Recombinant African swine fever virus Major structural protein p17 (Ba71V-107)

What is ASFV protein p17 and what are its basic structural characteristics?

Protein p17 is an abundant structural protein encoded by the late gene D117L in the BA71V strain of African swine fever virus. It is a small (17-kDa) integral membrane protein with a highly conserved sequence among ASFV isolates . Structurally, p17 is predicted to insert into membranes with a Singer type I topology, functioning as a transmembrane protein . The protein contains a transmembrane domain between amino acids 39-59 that is critical for its functionality . Unlike many viral proteins, p17's sequence does not show significant similarity with sequences present in current databases, making it a unique component of ASFV .

Where is p17 localized within the mature ASFV particle?

Protein p17 has been localized to the viral internal envelope, which is the only membranous structure of the intracellular particles . Immunolocalization studies have detected p17 in envelope precursors as well as in both intracellular and extracellular mature particles . This consistent presence in the viral envelope across different stages of viral maturation suggests a fundamental structural role in maintaining envelope integrity throughout the viral life cycle.

What is the subcellular localization of p17 when expressed outside of viral infection?

When p17 is expressed in transfected cells outside the context of ASFV infection, it demonstrates an intrinsic affinity for endoplasmic reticulum (ER) membranes. Confocal immunofluorescence studies of cells transiently expressing p17 show that the fluorescence signal associated with anti-p17 antibody essentially colocalizes with ER-specific labeling (using protein disulfide isomerase as an ER marker) . This intrinsic affinity for the ER is believed to be related to p17's role in recruiting and modifying ER-derived membranes that serve as starting material for viral precursor membranes .

Why is p17 considered essential for ASFV viability?

Research using inducible expression systems has demonstrated that p17 is absolutely essential for virus viability . When p17 expression is repressed using an IPTG-dependent lethal conditional virus system, viral replication is completely inhibited. The essential nature of p17 is evidenced by several consequences of its repression:

• Blocking of the proteolytic processing of polyproteins pp220 and pp62

• Prevention of viral morphogenesis progression beyond the formation of viral precursor membranes

• Accumulation of viral precursor membranes at the viral factory

• Delocalization of major components of the capsid and core shell domains

These effects collectively result in a non-productive infection, confirming p17's status as an essential viral protein .

How can researchers generate and utilize ASFV recombinants with inducible p17 expression?

To study essential viral proteins like p17, researchers have developed inducible expression systems that allow controlled protein production. The methodology involves:

• Starting with a parental virus (vGUSREP) that constitutively expresses the E. coli lac repressor

• Constructing plasmids containing the D117L gene under control of an inducible promoter

• Generating the recombinant virus through homologous recombination

• Using IPTG as an inducer to control p17 expression

The detailed plasmid construction involves:

• Amplifying the flanking region from -516 to +10 relative to the translation initiation codon of D117L using PCR

• Mutating the initiation codon to prevent unwanted translation

• Inserting the PCR product into appropriate vectors

• Placing the D117L ORF under control of an inducible promoter derived from the promoter of late protein p72

This system enables researchers to study the function of p17 by comparing viral behavior under permissive (with IPTG) and restrictive (without IPTG) conditions.

What imaging techniques are most effective for analyzing p17's role in viral assembly?

Electron microscopy techniques, particularly transmission electron microscopy of ultrathin serial sections, have proven invaluable for studying p17's role in viral assembly. These approaches have revealed:

• The accumulation of viral precursor membranes in the absence of p17

• The formation of zipper-like structures outside viral factories when p17 is repressed

• The presence of previously undetected large helicoidal structures from which immature particles are produced

For protein localization studies, confocal immunofluorescence with double immunolabeling (using anti-p17 antibody and markers for cellular compartments) has been effectively used to demonstrate p17's targeting to the ER . Additionally, immunoprecipitation techniques are necessary to detect p17 expression levels, as the protein is not readily detectable in total cell extracts .

How does p17 contribute to ASFV immune evasion strategies?

Recent research has demonstrated that p17 plays a significant role in ASFV immune evasion by inhibiting the cGAS-STING signaling pathway . This pathway is a critical component of the innate immune response against DNA viruses. Functional studies show that p17 exerts a negative regulatory effect on cGAS-STING signaling and interferes with STING-dependent antiviral functions against other viruses like HSV1 and VSV .

The mechanism involves p17 interaction with STING (stimulator of interferon genes), which prevents STING from recruiting TBK1 and IKKε, essential components for downstream signaling . This interference ultimately suppresses the production of type I interferons and inflammatory cytokines, allowing the virus to evade host innate immunity.

Which domains of p17 are critical for its immunomodulatory functions?

Research has identified that the transmembrane domain of p17, spanning amino acids 39-59, is required for:

• Interaction with STING

• Inhibition of the cGAS-STING pathway

This finding suggests that the membrane-anchoring capacity of p17 is not only important for its structural role in viral morphogenesis but also for its function in immune evasion. The transmembrane domain likely facilitates p17's localization to the appropriate cellular compartments (ER and Golgi apparatus) where it can interact with STING and interfere with immune signaling .

How can researchers experimentally verify p17's impact on host immune responses?

To investigate p17's effect on host immune responses, researchers have employed several methodological approaches:

• RNA interference: Using p17-specific siRNA in ASFV-infected primary porcine alveolar macrophages (PAMs) to observe changes in immune-related gene expression

• Gene expression analysis: Measuring transcription levels of IFN-β, ISG15, ISG56, IL-6, and IL-8 in the presence or absence of p17

• Protein interaction studies: Investigating p17's interaction with STING and interference with TBK1 and IKKε recruitment

Results from these approaches demonstrate that when p17 expression is suppressed during ASFV infection, there is upregulation of interferon and inflammatory cytokine gene transcription, confirming p17's immunosuppressive role .

What novel viral assembly intermediates have been identified through p17 research?

Studies of p17's role in viral morphogenesis have unexpectedly revealed the presence of large helicoidal structures from which immature particles are produced . These structures represent a previously undetected viral intermediate in ASFV assembly. Through analysis of ultrathin serial sections of viral factories in cells infected with BA71V or the inducible virus under permissive conditions, researchers observed patterns of electron density that displayed constant length and width between sections .

When sectioned longitudinally, these structures produce three different and characteristic patterns that, when superposed, reveal two-dimensional projections of a two-strand helicoidal structure . This discovery suggests that during assembly, viral precursor membranes and core material organize into large helicoidal intermediates from which icosahedral particles emerge, adding a new level of complexity to our understanding of ASFV morphogenesis.

What aberrant structures form when p17 expression is repressed?

When p17 expression is repressed during ASFV infection, two main types of aberrant structures are observed:

• Accumulated viral precursor membranes within the viral factories, representing the arrest of morphogenesis at this early stage

• Zipper-like structures outside the viral factories, in tight association with ER cisternae

These zipper-like structures are elongated laminar domains composed of unprocessed core shell precursors pp220 and pp62. They appear as large irregular areas when labeled with anti-pp220 antibody in immunofluorescence experiments . Similar aberrant structures have been observed in cells infected with ASFV recombinants where other structural proteins (p72, pB602L, or p54) are repressed, suggesting a common morphogenetic defect when key structural proteins are missing .

How does p17's function compare with other ASFV structural membrane proteins?

Protein p17 shares functional similarities with other ASFV structural membrane proteins, particularly p54. Both proteins are essential for virus viability and play crucial roles in viral morphogenesis, though at different stages:

• p54 is essential for the generation of viral precursor membranes

• p17 is required for the progression of these precursor membranes toward icosahedral particles