Recombinant Burkholderia cepacia ATP synthase subunit b (atpF)

What is the function of ATP synthase subunit b (atpF) in Burkholderia cepacia?

ATP synthase subunit b (atpF) in Burkholderia cepacia serves as a critical structural component of the F-type ATP synthase complex. It forms part of the peripheral stalk, connecting the membrane-embedded F₀ domain to the catalytic F₁ domain. This connection is essential for maintaining the structural integrity of the ATP synthase complex during the rotational catalysis process. The subunit b is crucial for energy transduction, allowing the proton gradient established across the membrane to be harnessed for ATP synthesis. In Burkholderia species, ATP-utilizing enzymes play significant roles in bacterial survival and virulence, particularly in clinical isolates where they can modulate external ATP levels affecting host phagocytic cells .

How does the atpF gene differ between clinical and environmental isolates of B. cepacia?

The atpF gene and its expression can vary significantly between clinical and environmental isolates of B. cepacia. Clinical isolates, particularly those from cystic fibrosis patients, often show enhanced expression of ATP-utilizing enzymes compared to environmental strains. Several environmental strains have been found deficient in releasing ATP-utilizing enzymes that act as cytotoxic factors in the presence of millimolar ATP concentrations . The secretion of these ATP-related proteins in clinical isolates (such as strain 38) is notably enhanced in the presence of host proteins like α2-macroglobulin, suggesting adaptation to the host environment . These differences indicate potential evolutionary adaptations in the ATP synthase components, including the atpF gene product, that may contribute to virulence in clinical settings.

What structural characteristics define the ATP synthase subunit b in Burkholderia species?

In Burkholderia species, ATP synthase subunit b typically displays a characteristic structure consisting of a membrane-anchored N-terminal domain and an extended α-helical C-terminal domain that interacts with the F₁ sector. While specific structural data for B. cepacia atpF is limited, insights can be drawn from related Burkholderia species. For instance, B. pseudomallei possesses both conventional F-type ATP synthase and an N-type ATPase with unique structural features . The N-type ATPase contains a heptadecameric c-ring (17 subunits) with a molecular mass of approximately 141 kDa, resulting in an unusually high ion-to-ATP ratio of 5.7 . This structural arrangement suggests Burkholderia ATP synthases have evolved specialized configurations that may enhance their efficiency in challenging environments, such as the acidic conditions of phagosomes during infection .

What are the optimal conditions for expressing recombinant B. cepacia atpF in heterologous systems?

For optimal expression of recombinant B. cepacia atpF, E. coli BL21(DE3) or similar expression strains are recommended with the following conditions:

When expressing membrane-associated proteins like atpF, the use of specialized E. coli strains such as C41(DE3) or C43(DE3) may improve yield and reduce toxicity. For quasi-homologous expression, B. thailandensis can be used with arabinose induction (0.2%) at OD₆₀₀ of 0.6, with cells harvested after 4 hours at 37°C as demonstrated for similar Burkholderia ATPase components . Due to the high GC content (~70%) typical of Burkholderia genes, the use of combinatorial enhancer solutions (CES) during cloning is recommended .

What purification strategies effectively isolate functional recombinant atpF protein?

Purification of recombinant B. cepacia atpF protein requires careful consideration of its membrane-associated nature. The following stepwise protocol is recommended:

• Cell disruption using French press (1,000 bar) in buffer containing 20 mM Tris/HCl pH 8.0, 1 mM Pefablock, 1 mM DTT, and trace DNase A .

• Differential centrifugation: Remove cell debris (24,000 g, 35 min, 4°C), then isolate membrane vesicles (230,000 g, 60 min, 4°C) .

• Membrane protein solubilization: Resuspend membrane fraction in buffer (20 mM Tris/HCl pH 8.0, 100 mM NaCl, 5% glycerol) with appropriate detergent. For ATP synthase components, LDAO (lauryldimethylamine oxide) has shown superior results compared to other detergents in terms of protein stability and structural integrity .

• Affinity chromatography: If His-tagged, use Ni-NTA resin with gradient elution (20-250 mM imidazole).

