Recombinant Pasteurella multocida Fumarate reductase subunit C (frdC)

What is Fumarate reductase subunit C and its role in Pasteurella multocida metabolism?

Fumarate reductase subunit C (frdC) is a membrane-anchoring protein component of the fumarate reductase complex in Pasteurella multocida. This complex catalyzes the reduction of fumarate to succinate during anaerobic respiration, allowing the bacterium to use fumarate as a terminal electron acceptor when oxygen is limited. In P. multocida, this enzyme plays a crucial role in energy metabolism under anaerobic conditions, which is particularly relevant during infection when oxygen availability may be restricted.

The complete protein consists of 132 amino acids with a molecular weight of approximately 15 kDa. The amino acid sequence (MTATTSKRKKYVREMKPTWWKKLDFYKLYIAREATAIPTLWFCLVLLYGVISLGSLDSFGNFISFLKNPIVIILNIITLGAMLLNTVTYYVMTPKVLNIIVKNERINPNIITMALWAVTAFISLVILVFMYV) contains predominantly hydrophobic residues, consistent with its role as a membrane anchor .

How does frdC expression change under different environmental conditions?

Fumarate reductase expression in Pasteurella multocida, including the frdC subunit, is influenced by oxygen availability and the presence of alternative electron acceptors. Similar to other respiratory enzymes in bacteria:

• Under aerobic conditions: frdC expression is typically repressed as the bacterium preferentially uses aerobic respiration pathways.

• Under anaerobic conditions: frdC expression is upregulated, particularly when fumarate is available as an electron acceptor.

• Under iron limitation: While specific data for frdC is limited, P. multocida shows significant changes in gene expression patterns under iron-limited conditions, which likely affects anaerobic respiration components .

This regulation pattern is similar to what has been observed with the nitrite reductase system (nrfE) in P. multocida, which is preferentially expressed in vivo during infection and is essential for nitrite reduction under both aerobic and anaerobic conditions .

What is the relationship between frdC and bacterial virulence in P. multocida?

While direct evidence linking frdC to virulence in P. multocida is limited, anaerobic respiration enzymes often contribute to bacterial pathogenesis by:

• Enabling survival in oxygen-limited infection sites

• Contributing to energy production during host colonization

• Potentially affecting redox balance during infection

Research on related respiratory enzymes, such as the nitrite reductase system, has shown that disruption of the nrfE gene did not affect virulence in mouse models despite being upregulated during infection . This suggests a complex relationship between respiratory enzymes and virulence that may be context-dependent.

What expression systems are most effective for producing recombinant P. multocida frdC?

Based on successful expression of other P. multocida proteins, the following expression systems have proven effective and could be applied to frdC:

Escherichia coli Expression System:

• BL21(DE3) strain is commonly used for expression of P. multocida proteins

• pET vector systems (such as pET43.1a) have been successfully employed for P. multocida protein expression

• Expression conditions typically involve induction with IPTG at 0.5-1.0 mM when cultures reach OD600 of 0.6-0.8

For membrane proteins like frdC, consider these optimizations:

• Lower induction temperatures (16-25°C) to improve proper folding

• Use of E. coli strains designed for membrane protein expression (C41/C43)

• Addition of detergents during cell lysis and purification

What purification strategies yield high-purity recombinant frdC protein?

For effective purification of recombinant frdC, consider the following approach:

Affinity Chromatography:

• His-tag purification using nickel-nitrilotriacetic acid (Ni-NTA) affinity chromatography has been successful for other P. multocida recombinant proteins

• For membrane proteins like frdC, include 0.1-1% detergent (e.g., DDM, LDAO) throughout purification

Additional Purification Steps:

• Size exclusion chromatography to separate properly folded protein from aggregates

• Ion exchange chromatography for removing contaminating proteins

Typical Buffer Conditions:

• Extraction buffer: 50 mM Tris-HCl pH 8.0, 300 mM NaCl, 10% glycerol, 0.5% detergent

• Storage buffer: Tris-based buffer with 50% glycerol as used for commercial preparations

How can I verify the proper folding and functionality of recombinant frdC?

