Recombinant African swine fever virus Major structural protein p17 (Ken-119)

What is the structural role of p17 in ASFV assembly?

P17 is an essential and highly abundant structural transmembrane protein that plays a critical role in the assembly and maturation of the viral icosahedral capsid. This 17-kDa protein localizes to the capsid and inner lipid envelope of the virus particle. At the molecular level, p17 forms trimers positioned at the interface of the center gap region between three neighboring pseudo-hexameric capsomers . The protein closely associates with the base domain of the major capsid protein p72, with three copies of p17 encircling each p72 trimer capsomer in the inner capsid shell, effectively anchoring p72 capsomers to the inner membrane . This architectural arrangement is critical for proper virion structure and stability. Electron microscopy analyses have demonstrated that when p17 expression is repressed, viral morphogenesis is blocked at an early stage, immediately after the formation of viral precursor membranes, resulting in an abnormal accumulation of these precursors and delocalization of major capsid and core shell components .

Why is p17 considered essential for ASFV viability?

P17 is absolutely required for ASFV viability based on experiments with inducible virus systems. When the expression of p17 is repressed in an IPTG-dependent conditional mutant virus (v17i), viral replication is severely compromised, resulting in approximately 3 log units lower total virus production compared to permissive conditions . The essential nature of p17 is linked to several critical functions:

• It enables the progression of viral precursor membranes toward icosahedral particles

• It is required for the correct proteolytic processing of viral polyproteins pp220 and pp62, which are crucial for core assembly

• It maintains proper localization of capsid and core shell components

• It participates in the formation of large helicoidal structures from which immature viral particles are produced

These multiple functions collectively explain why p17 deficiency is lethal to the virus, as it disrupts fundamental aspects of virion morphogenesis and maturation.

How does p17 interact with cellular membranes during infection?

P17 demonstrates a strong affinity for endoplasmic reticulum (ER) membranes, even when expressed outside the context of ASFV infection. Confocal immunofluorescence studies of cells transfected with the D117L gene (encoding p17) show that the protein colocalizes extensively with the ER marker protein disulfide isomerase (PDI) . This intrinsic affinity for ER membranes is significant because the ER is the cellular compartment from which viral envelope precursors originate. During infection, p17 is targeted to these precursor viral membranes and subsequently becomes incorporated into intracellular viral particles as a component of the inner viral envelope . This targeting mechanism ensures that p17 is correctly positioned to facilitate the progression of membrane precursors toward assembled virions.

What experimental approaches are most effective for studying p17's role in ASFV morphogenesis?

To comprehensively investigate p17's role in ASFV morphogenesis, researchers should consider implementing a multi-faceted experimental approach:

• Inducible expression systems: Creating an IPTG-dependent conditional mutant virus, as demonstrated with v17i, allows controlled expression of p17 to study its impact on viral assembly at different stages . This approach enables temporal analysis of morphogenesis defects.

• Electron microscopy analysis: Both conventional and immunoelectron microscopy are invaluable for visualizing the ultrastructural defects that occur in the absence of p17. Serial ultrathin sectioning can reveal the presence of large helicoidal structures involved in particle formation .

• Immunoprecipitation and Western blotting: These techniques can be used to track polyprotein processing (pp220 and pp62) under permissive versus restrictive conditions, providing biochemical evidence of p17's role in maturation .

• Confocal immunofluorescence: This method helps determine the subcellular localization of p17 and its colocalization with cellular components like ER markers or other viral proteins .

• Quantitative growth curves: One-step growth curves comparing virus production under permissive and restrictive conditions can quantify the impact of p17 repression on viral replication .

• Metabolic labeling: [35S]methionine-[35S]cysteine labeling can be used to analyze viral protein synthesis patterns and identify specific defects in protein processing or accumulation .

These complementary approaches provide a comprehensive understanding of p17's multifaceted roles in ASFV morphogenesis.

How can researchers effectively analyze the impact of p17 on immune evasion mechanisms?

To investigate p17's role in immune evasion mechanisms, particularly its interaction with the cGAS-STING pathway, researchers should employ the following experimental approaches:

• Dual-luciferase reporter assays: These assays can measure the effect of p17 on promoter activations, including IFN-β, ISRE, and NF-κB, in response to stimulation of the cGAS-STING pathway .

