes enwrapped by a membrane cisterna derived from the intermediate compartment between the ER and the Golgi stacks, thus acquiring two membranes in one step. Phospholipid analysis of purified INV supports its derivation from an early biosynthetic compartment. This unique assembly process is repeated once more when the INV becomes enwrapped by an additional membrane cisterna, in agreement with earlier reports. The available data suggest that after fusion between the outer envelope and the plasma membrane, mature EEV is released from the cell.

Coronavirus M proteins accumulate in the Golgi complex beyond the site of virion budding

The prevailing hypothesis is that the intracellular site of budding of coronaviruses is determined by the localization of its membrane protein M (previously called E1). We tested this by analyzing the site of budding of four different coronaviruses in relation to the intracellular localization of their M proteins. Mouse hepatitis virus (MHV) and infectious bronchitis virus (IBV) grown in Sac(-) cells, and feline infectious peritonitis virus (FIPV) and transmissible gastroenteritis virus (TGEV) grown in CrFK cells, all budded exclusively into smooth-walled, tubulovesicular membranes located intermediately between the rough endoplasmic reticulum and Golgi complex, identical to the so-called budding compartment previously identified for MHV. Indirect immunofluorescence staining of the infected cells showed that all four M proteins accumulated in a perinuclear region. Immunogold microscopy localized MHV M and IBV M in the budding compartment; in addition, a dense labeling in the Golgi complex occurred, MHV M predominantly in trans-Golgi cisternae and trans-Golgi reticulum and IBV M mainly in the cis and medial Golgi cisternae. The corresponding M proteins of the four viruses, when independently expressed in a recombinant vaccinia virus system, also accumulated in the perinuclear area. Quantitative pulse-chase analysis of metabolically labeled cells showed that in each case the majority of the M glycoproteins carried oligosaccharide side chains with Golgi-specific modifications within 4 h after synthesis. Immunoelectron microscopy localized recombinant MHV M and IBV M to the same membranes as the respective proteins in coronavirus-infected cells, with the same cis-trans distribution over the Golgi complex. Our results demonstrate that some of the M proteins of the four viruses are transported beyond the budding compartment and are differentially retained by intrinsic retention signals; in addition to M, other viral and/or cellular factors are probably required to determine the site of budding.

Intracellular transport of newly synthesized varicella-zoster virus: final envelopment in the trans-Golgi network

The maturation and envelopment of varicella-zoster virus (VZV) was studied in infected human embryonic lung fibroblasts. Transmission electron microscopy confirmed that nucleocapsids acquire an envelope from the inner nuclear membrane as they enter the perinuclear-cisterna-rough endoplasmic reticulum (RER). Tegument is not detectable in these virions; moreover, in contrast to the mature VZV envelope, the envelope of VZV in the RER is not radioautographically labeled in pulse-chase experiments with [3H]mannose, and it lacks gpI immunoreactivity and complex oligosaccharides. This primary envelope fuses with the RER membrane (detected in cells incubated at 20 degrees C), thereby releasing nucleocapsids to the cytosol. Viral glycoproteins, traced by transmission electron microscopy radioautography in pulse-chase experiments with [3H]mannose, are transported to the trans-Golgi network (TGN) by a pathway that runs from the RER through an intermediate compartment and the Golgi stack. At later chase intervals, [3H]mannose labeling becomes associated with enveloped virions in post-Golgi locations (prelysosomes and plasma membrane). Nucleocapsids appear to be enveloped by wrapping in specialized cisternae, identified as the TGN with specific markers. Tegument-like material adheres to the cytosolic face of the concave surface of TGN sacs; nucleocapsids adhere to this protein, which is thus trapped between the nucleocapsid and the TGN-derived membrane that wraps around it. Experiments with brefeldin A suggest that tegument may bind to the cytosolic tails of viral glycoproteins. Fusion and fission convert the TGN-derived wrapping sacs into an inner enveloped virion and an outer transport vesicle that carries newly enveloped virions to cytoplasmic vacuoles. These vacuoles are acidic and were identified as prelysosomes. It is postulated that secreted virions are partially degraded by their exposure to the prelysosomal internal milieu and rendered noninfectious. This process explains the cell-associated nature of VZV in vitro; however, the mechanism by which the virus escapes diversion from the secretory pathway to the lysosomal pathway in vivo remains to be determined.

