Recombinant African swine fever virus Protein E248R (Ken-144)

What is the structural composition of ASFV protein E248R?

E248R is a type I transmembrane protein with distinct structural domains that contribute to its function in viral pathogenesis. The protein contains an N-terminal domain oriented toward the interior of the viral particle, a transmembrane domain (194SAVFK), and a C-terminal extracellular region of approximately 28 amino acids . E248R is enriched with cysteine residues that facilitate the formation of disulfide bonds, which are critical for its structural stability and function. Comparative analysis with related viral fusion proteins indicates that E248R shares 16.2% identity and 30.7% similarity with the vaccinia virus (VACV) L1R fusion protein . The distribution of cysteine residues suggests that E248R may form disulfide bonds similar to those found in L1R between amino acids 28-62 and 122-156 .

What are the primary functions of E248R in ASFV infection?

E248R serves as a crucial component in ASFV entry and fusion mechanisms. The protein facilitates viral membrane fusion with endosomal membranes, enabling viral core penetration into the host cell cytoplasm . As the final substrate of the ASFV-encoded redox system, E248R likely undergoes post-translational modifications that regulate its fusogenic activity . Research has demonstrated that E248R directly interacts with the host cell cholesterol transporter protein NPC1 (Niemann-Pick C type 1), particularly through its transmembrane domain binding to the C domain of NPC1 . This interaction appears to be analogous to that observed between Ebola virus glycoprotein and NPC1, suggesting a conserved mechanism of viral entry across different viral families .

How is E248R post-translationally modified, and what implications do these modifications have for protein function?

E248R undergoes significant post-translational modifications, most notably myristoylation, which contributes to its membrane-association properties . This fatty acid modification typically occurs at an N-terminal glycine residue (2GGSTSK7) and facilitates protein anchoring to cellular membranes . Additionally, as the final substrate of the ASFV-encoded redox system, E248R likely contains intramolecular disulfide bonds formed during viral maturation . These modifications collectively enhance the protein's stability and facilitate its proper localization at viral and cellular membranes, which is essential for mediating fusion events during infection. The specific arrangement of these modifications may also influence E248R's interaction with host factors such as NPC1, potentially modulating viral tropism and pathogenicity.

What techniques are most effective for expressing and purifying recombinant E248R protein?

For optimal expression and purification of recombinant E248R protein, a bacterial expression system using E. coli BL21(DE3) transformed with a pET-based vector containing the E248R gene sequence has proven effective. When expressing membrane proteins like E248R, consider the following protocol:

• Clone the E248R gene into a pET vector with an N-terminal His-tag to facilitate purification

• Transform into E. coli BL21(DE3)

• Culture in LB medium at 37°C until OD600 reaches 0.6-0.8

• Induce protein expression with 0.5-1.0 mM IPTG

• Reduce temperature to 16-18°C for overnight expression to improve protein folding

• Harvest cells and lyse in buffer containing detergents (0.5% Triton X-100 or 1% CHAPS)

• Purify using nickel affinity chromatography followed by size exclusion chromatography

For functional studies requiring proper protein folding and post-translational modifications, mammalian or insect cell expression systems may be preferable. HEK293T cells have been successfully used for expressing E248R with epitope tags (such as HA) for co-immunoprecipitation experiments with host factors like NPC1 .

What methods are recommended for studying E248R interactions with host cellular factors?

Several complementary approaches have proven effective for investigating E248R interactions with host factors:

• Co-immunoprecipitation (Co-IP): This technique has successfully demonstrated the interaction between E248R and NPC1. Expression of HA-tagged E248R and Flag-tagged NPC1 in HEK293T cells, followed by immunoprecipitation with anti-Flag antibodies and western blot detection with anti-HA antibodies, revealed their specific interaction .

• Domain mapping: Constructing deletion mutants of E248R (ΔExt, ΔTM, and ΔExt+TM) has helped identify the transmembrane domain as critical for NPC1 binding . Similarly, expressing individual domains of NPC1 (A, C, and I domains) demonstrated that E248R specifically interacts with the C domain of NPC1 .

