Recombinant African swine fever virus Uncharacterized protein B117L (Mal-091)

What is the structural composition of the B117L protein?

B117L is a 115-amino-acid integral membrane protein with a distinctive structural organization. Hydrophobicity distribution analysis confirms the presence of a single transmembrane helix, which together with flanking amphipathic sequences forms a membrane-associated C-terminal domain of approximately 50 amino acids . The protein contains an N-terminal small globular ectodomain followed by an amphipathic-strand–hydrophobic-helix–amphipathic-helix membrane domain . This structure is reminiscent of other viral proteins with membrane-permeabilizing functions, suggesting a specialized role in viral infection processes.

When during the viral replication cycle is B117L expressed?

Time course experiments in swine macrophage cultures infected with ASFV strain Georgia (ASFV-G) demonstrate that B117L is transcribed as a late gene during the viral replication cycle. RNA transcription analysis detected B117L expression beginning at 4 hours post-infection (hpi) and remaining stable until 24 hpi . This expression pattern closely resembles that of the well-characterized late protein p72 (B646L) rather than early proteins such as p30 (CP204L) . The late expression timing aligns with B117L's proposed functional role during viral entry in subsequent infection cycles.

Where does the B117L protein localize within infected cells?

Ectopic transient cell expression studies using B117L as a green fluorescent protein (GFP) fusion protein revealed colocalization with markers of the endoplasmic reticulum (ER) . Further analysis of intracellular localization using various B117L constructs showed patterns consistent with the formation of organized smooth ER (OSER) structures . This localization pattern is compatible with the presence of a single transmembrane helix with a cytoplasmic carboxy terminus, suggesting B117L integrates into ER membranes in a specific orientation that may be crucial for its function .

How conserved is the B117L gene across different ASFV isolates?

Evolutionary analysis has demonstrated high conservation of the transmembrane domain during the evolution of the B117L gene . This conservation indicates that the integrity of this domain is preserved by the action of purifying selection, suggesting functional importance . Analysis of recombination during B117L evolution revealed potential break points at nucleotides 87 and 170, with topology incongruences among different gene segments . Notably, the absence of potential breakpoints at the membrane domain further emphasizes the preservation of this domain during evolution, highlighting its essential role in viral biology.

What methodologies are most effective for studying B117L membrane interactions?

Research on B117L membrane interactions employs multiple complementary approaches. Hydrophobicity distribution analysis provides initial insights into the protein's potential membrane-associating regions . Transient expression of GFP-tagged B117L constructs allows visualization of cellular localization and membrane integration patterns . For detailed analysis of membrane-permeabilizing activities, synthetic peptides representing the membrane domain can be used in artificial membrane systems to measure ion conductance and pore formation, particularly under varying pH conditions that mimic endosomal environments . These methods can be supplemented with mutagenesis studies targeting key residues in the transmembrane helix to establish structure-function relationships. Computational modeling of protein-membrane interactions can further predict how B117L integrates into lipid bilayers.

Why have attempts to create B117L gene deletion mutants been unsuccessful?

Despite using established methods that successfully deleted other individual ASFV genes, attempts to delete the B117L gene from the genome of the parental ASFV strain Georgia yielded only mixed virus populations rather than pure recombinant viruses lacking B117L . This observation suggests that B117L likely provides essential functions for viral replication that cannot be completely complemented by other viral proteins. The partial success in obtaining mixed populations indicates that some aspects of B117L function might be complemented by the presence of B117L in other viral parental genomes within the mixed population . This finding indirectly supports the critical nature of B117L for ASFV viability and highlights the challenges in creating gene deletion mutants for essential viral components.

What cell culture systems are optimal for studying B117L function?

