Structural proteins of tacaribe and tamiami virions

Membrane Glycoproteins of Enveloped Viruses

This chapter focuses on the recent information of the glycoprotein components of enveloped viruses and points out specific findings on viral envelopes. Although enveloped viruses of different major groups vary in size and shape, as well as in the molecular weight of their structural polypeptides, there are general similarities in the types of polypeptide components present in virions. The types of structural components found in viral membranes are summarized briefly in the chapter. All the enveloped viruses studied to date possess one or more glycoprotein species and lipid as a major structural component. The presence of carbohydrate covalently linked to proteins is demonstrated by the incorporation of a radioactive precursor, such as glucosamine or fucose, into viral polypeptides, which is resolved by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. Enveloped viruses share many common features in the organization of their structural components, as indicated by several approaches, including electron microscopy, surface-labeling, and proteolytic digestion experiments, and the isolation of subviral components. The chapter summarizes the detailed structure of the glycoproteins of four virus groups: (1) influenza virus glycoproteins, (2) rhabdovirus G protein, (3) togavirus glycoprotein, and (4) paramyxovirus glycoproteins The information obtained includes the size and shape of viral glycoproteins, the number of polypeptide chains in the complete glycoprotein structure, and compositional data on the polypeptide and oligosaccharide portions of the molecules.

Complementarity in the supramolecular design of arenaviruses and retroviruses revealed by electron cryomicroscopy and image analysis

Arenaviruses are rodent-borne agents of diseases, including potentially lethal human hemorrhagic fevers. These enveloped viruses encapsidate a bisegmented ambisense single-stranded RNA genome that can be packaged in variable copy number. Electron cryomicroscopy and image analysis of New World Pichinde and Tacaribe arenaviruses and Old World lymphocytic choriomeningitis virus revealed pleomorphic enveloped particles ranging in diameter from approximately 400 to approximately 2,000 A. The surface spikes were spaced approximately 100 A apart and extended approximately 90 A from the maximum phospholipid headgroup density of the outer bilayer leaflet. Distinctive stalk and head regions extended radially approximately 30 and approximately 60 A from the outer bilayer leaflet, respectively. Two interior layers of density apposed to the inner leaflet of the viral lipid bilayer were assigned as protein Z and nucleoprotein (NP) molecules on the basis of their appearance, spacing, and projected volume. Analysis of en face views of virions lacking the GP-C spikes showed reflections consistent with paracrystalline packing of the NP molecules in a lattice with edges of approximately 57 and approximately 74 A. The structural proteins of retroviruses and arenaviruses assemble with similar radial density distributions, using common cellular components.

Immunological identification of Tacaribe virus proteins

Tacaribe virus (TV), an arenavirus, is an enveloped virus with genetic information encoded in two segments of single-stranded RNA. The completed sequence of TV led to the identification of four open reading frames (ORF). In order to establish a direct link between ORFs in the sequence of TV and proteins present in virus particles and virus-infected cells, segments of the molecularly cloned TV genome were engineered so as to be expressed in Escherichia coli to produce fusion proteins that were used to raise antisera. The antisera were in turn employed to identify the TV gene products. Serum to the putative nucleocapsid (N) protein reacted with a 68-kDa protein, both in TV particles and in the infected cells. Sera raised to the glycoprotein precursor (GPC) immunoprecipitated two proteins of 68 and 70 kDa from infected cell lysates. Analysis of GPC synthesis in the presence of tunicamycin revealed that the unglycosylated GPC appeared as two polypeptides of 43 and 46 kDa. The putative RNA polymerase gene product (L) was detected as a approximately 240-kDa protein. Serum to the small zinc-binding domain protein (p11-Z) recognized a protein of approximately 11kDa. Immunological evidence is presented that in addition to N and L, two glycoproteins (GP1 and GP2) and p11-Z are structural components of Tacaribe virions.

