Recombinant Listeria monocytogenes serovar 1/2a ATP synthase subunit b (atpF)

What is the genetic context and evolutionary significance of atpF in Listeria monocytogenes serovar 1/2a?

The atpF gene in Listeria monocytogenes serovar 1/2a (strain ATCC BAA-679/EGD-e) encodes ATP synthase subunit b, which is part of the F0 sector of the bacterial ATP synthase complex. This gene is located at the lmo2533 locus in the bacterial chromosome .

L. monocytogenes strains are classified into different evolutionary lineages, with serovar 1/2a predominantly found in lineage II, while serovars 4b and 1/2b are primarily associated with lineage I . These lineage distinctions are important as they reflect different evolutionary histories and potentially different functional adaptations of ATP synthase components.

Genomic analyses have shown that certain genes, including those in the ATP synthase complex, can exhibit specific patterns associated with particular serovars. When comparing whole-genome data, researchers identified 51 genes specific to serovar 4b and 83 genes specific to serovar 1/2a . Although atpF is not serovar-specific, its sequence variations might contribute to strain-specific metabolic characteristics.

Methodology for investigating evolutionary relationships:

• Whole genome sequencing and comparison

• Multi-locus sequence typing (MLST)

• Core genome alignment (as seen in Ecuadorian L. monocytogenes isolates studies)

• Phylogenetic tree construction based on conserved genes

How do storage and handling conditions affect the stability of recombinant L. monocytogenes atpF?

Optimal storage and handling conditions for recombinant L. monocytogenes atpF include:

• Storage buffer: Tris-based buffer with 50% glycerol, specifically optimized for this protein

• Storage temperature: -20°C for routine storage, -80°C for extended storage periods

• Stability considerations:

  • Avoid repeated freeze-thaw cycles which can lead to protein denaturation

  • Working aliquots should be stored at 4°C for up to one week

  • The high glycerol concentration (50%) acts as a cryoprotectant and helps maintain protein solubility

Methodologies for assessing protein stability:

• Thermal shift assays to determine melting temperature

• Size-exclusion chromatography to monitor aggregation

• SDS-PAGE to track degradation over time

• Activity assays to measure functional preservation

• Dynamic light scattering to assess protein homogeneity

How can recombinant L. monocytogenes atpF be utilized in Listeria-based vaccine development strategies?

Recombinant L. monocytogenes proteins, including components like atpF, can be incorporated into vaccine development strategies through several approaches:

• Listeria-based vector systems: L. monocytogenes has been developed as a vaccine vector due to its ability to induce robust cellular immune responses. Recombinant atpF could be expressed in these vector systems as:

  • A fusion protein with tumor antigens

  • Part of a multi-antigen construct

  • A component for enhancing immunogenicity

• Attenuated Listeria strains: Several attenuated platforms exist that could express atpF:

  • LADD (live-attenuated double deletion) strains

  • Listeria monocytogenes recombinase-induced intracellular death (Lm-RIID) strains

  • Suicidal Listeria strains that undergo autolysis upon entry into host cell cytosol

• Novel delivery approaches: The recombinant suicidal L. monocytogenes strain (rsΔ2) can deliver both protein antigens and eukaryotic expression vectors encoding the same antigen . This dual delivery approach could be applied to atpF.

Researchers have demonstrated that these vector systems can induce both humoral and cell-mediated immune responses. For example, an oral or intramuscular delivery of a suicidal L. monocytogenes strain expressing ovalbumin induced both humoral and cytolytic responses .

Methodology for vaccine development using atpF:

• Construction of expression vectors with atpF as an antigen

• In vitro testing of antigen presentation in dendritic cells

• Assessment of immunogenicity in animal models

• Evaluation of protection in challenge studies

• Compatibility testing with various adjuvants

What is the role of ATP synthase subunit b in L. monocytogenes stress response and virulence?

