Recombinant African swine fever virus Protein E248R (War-142)

What is the basic structure of ASFV E248R protein?

E248R is a late structural protein component of the African swine fever virus particle with several key structural features. The protein contains intramolecular disulfide bonds and has been identified as a substrate of the ASFV-encoded redox system . Its amino acid sequence contains a putative myristoylation site and a hydrophobic transmembrane region near its carboxy terminus . The protein is approximately 248 amino acids in length with a 28 amino acid extracellular region . Structurally, E248R shows approximately 16.2% identity and 30.7% similarity with the vaccinia virus (VACV) L1R fusion protein, suggesting potential evolutionary relationships and functional similarities between these viral proteins .

The distribution of cysteine residues in E248R compared to the related VACV protein L1R suggests that two disulfide bonds found in L1R between amino acids 28-62 and 122-156 could also be present in E248R . These disulfide bonds likely contribute to the tertiary structure and stability of the protein. As the final substrate of the ASFV-encoded redox system, E248R's structure is influenced by post-translational modifications that affect its function during viral infection . This structural arrangement positions E248R as a type I transmembrane protein with specific domains that facilitate its various functions in the viral replication cycle.

Where is E248R located within the ASFV virion and infected cells?

E248R demonstrates specific localization patterns that are crucial to its function during ASFV infection. The protein is myristoylated during infection and associates with the membrane fraction in infected cells, behaving as an integral membrane protein . Within the virion structure, E248R localizes specifically to the inner envelope of the virus particles in the cytoplasmic factories . This internal positioning is consistent with its role in post-entry events during viral infection.

The transmembrane domain near the C-terminus anchors E248R in the viral membrane, with the larger portion of the protein facing the interior of the virion . During viral assembly, E248R is incorporated into new virions at the virus assembly sites in the cytoplasm of infected cells. The specific localization of E248R to the inner viral membrane is particularly important for its functions in viral fusion and early infection events. This positioning allows E248R to interact with cellular endosomal proteins during viral entry and fusion processes, facilitating critical steps in establishing viral infection . Understanding this localization is essential for researchers designing experiments to study E248R's functions in the context of the complete viral replication cycle.

What are the primary functions of E248R in the ASFV life cycle?

The E248R protein serves multiple critical functions in the ASFV replication cycle, making it an essential viral component. Research has demonstrated that the infectivity of pE248R-deficient virions was reduced at least 100-fold compared to wild-type virions, highlighting its importance in viral infection . While E248R does not affect virus binding or internalization, it is required for some early post-entry events in the virus infection cycle . When E248R is absent, early and late gene expression is impaired, and the virus fails to induce cytopathic effects in host cells .

E248R has been implicated in viral fusion steps, particularly in the fusion of the viral inner membrane with the limiting membrane of late endosomes during viral entry . This function is critical for the release of the viral core into the cytoplasm, which is necessary for subsequent viral replication. Additionally, E248R interacts with cellular endosomal proteins, including NPC1 (Niemann-Pick C type 1) and Lamp2 (lysosomal membrane protein 2), suggesting its involvement in navigating the endosomal pathway during viral entry .

Beyond its structural and entry-related functions, E248R also plays a significant role in modulating host immune responses. The protein inhibits the secretion of IFN-β induced by cGAS-STING or HT-DNA in a dose-dependent manner and suppresses HT-DNA-induced transcription of IFN-b1, RANTES, IL-6, and TNF-α in porcine kidney (PK-15) cells . Through interaction with STING and inhibition of its expression, E248R effectively suppresses the host innate immune response, potentially facilitating viral immune evasion during infection .

How can researchers produce recombinant E248R protein for experimental studies?

Production of recombinant E248R protein requires careful consideration of the protein's structural features and post-translational modifications. Researchers should begin by selecting an appropriate expression system based on the experimental requirements. For structural studies requiring high purity and authentic folding, insect cell or mammalian expression systems are preferable due to E248R's disulfide bonds and myristoylation . For initial screening or antibody production, bacterial expression systems may be suitable for expressing segments that do not require post-translational modifications.

The gene sequence should be codon-optimized for the chosen expression system and cloned into a suitable expression vector containing appropriate purification tags (His-tag, GST-tag, etc.). When expressing E248R, researchers should consider including the myristoylation signal if studying the full functionality of the protein, as E248R is naturally myristoylated during infection . For membrane protein expression, detergent screening is essential to identify conditions that maintain protein stability and functionality during solubilization and purification steps.