Arabidopsis thaliana subcellular responses to compatible Erysiphe cichoracearum infections

Subcellular events of Erysiphe cichoracearum infections of epidermal cells were visualized in living tissues of Arabidopsis plants carrying various green fluorescent protein (GFP)-tagged organelles via laser scanning confocal microscopy. Early in the infection sequence, cytoplasm and organelles moved towards penetration sites and accumulated near penetration pegs. Peroxisomes appeared to accumulate preferentially relative to the cytoplasm at penetration sites. Another early event, which preceded haustorium formation, was the aggregation of some GFP-tagged plasma membrane marker proteins into rings around penetration sites, which extended across cell-wall boundaries into neighboring cells. This feature localized to sites where papillae were deposited. The extrahaustorial membrane (EHM) encases the fungal feeding structure, the haustorium, separating it from the host cytoplasm. Eight plasma membrane markers were excluded from the EHM and remained in a collar-like formation around the haustorial neck. These observations support the suggestions that the EHM is a unique, specialized membrane and is different from the plasma membrane. Our results suggested two possibilities for the origin of the EHM: invagination of the plasma membrane coupled with membrane differentiation; or de novo synthesis of the EHM by targeted vesicle trafficking.

Site of addition of N-acetyl-galactosamine to the E1 glycoprotein of mouse hepatitis virus-A59

By pulse-chase labeling with [35S]methionine and long-term labeling with 3H-sugars, the E1 glycoprotein of coronavirus MHV-A59 has been shown to acquire O-linked oligosaccharides in a two-step process. About 10 min after synthesis of the E1 protein, N-acetyl-galactosamine was added. This was followed approximately 10 min later by the addition of both galactose and sialic acid to give the mature oligosaccharides. This sequence of additions was confirmed by analyzing the 3H-labeled oligosaccharides bound to each of the E1 forms using gel filtration on P4 columns. The intracellular location of the first step was determined by exploiting the temperature sensitivity of virus release. The virus normally buds first into a smooth membrane compartment lying between the rough endoplasmic reticulum and the cis side of the Golgi stack (Tooze et al., 1984). At 31 degrees C the virus is assembled but does not appear to enter the Golgi stacks. The addition of N-acetyl-galactosamine is unaffected although the addition of galactose and sialic acid is inhibited. These results strongly suggest that addition of N-acetyl-galactosamine occurs in this budding compartment, the morphology of which is similar to that of transitional elements and vesicles.

Sorting of progeny coronavirus from condensed secretory proteins at the exit from the trans-Golgi network of AtT20 cells

Murine hepatitis virus (strain A59), (MHV-A59) is a coronavirus that buds into pre-Golgi compartments and then exploits the exocytic pathway of the host cell to reach the exterior. The fibroblastic cells in which replication of this virus is usually studied have only a constitutive exocytic pathway that the virus uses. MHV-A59 also infects, albeit inefficiently, AtT20 cells, murine pituitary tumor cells with a regulated as well as a constitutive exocytic pathway. Here we examine AtT20 cells at early times after the infection, when the Golgi apparatus retains its morphological and biochemical integrity. We observe that progeny coronavirus and secretory protein destined for the secretory granules of the regulated exocytic pathway traverse the same Golgi stacks and accumulate in the trans-Golgi network. Their pathways diverge at this site, the condensed secretory proteins including the ACTH going to the secretory granules and the coronavirus to post-Golgi transport vesicles devoid of ACTH. On very rare occasions there is missorting such that aggregates of condensed secretory proteins and viruses occur together in post-Golgi vesicles. We conclude that the constitutive and regulated exocytic pathways, identified respectively by the progeny virions and the secretory protein ACTH, diverge at the exit from the trans-Golgi network.

