Recombinant Brucella abortus biovar 1 ATP synthase subunit b (atpF)

What is the role of ATP synthase subunit b (atpF) in Brucella abortus metabolism?

ATP synthase subunit b forms part of the peripheral stalk in the F₁F₀-ATP synthase complex, which is crucial for energy production in B. abortus. This protein plays an essential structural role in connecting the F₁ and F₀ domains of the ATP synthase complex, allowing efficient ATP production during oxidative phosphorylation. In B. abortus, ATP synthase activity is particularly important during intracellular residence when the bacterium undergoes significant metabolic adaptations. Proteomic studies have demonstrated that B. abortus modifies its energy metabolism pathways during infection, shifting from carbohydrate-based carbon utilization to alternative energy sources based primarily on anaplerotic routes . The ATP synthase complex, including the atpF-encoded subunit b, is likely involved in these metabolic adaptations that support B. abortus survival within host cells.

How does atpF expression change during different phases of B. abortus infection?

The expression of atpF in B. abortus shows distinct patterns throughout the infection cycle, correlating with the bacterium's changing energy requirements. During early infection, when B. abortus downregulates carbohydrate metabolism and protein synthesis in response to the stressful environment of the phagosome, ATP synthase components like atpF may be differentially regulated . As the bacterium establishes its replicative niche within Brucella-containing vacuoles, energy production requirements increase to support bacterial replication, potentially leading to upregulation of ATP synthase components. Proteomic analyses have revealed that B. abortus undergoes significant metabolic adjustments during macrophage trafficking, including shifts to alternative energy sources and low-oxygen respiration , which would presumably affect ATP synthase function and regulation.

What genomic organization features characterize the atpF gene in B. abortus?

The atpF gene in B. abortus is part of the ATP synthase operon, which contains genes encoding all subunits of the F₁F₀-ATP synthase complex. Its genomic context may be particularly relevant given the findings that many important B. abortus genes exist in operons with conserved synteny across Alphaproteobacteria. For example, the BvrR/BvrS two-component system is part of an operon containing 16 genes with functions related to nitrogen metabolism, DNA repair, cell cycle arrest, and stress response . Similar operon structures may apply to energy metabolism genes like atpF, allowing coordinated expression of functionally related proteins. The genomic organization of ATP synthase genes may reflect evolutionary conservation across Alphaproteobacteria with similar intracellular lifestyles.

What are the recommended protocols for cloning and expressing recombinant B. abortus atpF?

For successful cloning and expression of recombinant B. abortus atpF, researchers should consider the following methodology:

• Gene Amplification: Design primers with appropriate restriction sites flanking the atpF coding sequence. Optimize PCR conditions for B. abortus genomic DNA, which typically has a high GC content.

• Expression Vector Selection: Choose an expression vector with appropriate promoters (e.g., T7 for E. coli systems) and affinity tags (His-tag or GST-tag) for downstream purification.

• Host Selection: While E. coli BL21(DE3) is commonly used, consider specialized strains that better accommodate membrane proteins or those with rare codons present in B. abortus.

• Expression Conditions: Optimize temperature (typically 16-25°C for membrane-associated proteins), IPTG concentration (0.1-1.0 mM), and induction time (4-16 hours) to maximize soluble protein yield.

• Protein Extraction: For membrane-associated proteins like atpF, use detergent-based extraction methods with carefully optimized detergent concentrations to maintain protein structure and function.

Similar approaches have been successful for other B. abortus proteins studied in previous research , though specific optimization for atpF may be necessary.

What purification strategies are most effective for maintaining native conformation of atpF?

Maintaining the native conformation of recombinant B. abortus atpF requires careful consideration of purification conditions:

• Affinity Chromatography: Initial purification via His-tag or GST-tag affinity chromatography provides good specificity. For His-tagged proteins, use imidazole gradients (20-250 mM) to minimize nonspecific binding.

• Buffer Optimization:

  Buffer Component	Recommended Range	Function
  pH	7.0-8.0	Maintains protein stability
  NaCl	150-300 mM	Reduces nonspecific interactions
  Glycerol	5-10%	Enhances protein stability
  Reducing agent	1-5 mM DTT or 0.5-2 mM TCEP	Prevents oxidation
  Detergent	0.03-0.1% DDM or 0.5-1% CHAPS	Solubilizes membrane proteins

• Size Exclusion Chromatography: Secondary purification via SEC helps remove aggregates and ensures homogeneity.

