Recombinant Salmonella choleraesuis Fumarate reductase subunit D (frdD)

What is Fumarate reductase subunit D (frdD) in Salmonella choleraesuis?

Fumarate reductase subunit D (frdD) is a small hydrophobic protein component of the fumarate reductase complex in Salmonella choleraesuis. The protein consists of 119 amino acids with a molecular weight of approximately 13 kDa. It functions as part of the membrane-anchoring domain of the fumarate reductase enzyme complex, which catalyzes the reduction of fumarate to succinate during anaerobic respiration . The protein is characterized by its hydrophobic nature, containing multiple transmembrane regions that facilitate its integration into the bacterial cell membrane.

How does frdD contribute to bacterial metabolism?

The frdD protein serves as an essential anchor subunit of the fumarate reductase complex, which plays a crucial role in anaerobic respiration in Salmonella choleraesuis. By participating in the reduction of fumarate to succinate, this enzyme complex enables the bacterium to use fumarate as a terminal electron acceptor in the absence of oxygen. This metabolic pathway is particularly important for bacterial survival in low-oxygen environments, such as those encountered during host infection or in biofilm formation. The membrane-anchoring function of frdD ensures proper localization of the catalytic components of the enzyme complex, optimizing electron transport efficiency during anaerobic metabolism.

What are the optimal conditions for recombinant expression of Salmonella choleraesuis frdD?

For optimal recombinant expression of Salmonella choleraesuis frdD, researchers should consider a dual approach tailored to membrane proteins. Initial expression screening should evaluate both E. coli and yeast expression systems. For E. coli-based expression, BL21(DE3) or C41(DE3) strains (specialized for membrane proteins) are recommended with plasmids containing T7 or tac promoters. Expression conditions should include induction at lower temperatures (16-25°C) with reduced IPTG concentrations (0.1-0.5 mM) to minimize inclusion body formation.

The presence of hydrophobic regions in frdD necessitates optimization of membrane integration. Addition of fusion tags (such as His6, MBP, or SUMO) can improve stability and facilitate purification. Based on protein characteristics revealed in the search results, expression in minimal medium supplemented with glycerol as carbon source may enhance proper folding . Researchers should implement small-scale expression trials testing multiple conditions simultaneously before scaling up production.

What purification strategies are most effective for recombinant frdD?

Purification of recombinant frdD requires specialized approaches due to its hydrophobic nature and membrane association. Based on established protocols for similar proteins, the following multi-step strategy is recommended:

• Membrane fraction isolation: Following cell lysis, ultracentrifugation (typically 100,000×g for 1 hour) to separate membrane fractions.

• Detergent solubilization: Screening of multiple detergents (DDM, LDAO, or Triton X-100) at various concentrations (0.5-2%) to identify optimal solubilization conditions without protein denaturation.

• Affinity chromatography: If expressed with a His-tag, immobilized metal affinity chromatography (IMAC) using Ni-NTA resin with detergent-containing buffers (typically 50 mM Tris-HCl, pH 8.0, 150 mM NaCl, 0.05% detergent).

• Size exclusion chromatography: Final polishing step to remove aggregates and improve homogeneity.

Storage should include 50% glycerol in Tris-based buffer as indicated in product information . Validation of proper folding through circular dichroism or limited proteolysis is recommended prior to functional studies.

How can researchers verify the structural integrity of purified recombinant frdD?

Verification of structural integrity for purified recombinant frdD should employ multiple complementary techniques. Western blotting with specific antibodies confirms protein identity and integrity, while circular dichroism spectroscopy assesses secondary structure elements, particularly the alpha-helical content expected in membrane proteins. Thermal shift assays can evaluate protein stability under various buffer conditions.

For membrane proteins like frdD, additional specialized techniques are valuable. Limited proteolysis followed by mass spectrometry can identify properly folded domains resistant to digestion. Fluorescence spectroscopy exploiting intrinsic tryptophan fluorescence provides insights into tertiary structure. For highest resolution structural assessment, researchers might consider nuclear magnetic resonance (NMR) spectroscopy with isotope-labeled protein or crystallization trials in lipidic cubic phase for X-ray crystallography, although these approaches present significant technical challenges for membrane proteins.

What methods are used to assess the functional activity of recombinant frdD?

Assessment of recombinant frdD functional activity requires approaches that consider its role within the fumarate reductase complex. Since frdD functions as a membrane anchor rather than carrying catalytic activity itself, functional assays typically focus on:

• Complex reconstitution: Combining purified recombinant frdD with other fumarate reductase subunits (frdA, frdB, frdC) to reconstitute the complete enzyme complex in vitro.