• Size exclusion chromatography: Final purification step using Superdex 200 in buffer containing 0.05% suitable detergent.

For functional studies, it's critical to maintain the protein in a physiologically relevant state throughout purification. The choice of detergent is crucial – while LDAO may be optimal for structural studies of some ATP synthase components, milder detergents like DDM (n-dodecyl β-D-maltoside) might better preserve functional properties for enzymatic assays .

How can researchers assess the proper folding and functionality of purified recombinant atpF?

Multiple complementary techniques should be employed to confirm proper folding and functionality of recombinant atpF:

• Circular Dichroism (CD) Spectroscopy: To verify secondary structure composition, particularly the α-helical content expected in subunit b.

• Limited Proteolysis: Properly folded protein shows characteristic resistance patterns to controlled proteolytic digestion.

• Thermal Shift Assay: Monitors protein stability and can be used to optimize buffer conditions.

• Interaction Studies: Co-immunoprecipitation or pull-down assays with other ATP synthase subunits (particularly a and δ subunits) to confirm proper binding interfaces.

• Reconstitution Experiments: For functional assessment, reconstitution with other purified ATP synthase components to measure contribution to ATP hydrolysis/synthesis activity.

• NCD-4 Modification Reaction: Similar to techniques used for other ATP synthase components, fluorescence spectroscopy with NCD-4 (λₑₓ= 324 nm, λₑₘ= 440 nm) can detect conformational changes associated with functional protein states .

• Western Blot Analysis: Using specific antibodies against the recombinant protein or epitope tags to confirm expression and integrity of the full-length protein .

The combination of these approaches provides complementary data on structure, stability, and function of the recombinant atpF protein.

What techniques are most effective for determining the structural interactions of atpF within the ATP synthase complex?

The structural interactions of atpF within the ATP synthase complex can be effectively investigated using a combination of high-resolution techniques:

• Cryo-Electron Microscopy (Cryo-EM): Single-particle cryo-EM has emerged as the gold standard for visualizing large membrane protein complexes like ATP synthases. This technique has successfully resolved the structure of Burkholderia ATPase components to resolutions as high as 6.1 Å . For optimal results with B. cepacia ATP synthase components, the low-density, low-CMC detergent LDAO has proven superior in terms of map quality and resolution .

• Cross-linking Mass Spectrometry (XL-MS): This technique can capture dynamic interactions between subunit b and other components of the ATP synthase. Chemical cross-linkers like BS3 or EDC coupled with mass spectrometry analysis can identify proximity relationships and contact points.

• Förster Resonance Energy Transfer (FRET): By introducing fluorescent labels at strategic positions in atpF and interacting subunits, researchers can measure distances and conformational changes during the catalytic cycle.

• Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS): This approach identifies regions of atpF that are protected or exposed upon complex formation, providing insights into interaction interfaces.

• Computational Molecular Dynamics: Simulations based on homology models and experimental constraints can predict detailed interaction networks and conformational states.

When these complementary approaches are combined, they provide a comprehensive view of how atpF contributes to the structural integrity and function of the ATP synthase complex in B. cepacia.

How does atpF contribute to the proton conductance mechanism in B. cepacia ATP synthase?

While the atpF gene product (subunit b) does not directly participate in proton translocation, it plays a critical supporting role in the proton conductance mechanism of B. cepacia ATP synthase:

• Structural Scaffold: Subunit b forms part of the peripheral stalk that prevents rotation of the α₃β₃ hexamer during catalysis, enabling the conversion of proton flow through the F₀ sector into mechanical energy that drives ATP synthesis.

• Maintenance of Proton Channel Integrity: Through its interactions with subunit a, which contains essential components of the proton channel, subunit b helps position critical residues involved in proton translocation.

• Transmission of Conformational Changes: Subunit b may transmit conformational changes between the membrane and catalytic domains during proton translocation, optimizing the efficiency of energy conversion.

What post-translational modifications have been identified in B. cepacia atpF, and how do they affect function?