Involvement of membrane traffic in the replication of poliovirus genomes: effects of brefeldin A

Brefeldin A (BFA) is a macrolide antibiotic that has multiple targets in vesicular transport and blocks membrane traffic between the cis- and trans-Golgi compartments, leading to the disruption of the trans-Golgi apparatus (for a review see Pelham, 1991, Cell 67, 449-451). Consequently, BFA interferes with the maturation of viral glycoproteins and suppresses the formation of infectious viruses that contain a lipid envelope. We report that this antibiotic strongly inhibits poliovirus replication even though this virus lacks a lipid envelope and does not encode any glycoproteins. Addition of BFA from the beginning of poliovirus infection blocks the synthesis of late proteins but has no effect on p220 cleavage, indicating that the input viral RNA is translated to produce active 2Apro. The presence of BFA at later times has no effect on poliovirus protein synthesis, indicating that this step is not a direct target for the antibiotic. Indeed, the target of BFA is viral RNA synthesis, because addition of the antibiotic at any time after poliovirus infection drastically reduces the incorporation of labeled uridine into poliovirus RNA. Both plus- and minus-stranded RNA syntheses are diminished when BFA is present from the beginning of infection, but plus-stranded RNA synthesis is more affected when the inhibitor is added at later times. The replication of poliovirus RNA takes place in close association with membrane vesicles that fill the cytoplasm of the infected cells. Little is known about the origin and function of these vesicles that form part of the viral replication complexes. Our findings suggest that the replication of poliovirus genomes may require the maturation of membranous vesicles from a vesicular compartment that is affected by BFA. The effects of BFA on late protein synthesis by other animal viruses varies according to the virus species examined. Among picornaviruses, rhinoviruses are sensitive to the antibiotic, whereas encephalomyocarditis virus is resistant. A negative-stranded RNA virus such as vesicular stomatitis is blocked by BFA, whereas vaccinia virus, a cytoplasmic DNA virus, is resistant.

Fragmentation and dispersal of Golgi proteins and redistribution of glycoproteins and glycolipids processed through the Golgi apparatus after infection with herpes simplex virus 1

In Vero monkey cells and HEp-2 human epidermoid carcinoma cells infected with herpes simplex virus 1 the proteins beta-COP, galactosyltransferase, and alpha-mannosidase II associated with the Golgi apparatus appear to be associated with numerous smaller structures dispersed throughout the cytoplasm. Concomitantly, the intracytoplasmic ligands of lectins normally associated wholly (Helix pomatia or Ricinus communis agglutinin) or in part (wheat germ agglutinin) with the Golgi apparatus increased in amount and became dispersed. This phenomenon was seen in some of the baby hamster kidney cells analyzed but not in others and not in the human 143TK- cells. The fragmentation and dispersal of the Golgi apparatus was a late event in the reproductive cycle coinciding with virion assembly, processing of viral glycoproteins, and exocytosis from infected cells. The fragmentation of the Golgi apparatus is morphologically different from that seen with brefeldin A and may reflect disequilibration between the anterograde and retrograde Golgi transport caused by the huge influx of viral glycoproteins contained in virions and membranes flowing through the exocytic pathway.

LidA, a translocated substrate of the Legionella pneumophila type IV secretion system, interferes with the early secretory pathway

Legionella pneumophila uses a type IV secretion system to deliver effector molecules into the host cell and establish its replication vacuole. In this study, we investigated the role of LidA, a translocated substrate associated with the surface of the L. pneumophila-containing vacuole. LidA is secreted into the host cell throughout the replication cycle of the bacteria and associates with compartments of the early secretory pathway. When overexpressed in mammalian cells or yeast, LidA interferes with the early secretory pathway, probably via a domain predicted to be rich in coiled-coil structure. Finally, during intracellular replication, the replication vacuoles are in close contact with the endoplasmic reticulum-Golgi intermediate compartment and the Golgi apparatus, suggesting a positive correlation between intracellular growth and association of the vacuole with compartments of the early secretory pathway. We propose that LidA is involved in the recruitment of early secretory vesicles to the L. pneumophila-containing vacuole and that the vacuole associates with the secretory pathway to facilitate this process.

