Recombinant Oliveros virus Pre-glycoprotein polyprotein GP complex (GPC)

What is the molecular structure and function of the Oliveros virus pre-glycoprotein polyprotein GP complex?

The Oliveros virus pre-glycoprotein polyprotein GP complex (GPC) forms virion spikes and functions as a critical component in viral attachment and fusion. Similar to other mammarenaviruses, the GPC is cleaved into three main components: a stable signal peptide (SSP), the receptor-binding glycoprotein G1, and the fusion-mediating glycoprotein G2 . The SSP is retained as the third component of the GP complex and helps stabilize the spike complex in its native conformation, while also facilitating efficient glycoprotein expression, post-translational maturation, and transport to the cell surface . G1 mediates host receptor attachment, while G2 functions as a class I viral fusion protein that directs fusion of viral and host endosomal membranes through irreversible conformational changes induced by acidification of the endosome . This fusion process ultimately leads to the delivery of the viral nucleocapsid into the cytoplasm, initiating infection.

What expression systems are most effective for producing recombinant Oliveros virus GPC?

For recombinant expression of Oliveros virus GPC, researchers should consider several expression systems based on experimental goals:

• Mammalian cell systems: HEK293T or Vero E6 cells provide appropriate post-translational modifications for functional studies . These systems are particularly useful when native glycosylation patterns and protein folding are critical.

• Insect cell systems: Baculovirus-infected Sf9 or High Five cells can produce higher protein yields while maintaining many post-translational modifications. This system offers a balance between protein quantity and quality.

• Cell-free expression systems: These may be employed for structural studies requiring specific labeling or when rapid production is needed.

The optimal choice depends on research objectives, with mammalian systems generally preferred when studying functional properties of the GPC, especially receptor binding and fusion activities. Vero E6 cells have been specifically documented as effective for cultivating Oliveros virus isolates, as demonstrated in the isolation of Vello virus following passage of rodent lung tissue in these cells .

What are the key host factors that interact with Oliveros virus GPC during viral entry?

While specific interaction partners for Oliveros virus GPC have not been definitively established in the provided references, insights can be drawn from related mammarenaviruses. The receptor-binding function of the G1 subunit likely determines cellular tropism, similar to how Junín virus (a related Clade B mammarenavirus) uses transferrin receptor 1 (TfR1) for entry . Unlike Old World mammarenaviruses that primarily utilize α-dystroglycan as their receptor, New World mammarenaviruses in Clade B and potentially some in Clade C may use TfR1 or other alternate receptors .

Following receptor binding, host factors involved in clathrin-mediated endocytosis facilitate virion internalization, as described for other mammarenaviruses . The acidification of endosomes triggers conformational changes in the G2 fusion protein, enabling membrane fusion and release of viral contents into the cytoplasm. Studies examining receptor usage patterns across mammarenavirus clades suggest that receptor binding properties may influence pathogenicity and host range .

What methodologies can detect potential reassortment events involving Oliveros virus GPC in natural isolates?

Recent research has identified evidence of reassortment among Clade C mammarenaviruses, including Oliveros virus . To detect such events, researchers should implement a multi-faceted approach:

• Whole-genome sequencing: Deep sequencing of both S and L segments from multiple isolates, followed by phylogenetic analysis of individual genome segments.

• Tanglegram analysis: Comparing phylogenetic trees constructed from S and L segments to identify incongruent branching patterns that suggest segment exchange.

• Recombination detection algorithms: Software tools like RDP4, GARD, or SimPlot can identify potential breakpoints and reassortment signals.

• Percent identity matrices: Calculating nucleotide and amino acid identity percentages between segments of different viruses to identify discordant relatedness patterns.

The tanglegram approach has proven particularly valuable, as demonstrated in the identification of reassortment evidence between Vello virus, Ura virus, and Pampa virus . When examining potential reassortment events involving the GPC (encoded on the S segment), researchers should compare its phylogeny with that of the viral polymerase (L segment) to detect incongruent evolutionary relationships.

How can structural modifications be introduced to the Oliveros virus GPC to enhance its stability for research applications?

Enhancing stability of recombinant Oliveros virus GPC requires strategic modifications based on structure-function understanding:

• Disulfide bond engineering: Introduction of additional cysteine pairs to form stabilizing disulfide bonds, particularly in regions that undergo conformational changes during the fusion process.

• Pre-fusion stabilization: Introduction of proline residues at hinge regions can lock the protein in its pre-fusion conformation, similar to approaches used for other viral fusion proteins.

• Glycan shielding: Strategic addition of N-linked glycosylation sites can enhance solubility and reduce proteolytic degradation.

• Domain truncation: Removing the transmembrane domain while maintaining the ectodomain structure can improve solubility while preserving antigenic properties.

