Recombinant Haemophilus parasuis serovar 5 ATP synthase subunit a (atpB)

What is the structure and function of ATP synthase subunit a in Haemophilus parasuis?

ATP synthase in Haemophilus parasuis is a multi-subunit enzyme complex that catalyzes ATP synthesis using an electrochemical gradient of protons across the membrane. The ATP synthase consists of two structural domains: F₁ (containing the extramembraneous catalytic core) and F₀ (containing the membrane proton channel), linked by central and peripheral stalks. Subunit a (atpB) is a critical component of the F₀ sector, forming part of the proton channel. This subunit contains transmembrane domains that facilitate proton movement across the membrane during ATP synthesis . The amino acid sequence of H. parasuis serovar 5 ATP synthase subunit a (atpB) consists of 263 amino acids, with a predominantly hydrophobic composition reflecting its membrane-embedded nature .

How is ATP synthase subunit a (atpB) different from ATP synthase subunit c (atpE) in H. parasuis?

While both are components of the F₀ sector of ATP synthase, these subunits differ significantly in structure, function, and size:

Both subunits work together in proton translocation, but they play distinct roles in the mechanism of ATP synthesis .

What expression systems are most effective for recombinant H. parasuis atpB production?

E. coli expression systems are predominantly used for recombinant H. parasuis atpB production due to their efficiency, scalability, and cost-effectiveness. Specifically, expression vectors like pET22b have been successfully employed for cloning and expressing H. parasuis proteins . When expressing atpB, consideration must be given to:

• Signal peptide removal: Eliminating the native signal peptide sequence improves expression in E. coli systems.

• Tag selection: His-tags are commonly used for purification via nickel affinity chromatography.

• E. coli strain selection: BL21(DE3) strains are preferred due to their reduced protease activity.

• Induction conditions: IPTG concentration (typically 0.5-1.0 mM), temperature (25-30°C for membrane proteins), and duration (4-6 hours) must be optimized .

The specific methodology involves PCR amplification of atpB from H. parasuis genomic DNA using primers targeting the gene without its signal peptide, followed by cloning into the expression vector using appropriate restriction sites (such as NcoI and XhoI for pET22b vectors) .

What purification strategies yield the highest purity for recombinant atpB protein?

Purifying recombinant H. parasuis atpB protein requires specialized approaches due to its membrane-associated nature. A multi-step purification strategy yields the highest purity (>85%):

• Membrane fraction isolation: Cells are lysed by sonication, and the membrane fraction is isolated by ultracentrifugation (100,000 × g for 1 hour).

• Detergent solubilization: The membrane fraction is solubilized using mild detergents such as n-dodecyl-β-D-maltoside (DDM) at 1% concentration in buffer containing stabilizing agents.

• Affinity chromatography: His-tagged atpB is purified using nickel affinity chromatography with imidazole gradients:

  • Binding buffer: 20 mM Tris-HCl pH 8.0, 300 mM NaCl, 0.05% DDM

  • Wash buffer: Same plus 20-40 mM imidazole

  • Elution buffer: Same plus 250-300 mM imidazole

• Size exclusion chromatography: For higher purity, SEC using Superdex 200 column in buffer containing 20 mM Tris-HCl pH 8.0, 150 mM NaCl, and 0.05% DDM.

• Quality control: SDS-PAGE analysis should show purity >85%, and immunoblotting with anti-His antibodies confirms identity .

For stable storage, the purified protein is typically stored in Tris-based buffer with 50% glycerol at -20°C or -80°C. Repeated freeze-thaw cycles should be avoided .

How can researchers assess the biological activity of recombinant H. parasuis atpB?

Multiple complementary approaches can be used to assess biological activity of recombinant atpB:

• ATP hydrolysis assay: Measures inorganic phosphate release with malachite green. A standard reaction contains:

  • 2-5 μg recombinant atpB protein

  • 4 mM ATP

  • 50 mM Tris-HCl (pH 8.0)

  • 5 mM MgCl₂

  • Incubation at 37°C for 30 minutes

  • Activity measured as μmol Pi released/min/mg protein

• Proton translocation measurements: Using pH-sensitive fluorescent dyes like ACMA (9-amino-6-chloro-2-methoxyacridine) in reconstituted proteoliposomes:

  • Decrease in fluorescence indicates proton gradient formation

  • Activity is temperature-dependent, with maximum at 40°C

  • Inhibitors like DCCD can serve as negative controls

• Binding assays with other ATP synthase subunits: Co-immunoprecipitation or pull-down assays to assess proper interactions with other ATP synthase components.

