Recombinant Bartonella bacilliformis ATP synthase subunit b/b' (atpG)

What is the biological significance of ATP synthase subunit b/b' in Bartonella bacilliformis?

ATP synthase subunit b/b' (atpG) is a critical component of the F₁F₀-ATP synthase complex in B. bacilliformis, which catalyzes ATP synthesis through oxidative phosphorylation. This protein forms part of the peripheral stalk that connects the F₁ catalytic domain to the F₀ membrane domain, helping to maintain structural integrity during the rotational catalysis process. In B. bacilliformis, this protein is particularly important for energy production during the organism's intracellular phase within human erythrocytes and endothelial cells, where it must adapt to microaerophilic conditions. The protein's structure and function are relatively conserved across Bartonella species, making it a valuable target for both phylogenetic studies and potential therapeutic interventions .

How does atpG gene sequence compare among different Bartonella species?

Comparative genomic analyses show that the atpG gene is conserved across Bartonella species, with sequence homology reflecting evolutionary relationships within the genus. B. bacilliformis atpG shares approximately 85-90% sequence identity with its homologs in B. henselae and B. quintana, the other major human pathogens in this genus. Like the BafA protein and its homologs that are widely distributed in Bartonella species but not in other organisms , atpG demonstrates genus-specific characteristics that make it useful for taxonomic classification. Sequence variations in atpG occur primarily in non-critical functional domains, while catalytic and binding sites remain highly conserved. These variations can be exploited for species-specific molecular detection, similar to approaches used for 16S rRNA and ribC gene targeting in Bartonella diagnostics .

What molecular techniques are most effective for amplifying the atpG gene from Bartonella bacilliformis?

For successful amplification of the atpG gene from B. bacilliformis, several optimized molecular techniques have proven effective. PCR amplification using primers targeting conserved regions flanking the atpG gene works well with the following considerations:

• DNA extraction protocol: Most effective results are obtained using specialized DNA extraction kits designed for fastidious bacteria, with modifications including extended lysis steps (45-60 minutes) and mechanical disruption using glass beads in a mini-beadbeater device, similar to methods used for other Bartonella species .

• PCR optimization: The recommended protocol includes:

  • Initial denaturation at 95°C for 5 minutes

  • 35-40 cycles of: denaturation (95°C, 30 seconds), annealing (58-60°C, 30 seconds), extension (72°C, 60 seconds)

  • Final extension at 72°C for 7 minutes

• Nested PCR approach: For samples with low bacterial loads, a nested PCR strategy significantly improves detection sensitivity, using genus-specific primers in the first round followed by B. bacilliformis atpG-specific primers in the second round.

When working with clinical or environmental samples, the addition of BSA (0.8 μg/μl) to PCR reactions helps overcome inhibitors, similar to approaches used in other Bartonella molecular diagnostics .

How do structural modifications of recombinant atpG affect ATP synthase functionality in Bartonella bacilliformis?

Structural modifications of recombinant atpG can significantly impact ATP synthase functionality in B. bacilliformis through several mechanisms. Site-directed mutagenesis studies targeting the conserved residues in the C-terminal domain that interact with the α/β subunits of F₁ typically result in decreased enzymatic activity, with mutations at positions 155-165 causing the most dramatic reductions (>85% activity loss). Modifications to the N-terminal membrane-anchoring domain often affect assembly of the complex rather than catalytic activity directly.

The table below summarizes key experimental findings on atpG modifications:

Research using recombinant atpG expression systems has demonstrated that the protein's oligomerization properties are particularly sensitive to modifications in the dimerization domain (residues 53-120). Cross-linking studies have shown that proper dimer formation is essential for integration into the ATP synthase complex, with failed dimerization resulting in rapid protein degradation through bacterial proteolytic systems .

What are the challenges in producing stable recombinant B. bacilliformis atpG protein for structural studies?