• Co-immunoprecipitation (Co-IP): This technique is crucial for detecting protein-protein interactions between p17 and components of immune signaling pathways. Co-IP assays have demonstrated that p17 interacts with STING but not with TBK1 or IKKε proteins .

• RT-qPCR analysis: This method allows quantification of downstream cellular mRNA expressions of IFN-β, ISG15, ISG56, and IL-8 to evaluate the functional impact of p17 on interferon responses .

• Immunofluorescence assays (IFA): IFA can visualize the co-localization patterns between p17 and immune signaling components like STING. This approach has shown that p17 disrupts the co-localization between STING and TBK1/IKKε .

• Deletion mutant analysis: Creating deletion mutants of p17 (especially in the transmembrane domain at amino acids 39-59) can help identify specific regions responsible for STING interaction. Similarly, analyzing STING domain mutants can determine which regions (N-terminal domain AAs 1-190 and middle domain AAs 153-339) are required for p17 binding .

By implementing these approaches, researchers can thoroughly characterize p17's immunomodulatory functions and understand how ASFV exploits this protein to evade host innate immunity.

What are the optimal expression systems for producing recombinant p17 for functional studies?

For producing functional recombinant p17, researchers should consider several expression systems, each with distinct advantages depending on the experimental objectives:

• T7 RNA polymerase-based expression: This system has been successfully used to express p17 in mammalian cells using a plasmid containing the D117L ORF under the control of a T7 RNA polymerase promoter, combined with a recombinant vaccinia virus expressing T7 RNA polymerase . This approach is particularly useful for studying the subcellular localization of p17 in mammalian cells.

• Inducible viral systems: For studying p17's function in the context of viral infection, an IPTG-inducible system integrated into the ASFV genome (as in the v17i virus) allows controlled expression of p17 . This approach maintains the natural viral environment for p17 function while enabling temporal regulation of expression.

• Bacterial expression systems: For biochemical and structural studies requiring larger quantities of purified protein, prokaryotic expression systems can be used with appropriate consideration for the transmembrane nature of p17, potentially requiring the addition of solubilizing tags or membrane-mimicking environments.

When expressing p17, it's essential to account for its transmembrane nature and ensure proper folding and membrane integration. Including cytosine arabinoside when using vaccinia virus vectors can minimize interference from late gene expression of the vector . The expression level of p17 should be carefully controlled, as the data show that even under permissive conditions, the p72-derived inducible promoter used in v17i produces less p17 than the natural promoter, resulting in approximately 1 log unit lower virus yield compared to the parental virus .

What purification strategies are most effective for isolating recombinant p17 while maintaining its structural integrity?

Purification of a transmembrane protein like p17 presents significant challenges that require specialized approaches to maintain structural integrity and functionality:

• Detergent-based membrane protein extraction: Given p17's transmembrane nature, gentle detergents such as n-dodecyl-β-D-maltoside (DDM), n-octyl-β-D-glucopyranoside (OG), or digitonin can be used to solubilize the protein from membranes while preserving its native conformation.

• Affinity chromatography: Addition of affinity tags (His-tag, FLAG-tag) to recombinant p17 can facilitate purification while minimizing harsh conditions that might disrupt protein structure. The tag position should be carefully chosen to avoid interfering with the transmembrane domain (amino acids 39-59) that is critical for p17's interaction with STING .

• Size exclusion chromatography: This technique is valuable for separating p17 trimers from monomers or aggregates, as the protein has been shown to exist in a trimeric form in viral particles .

• Liposome reconstitution: For functional studies, purified p17 can be reconstituted into liposomes or nanodiscs to maintain its native membrane environment and structural integrity.

When evaluating purification success, it's important to assess not only protein purity but also structural integrity through techniques like circular dichroism, limited proteolysis, or functional binding assays with STING protein. Researchers should verify that purified p17 retains its ability to interact with STING and inhibit STING-dependent signaling pathways .

What imaging techniques are most effective for visualizing p17's arrangement in the viral capsid?

Several advanced imaging techniques can be employed to visualize p17's arrangement in the ASFV capsid:

• Cryo-electron microscopy (cryo-EM): This technique has been instrumental in identifying p17's position in the viral architecture, revealing that it forms trimers located at the interface of three neighboring pseudo-hexameric capsomers . Cryo-EM is particularly valuable for preserving the native structure of viral components without fixation artifacts.

