Recombinant Actinobacillus pleuropneumoniae serotype 7 ATP synthase subunit b (atpF)

What is Actinobacillus pleuropneumoniae and why is it significant?

Actinobacillus pleuropneumoniae (APP) is the bacterial pathogen responsible for porcine contagious pleuropneumonia (PCP), a highly contagious respiratory disease characterized by severe fibrinous necrotizing hemorrhagic pleuropneumonia in swine . The disease represents a significant threat to the global swine industry due to its high morbidity and mortality rates. Currently, APP is classified into 18 distinct serovars, which complicates vaccine development due to varying cross-protection efficacy between serovars . Understanding the molecular biology of APP, including proteins like ATP synthase subunit b, is crucial for developing effective prevention and control strategies for this economically important pathogen.

What is ATP synthase subunit b (atpF) and what is its function in bacteria?

ATP synthase subunit b, encoded by the atpF gene, is an essential component of the F₀F₁-ATP synthase complex in bacteria. In the ATP synthase structure, the b subunit exists as a dimer and serves as part of the peripheral stalk that connects the membrane-embedded F₀ sector to the catalytic F₁ sector . This connection prevents rotation of the F₁ component, enabling the enzyme to function as a molecular motor for ATP synthesis. Specifically, residues 62-122 of the b subunit are crucial for mediating dimerization . Structural studies using analytical ultracentrifugation and small-angle X-ray scattering indicate that the b(62-122) dimer is extremely elongated, with a maximal dimension of 95 Å and a radius of gyration of 27 Å, consistent with an α-helical coiled-coil structure . This structural arrangement is critical for proper functioning of ATP synthase in energy production.

How is the atpF gene conserved across species?

The atpF gene shows interesting evolutionary conservation across various species. In mitochondrial genomes, a gene called ymf39 has been identified as the atpF homolog . This discovery came from sequence similarity analysis between Ymf39 from green algae and atpF gene products from bacteria like Bradyrhizobium. Research on jakobids (protists with minimally derived mitochondrial genomes) confirmed that Ymf39 is indeed an ATP synthase subunit through protein sequencing of the isolated mitochondrial ATP synthase complex . Statistical tests assessing sequence similarity provide clear evidence that ymf39 is an atpF homolog, while ATP4/ATP5F genes in fungi and animals appear to be highly diverged forms of ymf39 that have migrated to the nucleus . This evolutionary relationship highlights the conservation of this important bioenergetic component across diverse organisms.

What methods are most effective for cloning and expressing recombinant A. pleuropneumoniae atpF?

For successful cloning and expression of recombinant A. pleuropneumoniae atpF, researchers typically employ the following methodological approach:

• Gene Identification and Primer Design: Begin by identifying the complete atpF sequence from A. pleuropneumoniae serotype 7 genome databases. Design specific primers with appropriate restriction enzyme sites for subsequent cloning steps.

• PCR Amplification and Vector Selection: Amplify the atpF gene using high-fidelity polymerase and optimize PCR conditions (typically 95°C for initial denaturation, followed by 30-35 cycles of 95°C for 30 seconds, 55-60°C for 30 seconds, and 72°C for 1 minute). For expression, pET-series vectors (particularly pET-28a) are commonly used for their strong T7 promoter and optional N-terminal or C-terminal His-tag for purification .

• Expression System: Transform the recombinant plasmid into E. coli BL21(DE3) or similar expression strains. For induction, IPTG concentrations of 0.5-1.0 mM at 16-30°C for 4-16 hours usually yield optimal protein expression, with lower temperatures often preferred to enhance protein solubility.

• Protein Purification: For His-tagged atpF, nickel affinity chromatography followed by size exclusion chromatography produces high-purity protein. Depending on downstream applications, additional steps like ion exchange chromatography may be necessary.

• Verification: Confirm successful expression through SDS-PAGE, Western blotting, and mass spectrometry. For functional studies, assess the purified protein's secondary structure using circular dichroism spectroscopy to verify proper folding.

This methodological approach has been adapted from successful recombinant protein expression strategies used for similar APP antigens in vaccine development studies .

How does atpF contribute to A. pleuropneumoniae virulence and pathogenicity?

While the direct role of atpF in A. pleuropneumoniae virulence has not been extensively characterized in the provided search results, insights can be drawn from studies of similar outer membrane proteins in APP. The ATP synthase complex plays a critical role in bacterial energy metabolism, which indirectly supports various virulence mechanisms.

Studies of membrane proteins in APP have shown that mutations in membrane components can significantly affect bacterial phenotypes. For example, deletion of the ompW gene in APP altered bacterial morphology during steady growth, decreased oxidative tolerance, and changed susceptibility to antibiotics including polymyxin B, kanamycin, and penicillin . Similar to ompW, atpF as a membrane-associated protein may influence these virulence-related characteristics.