Protein-protein interactions in lymphocytic choriomeningitis virus

The structural organization of the lymphocytic choriomeningitis virus (LCMV) particle has been examined by Triton X-114 phase separation and nearest neighbor analyses in order to define protein-protein interactions in the virion. Extraction with Triton X-114 established that the 44-kDa membrane glycoprotein, GP-1, is a peripheral protein and that the 35-kDa glycoprotein, GP-2, is an integral membrane protein. Membrane permeable and membrane impermeable crosslinking reagents were used to establish the structural organization of the virion. Results obtained with both types of crosslinking reagents demonstrated that both GP-1 and GP-2 were assembled as native homotetramers. No covalent or disulfide linkages were found between GP-1 and GP-2, nor were these glycoproteins crosslinked. Protein complexes composed of GP-2 and NP were observed after treatment with a membrane permeable crosslinker (DMS) but not after treatment with the membrane impermeable crosslinker (DTSSP), localizing the site of the GP-2:nucleocapsid protein (NP) interaction to the interior of the virion. The interaction of GP-2 with NP may be important in directing the maturation and budding of LCM virions.

The 3' end termini of the Tacaribe arenavirus subgenomic RNAs

Tacaribe virus (TV), a member of the Arenaviridae family, contains two single-stranded RNA genome segments called S and L. Two proteins, in an ambisense coding strategy, are encoded in both the S RNA and the L RNA. The 3' ends of the TV four putative mRNAs have been characterized using S1 nuclease mapping. The experiments revealed that the transcripts terminate within the intergenic region in each RNA segment. No special sequences that might function as termination signals were evident. The 3' end sequences of the four putative mRNAs can be predicted to adopt GC-rich stable hairpin configurations (delta G greater than or equal to -25 kcal). These observations suggest that the transcript structure rather than particular sequences might be the signal involved in the termination of arenavirus transcription.

Nontemplated bases at the 5' ends of Tacaribe virus mRNAs

Centrifugation of Tacaribe arenavirus-infected cell extracts on CsCl density gradients was used to separate genomes and antigenomes, which band at 1.31 g/ml as nucleocapsids, from mRNAs which pellet. Primer extensions on the banded RNAs showed that the 5' ends of the genomes and antigenomes were unique, whereas primer extensions on the mRNAs showed that their 5' ends were heterogenous in length, extending 0-4 bases beyond the 3' ends of the templates for their synthesis. This suggests that arenavirus mRNAs may initiate by a cap-snatching mechanism, somewhat similar to influenza viruses and bunyaviruses. We also found an extra G residue at the 5' end of the genome RNA, which was not predicted according to current models. This is now the third time that the unexpected G residue has been found at the 5' end of arenavirus genomes.

Epitope mapping of the Lassa virus nucleoprotein using monoclonal anti-nucleocapsid antibodies

Monoclonal antibodies with differing specificity were prepared against the Josiah strain of the Lassa virus. All monoclonal antibodies were characterized by subclass determination and the immunofluorescence test against Lassa, LCM (WE & Arm strain), Junin, Machupo, and other arenavirus antigens. In radioimmune precipitation tests using purified Lassa virus antigen all monoclonal antibodies precipitated a single band of 60 kd, specific for the viral nucleoprotein (p 60). Three domains (A, B, C) were identified on the surface of the Lassa virus nucleoprotein using an ELISA-inhibition test. All domains carried different Lassa virus specific epitopes. In addition, the A-domain carried a group specific epitope present within the arenavirus family as a whole as shown by cross-reaction in immunofluorescence tests. The B-domain only carries Lassa virus specific epitopes, whereas the C-domain has a type specific and a subgroup specific (Lassa, LCM) epitope.