Post-translational modifications (PTMs) of B. cepacia atpF represent an emerging area of research with important functional implications:

The identification and characterization of these modifications require sophisticated proteomic approaches:

Understanding these modifications will provide insights into how B. cepacia fine-tunes its energy production in response to environmental changes, particularly during infection processes.

How does atpF expression change during B. cepacia infection in cystic fibrosis patients?

During B. cepacia infection in cystic fibrosis patients, atpF expression undergoes significant modulation as part of the bacterium's adaptation to the host environment:

• Enhanced Expression in Clinical Isolates: Clinical isolates of B. cepacia, particularly from cystic fibrosis patients, show upregulation of ATP-utilizing enzymes compared to environmental strains . This suggests that ATP synthase components, potentially including atpF, are differentially regulated during infection.

• Host Factor Responsiveness: The presence of host factors, such as α2-macroglobulin, has been shown to greatly enhance the secretion of ATP-utilizing enzymes in cystic fibrosis isolates (strain 38) . This indicates that B. cepacia can sense and respond to the host environment by modulating its energy metabolism.

• Adaptation to Microenvironments: Within the CF lung, B. cepacia encounters various microenvironments with different oxygen availability and pH conditions. Studies suggest that ATP synthase expression is dynamically regulated to optimize energy production under these changing conditions.

• Relation to Virulence: Clinical isolates secrete ATP-utilizing enzymes that can modulate external ATP levels, affecting host phagocytic cells and potentially contributing to bacterial survival . The structural components of ATP synthase, including atpF, may play supporting roles in this process.

These changes in expression patterns highlight the importance of energy metabolism in B. cepacia pathogenesis and suggest that ATP synthase components could be targets for therapeutic intervention.

What experimental approaches can assess the role of atpF in B. cepacia biofilm formation?

To investigate the role of atpF in B. cepacia biofilm formation, researchers can employ the following experimental approaches:

• Gene Knockout and Complementation Studies:

  • Generate atpF deletion mutants using homologous recombination or CRISPR-Cas9

  • Complement with wild-type and mutated versions of atpF

  • Compare biofilm formation quantitatively and qualitatively

• Microscopy Techniques:

  • Confocal laser scanning microscopy with fluorescent probes to analyze biofilm architecture

  • Scanning electron microscopy to examine detailed surface structures

  • Live-cell imaging to track biofilm development over time

• Transcriptional Analysis:

  • RNA-Seq to compare global gene expression between planktonic and biofilm states

  • qRT-PCR to quantify atpF expression during different stages of biofilm formation

  • Transcriptional reporter fusions to monitor atpF expression in situ

• Biochemical Assays:

  • Crystal violet assays to quantify total biofilm biomass

  • XTT/resazurin assays to measure metabolic activity within biofilms

  • Extracellular polymeric substance (EPS) quantification

• Co-culture Experiments:

  • Mixed species biofilms to assess competitive fitness

  • Host cell interaction models to evaluate adhesion and invasion

Related research has shown that mutations in BceF, a protein kinase involved in exopolysaccharide production in B. cepacia, lead to alterations in biofilm architecture . Given that energy metabolism is crucial for biofilm formation and maintenance, atpF likely plays a role in this process by supporting ATP production necessary for exopolysaccharide synthesis and other biofilm-related functions.

How might targeting atpF impact B. cepacia survival under stress conditions relevant to infection?

Targeting atpF could significantly impact B. cepacia survival under various stress conditions encountered during infection:

• Acid Stress Tolerance: B. cepacia must survive in acidic microenvironments, including phagolysosomes. Related research in B. pseudomallei suggests that ATPases function as efficient proton pumps that help bacteria maintain pH homeostasis in phagosomes . Disruption of atpF function could compromise this ability, reducing bacterial survival.

• Oxidative Stress Response: During infection, B. cepacia encounters reactive oxygen species generated by host immune cells. ATP synthase function is linked to maintaining redox balance and energy supply during oxidative stress response. Comparative transcriptome analysis between wild-type and mutant B. cepacia has shown that disruption of related pathways (bceF mutant) leads to reduced survival under stress conditions, including UV light exposure .