• Reverse pull-downs: Performing reciprocal co-immunoprecipitations against HA-E248R using protein G-beads and specific monoclonal antibodies confirmed the interaction specifically with NPC1's C domain .

• Confocal microscopy: For studying co-localization of E248R with host factors during infection, immunofluorescence techniques with antibodies against E248R and cellular markers (e.g., Rab7 for late endosomes) can reveal spatial relationships during viral entry and replication.

• Proximity ligation assays: These provide higher sensitivity for detecting protein-protein interactions in situ with spatial resolution.

How can researchers effectively evaluate the role of E248R in membrane fusion?

To assess E248R's role in membrane fusion, researchers should employ a multi-faceted approach:

• Cell-cell fusion assays: Express E248R in one cell population and potential receptor proteins in another, then measure fusion events using fluorescent markers or reporter systems.

• Liposome fusion assays: Reconstitute purified E248R into liposomes and measure lipid mixing or content mixing with target liposomes containing appropriate receptor proteins.

• Site-directed mutagenesis: Systematically mutate cysteine residues or other conserved amino acids in E248R to identify critical residues for fusion activity.

• Inhibition studies: Use antibodies against specific domains of E248R or small molecule inhibitors targeting the NPC1-E248R interaction to assess their impact on fusion events.

• Recombinant viruses: Generate ASFV mutants with inducible E248R expression to directly assess the protein's role in viral entry and fusion in the context of viral infection .

• Electron microscopy: Visualize fusion intermediates at the ultrastructural level during ASFV entry in the presence or absence of functional E248R.

What sequence variations exist in E248R across different ASFV isolates, and how might they impact protein function?

Analysis of E248R sequences across various ASFV isolates reveals both conserved and variable regions that may influence protein function and viral pathogenicity. The transmembrane domain and cysteine residues involved in disulfide bond formation tend to be highly conserved (>80% identity), suggesting their critical importance for basic viral functions including membrane fusion . In contrast, regions of the protein exposed to the host immune system, particularly in the extracellular domain, show greater variability.

The Ken-144 strain's E248R protein contains specific amino acid substitutions that may affect:

• Receptor binding affinity

• Fusion kinetics

• pH threshold for activation

• Thermal stability

• Susceptibility to neutralizing antibodies

These variations could contribute to differences in viral tropism, pathogenicity, and immune evasion strategies across ASFV isolates. Comparative studies examining fusion efficiency between isolates with different E248R sequences would provide valuable insights into the functional consequences of these genetic variations.

How can researchers design effective gene editing strategies to study E248R function?

For comprehensive functional analysis of E248R, researchers should consider the following gene editing approaches:

• CRISPR-Cas9 system for host factor manipulation: Generate knockout cell lines for host interaction partners like NPC1 and NPC2 using CRISPR-Cas9 technology. This approach has successfully demonstrated reduced ASFV infectivity in NPC1-KO cells .

• Recombinant ASFV with inducible E248R expression: Develop viral systems where E248R expression can be controlled through inducible promoters, allowing for temporal regulation of protein expression during different stages of infection .

• Site-directed mutagenesis: Design comprehensive mutagenesis studies targeting:

  • Cysteine residues involved in disulfide bonding

  • The myristoylation site (2GGSTSK7)

  • Transmembrane domain residues that interact with NPC1

  • Conserved residues identified through comparative analysis with VACV L1R

• Domain swapping experiments: Create chimeric proteins by swapping domains between E248R and homologous proteins from other viruses (e.g., VACV L1R) to identify functional conservation and specificity.

• Reporter fusion systems: Engineer E248R fused to fluorescent or enzymatic reporters to track protein localization and interactions in real-time during infection.

Each approach should include appropriate controls and be complemented with functional assays to determine the impact of genetic modifications on viral entry, replication, and pathogenesis.

What is known about the interaction between E248R and NPC1, and how does this compare to similar interactions in other viruses?

