Recombinant Shigella flexneri serotype 5b Fumarate reductase subunit C (frdC)

How does frdC differ from frdD in the fumarate reductase complex?

Both frdC and frdD are hydrophobic membrane anchor proteins in the fumarate reductase complex, but they differ in several key aspects:

What expression systems are optimal for recombinant production of S. flexneri serotype 5b frdC?

For successful expression of recombinant S. flexneri serotype 5b frdC, E. coli is the preferred heterologous host due to phylogenetic proximity to Shigella. Research indicates several optimized methodologies:

• Expression vectors: pET-based vectors with T7 promoter systems provide high-level expression with tight regulation using IPTG induction . The addition of fusion tags (particularly N-terminal His-tags) facilitates subsequent purification while maintaining protein functionality.

• Host strain selection: E. coli BL21(DE3) and its derivatives show enhanced expression of membrane proteins. Specialized strains like C41(DE3) or C43(DE3) are particularly effective for potentially toxic membrane proteins like frdC.

• Expression conditions:

  • Induction at lower temperatures (16-25°C) improves proper folding

  • Lower inducer concentrations (0.1-0.5 mM IPTG) prevent inclusion body formation

  • Rich media supplemented with glycerol can increase yields

• Membrane integration: Addition of mild detergents (0.1% Triton X-100) during expression can improve membrane insertion efficiency.

Most commercial preparations of recombinant frdC use E. coli expression systems with N-terminal His-tags, which have proven effective for maintaining protein structure and function .

What are the critical considerations for purification of recombinant frdC protein?

Purification of recombinant frdC requires specialized approaches due to its hydrophobic nature and membrane localization:

• Cell lysis and membrane preparation:

  • Mechanical disruption (sonication or French press) in buffer containing protease inhibitors

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

  • Membrane solubilization using appropriate detergents (n-dodecyl-β-D-maltoside, CHAPS, or Triton X-100)

• Affinity chromatography:

  • For His-tagged proteins, Ni-NTA or Co-NTA resins with imidazole gradient elution

  • Critical detergent concentration must be maintained throughout purification

  • Inclusion of glycerol (5-10%) improves stability

• Further purification:

  • Size exclusion chromatography to remove aggregates

  • Ion exchange chromatography for additional purification

• Quality assessment:

  • SDS-PAGE analysis (>90% purity should be achievable)

  • Western blotting with anti-His antibodies

  • Mass spectrometry for identity confirmation

  • Circular dichroism to verify secondary structure integrity

Maintaining appropriate detergent concentrations above the critical micelle concentration throughout all purification steps is essential for preventing protein aggregation and precipitation.

What storage conditions maintain stability and activity of purified recombinant frdC?

Optimal storage conditions for maintaining recombinant frdC stability include:

• Short-term storage (up to one week):

  • 4°C in purification buffer containing appropriate detergent

  • Addition of protease inhibitors to prevent degradation

• Long-term storage:

  • Lyophilization is preferred for maximum stability

  • Storage at -20°C/-80°C with addition of cryo-protectants

  • Glycerol at 50% final concentration prevents freeze-damage

  • Aliquoting to avoid repeated freeze-thaw cycles

• Reconstitution protocol:

  • Brief centrifugation prior to opening vials

  • Reconstitution in deionized sterile water to 0.1-1.0 mg/mL

  • Addition of appropriate detergent if not present in the lyophilized material

  • Buffer containing 6% trehalose at pH 8.0 enhances stability

• Activity preservation:

  • Avoid repeated freeze-thaw cycles which significantly reduce functional activity

  • Monitor protein stability over time using activity assays

Research indicates that reconstitution into synthetic lipid membranes or nanodiscs may better preserve the native conformation and activity for functional studies compared to detergent solutions.

How can recombinant frdC contribute to molecular serotyping methods for Shigella flexneri?

Although traditional Shigella serotyping relies primarily on O-antigen variation, recombinant frdC can enhance molecular serotyping approaches through several methodological strategies:

• PCR-based differentiation:

  • Development of serotype-specific primers targeting variable regions of frdC

  • Integration into multiplex PCR assays alongside O-antigen modification genes (gtr and oac)

  • Real-time PCR assays with serotype-specific probes for rapid identification

• Antibody-based detection:

  • Generation of serotype-specific antibodies against variable regions of frdC

  • Implementation in ELISA or immunofluorescence assays

  • Development of lateral flow immunoassays for field detection

• Genomic fingerprinting:

  • Single nucleotide polymorphism (SNP) analysis of frdC sequences

  • Integration with whole genome sequence analysis for comprehensive strain typing

  • Correlation with existing serotyping schemes

Current molecular serotyping methods achieve over 97% sensitivity and 99.9% specificity compared to conventional serotyping . Including genetic markers like frdC could potentially improve these metrics, particularly for direct detection from clinical specimens.

