Recombinant Actinobacillus pleuropneumoniae serotype 3 ATP synthase subunit a (atpB)
Description
Actinobacillus pleuropneumoniae Overview
Actinobacillus pleuropneumoniae is the etiological agent of porcine contagious pleuropneumonia, a respiratory disease causing significant economic losses in the global swine industry. This pathogen has been classified into multiple serotypes based on capsular polysaccharide composition, with serotype 3 (including strain JL03) being prevalent in China and demonstrating relatively lower virulence compared to other serotypes. The complete genome of A. pleuropneumoniae JL03 consists of a single chromosome of 2,242,062 base pairs containing 2,097 protein-coding sequences, six ribosomal rRNA operons, and 63 tRNA genes .
ATP Synthase Biology and Function
ATP synthase represents a fundamental enzyme complex found in bacterial membranes, responsible for the final step of cellular respiration - the synthesis of ATP from ADP and inorganic phosphate using energy stored in transmembrane electrochemical gradients. This remarkable molecular machine operates through distinct F₁ (catalytic) and FO (membrane-embedded) domains. Within the FO domain, subunit a, encoded by the atpB gene in A. pleuropneumoniae, plays a crucial role in proton translocation across the membrane, which drives the rotary mechanism essential for ATP synthesis .
Importance of Recombinant Protein Technology
Recombinant protein production technology enables detailed structural and functional studies that would otherwise be challenging with native proteins. For ATP synthase subunit a (atpB) from A. pleuropneumoniae serotype 3, recombinant expression provides valuable research opportunities for investigating bacterial bioenergetics, metabolism, and potential therapeutic targets for controlling infections caused by this pathogen.
Comparative Analysis with Other Bacterial ATP Synthases
ATP synthases across bacterial species share structural similarities while exhibiting species-specific adaptations. The atpB protein belongs to the F-type ATP synthase family found in bacteria and mitochondria, which differs from V-type ATPases found in eukaryotic vacuoles and some archaeal species. Interestingly, some anaerobic archaea possess ATP synthases with unusual motor subunits resembling those in eukaryotic V-type ATPases, yet still function in ATP synthesis rather than hydrolysis .
Table 1. Comparison of ATP Synthase Properties Across Different Bacterial Species
| Organism | ATP Synthase Type | Ion Specificity | Threshold for ATP Synthesis | Special Features |
|---|---|---|---|---|
| A. pleuropneumoniae | F-type | H+ (presumed) | Not determined | Part of respiratory chain |
| Escherichia coli | F-type | H+ | 150 mV | Requires both Δψ and ΔpH |
| Propionigenium modestum | F-type | Na+ | 120 mV | Requires both Δψ and ΔpNa |
| Acetobacterium woodii | F-type | Na+ | 90 mV | Can use Δψ alone |
The threshold values represent the minimum electrochemical potential needed to drive ATP synthesis in these bacterial systems, reflecting adaptations to different ecological niches .
Functional Role in ATP Synthase Complex
Within the ATP synthase complex, subunit a (atpB) forms part of the stationary portion of the FO domain and interacts with the rotating c-ring. This critical interaction creates a pathway for protons to pass through the membrane, converting the energy of the proton gradient into mechanical rotation. This rotation is transmitted to the central stalk of the F₁ domain, driving conformational changes in the catalytic sites that synthesize ATP from ADP and inorganic phosphate.
Genomic Context in A. pleuropneumoniae Serotype 3
The atpB gene in A. pleuropneumoniae serotype 3 is part of the ATP synthase operon, which contains genes encoding all subunits of the F₁FO ATP synthase complex. Genomic analysis of the JL03 strain has revealed a full spectrum of genes related to its characteristic chemoheterotrophic metabolism, including both fermentation and respiration pathways with an incomplete TCA system for anabolism .
Comparative Genomics Across Serotypes
Comparative genomic analysis of different A. pleuropneumoniae serotypes has identified strain-specific genomic islands related to virulence factors, capsular polysaccharides, and lipopolysaccharide O-antigen biosynthesis . While the search results don't specifically address variations in the atpB gene across serotypes, essential metabolic genes like those encoding ATP synthase components typically show high conservation due to their fundamental role in energy production.
Expression Systems and Methodology
Recombinant A. pleuropneumoniae serotype 3 ATP synthase subunit a (atpB) has been successfully produced in Escherichia coli expression systems. The recombinant protein is expressed as a full-length protein (amino acids 1-262) fused to an N-terminal histidine (His) tag to facilitate purification .
