Protein structure and expression among arenaviruses

The underlying mechanisms of arenaviral entry through matriglycan

Matriglycan, a recently characterized linear polysaccharide, is composed of alternating xylose and glucuronic acid subunits bound to the ubiquitously expressed protein α-dystroglycan (α-DG). Pathogenic arenaviruses, like the Lassa virus (LASV), hijack this long linear polysaccharide to gain cellular entry. Until recently, it was unclear through what mechanisms LASV engages its matriglycan receptor to initiate infection. Additionally, how matriglycan is synthesized onto α-DG by the Golgi-resident glycosyltransferase LARGE1 remained enigmatic. Recent structural data for LARGE1 and for the LASV spike complex informs us about the synthesis of matriglycan as well as its usage as an entry receptor by arenaviruses. In this review, we discuss structural insights into the system of matriglycan generation and eventual recognition by pathogenic viruses. We also highlight the unique usage of matriglycan as a high-affinity host receptor compared with other polysaccharides that decorate cells.

Arenavirus variations due to host-specific adaptation

Arenavirus particles are enveloped and contain two single-strand RNA genomic segments with ambisense coding. Genetic plasticity of the arenaviruses comes from transcription errors, segment reassortment, and permissive genomic packaging, and results in their remarkable ability, as a group, to infect a wide variety of hosts. In this review, we discuss some in vitro studies of virus genetic and phenotypic variation after exposure to selective pressures such as high viral dose, mutagens and antivirals. Additionally, we discuss the variation in vivo of selected isolates of Old World arenaviruses, particularly after infection of different animal species. We also discuss the recent emergence of new arenaviruses in the context of our observations of sequence variations that appear to be host-specific.

Identification, characterization, and in vitro culture of highly divergent arenaviruses from boa constrictors and annulated tree boas: candidate etiological agents for snake inclusion body disease

Inclusion body disease (IBD) is an infectious fatal disease of snakes typified by behavioral abnormalities, wasting, and secondary infections. At a histopathological level, the disease is identified by the presence of large eosinophilic cytoplasmic inclusions in multiple tissues. To date, no virus or other pathogen has been definitively characterized or associated with the disease. Using a metagenomic approach to search for candidate etiologic agents in snakes with confirmed IBD, we identified and de novo assembled the complete genomic sequences of two viruses related to arenaviruses, and a third arenavirus-like sequence was discovered by screening an additional set of samples. A continuous boa constrictor cell line was established and used to propagate and isolate one of the viruses in culture. Viral nucleoprotein was localized and concentrated within large cytoplasmic inclusions in infected cells in culture and tissues from diseased snakes. In total, viral RNA was detected in 6/8 confirmed IBD cases and 0/18 controls. These viruses have a typical arenavirus genome organization but are highly divergent, belonging to a lineage separate from that of the Old and New World arenaviruses. Furthermore, these viruses encode envelope glycoproteins that are more similar to those of filoviruses than to those of other arenaviruses. These findings implicate these viruses as candidate etiologic agents of IBD. The presence of arenaviruses outside mammals reveals that these viruses infect an unexpectedly broad range of species and represent a new reservoir of potential human pathogens.

IMPORTANCE

Inclusion body disease (IBD) is a common infectious disease of captive snakes. IBD is fatal and can cause the loss of entire animal collections. The cause of the disease has remained elusive, and no treatment exists. In addition to being important to pet owners, veterinarians, breeders, zoological parks, and aquariums, the study of animal disease is significant since animals are the source of virtually every emerging infectious human disease. We searched for candidate causative agents in snakes diagnosed with IBD and found a group of novel viruses distantly related mainly to arenaviruses but also to filoviruses, both of which can cause fatal hemorrhagic fevers when transmitted from animals to humans. In addition to providing evidence that strongly suggests that these viruses cause snake IBD, this discovery reveals a new and unanticipated domain of virus biology and evolution.