While the ATP synthase complex primarily functions in energy metabolism, its components, including subunit b, may contribute to stress response and virulence in L. monocytogenes:

• Relationship to stress survival islets: L. monocytogenes contains various stress survival islets (SSI) that help the bacterium adapt to environmental challenges. The relationship between ATP synthase function and these islets is an area of research interest:

  • SSI-1: Provides tolerance to acidic and salt stress

  • SSI-2: Confers tolerance to alkaline and oxidative stress (though not typically found in L. monocytogenes serovar 1/2a)

  • SSI-F2365: Present in most L. monocytogenes isolates

• Metabolic adaptation during infection: ATP synthase activity is crucial for bacterial adaptation to the intracellular environment:

  • The protein may be differentially regulated during various stages of infection

  • ATP production is essential for powering virulence mechanisms

• Potential interaction with virulence factors: ATP synthase components may interact with or influence the expression of established virulence factors:

  • LIPI-1 genes (crucial for intracellular lifecycle)

  • Internalins like inlA and inlB (important for host cell invasion)

  • Genes involved in biofilm formation (flaA, luxS, cheY)

Methodological approaches to study these relationships:

• Transcriptomic analysis under various stress conditions

• Construction of atpF deletion or point mutants

• Virulence assessment in cell culture and animal models

• Protein-protein interaction studies

• Metabolomic profiling during infection

What experimental techniques are most effective for studying protein-protein interactions involving L. monocytogenes atpF?

Several advanced techniques can be employed to study protein-protein interactions involving recombinant L. monocytogenes atpF:

• Co-immunoprecipitation (Co-IP):

  • Allows isolation of protein complexes using antibodies against atpF

  • Can be coupled with mass spectrometry for identification of interaction partners

  • Requires development of specific antibodies against L. monocytogenes atpF

• Bacterial two-hybrid systems:

  • Adapted for membrane proteins like ATP synthase components

  • Can screen for interactions between atpF and other bacterial proteins

  • Allows quantification of interaction strengths

• Surface Plasmon Resonance (SPR):

  • Provides real-time binding kinetics

  • Requires purified recombinant atpF and potential interaction partners

  • Can determine association/dissociation constants

• Crosslinking coupled with mass spectrometry:

  • Chemical crosslinkers can capture transient interactions

  • MS analysis identifies crosslinked peptides

  • Provides spatial constraints for structural modeling

• Förster Resonance Energy Transfer (FRET):

  • Labeled atpF and partner proteins enable monitoring of interactions in real-time

  • Can be used in living bacterial cells

  • Requires fluorescent protein fusions that maintain native function

Methodological workflow:

• Expression and purification of recombinant atpF with appropriate tags

• Verification of protein folding and activity

• Initial screening for potential interaction partners

• Validation with multiple orthogonal techniques

• Functional characterization of identified interactions

What are the optimal conditions for expressing and purifying functional recombinant L. monocytogenes atpF?

Expression and purification of functional recombinant L. monocytogenes atpF requires careful optimization:

Expression systems:

• E. coli expression systems:

  • BL21(DE3) for high-level expression

  • C41(DE3) or C43(DE3) for membrane proteins

  • Tuner cells for controlled expression levels

• Expression vectors:

  • pET system with T7 promoter for high expression

  • pBAD system for arabinose-inducible, titratable expression

  • pCold for cold-shock induced expression to aid folding

• Induction conditions:

  • IPTG concentration: 0.1-0.5 mM for balanced expression

  • Temperature: 16-18°C for slow expression to aid folding

  • Induction time: Extended periods (16-20 hours) at lower temperatures

Purification strategy:

• Initial extraction:

  • Detergent selection critical for membrane protein extraction (DDM, LDAO, or C12E8)

  • Gentle cell lysis methods to preserve protein structure

  • Stabilizing additives (glycerol, reducing agents)

• Purification methods:

  • Affinity chromatography (Ni-NTA for His-tagged constructs)

  • Ion exchange chromatography

  • Size exclusion chromatography for final polishing

• Quality control:

  • Western blotting for identity confirmation

  • Circular dichroism for secondary structure verification

  • Mass spectrometry for exact mass determination

  • Activity assays to confirm functionality

Typical yield:
Bacterial expression systems typically yield 1-5 mg of purified recombinant atpF per liter of culture, though this varies with expression conditions and purification efficiency.

How can researchers design effective experimental controls when working with recombinant L. monocytogenes atpF?