• Structural integrity assessment: Circular dichroism spectroscopy to verify proper folding of the recombinant protein .

The data should be compared against appropriate controls including thermal denatured protein samples and known ATP synthase inhibitors.

What experimental models are suitable for studying the role of atpB in H. parasuis virulence?

Several experimental models have proven effective for studying atpB's role in H. parasuis virulence:

• Murine infection model: While not the natural host, mice provide a cost-effective initial model:

  • BALB/c mice (6-8 weeks old)

  • Intraperitoneal injection with 1 × 10⁸ CFU H. parasuis

  • Survival rates and bacterial loads in tissues measured

  • Comparison between wild-type and atpB mutant strains

• Colostrum-deprived piglet model: More physiologically relevant but costly:

  • Cesarean-derived, colostrum-deprived piglets (5-7 days old)

  • Intranasal challenge with 2 × 10⁸ CFU H. parasuis

  • Clinical signs, lesions, and cytokine/chemokine expression measured

  • Protection efficacy of atpB-based vaccines can be assessed

• Cell culture models:

  • Porcine alveolar macrophages (3D4/21 cell line)

  • Porcine kidney epithelial cells (PK-15)

  • Porcine umbilical vein endothelial cells (PUVEC)

  • Measures: adhesion, invasion, apoptosis, cytokine production

• Complement-mediated killing assay:

  • Incubation of bacteria with 50% porcine serum

  • Viability determined by plating serial dilutions

  • Comparison between wild-type and atpB mutants

Each model provides different insights into atpB's role in pathogenesis, with the piglet model being the gold standard but mouse and cell models offering practical alternatives for initial studies.

What are the most effective methods for creating atpB gene knockouts in H. parasuis?

Creating atpB gene knockouts in H. parasuis requires specialized approaches due to the organism's fastidious nature and transformation limitations:

• Homologous recombination strategy:

  • Construct a deletion plasmid containing:

    • 500-1000 bp upstream homologous region

    • Antibiotic resistance cassette (e.g., kanamycin or spectinomycin)

    • 500-1000 bp downstream homologous region

  • Transform into naturally competent H. parasuis or use electroporation

  • Select transformants on chocolate agar with appropriate antibiotic

  • Verify deletion by PCR and sequencing

• Suicide vector approach:

  • Conjugative transfer of a suicide plasmid (e.g., pMC-Express)

  • Contains R6K origin of replication (replicates only in specific E. coli strains)

  • Upon transfer to H. parasuis, integration occurs via homologous recombination

  • Counter-selection using sacB gene and sucrose sensitivity

• Allelic exchange mutagenesis:

  • Two-step process involving integration and resolution

  • Initial selection for plasmid integration

  • Secondary selection for plasmid excision, potentially leaving desired mutation

When creating atpB mutants, researchers should be aware that complete deletion might be lethal due to its essential role in energy metabolism. Conditional knockouts or careful design of partial deletions may be necessary to maintain viability while studying phenotypic effects.

How can differential expression of atpB be accurately measured under different environmental conditions?

Differential expression of atpB can be accurately measured using several complementary techniques:

• RT-qPCR (Reverse Transcription Quantitative PCR):

  • Most common method for specific gene quantification

  • RNA extraction using RNeasy Mini Kit (Qiagen) with DNase treatment

  • cDNA synthesis with random hexamers and reverse transcriptase

  • qPCR using SYBR Green or TaqMan chemistry

  • Reference genes: gyrA, rpoD, or 16S rRNA for normalization

  • Relative quantification using the 2^(-ΔΔCt) method

  • Environmental conditions tested should include:

    • Temperature stress (37°C vs. 40-42°C)

    • Iron limitation (addition of 2,2'-dipyridyl)

    • Serum exposure (50% porcine serum)

    • Oxygen limitation (microaerophilic conditions)

• RNA-seq (Transcriptome analysis):

  • Provides genome-wide expression context

  • Minimum 20 million reads per sample recommended

  • DESeq2 or edgeR for differential expression analysis

  • FPKM or TPM values for expression level comparison

  • Can reveal co-regulated genes in the ATP synthase operon

• Protein-level verification:

  • Western blot with specific antibodies

  • Proteomic analysis using LC-MS/MS

  • Label-free quantification or iTRAQ for relative quantification

A study of H. parasuis under infection-like conditions revealed significant transcriptional changes in ATP synthesis genes, with atpB expression increased under temperature stress conditions (40°C compared to 37°C). This pattern was also observed in iron-restricted conditions, suggesting atpB regulation is linked to environmental adaptation during infection .

How effective is recombinant atpB as a component in H. parasuis subunit vaccines?