Identification of an N-terminal trimeric coiled-coil core within arenavirus glycoprotein 2 permits assignment to class I viral fusion proteins

The lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP) consists of the transmembrane subunit GP-2 and the receptor binding subunit GP-1. Both are synthesized as one precursor protein and stay noncovalently attached after cleavage. In this study, we determined the oligomeric state of the LCMV GP and expressed it in two different conformations suitable for structural analysis. Sequence analysis of GP-2 identified a trimeric heptad repeat pattern containing an N-terminal alpha-helix. An alpha-helical peptide matching this region formed a stable oligomer as revealed by gel filtration chromatography and dynamic light scattering. In contrast, a second alpha-helical peptide corresponding to a predicted C-terminal alpha-helix within GP-2 did not oligomerize. Refolding of the complete GP-2 ectodomain revealed trimeric all-alpha complexes probably representing the six-helix bundle state that is considered a hallmark of class I viral fusion proteins. Based on these results, we generated a construct consisting of the complete uncleavable LCMV GP ectodomain fused C-terminally to the trimeric motif of fibritin. Gel filtration analysis of the secreted fusion protein identified two complexes of approximately 230 and approximately 440 kDa. Both complexes bound to a set of conformational and linear antibodies. Cross-linking confirmed the 230-kDa complex to be a trimer. The 440-kDa complexes were found to represent disulfide-linked pairs of trimers, since partial reduction converted them to a complex species migrating at 250 kDa. By electron microscopy, the 230-kDa complexes appeared as single spherical particles and showed no signs of rosette formation. Our results clearly demonstrate that the arenavirus GP is a trimer and must be considered a member of the class I viral fusion protein family.

Molecular characteristics of Junin virus

Results suggest that protein, glycerophospolipid, galactoside, and sialyl glycoside residues are present in Junin virus (JV), are accessible to enzymatic digestion, and play an important role in infection. Four major protein bands with molecular masses (Mr) 64 +/- 2, 56 +/- 2, 52 +/- 3 (mean +/- standard deviation, n = 4) and approximately 12-18 kDa were consistently detected after denaturing gel electrophoresis analysis of purified attenuated JV. The 52 kDa protein showed a diffuse tail in the 52-56 kDa range and was considered to be the JV glycoprotein. By Western blotting, the 64 kDa protein bound a JV neutralizing antibody and was considered to be the viral nucleoprotein. Additional bands corresponding to larger proteins (approximately 200, 96, 86, and 78 80 kDa in size), as well as fainter and broader bands in the 23-44 kDa region were also present in purified JV preparations. The relative resistance of virus infectivity to RNase digestion demonstrates that the genome of JV is protected from enzymatic attack. Analysis of purified JV virions by electrophoresis resolved the viral small (S) RNA and large (L) RNA species, 3636 +/- 54 bases and 7667 +/- 154 bases long, respectively (average length +/- range, in four determinations). The (S) RNA of attenuated JV appeared slightly larger than that reported for a pathogenic strain, ruling out a large sequence deletion as a reason for attenuation.

The entry of Junin virus into Vero cells

The entry mechanism of Junin virus (JV) into Vero cells was studied analyzing the effect of lysosomotropic compounds and acid pH on JV infection. Ammonium chloride, amantadine, chlorpheniramine and procaine inhibited JV production. The action of ammonium chloride was exerted at early times of infection. Virus internalization was inhibited and viral protein expression was not detected. When the extracellular medium was buffered at low pH, the ammonium chloride induced block on JV infection was overcome. Furthermore, JV was able to induce fusion of infected cells at pH 5.5 leading to polykaryocyte formation. Taken together, these results demonstrate that JV entry occurs through an endocytic mechanism requiring a low pH dependent membrane fusion.

Generation of reassortants between African arenaviruses

Lassa (LAS) and Mopeia (MOP) viruses are African arenaviruses which are carried by wild rodents and occasionally transferred to humans. In humans and nonhuman primates, Lassa causes mortality in 60% of untreated cases, whereas Mopeia does not cause mortality and has been known to protect monkeys from lethal challenge with Lassa. These two African arenaviruses also differ in their lethality for suckling outbred mice and in their plaque sizes under agar overlay. MOP virus induces small plaques and lethal infection after intracerebral (ic) inoculation. In contrast, LAS inoculation does not kill mice and the virus induces large plaques. After coinfection of Vero cells with LAS and MOP viruses some phenotypic reassortants which produced small plaques and were not lethal for outbred mice were isolated and plaque-purified. Dot-blot hybridization using LAS and MOP cDNA probes specific for L and S RNA segments revealed a genotype consisting of the L RNA of MOP and the S RNA of LAS (MOP/LAS reassortant). Adoptive transfer experiments demonstrated an ability of immune splenocytes from CBA mice intraperitoneally infected with the MOP/LAS reassortants to protect recipient mice against lethal disease after ic inoculation with LAS virus.