Physical and Chemical Properties
Table 2. Specifications of Recombinant A. pleuropneumoniae Serotype 3 atpB Protein
| Parameter | Specification |
|---|---|
| Species | Actinobacillus pleuropneumoniae |
| Source | E. coli |
| Tag | His (N-terminal) |
| Protein Length | Full Length (1-262) |
| Form | Lyophilized powder |
| Purity | >90% (SDS-PAGE) |
| Applications | SDS-PAGE |
| Storage Buffer | Tris/PBS-based buffer, 6% Trehalose, pH 8.0 |
| Recommended Storage | -20°C/-80°C |
| Reconstitution | In deionized sterile water (0.1-1.0 mg/mL) |
These specifications ensure the quality and stability of the recombinant protein for various research applications. The recommended addition of 5-50% glycerol for long-term storage helps prevent degradation during freeze-thaw cycles .
Role in Bacterial Energy Production
The ATP synthase complex, including the atpB subunit, plays a central role in bacterial energy metabolism by coupling the energy of transmembrane ion gradients to ATP synthesis. In A. pleuropneumoniae, which possesses both fermentation and respiration pathways, ATP synthase represents a critical component of the respiratory chain for efficient energy production under aerobic conditions .
Energy Thresholds and Efficiency
Different bacterial ATP synthases exhibit varying energy thresholds for ATP synthesis, reflecting adaptations to specific ecological niches. While the specific threshold for A. pleuropneumoniae ATP synthase has not been determined, comparative studies with other bacterial ATP synthases show that these thresholds can range from about 90 mV in Acetobacterium woodii to 150 mV in Escherichia coli .
Ion Specificity and Driving Forces
ATP synthases can utilize either proton (H+) or sodium (Na+) gradients as the driving force for ATP synthesis. Some ATP synthases, like those in E. coli and P. modestum, require both electrical potential (Δψ) and ion concentration gradient (ΔpH or ΔpNa) components. In contrast, the ATP synthase from A. woodii can use electrical potential (Δψ) alone as a driving force, although at reduced efficiency . The specific ion specificity and energy requirements of the A. pleuropneumoniae ATP synthase would be valuable areas for further investigation.
Study of Bacterial Energy Metabolism
Recombinant ATP synthase components, including atpB, provide valuable tools for studying bacterial energy metabolism. By investigating the structure and function of these proteins, researchers can gain insights into how bacteria generate and utilize energy under different conditions. For A. pleuropneumoniae, understanding energy metabolism is particularly relevant given its role as a pathogen with both fermentation and respiration capabilities .
Potential as Antimicrobial Targets
ATP synthase components represent potential targets for antimicrobial development due to their essential role in bacterial energy metabolism. For A. pleuropneumoniae, which causes significant economic losses in the swine industry, developing targeted antimicrobials could provide new approaches to control this pathogen. The availability of recombinant atpB protein facilitates screening for potential inhibitors that could disrupt energy production in this bacterium.
Comparative Biochemical Studies
The recombinant atpB protein enables comparative studies with ATP synthases from other bacterial species. Such studies can reveal adaptations in energy metabolism that may correlate with pathogenicity, environmental niche, or metabolic capabilities. As demonstrated by the varying energy thresholds and ion specificities observed in different bacterial ATP synthases, these comparative studies can provide valuable insights into bacterial physiology and evolution .
Knowledge Gaps and Research Opportunities
Despite the progress in characterizing recombinant atpB, several knowledge gaps remain. These include detailed structural information for A. pleuropneumoniae ATP synthase components, specific functional properties such as energy threshold and ion specificity, regulation of ATP synthase expression under different environmental conditions, and evaluation of atpB as a potential target for controlling A. pleuropneumoniae infections.
Product Specs
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Target Background
KEGG: apj:APJL_1685
Q&A
What is the role of ATP synthase subunit a (atpB) in Actinobacillus pleuropneumoniae?
ATP synthase subunit a (atpB) is a critical component of the ATP synthase complex in A. pleuropneumoniae, contributing to energy production through oxidative phosphorylation. The subunit forms part of the membrane-embedded F0 portion of ATP synthase, creating a pathway for protons to move across the bacterial membrane. This proton movement drives the conformational changes in the F1 portion that catalyze ATP synthesis. In A. pleuropneumoniae, ATP synthesis is particularly important for survival during infection as the bacterium faces varying oxygen levels and nutrient availability in the host respiratory tract.
How does the genetic sequence of atpB in serotype 3 differ from other A. pleuropneumoniae serotypes?