• Trimerization domains: Addition of heterologous trimerization domains (e.g., foldon) can stabilize the native trimeric structure.

These approaches should be guided by comparative structural analysis with related mammarenaviruses. When designing modifications, researchers should consider preserving epitopes critical for immunological studies and functional domains essential for mechanistic investigations.

What are the molecular determinants of Oliveros virus GPC that influence its potential for cross-species transmission?

The molecular determinants in Oliveros virus GPC that influence cross-species transmission potential include:

• Receptor-binding domain variations: The G1 subunit contains regions that determine receptor specificity. Sequence variations that affect receptor binding might expand or restrict host range .

• Fusion mechanism adaptations: The G2 subunit's fusion peptide and surrounding regions that mediate membrane fusion may contain adaptations that function optimally at specific pH ranges or membrane compositions found in particular host species.

• Glycosylation patterns: N-linked glycan positions can influence immune evasion capabilities and receptor interactions across different host species.

• Cleavage site characteristics: The recognition sequence between G1 and G2 that is cleaved by host proteases may require adaptation to efficiently utilize proteases present in new host species.

Research suggests that Oliveros virus naturally infects multiple rodent species, including Necromys obscurus (formerly Bolomys obscurus), Akodon dolores, Necromys benefactus, and Calomys tener . This multi-host tropism indicates pre-existing adaptability that could potentially facilitate wider host range expansion. Studying the molecular basis of this adaptability could provide insights into the mechanisms underlying cross-species transmission events.

How does the reassortment potential between Oliveros virus and other mammarenaviruses impact vaccine development strategies?

The documented evidence of reassortment among Clade C mammarenaviruses, including Oliveros virus , raises important considerations for vaccine development:

• Conserved epitope targeting: Vaccine designs should prioritize targeting conserved epitopes across potential reassortants to ensure broad protection against emergent viruses.

• Multi-segment immunization: Vaccines incorporating antigens from both S and L segments may provide more comprehensive protection against potential reassortants.

• Surveillance integration: Continuous monitoring of circulating strains should inform vaccine updates, similar to influenza vaccine strategy.

• Multivalent approaches: Development of multivalent vaccines covering multiple mammarenavirus species within geographic regions where co-circulation occurs.

The co-circulation of different mammarenavirus species in the same geographic regions and even within the same rodent host species creates conditions favorable for reassortment events. This genetic exchange could potentially generate viruses with novel properties, including altered pathogenicity or host range. Vaccine design strategies must account for this evolutionary potential to ensure long-term efficacy.

Table 1: Nucleotide identity percentages between mammarenavirus segments and evidence for reassortment based on phylogenetic analysis

What are the optimal conditions for expressing and purifying recombinant Oliveros virus GPC for structural studies?

For structural studies of recombinant Oliveros virus GPC, researchers should implement the following optimized protocol:

• Expression system selection:

  • Use HEK293T or Expi293F cells for mammalian expression to maintain native glycosylation patterns

  • Incorporate a C-terminal dual tag system (His8 and Twin-Strep-tag) for two-step affinity purification

  • Consider using GnTI-deficient cell lines (HEK293S) for structural studies requiring homogeneous glycosylation

• Vector design considerations:

  • Include a strong promoter (CMV) for high-level expression

  • Optimize codon usage for mammalian expression

  • Add a cleavable signal peptide for proper membrane targeting

  • Consider replacing the transmembrane domain with a heterologous trimerization domain for soluble expression

  • Incorporate a TEV protease cleavage site between the protein and affinity tags

• Purification strategy:

  • Initial capture via immobilized metal affinity chromatography (IMAC)

  • Secondary purification using Strep-Tactin affinity chromatography

  • Size exclusion chromatography as a final polishing step

  • Maintain pH 7.4-8.0 and include 150-300 mM NaCl throughout purification

  • Add 0.01-0.05% mild detergent (e.g., LMNG or DDM) if including transmembrane domains

• Protein stabilization:

  • Add 5-10% glycerol to all buffers

  • Include reducing agents (2-5 mM DTT or 0.5-1 mM TCEP) to prevent non-native disulfide formation

  • Consider adding specific ligands or nanobodies that lock the protein in preferred conformations

This methodology has been adapted from successful approaches used with other mammarenaviruses, with modifications to address the specific characteristics of Oliveros virus GPC.

What techniques can be employed to assess the interaction between recombinant Oliveros virus GPC and potential cell receptors?