What techniques can assess the interaction between frdC and other fumarate reductase subunits?

Understanding protein-protein interactions between frdC and other fumarate reductase subunits requires multiple complementary approaches:

• Co-purification strategies:

  • Co-expression of multiple subunits with differential tagging

  • Pull-down assays using tag-specific matrices

  • Chemical cross-linking followed by mass spectrometry (XL-MS)

  • Blue native PAGE to preserve native complexes

• Biophysical interaction analysis:

  • Surface plasmon resonance (SPR) with immobilized frdC

  • Isothermal titration calorimetry (ITC) for thermodynamic parameters

  • Microscale thermophoresis (MST) for solution-based interaction analysis

  • Fluorescence resonance energy transfer (FRET) for proximity assessment

• Structural biology:

  • Cryo-electron microscopy of the assembled complex

  • X-ray crystallography with lipidic cubic phase crystallization

  • Hydrogen-deuterium exchange mass spectrometry to map interaction surfaces

  • Solid-state NMR for membrane protein complexes

• Functional complementation:

  • Reconstitution of individual subunits into proteoliposomes

  • Activity assays measuring electron transfer or fumarate reduction

  • Mutational analysis targeting potential interaction sites

These methodologies can elucidate the quaternary structure and assembly mechanism of the fumarate reductase complex, which is essential for understanding its functional properties.

How should researchers assess functional activity of recombinant frdC?

Assessing the functional activity of recombinant frdC requires specialized approaches that account for its role within the fumarate reductase complex:

• Reconstitution assays:

  • Incorporation of purified frdC into liposomes

  • Co-reconstitution with other fumarate reductase subunits

  • Monitoring membrane potential using fluorescent dyes

  • Measurement of proton translocation across membranes

• Electron transfer measurements:

  • Spectrophotometric assays monitoring reduction of artificial electron acceptors

  • Electrochemical approaches measuring electron transfer capabilities

  • EPR spectroscopy to analyze paramagnetic centers involved in electron transfer

• Complex assembly assessment:

  • Analysis of protein-protein interactions with frdA, frdB, and frdD

  • Size exclusion chromatography to verify complex formation

  • Native PAGE to assess complex integrity

  • Mass spectrometry of intact complexes

• Complementation studies:

  • Expression in ΔfrdC bacterial strains

  • Assessment of growth under anaerobic conditions with fumarate

  • Measurement of fumarate reductase activity in membrane preparations

Importantly, functional activity assessment should include appropriate controls with known inactive mutants to validate assay specificity.

How does S. flexneri serotype 5b frdC compare with homologous proteins in other Shigella serotypes?

Comparative analysis of frdC across different Shigella flexneri serotypes reveals important evolutionary and functional insights:

• Sequence conservation analysis:

  • Core functional regions show high conservation (>95% identity) across serotypes

  • Variation primarily occurs in surface-exposed regions

  • Analysis of positive selection signatures within specific lineages

• Serotype-specific variations:

  • S. flexneri serotype 5b frdC shows distinctive amino acid substitutions compared to serotypes 2a, 3a, and 6

  • These variations may reflect adaptation to different metabolic niches

  • Correlation with serotype conversion events and genomic rearrangements

• Evolutionary context:

  • Phylogenetic analysis indicates frdC was present before serotype diversification

  • Most S. flexneri serotypes arose through horizontal acquisition of serotype conversion genes rather than through frdC mutation

  • Clustering of frdC sequences corresponds to the phylogenetic groups identified by whole genome analysis

This comparative analysis suggests that while frdC is not the primary determinant of serotype, its variations may contribute to metabolic differences between serotypes that influence pathogenicity and environmental adaptation.

What is the relationship between frdC sequence variation and S. flexneri pathogenicity?

The relationship between frdC sequence variation and pathogenicity remains an area of active investigation:

• Metabolic adaptation:

  • Variations in frdC may affect the efficiency of anaerobic respiration

  • Enhanced metabolic flexibility correlates with virulence in enteric pathogens

  • Comparative growth studies under oxygen-limited conditions reveal serotype-specific differences

• Expression analysis during infection:

  • Transcriptomic data indicates differential expression of frdC during cellular invasion

  • Upregulation coincides with transition to anaerobic metabolism in the intracellular environment

  • Correlation with expression of known virulence factors

• Phenotypic associations:

  • Mutations in frdC affect colonization and persistence in animal models

  • Certain frdC variants correlate with antibiotic resistance profiles

  • Association with extensively drug-resistant (XDR) phenotypes in clinical isolates

• Host response interactions:

  • Variations in surface-exposed regions may affect recognition by host immune system

  • Potential epitope masking through serotype-specific modifications

  • Altered inflammatory responses to different serotypes

How does frdC contribute to the metabolic adaptation of S. flexneri during infection?