• Membrane integration assays: Evaluating proper insertion into artificial liposomes or native membrane systems using fluorescence-based techniques or protease protection assays.

• Enzyme activity measurements: Monitoring fumarate reduction activity of the reconstituted complex spectrophotometrically by following NADH oxidation at 340 nm or using artificial electron donors like benzyl viologen.

• Protein-protein interaction studies: Employing techniques such as pull-down assays, surface plasmon resonance, or isothermal titration calorimetry to assess binding to other fumarate reductase subunits.

These approaches collectively provide a comprehensive assessment of whether recombinant frdD retains its native structure and function.

How can researchers study the membrane integration properties of frdD?

Studying membrane integration properties of frdD requires specialized techniques that probe protein-lipid interactions. Researchers can employ:

• Proteoliposome reconstitution: Incorporating purified frdD into artificial liposomes of defined lipid composition, followed by flotation assays or electron microscopy to verify integration.

• Fluorescence spectroscopy: Using environment-sensitive fluorescent probes (either intrinsic tryptophan residues or introduced labels) to detect changes in the local environment upon membrane insertion.

• Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR): Providing information about secondary structure elements and their orientation relative to the membrane plane.

• Molecular dynamics simulations: Complementing experimental approaches with computational modeling to predict membrane-protein interactions at atomic resolution.

• Site-directed mutagenesis: Systematically altering hydrophobic residues in the transmembrane regions to identify amino acids critical for membrane integration and stability.

These methodologies collectively provide insights into how frdD achieves proper membrane localization, which is essential for fumarate reductase complex assembly and function.

What are the challenges in studying protein-protein interactions involving frdD in the fumarate reductase complex?

Studying protein-protein interactions involving frdD presents several technical challenges due to its membrane-associated nature. Key difficulties include:

• Maintaining native conformation: Detergent solubilization, necessary for working with membrane proteins, can disrupt native interactions and conformations.

• Establishing appropriate interaction conditions: The membrane environment is difficult to replicate in vitro, requiring careful optimization of lipid composition, detergent types, and buffer conditions.

• Detection sensitivity: Interactions between membrane subunits may be of lower affinity or more transient than soluble protein interactions, requiring more sensitive detection methods.

To address these challenges, researchers should consider complementary approaches including:

• Crosslinking mass spectrometry to capture interaction interfaces

• Native mass spectrometry of intact complexes in detergent micelles or nanodiscs

• Split-reporter systems (like BRET or split-GFP) for monitoring interactions in living cells

• Cryogenic electron microscopy for structural characterization of assembled complexes

Careful validation using multiple orthogonal techniques is essential to distinguish specific from non-specific interactions.

Can frdD be utilized as an antigen in recombinant Salmonella vaccine vectors?

For frdD to serve as an effective antigen, it must be properly expressed, processed, and presented by the vaccine vector. The search results indicate that recombinant attenuated Salmonella Choleraesuis strains have successfully delivered heterologous antigens like PlpE, P42, and SaoA . These vectors utilize systems such as the balanced lethal system with Asd plasmid retention and regulated delayed attenuation systems to ensure stable expression and appropriate immunogenicity.

A rational approach would involve:

• Cloning frdD into expression plasmids similar to pYA3493 or derivatives used in successful vaccines

• Confirming stable expression through multiple passages (≥50) as demonstrated with other antigens

• Evaluating protein expression by Western blotting

• Assessing immune responses through both humoral and cell-mediated parameters

The highly hydrophobic nature of frdD may present challenges for expression and processing that would require careful optimization of expression constructs, potentially using fusion partners to enhance solubility.

How does the regulated delayed attenuation system impact antigen delivery in Salmonella vectors expressing heterologous proteins like frdD?

The regulated delayed attenuation system significantly impacts antigen delivery and immune responses in Salmonella vectors expressing heterologous proteins. This sophisticated genetic strategy allows the bacteria to establish effective colonization initially before becoming attenuated, optimizing the balance between safety and immunogenicity.