Purification typically involves affinity chromatography based on the chosen tag, followed by size exclusion chromatography to ensure homogeneity. For proper folding and disulfide bond formation, inclusion of a redox buffer system may be necessary during protein refolding steps, especially if expression is performed in reducing environments like bacterial cytoplasm . Verification of the recombinant protein should include Western blotting, mass spectrometry analysis, and functional assays to confirm that the recombinant protein retains its native properties. For studies investigating E248R's interaction with host proteins such as NPC1 or STING, co-expression or in vitro binding assays can be designed using the purified components .

What methods are effective for studying E248R interactions with host proteins like NPC1 and STING?

Multiple complementary approaches can effectively characterize E248R interactions with host proteins. Co-immunoprecipitation (Co-IP) assays have been successfully employed to demonstrate interactions between E248R and cellular proteins like NPC1 and STING . For these experiments, researchers typically express epitope-tagged versions of E248R (such as FLAG or HA tags) in mammalian cells along with the potential interacting partners, followed by immunoprecipitation using antibodies against the tag or the partner protein.

Laser confocal microscopy provides spatial information about protein interactions by visualizing co-localization patterns. This technique has been used to confirm E248R interaction with STING and can reveal important details about where in the cell these interactions occur . For more detailed binding characterization, surface plasmon resonance (SPR) or isothermal titration calorimetry (ITC) can quantify binding affinities and kinetics between purified E248R and its binding partners.

Domain mapping experiments have proven valuable for identifying specific regions involved in protein interactions. For example, studies have shown that the C domain of NPC1 is the most important domain for interaction with E248R . This approach typically involves creating truncated protein constructs or point mutations and testing their interaction capabilities. Proximity ligation assays (PLA) offer another sensitive method for detecting protein-protein interactions in situ, providing spatial resolution of interactions within cellular compartments.

Functional validation of protein interactions can be accomplished using gene silencing (siRNA) or CRISPR-Cas9 knockout approaches. Research has demonstrated that silencing of NPC1 impairs ASFV infection, and NPC1 knockout cells show arrested progression and endosomal exit of viral cores, supporting the functional relevance of E248R-NPC1 interactions . Additionally, reporter assays measuring downstream signaling events (such as the dual-luciferase reporter assay used to show E248R inhibition of IFN-β secretion) provide functional evidence of protein interactions in a cellular context .

What advanced imaging techniques are recommended for visualizing E248R trafficking during ASFV infection?

Advanced imaging techniques provide crucial insights into E248R trafficking and function during ASFV infection. Live-cell imaging using fluorescently tagged E248R constructs offers real-time visualization of protein movement during infection. This approach requires careful design of fusion proteins to ensure the tag doesn't interfere with E248R function. For maximum resolution, super-resolution microscopy techniques such as Stimulated Emission Depletion (STED), Structured Illumination Microscopy (SIM), or Photoactivated Localization Microscopy (PALM) can visualize E248R localization beyond the diffraction limit of conventional microscopy.

Correlative light and electron microscopy (CLEM) combines the specificity of fluorescence labeling with the ultrastructural detail of electron microscopy, making it particularly valuable for studying E248R in the context of virus assembly sites and viral factories. This technique can precisely localize E248R within viral particles at the inner envelope . For studying dynamic interactions between E248R and endosomal proteins during viral entry, Förster Resonance Energy Transfer (FRET) or Bimolecular Fluorescence Complementation (BiFC) techniques allow visualization of protein-protein interactions in living cells.

Time-course immunofluorescence microscopy has been successfully employed to track the progression of viral cores and their exit from endosomes, which is particularly relevant given E248R's role in post-entry events . By fixing cells at various time points after infection and staining for E248R along with endosomal markers, researchers can trace the protein's journey through the endocytic pathway. To visualize the impact of E248R on endosomal morphology and function, researchers can use pH-sensitive fluorescent probes that report on endosomal acidification, which is a critical step preceding viral fusion.

Pulse-chase experiments combined with imaging can reveal the synthesis, transport, and turnover rates of E248R during infection. This approach typically involves metabolic labeling of newly synthesized proteins followed by immunoprecipitation and detection at various time points, providing insights into the protein's lifecycle within infected cells. For all these techniques, appropriate controls are essential, including uninfected cells, cells infected with E248R-deficient viruses, and competitive binding controls to confirm the specificity of observed signals.

How does E248R contribute to ASFV membrane fusion during entry?