To assess proper folding and functionality of recombinant frdC:

Structural Analysis:

• Circular dichroism (CD) spectroscopy to evaluate secondary structure content

• Thermal shift assays to assess protein stability

• Size exclusion chromatography to detect proper oligomerization state

Functional Assays:

• Measure fumarate reduction activity in reconstituted membrane systems

• Assess membrane integration using liposome incorporation assays

• Fumarate reductase activity can be measured spectrophotometrically by monitoring the oxidation of reduced benzyl viologen in the presence of fumarate

Similar approaches have been used to assess functionality of other respiratory enzymes in P. multocida, such as the nitrite reduction assay described for nrfE .

How can recombinant frdC be utilized in vaccine development against P. multocida?

Based on successful approaches with other P. multocida antigens, recombinant frdC could be utilized in vaccine development through:

Subunit Vaccine Approaches:

• Formulation with appropriate adjuvants: Single-phase water-in-oil adjuvants have shown success with other P. multocida recombinant proteins

• Combination with other immunogenic proteins: Multi-antigen formulations using different membrane proteins (similar to the VacJ, PlpE, and OmpH combination) have demonstrated enhanced protection compared to single-antigen vaccines

Evaluation Protocol:

• Assess antibody responses using ELISA to measure antigen-specific titers

• Challenge studies with virulent P. multocida strains to determine protective efficacy

• Histopathological examination and bacterial load detection to evaluate tissue protection

A systematic evaluation approach similar to that used for recombinant VacJ, PlpE, and OmpH proteins could be applied, where:

• Immunization with single antigens provided 33.3-83.33% protection

• Combination of multiple antigens achieved 100% protection against homologous challenge

• Killed whole-cell vaccines provided 50% protection

What role does frdC play in P. multocida adaptation to different host environments?

Fumarate reductase is critically important for bacterial adaptation to microaerobic and anaerobic environments within hosts:

Environmental Adaptation:

• During infection, P. multocida encounters varying oxygen concentrations in different host tissues

• Expression of fumarate reductase allows utilization of alternative electron acceptors when oxygen is limited

• This metabolic flexibility contributes to bacterial persistence in diverse host niches

Expression Patterns:

• Similar to nrfE, frdC expression may be upregulated during in vivo growth

• Real-time RT-PCR analysis could be used to quantify expression changes under different conditions, as was done for nrfE

• Understanding expression patterns can reveal when this metabolic pathway is most important during infection

How does the structure of frdC differ between P. multocida strains, and what are the implications for cross-protection?

Structural Variation Analysis:

• Sequence alignment of frdC from different P. multocida serotypes shows high conservation in membrane-spanning regions

• Variation typically occurs in loop regions that may be exposed to the periplasm

• These variations could affect antibody recognition and cross-protective potential

Implications for Vaccine Development:

• Highly conserved epitopes represent targets for broad cross-protection

• Strain-specific variations may necessitate multivalent vaccine approaches

• Epitope mapping using truncated recombinant fragments could identify immunodominant regions

This approach mirrors successful epitope mapping strategies used for PMT, where recombinant fragments (PMT-A, PMT-B, PMT-C, and PMT2.3) were evaluated for antigenicity and protection .

What are the most effective methods for studying frdC interactions with other fumarate reductase complex components?

In Vitro Protein-Protein Interaction Studies:

• Co-immunoprecipitation with antibodies against frdC or other complex components

• Bacterial two-hybrid systems to map interaction domains

• Surface plasmon resonance (SPR) to determine binding kinetics

Structural Studies:

• Cryo-electron microscopy of reconstituted complexes

• X-ray crystallography of the assembled complex

• Cross-linking mass spectrometry to identify interaction interfaces

Genetic Approaches:

• Site-directed mutagenesis to identify critical residues for complex assembly

• Complementation studies in frdC knockout strains

• Suppressor mutation analysis to identify functional interactions

How can gene knockout and complementation approaches be used to study frdC function?