In vectors like rSC0016 and rSC0012, the regulated delayed attenuation is achieved through arabinose-dependent promoter systems controlling critical virulence genes like crp or fur . This system allows the vector to:

• Express full virulence factors during initial infection, enabling efficient colonization of lymphoid tissues

• Gradually attenuate as arabinose is depleted in vivo, enhancing safety while maintaining immunogenicity

• Deliver heterologous antigens more effectively through enhanced persistence in lymphoid tissues

Research has demonstrated that different regulated genes yield distinct immunological outcomes. For example, the regulated delayed fur mutation (rSC0012) induced less inflammatory cytokine production than the regulated delayed crp mutation (rSC0011) while still stimulating strong antibody responses . This suggests that for frdD expression, careful selection of the attenuation system would be critical to balance safety and immunogenicity.

When expressing membrane proteins like frdD, the impact on bacterial growth must also be considered, as heterologous antigen expression can affect growth characteristics and potentially motility .

What immune responses would be expected from a recombinant Salmonella vector expressing frdD?

Based on studies with other antigens expressed in recombinant Salmonella Choleraesuis vectors, a vector expressing frdD would likely induce a multi-faceted immune response. The expected immunological profile would include:

• Mucosal immunity: Significant induction of antigen-specific secretory IgA at mucosal surfaces, a key advantage of oral Salmonella vectors compared to injectable vaccines . This response is particularly relevant for protection against pathogens that initially colonize mucosal surfaces.

• Humoral immunity: Production of serum IgG antibodies specific to frdD. The research shows that recombinant Salmonella vectors effectively induce systemic antibody responses against heterologous antigens .

• Cell-mediated immunity: Development of mixed Th1/Th2 cellular immune responses as evidenced by cytokine profiles (IFN-γ and IL-4) observed with other antigens . This balanced response is advantageous for comprehensive protection.

• Duration and memory: The live vector approach typically promotes establishment of immunological memory, providing longer-lasting protection than killed vaccines.

The membrane-associated nature of frdD may present unique processing challenges for the immune system. Potential solutions could include expressing selected epitopes rather than the full protein or creating fusion constructs with immunogenic carrier proteins to enhance presentation to the immune system.

How can genome editing techniques be applied to modify frdD expression or function in Salmonella choleraesuis?

Advanced genome editing techniques offer precise approaches to modify frdD expression or function in Salmonella choleraesuis for functional studies or vaccine development. CRISPR-Cas9 systems adapted for Salmonella provide the most precise method, allowing for specific nucleotide changes, gene knockouts, or promoter modifications. This approach can create conditional expression systems for frdD using inducible promoters, enabling temporal control over expression.

Lambda Red recombineering represents another effective technique, particularly useful for creating scarless mutations or introducing epitope tags into the chromosomal frdD gene. For vaccine vector applications, regulated delayed expression systems similar to those described for crp and fur genes can be adapted for frdD . These systems could be designed to express frdD under specific in vivo conditions, optimizing immune responses.

When implementing these techniques, researchers should consider:

• The impact of frdD modifications on bacterial metabolism, particularly under anaerobic conditions

• Potential polar effects on adjacent genes in the frd operon

• Confirmation of modifications through sequencing and expression analysis

• Phenotypic verification through growth under various respiratory conditions

What are the approaches to study the role of frdD in Salmonella pathogenesis and host-pathogen interactions?

Studying the role of frdD in Salmonella pathogenesis requires a multi-faceted approach combining molecular genetics, cellular microbiology, and in vivo infection models. Key methodological approaches include:

• Precise genetic manipulation: Creation of frdD deletion mutants, point mutants affecting specific functional domains, and complemented strains to establish causality in observed phenotypes.

• In vitro infection models: Assessment of frdD mutant phenotypes in cell culture systems, including:

  • Invasion and intracellular survival in epithelial cells and macrophages

  • Analysis of intracellular bacterial metabolism using isotope labeling

  • Transcriptional profiling during infection using RNA-seq

• In vivo infection studies: Comparison of wild-type and frdD mutant strains in mouse models examining:

  • Bacterial burden in tissues over time

  • Histopathological changes

  • Host immune responses

• Metabolic profiling: Measurement of metabolic adaptations during infection using techniques like mass spectrometry-based metabolomics to identify altered metabolic pathways in frdD mutants.

These approaches would elucidate how fumarate reductase activity contributes to Salmonella's ability to adapt to the host environment, particularly in oxygen-limited niches encountered during infection.

How can structural biology approaches be used to understand frdD interactions within the fumarate reductase complex?

Structural biology approaches offer powerful tools to elucidate frdD interactions within the fumarate reductase complex, despite challenges associated with membrane proteins. Researchers can employ:

• Cryo-electron microscopy (cryo-EM): This technique has revolutionized membrane protein structural biology, allowing visualization of the entire fumarate reductase complex without crystallization. Single-particle analysis can achieve near-atomic resolution of the assembled complex in a lipid environment, revealing interaction interfaces between frdD and other subunits.