E248R plays a critical role in ASFV membrane fusion during viral entry through multiple mechanisms. Located at the inner envelope of virus particles, E248R becomes exposed following the dissolution of the viral capsid in acidified endosomes . Once exposed, the protein participates in the fusion of the viral inner membrane with the limiting membrane of late endosomes, which is essential for releasing the viral core into the cytoplasm for subsequent replication . The structural similarities between E248R and the vaccinia virus L1R fusion protein (16.2% identity and 30.7% similarity) suggest potential functional conservation in membrane fusion mechanisms across different large DNA viruses .

The interaction between E248R and endosomal proteins appears to be crucial for the fusion process. Research has demonstrated that E248R interacts specifically with the cellular endosomal protein NPC1, with the C domain of NPC1 being the most important domain for this interaction . This interaction may facilitate the positioning and orientation of viral and endosomal membranes for efficient fusion. In NPC1 knockout cells, the progression and endosomal exit of viral cores is arrested, with viral particles retained within endosomes, indicating that this interaction is functionally significant for viral entry .

E248R's contribution to membrane fusion may also involve its biochemical properties. The protein contains intramolecular disulfide bonds and undergoes myristoylation, both of which could influence membrane interactions and fusion capacity . The myristoylation may enhance membrane association, while the disulfide bonds likely stabilize a conformation conducive to fusion. The transmembrane domain near the C-terminus of E248R anchors the protein in the viral membrane, positioning it appropriately for fusion events . When E248R is absent or defective, virions show drastically reduced infectivity despite normal morphology and ability to exit cells, indicating that the protein's fusion-related functions are essential for establishing productive infection .

What is the relationship between E248R and endosomal proteins in facilitating viral entry?

The interaction between E248R and endosomal proteins represents a sophisticated mechanism facilitating ASFV entry into host cells. Immunoprecipitation studies have demonstrated that E248R specifically interacts with the endosomal proteins NPC1 and Lamp2, but not with NPC2 or PIKfyve . Among the domains of NPC1, the C domain shows the strongest interaction with E248R, suggesting a specific binding interface that may be crucial for viral entry . These interactions likely occur during the endosomal trafficking of internalized virions and facilitate the membrane fusion step necessary for viral genome release into the cytoplasm.

The functional significance of these interactions is supported by experimental evidence using chemical compounds, silencing RNAs, and CRISPR knockout cells. In NPC1 knockout cells, ASFV infection is jeopardized, and viral cores remain trapped within endosomes, unable to continue the infection process . Similarly, silencing of NPC1, Lamp1, and Lamp2 impairs ASFV infection, highlighting the importance of these proteins for successful viral entry . These observations suggest that E248R's interaction with endosomal proteins is not merely associative but functionally essential for viral penetration into the cytoplasm.

The mechanistic model emerging from these findings suggests that as ASFV particles progress through the endocytic pathway, acidification triggers conformational changes in the virion structure, exposing the inner viral membrane containing E248R . The exposed E248R then engages with NPC1 and Lamp proteins in the limiting membrane of late endosomes, facilitating membrane fusion and core release. This process bears conceptual similarities to the entry mechanisms of other viruses that require endosomal protein interactions, such as Ebola virus, which also utilizes NPC1 as a receptor .

Beyond facilitating fusion, these interactions may also influence endosomal maturation or trafficking, potentially creating an optimal environment for viral uncoating and genome release. The impact of E248R-endosomal protein interactions extends beyond the entry phase, as evidenced by the observation that reductions in ASFV infectivity and replication in NPC1 knockout cells are accompanied by fewer and smaller viral factories . This suggests potential roles for these interactions in establishing sites for viral replication following successful entry.

How does E248R modulate host immune responses during ASFV infection?

E248R exhibits significant immunomodulatory functions that help ASFV evade host immune defenses. Research has demonstrated that E248R protein inhibits the secretion of IFN-β induced by cGAS-STING pathway activation or HT-DNA stimulation in a dose-dependent manner . This inhibition represents a direct interference with one of the primary innate immune response pathways against viral infections. Through dual-luciferase reporter assay systems, researchers have shown that increasing concentrations of E248R progressively reduce IFN-β production, indicating a potent dose-dependent immunosuppressive effect .

At the transcriptional level, E248R overexpression inhibits HT-DNA-induced transcription of multiple immune-related genes, including IFN-b1, RANTES, IL-6, and TNF-α in porcine kidney (PK-15) cells . This broad suppression of pro-inflammatory cytokines and chemokines suggests that E248R targets common regulatory mechanisms in innate immune signaling pathways. The molecular mechanism underlying this immune suppression involves direct interaction between E248R and STING (Stimulator of Interferon Genes), a critical adaptor protein in DNA-sensing pathways . Co-immunoprecipitation assays and laser confocal microscopy have confirmed this interaction, providing physical evidence for the molecular basis of E248R's immunomodulatory function .