E248R establishes a direct interaction with the cholesterol transporter protein NPC1 through specific structural domains. Experimental evidence demonstrates that:

• E248R binds to NPC1 through its transmembrane domain, as deletion mutants lacking this domain failed to interact with NPC1 in co-immunoprecipitation assays .

• The interaction occurs specifically with the C domain of NPC1, the same domain targeted by Ebola virus (EBOV) glycoprotein during entry .

• This interaction appears mechanistically similar to that observed between vaccinia virus (VACV) L1R protein and NPC1, suggesting a conserved viral entry strategy across different viral families .

Comparative analysis with other viral fusion systems reveals intriguing parallels:

These similarities suggest that targeting the viral protein-NPC1 interaction could potentially offer broad-spectrum antiviral strategies effective against multiple viral families.

How does the absence of NPC1 or NPC2 affect ASFV infection, and what compensatory mechanisms exist?

The absence of NPC1 or NPC2 significantly impacts ASFV infection through various mechanisms:

• NPC1 knockout effects:

  • CRISPR-Cas9-generated NPC1 knockout Vero cells showed significantly reduced ASFV infection levels

  • Infection reductions were accompanied by smaller viral factories lacking the typical cohesive morphology between endosomes and viral proteins

  • The phenotype resembled cholesterol accumulation in dilated acidic vesicles, similar to cells treated with the NPC1 inhibitor U18666A

• NPC2 involvement and compensation:

  • NPC1 knockout cells showed elevated NPC2 expression levels, suggesting a compensatory mechanism

  • Silencing NPC2 in wild-type cells reduced ASFV infection

  • Combined NPC1 knockout and NPC2 silencing resulted in enhanced inhibition of viral infection compared to either condition alone

• Mechanistic implications:

  • The data suggests both proteins contribute to ASFV entry and fusion processes

  • While NPC1 directly interacts with viral proteins E248R and E199L, NPC2 may facilitate this interaction by promoting proper cholesterol distribution

  • The compensatory upregulation of NPC2 in NPC1-deficient cells indicates interconnected regulatory pathways

This compensatory relationship between NPC1 and NPC2 highlights the complexity of host-pathogen interactions and suggests that targeting both proteins simultaneously might provide more effective antiviral strategies than targeting either one alone.

What methodologies are effective for identifying new host factors that interact with E248R?

To comprehensively identify novel host factors interacting with E248R, researchers should employ complementary approaches:

• Proximity-based proteomics:

  • BioID or APEX2 fusion with E248R to identify proximal proteins in living cells

  • These techniques involve fusing E248R to a biotin ligase that biotinylates nearby proteins, which can then be purified and identified by mass spectrometry

• Affinity purification coupled with mass spectrometry (AP-MS):

  • Express epitope-tagged E248R in relevant cell types

  • Perform immunoprecipitation under various conditions (different detergents, crosslinking)

  • Identify co-precipitating proteins using sensitive mass spectrometry approaches

  • Validate top candidates with orthogonal methods such as co-immunoprecipitation

• Yeast two-hybrid screening:

  • Use E248R domains as bait to screen human cDNA libraries

  • Perform directed tests against specific candidate proteins

  • Consider membrane yeast two-hybrid systems for improved detection of transmembrane protein interactions

• CRISPR screens:

  • Genome-wide or targeted CRISPR screens to identify host factors whose absence affects ASFV infection

  • Compare results with similar screens for other viruses that utilize NPC1 for entry

• Computational approaches:

  • Structural modeling of E248R to predict potential interaction interfaces

  • Sequence-based prediction of protein-protein interaction motifs

  • Network analysis integrating known viral-host protein interactions

Validation of identified interactions should include:

• Reciprocal co-immunoprecipitation experiments

• Domain mapping to identify specific interaction regions

• Functional assays to assess biological relevance

• Localization studies to confirm spatial and temporal co-occurrence

How might understanding E248R structure and function contribute to ASFV vaccine development?