For studying B117L function, primary swine macrophages represent the gold standard as they are natural host cells for ASFV infection . These cells allow for analysis of B117L expression kinetics and localization in a physiologically relevant context. Time course experiments in macrophage cultures infected with ASFV at controlled multiplicity of infection (MOI) enable precise tracking of B117L transcription relative to early and late viral genes . For protein localization studies, both infected macrophages and transfected cell lines expressing B117L constructs can be valuable. When establishing experimental systems, researchers should consider:

• Cell type specificity (primary macrophages vs. cell lines)

• Expression systems (viral infection vs. plasmid-based expression)

• Detection methods (antibody-based vs. fusion protein approaches)

• Temporal dynamics (early vs. late infection stages)

• Subcellular compartment markers for colocalization studies

Each approach offers distinct advantages depending on the specific research question being addressed.

What techniques are most suitable for analyzing B117L's membrane-permeabilizing activity?

Multiple complementary techniques can assess B117L's membrane-permeabilizing properties:

Research has demonstrated that the B117L transmembrane helix can establish ion channels in membranes particularly at low pH, suggesting activation during endosomal acidification . When designing experiments, researchers should include appropriate controls and consider physiologically relevant conditions that reflect the endosomal environment where B117L likely functions during viral entry.

How should evolutionary analyses of B117L be structured to identify functionally important regions?

To identify functionally important regions of B117L through evolutionary analysis, researchers should implement a multi-faceted approach:

• Sequence alignment of B117L across multiple ASFV isolates to identify conserved residues

• Calculation of selection pressures (dN/dS ratios) across different protein domains to identify regions under purifying selection

• Analysis of potential recombination events using algorithms like GARD to detect breakpoints, as previously identified at nucleotides 87 and 170

• Comparative topology analysis of different gene segments to identify incongruences that might indicate evolutionary constraints

• Focused analysis on the transmembrane domain, which shows high conservation suggesting functional importance

Special attention should be given to the membrane domain, which appears preserved during evolution due to its critical functional role. Combining evolutionary data with structural predictions and experimental functional results provides a comprehensive understanding of B117L's essential regions.

What approaches can determine if B117L is essential for ASFV replication?

Several approaches can help determine whether B117L is essential for ASFV replication:

• Gene deletion attempts: Previous efforts to delete B117L resulted in mixed virus populations rather than pure deletion mutants, suggesting essentiality . This approach can be refined using inducible systems or complementation strategies.

• Conditional expression systems: Developing ASFV strains with B117L under control of inducible promoters would allow controlled reduction of expression to assess impact on viral replication.

• Dominant-negative mutants: Expressing modified B117L variants that interfere with wild-type function could reveal dependency on specific protein domains.

• Small interfering RNA (siRNA) or CRISPR interference: Targeted reduction of B117L expression during infection could reveal replication defects.

• Complementation analysis: Providing B117L in trans while attempting to delete the genomic copy could distinguish between cell-autonomous and trans-complementable functions.

The evidence suggesting mixed populations during deletion attempts indicates B117L likely provides critical functions that cannot be entirely complemented by other viral proteins . Each experimental approach has advantages and limitations, necessitating multiple strategies to conclusively establish essentiality.

How can researchers effectively study B117L's interactions with cellular membranes?

To study B117L interactions with cellular membranes, researchers can employ these methodological approaches:

• Fluorescence microscopy with tagged B117L constructs to visualize subcellular localization .

• Fractionation studies to biochemically separate membrane-bound from soluble B117L.

• Fluorescence resonance energy transfer (FRET) to detect protein-protein interactions within membrane complexes.

• Protease protection assays to determine topology of B117L in membranes.

• Lipid binding assays to assess preference for specific membrane compositions.

• Cryo-electron microscopy to visualize B117L integration into membranes at high resolution.

• Crosslinking studies to capture transient interactions with other membrane components.

Previous research has established B117L's localization to the ER and its ability to form organized smooth ER (OSER) structures . Further studies should focus on defining the precise orientation of the protein in membranes and identifying potential interacting partners that might regulate its membrane-permeabilizing activity during viral infection.

What protocols are recommended for analyzing B117L ion channel formation?

