Recombinant Salmonella choleraesuis ATP synthase subunit a (atpB)

What is the structure and function of ATP synthase subunit a (atpB) in Salmonella choleraesuis?

ATP synthase subunit a (atpB) in Salmonella choleraesuis is a membrane protein that forms part of the F0 sector of ATP synthase. The protein consists of 271 amino acids and functions as a critical component in the proton channel that drives ATP synthesis. The complete amino acid sequence is: MASENMTPQEYIGHHLNNLQLDLRTFSLVDPQNPPATFWTLNIDSMFFSVVLGLLFLVMFRSVAKKATSGVPGKFQTAIELIVGFVHGSVKDMYHGKSKLIAPLALTIFVWVFLMNLMDLLPIDLLPYIAEHWLGLPATRVVPSADVNITLSMALGVFILILFYSIKMKGIGGFAKELTLQPFNHWAFIPVNLILEGVSLLSKPVSLGLRLFGNMYAGELIFILIAGLLPWWSQWILNVPWAIFHILIITLQAFIFMVLTIVYLSMASEEH .

The protein is anchored in the inner membrane and forms a critical channel through which protons move, driving the rotary mechanism of ATP synthesis. This process is fundamental to cellular energetics and bacterial survival under various environmental conditions.

How should recombinant Salmonella choleraesuis atpB protein be stored for optimal stability?

For optimal stability of recombinant Salmonella choleraesuis atpB protein, the recommended storage protocol involves keeping the protein at -20°C in a Tris-based buffer with 50% glycerol, which has been optimized specifically for this protein . For extended storage periods, conservation at -80°C is advisable to prevent degradation.

It's important to note that repeated freeze-thaw cycles significantly compromise protein integrity and should be avoided. When working with the protein, prepare small working aliquots that can be stored at 4°C for up to one week . This approach minimizes the need for repeated freezing and thawing of the stock solution while ensuring experimental reproducibility.

What role does atpB play in Salmonella choleraesuis virulence?

While atpB itself is not directly classified as a virulence factor, ATP synthesis is critical for bacterial metabolism and survival during infection. Salmonella choleraesuis possesses several Salmonella pathogenicity islands (SPIs) that contribute directly to its virulence mechanisms, enabling it to cause extraintestinal infections in humans and pigs .

The five SPIs present in S. Choleraesuis strains (SPI-1, SPI-5, SPI-9, SPI-13, and CS54) all contribute to virulence through different mechanisms . For instance, SPI-1 encodes a type III secretion system (T3SS) required for invasion of host cells, while SPI-5 encodes effector proteins for this T3SS. As a fundamental energy generation component, atpB supports these virulence mechanisms by ensuring adequate ATP supply for the energetically demanding processes of invasion and replication within host cells.

How can researchers differentiate between the roles of atpB and other ATP synthase subunits in Salmonella pathogenesis?

Differentiating between the roles of atpB and other ATP synthase subunits in Salmonella pathogenesis requires a multifaceted experimental approach:

• Gene Knockout Studies: Create targeted knockout mutants of individual ATP synthase subunits, including atpB, and compare their virulence profiles in appropriate infection models. This approach allows for the assessment of each subunit's contribution to bacterial fitness during infection.

• Complementation Assays: Perform genetic complementation of the knockout strains with functional copies of the respective genes to confirm phenotypic restoration, validating the specificity of the observed effects.

• Site-Directed Mutagenesis: Introduce specific mutations in conserved domains of atpB to disrupt function while minimizing structural changes, then assess how these mutations affect ATP synthesis and virulence.

• Transcriptional Analysis: Analyze the expression patterns of ATP synthase genes, including atpB, during different stages of infection using RNA-Seq or qRT-PCR to identify potential differential regulation patterns that might suggest specialized roles.

• Proteomic Approaches: Use mass spectrometry-based proteomics to quantify the abundance of different ATP synthase subunits during infection and correlate with virulence phenotypes.

Research has shown that Salmonella virulence is closely tied to SPI-encoded virulence factors . The connection between basic metabolic functions (like ATP synthesis) and the activation or function of virulence determinants represents an important area for investigation in understanding Salmonella pathogenesis mechanisms.

What is the relationship between atpB function and the effectiveness of Salmonella Choleraesuis as a vaccine vector?

The relationship between atpB function and Salmonella Choleraesuis vaccine vector effectiveness involves several interconnected aspects:

Energy Homeostasis and Attenuation: ATP synthase activity directly affects bacterial energy metabolism. Controlled attenuation through modifications of energy-related genes like atpB can create balanced attenuation that maintains immunogenicity while reducing virulence.

Survival in Host Environments: Functional RpoS in attenuated S. Choleraesuis enhances strain viability in mice by improving survival in the gastrointestinal environment and facilitating entry into mucosa-associated lymphoid tissue . Since ATP production supports stress responses, atpB functionality indirectly influences this process.

Antigen Expression Capacity: Efficient ATP production is necessary for the expression of heterologous antigens in vaccine vectors. Modifications to atpB that maintain minimal required ATP production while limiting growth can create an optimal balance for vaccine effectiveness.