Robust experimental design requires appropriate controls when working with recombinant L. monocytogenes atpF:

• Negative controls:

  • Empty vector constructs processed identically to atpF expression vectors

  • Non-related proteins expressed and purified under identical conditions

  • Inactivated atpF protein (heat-denatured or specific inhibitors)

  • Buffer-only controls for activity and binding assays

• Positive controls:

  • Well-characterized ATP synthase subunits from model organisms (E. coli)

  • Previously validated batches of recombinant atpF

  • Known interaction partners or substrates for functional assays

• Internal controls:

  • Housekeeping proteins for expression studies

  • Spiked standards for quantitative analyses

  • Tagged versions of atpF with established properties

• Validation controls:

  • Multiple methods to confirm the same result

  • Dose-response experiments to establish specificity

  • Competition assays to verify binding specificity

A control matrix should be established for each experiment, with systematic variation of key parameters to ensure robust and reproducible results.

How can atpF be utilized in developing diagnostic tools for L. monocytogenes detection?

Recombinant L. monocytogenes atpF offers several possibilities for developing sensitive and specific diagnostic tools:

• Antibody-based detection systems:

  • ELISA systems using anti-atpF antibodies

  • Lateral flow assays for rapid field detection

  • Immunofluorescence for microscopic visualization

• Nucleic acid-based detection:

  • PCR primers targeting the atpF gene

  • LAMP (Loop-mediated isothermal amplification) assays

  • DNA microarrays incorporating atpF sequences

• Biosensor development:

  • Surface plasmon resonance biosensors with immobilized anti-atpF antibodies

  • Electrochemical biosensors using atpF-specific aptamers

  • Piezoelectric biosensors for rapid detection

Comparative detection sensitivity table:

Methodological considerations for diagnostic development:

• Cross-reactivity testing with other Listeria species and common foodborne bacteria

• Validation across different food matrices

• Optimization for minimal sample preparation

• Field testing in food processing environments

What insights can comparative analysis of atpF across different L. monocytogenes serovars provide for evolutionary studies?

Comparative analysis of atpF across different L. monocytogenes serovars can provide valuable evolutionary insights:

• Phylogenetic relationships:

  • Core genome alignment incorporating atpF sequences helps establish evolutionary relationships between serovars

  • L. monocytogenes strains cluster into distinct evolutionary lineages, with serovar 1/2a predominantly in lineage II and serovars 4b and 1/2b in lineage I

• Selective pressure analysis:

  • Ratio of synonymous to non-synonymous mutations in atpF can indicate selective pressures

  • Conservation patterns suggest functional constraints on specific protein domains

  • Hypervariable regions may indicate adaptation to different environments

• Host adaptation signatures:

  • Comparing atpF sequences from clinical versus environmental isolates

  • Analysis of atpF sequences from persistent strains in food processing environments

  • Correlation with stress survival islet (SSI) distribution patterns

• Recombination and horizontal gene transfer:

  • Analysis of homologous recombination within the ATP synthase operon

  • The role of RecA in genetic exchange affecting atpF

  • Identification of potential recombination hotspots

Methodological approaches:

• Whole genome sequencing of diverse L. monocytogenes isolates

• Maximum likelihood phylogenetic analysis of atpF sequences

• Selection of representative strains from each major lineage and serovar

• Integration with proteomic data to correlate genetic changes with phenotypic effects

• Analysis of core genome alignment encompassing 2,112 genes spanning 1,896,388 nucleotides

What approaches can resolve solubility and stability issues with recombinant L. monocytogenes atpF?

Membrane proteins like ATP synthase subunit b often present solubility and stability challenges. Researchers can employ several strategies to overcome these issues:

• Solubility enhancement strategies:

  • Fusion partners: MBP, SUMO, or thioredoxin tags

  • Detergent screening: Systematic testing of different detergent classes

  • Lipid nanodisc incorporation for native-like membrane environment

  • Co-expression with ATP synthase partner subunits

• Stability optimization:

  • Buffer optimization: pH, ionic strength, specific ions

  • Addition of stabilizing additives: glycerol (at 50% for long-term storage) , specific lipids, reducing agents

  • Thermal stability screening using differential scanning fluorimetry

  • Limited proteolysis to identify stable domains

• Expression modifications:

  • Temperature reduction during expression (16-20°C)

  • Codon optimization for expression host

  • Signal sequence optimization

  • Use of specialized E. coli strains with enhanced membrane protein expression capabilities

Methodological workflow for troubleshooting:

• Systematic variation of expression conditions

• Small-scale parallel screening of purification conditions

• Stability assessment under different buffer compositions

• Functional assays to verify biological activity of solubilized protein

• Implementation of quality control checkpoints throughout the purification process