• Nutritional Limitation: Infection sites often present nutrient-limited environments. ATP synthase efficiency becomes crucial for energy conservation under these conditions. The high ion-to-ATP ratio (5.7) observed in related Burkholderia species suggests specialized adaptations for energy efficiency .

• Temperature Variation: B. cepacia must adapt to temperature fluctuations during infection. Studies have shown that mutation of related pathways (bceF) leads to reduced survival under heat shock conditions , suggesting that proper energy metabolism is essential for thermotolerance.

• Antibiotic Exposure: Energy-dependent efflux pumps are key mechanisms of antimicrobial resistance in B. cepacia. ATP synthase inhibition could potentially enhance antibiotic efficacy by reducing available energy for these resistance mechanisms.

The table below summarizes the potential impact of atpF targeting on B. cepacia stress responses:

What techniques can resolve potential discrepancies in experimental data regarding atpF function?

When researchers encounter discrepancies in experimental data regarding atpF function, several advanced troubleshooting and validation approaches can help resolve these contradictions:

• Multi-system Validation:

  • Express atpF in different heterologous systems (E. coli, B. thailandensis, cell-free systems)

  • Compare protein behavior across systems to identify system-specific artifacts

  • Use quasi-homologous expression in related Burkholderia species for more native conditions

• Structural Integrity Confirmation:

  • Employ multiple structural analysis techniques in parallel

  • Utilize hydrogen-deuterium exchange mass spectrometry to assess folding state

  • Compare results from detergent-solubilized samples with membrane nanodiscs or amphipols

• Advanced Functional Assays:

  • Develop real-time ATP synthesis/hydrolysis assays

  • Use reconstituted proteoliposomes with defined composition

  • Implement patch-clamp techniques to directly measure proton flux

• Conditional Mutants and Genetic Approaches:

  • Create temperature-sensitive or inducible atpF mutants

  • Use suppressor mutation analysis to identify functional interactions

  • Apply CRISPR interference for partial knockdown instead of complete knockout

• Controlled Environmental Parameters:

  • Systematically vary pH, ion concentrations, and temperature

  • Test function under conditions mimicking infection microenvironments

  • Consider the impact of oxygen tension on ATP synthase function

When contradictory results are obtained, it's essential to examine the specific experimental conditions used. For example, the choice of detergent significantly impacts the quality and resolution of structural data for ATP synthase components, with LDAO showing superior results compared to other detergents for Burkholderia ATPase components . Similarly, the high GC content (~70%) of Burkholderia genes may necessitate specialized approaches for successful cloning and expression .

How can evolutionary analysis of atpF across Burkholderia species inform understanding of its functional adaptations?

Evolutionary analysis of atpF across Burkholderia species provides valuable insights into functional adaptations and specialized roles:

• Sequence Conservation Patterns:

  • Perform multiple sequence alignment of atpF across Burkholderia species

  • Identify highly conserved regions likely essential for core functions

  • Detect variable regions potentially associated with species-specific adaptations

• Positive Selection Analysis:

  • Calculate dN/dS ratios to identify positions under positive selection

  • Apply branch-site models to detect lineage-specific selection

  • Correlate selected sites with known functional domains or interaction interfaces

• Structural Homology Modeling:

  • Develop comparative models based on resolved structures from related species

  • Map conserved and variable regions onto 3D structures

  • Predict functional consequences of sequence variations

• Genomic Context Analysis:

  • Compare organization of ATP synthase operons across species

  • Identify lineage-specific gene arrangements or additional components

  • Analyze presence of both F-type and N-type ATPases in different species

• Host Adaptation Signatures:

  • Compare atpF sequences between environmental and clinical isolates

  • Identify polymorphisms associated with host adaptation

  • Correlate genetic variations with virulence phenotypes

This comparative approach reveals that Burkholderia species have evolved specialized energy-generating systems. For example, B. pseudomallei harbors both a conventional F-type ATP synthase and an N-type ATPase with unique structural features, including a heptadecameric c-ring resulting in an unusually high ion-to-ATP ratio of 5.7 . Such adaptations likely help these bacteria survive in hostile environments like the acidic interior of phagosomes . Similar specialized adaptations may exist in the ATP synthase components of B. cepacia, particularly in clinical isolates where energy metabolism is crucial for virulence and persistence.