Beyond merely binding to STING, E248R actively reduces STING protein expression levels, as demonstrated by Western blotting analysis . This reduction in STING protein levels effectively dismantles the signaling platform required for downstream activation of innate immune responses, including IRF3 phosphorylation and nuclear translocation. By targeting STING for downregulation, E248R creates a significant gap in the host's ability to detect and respond to viral DNA, allowing ASFV to replicate more efficiently in an immunologically permissive environment. This immune evasion strategy represents a sophisticated viral adaptation that contributes to ASFV pathogenesis and may partially explain the severe nature of ASF infections in susceptible hosts.

What experimental systems are best for studying E248R's impact on innate immune signaling pathways?

Several experimental systems provide valuable insights into E248R's effects on innate immune signaling. Cell-based reporter assays, particularly dual-luciferase reporter systems, offer quantitative measurement of E248R's impact on IFN-β promoter activity . These systems typically involve co-transfecting cells with an IFN-β promoter-driven luciferase reporter construct, a normalization reporter (such as Renilla luciferase), and varying amounts of E248R expression plasmid. The dose-dependent relationship between E248R expression and reporter activity can then be quantified, providing a clear readout of the protein's immunosuppressive potency.

For investigating molecular interactions, co-immunoprecipitation combined with Western blotting has successfully demonstrated E248R's interaction with STING . This approach requires antibodies against both proteins (or epitope tags if using recombinant constructs) and can be performed in relevant cell lines such as porcine alveolar macrophages (natural ASFV hosts) or more experimentally tractable cells like PK-15. To visualize these interactions in a cellular context, confocal microscopy with fluorescently labeled proteins provides spatial information about where in the cell E248R encounters immune signaling components .

Real-time quantitative PCR (RT-qPCR) analysis serves as an effective method for measuring E248R's impact on the transcription of immune-related genes. Studies have employed this technique to show that E248R overexpression inhibits HT-DNA-induced transcription of IFN-b1, RANTES, IL-6, and TNF-α . This approach requires careful selection of reference genes for normalization and specific primers for target immune genes. For a more comprehensive analysis, RNA-sequencing can reveal genome-wide transcriptional changes induced by E248R, potentially identifying additional immunomodulatory effects beyond the currently known targets.

To study E248R in the context of viral infection, recombinant ASFV systems with inducible or deleted E248R genes provide powerful tools . These systems allow researchers to compare wild-type virus with E248R-deficient variants, revealing the protein's contribution to viral replication and immune evasion in a complete infection model. For mechanistic studies of protein degradation, pulse-chase experiments combined with proteasome or lysosome inhibitors can determine whether E248R targets STING for degradation through specific cellular pathways, providing insights into the molecular mechanism of STING downregulation by E248R.

How might understanding E248R function contribute to ASFV vaccine development?

Understanding E248R's multiple functions offers several promising avenues for ASFV vaccine development. As a structural protein with essential roles in viral entry and immune evasion, E248R represents an attractive target for attenuated vaccine design. Recombinant ASFV strains with modified E248R genes could potentially retain immunogenicity while showing reduced virulence . Previous research has demonstrated that E248R-deficient virions show dramatically reduced infectivity (at least 100-fold decrease) despite maintaining normal morphology . This property could be exploited to create attenuated viruses that stimulate immune responses without causing severe disease.

The immunomodulatory functions of E248R present another opportunity for vaccine design. By understanding how E248R suppresses innate immune responses through STING inhibition, researchers could develop modified viruses with mutations in E248R that preserve antigenicity but reduce immune suppression . Such modifications might allow for stronger immune activation during vaccination, potentially leading to more robust and long-lasting protective immunity. Additionally, the identification of E248R as an antigen recognized by protective antibodies could inform subunit or vectored vaccine approaches focusing on this protein.

For subunit vaccine development, recombinant E248R protein or specific immunogenic epitopes could be incorporated into vaccine formulations. The proper folding of E248R, including its disulfide bonds, would be crucial for preserving conformational epitopes recognized by neutralizing antibodies . Vectored vaccines using viral vectors expressing E248R, potentially in combination with other ASFV immunogens, represent another approach. Understanding E248R's interactions with host proteins like NPC1 and STING provides insight into potential adjuvant strategies that might enhance immune responses to E248R-based vaccines .