Post-translational processing of the glycoproteins of lymphocytic choriomeningitis virus

Intracellular events in the synthesis, glycosylation, and transport of the lymphocytic choriomeningitis virus (LCMV) glycoproteins have been examined. We have shown by N-glycanase digestion that LCMV strain Arm-4 bears five oligosaccharides on GP-1 and two on GP-2. By pulse-chase labeling experiments in the presence of drugs which inhibit N-linked oligosaccharide addition and processing we demonstrate that addition of high mannose precursor oligosaccharides is necessary for transport and cleavage of the viral GP-C glycoprotein. Moreover, in the presence of tunicamycin which inhibits en bloc addition of these mannose-rich side chains, virus budding was substantially decreased and infectious virions were reduced by more than 1000-fold in the supernatant medium. Incubation in the presence of castantospermine, which permits addition of oligomannosyl-rich chains but blocks further processing, restored transport and cleavage of GP-C and maturation of virions. Finally, by temperature block experiments we have determined that maturation of GP-C oligosaccharides to an endoglycosidase H resistant form precedes cleavage to GP-1 and GP-2. The latter process is most likely to occur in the Golgi or post-Golgi compartment.

Preferential usage of V alpha 4 and V beta 10 T cell receptor genes by lymphocytic choriomeningitis virus glycoprotein-specific H-2Db-restricted cytotoxic T cells

Correlations between the T cell receptor (TcR) V gene usage and the specificity of T cells have been primarily described for major histocompatibility complex (MHC) class II-restricted helper T cell responses. In the present study the TcR genes expressed by MHC class I-restricted murine cytotoxic T cells (CTL) specific for a major epitope of the lymphocytic choriomeningitis virus (LCMV), LCMV-GP2(275-289), were investigated. The TcR primary structure of an LCMV-GP2(275-289) specific H-2Db-restricted CTL clone has been determined. It uses a member of the V alpha 4 family joined to J alpha AN14.4 for the alpha chain and V beta 10 rearranged to D beta 2.1 and J beta 2.4 for its beta chain. Four other independent LCMV-GP2(275-289) specific H-2Db-restricted CTL clones also expressed V alpha 4 and V beta 10 gene elements. Furthermore, V alpha 4 and V beta 10 were preferentially expressed by polyclonal CTL of C57BL/6 origin specific for LCMV. These results suggest that both TcR V alpha and V beta regions are important for the recognition of the LCMV-GP2(275-289) epitope on H-2Db molecules.

The completed sequence of lymphocytic choriomeningitis virus reveals a unique RNA structure and a gene for a zinc finger protein

The arenavirus, lymphocytic choriomeningitis virus (LCMV) has a single-stranded RNA genome composed of a large (L) and a small (S) RNA segment. The completed sequence of LCMV, presented here, reveals a formerly unknown gene (Z) on the L genomic segment. This gene is encoded in the positive or message-sense of the viral genomic RNA, whereas the adjacent gene (L) is in the genome-complementary, or negative sense. The ambisense polarity of the genes on the L RNA reiterates the polarity of genes on the small (S) genomic segment. The Z gene encodes a 10-kDa protein containing a single zinc-finger sequence (Cys2His2). A small RNA representing the message sense of the Z gene is found in infected cells and within virions. In contrast to the known LCMV proteins having structural or enzymatic functions, the predicted Z gene product is most likely to be an RNA-binding protein with a regulatory role. The encapsidation of a message sense Z RNA suggests a role for this gene immediately following virus penetration. The L/Z intergenic region is rich in cytidylic acid (C) and presents an unusual RNA structure. All cDNA clones of the intergenic region differ from each other within a certain poly(C) stretch and lack a 30-base region present in the direct RNA sequence. Finally, the completed sequence establishes that the L RNA 5' end is complementary to its 3' end. The L RNA termini, similar to the S RNA termini, have a small but potentially important asymmetry of sequence. LCMV is the first arenavirus to be completely sequenced.