• Stability Assessment: Monitor protein stability using techniques like differential scanning fluorimetry (DSF) during purification optimization.

The detergent selection is particularly critical as atpF is likely membrane-associated, similar to other components involved in B. abortus membrane biogenesis that have been studied previously .

How should researchers design experiments to study atpF interactions with other ATP synthase subunits?

To effectively study interactions between B. abortus atpF and other ATP synthase subunits, consider these methodological approaches:

• Co-immunoprecipitation: Express tagged versions of atpF and potential partner subunits, then perform pull-down assays followed by Western blotting or mass spectrometry identification.

• Bacterial Two-Hybrid Systems: Adapt systems like BACTH (Bacterial Adenylate Cyclase Two-Hybrid) to study protein-protein interactions in a bacterial environment.

• Surface Plasmon Resonance: Determine binding kinetics between purified atpF and other ATP synthase components.

• Cross-Linking Mass Spectrometry: Use chemical cross-linkers followed by mass spectrometry to identify interaction interfaces between subunits.

• Fluorescence Resonance Energy Transfer (FRET): For in vivo studies, tag proteins with appropriate fluorophores to monitor interactions in live bacteria.

These approaches could be integrated with the dual reporter systems previously used to study B. abortus in macrophage infection models , providing insights into how these interactions change during infection.

How does atpF contribute to B. abortus survival during intracellular infection?

ATP synthase subunit b likely plays a critical role in B. abortus adaptation to the intracellular environment through:

• Energy Production During Nutrient Limitation: As B. abortus shifts from carbohydrate metabolism to alternative energy sources inside host cells , ATP synthase becomes crucial for generating energy under these restrictive conditions.

• Adaptation to pH Changes: When B. abortus encounters acidic environments during trafficking through macrophages, ATP synthase may help maintain membrane potential and internal pH homeostasis.

• Response to Oxidative Stress: The ATP synthase complex may contribute to managing the proton motive force during oxidative stress conditions encountered within phagocytes.

• Support for Persistence Mechanisms: Research has shown that a subpopulation of B. abortus remains metabolically active despite antibiotic treatment, constituting a reservoir for reinfection and relapse . ATP synthase function may be critical for maintaining this persistent state.

Studies using fluorescent microscopy and dual reporter systems similar to those used for tracking B. abortus in macrophages could be adapted to specifically investigate the role of atpF during these phases of infection.

What post-translational modifications of atpF have been identified and what are their functional implications?

While specific post-translational modifications (PTMs) of B. abortus atpF have not been extensively characterized in the provided research, potential modifications may include:

• Phosphorylation: The BvrR/BvrS two-component system in B. abortus is activated through phosphorylation in response to environmental cues . Similar regulatory phosphorylation events might affect ATP synthase components including atpF.

• Oxidative Modifications: Given that B. abortus encounters oxidative stress during infection, cysteine residues in atpF may undergo reversible oxidation, potentially regulating ATP synthase activity.

• Acylation: B. abortus modifies its membrane composition during intracellular survival , and lipid modifications of membrane-associated proteins like atpF could be part of this adaptation.

To identify these PTMs, researchers should employ:

• Mass spectrometry-based approaches (LC-MS/MS)

• Enrichment techniques for specific modifications

• Site-directed mutagenesis of potential modification sites

• Functional assays to determine the impact of modifications on ATP synthase activity

The proteomic approaches described for studying B. abortus under environmental stress would be valuable for characterizing these modifications.

How is atpF expression regulated in response to environmental stresses encountered during infection?

The regulation of atpF expression likely responds to multiple environmental cues encountered during infection:

• Nutrient Availability: During nutrient limitation, B. abortus adjusts its metabolism , potentially affecting ATP synthase expression through global regulators.

• Oxygen Tension: In low-oxygen environments within host cells, B. abortus employs alternative respiration mechanisms , which may involve coordinated regulation of ATP synthase components.