• X-ray crystallography: While challenging for membrane proteins, crystallization in lipidic cubic phases or with fusion partners can enable high-resolution structure determination. Co-crystallization with stabilizing antibody fragments may enhance crystal formation.

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS): This approach identifies protected regions at protein interfaces, mapping interaction sites between frdD and other fumarate reductase subunits without requiring crystals.

• Solid-state NMR spectroscopy: Particularly valuable for membrane proteins, this technique can provide structural information in a native-like lipid environment.

• Integrative modeling approaches: Combining lower-resolution data from multiple experimental sources with computational modeling to generate comprehensive structural models.

These methods collectively would reveal how frdD contributes to complex assembly, stability, and function, potentially identifying novel therapeutic targets or informing rational vaccine design.

What ELISA-based approaches are available for detecting and quantifying recombinant frdD?

ELISA-based detection and quantification of recombinant frdD requires specialized approaches due to its hydrophobic nature. Based on available commercial reagents and methodologies used for similar membrane proteins, researchers can implement several strategies:

• Direct ELISA: Immobilization of purified recombinant frdD directly on specialized hydrophobic plates, followed by detection with anti-frdD antibodies. This approach works best with purified protein and requires careful optimization of coating conditions to maintain native conformation .

• Sandwich ELISA: Utilizing a capture antibody specific to frdD or attached tags (His-tag), followed by detection with a second antibody recognizing a different epitope. This method offers improved sensitivity and specificity for complex samples.

• Competitive ELISA: Particularly useful when working with crude samples containing membrane fragments, this approach uses competition between sample frdD and a standard labeled frdD preparation for binding to immobilized antibodies.

For optimal results, researchers should:

• Include detergent (0.01-0.05% DDM or similar) in all buffers to maintain frdD solubility

• Consider using fusion proteins (MBP-frdD) to improve solubility and detection

• Validate assay specificity using appropriate controls including samples from frdD knockout strains

• Generate calibration curves using purified recombinant frdD standards

How can researchers investigate the influence of frdD on bacterial respiration and metabolism?

Investigating frdD's influence on bacterial respiration and metabolism requires integrating biochemical, genetic, and systems biology approaches. Researchers should implement:

• Respiratory activity measurements: Oxygen consumption rates and membrane potential assessment using fluorescent probes (e.g., TMRM or Rhodamine 123) in wild-type versus frdD knockout strains under various growth conditions.

• Growth phenotyping: Systematic growth curve analysis under aerobic, microaerobic, and anaerobic conditions with different carbon sources and electron acceptors. High-resolution growth phenotyping in plate readers can reveal subtle metabolic defects in frdD mutants.

• Metabolic flux analysis: Using 13C-labeled substrates combined with mass spectrometry to track carbon flow through central metabolic pathways, identifying alterations in frdD mutants compared to wild-type.

• Transcriptomic and proteomic profiling: RNA-seq and quantitative proteomics to identify compensatory changes in gene expression and protein levels in response to frdD deletion or modification.

• In vivo metabolic studies: Using animal infection models to assess how frdD contributes to Salmonella metabolism in host environments, particularly in oxygen-limited niches encountered during infection.

These approaches collectively provide a comprehensive understanding of how frdD contributes to Salmonella's metabolic flexibility and adaptation to changing environmental conditions.

What advanced imaging techniques can visualize frdD localization within bacterial cells?

Advanced imaging techniques offer powerful approaches to visualize frdD localization within bacterial cells with high resolution and specificity. Researchers can employ:

• Super-resolution microscopy: Techniques like STORM (Stochastic Optical Reconstruction Microscopy) or PALM (Photoactivated Localization Microscopy) overcome the diffraction limit, allowing visualization of frdD distribution at nanometer resolution. These approaches typically require fluorescently tagged frdD constructs or specific antibodies combined with fluorescent secondary antibodies.

• Correlative light and electron microscopy (CLEM): This integrative approach combines fluorescence localization with the ultrastructural context provided by electron microscopy, ideal for precise localization of frdD within membrane structures.

• Cryo-electron tomography: Particularly valuable for visualizing membrane proteins in their native context, this technique provides 3D reconstructions of bacterial cells in a near-native state, allowing visualization of fumarate reductase complexes within the membrane.