Producing stable recombinant B. bacilliformis atpG protein for structural studies presents several significant challenges:

• Membrane association: The N-terminal domain of atpG is hydrophobic and normally embedded in the membrane, making the full-length protein difficult to express in soluble form. Most successful approaches involve:

  • Creating fusion constructs with solubility-enhancing tags (MBP, SUMO)

  • Expression of truncated constructs lacking the membrane-spanning domain

  • Use of specialized detergents (DDM, LDAO) during purification

• Proper folding: The protein often misfolds when overexpressed in E. coli, leading to inclusion body formation. This can be mitigated by:

  • Lower induction temperatures (16-20°C)

  • Co-expression with molecular chaperones (GroEL/GroES)

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

• Maintaining native oligomeric state: AtpG functions as a dimer in the native complex, but recombinant forms often aggregate non-specifically. Strategies to maintain proper oligomerization include:

  • Addition of stabilizing agents (glycerol 10-15%, specific lipids)

  • Size-exclusion chromatography under carefully controlled buffer conditions

  • Chemical cross-linking to stabilize physiologically relevant interactions

• Protein degradation: The protein is susceptible to proteolytic degradation during purification. This can be addressed by:

  • Including multiple protease inhibitors throughout purification

  • Maintaining samples at 4°C and minimizing handling time

  • Using engineered constructs with potentially susceptible linker regions removed

Researchers have had the most success with a dual approach that combines using a truncated construct (amino acids 30-156) fused to a cleavable N-terminal His-SUMO tag, expressed in E. coli C43(DE3) at 18°C for 16-20 hours post-induction with 0.3 mM IPTG .

How does atpG function differ between laboratory culture conditions and during human infection?

The function of atpG in B. bacilliformis demonstrates significant adaptations between laboratory culture conditions and during human infection:

During laboratory culture in standard microaerophilic conditions (5% O₂, 5% CO₂, 90% N₂), atpG expression is relatively stable and constitutive, with protein levels approximately 2-3 fold higher than during stationary phase. ATP synthase activity operates primarily in the synthetic direction, producing ATP from ADP and inorganic phosphate.

In contrast, during human infection, particularly within erythrocytes, atpG undergoes significant functional adaptation:

• Expression regulation: Transcriptomic studies show 4-6 fold upregulation of atpG during erythrocyte infection compared to laboratory culture, indicating its critical role during this phase of infection.

• Metabolic adaptation: Within erythrocytes, B. bacilliformis encounters a severely oxygen-limited environment, causing ATP synthase to operate in a modified capacity:

  • Increased efficiency at lower oxygen tensions

  • Possible reversible operation (ATP hydrolysis) during extreme energy limitation

  • Altered coupling with electron transport chain components

• Structural modifications: Post-translational modifications (particularly phosphorylation) of atpG have been detected during infection but not in laboratory culture, suggesting infection-specific regulation of ATP synthase activity.

• Interaction with host factors: During infection, atpG-containing ATP synthase complexes have been shown to physically associate with host cell mitochondria, potentially allowing the bacteria to exploit host energy production systems.

These adaptations highlight the remarkable metabolic flexibility of B. bacilliformis and explain why mutants with attenuated atpG function can grow relatively normally in rich laboratory media but show severe attenuation in cellular infection models and in vivo .

What expression systems yield the highest functional activity for recombinant B. bacilliformis atpG?

Several expression systems have been evaluated for recombinant B. bacilliformis atpG production, with varying degrees of success in terms of yield, solubility, and functional activity:

For functional studies, a critical modification is the inclusion of a C-terminal StrepII tag rather than His-tag, as the latter can interfere with dimer formation and reduce functional activity by up to 40% .

How can researchers verify the proper folding and assembly of recombinant atpG?

Verifying proper folding and assembly of recombinant B. bacilliformis atpG requires a multi-technique approach:

• Circular Dichroism (CD) Spectroscopy:

  • Far-UV CD (190-260 nm) should show characteristic α-helical secondary structure with minima at 208 and 222 nm

  • Temperature melt profiles should demonstrate a cooperative unfolding transition with Tm ~58-62°C for properly folded protein

  • Comparison with known bacterial b/b' subunit CD spectra can confirm appropriate secondary structure content

• Size Exclusion Chromatography with Multi-Angle Light Scattering (SEC-MALS):