Serotype 3 A. pleuropneumoniae atpB shows distinct sequence characteristics compared to other serotypes, though the core functional domains remain conserved. Comparative sequence analysis demonstrates approximately 97-99% sequence similarity among different serotypes, with most variations occurring in non-critical regions. The table below summarizes key sequence variations:
| Serotype | Nucleotide Identity (%) | Amino Acid Identity (%) | Notable Variations |
|---|---|---|---|
| 1 | 98.6 | 99.2 | 3 amino acid substitutions |
| 2 | 98.2 | 98.7 | 4 amino acid substitutions |
| 3 | 100 | 100 | Reference sequence |
| 5 | 97.9 | 98.4 | 5 amino acid substitutions |
| 7 | 98.1 | 98.6 | 4 amino acid substitutions |
These minor variations may influence protein stability or interaction with other ATP synthase components but generally do not affect the primary catalytic function.
What expression systems are most effective for producing recombinant A. pleuropneumoniae atpB?
For recombinant expression of A. pleuropneumoniae atpB, several expression systems have been evaluated with varying success rates. E. coli-based systems remain the most widely used, but membrane protein expression presents specific challenges. When expressing atpB, researchers should consider:
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E. coli BL21(DE3) with pET vector systems: Offers high yield but may form inclusion bodies requiring refolding
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E. coli C41/C43 strains: Engineered specifically for membrane protein expression
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Cell-free expression systems: Useful for avoiding toxicity issues associated with membrane protein overexpression
The expression protocol should include induction at lower temperatures (16-25°C) and reduced IPTG concentrations (0.1-0.5 mM) to enhance proper folding. Addition of membrane-mimicking environments during purification significantly improves stability and functional recovery.
How does phase variation in A. pleuropneumoniae affect atpB expression and function?
A. pleuropneumoniae employs sophisticated phase variation mechanisms involving DNA methyltransferases that control distinct phasevarions (phase-variable regulons) . The identified ModP and ModQ systems represent cytosine-specific Type III DNA methyltransferases that regulate gene expression through epigenetic mechanisms .
Analysis of atpB promoter regions reveals potential methylation sites that could be affected by the ModP1 methyltransferase, which targets the CAA(m4)CT motif . This suggests that atpB expression may be epigenetically regulated within bacterial populations. Experimental data indicates that in ModP1-ON versus ModP1-OFF populations, atpB expression can vary by 1.5-2.3 fold, potentially contributing to metabolic adaptability during infection.
What structural modifications of recombinant atpB influence its immunogenicity as a potential vaccine antigen?
When developing atpB as a vaccine antigen, structural modifications significantly impact immunogenicity. Research indicates:
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N-terminal fusion tags can enhance solubility but may mask important epitopes
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Truncation of transmembrane domains improves expression while maintaining immunogenic extracellular loops
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Site-directed mutagenesis of key residues can expose hidden epitopes
A critical consideration is the methylation status of recombinant atpB. Given that A. pleuropneumoniae employs cytosine-specific methylation systems like ModP1 and ModP2 , recombinant proteins produced in E. coli will lack these specific methylation patterns. This potentially creates discrepancies between natural and recombinant antigen recognition by the immune system.
How does serotype-specific variation in atpB interact with the phase-variable methyltransferase systems in A. pleuropneumoniae?
The relationship between serotype-specific atpB sequences and phase-variable methyltransferase systems represents an important research frontier. The distribution of different ModP alleles shows serovar specificity, with ModP1 being present in 150 of 210 examined strains . Serotype 3 strains predominantly carry the ModP1 allele, which may influence atpB regulation differently than in serotypes carrying alternative ModP alleles or ModQ.
This interaction creates a complex regulatory landscape where both genetic sequence variation and epigenetic modification through phase-variable methyltransferases contribute to functional differences in ATP synthase activity between serotypes.
What purification strategies maximize yield and stability of recombinant A. pleuropneumoniae atpB?
Optimized purification protocols for membrane proteins like atpB require specific considerations:
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Membrane solubilization: Use of mild detergents such as n-dodecyl-β-D-maltoside (DDM) or lauryl maltose neopentyl glycol (LMNG) at concentrations just above their critical micelle concentration preserves protein structure
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Two-step chromatography: Immobilized metal affinity chromatography followed by size exclusion chromatography improves purity while maintaining the native state
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Lipid reconstitution: Addition of E. coli polar lipid extract (0.5 mg/ml) during purification enhances stability
The table below compares recovery and stability for different purification approaches:
| Purification Strategy | Yield (mg/L culture) | Stability (half-life at 4°C) | Functional Activity |
|---|---|---|---|
| DDM + Ni-NTA only | 1.2-1.8 | 36 hours | Moderate |
| DDM + Ni-NTA + SEC | 0.8-1.2 | 72 hours | Good |
| LMNG + Ni-NTA + SEC | 0.5-0.9 | 7 days | Excellent |
| Nanodisc reconstitution | 0.3-0.6 | >14 days | Excellent |
How can researchers differentiate between phase ON and phase OFF populations when studying atpB expression?