Specific membranous structures associated with the replication of group A arboviruses

INTRACYTOPLASMIC MEMBRANOUS STRUCTURES OF A UNIQUE TYPE WERE ASSOCIATED WITH THE REPLICATION OF THREE GROUP A ARBOVIRUSES: Semliki Forest virus (SFV), Sindbis virus, or Western equine encephalomyelitis virus. The structures, referred to as type 1 cytopathic vacuoles (CPV-1), were membrane-limited and characteristically lined by regular membranous spherules measuring 50 nm in diameter. The membranous spherules typically contained a fine central density, but were neither virus cores nor virions. Detection of CPV-1 by electron microscopy at 3 to 6 hr postinfection coincided with the time of rapid virus growth and preceded the accumulation of virus nucleocapsids. A range of 20 to 100 CPV-1 profiles were counted per 100 ultrathin cell sections at 6 to 9 hr postinfection when viruses were grown in chick embryo, baby hamster kidney, or mouse L cells. Maximum counts remained in the same range even when the multiplicity of infection was varied over 100-fold. Inhibition of cellular ribonucleic acid (RNA) and protein synthesis by actinomycin D during SFV infection did not decrease the counts of CPV-1; however, biogenesis of CPV-1 was decreased when viral replication was limited by inhibitors of viral RNA synthesis (guanidine) or of viral protein synthesis (cycloheximide). On the basis of present and earlier findings, we concluded that formation of CPV-1 must result from a virus-specified modification of pre-existing host cell macromolecules.

The CVS strain of rabies virus as transneuronal tracer in the olfactory system of mice

The sequential distribution of transneuronally infected neurons was studied in the olfactory pathway of mice after unilateral inoculation of the challenge virus standard (CVS) strain in the nasal cavity. A first cycle of viral multiplication was observed in a subpopulation of receptor cells scattered in the main olfactory epithelium and in the septal organ. No viral spread from cell body to cell body was reported even in later stages of infection. The second round of viral replication which took place in the ipsilateral main olfactory bulb at 2 and 2.5 days post-inoculation (p.i.), involved second order neurons and periglomerular cells, known to be directly connected with the axon terminals of receptor cells. Also reported as a result of a second cycle of viral replication, was surprisingly the spread of CVS at 2 and 2.5 days p.i. in bulbar interneurons located in the internal plexiform layer and in the superficial granule cell layer, as well as that of 2 ipsilateral cerebral nuclei, the anterior olfactory nucleus and the horizontal limb of the diagonal band. From day 3, a rapid spread of CVS was suggested by detection of virus in all ipsilateral direct terminal regions of the second order neurons and in most tertiary olfactory projections. The locus coeruleus, a noradrenergic nucleus which sends direct afferents to the olfactory bulb, never appeared immunoreactive. In spite of a certain inability of CVS to infect some neuron types, the virus appears relevant to provide new information regarding the complex network of olfactory-related neurons into the CNS.

Brefeldin A arrests the maturation and egress of herpes simplex virus particles during infection

Herpes simplex virus (HSV) requires the host cell secretory apparatus for transport and processing of membrane glycoproteins during the course of virus assembly. Brefeldin A (BFA) has been reported to induce retrograde movement of molecules from the Golgi to the endoplasmic reticulum and to cause disassembly of the Golgi complex. We examined the effects of BFA on propagation of HSV type 1. Release of virions into the extracellular medium was blocked by as little as 0.3 microgram of BFA per ml when present from 2 h postinfection. Characterization of infected cells revealed that BFA inhibited infectious viral particle formation without affecting nucleocapsid formation. Electron microscopic analyses of BFA-treated and untreated cells (as in control cells) demonstrated that viral particles were enveloped at the inner nuclear membrane in BFA-treated cells and accumulated aberrantly in this region. Most of the progeny virus particles observed in the cytoplasm of control cells, but not that of BFA-treated cells, were enveloped and contained within membrane vesicles, whereas many unenveloped nucleocapsids were detected in the cytoplasm of BFA-treated cells. This suggests that BFA prevents the transport of enveloped particles from the perinuclear space to the cytoplasmic vesicles. These findings indicate that BFA-induced retrograde movement of molecules from the Golgi complex to the endoplasmic reticulum early in infection arrests the ability of host cells to support maturation and egress of enveloped viral particles. Furthermore, we demonstrate that the effects of BFA on HSV propagation are not fully reversible, indicating that maturation and egress of HSV type 1 particles relies on a series of events which cannot be easily reconstituted after the block to secretion is relieved.