Several complementary techniques can effectively characterize the interactions between recombinant Oliveros virus GPC and potential cellular receptors:

• Surface Plasmon Resonance (SPR):

  • Immobilize purified recombinant GPC or receptor candidates on sensor chips

  • Measure binding kinetics (kon, koff) and affinity (KD)

  • Compare binding profiles with related mammarenaviruses like Junín virus

  • Assess how pH changes affect binding stability

• Bio-Layer Interferometry (BLI):

  • Alternative to SPR with similar applications but different experimental setup

  • Particularly useful for screening multiple receptor candidates simultaneously

• Cell-based binding assays:

  • Flow cytometry with fluorescently labeled GPC to quantify binding to various cell types

  • Competitive binding assays using unlabeled GPC or receptor antibodies

  • Reverse genetic approaches expressing mutant forms of potential receptors

• Virus-like particle (VLP) binding studies:

  • Generate VLPs displaying Oliveros virus GPC

  • Assess VLP binding to cells expressing candidate receptors

  • Perform inhibition studies with soluble receptor fragments or antibodies

• Cryo-electron microscopy:

  • Visualize GPC-receptor complexes to determine binding interfaces

  • Compare structural conformations with and without receptor binding

Considering that Junín virus (another South American arenavirus) uses transferrin receptor 1 (TfR1) for cell entry , researchers should prioritize testing this receptor with Oliveros virus GPC, while also examining α-dystroglycan and other alternative receptors that might be utilized by this Clade C mammarenavirus .

How can researchers develop neutralizing antibodies against Oliveros virus GPC for diagnostic and therapeutic applications?

Developing neutralizing antibodies against Oliveros virus GPC requires a strategic approach:

• Immunization strategies:

  • Use DNA vaccines encoding full-length GPC followed by protein boost with purified recombinant GPC

  • Implement prime-boost regimens alternating between different forms of the antigen

  • Consider various animal models including mice, rabbits, and non-human primates

  • Utilize adjuvants that promote strong neutralizing antibody responses (e.g., AddaVax or AS01)

• Screening methodologies:

  • Develop pseudotyped virus neutralization assays using vesicular stomatitis virus (VSV) or lentiviral vectors bearing Oliveros virus GPC

  • Establish cell-based fusion inhibition assays to identify antibodies blocking the fusion function

  • Perform competitive binding ELISAs to identify antibodies targeting the receptor-binding domain

• Antibody production and characterization:

  • Isolate B cells from immunized animals and perform single-cell sorting of antigen-specific cells

  • Use hybridoma technology or phage display libraries to generate monoclonal antibodies

  • Characterize binding epitopes through peptide mapping, competition assays, and structural studies

  • Evaluate cross-reactivity with related mammarenaviruses to identify broadly neutralizing antibodies

• Therapeutic development considerations:

  • Humanize promising antibody candidates for potential therapeutic applications

  • Evaluate antibody combinations targeting non-overlapping epitopes to prevent escape mutations

  • Assess stability, half-life, and tissue distribution of antibody candidates

This methodological framework enables the systematic development of neutralizing antibodies that could serve both as research tools and as potential therapeutic or diagnostic agents. Given that Oliveros virus is phylogenetically distinct from other mammarenaviruses but shares common structural features , antibodies developed against its GPC might provide valuable insights into conserved neutralization determinants across the virus family.

What approaches can be used to study the membrane fusion mechanism of Oliveros virus GPC?

The membrane fusion mechanism of Oliveros virus GPC can be investigated through multiple complementary approaches:

• pH-dependent conformational change studies:

  • Circular dichroism spectroscopy to monitor secondary structure changes at varying pH

  • Intrinsic tryptophan fluorescence to detect exposure of hydrophobic domains

  • Protease sensitivity assays to identify pH-dependent structural rearrangements

  • Differential scanning calorimetry to measure stability changes at different pH values

• Cell-cell fusion assays:

  • Co-culture GPC-expressing cells with target cells containing fluorescent markers

  • Monitor syncytia formation microscopically following pH reduction

  • Quantify fusion using split reporter systems that activate upon cytoplasmic mixing

  • Compare fusion efficiency of Oliveros GPC with other mammarenavirus GPCs

• Single-particle fusion assays:

  • Generate GPC-containing liposomes or virus-like particles with encapsulated fluorescent dyes

  • Monitor lipid mixing and content mixing using FRET-based assays upon exposure to low pH

  • Track individual fusion events using total internal reflection fluorescence microscopy

• Mutagenesis studies:

  • Introduce systematic mutations in predicted fusion peptide and associated domains

  • Assess impact on fusion function using the above assays

  • Identify critical residues involved in the fusion process

• Structural biology approaches:

  • Obtain cryo-EM structures of GPC in pre-fusion and post-fusion conformations

  • Capture intermediates using conformation-specific antibodies or small molecules

  • Compare with known structures of other class I viral fusion proteins to identify conserved mechanisms

Based on understanding of related viral fusion proteins , the G2 subunit of Oliveros virus GPC likely undergoes dramatic conformational changes triggered by acidification in the endosome, transitioning from a metastable pre-fusion state to a stable post-fusion six-helix bundle configuration that brings the viral and cellular membranes into proximity for fusion.