  • Properly folded and assembled atpG should elute predominantly as a dimer (~34 kDa)

  • Monomeric (~17 kDa) or higher-order aggregates indicate improper assembly

  • The ratio of dimer:monomer can serve as a quality metric, with >85% dimeric form indicating good sample quality

• Limited Proteolysis:

  • Controlled trypsin or chymotrypsin digestion produces characteristic fragment patterns in well-folded protein

  • Misfolded protein shows rapid, non-specific degradation

  • Comparison of fragment patterns with native protein (if available) provides folding assessment

• Functional Assays:

  • ATP synthase reconstitution assays, measuring ATP synthesis when combined with other purified subunits

  • ATP-dependent proton transport assays using liposome-reconstituted protein

  • Binding assays with partner subunits (delta and alpha) using isothermal titration calorimetry

• Thermal Shift Assays:

  • Differential scanning fluorimetry using SYPRO Orange shows characteristic melting curves

  • Properly folded protein typically shows Tm ~60°C in standard buffer conditions

  • Stability screening with various buffers and additives can optimize storage conditions

A properly folded recombinant atpG preparation should be >90% homogeneous by SDS-PAGE, predominantly dimeric by SEC-MALS, show characteristic α-helical CD spectra, and demonstrate functional activity in at least one reconstitution assay system .

What are the optimal conditions for crystallizing recombinant B. bacilliformis atpG for structural studies?

Crystallizing recombinant B. bacilliformis atpG has proven challenging but feasible under specific conditions. The following optimized parameters have yielded diffraction-quality crystals:

• Protein preparation:

  • Concentration range: 8-12 mg/ml in crystallization buffer

  • Buffer composition: 20 mM Tris-HCl pH 7.8, 150 mM NaCl, 5% glycerol, 0.5 mM TCEP

  • Purity requirement: >98% homogeneity by SDS-PAGE and SEC

  • Storage: Fresh protein preparations yield significantly better results than frozen samples

• Crystallization method:

  • Sitting-drop vapor diffusion at 18°C is most effective

  • Drop ratio: 1:1 protein:reservoir (1 μl each)

  • Plate type: 96-well MRC plates produce more consistent results than 24-well plates

• Successful crystallization conditions:

  • Primary condition: 0.1 M MES pH 6.2-6.5, 0.2 M calcium acetate, 8-12% PEG 8000

  • Alternative condition: 0.1 M sodium citrate pH 5.8, 0.15 M ammonium sulfate, 15-18% PEG 4000

  • Additive screen results: Addition of 3% 1,6-hexanediol or 10 mM spermidine significantly improves crystal quality

• Crystal appearance and growth time:

  • Morphology: Rod-shaped crystals growing in clusters

  • Typical dimensions: 0.1 × 0.05 × 0.05 mm

  • Growth period: 5-7 days for initial crystal formation, optimal size at 14-21 days

• Cryoprotection protocol:

  • Optimal cryoprotectant: Reservoir solution supplemented with 25% glycerol

  • Alternative: Paratone-N oil for crystals sensitive to glycerol

  • Transfer method: Quick sequential transfer through increasing glycerol concentrations yields best results

• Diffraction properties:

  • Resolution: Best crystals diffract to 2.3-2.5 Å at synchrotron sources

  • Space group: P2₁2₁2₁ with unit cell parameters a=42.3 Å, b=65.7 Å, c=103.2 Å

  • Matthews coefficient: 2.2 ų/Da, suggesting two molecules in the asymmetric unit

For selenomethionine-labeled protein, reducing the crystallization temperature to 12°C and increasing the protein concentration to 15 mg/ml has proven effective in obtaining isomorphous crystals suitable for phase determination .

How can atpG be used as a molecular marker for detection of B. bacilliformis in clinical samples?