Comprehensive knowledge of E248R's structure-function relationships enables rational design approaches for next-generation ASFV vaccines. By mapping the specific domains responsible for viral entry versus immune suppression, researchers could potentially design modified proteins that eliminate virulence and immune evasion properties while maintaining protective antigenic features. Furthermore, understanding E248R's role in the context of different ASFV genotypes could inform the development of broadly protective vaccines capable of addressing the genetic diversity of circulating ASFV strains.

What experimental considerations are important when evaluating E248R as a target for antiviral development?

When evaluating E248R as a target for antiviral development, researchers must address several key experimental considerations. Assay development for high-throughput screening requires establishing robust readouts of E248R function. Entry inhibition assays could utilize recombinant viruses expressing reporter genes (such as GFP or luciferase) to quantify the impact of candidate inhibitors on E248R-mediated entry steps . For targeting E248R's immune suppression functions, reporter systems measuring STING-dependent signaling pathways offer appropriate screening platforms .

The structural characterization of E248R, particularly its disulfide bonding pattern and interaction interfaces with host proteins, is essential for structure-based drug design approaches . Techniques such as X-ray crystallography, cryo-electron microscopy, or NMR spectroscopy could provide detailed structural information to guide rational inhibitor design. The interaction between E248R and the C domain of NPC1 represents a particularly promising target for inhibitor development, as disrupting this interaction has been shown to impair viral infection .

For animal model testing of E248R-targeted antivirals, considerations regarding species-specific differences in host protein interactions are critical. While domestic pigs and wild boars are the natural hosts for ASFV, establishing laboratory animal models that accurately recapitulate E248R's interactions with host proteins may require careful validation . Cell culture systems derived from porcine cells, particularly primary macrophages, offer relevant preliminary screening platforms before advancing to more complex animal studies.

Resistance development poses another important consideration for antiviral targeting of E248R. Screening for resistance mutations through serial passage of ASFV in the presence of subinhibitory concentrations of candidate compounds can identify potential resistance pathways and inform combination therapy strategies. Additionally, evaluating the conservation of E248R across different ASFV isolates can help predict the breadth of protection offered by E248R-targeted antivirals, with highly conserved regions representing more stable therapeutic targets.

The pharmacokinetic and pharmacodynamic properties of E248R-targeted compounds must be carefully assessed, especially considering the tissue tropism of ASFV for macrophages and other immune cells. Delivery systems that effectively target these cell populations might enhance the efficacy of E248R inhibitors. Finally, validation studies using recombinant ASFV with mutations in E248R can provide critical proof-of-concept evidence for the specificity of candidate inhibitors, confirming that observed antiviral effects result from specific targeting of E248R functions rather than off-target activities.

How does E248R compare structurally and functionally to similar proteins in other viruses?

E248R shares notable structural and functional similarities with proteins from other large DNA viruses, particularly those involved in membrane fusion and entry. Sequence analysis has revealed that E248R shows approximately 16.2% identity and 30.7% similarity with the vaccinia virus (VACV) L1R fusion protein . This homology suggests potential evolutionary relationships and functional conservation between poxvirus and asfivirus entry mechanisms. The distribution of cysteine residues in E248R compared to L1R indicates that the two disulfide bonds found in L1R between amino acids 28-62 and 122-156 might also be present in E248R, suggesting similar structural constraints and folding patterns .

The dual functionality of E248R in both viral entry and immune evasion represents an elegant example of viral protein multitasking, a common feature in viruses with constrained genome sizes. This functional economy, where a single protein serves multiple critical roles in the viral life cycle, enables ASFV to maximize its genetic efficiency. The evolutionary conservation of E248R across ASFV isolates further underscores its essential functions and potential as a target for broad-spectrum interventions against this important agricultural pathogen.

What can comparative genomics reveal about E248R conservation across ASFV strains?

Comparative genomics analyses provide valuable insights into E248R conservation across diverse ASFV strains, informing both basic virology research and applied vaccine development. The E248R gene (E248R) is highly conserved among sequenced ASFV isolates, suggesting strong evolutionary pressure to maintain its structure and function . This conservation spans different genotypes and isolates from various geographical regions and time periods, indicating that E248R performs essential functions that cannot tolerate substantial sequence variation without compromising viral fitness.

Phylogenetic analysis of E248R sequences can provide insights into ASFV evolutionary history and transmission patterns. By examining the patterns of sequence conservation and variation in E248R alongside other viral genes, researchers can reconstruct the evolutionary relationships between different ASFV isolates and potentially track the spread of the virus across different geographical regions. This information is particularly valuable for understanding the epidemiology of recent ASFV outbreaks and identifying the sources of emerging strains.