Metabolic Burden Management: Recombinant antigen expression creates a metabolic burden on bacterial vectors. Understanding and optimizing atpB function can help manage this burden to ensure sufficient antigen production without compromising vector viability.

When developing live attenuated Salmonella vaccines, researchers must consider how modifications to energy metabolism genes like atpB might affect both attenuation level and immunogenicity. The ideal balance maintains sufficient metabolic activity for antigen expression and immune stimulation while preventing pathology.

How do sequence variations in atpB across different Salmonella Choleraesuis strains correlate with virulence and host adaptation?

Sequence variations in atpB across different Salmonella Choleraesuis strains can be analyzed using comparative genomics approaches to identify correlations with virulence and host adaptation:

The majority of S. Choleraesuis isolates are identified as ST-145 (formerly ST-1804) with a characteristic allele profile: aroc-36, dnan-31, hemd-35, hisd-14, pure-26, suca-6, thra-8 . Some isolates exhibit single locus variants, such as:

• Strains from Estonia and the United States: aroc-129 variant (ST-363) and aroc-34 variant (ST-66)

• Strains from Germany, Italy, and Austria: suca-34 variant (ST-68)

These sequence variations can be correlated with virulence by:

• Phylogenomic Analysis: Constructing phylogenetic trees based on whole-genome sequences that include atpB and other genes to identify relationships between sequence variants and virulence phenotypes.

• Structural Modeling: Performing protein structure prediction to determine how amino acid substitutions in atpB might affect proton channel function and energy generation efficiency.

• Host Adaptation Studies: Comparing atpB sequences from strains isolated from different hosts (humans vs. pigs) to identify host-specific adaptations.

• Experimental Validation: Swapping atpB alleles between strains with different virulence profiles to directly test the contribution of specific variants to pathogenicity.

Understanding these correlations can provide insights into the evolutionary pressures on energy metabolism genes during host adaptation and may reveal whether atpB variants contribute directly or indirectly to the documented variations in virulence across Salmonella Choleraesuis lineages.

What are the optimal conditions for expressing and purifying recombinant Salmonella choleraesuis atpB protein?

Expression System Selection:
For membrane proteins like atpB, E. coli-based expression systems using C41(DE3) or C43(DE3) strains are recommended as they are engineered specifically for toxic or membrane protein expression. Alternative systems include cell-free expression systems that can directly incorporate the protein into nanodiscs or liposomes.

Expression Protocol:

• Clone the atpB gene (SCH_3783) into an expression vector with an appropriate tag (His-tag commonly used)

• Transform into expression host cells

• Culture in LB medium at 37°C until OD600 reaches 0.6-0.8

• Induce with IPTG (0.1-0.5 mM) and reduce temperature to 16-20°C

• Continue expression for 16-18 hours

• Harvest cells by centrifugation at 5000×g for 15 minutes at 4°C

Purification Method:

• Resuspend cell pellet in lysis buffer (typically Tris-based with protease inhibitors)

• Disrupt cells by sonication or French press

• Isolate membrane fraction by ultracentrifugation (100,000×g for 1 hour)

• Solubilize membrane proteins with appropriate detergent (DDM or LDAO at 1%)

• Perform affinity chromatography using the tag (Ni-NTA for His-tagged proteins)

• Consider size exclusion chromatography as a polishing step

• Store in Tris-based buffer with 50% glycerol at -20°C for short-term or -80°C for long-term storage

Quality Control:
Verify protein purity by SDS-PAGE and Western blotting. Assess functionality through ATP synthase activity assays if the protein is to be used for functional studies.

What experimental designs are most effective for studying atpB's role in Salmonella choleraesuis pathogenicity?

Genetic Manipulation Approaches:

• Conditional Knockdowns: Use inducible promoters to control atpB expression levels, allowing for temporal regulation during infection studies

• Point Mutations: Generate site-directed mutations in conserved regions of atpB to create strains with altered ATP synthase function without complete loss

• Domain Swapping: Exchange domains between atpB from S. Choleraesuis and non-pathogenic bacteria to identify regions important for pathogenicity

In Vitro Infection Models:

• Cell Invasion Assays: Compare invasion efficiency of wild-type and atpB-modified strains in relevant cell lines (e.g., intestinal epithelial cells, macrophages)

• Intracellular Survival Assays: Measure bacterial persistence within host cells over time

• Trans-epithelial Migration Studies: Assess the ability of bacteria to cross polarized epithelial monolayers

In Vivo Experimental Designs:

• Competitive Index Studies: Co-infect animal models with wild-type and atpB-modified strains to directly compare fitness

• Tissue Distribution Analysis: Track bacterial spread through different organs after infection

• Immune Response Profiling: Measure host immune responses to different bacterial strains

Integrative Approaches:

• Metabolomics: Analyze changes in bacterial and host cell metabolism during infection

• Transcriptomics: Profile gene expression in both pathogen and host during infection

• Proteomics: Identify protein-protein interactions between bacterial atpB-associated complexes and host factors

These methodologies should be designed to differentiate between direct effects of atpB alterations on virulence and indirect effects resulting from general metabolic impairment. Controls should include complementation studies and comparative analyses with mutations in other ATP synthase subunits to establish specificity.