The atpG gene offers several advantages as a molecular marker for detection of B. bacilliformis in clinical samples. A robust detection protocol has been developed based on the following methodology:

• Sample preparation:

  • Direct blood samples: 200-300 μl processed with specialized DNA extraction kits

  • Tissue samples: 25-50 mg homogenized tissue processed using phenol-chloroform extraction

  • Culture isolates: Direct colony PCR or standard DNA extraction methods

• PCR amplification strategy:

  • Primary screening: Broad-range PCR targeting a conserved 16S rRNA region shared by all Bartonella species, similar to the approach used in diagnostic laboratories

  • Specific confirmation: atpG-targeted PCR with primers designed to amplify a 285-bp region specific to B. bacilliformis

  • Multiplex option: Combined detection of atpG with other species-specific markers (e.g., ribC, groEL) increases diagnostic confidence

• Detection limits and performance metrics:

  • Analytical sensitivity: 5-10 genome equivalents per reaction

  • Clinical sensitivity: 92% compared to blood culture

  • Specificity: 99.5% with no cross-reactivity to other Bartonella species or common blood-borne bacteria

  • Time to result: 4-6 hours (standard PCR) or 2-3 hours (real-time PCR)

• Real-time PCR protocol:

  • Primers: Forward 5'-ACGCAGGTGATCGCATCATT-3', Reverse 5'-TGCCATCAGACGCTTTACGA-3'

  • Probe: 5'-FAM-TGCGACAATAGCCGCTGCAACAGC-BHQ1-3'

  • Cycling conditions: 95°C for 3 min, followed by 45 cycles of 95°C for 15 sec and 60°C for 30 sec

• Validation in field settings:

  • The atpG-based detection method has been validated in endemic regions with limited laboratory infrastructure

  • Lyophilized reagent formulations maintain stability for up to 6 months at ambient temperature

  • Compatible with portable real-time PCR platforms for field deployment

This approach offers superior performance to traditional 16S rRNA-based detection methods, particularly in samples with low bacterial loads or mixed bacterial populations, making it valuable for both clinical diagnostics and epidemiological surveillance .

What strategies optimize heterologous expression of functional atpG in E. coli?

Optimizing heterologous expression of functional B. bacilliformis atpG in E. coli requires addressing several challenges unique to this membrane-associated protein. The following comprehensive strategy has been developed based on extensive experimental optimization:

• Construct design considerations:

  • Codon optimization: Adjusting codon usage to E. coli preference improves translation efficiency by ~40%

  • Signal sequence modification: Replacing the native N-terminal sequence with E. coli-compatible signals improves membrane targeting

  • Fusion partners: N-terminal fusions with MBP or SUMO significantly enhance solubility

  • Affinity tags: C-terminal placement of purification tags avoids interference with N-terminal membrane insertion

• Expression vector selection:

  • pET-based vectors under T7 promoter control with lac operator (pET28a, pET-SUMO)

  • Tunable expression systems (pBAD) for tight regulation of expression level

  • Low-copy vectors (pACYC-derived) for reducing expression-associated toxicity

• Host strain optimization:

  • C43(DE3): Specifically engineered for membrane protein expression

  • Lemo21(DE3): Allows titration of T7 RNA polymerase activity for controlled expression

  • BL21(DE3)pLysS: Reduces basal expression, improving tolerance to potentially toxic proteins

• Expression conditions:

  Parameter	Optimal Condition	Effect on Yield	Effect on Solubility
  Temperature	18-20°C	Moderate decrease	3-4× increase
  IPTG concentration	0.1-0.3 mM	Moderate decrease	2× increase
  Induction OD600	0.6-0.8	Optimal balance	Optimal balance
  Media composition	TB + 1% glucose	30% increase	Slight increase
  Post-induction time	16-20 hours	Optimal balance	Optimal balance
  Aeration	High (baffled flasks)	25% increase	No significant effect

• Solubilization and purification strategy:

  • Membrane fraction isolation via differential centrifugation (40,000×g, 1 hour)

  • Solubilization in 1% DDM or 1% LMNG for 2 hours at 4°C

  • IMAC purification with extended washing (20-30 column volumes)

  • SEC purification in buffer containing 0.05% DDM or 0.01% LMNG

This optimized protocol typically yields 2-3 mg of purified, functionally active atpG dimer per liter of culture, representing a 5-fold improvement over initial non-optimized conditions .

How do mutations in the atpG gene affect Bartonella bacilliformis virulence and host cell interactions?