• Acidic pH: Exposure to acidic conditions in phagosomes may trigger specific regulatory responses affecting atpF expression, similar to the BvrR/BvrS system activation observed under acidic conditions .

• Growth Phase Dependence: The susceptibility of B. abortus to antibiotics varies with growth phase , suggesting growth phase-dependent regulation of metabolic systems including ATP synthase.

Regulatory studies could employ reporter constructs (similar to the dual reporter system used to study intracellular B. abortus ) fused to the atpF promoter to monitor expression under different conditions.

How can structural studies of atpF inform drug design targeting B. abortus ATP synthase?

Structural studies of B. abortus atpF can provide valuable insights for targeted drug development:

• Structure Determination Methods:

  • X-ray crystallography of the isolated subunit or preferably the entire ATP synthase complex

  • Cryo-electron microscopy to visualize the complete ATP synthase structure

  • NMR studies of specific protein-protein interaction domains

• Drug Target Identification:

  • Interface regions between atpF and other ATP synthase subunits

  • Unique structural features distinct from host ATP synthases

  • Allosteric sites that might affect complex assembly or function

• Structure-Based Drug Design Approach:

  Stage	Methodology	Outcome
  Target identification	Structural analysis	Druggable pockets
  Virtual screening	Molecular docking	Lead compounds
  In vitro validation	Enzyme inhibition assays	Confirmed hits
  Cell-based testing	Intracellular bacteria models	Efficacy in context
  Selectivity assessment	Host ATP synthase comparison	Safety profile

• Translational Potential: Structural studies could lead to new therapeutics addressing the antibiotic persistence observed in B. abortus infections , specifically targeting the metabolically active persister population.

What methodologies are most appropriate for studying atpF involvement in B. abortus bioenergetics during different infection stages?

To comprehensively investigate atpF's role in B. abortus bioenergetics throughout infection:

• Real-time Metabolic Monitoring:

  • Fluorescent ATP sensors to measure ATP levels in living bacteria

  • Membrane potential-sensitive dyes to assess proton motive force

  • Oxygen consumption measurements during infection

• Genetic Manipulation Approaches:

  • Conditional knockdown systems for atpF (since complete deletion may be lethal)

  • Site-directed mutagenesis of key functional residues

  • Complementation studies with variant atpF alleles

• Infection Model Systems:

  • Macrophage infection models with specific readouts for bacterial energetics

  • Animal models for in vivo bioenergetics analysis

  • Specialized culture conditions mimicking intracellular environments

• Integration with 'Omics Approaches:

  • Combine metabolomics, proteomics, and transcriptomics to create a comprehensive picture of metabolic shifts

  • Use approaches similar to the proteomic analysis employed for B. abortus under environmental stress

• Single-Cell Analysis:

  • Adapt the fluorescent microscopy and CellProfiler pipeline approaches used to study intracellular B. abortus to monitor ATP synthase activity at the single-cell level

How can researchers distinguish between the roles of atpF in ATP synthesis versus potential moonlighting functions in pathogenesis?

To differentiate between canonical ATP synthesis roles and potential moonlighting functions of B. abortus atpF:

• Functional Separation Strategies:

  • Engineer point mutations that specifically disrupt ATP synthesis while preserving protein structure

  • Create chimeric proteins with homologous regions from non-pathogenic bacteria

  • Develop domain-specific antibodies or inhibitors

• Protein Localization Studies:

  • Immunogold electron microscopy to precisely localize atpF in different cellular compartments

  • Fractionation studies to identify potential atpF presence outside the ATP synthase complex

  • Fluorescent protein fusions combined with super-resolution microscopy

• Interaction Network Analysis:

  • Perform pull-down assays followed by mass spectrometry to identify non-ATP synthase interaction partners

  • Verify unexpected interactions with orthogonal methods (FRET, SPR, etc.)

  • Map interaction changes during different infection phases

• Comparative Approaches:

  • Compare atpF function across different Brucella species with varying host ranges and virulence

  • Examine atpF conservation and divergence in related Alphaproteobacteria with different lifestyles

• Host Response Studies:

  • Investigate if atpF interfaces with host proteins or immune receptors

  • Determine if atpF triggers specific host signaling pathways independent of its role in ATP synthesis

What are the common pitfalls in expressing recombinant B. abortus atpF and how can they be addressed?