For analyzing B117L ion channel formation, researchers should consider these specialized protocols:

• Planar lipid bilayer electrophysiology: This technique allows direct measurement of ion conductance across membranes containing B117L or synthetic peptides representing its transmembrane domain . Experiments should test channel formation under varying pH conditions to mimic the endosomal environment.

• Liposome-based flux assays: Fluorescent dyes entrapped in liposomes can detect membrane permeabilization upon addition of B117L peptides or protein.

• Cell-based permeabilization assays: Cells expressing B117L can be exposed to membrane-impermeable dyes to assess increased membrane permeability.

• Ion selectivity measurements: Comparing conductance of different ions (H+, Na+, K+, Ca2+, Cl-) can determine channel selectivity properties.

• pH-dependent activation studies: Testing channel activity across pH ranges from neutral to acidic can establish activation thresholds relevant to endosomal entry.

Research has already demonstrated that B117L forms ion channels with weak ion selectivity, unlike the highly proton-selective influenza A/M2 channel . Further characterization should focus on determining the minimal peptide sequence needed for channel activity, identifying gating mechanisms, and establishing the stoichiometry of the functional channel complex.

What computational methods can predict B117L structure-function relationships?

Multiple computational approaches can help predict B117L structure-function relationships:

• Transmembrane domain prediction: Tools like TMHMM, Phobius, and MEMSAT can identify the transmembrane helix with high confidence .

• Secondary structure prediction: Programs like PSIPRED and JPred can predict helical content beyond the transmembrane region, identifying potential amphipathic helices.

• 3D structure prediction: AlphaFold or RoseTTAFold can generate structural models of B117L to visualize potential channel-forming configurations.

• Molecular dynamics simulations: Can model B117L integration into lipid bilayers and predict conformational changes under different pH conditions.

• Evolutionary coupling analysis: Co-evolving residues often indicate structural contacts, helping validate structural models.

• Homology detection: Despite no obvious homologs, sensitive profile-based methods might detect distant relationships with known channel proteins.

Hydrophobicity analysis has already confirmed a single transmembrane helix in B117L , while evolutionary analysis showed high conservation of this domain . Integrating computational predictions with experimental data will help generate testable hypotheses about specific residues involved in channel formation and gating.

How does recombination influence B117L gene evolution across ASFV isolates?

Analysis using the GARD algorithm has revealed evidence of recombination in B117L evolution, with two potential breakpoints identified at nucleotides 87 and 170 . This recombination pattern has several important implications:

• Recombination appears to introduce topology incongruences among different gene segments, suggesting distinct evolutionary pressures on different regions of the protein .

• The membrane domain lacks recombination breakpoints, indicating this region's importance is preserved during evolution regardless of recombination events elsewhere in the gene .

• Recombination may contribute to functional diversification while maintaining core activities, potentially enabling adaptation to different host environments.

• The improvement in the Akaike Information Criterion score (AIC) from 1678.90 for a single partition model to 1650.65 for a multiple breakpoints model statistically supports the role of recombination in B117L evolution .

What structural similarities exist between B117L and other viral membrane proteins?

B117L shares several structural similarities with membrane proteins from other viruses, particularly influenza A virus M2 protein :

• Both are single-pass, type III integral membrane proteins with similar domain organization.

• Both contain an N-terminal ectodomain, a middle transmembrane helix, and a C-terminal amphipathic helix.

• In both proteins, the transmembrane helix assembles the membrane channel crucial for function.

• Unlike the proton-selective A/M2, B117L shows weak discrimination for ions and small charged solutes, suggesting broader permeabilization functions .

• B117L is not inhibited by amantadine, an effective inhibitor of influenza A virus infection targeting A/M2 .

• The amino acid composition of the B117L pore-forming domain differs significantly from A/M2, explaining differential susceptibility to channel blockers .

These comparative analyses provide valuable insights into potential functional mechanisms and highlight distinctive features that might be targeted for specific anti-ASFV interventions. Understanding these similarities and differences can inform both basic research and applied antiviral development strategies.