Fumarate reductase subunit C plays critical roles in metabolic adaptation during Shigella infection:

• Anaerobic respiration:

  • Enables energy generation in the oxygen-limited gut environment

  • Supports growth when oxygen is unavailable as terminal electron acceptor

  • Provides metabolic flexibility during transitions between aerobic and anaerobic conditions

• Intracellular survival:

  • Facilitates adaptation to the cytosolic environment after escape from the phagosome

  • Contributes to pH homeostasis during exposure to acidic environments

  • Supports metabolic shifts required for persistent infection

• Nutrient acquisition:

  • Enables utilization of alternative carbon sources during nutrient limitation

  • Contributes to competitive fitness against commensal bacteria

  • Supports growth in different intestinal microenvironments

• Stress response integration:

  • Coordinates metabolic adaptation with expression of virulence factors

  • Contributes to tolerance of host-derived antimicrobial factors

  • Links metabolic state to type III secretion system regulation

Experimental approaches to study these adaptations include metabolic flux analysis with isotope-labeled substrates, transcriptional profiling during infection, and comparative growth studies with wild-type and frdC mutant strains under various environmental conditions.

How can CRISPR-Cas9 genome editing be applied to study frdC function in S. flexneri?

CRISPR-Cas9 technology offers powerful approaches for investigating frdC function in S. flexneri:

• Gene knockout strategies:

  • Design of guide RNAs targeting conserved regions of frdC

  • Development of S. flexneri-optimized CRISPR-Cas9 delivery systems

  • Creation of scarless deletions to minimize polar effects on adjacent genes

  • Complementation studies with wild-type and mutant variants

• Site-directed mutagenesis:

  • Precise modification of key residues to study structure-function relationships

  • Introduction of serotype-specific variations to assess their functional impact

  • Creation of epitope tags for in situ localization studies

  • Engineering of reporter fusions for expression analysis

• Regulatory studies:

  • Targeted modification of frdC promoter elements

  • CRISPRi (interference) for tunable repression of frdC expression

  • CRISPRa (activation) for upregulation studies

  • Implementation of inducible expression systems

• Mutant characterization:

  • Growth phenotyping under aerobic and anaerobic conditions

  • Metabolomic profiling to assess metabolic pathway alterations

  • Virulence assessment in cellular and animal infection models

  • Competitive fitness assays in mixed populations

When implementing these approaches, researchers should consider potential off-target effects, optimize transformation efficiency for Shigella, and validate all genetic modifications through whole genome sequencing.

What role might frdC play in vaccine development against S. flexneri?

The potential applications of frdC in vaccine development against S. flexneri include:

• As an antigen carrier:

  • Fusion of serotype-specific epitopes to frdC for enhanced presentation

  • Incorporation into recombinant protein vaccines alongside established antigens like IpaD

  • Design of chimeric proteins displaying multiple protective epitopes

  • Integration with regulated antigen expression systems

• In vaccine vector design:

  • Incorporation into outer membrane vesicle (OMV) vaccines

  • Expression in attenuated live vaccine strains

  • Utilization in DNA or RNA vaccine constructs

  • Implementation in regulated delayed antigen synthesis (RDAS) systems

• For cross-protection strategies:

  • Identification of conserved epitopes across serotypes

  • Design of multivalent vaccines targeting conserved regions of frdC

  • Integration with O-antigen-based vaccination approaches

  • Assessment of cross-protection against multiple serotypes

• In vaccine evaluation:

  • Development of serological assays measuring anti-frdC antibodies

  • Use as challenge strain markers in protective immunity studies

  • Assessment of cell-mediated immune responses to frdC epitopes

  • Correlation of anti-frdC responses with protection

Recent research on cross-protective antigens like the DBF fusion (combining IpaD, IpaB, and LTB) demonstrates the feasibility of generating broad protection against multiple Shigella serotypes , suggesting that incorporating metabolic proteins like frdC might enhance vaccine efficacy.

How can structural biology approaches be optimized for frdC membrane protein analysis?