Tacaribe virus L gene encodes a protein of 2210 amino acid residues

The nucleotide sequence of Tacaribe virus (TV) L gene was obtained from two sets of overlapping cDNA clones constructed by walking along the virus L RNA using two successive synthetic DNA primers. Analysis of the sequence indicated the existence of a unique long open reading frame in the viral complementary strand. The first in-phase AUG codon is in positions 31-33 from the 5' end of the viral complementary L RNA surrounded by a sequence favorable for initiation of protein synthesis. The open reading frame ends at positions 6661-6663. The predicted TV L protein is a 2210 amino acid long polypeptide with an estimated molecular weight of 251,942. Comparison of the amino acid sequence of TV L protein with peptide sequences predicted from L-derived cDNA clones of lymphocytic choriomeningitis virus shows an overall 42% of homology.

Molecular structure and early events in the replication of Tacaribe arenavirus S RNA

Tacaribe arenavirus S RNA was cloned and analysis of its nucleotide sequence revealed two open reading frames of significant size, one in the virus-sense strand, the other in the virus-complementary strand. The predicted amino acid sequences of the two reading frames were compared with the predicted primary structures of the nucleoprotein (N) and glycoprotein precursor (GPC) of LCM, Pichinde and Lassa viruses. The results indicated a high degree of homology between the proteins of similar properties. It was also found that in Tacaribe virus-infected cells a subgenomic viral-sense GPC RNA and a subgenomic viral-complementary N RNA are synthesized in addition to the full length viral (v) RNA and viral complementary (vc) RNAs. These results support the conclusion that in Tacaribe virus--as in Pichinde and lymphocytic choriomeningitis arenavirus-S RNA encodes the viral N and GPC proteins and has an 'ambisense' coding strategy. Analysis of the S-derived RNA species at early times post-infection in cells incubated with or without inhibitors of protein synthesis indicated that for primary transcription of the N mRNA, protein synthesis is not required; whereas synthesis of the vc RNA, GPC mRNA and v RNA does require protein synthesis to take place.

Ambisense RNA genomes of arenaviruses and phleboviruses

This chapter reviews the evidence that shows that arenaviruses and members of one genus of the Bunyaviridae (phleboviruses) have some proteins coded in subgenomic, viral-sense mRNA species and other proteins coded in subgenomic, viral-complementary mRNA sequences. This unique feature is discussed in relation to the implications it has on the intracellular infection process and how such a coding arrangement may have evolved. The chapter presents a list of the known members of the arenaviridae, their origins, and the vertebrate hosts from which isolates have been reported. It discusses the structural components, the infection cycle, and genetic attributes of arenaviruses. In order to determine how arenaviruses code for gene products, the S RNA species of Pichinde virus and that of a viscerotropic strain of LCM virus (LCM-WE) have been cloned into DNA and sequenced. The arenavirus S RNA is described as having an ambisense strategy, to denote the fact that both viral and viral-complementary sequences are used to make gene products. The chapter discusses the infection cycle, the structural and genetic properties of bunyaviridae member.

Synthesis of Tacaribe virus polypeptides in an in vitro coupled transcription and translation system

We have analyzed polypeptides synthesized in a coupled in vitro transcription and translation system in response to detergent-disrupted Tacaribe virus. Analysis of the major Tacaribe virus-specified product by two-dimensional polyacrylamide gel electrophoresis indicated that it had an isoelectric point similar to that of the Tacaribe nucleocapsid polypeptide N; however, the in vitro product had an approximate mol. wt. of 73 000, compared to a mol. wt. of 68 000 for the N protein. The 73 000 dalton product was found to yield proteolytic cleavage products with similar electrophoretic mobilities to those obtained from the virion P and N proteins. These results, as well as pulse-chase experiments in Tacaribe virus-infected cells, suggest that a 73 000 dalton polypeptide may be processed to yield the N polypeptide. The polypeptides synthesized in the coupled system depended on the amount and type of virus added; addition of purified Shark River (SR) virus, a member of the Patois group of bunyaviruses, resulted in synthesis of a polypeptide of mol. wt. 22 000 which corresponds to the SR nucleocapsid protein.