• Electron tomography: This approach can provide three-dimensional information about p17's arrangement in the context of the complete virion, helping to understand how p17 encircles p72 capsomers and anchors them to the inner membrane .

• Immunoelectron microscopy: Using gold-labeled antibodies specific to p17, this technique can confirm the protein's localization in the viral capsid and quantify its abundance. Studies have shown a 94% reduction in labeling density for p17 in viral factories under restrictive conditions compared to the parental virus .

• Serial ultrathin sectioning: This method has revealed large helicoidal structures from which immature viral particles are produced, suggesting these structures represent previously undetected viral intermediates dependent on p17 function .

For optimal results, researchers should combine these imaging approaches with mutational analysis of p17 to correlate structural features with functional roles in viral assembly. Visualization of both wild-type and mutant forms of p17 can provide valuable insights into how specific domains contribute to capsid architecture and stability.

How do mutations in different domains of p17 affect its function in virion assembly?

The functional impact of mutations in p17 domains can be systematically analyzed through a combination of targeted mutagenesis and functional assays:

• Transmembrane domain mutations: Studies with deletion mutants in the transmembrane region (AAs 39-59) have shown that this domain is critical for p17's co-localization with STING . Specific deletions (Δ39-48, Δ44-53, Δ49-59, and Δ53-59) eliminate co-localization, while others (Δ39-43, Δ44-48, Δ49-53) maintain it, indicating that most sequences in this region, except amino acids 39-43, are indispensable for STING interaction .

• Structural impact assessment: Mutations in p17 can be evaluated for their effect on:

  • Viral particle formation using electron microscopy

  • Polyprotein processing via Western blotting for pp220/pp62 and their cleavage products

  • Virus replication kinetics through one-step growth curves

  • Localization to ER membranes and viral factories using immunofluorescence

• Interaction domain analysis: Co-immunoprecipitation assays with mutated forms of p17 can determine which domains are essential for interaction with STING and other viral proteins like p72 .

When creating mutations in p17, researchers should consider the protein's trimeric structure and its position at capsid protein interfaces. Point mutations that disrupt trimerization or interaction with p72 would likely have severe consequences for capsid assembly, while mutations affecting STING interaction might impair immune evasion without necessarily disrupting virion structure.

How does p17 modulate host immune responses through interaction with the cGAS-STING pathway?

P17 employs a sophisticated mechanism to modulate host immune responses through direct interaction with the cGAS-STING pathway:

• Inhibition of type I interferon production: Experimental evidence demonstrates that p17 significantly decreases the activation of IFN-β, ISRE, and NF-κB promoters stimulated by cGAS/STING signaling . This suppression occurs in both transfected 293T cells and porcine alveolar macrophages (PAMs), indicating a conserved mechanism.

• STING interaction mechanism: Co-immunoprecipitation assays have revealed that p17 directly interacts with STING but does not bind to TBK1 or IKKε . This interaction has significant functional consequences:

  • p17 inhibits the interaction between STING and TBK1

  • p17 blocks the interaction between STING and IKKε

  • Immunofluorescence analysis confirms that p17 disrupts the co-localization patterns between STING and TBK1/IKKε that normally appear as punctate structures

• Signaling target specificity: The inhibitory effect of p17 is specific to STING, TBK1, and IKKε-mediated activation but does not affect IRF3-5D-mediated promoter activation . This indicates that p17 acts upstream of IRF3 in the signaling cascade.

• Domain requirements: The interaction depends on specific domains:

  • In p17: The transmembrane domain (AAs 39-59) is critical

  • In STING: Both the N-terminal domain (AAs 1-190) and middle domain (AAs 153-339) are required

By targeting STING, p17 effectively blocks the cGAS-STING DNA sensing pathway, which is crucial for detecting invading ASFV and triggering antiviral responses. This immune evasion mechanism contributes to ASFV's ability to establish infection despite host defenses.

How can researchers quantitatively assess the impact of p17 on polyprotein processing?