E248R's critical role in ASFV entry and its conservation across viral isolates make it a promising target for vaccine development. Research insights can contribute to vaccine strategies through multiple approaches:

• Subunit vaccine development:

  • E248R protein or specific domains could serve as antigens in subunit vaccines

  • Focus on regions that elicit neutralizing antibodies targeting the fusion mechanism

  • Consider combining E248R with other structural proteins like E199L for broader protection

• Rational attenuation strategies:

  • Engineer recombinant ASFV with modified E248R to create attenuated strains

  • Mutations in the transmembrane domain that disrupt NPC1 binding could reduce virulence while maintaining immunogenicity

  • Temperature-sensitive mutations in E248R could restrict viral replication to specific tissues

• Epitope mapping for peptide vaccines:

  • Identify conserved, surface-exposed epitopes in E248R that elicit strong neutralizing antibody responses

  • Design peptide vaccines targeting these specific epitopes

  • Use structural information about E248R-NPC1 interaction to develop peptides that mimic interaction interfaces

• Vector-based vaccines:

  • Express E248R in viral vectors (adenovirus, modified vaccinia Ankara) to induce robust immune responses

  • Engineer chimeric proteins combining immunogenic domains from E248R with other ASFV proteins

• Adjuvant selection and optimization:

  • Pair E248R-based immunogens with adjuvants that enhance both humoral and cellular immunity

  • Investigate toll-like receptor agonists that complement the immune response to E248R

Understanding the structural aspects of E248R and its interaction with host factors provides crucial information for designing vaccines that specifically target and neutralize this essential viral function.

What methodologies are recommended for evaluating immune responses against E248R?

To comprehensively evaluate immune responses against E248R, researchers should employ the following methodologies:

• Antibody response characterization:

  • Enzyme-linked immunosorbent assays (ELISAs) using recombinant E248R protein to measure antibody titers

  • Epitope mapping through peptide arrays or phage display to identify immunodominant regions

  • Neutralization assays to assess whether antibodies block E248R-mediated fusion

  • Antibody-dependent cellular cytotoxicity (ADCC) assays to evaluate Fc-mediated immune functions

• T-cell response evaluation:

  • ELISpot assays to quantify E248R-specific T-cell responses

  • Intracellular cytokine staining to characterize T-cell polarization (Th1/Th2/Th17)

  • Cytotoxic T-lymphocyte (CTL) assays to assess killing of cells expressing E248R

  • MHC tetramer staining to identify and track E248R-specific T cells

• Structural analysis of immune recognition:

  • X-ray crystallography or cryo-EM of antibody-E248R complexes to understand recognition at atomic resolution

  • Hydrogen-deuterium exchange mass spectrometry to map conformational epitopes

  • Competition binding assays to characterize overlapping epitopes

• Systems immunology approaches:

  • Transcriptomics or proteomics of immune cells responding to E248R

  • Cytokine profiling to characterize the inflammatory response

  • Single-cell analysis to identify rare but potentially important immune cell populations

• In vivo protection assessment:

  • Challenge studies in appropriate animal models to correlate immune parameters with protection

  • Passive transfer experiments to determine the protective capacity of anti-E248R antibodies

  • T-cell depletion studies to assess the contribution of cellular immunity

These methodologies should be implemented in a coordinated manner to develop comprehensive immune correlates of protection against ASFV infection.

How does the E248R-NPC1 interaction affect endosomal trafficking during ASFV infection?

The interaction between E248R and NPC1 significantly influences endosomal trafficking during ASFV infection, with implications for viral entry and replication processes. Research findings indicate:

• ASFV internalization occurs via clathrin/dynamin-mediated endocytosis, with the virus progressing through the endosomal pathway where it encounters NPC1 in late endosomes .

• The binding of E248R to the C loop of NPC1 appears to trigger changes in endosomal membrane properties that facilitate viral fusion and core release into the cytoplasm .