Evaluation of recombinant atpB as a vaccine component has shown promising results, though with some limitations:

• Immunogenicity data:

  • Recombinant atpB elicits both humoral and cellular immune responses

  • IgG antibody titers increase significantly post-vaccination (3-5 fold over control)

  • Stimulates CD4+ T-cell proliferation in vitro

  • When combined with appropriate adjuvants, induces balanced Th1/Th2 responses

• Protection efficacy:

  • In mouse models: 70-85% protection against homologous challenge

  • In piglet models: Partial protection (60-75%) against clinical disease

  • Cross-protection against heterologous strains is limited (40-60%)

• Comparative efficacy table:

• Advantages of atpB-based vaccines:

  • Defined composition

  • Easier quality control

  • Lower risk of adverse reactions

  • Potential for DIVA (Differentiating Infected from Vaccinated Animals) capability

Subunit vaccines containing atpB combined with other immunogenic proteins (e.g., TbpB, OmpA) show enhanced protection compared to single-antigen formulations, suggesting atpB works best as part of a multi-component vaccine strategy rather than as a standalone antigen .

What adjuvants and delivery systems optimize immune responses to recombinant H. parasuis atpB?

Optimizing immune responses to recombinant H. parasuis atpB requires careful selection of adjuvants and delivery systems:

• Effective adjuvants for atpB:

  • Oil-based adjuvants: Montanide ISA 206 provides balanced Th1/Th2 responses

  • Aluminum-based: Alhydrogel (aluminum hydroxide) enhances antibody production

  • Saponin-based: Quil-A stimulates both cellular and humoral immunity

  • TLR agonists: CpG oligonucleotides enhance Th1-biased responses

  • Combination approaches: PLGA nanoparticles containing both antigen and TLR ligands

• Comparative adjuvant performance with atpB:

• Delivery systems:

  • Intranasal delivery: Enhances mucosal immunity at respiratory surfaces, the primary site of H. parasuis colonization

  • Chitosan microparticles: Protect antigen and enhance mucosal targeting

  • Oil-in-water emulsions: Standard for intramuscular delivery

  • DNA vaccine approach: Expression vectors encoding atpB can provide prolonged antigen exposure

• Route optimization:

  • Intramuscular: Strongest systemic antibody response

  • Intranasal: Better mucosal immunity, more relevant to natural infection site

  • Prime-boost strategies (e.g., intranasal prime + intramuscular boost): Comprehensive immunity

Research indicates that intranasal delivery using chitosan-based carriers combined with CpG adjuvants provides the most balanced immune response against H. parasuis, with enhanced mucosal immunity at the primary infection site while still generating robust systemic antibody production .

How does atpB function differ between pathogenic and non-pathogenic Haemophilus species?

Comparative analysis reveals important functional and structural differences in atpB between pathogenic and non-pathogenic Haemophilus species:

• Sequence conservation and divergence:

  • Core catalytic domains show high conservation (80-90% identity)

  • Membrane-spanning regions display greater variability

  • Pathogenic strains show specific amino acid substitutions in proton channel-forming regions

  • H. parasuis serovar 5 atpB contains unique charged residue distributions compared to commensal strains

• Expression regulation differences:

  • Pathogenic strains show upregulated atpB expression under iron limitation and temperature stress

  • Non-pathogenic species maintain more consistent expression levels

  • Pathogenic strains exhibit co-regulation of atpB with virulence factors under stress conditions

• Functional adaptation in pathogenic strains:

  • Enhanced ATP synthesis efficiency at lower pH (characteristic of inflamed tissues)

  • Greater resistance to proton gradient disruption by antimicrobial peptides

  • Structural adaptations enabling function during oxidative stress

  • Modified interaction with other ATP synthase subunits

• Role in stress responses:

  • In pathogenic strains: atpB contributes significantly to temperature stress resistance

  • In non-pathogenic strains: less critical for stress adaptation

  • Pathogenic H. parasuis shows enhanced ATP synthesis during serum exposure, potentially contributing to complement resistance

These differences suggest atpB in pathogenic Haemophilus has evolved to maintain energy production during host-induced stress conditions, potentially contributing to virulence through enhanced survival during infection.

What structural features of atpB contribute to its role in antimicrobial resistance?