Determining the high-resolution structure of membrane proteins like frdC requires specialized approaches:

• Sample preparation optimization:

  • Screening of detergents for optimal extraction and stability

  • Development of lipid nanodiscs or amphipol systems for native-like environments

  • Reconstitution into liposomes or bicelles for functional studies

  • Implementation of fusion partners to enhance stability and crystallizability

• Cryo-electron microscopy approaches:

  • Single particle analysis of the complete fumarate reductase complex

  • Optimization of grid preparation for membrane proteins

  • Implementation of focused classification for heterogeneous samples

  • Use of Volta phase plates to enhance contrast for smaller complexes

• X-ray crystallography strategies:

  • Lipidic cubic phase (LCP) crystallization for membrane proteins

  • In meso crystallization with monoolein or other lipids

  • Use of antibody fragments or nanobodies as crystallization chaperones

  • Serial crystallography at synchrotron sources or X-ray free electron lasers

• Integrative structural biology:

  • Combination of data from multiple structural techniques

  • Validation with cross-linking mass spectrometry

  • Integration of molecular dynamics simulations

  • Correlation of structural insights with functional data from mutagenesis

These approaches have successfully resolved structures of membrane protein complexes similar to fumarate reductase, providing templates for experimental design and optimization.

What emerging technologies show promise for studying frdC protein dynamics and interactions?

Several cutting-edge technologies are transforming research on membrane proteins like frdC:

• Single-molecule approaches:

  • Single-molecule FRET to study conformational changes

  • Atomic force microscopy for topographical analysis

  • Nanopore recording for single-molecule electrical measurements

  • Total internal reflection fluorescence microscopy for membrane dynamics

• Advanced imaging methods:

  • Super-resolution microscopy (STORM, PALM) for in situ localization

  • Correlative light and electron microscopy (CLEM)

  • Cryo-electron tomography of bacterial cells

  • Label-free imaging with coherent Raman scattering

• High-throughput interaction analysis:

  • Microfluidic-based binding assays

  • Protein complementation assays in living cells

  • Thermal proteome profiling for interaction networks

  • Proximity-dependent biotin labeling (BioID, APEX)

• Real-time dynamics assessment:

  • Time-resolved spectroscopy for electron transfer kinetics

  • Fast relaxation imaging for conformational changes

  • Hydrogen-deuterium exchange with rapid quench for dynamic regions

  • Microsecond mixing devices for transient intermediates

These technologies will enable unprecedented insights into the dynamic behavior of frdC within the context of the complete fumarate reductase complex and its interactions with other cellular components.

How might antimicrobial resistance mechanisms involve frdC in S. flexneri?

The potential role of frdC in antimicrobial resistance presents an important research frontier:

• Metabolic adaptation and persistence:

  • Fumarate reductase activity supports metabolic flexibility during antibiotic stress

  • Alterations in electron transport chain components affect susceptibility to respiratory inhibitors

  • Metabolic dormancy mediated by fumarate reductase activity contributes to persister cell formation

• Direct involvement in resistance:

  • Mutations in frdC associated with resistance to specific antibiotics

  • Correlation with extensively drug-resistant (XDR) phenotypes in clinical isolates

  • Co-localization of resistance determinants with frdC variants

• Resistance gene co-evolution:

  • Acquisition of resistance genes alongside serotype conversion

  • Correlation between frdC variants and plasmid-mediated resistance

  • Analysis of genetic linkage between metabolic genes and mobile genetic elements

• Experimental approaches:

  • Genome-wide association studies correlating frdC variations with resistance phenotypes

  • Directed evolution experiments under antibiotic selection pressure

  • Analysis of frdC expression during antibiotic exposure

  • Metabolic profiling of resistant versus susceptible strains

Recent studies identified XDR S. flexneri strains carrying both serotype-specific elements and mobilizable resistance determinants , suggesting potential functional relationships between metabolism and antimicrobial resistance that warrant further investigation.

How can systems biology approaches integrate frdC function into broader metabolic networks of S. flexneri?

Systems biology offers comprehensive frameworks for understanding frdC's role within Shigella metabolism:

• Genome-scale metabolic modeling:

  • Integration of frdC into genome-scale metabolic models of S. flexneri

  • Flux balance analysis to predict metabolic rewiring during infection

  • Simulation of frdC knockout effects on global metabolism

  • Identification of condition-specific metabolic vulnerabilities

• Multi-omics integration:

  • Correlation of frdC expression with global transcriptomic changes

  • Proteomic analysis of protein complex remodeling during environmental shifts

  • Metabolomic profiling to track metabolic flux through fumarate reductase

  • Integration of genomic variation with phenotypic outcomes

• Network analysis:

  • Mapping of protein-protein interaction networks centered on frdC

  • Regulatory network reconstruction to identify coordinated expression patterns

  • Identification of metabolic control points for therapeutic targeting

  • Comparative network analysis across Shigella serotypes

• Implementation methods:

  • Development of S. flexneri-specific constraint-based metabolic models

  • Application of machine learning for multi-omics data integration

  • Validation of model predictions through targeted experiments

  • Comparison with related enteric pathogens

These systems-level approaches enable contextualization of frdC function within the broader metabolic landscape of S. flexneri, revealing emergent properties not evident from reductionist studies and identifying potential therapeutic interventions.