Based on successful gene disruption methods used for the nrfE gene in P. multocida :

Gene Knockout Strategy:

• PCR amplification of the target gene and flanking regions

• Insertion of a tetracycline resistance cassette (tetM) into the coding sequence

• Electroporation of the construct into P. multocida after dam methylation

• Selection of recombinants on media containing tetracycline (2.5 μg/ml)

• Verification by PCR and sequencing

Complementation Approach:

• Cloning of the wild-type frdC gene into a shuttle vector

• Introduction of the complementation construct into the knockout strain

• Functional assays to confirm restoration of activity

Phenotypic Analysis:

• Growth curves under aerobic vs. anaerobic conditions

• Fumarate reduction assays with cell lysates or membrane fractions

• Virulence assessment in appropriate animal models

What protocols can be used to assess the immunogenicity of recombinant frdC for vaccine development?

Immunization Protocol:

• Prepare recombinant frdC protein (100 μg per dose) formulated with water-in-oil adjuvant (2:3 ratio)

• Vaccinate animal groups (n=6-10) via appropriate route (subcutaneous or intramuscular)

• Administer booster vaccinations at 2-3 week intervals

• Collect serum samples before vaccination and at regular intervals post-vaccination

Immunological Assays:

• ELISA to measure specific antibody titers:

  • Coat plates with purified native or recombinant frdC

  • Incubate with serial dilutions of test sera

  • Detect using species-appropriate secondary antibodies

  • Calculate endpoint titers or OD50 values

• Western blotting to confirm antibody specificity:

  • Run purified frdC and P. multocida lysates on SDS-PAGE

  • Transfer to membranes and probe with immune sera

  • Visualize with appropriate detection systems

Protection Studies:

• Challenge vaccinated animals with virulent P. multocida (typically 20 LD50)

• Monitor clinical signs, mortality, and bacterial colonization

• Perform histopathological examination to assess tissue protection

What are common challenges in expressing membrane proteins like frdC, and how can they be addressed?

Challenge 1: Protein Insolubility and Inclusion Body Formation

Challenge 2: Low Yield and Purification Difficulties

Challenge 3: Loss of Functional Activity

How can immune responses to frdC be enhanced for improved vaccine efficacy?

Based on approaches successfully used with other P. multocida antigens:

Adjuvant Selection and Optimization:

• Single-phase water-in-oil adjuvants have shown success with recombinant P. multocida proteins

• Optimal protein:adjuvant ratios are typically 2:3 with final concentration of 100 μg protein per 500 μl dose

Multi-Antigen Approaches:

• Combining frdC with other membrane proteins could enhance protection

• The combination of three recombinant proteins (VacJ, PlpE, OmpH) provided 100% protection against P. multocida challenge in ducks, while individual proteins gave 33.3-83.33% protection

Molecular Adjuvant Fusion:

• Fusion of complement component C3d to antigen peptides enhances immune responses

• Multiple C3d units (up to 6 repeats) can be linked to target epitopes using GS linkers

• This approach was successful with P. multocida toxin (PMT) epitopes

Epitope Enhancement:

• Identify immunodominant epitopes through epitope mapping

• Similar to PMT, where specific regions (PMT2.3) showed higher protection

• Link multiple epitopes with appropriate linker sequences

What bioinformatic approaches can identify functional domains and epitopes in frdC?

Functional Domain Prediction:

• Transmembrane topology prediction using TMHMM, MEMSAT, or Phobius

• Conserved domain analysis through InterPro, CDD, or Pfam

• Functional site prediction with ConSurf based on evolutionary conservation

Epitope Mapping:

• B-cell epitope prediction tools:

  • BepiPred for linear epitopes

  • DiscoTope for conformational epitopes

  • EPCES for surface accessibility analysis

• T-cell epitope prediction:

  • NetMHCpan for MHC-I binding prediction

  • NetMHCIIpan for MHC-II binding prediction

  • SYFPEITHI for proteasomal processing prediction

Structural Analysis:

• Homology modeling based on related fumarate reductase structures

• Molecular dynamics simulations to identify stable conformations

• Docking studies to predict interactions with other complex components

This bioinformatic approach can guide experimental design, similar to how PMT epitopes were predicted and used in the design of recombinant P. multocida toxin constructs .