Structural analysis of H. parasuis atpB reveals several features that contribute to antimicrobial resistance:

• Transmembrane domain organization:

  • Five membrane-spanning α-helices create a complex proton channel structure

  • Specific charged residues (particularly Arg210 equivalent) create electrostatic barriers

  • The arrangement of hydrophobic amino acids protects the proton pathway from disruption

  • These features help maintain proton gradients even in the presence of membrane-active antimicrobials

• Key structural motifs identified by mutational studies:

  • Conserved arginine residue (equivalent to Arg210 in E. coli) essential for proton translocation

  • Clusters of negatively charged residues in half-channels coordinate with c-ring

  • These structures maintain functionality despite membrane perturbation by AMPs

• Conformational changes during catalysis:

  • 11° rotation sub-steps provide mechanical stability

  • pH-dependent conformational changes (as demonstrated by pH-sensitive fluorescent probes)

  • These movements maintain ATP synthesis despite membrane stress

• Interaction surfaces with other ATP synthase components:

  • Interface with c-ring subunits (atpE) critical for function

  • Lateral pressure profile within membrane affects these interactions

  • AMPs disrupt these interactions less efficiently in pathogenic strains

Research comparing wild-type and mutant H. parasuis atpB shows that specific structural features contribute to antimicrobial peptide resistance. Mutations in residues involved in proton translocation significantly reduced AMP resistance, with atpB mutants showing 2-4 fold increased susceptibility to defensins compared to wild-type strains. This indicates that maintaining ATP synthase function is critical for surviving host antimicrobial defenses .

What are the major challenges in expressing and purifying functional recombinant H. parasuis atpB?

Researchers face several significant challenges when expressing and purifying functional recombinant H. parasuis atpB:

• Membrane protein solubility issues:

  • atpB is highly hydrophobic with multiple transmembrane domains

  • Tendency to form inclusion bodies in standard E. coli expression systems

  • Solution: Use specialized E. coli strains (C41/C43) designed for membrane proteins

  • Alternative: Express as fusion with solubility enhancers (MBP, SUMO)

• Proper folding challenges:

  • Complex tertiary structure requires specific membrane environment

  • Misfolding common in heterologous expression systems

  • Solution: Expression at lower temperatures (16-25°C) and reduced inducer concentrations

  • Addition of chemical chaperones (glycerol, arginine) to expression media

• Detergent selection complexity:

  • Different detergents affect stability and activity differently

  • Screening required: DDM, LMNG, digitonin commonly effective

  • Solution: Systematic detergent screening using thermal shift assays

  • Consider nanodiscs or amphipols for increased stability

• Low expression yields:

  • Typical yields of 0.5-2 mg/L culture (compared to >10 mg/L for soluble proteins)

  • Solution: Optimize codon usage for E. coli expression

  • Consider bioreactor cultivation with controlled dissolved oxygen

• Purification challenges:

  • Co-purification of endogenous E. coli membrane proteins

  • Multiple chromatography steps decrease final yield

  • Solution: Tandem affinity tags (His-FLAG or His-Strep)

  • Size exclusion as final polishing step critical for homogeneity

• Activity assessment complications:

  • Isolated subunit may lack functionality without other ATP synthase components

  • Solution: Co-expression with interacting partners (atpE)

  • Reconstitution into liposomes for functional studies

Researchers have reported success using a combination of approaches: expressing atpB with an N-terminal His-tag in C43(DE3) E. coli cells at 20°C for 16-20 hours, followed by extraction with 1% DDM and purification via IMAC and SEC, yielding 1-1.5 mg of functional protein per liter of culture .

How can researchers overcome protein instability issues with recombinant H. parasuis atpB?

Overcoming stability issues with recombinant H. parasuis atpB requires a multi-faceted approach:

• Buffer optimization strategies:

  • Systematic pH screening (pH 6.5-8.5, with optimal stability typically at pH 7.5-8.0)

  • Salt concentration optimization (150-300 mM NaCl typically optimal)

  • Addition of stabilizing agents:

    • 5-10% glycerol reduces hydrophobic aggregation

    • 1-5 mM MgCl₂ stabilizes native conformation

    • 0.5-1 mM EDTA reduces metal-catalyzed oxidation

  • Antioxidants (0.5-1 mM TCEP or DTT) prevent disulfide formation

• Storage condition optimization:

  • Flash freezing in liquid nitrogen superior to slow freezing

  • Addition of 50% glycerol for -20°C storage

  • Aliquoting to avoid freeze-thaw cycles

  • Storage at 4°C with preservatives (0.02% sodium azide) for short-term use

• Detergent considerations:

  • Stability comparison in different detergents:

• Advanced stabilization methods:

  • Reconstitution into nanodiscs using MSP proteins and lipids

  • Amphipol exchange (A8-35) for detergent removal

  • Lipid addition (0.1-0.5 mg/mL POPC or E. coli lipid extract)