Major histocompatibility complex--dependent T cell epitopes of lymphocytic choriomeningitis virus nucleoprotein and their protective capacity against viral disease

In mice the immune response to infection with lymphocytic choriomeningitis virus (LCMV), a member of the arenavirus family, is mainly based on the activity of cytotoxic T cells. The immunogenic epitopes of the viral nucleoprotein recognized by cytotoxic T cells in various inbred strains of mice were defined. These epitopes were located in H-2d and H-2q mice in the amino-terminal region and in H-2b mice in the carboxy-terminal region of the nucleoprotein. A detailed analysis with synthetic peptides allowed the definition of a common epitope of 9 amino acids in H-2d and H-2q mice and of about 15 amino acids in H-2b mice. These T cell epitopes were all recognized in association with H-2 D or L transplantation antigen. The protective capacity of recombinant vaccinia viruses expressing these epitopes was documented by assaying prevention of virus replication, protection against LCM and prevention of the local footpad swelling reaction. Thus, distinct T cell epitopes on the same internal viral protein mediate protection in a major histocompatibility complex-restricted manner.

Homologous interference of lymphocytic choriomeningitis virus involves a ribavirin-susceptible block in virus replication

Depending on the multiplicity of infection (MOI), infection of L929 cells results in either productive lymphocytic choriomeningitis virus replication or homologous interference M. Bruns, A. Gessner, H. Lother, and F. Lehmann-Grube, Virology 166:133-139, 1988). As shown in this communication, productive lymphocytic choriomeningitis virus replication as observed at a low MOI was effectively inhibited by ribavirin. In contrast, virus yields increased if cells were infected with a high MOI and in the presence of 5 microM of the antiviral compound. This drug-dependent release of infectious virus was preceded by enhanced nucleoprotein (NP) synthesis, a change in intracellular NP distribution, and by an onset of glycoprotein synthesis. It is therefore proposed that this block in viral replication is brought about by a posttranslational effect on a viral gene product, probably the NP, present in reasonably large quantities both during homologous interference as well as persistent infection.

The primary structure of the lymphocytic choriomeningitis virus L gene encodes a putative RNA polymerase

The complete RNA sequence of the L protein gene of lymphocytic choriomeningitis virus (LCMV) is presented. It is the first L protein sequence to be obtained for the Arenaviridae, a family of single-stranded RNA viruses which includes Lassa fever virus, and the Tacaribe complex viruses such as Pichinde and the Argentine and Bolivian hemorrhagic fever viruses. It is the largest open reading frame on the L RNA spanning 6633 nucleotides and coding for a 2210 amino acid protein with a calculated molecular weight of 254,529. Antipeptide sera identify a gene product encoded on the L RNA: it has a mass of approximately 200,000 Da and is found in virions and ribonucleoprotein complexes from infected cells (M. Singh, F. Fuller-Pace, M. J. Buchmeier, and P. J. Southern, 1987, Virology, 161, 448-456). Mutations mapped to the L gene affect plaque morphology (Kirk et al., 1980), the lethality of a virulent LCMV strain on guinea pigs (Y. Riviere, R. Ahmed, P. J. Southern, M. J. Buchmeier, and M. B. A. Oldstone, 1985, J. Virol., 55, 704-709), and the ability of a variant strain of LCMV to suppress the cytotoxic T-cell response and initiate persistent infection (M. Salvato, E. Shimomaye, P. Southern, and M. B. A. Oldstone, 1988, Virology, 164, 517-522; Ahmed et al., 1988). All of these phenotypes indicate that the viral genes on the L strand are critical elements controlling virus replication and the pattern of LCMV infection. The L gene sequence encodes a viral polymerase although this protein bears little resemblance to the published sequences of other RNA virus polymerases. Therefore the LCMV polymerase likely represents a distinct category of viral transcriptase.