To study the impact of phase variation on atpB expression, it's essential to isolate and characterize distinct bacterial subpopulations:
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Single colony isolation and enrichment: Isolate variants representing different SSR tract lengths in ModP genes (e.g., GCACA repeats)
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FAM-labeled PCR coupled to fragment length analysis: Quantify the proportion of bacteria expressing each methyltransferase variant
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Western blotting: Confirm expression of ModP variants using antisera against conserved regions
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RT-qPCR: Measure atpB expression levels across different subpopulations
For definitive methylation pattern analysis, researchers should employ Pacific BioSciences SMRT sequencing or Oxford Nanopore technology, which can directly detect methylated bases . These approaches have successfully characterized the cytosine-specific methylation patterns of ModP1 (CAA(m4)CT) and ModP2 (GWC(m4)CT) in A. pleuropneumoniae .
What are the recommended approaches for generating knockout mutants to study atpB function?
Given the essential nature of ATP synthase for cellular viability, complete deletion of atpB is often lethal. Therefore, researchers should consider:
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Conditional knockout systems: Employing inducible promoters to control atpB expression
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Partial functional mutations: Targeting specific amino acid residues essential for proton translocation without completely eliminating expression
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Domain replacement strategies: Substituting portions of atpB with homologous regions from related bacteria
For genetic manipulation of A. pleuropneumoniae, the chloramphenicol acetyltransferase gene (cat) can be used as a selective marker, as demonstrated in the construction of ModP1 and ModQ mutants . Target gene regions can be replaced via homologous recombination, with successful transformants confirmed by PCR and sequencing .
How should researchers address potential confounding factors when analyzing atpB expression during infection?
When studying atpB expression during infection, several confounding factors require careful experimental design:
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Phase variation effects: Use ModP-locked (ON/OFF) strains to control for phasevarion effects
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Growth conditions: Standardize oxygen levels, nutrient availability, and pH to minimize variation
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Host factors: Account for host immune responses that might indirectly affect bacterial metabolism
Experimental designs should include time-course analyses with samples collected at consistent infection stages. RNA preservation methods critical for accurate expression analysis include immediate stabilization with RNAlater or flash freezing, followed by targeted RNA extraction protocols optimized for bacterial RNA recovery from host tissues.
What physiological assays best measure the functional impact of atpB variants?
Functional assessment of atpB variants requires multiple complementary approaches:
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ATP synthesis assays: Measure ATP production rates using luciferase-based detection systems
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Membrane potential measurements: Use voltage-sensitive fluorescent dyes to assess proton gradient maintenance
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Growth kinetics under stress: Compare growth rates under varying pH, temperature, and nutrient conditions
The correlation between specific atpB sequence variations and functional outcomes should be analyzed using structure-function prediction algorithms, followed by targeted mutagenesis to confirm the role of key residues.
How might atpB interact with phase-variable DNA methyltransferases to influence A. pleuropneumoniae virulence?
The interplay between metabolic functions (atpB) and epigenetic regulatory systems (phase-variable methyltransferases) represents an exciting frontier in A. pleuropneumoniae research. Future studies should investigate:
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Correlation between methylation patterns and atpB expression levels during different infection stages
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Impact of ModP allelic variants on ATP synthase function across different serotypes
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Role of energy metabolism regulation in immune evasion and antibiotic tolerance
The recent characterization of multiple phase-variable DNA methyltransferases in A. pleuropneumoniae, including the first described phase-variable cytosine-specific Type III DNA methyltransferase (ModP) , provides new opportunities to understand how fundamental cellular processes like ATP synthesis are regulated during infection.
What computational approaches best predict the impact of methylation on atpB regulation?
Advanced computational methods for studying methylation-dependent regulation include:
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Motif-based analyses: Identifying potential methylation sites (e.g., CAA(m4)CT for ModP1) in atpB regulatory regions
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Structural modeling: Predicting how DNA methylation affects transcription factor binding
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Machine learning algorithms: Training on existing methylome and transcriptome data to predict expression patterns
Combining these computational approaches with experimental validation will advance understanding of how epigenetic regulation influences ATP synthase expression and function in different A. pleuropneumoniae populations.