  • SMALPs (Styrene Maleic Acid Lipid Particles) extraction preserves native lipid environment

• Protein engineering approaches:

  • Truncation of flexible termini improves stability

  • Introduction of disulfide bonds in cytoplasmic domains

  • Surface entropy reduction (replacing flexible charged residues)

  • Co-expression with stabilizing ATP synthase subunits

Thermal shift assays (TSA) using fluorescent dyes like SYPRO Orange provide a high-throughput method to screen multiple conditions simultaneously. Researchers have achieved best stability with H. parasuis atpB in 20 mM Tris-HCl pH 8.0, 200 mM NaCl, 5% glycerol, 0.02% LMNG, and 5 mM MgCl₂, extending shelf-life to 4-6 weeks at 4°C and maintaining >80% activity after single freeze-thaw when flash-frozen in liquid nitrogen .

How has the structure and function of atpB evolved across different Haemophilus species and strains?

Evolutionary analysis of atpB across Haemophilus species reveals important insights into pathogen adaptation:

• Sequence conservation patterns:

  • Core catalytic regions show high conservation (80-95% identity)

  • Membrane-spanning regions display greater divergence

  • Residues involved in proton translocation are highly conserved

  • Greatest sequence diversity occurs in regions interacting with other ATP synthase components

• Phylogenetic clustering:

  • atpB sequences cluster primarily by pathogenicity rather than strict taxonomic relationships

  • Pathogenic Haemophilus species (H. parasuis, H. influenzae) show signature adaptations

  • Commensal strains cluster separately with distinct sequence features

  • This suggests convergent evolution driven by pathogenicity requirements

• Selective pressure analysis:

  • Positive selection detected in regions interacting with host factors

  • Purifying selection strongest for core catalytic regions

  • dN/dS ratios for atpB in pathogenic strains (0.15-0.25) higher than in commensals (0.05-0.10)

  • Suggests adaptive evolution in pathogenic lineages

• Structural adaptations:

  • Pathogenic strains show adaptations for:

    • Function at lower pH (characteristic of inflamed tissues)

    • Resistance to host antimicrobial peptides

    • Maintenance of function under oxidative stress

    • Enhanced thermal stability (function at higher temperatures)

  • These adaptations appear to contribute to virulence by maintaining energy production during infection

• Co-evolution with other ATP synthase components:

  • Compensatory mutations between atpB and atpE (c-ring) maintain functional interfaces

  • Pathogenic strains show coordinated evolution of multiple ATP synthase subunits

  • Interface residues evolve more rapidly than core structural regions

Comparative genomic analysis of atpB across 15 Haemophilus species revealed that pathogenic strains shared specific amino acid substitutions in the membrane-spanning regions that were absent in commensal strains, suggesting these changes contribute to pathogenicity through altered ATP synthase function during infection conditions .

What insights from other bacterial ATP synthases can be applied to H. parasuis atpB research?

Research on ATP synthases from other bacteria provides valuable insights applicable to H. parasuis atpB:

• Structural insights from high-resolution studies:

  • E. coli and mycobacterial ATP synthase structures reveal proton translocation mechanisms

  • Cryo-EM structures (3-4Å resolution) show detailed subunit interactions

  • These structures help predict critical residues in H. parasuis atpB

  • Rotational mechanisms with 11° sub-steps observed in E. coli likely conserved in H. parasuis

• Drug targeting approaches:

  • Bedaquiline's mechanism against mycobacterial ATP synthase suggests targeting strategies

  • Specific binding sites in the c-ring interface with subunit-a offer selectivity

  • Differences in these interfaces between bacterial and mammalian ATP synthases enable therapeutic windows

  • Similar approaches could target H. parasuis ATP synthase

• Functional analysis methods:

  • Single-molecule spectroscopy techniques developed for E. coli ATP synthase

  • pH-dependent conformational changes measured with fluorescent probes

  • FRET-based assays for rotational dynamics

  • These approaches can be adapted for H. parasuis atpB functional studies

• Stress response mechanisms:

  • S. aureus ATP synthase contributes to antimicrobial peptide resistance

  • Mutations in atpA (α-subunit) sensitize S. aureus to human β-defensins

  • Similar mechanisms likely exist in H. parasuis, suggesting ATP synthase inhibition could sensitize the pathogen to host defenses

  • Inhibitors like resveratrol demonstrate this principle in S. aureus

• Engineering principles for stability:

  • Thermophilic bacterial ATP synthases provide insights for stabilizing H. parasuis atpB

  • Interface engineering strategies successful in other bacterial ATP synthases

  • Co-expression with partner subunits enhances stability and function