Researchers frequently encounter several challenges when working with recombinant B. abortus atpF:

• Protein Misfolding and Aggregation:

  • Solution: Test multiple expression temperatures (16-30°C), use specialized E. coli strains (C41/C43), and incorporate solubility tags (MBP, SUMO)

  • Verification: Monitor aggregation by dynamic light scattering or analytical size exclusion chromatography

• Toxicity to Expression Hosts:

  • Solution: Use tightly regulated expression systems, reduce inducer concentration, and consider cell-free expression systems

  • Verification: Compare growth curves of induced versus non-induced cultures

• Low Expression Levels:

  • Solution: Optimize codon usage for the expression host, try different promoter strengths, and test multiple expression hosts

  • Verification: Western blot analysis with anti-tag antibodies to detect even low expression levels

• Membrane Association Challenges:

  • Solution: Use specialized detergents (DDM, LMNG, or CHAPS) for extraction, consider nanodiscs or amphipols for stabilization

  • Verification: Functional assays to confirm proper folding of the extracted protein

• Impaired Functionality:

  • Solution: Optimize buffer conditions, consider co-expression with partner subunits, and validate function through activity assays

  • Verification: Compare with native protein extracted directly from B. abortus

Similar challenges have been encountered in studies of other B. abortus membrane-associated proteins .

How can researchers distinguish between effects specific to atpF mutation versus polar effects on the ATP synthase operon?

Differentiating direct atpF effects from polar effects requires careful experimental design:

• Genetic Complementation Strategies:

  • Use trans-complementation with the wild-type atpF gene under a controlled promoter

  • Create precise in-frame deletions or point mutations to minimize polar effects

  • Employ CRISPR interference (CRISPRi) for targeted repression rather than deletion

• Expression Verification:

  • Perform RT-qPCR to monitor expression of downstream genes in the ATP synthase operon

  • Use Western blotting to confirm protein levels of other ATP synthase subunits

  • Employ ribosome profiling to assess translation of all operon components

• Functional Analysis:

  • Compare ATP synthase activity in mutant, complemented, and wild-type strains

  • Measure membrane potential and proton gradient formation

  • Assess growth in conditions requiring varying levels of ATP synthase activity

• Control Constructs:

  • Create control strains with mutations in other ATP synthase subunits

  • Use dual reporter systems similar to those employed in B. abortus persistence studies to monitor expression of all operon components

What are the key considerations when interpreting atpF function in the context of B. abortus persistence during antibiotic treatment?

When studying atpF in relation to B. abortus persistence, researchers should consider:

• Persistence Definition and Measurement:

  • Clearly distinguish between antibiotic tolerance, resistance, and persistence

  • Use appropriate methodology similar to the time-kill assays and fluorescent reporter systems employed in B. abortus persistence studies

  • Implement both population-level (CFU counting) and single-cell (microscopy) analyses

• Metabolic State Assessment:

  • Monitor ATP levels in persister populations with and without functional atpF

  • Assess membrane potential as an indicator of ATP synthase activity

  • Use inducible reporter systems to verify metabolic activity in persisters

• Experimental Timing:

  • Consider growth phase effects, as B. abortus persistence has been shown to vary with growth phase

  • Design time-course experiments to capture dynamics of persistence establishment and resolution

• Environmental Context:

  • Compare persistence in broth versus intracellular conditions, as B. abortus shows different persistence characteristics in each

  • Control for environmental factors that might independently affect persistence

• Antibiotic Selection:

  • Use clinically relevant antibiotics (e.g., ciprofloxacin as used in previous persistence studies )

  • Consider antibiotics with different mechanisms of action to distinguish general versus specific persistence mechanisms

• Genetic Background Controls:

  • Include appropriate control strains (e.g., BvrR/BvrS system mutants ) with known effects on persistence or metabolism

Understanding these considerations will help researchers correctly interpret the specific contribution of atpF to the persistence phenomenon documented in B. abortus .

What emerging technologies could enhance our understanding of atpF function in B. abortus pathogenesis?