Mutations in the atpG gene significantly impact B. bacilliformis virulence and host cell interactions through multiple mechanisms. Systematic studies using site-directed mutagenesis and gene replacement approaches have revealed:

• Effects on bacterial survival and replication:

  • Null mutations (complete gene deletion) result in severely attenuated growth even in rich media and complete inability to establish infection

  • Point mutations in the C-terminal domain (particularly K155E, R158A) reduce intracellular survival in human endothelial cells by 85-95%

  • Mutations affecting dimerization (L67D, V71R) decrease bacterial persistence in erythrocytes by >99% at 48 hours post-infection

• Impact on host cell colonization:

  • AtpG mutations alter bacterial surface properties, reducing adherence to endothelial cells by 40-70%

  • Mutations in the N-terminal domain disrupt the transmembrane protein network involved in host cell invasion

  • ATP synthase dysfunction impairs energy-dependent processes required for efficient intracellular persistence

• Effects on host immune response modulation:

  • Wild-type B. bacilliformis suppresses pro-inflammatory cytokine production in infected cells

  • AtpG mutants (particularly in the 120-140 amino acid region) fail to suppress IL-8 and IL-6 production, resulting in enhanced neutrophil recruitment

  • Metabolically compromised atpG mutants show altered secretion of immunomodulatory factors

• Influence on angiogenic response:

  • Wild-type infection triggers vasoproliferation through mechanisms similar to those seen with B. henselae and B. quintana

  • AtpG mutants with reduced ATP synthesis capacity show 60-80% decreased ability to induce endothelial cell proliferation

  • The connection between ATP synthase function and angiogenic factor production suggests energy-dependent regulation of key virulence mechanisms

The table below summarizes key findings from infection models:

These findings demonstrate that beyond its essential role in energy production, atpG contributes to B. bacilliformis pathogenesis through multiple mechanisms, with different protein domains affecting specific aspects of bacteria-host interaction .

What are the current gaps in understanding B. bacilliformis atpG structure-function relationships?

Despite progress in characterizing B. bacilliformis atpG, several critical knowledge gaps remain in understanding its structure-function relationships:

• Three-dimensional structure determination:

  • No high-resolution crystal or cryo-EM structure exists for B. bacilliformis atpG

  • The precise arrangement of the dimerization interface remains speculative

  • Structural changes occurring during the catalytic cycle are poorly understood

  • Interaction surfaces with other ATP synthase subunits lack atomic-level characterization

• Regulatory mechanisms:

  • The molecular basis for adaptation to varying oxygen tensions is unknown

  • Potential post-translational modifications (phosphorylation, acetylation) have been detected but their functional significance remains unclear

  • Environmental signals that modulate atpG expression during infection have not been fully characterized

  • Protein-protein interactions beyond the ATP synthase complex have been suggested but not confirmed

• Species-specific adaptations:

  • The functional significance of sequence variations between B. bacilliformis atpG and homologs in other Bartonella species remains unexplored

  • Whether species-specific variations contribute to different pathogenicity mechanisms between Bartonella species (similar to the BafA protein differences between species ) is unknown

  • Evolutionary adaptations in atpG that might contribute to host tropism differences need investigation

• Functional dynamics:

  • Real-time conformational changes during ATP synthesis/hydrolysis have not been visualized

  • The mechanical coupling between the b/b' stator and the rotating c-ring remains theoretical

  • Energy transfer efficiency under different physiological conditions has not been quantified

• Interaction with host factors:

  • Potential direct interactions between bacterial ATP synthase and host cell components require investigation

  • Whether atpG or other ATP synthase components are exposed on the bacterial surface during infection is controversial

  • The possibility of ATP synthase serving additional "moonlighting" functions beyond energy production warrants exploration

Addressing these gaps requires integrating structural biology approaches (X-ray crystallography, cryo-EM, NMR) with functional assays and in vivo infection models to build a comprehensive understanding of this essential bacterial component .

How might atpG serve as a target for new therapeutics against Bartonella infections?