How can researchers accurately measure the effect of environmental conditions on atpB expression and function in Salmonella Choleraesuis?

Quantitative Expression Analysis:

• qRT-PCR Protocol:

  • Extract total RNA from S. Choleraesuis grown under different conditions

  • Treat with DNase to remove genomic DNA contamination

  • Synthesize cDNA using reverse transcriptase

  • Perform qPCR with atpB-specific primers

  • Normalize to appropriate reference genes (rpoD or 16S rRNA)

  • Calculate relative expression using the 2^(-ΔΔCt) method

• Western Blot Analysis:

  • Prepare bacterial lysates from different conditions

  • Separate proteins by SDS-PAGE

  • Transfer to PVDF membrane

  • Probe with anti-atpB antibodies

  • Visualize and quantify band intensity

  • Normalize to constitutive proteins (e.g., GroEL)

Functional Assays:

• ATP Synthesis Measurement:

  • Isolate bacterial membrane vesicles

  • Establish a proton gradient (pH differential)

  • Add ADP and inorganic phosphate

  • Measure ATP production over time using luciferase-based assays

  • Compare rates across different growth conditions

• Membrane Potential Assessment:

  • Incubate bacteria with potential-sensitive fluorescent dyes (e.g., DiSC3(5))

  • Measure fluorescence changes in response to membrane energization

  • Correlate with ATP synthase activity

Environmental Condition Matrix:

Data Integration:
Correlate expression and functional data with virulence phenotypes to establish relationships between environmental adaptation of atpB, energy metabolism, and pathogenicity. This approach will provide insights into how S. Choleraesuis modulates energy production through atpB regulation during infection and environmental transitions.

What are the best approaches for studying interactions between atpB and Salmonella pathogenicity islands (SPIs)?

Genetic Correlation Analysis:

• Simultaneous Mutation Studies:

  • Generate strains with mutations in both atpB and key SPI genes

  • Compare single and double mutant phenotypes to identify genetic interactions

  • Look for synthetic lethality or suppressor effects

• Conditional Expression Systems:

  • Create strains where atpB or SPI genes are under inducible promoters

  • Assess how modulation of one system affects the function of the other

  • Measure virulence factor expression when ATP synthesis is limited

Molecular Interaction Analyses:

• Protein-Protein Interaction Screening:

  • Perform bacterial two-hybrid or pull-down assays with atpB and SPI-encoded proteins

  • Validate interactions using co-immunoprecipitation or FRET

  • Map interaction domains through truncation mutants

• Co-localization Studies:

  • Use fluorescently tagged proteins to visualize subcellular distribution

  • Perform super-resolution microscopy to detect proximity during infection

  • Correlate spatial arrangements with functional outcomes

Metabolic and Energetic Coupling:

• Energy Requirement Profiling:

  • Measure ATP consumption during SPI gene expression and secretion system assembly

  • Correlate ATP synthase activity with T3SS function

  • Develop ATP biosensors to monitor local ATP availability at virulence factor assembly sites

• Membrane Potential Distribution:

  • Use voltage-sensitive dyes to track membrane potential changes during virulence factor deployment

  • Assess how atpB mutations affect the proton motive force required for secretion systems

SPI Expression and Function Analysis:

Salmonella Choleraesuis contains five major SPIs with high identity (>95%): SPI-1, SPI-5, SPI-9, SPI-13, and CS54 . These SPIs contribute to virulence through specific mechanisms:

By implementing these methodological approaches, researchers can establish mechanistic links between Salmonella's energy metabolism through atpB function and the energetically demanding virulence mechanisms encoded by SPIs, providing valuable insights into how bacterial pathogens coordinate basic physiology with virulence.

What are the emerging research directions for understanding atpB's role in Salmonella choleraesuis biology and pathogenesis?

Emerging research directions for atpB in Salmonella choleraesuis include:

• Systems Biology Integration: Developing comprehensive models that connect energy metabolism through atpB function with virulence networks, providing predictive frameworks for understanding how metabolic perturbations affect pathogenicity.

• Host-Pathogen Energetic Interfaces: Investigating how S. Choleraesuis modulates host cell energy production during infection, potentially through interactions between bacterial ATP synthase components and host mitochondria.

• Strain-Specific ATP Synthase Adaptations: Comparative analysis of atpB sequence and function across different S. Choleraesuis lineages (ST-145, ST-363, ST-66, ST-68) to identify evolutionary adaptations that optimize energy production in specific host environments.

• Novel Attenuation Strategies: Developing precisely engineered atpB modifications for creating new live attenuated vaccine strains with optimized balance between attenuation and immunogenicity, leveraging the understanding that RpoS-functional strains with controlled energy metabolism may have enhanced vaccine potential .

• Nanoscale Visualization: Applying advanced microscopy techniques to visualize ATP synthase dynamics during the infection process, potentially revealing spatial and temporal regulation of energy production in response to host environments.