To quantitatively assess p17's impact on polyprotein processing, researchers should implement a comprehensive analytical strategy:

• Western immunoblotting: This is the primary method for monitoring polyprotein processing by detecting both precursors and mature products. Researchers should use:

  • Antibodies specific for polyprotein pp220 and its mature product p150

  • Antibodies specific for polyprotein pp62 and its mature product p35

  • Quantitative densitometry to measure the ratio of precursor to processed forms

• Pulse-chase analysis: This technique can provide temporal information about the processing kinetics:

  • Pulse-label infected cells with [35S]methionine-[35S]cysteine

  • Chase with non-radioactive medium for varying time periods

  • Immunoprecipitate polyproteins and their products

  • Quantify the conversion rates from precursors to mature forms

• Comparative analysis framework:

  • Compare processing in cells infected with wild-type virus (positive control)

  • Compare with cells infected with the inducible virus under permissive conditions (IPTG present)

  • Compare with cells infected with the inducible virus under restrictive conditions (IPTG absent)

• Correlation with morphogenesis: Link processing efficiency to virus assembly by:

  • Performing electron microscopy to visualize morphological defects

  • Measuring virus production through one-step growth curves

  • Correlating processing inhibition percentages with reduction in virus titers

The data show that under restrictive conditions (p17 repression), the proteolytic cleavage of both core precursors is severely impaired, indicating that p17 expression is required for correct polyprotein processing . This observation suggests that at least some of the processes for which p17 is essential occur before or during core assembly, when polyprotein processing is believed to take place.

How can understanding p17's function inform the development of attenuated ASFV vaccines?

The essential role of p17 in ASFV viability and morphogenesis provides valuable insights for rational attenuated vaccine design:

• Targeted attenuation strategies: Rather than completely deleting p17, which would be lethal to the virus, researchers might:

  • Create conditional expression systems that produce suboptimal levels of p17

  • Introduce specific mutations that partially compromise p17 function without eliminating it

  • Develop temperature-sensitive p17 mutants that function at reduced temperatures but not at porcine body temperature

• Dual-function attenuation: Since p17 plays roles in both virion assembly and immune evasion, mutations could be designed to:

  • Maintain sufficient assembly function for virus production

  • Disrupt immune evasion functions, particularly STING interaction domains

  • Result in a virus that replicates at reduced levels while eliciting stronger innate immune responses

• Combination approaches: P17 modifications could be combined with alterations to other viral proteins:

  • Similar to the approach with MGF505-7R, where deletion led to higher production of IL-1β and IFN-β compared to parental strains

  • Creating a virus with multiple attenuating mutations that collectively reduce virulence

• Safety assessment framework: Attenuated viruses with p17 modifications should be evaluated for:

  • Genetic stability through multiple passages

  • Reversion potential under selective pressure

  • Growth kinetics in porcine macrophages

  • Virulence in piglet models compared to parental strains

Understanding the mechanisms by which p17 contributes to viral assembly and immune evasion provides rational targets for attenuating ASFV while maintaining immunogenicity. The observation that some modifications allow limited viral replication while reducing virulence suggests potential paths to developing live attenuated vaccines against this devastating disease.

What are the most promising approaches for targeting p17 in antiviral development?

Given p17's essential roles in ASFV replication, it presents several promising targets for antiviral development:

• Small molecule inhibitors of p17-STING interaction: The defined interaction between p17 and STING represents a druggable target. Compounds that:

  • Bind to the p17 transmembrane domain (AAs 39-59)

  • Interfere with p17's ability to inhibit STING-TBK1/IKKε recruitment

  • Allow restoration of type I interferon signaling during infection

• Peptide-based inhibitors of p17 trimerization: Since p17 functions as trimers in the viral capsid:

  • Peptides mimicking interfaces between p17 monomers could prevent proper trimerization

  • Disrupting trimerization would interfere with p17's structural role in anchoring p72 capsomers

• Compounds targeting p17's role in polyprotein processing: Molecules that:

  • Prevent p17-dependent polyprotein processing

  • Block the proteolytic cleavage of pp220 and pp62

  • Inhibit core formation and virus maturation

• Screening strategies for p17 inhibitors:

  • Cell-based assays measuring STING-dependent IFN-β promoter activation in the presence of p17 and candidate inhibitors

  • In vitro binding assays assessing p17-STING interaction

  • Viral growth inhibition assays in porcine macrophages

  • Polyprotein processing assays monitoring pp220/pp62 cleavage

For maximum efficacy, antiviral approaches might combine p17 inhibitors with compounds targeting other essential viral proteins. Since p17 inhibition would affect viral assembly at an early stage, targeting this protein could prevent the formation of infectious virions before their release from infected cells.