What are the cutting-edge methodologies for investigating atpF interactions with host immune factors during infection?

Investigating interactions between B. cepacia atpF and host immune factors requires sophisticated methodologies that bridge microbiology, immunology, and molecular biology:

• Advanced Imaging Techniques:

  • Super-resolution microscopy (STORM, PALM) to visualize bacterial-host interactions at nanoscale

  • Live-cell imaging with fluorescently tagged atpF to track dynamics during infection

  • Correlative light and electron microscopy (CLEM) to combine functional and ultrastructural data

• Host-Pathogen Protein Interaction Studies:

  • Proximity labeling techniques (BioID, APEX) to identify host proteins interacting with atpF

  • Cross-linking mass spectrometry to capture transient interactions

  • Surface plasmon resonance to measure binding kinetics with purified host factors

• Ex Vivo Infection Models:

  • Human airway epithelial cells grown at air-liquid interface

  • Precision-cut lung slices from CF patient-derived tissues

  • Organoid models that recapitulate CF airway microenvironments

• Single-Cell Approaches:

  • Single-cell RNA-seq of infected host cells to identify transcriptional responses

  • Mass cytometry (CyTOF) to phenotype immune cell populations during infection

  • Spatial transcriptomics to map host-pathogen interactions in tissue context

• Advanced Genetic Systems:

  • Conditional expression systems to modulate atpF levels during infection

  • Reporter strains to monitor ATP synthesis activity in vivo

  • CRISPR screens to identify host factors affecting bacterial ATP synthase function

Research has shown that host factors like α2-macroglobulin can significantly enhance the secretion of ATP-utilizing enzymes in clinical isolates of B. cepacia . This suggests complex interactions between host proteins and bacterial energy metabolism components. Additionally, the finding that B. cepacia isolates secrete ATP-utilizing enzymes that modulate external ATP levels, affecting host phagocytic cells , indicates that ATP-related bacterial proteins may directly interface with host immune responses.

What are the most promising future research directions for B. cepacia atpF studies?

The field of B. cepacia atpF research presents several promising avenues for future investigation that could significantly advance our understanding of bacterial energy metabolism and pathogenesis:

• Structural Biology Advances:

  • High-resolution cryo-EM studies of the complete B. cepacia ATP synthase complex

  • Time-resolved structural analysis to capture conformational changes during catalysis

  • Comparative structural analysis between F-type and potential N-type ATPases in B. cepacia

• Host-Pathogen Interface:

  • Mechanistic understanding of how host factors like α2-macroglobulin enhance ATP-utilizing enzyme secretion

  • Investigation of ATP synthase components as potential pathogen-associated molecular patterns (PAMPs)

  • Role of ATP synthase in modulating host cell death pathways during infection

• Therapeutic Applications:

  • Development of atpF-targeted inhibitors as potential novel antibiotics

  • Exploration of ATP synthase components as vaccine candidates

  • Screening for compounds that disrupt ATP synthase assembly rather than function

• Systems Biology Approaches:

  • Integration of proteomics, transcriptomics, and metabolomics data to place atpF in broader regulatory networks

  • Mathematical modeling of energy metabolism during different infection stages

  • Identification of synthetic lethal interactions with atpF for combination therapy approaches

• Environmental Adaptation Mechanisms:

  • Comparative analysis of atpF regulation between clinical and environmental isolates

  • Investigation of how atpF contributes to bacterial persistence in biofilms

  • Role of ATP synthase in adaptation to the challenging CF lung environment

These research directions will benefit from emerging technologies such as cryo-electron tomography for in situ structural studies, CRISPR-based approaches for precise genetic manipulation, and advanced computational methods for integrating diverse datasets. The unusual structural features observed in related Burkholderia ATPases, such as the heptadecameric c-ring in B. pseudomallei N-ATPase , suggest that B. cepacia may possess similarly specialized adaptations that could be exploited for therapeutic development.