• In NPC1 knockout cells, viral particles are retained within late endosomes and lysosomes, suggesting failed membrane fusion events . This retention correlates with smaller, morphologically abnormal viral factories lacking the typical cohesive structure between endosomes and viral proteins .

• The interaction may disrupt normal cholesterol trafficking mediated by NPC1/NPC2, potentially creating a modified lipid environment that favors viral fusion. This hypothesis is supported by the observation that both NPC1 knockout and NPC2 silencing reduce ASFV infection .

• Confocal microscopy of Rab7 (a late endosome marker) and ASFV p72 in wild-type versus NPC1 knockout cells reveals altered distribution patterns of viral components, suggesting that the E248R-NPC1 interaction influences the spatial organization of viral replication sites .

Future research should investigate whether E248R binding to NPC1 alters endosomal pH, calcium signaling, or recruitment of other host factors that may collectively create an optimal environment for viral fusion and genome release.

What is the molecular mechanism by which E248R and other ASFV proteins coordinate membrane fusion?

The molecular mechanism of ASFV membrane fusion likely involves a coordinated action of multiple viral proteins, with E248R playing a central role. Current evidence suggests the following model:

• Initiation phase:

  • E248R, together with E199L (another ASFV membrane protein), localizes to the inner viral membrane

  • Both proteins are enriched with cysteine residues capable of forming disulfide bonds, similar to the fusion machinery in vaccinia virus

  • E248R undergoes myristoylation, enhancing its membrane association properties

• Receptor engagement phase:

  • E248R specifically binds to the C domain of NPC1 through its transmembrane domain

  • E199L also interacts with NPC1, potentially stabilizing the fusion complex

  • These interactions may trigger conformational changes in both viral and endosomal membranes

• Fusion execution phase:

  • The transmembrane domains of E248R and E199L likely undergo structural rearrangements that bring the viral and endosomal membranes into close proximity

  • Disulfide bond isomerization, potentially catalyzed by the ASFV-encoded redox system, may regulate these conformational changes

  • The specific arrangement of cysteine residues in E248R, similar to those in VACV L1R, suggests a conserved fusion mechanism

This coordinated process results in lipid mixing between viral and endosomal membranes, formation of a fusion pore, and release of the viral core into the cytoplasm. The precise temporal coordination of these events and the contribution of additional viral or host factors remain important areas for future investigation.

How might variations in cellular NPC1/NPC2 expression levels across different host tissues influence ASFV tropism?

Variations in NPC1 and NPC2 expression levels across different host tissues may significantly impact ASFV tropism, potentially explaining some aspects of viral pathogenesis. This hypothesis is supported by several observations:

• Differential susceptibility:

  • Experimental evidence shows that NPC1 knockout cells are significantly less susceptible to ASFV infection

  • Similarly, NPC2 silencing reduces viral infectivity and replication

  • Tissues with naturally lower expression of these proteins might be inherently more resistant to infection

• Compensatory mechanisms:

  • NPC1 knockout cells show elevated NPC2 expression, suggesting a compensatory relationship

  • Tissues with varying ratios of NPC1:NPC2 expression might support different levels of viral replication

  • The combined effect of both proteins may create specific "permissiveness thresholds" in different tissues

• Impact on viral dissemination:

  • Cells with higher NPC1/NPC2 expression might serve as primary replication sites, producing higher viral titers

  • This could influence the kinetics and patterns of viral spread within the host

  • Macrophages, which are primary targets for ASFV, have specific endosomal/lysosomal characteristics that may optimize NPC1/NPC2 availability for viral entry

• Implications for pathogenesis:

  • Tissue-specific pathology in ASFV infection might correlate with NPC1/NPC2 expression patterns

  • The severe hemorrhagic symptoms characteristic of ASFV might be linked to infection of endothelial cells with particular NPC1/NPC2 expression profiles

Future research should systematically map NPC1 and NPC2 expression across tissues relevant to ASFV infection and correlate these patterns with viral replication efficiency and pathological outcomes. This approach could reveal new insights into the molecular basis of ASFV tissue tropism and inform targeted antiviral strategies.