The atpF protein, as part of the ATP synthase complex, ensures efficient energy production, which is critical for:

• Maintaining membrane potential

• Powering secretion systems for virulence factors

• Supporting growth and division in host environments

• Enabling stress responses during infection

What is known about the immunogenicity of recombinant atpF compared to other A. pleuropneumoniae antigens?

Research on A. pleuropneumoniae vaccine development has primarily focused on RTX toxins (ApxI, ApxII, and ApxIII) and outer membrane proteins as primary immunogens . While specific immunogenicity data for atpF alone is limited in the search results, comparative analysis can provide valuable insights.

Multicomponent recombinant vaccines containing various APP antigens have shown promising results. In one study, a vaccine composed of rApxI, rApxII, rApxIII, and rOMP demonstrated significant immunoprotection, with higher antibody titers and survival rates compared to single-component or partial combinations . The effectiveness of such multicomponent approaches suggests that combining atpF with established immunogens could enhance vaccine efficacy.

When incorporating atpF into vaccine formulations, its membrane-associated nature suggests it may provide complementary immunogenicity to the established APP antigens. Membrane proteins often expose epitopes that are accessible to antibodies during natural infection, potentially conferring protection through multiple immune mechanisms.

For immunization protocols, a recommended approach based on successful APP antigen studies would be:

• Primary immunization with 200-300 μg of purified recombinant atpF

• Booster immunization 2-3 weeks later

• Adjuvant selection: oil-based adjuvants (like Freund's) for experimental animals; aluminum hydroxide or oil-in-water emulsions for field applications

How effective are epitope-based vaccines containing atpF sequences for cross-protection against multiple A. pleuropneumoniae serovars?

Recent research has shown that recombinant tandem epitope (RTA) vaccines can provide cross-protection against different APP serovars . In one study, researchers predicted B cell epitopes from trimeric autotransporter adhesin and constructed a recombinant tandem antigen by connecting selected epitopes (Ba1, Bb5, C1, PH1, and PH2) using linker sequences . The resulting RTA protein demonstrated promising cross-protection.

For atpF-based epitope vaccines, identifying conserved, immunogenic epitopes across serovars is crucial. Potential methodological approaches include:

• In silico epitope prediction: Analyzing atpF sequences from multiple APP serovars using algorithms like BepiPred, ABCpred, and IEDB to identify conserved B-cell epitopes.

• Epitope validation: Testing predicted epitopes using:

  • Synthetic peptide ELISA with sera from convalescent pigs

  • Epitope-specific antibody generation and functional testing

  • Phage display libraries to confirm antibody binding

• Construction of multi-epitope antigens: Similar to the RTA approach, connecting validated atpF epitopes using flexible linkers (typically GGGGS or EAAAK sequences) to create a recombinant tandem construct.

Experimental evidence suggests that combining epitope-based vaccines with inactivated bacteria significantly enhances cross-protection. As demonstrated in one study, while the RTA immune group had a 40% survival rate after APP infection, combining RTA with inactivated APP produced substantially stronger cross-immune protection, ranging from 50% (RTA IB1 + C5) to 100% (RTA IB5 + C1) . This suggests that an effective strategy for atpF-based vaccines would be to incorporate identified atpF epitopes into a broader epitope cocktail, potentially combined with inactivated bacteria.

The most promising approach based on current evidence would be a combination of:

• Multiple conserved atpF epitopes

• Epitopes from other protective antigens (ApxI, ApxII, ApxIII)

• Inactivated APP representing prevalent serovars

What molecular interactions occur between ATP synthase subunit b dimers and how do they affect protein function in A. pleuropneumoniae?

ATP synthase subunit b exists as a dimer that plays a crucial role in the structure and function of the ATP synthase complex. The dimerization domain, specifically residues 62-122, is essential for this function . Molecular studies have revealed important structural characteristics of this dimerization.

The molecular interactions that stabilize the b subunit dimer include:

• Hydrophobic interactions: Regular packing of hydrophobic residues at the dimer interface creates a stable hydrophobic core.

• Ionic interactions: Charged residues form salt bridges across the dimer interface, contributing to stability.

• Hydrogen bonding networks: These provide additional stabilization of the coiled-coil structure.

In the functional ATP synthase complex, the b dimer serves as a critical part of the peripheral stalk, connecting the membrane-embedded F₀ sector to the catalytic F₁ sector. This connection prevents rotation of F₁ during catalysis, which is essential for the enzyme's function as a molecular motor . Mutations that disrupt the dimerization interface would likely compromise this structural role, leading to decreased ATP synthesis efficiency.