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

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

A Brucella Type IV Effector Targets the COG Tethering Complex to Remodel Host Secretory Traffic and Promote Intracellular Replication

Many intracellular pathogens exploit host secretory trafficking to support their intracellular cycle, but knowledge of these pathogenic processes is limited. The bacterium Brucella abortus uses a type IV secretion system (VirB T4SS) to generate a replication-permissive Brucella-containing vacuole (rBCV) derived from the host ER, a process that requires host early secretory trafficking. Here we show that the VirB T4SS effector BspB contributes to rBCV biogenesis and Brucella replication by interacting with the conserved oligomeric Golgi (COG) tethering complex, a major coordinator of Golgi vesicular trafficking, thus remodeling Golgi membrane traffic and redirecting Golgi-derived vesicles to the BCV. Altogether, these findings demonstrate that Brucella modulates COG-dependent trafficking via delivery of a T4SS effector to promote rBCV biogenesis and intracellular proliferation, providing mechanistic insight into how bacterial exploitation of host secretory functions promotes pathogenesis.

Infection of the basal ganglia by a murine coronavirus

The coronavirus, mouse hepatitis virus strain A59 (MHV-A59), causes mild encephalitis and chronic demyelination. Immunohistochemical techniques showed that MHV-A59-infected C57BL/6 mice contained dense deposits of viral antigen in the subthalamic nucleus and substantia nigra, with fewer signs of infection in other regions of the brain. The animals showed extra- and intracellular vacuolation, neuronal loss, and gliosis in the subthalamic-nigral region. Such localization is unprecedented among known viral encephalitides of humans and other species. This infection by a member of a viral class capable of causing both encephalitis and persistent infection in several species may be related to postencephalitic parkinsonism.

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.

The M protein of vesicular stomatitis virus associates specifically with the basolateral membranes of polarized epithelial cells independently of the G protein

Using monoclonal antibodies and indirect immunofluorescence microscopy, we investigated the distribution of the M protein in situ in vesicular stomatitis virus-(VSV) infected MDCK cells. M protein was observed free in the cytoplasm and associated with the plasma membrane. Using the ts045 mutant of VSV to uncouple the synthesis and transport of the VSV G protein we demonstrated that this distribution was not related to the presence of G protein on the cell surface. Sections of epon-embedded infected cells labeled with antibody to the M protein and processed for indirect horseradish peroxidase immunocytochemistry revealed that the M protein was associated specifically with the basolateral plasma membrane. The G and M proteins of VSV have therefore evolved features which bring them independently to the basolateral membrane of polarized epithelial cells and allow virus to bud specifically from that membrane.

Dictyostelium discoideum strains lacking the RtoA protein are defective for maturation of the Legionella pneumophila replication vacuole

To identify host proteins involved in Legionella pneumophila intracellular replication, the soil amoeba Dictyostelium discoideum was analysed. The absence of the amoebal RtoA protein is demonstrated here to depress L. pneumophila intracellular growth. Uptake of L. pneumophila into a D. discoideum rtoA(-) strain was marginally defective, but this effect was not sufficient to account for the defective intracellular growth of L. pneumophila. The rtoA mutant was also more resistant to high-multiplicity killing by the bacterium. A targeting assay testing the colocalization of L. pneumophila-containing vacuole with an endoplasmic reticulum/pre-Golgi intermediate compartment marker protein, GFP-HDEL, was used to analyse these defects. In parental D. discoideum, the L. pneumophila vacuole showed recruitment of GFP-HDEL within 40 min after introduction of bacteria to the amoebae. By 6 h after infection it was clear that the rtoA mutant acquired and retained the GFP-HDEL less efficiently than the parental strain, and that the mutant was defective for promoting the physical expansion of the membranous compartment surrounding the bacteria. Depressed intracellular growth of L. pneumophila in a D. discoideum rtoA(-) mutant therefore appeared to result from a lowered efficiency of vesicle trafficking events that are essential for the modification and expansion of the L. pneumophila-containing compartment.

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.

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.