Machupo virus polypeptides: identification by immunoprecipitation

The most abundant protein in purified Machupo virions (Corvallo strain) labelled with 14C-Protein hydrolysate is a 64 K polypeptide which is associated with virion RNAs. Another structural polypeptide, 37 K, solubilized by nonionic detergent seems to be a major surface glycoprotein. In addition to this, a 78 K polypeptide and a minor 50 K polypeptide have been detected. In Machupo virus infected cells three virus-specific polypeptides similar in size to those described for structural polypeptides were immunoprecipitated with anti-Machupo virus serum. The most abundant virus-specific polypeptide was nonglycosylated (64 K, NP), and the others were glycosylated polypeptides (78 K and 37 K). The synthesis of NP and 78 K polypeptides was recognized at the beginning of a log phase of virus replication. Pulse-chase experiments as well as experiments with an arginine analogue, canavanine (to block proteolytic processing) suggest that 78 K is a precursor for structural glycoproteins of Machupo virions.

Lymphocytic choriomeningitis virus. IV. Electron microscopic investigation of the virion

The structure of lymphocytic choriomeningitis virus (LCM virus) was investigated by a variety of conventional as well as novel electron microscopic procedures. Thin sections of infected cells revealed the characteristic arenavirus entities whose interiors contain ribosome-like granules but look otherwise empty. In contrast, most thin-sectioned virus particles from infectious cell culture fluid, both untreated and highly purified with little loss of initial infectivity, appeared to be filled with rather homogeneous cores. Cores rather than granules were also found in positively contrasted whole and thin-sectioned virus particles. We favor the explanation that the sandy grains, which have given this group of viruses its name, are altered cores that happen to look like ribosomes. However, the alternative cannot yet be excluded, namely, that LCM virus-infected cells produce two types of particles, of which only the core-containing ones represent virions.

Analysis of polypeptides in Tacaribe virus-infected cells

Two virus-induced polypeptides designated p79 (mol wt, 79,000) and p105 (mol wt, 105,000) in BHK21 or Vero cells infected with Tacaribe (Tac) virus have been identified. Both polypeptides were found in immune precipitates with antiserum to Tac virus, suggesting that they are virus specific. Two-dimensional gel analysis of Tac virus-infected Vero cell extracts indicated that p79 and p105 were acidic proteins which did not comigrate with any polypeptides from uninfected cells. Neither of the polypeptides was found to be phosphorylated under conditions in which phosphorylation of the N (nucleocapsid) protein was detected. Comparison of one-dimensional peptide maps of the p79 polypeptide and the nucleoprotein indicated that they are unrelated in primary sequences.

Virion RNA species of the arenaviruses Pichinde, Tacaribe, and Tamiami

The principal RNA species isolated from labeled preparations of the arenavirus Pichinde usually include a large viral RNA species L (apparent molecular weight = 3.2 X 10(6)), and a smaller viral RNA species S (apparent molecular weight = 1.6 X 10(6)). In addition, either little or considerable quantities of 28S rRNA as well as 18S rRNA can also be obtained in virus extracts, depending on the virus stock and growth conditions used to generate virus preparations. Similar RNA species have been identified in RNA extracted from Tacaribe and Tamiami arenavirus preparations. Oligonucleotide fingerprint analyses have confirmed the host ribosomal origin of the 28S and 18S species. Such analyses have also indicated that the Pichinde viral L and S RNA species each contain unique nucleotide sequences. Viral RNA preparations isolated by conventional phenol-sodium dodecyl sulfate extraction often have much of their L and S RNA species in the form of aggregates as visualized by either electron microscopy or oligonucleotide fingerprinting of material recovered from the top of gels (run by using undenatured RNA preparations). Circular and linear RNA forms have also been seen in electron micrographs of undenatured RNA preparations, although denatured viral RNA preparations have yielded mostly linear RNA species with few RNA aggregates or circular forms.