In A. pleuropneumoniae, this energy production machinery is particularly important during infection, as the bacterium must adapt to the host environment and power various virulence mechanisms. The integrity of the b subunit dimer is therefore likely crucial for full virulence potential, making it both a potential therapeutic target and vaccine candidate.

How does atpF gene regulation respond to environmental stresses in A. pleuropneumoniae?

Studies on the outer membrane protein W gene (ompW) in A. pleuropneumoniae provide relevant parallels. When ompW was deleted, significant changes occurred in the expression of ribosome synthesis-related genes and ABC transporter genes . Specifically, genes rpmA, rpmB, and rplT were significantly upregulated, while rpsC, rplU, and rplC were significantly downregulated within the ribosome synthesis pathway. For ABC transporters, xylG and modA were upregulated, while phnS_2, PROKKA_00896, and fhuB were downregulated .

This suggests a complex regulatory network connecting membrane proteins to central cellular processes. For atpF regulation, similar connections to key metabolic and transport pathways likely exist, with probable responses to:

A recommended experimental approach to study atpF regulation would include:

• qRT-PCR analysis of atpF expression under defined stress conditions

• Reporter gene fusions to identify promoter elements and regulatory factors

• Chromatin immunoprecipitation to identify transcription factors binding the atpF promoter

• Metabolic labeling to measure atpF protein synthesis rates during stress responses

What are the optimal conditions for purifying functional recombinant atpF protein?

Purifying functional recombinant atpF protein requires careful attention to maintain the protein's native structure and function. Based on techniques used for similar membrane-associated proteins, the following optimized protocol is recommended:

Expression System Selection:

• E. coli BL21(DE3) or C43(DE3)/C41(DE3) strains (the latter two specifically designed for membrane protein expression)

• pET expression system with T7 promoter

• Consider fusion tags: N-terminal His₆-tag with TEV protease cleavage site

Cultivation Conditions:

• Grow cultures at 37°C until OD₆₀₀ reaches 0.6-0.8

• Induce with 0.1-0.5 mM IPTG

• Shift temperature to 16-18°C after induction

• Continue expression for 16-18 hours at reduced temperature

Cell Lysis and Membrane Preparation:

• Resuspend cells in buffer containing 50 mM Tris-HCl pH 8.0, 200 mM NaCl, 5% glycerol, 1 mM EDTA, protease inhibitor cocktail

• Disrupt cells using French press (15,000 psi) or sonication (10 cycles of 30s on/30s off)

• Remove unbroken cells and debris by centrifugation at 10,000×g for 20 minutes

• Collect membranes by ultracentrifugation at 100,000×g for 1 hour

Solubilization and Purification:

• Solubilize membranes in buffer containing 50 mM Tris-HCl pH 8.0, 200 mM NaCl, 1% n-dodecyl β-D-maltoside (DDM) or 1% digitonin

• Incubate with gentle rotation for 1 hour at 4°C

• Clear insoluble material by ultracentrifugation at 100,000×g for 30 minutes

• Apply supernatant to Ni-NTA affinity column equilibrated with 50 mM Tris-HCl pH 8.0, 200 mM NaCl, 0.05% DDM

• Wash with the same buffer containing 20-40 mM imidazole

• Elute with buffer containing 250-300 mM imidazole

Secondary Purification:

• Apply eluted protein to size exclusion chromatography column (Superdex 200) equilibrated with 50 mM Tris-HCl pH 8.0, 150 mM NaCl, 0.05% DDM

• Collect fractions containing pure atpF protein

• Optional: Remove His-tag using TEV protease followed by reverse Ni-NTA chromatography

Quality Control Assessments:

• Purity: SDS-PAGE (>90% homogeneity)

• Identity: Western blot and mass spectrometry

• Structure: Circular dichroism to confirm α-helical content

• Oligomeric state: Analytical ultracentrifugation to verify dimer formation

• Functionality: ATP synthase reconstitution assays or dimerization analysis through native PAGE

This optimized protocol takes into account the α-helical, dimeric nature of atpF and its membrane association, adapting techniques that have been successful for similar proteins in the ATP synthase complex.

What experimental designs are most effective for evaluating atpF-based vaccines against A. pleuropneumoniae?