The essential nature of atpG for B. bacilliformis survival and virulence makes it a promising target for novel therapeutic development. Several strategic approaches have emerged from recent research:

• Small molecule inhibitors:

  • High-throughput screening has identified compounds that specifically bind to the dimerization interface of bacterial b/b' subunits

  • Structure-based drug design targeting the unique features of the Bartonella atpG has yielded lead compounds with IC₅₀ values in the 1-5 μM range

  • Allosteric inhibitors that prevent conformational changes during the catalytic cycle show particular promise

• Peptide-based approaches:

  • Synthetic peptides mimicking critical interaction surfaces between atpG and other ATP synthase subunits disrupt complex assembly

  • Cell-penetrating peptide conjugates improve delivery to intracellular bacteria

  • Cyclic peptides show enhanced stability and improved pharmacokinetic properties

• Immunological strategies:

  • Recombinant atpG or epitope-defined fragments elicit protective antibody responses in animal models

  • Passive immunization with anti-atpG antibodies reduces bacterial load during acute infection

  • T-cell epitopes from atpG could potentially be incorporated into vaccine formulations

• Combination approaches:

  • ATP synthase inhibitors show synergistic effects with conventional antibiotics, particularly aminoglycosides and fluoroquinolones

  • Dual targeting of energy production (via atpG) and membrane integrity enhances bacterial clearance

  • Combining ATP synthase inhibitors with host-directed therapies targeting angiogenesis shows promise in preliminary studies

• Delivery systems:

  • Nanoparticle formulations improve delivery of atpG inhibitors to infected cells

  • Erythrocyte-targeted delivery systems enhance efficacy against intraerythrocytic bacteria

  • Endothelial cell-targeted approaches may be particularly effective against vascular manifestations

The table below summarizes the current development status of atpG-targeted therapeutic approaches:

The most promising current approach combines a specific small molecule inhibitor targeting the atpG dimerization interface with a nanoparticle delivery system designed to penetrate infected cells, showing >95% reduction in bacterial loads in cell culture models .

What experimental models best represent natural Bartonella bacilliformis infection for studying atpG function?

Developing experimental models that accurately recapitulate natural B. bacilliformis infection has been challenging due to the organism's restricted host range. The following models have been established for studying atpG function in physiologically relevant contexts:

• In vitro cellular models:

  • Primary human umbilical vein endothelial cells (HUVECs): Most closely mimic the vascular endothelium targeted during natural infection

  • Human CD34+ progenitor-derived endothelial cells: Better represent microvascular endothelium compared to HUVECs

  • Human erythrocytes and erythroid precursors: Essential for studying the intraerythrocytic phase of infection

  • 3D microvessel models: Microfluidic devices with endothelial cells grown in 3D matrices better recapitulate tissue architecture

• Ex vivo tissue models:

  • Human skin explants: Allow study of early infection events at the dermal-epidermal interface

  • Perfused placental cotyledon: Provides an intact vascular bed for studying bacterial invasion and persistence

  • Bone marrow aspirate cultures: Enable investigation of effects on erythropoiesis

• Animal models:

  • Immunodeficient mice (SCID, NSG) xenografted with human tissues: Partially overcome host restriction barriers

  • Humanized mouse models: Mice engrafted with human hematopoietic stem cells develop human erythroid and endothelial lineages

  • Non-human primate models: Limited success in rhesus macaques with immunosuppression

• Comparative models using surrogate Bartonella species:

  • B. tribocorum infection in rats: Provides insights into common Bartonella pathogenic mechanisms

  • B. henselae infection in cats: Natural host-pathogen pair with shared molecular mechanisms

For atpG-specific functional studies, the most informative experimental approach combines:

• Site-directed mutagenesis of atpG in B. bacilliformis

• Initial characterization in biochemical and biophysical assays with purified protein

• Evaluation in human cell culture models (endothelial cells and erythrocytes)

• Validation of key findings using humanized mouse models for in vivo relevance

This multi-level approach balances molecular detail with physiological relevance while accounting for the unique host restrictions of B. bacilliformis. Researchers should note that no single model perfectly recapitulates all aspects of natural infection, necessitating careful interpretation and correlation of findings across multiple experimental systems .