Recombinant African swine fever virus Uncharacterized protein B117L (Pret-095)

What is the B117L protein of African swine fever virus?

B117L is a 115-amino-acid integral membrane protein encoded by the ASFV genome. It is transcribed late in the virus replication cycle and shows no homology to any previously published protein. The protein contains a single transmembrane helix and functions as a viroporin-like assistant during viral entry into host cells. Its structure includes an N-terminal small globular ectodomain followed by an amphipathic-strand-hydrophobic-helix-amphipathic-helix membrane domain spanning approximately 50 amino acids .

How conserved is the B117L gene across different ASFV strains?

Evolutionary analysis has demonstrated high conservation of the transmembrane domain during the evolution of the B117L gene. This conservation suggests that the integrity of this domain is preserved through purifying selection, indicating its functional importance in the viral life cycle. The high degree of conservation makes B117L a potential target for broad-spectrum antiviral strategies against different ASFV strains and isolates .

What structural features characterize the B117L protein?

The B117L protein exhibits a distinct structural organization with several key features:

• An N-terminal globular ectodomain

• A single transmembrane helix (TMH)

• A C-terminal amphipathic helix

• A membrane-associated C-terminal domain of approximately 50 amino acids

Hydrophobicity distribution analysis confirms the presence of the transmembrane helix, which combines with flanking amphipathic sequences to form the membrane-associated domain. This structural arrangement is crucial for the protein's localization and function in host cells .

What cellular localization patterns are observed for B117L, and how can researchers study this?

When expressed in cells as a green fluorescent protein (GFP) fusion protein, B117L colocalizes with markers of the endoplasmic reticulum (ER). Researchers can study this localization through ectopic transient cell expression systems followed by confocal microscopy with appropriate ER markers. The intracellular localization of various B117L constructs displays a pattern for the formation of organized smooth ER (OSER) structures, which is compatible with the presence of a single transmembrane helix with a cytoplasmic carboxy terminus .

What methodological approaches can be used to assess the membrane-permeabilizing activity of B117L?

Researchers can assess the membrane-permeabilizing activity of B117L using synthetic peptides representing the membrane domain sequence. These peptides can be tested for their ability to:

• Interact with artificial membrane systems

• Establish spores and ion channels in membranes, particularly at low pH

• Permeabilize membranes to ions and small molecules

Such experiments typically involve lipid vesicle-based assays, electrophysiological measurements, and fluorescent dye release assays to quantify membrane permeability under various conditions. These approaches have demonstrated that the B117L transmembrane helix can establish ion channels in membranes, particularly in acidic environments .

How can researchers attempt gene deletion studies for B117L?

Gene deletion studies for B117L have proven challenging. Previous attempts to delete the B117L gene from the ASFV Georgia strain resulted only in mixed virus populations rather than pure recombinant viruses lacking B117L. Researchers approaching this problem should:

• Use established methods for ASFV gene deletion

• Implement next-generation sequencing to verify recombination events

• Consider complementation strategies to overcome potential lethality

• Explore conditional expression systems to study essential genes

The difficulty in obtaining a pure deletion mutant suggests that B117L may have critical functions in the viral life cycle that cannot be fully complemented in mixed populations .

How does the pH-dependent activity of B117L relate to its role in ASFV entry?

The B117L transmembrane helix demonstrates enhanced capacity to establish ion channels in membranes specifically at low pH. This pH-dependent activity correlates with the conditions encountered during endosomal entry of ASFV, suggesting that B117L activity is triggered by the acidic environment of the endosome. This mechanism would allow the protein to remain inactive until the virus reaches the appropriate cellular compartment, then activate to facilitate membrane permeabilization and subsequent steps of viral entry. Researchers investigating this property should consider designing experiments that specifically examine protein function across a range of physiologically relevant pH values .

How might B117L interact with other ASFV proteins during viral entry and replication?

This advanced research question requires investigation of protein-protein interactions within the complex ASFV virion. Potential experimental approaches include:

• Co-immunoprecipitation studies with B117L and other structural proteins

• Proximity labeling techniques to identify interaction partners

• Cryo-electron microscopy of virions with immunogold labeling for B117L

• Yeast two-hybrid or mammalian two-hybrid screening

The large genome of ASFV encodes more than 150 genes, most with uncharacterized functions. Understanding how B117L interacts with other viral proteins would provide insight into the coordinated process of viral entry and the potential for developing targeted interventions .

What are the molecular determinants of B117L ion channel selectivity and conductance?

While B117L has been shown to form ion channels, the specific molecular determinants governing selectivity and conductance remain unknown. Researchers investigating this question should consider:

• Site-directed mutagenesis of conserved residues within the transmembrane domain

• Electrophysiological characterization of wild-type and mutant channels

• Molecular dynamics simulations to model ion flow through the channel

• Structural studies using NMR or X-ray crystallography of the transmembrane domain

Understanding these molecular details could provide targets for the design of specific inhibitors that block B117L activity and potentially inhibit ASFV infection .

How does B117L compare structurally and functionally to ion channels in other viruses?

A comparative analysis approach could include:

• Sequence alignment of B117L with known viral ion channels

• Structural modeling and comparison of transmembrane domains

• Functional assays comparing ion selectivity and inhibitor susceptibility

• Evolutionary analysis of conserved motifs across viral families

Such comparative studies would place B117L in the broader context of viral ion channels and potentially reveal conserved mechanisms that could be targeted for antiviral development .

What experimental approaches could determine if B117L is essential for ASFV replication?

The inability to generate pure B117L deletion mutants suggests this gene may be essential for viral replication. Researchers investigating this question should consider:

• Conditional expression systems where B117L can be regulated

• CRISPR interference approaches to reduce rather than eliminate expression

• Trans-complementation strategies providing B117L function from a separate genetic element

• Temperature-sensitive mutants that allow function at permissive but not restrictive temperatures

These approaches would help determine whether B117L is truly essential and under what conditions its function can be complemented or bypassed .

Could B117L serve as a target for antiviral development against ASFV?

Based on its apparent essential role and conserved nature, B117L represents a potential target for antiviral development. Researchers exploring this avenue should consider:

• High-throughput screening of small molecule libraries for inhibitors of B117L channel activity

• Structure-based drug design targeting the conserved transmembrane domain

• Evaluation of peptide-based inhibitors that might interfere with channel assembly

• Assessment of combination approaches targeting multiple viral components

What is the potential role of B117L in live-attenuated vaccine development for ASFV?

Live-attenuated vaccines represent a promising approach for ASFV control, but their development has been challenging. With respect to B117L:

• Complete deletion appears non-viable, suggesting mutation rather than deletion might be more appropriate

• Conditional mutations affecting channel function but not protein expression could create attenuated strains

• Engineering pH-dependence changes might create viruses that fail to enter cells efficiently

Recent attempts at developing live-attenuated ASFV vaccines have shown variable results. For example, recombinant ASFV strains Lv17/WB/Rie1-∆24 and Lv17/WB/Rie1/∆CD-∆GL induced mild clinical signs in pigs but failed to protect against virulent Armenia/07 challenge, highlighting the complexity of developing effective ASFV vaccines .