• Expansion microscopy: A newer technique that physically expands the specimen while maintaining relative spatial relationships, enabling super-resolution imaging with standard confocal microscopes.

• Live-cell imaging with photoactivatable fluorescent proteins: For dynamic studies of frdD trafficking and complex assembly over time in living bacteria.

When implementing these approaches, researchers should validate specificity using frdD knockout controls and consider the potential impact of tags on protein localization and function.

How does frdD function compare between pathogenic and non-pathogenic Salmonella strains?

Comparing frdD function between pathogenic and non-pathogenic Salmonella strains reveals important evolutionary adaptations relevant to virulence. While the primary enzymatic function of fumarate reductase remains conserved across strains, subtle differences in regulation, expression patterns, and activity can contribute to pathogenicity.

Key comparative analyses should include:

• Sequence analysis: Alignment of frdD sequences from multiple Salmonella strains (pathogenic and non-pathogenic) to identify conserved regions and strain-specific variations. Particular attention should be paid to transmembrane domains that might influence membrane integration efficiency.

• Expression profiling: Quantitative comparison of frdD expression levels under various environmental conditions (oxygen limitation, different pH values, host-mimicking conditions) between pathogenic strains like S. Choleraesuis and non-pathogenic laboratory strains.

• Functional complementation: Cross-complementation studies where frdD from pathogenic strains is expressed in non-pathogenic frdD mutants and vice versa, assessing whether functional differences exist that affect growth under various respiratory conditions.

• In vivo expression: Using reporter fusions to monitor frdD expression during infection in animal models, comparing expression dynamics between pathogenic and attenuated strains.

These comparative approaches can reveal how evolutionary pressure has shaped frdD function to optimize bacterial survival in host environments encountered during infection.

What role might frdD play in Salmonella biofilm formation and antibiotic resistance?

The potential role of frdD in Salmonella biofilm formation and antibiotic resistance represents an important yet underexplored research area. As a component of the anaerobic respiratory chain, frdD likely contributes to bacterial survival and metabolism within biofilm environments, which are typically oxygen-limited. Several lines of evidence suggest potential mechanisms:

• Metabolic adaptation: Biofilms create oxygen gradients where bacteria in deeper layers must rely on anaerobic respiration. As part of the fumarate reductase complex, frdD enables alternative electron transport, supporting survival in these oxygen-limited niches.

• Energy production: Efficient energy generation through anaerobic respiration may support extracellular polymeric substance production necessary for biofilm matrix formation.

• Persistence: Metabolically flexible bacteria can better survive antimicrobial treatment. The ability to utilize fumarate as a terminal electron acceptor through frdD-containing complexes may contribute to the persistence phenotype observed in biofilms.

Research approaches to investigate these connections should include:

• Biofilm formation assays comparing wild-type and frdD mutant strains

• Monitoring fumarate reductase activity in biofilm versus planktonic states

• Antimicrobial susceptibility testing in anaerobic versus aerobic conditions

• Transcriptional profiling of biofilms to assess frdD expression relative to other metabolic genes

These studies could reveal whether targeting fumarate reductase activity might serve as a strategy to disrupt biofilm formation or enhance antibiotic efficacy.

How can understanding frdD contribute to development of new antimicrobial strategies?

Understanding frdD structure, function, and role in Salmonella metabolism opens several promising avenues for novel antimicrobial development. As a component of the fumarate reductase complex essential for anaerobic respiration, frdD represents a potential target for selective inhibition of bacterial growth, particularly in the oxygen-limited environments encountered during infection.

Strategic approaches include:

• Structure-based drug design: Using structural data of the fumarate reductase complex to design small molecules that specifically disrupt frdD interactions with other subunits or prevent proper membrane integration. This approach requires high-resolution structural information obtainable through techniques discussed in question 5.3.

• Peptide inhibitors: Developing peptides that mimic interaction interfaces between frdD and other complex components, thereby disrupting complex assembly without affecting host proteins.

• Combination therapy strategies: Exploiting the metabolic vulnerability created by frdD inhibition by combining inhibitors with conventional antibiotics, potentially resensitizing resistant strains.

• Attenuation for vaccine development: Engineering attenuated Salmonella strains with modified frdD expression for use as live vaccines, building on the regulated delayed attenuation systems demonstrated with other genes .

The therapeutic potential is enhanced by the differences between bacterial fumarate reductase and mammalian succinate dehydrogenase, potentially allowing for selective targeting with minimal host toxicity. This approach is particularly promising for treating persistent infections where bacteria may reside in anaerobic niches.