Host cell-dependent homologous interference in lymphocytic choriomeningitis virus infection

The generation of virus progeny as well as transcription, translation, and replication of the viral small RNA (S-RNA), which codes for the nucleoprotein (NP) and the glycoprotein precursor (GPC), was followed in L and MDCK cells after infection with multiplicities (m.o.i.) ranging from 0.01 to 100. In L cells, the yields of both plaque-forming units and interfering particles varied inversely with the m.o.i. Northern blot analysis revealed that early after infection with high multiplicity NP-mRNA was present, but later few or no signals of any specificity were registered. After low m.o.i. the results were negative at 8 hr, but large quantities of mRNAs for NP and GPC as well as viral genomic S-RNA and genomic-sized complementary S-RNA had been synthesized at 48 hr. In MDCK cells, throughout the range of m.o.i. both entities attained lower levels and most were generated at m.o.i. one. The degree of hybridization correlated roughly with the quantity of infectious virus to which the cells had been exposed. In the cells of both lines the NP-mRNA corresponded to the synthesis of its translation product, but once produced, most of it appeared to be retained in the phosphorylated form. We assume that the homologous interference seen in L cells after infection with high m.o.i. results from a host-dependent inhibition of viral transcription and replication mediated by NP.

Fine mapping of a peptide sequence containing an antigenic site conserved among arenaviruses

The lymphocytic choriomeningitis virus (LCMV) structural glycoproteins GP-1 (Mr 44K) and GP-2 (Mr 35K) are encoded on a single intracellular proteolytic cleavage precursor glycoprotein, GP-C (Mr 76K). We have used a series of synthetic peptides derived from the deduced amino acid sequence of LCMV GP-C to define an antigenic site containing two topographically overlapping epitopes. Three mouse monoclonal antibodies directed against two epitopes on GP-2 were assayed for binding in solution phase blocking and solid-phase enzyme-linked immunoadsorbant assays to a series of peptides representing the sequence of the intracellular precursor glycopeptide GP-C. Both epitopes were initially localized to a single peptide GP-C 370-382 (Cys-Asn-Tyr-Ser-Lys-Phe-Trp-Tyr-Leu-Glu-His-Ala-Lys) in the GP-2 segment of GP-C. Further analysis demonstrated that both epitopes were contained within a nine amino acid segment, GP-C 370-378, which contains five residues conserved among LCMV, Lassa, Pichinde, and Tacaribe viruses. Assays with N-terminal deletions from this sequence suggested that the minimal epitope recognized by the broadly cross-reactive monoclonal 33.6 (epitope GP-2a) consisted of five amino acids, GP-C 374-378 (Lys-Phe-Trp-Tyr-Leu). Reactivity of a second monoclonal, 9-7.9 (epitope GP-2B) but not 33.6, was abolished when substitution of tyrosine for phenylalanine was made at position 375 in the antigenic sequence corresponding to a naturally occurring sequence difference between LCM and Lassa viruses. Polyclonal sera from human cases and from animals experimentally infected with Junin, LCM, and Lassa viruses, respectively, bound to the antigenic peptide GP-C 370-382 but not to control peptides. As was the case with the monoclonals, this binding activity was abrogated by blocking with the antigenic peptide but not with control peptides in solution.

Analysis of the genomic L RNA segment from lymphocytic choriomeningitis virus

The arenavirus genomic L RNA segment represents approximately 70% of the viral genetic material but current understanding of the organization, regulation, and functioning of the viral L products remains limited. Biological studies with reassortant viruses have implicated the L RNA segment in the lethal infection of adult guinea pigs produced by LCMV-WE but no further explanation of the pathogenic process is presently available. We have initiated a detailed molecular analysis of LCMV L products based on construction and characterization of L-specific cDNA clones and synthesis of L-specific hybridization probes. Nucleotide sequencing studies have allowed the derivation of a partial amino acid sequence for a predicted L protein and antisera raised against synthetic peptides have demonstrated an L protein in Western blotting experiments. Using this approach we have identified a single high molecular weight protein (approximately 200,000 Da) in purified virions and in viral ribonucleoprotein complexes extracted from acutely infected tissue culture cells. This L protein is translated from a nonpolyadenylated, genomic complementary L mRNA and potentially represents part or all of the viral RNA-dependent RNA polymerase.