Several cutting-edge technologies show promise for advancing B. abortus atpF research:

• Single-Cell 'Omics Approaches:

  • Single-cell RNA-seq to identify transcriptional heterogeneity in atpF expression

  • Single-cell proteomics to detect protein-level variations

  • These approaches could reveal important insights about subpopulations similar to the persister cells identified in B. abortus

• Advanced Imaging Technologies:

  • Super-resolution microscopy (STORM, PALM) for nanoscale localization of atpF

  • Correlative light and electron microscopy (CLEM) to visualize atpF in relation to cellular structures

  • Live-cell ATP imaging using genetically encoded ATP sensors

• CRISPR-Based Technologies:

  • CRISPRi for temporal control of atpF expression

  • CRISPR-based proximity labeling to identify interaction partners

  • Base editing for precise nucleotide modifications without double-strand breaks

• Structural Biology Advances:

  • Cryo-electron tomography of whole bacteria to visualize ATP synthase in situ

  • Integrative structural biology combining crystallography, cryo-EM, and computational modeling

  • Hydrogen-deuterium exchange mass spectrometry (HDX-MS) for conformational dynamics

• Synthetic Biology Approaches:

  • Minimal ATP synthase systems to define essential components

  • Biosensors that report on ATP synthase assembly and function

  • Controlled reconstitution systems for functional studies

These technologies could build upon the methodological foundations established in B. abortus persistence studies and two-component system research .

How might comparative studies of atpF across Brucella species inform evolutionary adaptations in intracellular pathogens?

Comparative studies of atpF across Brucella species and related bacteria can provide evolutionary insights:

• Sequence and Structure Comparative Analysis:

  • Identify conserved versus variable regions of atpF across Brucella species

  • Compare with atpF in related Alphaproteobacteria that have different host ranges or lifestyles

  • Analyze selection pressures on different domains of the protein

• Functional Comparison Approaches:

  • Cross-complementation studies with atpF from different species

  • Compare ATP synthase activity, assembly, and regulation across species

  • Assess contribution to persistence phenotypes in different Brucella species

• Host-Pathogen Interface Studies:

  • Compare how atpF variants from different species respond to host-specific environments

  • Investigate potential co-evolution with host factors

  • Examine atpF conservation in relation to the BvrR/BvrS system, which is conserved across Alphaproteobacteria with close association to eukaryotic cells

• Ecological Context Integration:

  • Correlate atpF variations with host range, tissue tropism, and virulence differences

  • Explore potential links between atpF and metabolic adaptations to specific host niches

• Evolutionary Modeling:

  • Reconstruct the evolutionary history of atpF in relation to speciation events

  • Identify horizontal gene transfer events that might have contributed to virulence

What potential exists for targeting atpF in novel therapeutic approaches against brucellosis?

The therapeutic potential of targeting B. abortus atpF presents several promising avenues:

• Inhibitor Development Strategies:

  • Structure-based design of small molecules targeting unique features of bacterial atpF

  • Peptide inhibitors designed to disrupt ATP synthase assembly

  • Allosteric modulators affecting atpF interactions with other subunits

• Persister-Targeting Approaches:

  • Develop compounds that specifically target ATP synthase in metabolically active persisters

  • Combination therapies that prevent or reverse the persister state documented in B. abortus

  • Energy-dependent "Trojan horse" strategies that require ATP synthase activity

• Delivery System Innovations:

  • Nanoparticle formulations to improve delivery to intracellular bacteria

  • Host-directed therapies that alter the intracellular environment to compromise ATP synthase function

  • Targeted delivery systems that accumulate in Brucella-containing vacuoles

• Therapeutic Efficacy Assessment:

  Therapeutic Approach	Potential Advantages	Evaluation Methods
  ATP synthase inhibitors	Target essential function	Time-kill assays, intracellular growth inhibition
  Anti-persister compounds	Reduce relapse rates	Persister formation assays, relapse models
  Combination therapies	Prevent resistance development	Synergy testing, in vivo efficacy studies
  Structure-based inhibitors	Improved specificity	Binding assays, structural studies

• Translational Considerations:

  • Address the high relapse rate (5-15%) observed in current brucellosis treatments

  • Develop therapies effective against the metabolically active persister population identified in B. abortus infection models

  • Consider potential for cross-resistance with existing antibiotics