Designing rigorous experiments to evaluate atpF-based vaccines requires careful consideration of multiple factors. Based on successful approaches in APP vaccine studies, the following experimental design is recommended:

1. Animal Model Selection:

• Primary model: Specific pathogen-free (SPF) pigs, 5-6 weeks old

• Alternative model: BALB/c mice (for preliminary studies)

• Group size: Minimum 8-10 animals per group for statistical power

• Control groups must include: (a) unvaccinated challenged, (b) commercial vaccine, (c) adjuvant-only

2. Vaccine Formulation Options:

• Purified recombinant atpF protein (100-200 μg/dose)

• atpF combined with other antigens (ApxI, ApxII, ApxIII, OMP)

• Epitope-based vaccine using predicted B-cell epitopes from atpF

• DNA vaccine encoding atpF

• Prime-boost strategy combining DNA and protein approaches

3. Immunization Protocol:

• Two-dose regimen: Primary immunization followed by booster at day 21

• Route: Intramuscular injection

• Adjuvants: Oil-in-water emulsion for protein vaccines; CpG oligonucleotides for DNA vaccines

• Timeline: Challenge 2-3 weeks after booster vaccination

4. Challenge Model:

• Strain selection: Homologous (serotype 7) and heterologous (serotypes 1, 5) challenges to assess cross-protection

• Challenge dose: 2 × 10⁸ to 5 × 10⁸ CFU via intranasal instillation

• Monitoring period: 14 days post-challenge

5. Evaluation Parameters:

6. Statistical Analysis:

• Sample size calculation: Power analysis assuming 80% power to detect 30% difference in survival

• Survival analysis: Kaplan-Meier method with log-rank test

• Antibody titers and lesion scores: ANOVA with Tukey's post-hoc test

• Bacterial counts: Non-parametric Kruskal-Wallis with Dunn's post-hoc test

This comprehensive experimental design, adapted from studies evaluating multicomponent recombinant vaccines against APP , provides a robust framework for assessing the efficacy of atpF-based vaccine candidates. Particular attention should be paid to cross-protection against heterologous serotypes, as this remains a significant challenge in APP vaccine development .

How can structural biology techniques be applied to optimize atpF for vaccine development?

Structural biology techniques offer powerful tools for rational vaccine design based on detailed understanding of protein structure-function relationships. For optimizing atpF as a vaccine antigen, the following methodological approach is recommended:

1. High-Resolution Structure Determination:

• X-ray Crystallography: Following the methods used for ATP synthase b subunit dimerization domain , crystallize purified atpF and collect diffraction data at resolutions better than 2.0 Å. Conditions that successfully yielded 1.55 Å resolution for b(62-122) serve as a starting point.

• Cryo-Electron Microscopy: For full-length atpF in its native dimeric state or as part of the ATP synthase complex, cryo-EM can provide structural information at near-atomic resolution without the need for crystallization.

• NMR Spectroscopy: For flexible regions or domains of atpF that resist crystallization, solution NMR can provide structural and dynamic information, particularly useful for identifying mobile epitopes.

2. Epitope Mapping and Engineering:

• Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS): Identify surface-exposed regions that may contain protective epitopes by measuring the rate of hydrogen-deuterium exchange.

• Alanine-Scanning Mutagenesis: Systematically replace surface residues with alanine and assess the impact on antibody binding to precisely locate epitope residues.

• Epitope Grafting: Once protective epitopes are identified, they can be engineered onto scaffold proteins or incorporated into nanoparticle designs for enhanced presentation to the immune system.

3. Protein-Antibody Complex Structures:

• Co-crystallization: Generate and structurally characterize complexes between atpF and antibody Fab fragments from protected animals to understand the molecular basis of neutralization.

• Single-Particle Cryo-EM: For larger complexes or when crystallization proves difficult, cryo-EM can resolve protein-antibody interactions.

4. Structure-Based Immunogen Design:

• Conformational Stabilization: Based on structural data, introduce disulfide bonds or other stabilizing mutations to lock atpF in its most immunogenic conformation.

• Glycan Masking: If non-protective epitopes are identified, they can be masked by introducing N-linked glycosylation sites to focus the immune response on protective regions.

• Multimerization Strategies: Based on the natural dimeric state of atpF , engineer constructs that present multiple copies of protective epitopes through self-assembling nanoparticles.

5. Integrated Computational-Experimental Approach:

• Molecular Dynamics Simulations: Model the dynamics of atpF to identify transiently exposed epitopes not evident in static structures.

• Computational Epitope Prediction: Use tools like BepiPred, DiscoTope, and Ellipro to predict B-cell epitopes based on structural features.

• Experimental Validation Loop: Iterate between computational prediction, structure-based design, and experimental immunogenicity testing.

A particularly promising application of these techniques is the design of a chimeric recombinant tandem antigen similar to the approach used in other APP vaccine studies , but informed by detailed structural knowledge of atpF. This would involve connecting multiple protective epitopes from atpF and other APP antigens using flexible linkers, with the spatial arrangement optimized based on structural data to maximize immune recognition.