Recombinant Escherichia fergusonii ATP synthase subunit b (atpF)

What is the structure and function of ATP synthase subunit b in E. fergusonii?

E. fergusonii ATP synthase subunit b is a critical component of the F(1)F(0) ATP synthase complex that produces ATP from ADP in the presence of a proton or sodium gradient. The protein acts as a component of the F(0) channel, forming part of the peripheral stalk that links the F(1) and F(0) domains of the ATP synthase complex . The protein has a highly organized structure divided into four distinct domains: the N-terminal membrane-spanning domain, the tether domain, the dimerization domain, and the C-terminal delta-binding domain .

Functionally, subunit b serves as a stator to counter the tendency of the α₃β₃ hexamer to follow the rotation of the central stalk during ATP synthesis or hydrolysis. This mechanism is essential for maintaining the structural integrity of the complex during the energy coupling process between the two active domains .

How is the E. fergusonii ATP synthase subunit b organized structurally?

The E. fergusonii ATP synthase subunit b exists as a highly extended, helical dimer that spans from the membrane to near the top of the F(1) domain. Nuclear magnetic resonance (NMR) studies have revealed that the region between residues 39 and 72 forms an α-helix, with the b30-82 segment measuring approximately 48.07 Å in length .

Surface charge distribution analysis of b30-82 shows one side with a distinctive hydrophobic surface pattern formed by alanine residues. Research has demonstrated that when alanine residues at positions 61, 68, 70, and 72 were replaced by single cysteines, those at positions 61, 68, and 72 formed disulfide bonds, while position 70 did not. This pattern of disulfide bonding provides strong evidence for an adjacent arrangement of residues 61, 68, and 72 in both α-helices in the b22-156 segment .

What are the key protein-protein interactions of subunit b within the ATP synthase complex?

The b subunit forms several critical interactions within the ATP synthase complex that are essential for both structural integrity and functional coordination. Multiple crosslinking sites have been identified between subunit b and the a, α, β, and δ subunits of ATP synthase .

Specifically, the δ-binding domain in the C-terminal region (residues 123-156) interacts with the δ subunit near the top of the F(1) domain, while the N-terminal membrane domain interacts with the a subunit within the membrane. These interactions collectively create a stable peripheral stalk that serves as a structural support during rotational catalysis .

Recent research suggests that the b dimer may serve a more complex role beyond just providing structural support. Evidence indicates that it may function as an elastic element during rotational catalysis and directly influence the catalytic sites, suggesting a more active role in coupling the proton gradient to ATP synthesis .

What expression systems are most effective for producing recombinant E. fergusonii ATP synthase subunit b?

For successful expression of recombinant E. fergusonii ATP synthase subunit b, researchers should consider the following methodological approaches:

• Expression System Selection: E. coli BL21(DE3) strains have proven effective for expressing ATP synthase components due to their reduced protease activity and compatibility with T7 promoter-based expression vectors. For membrane proteins like the full-length subunit b, C41(DE3) or C43(DE3) strains may provide better results as they're engineered specifically for membrane protein expression.

• Vector Optimization: Vectors containing a C-terminal His-tag facilitate purification while minimizing interference with the N-terminal membrane domain. The pET system (particularly pET-28a) with an inducible T7 promoter provides controlled expression.

• Solubility Enhancement: For studies focusing on the soluble portion (residues 30-156), expressing this region without the membrane domain significantly improves yield and solubility. Alternatively, fusion partners such as MBP (maltose-binding protein) can enhance solubility of the full-length protein.

• Expression Conditions: Optimal results are typically achieved by inducing expression at OD₆₀₀ of 0.6-0.8 with 0.5-1.0 mM IPTG, followed by expression at reduced temperature (16-20°C) for 16-18 hours to maximize proper folding.

When specifically studying the solution structure of segments like b30-82, expression as a soluble domain followed by NMR analysis has successfully revealed structural details including the α-helical arrangement and surface charge distribution .

How can researchers effectively purify recombinant E. fergusonii ATP synthase subunit b for structural studies?

Purification of recombinant E. fergusonii ATP synthase subunit b requires a strategic approach depending on whether the study focuses on the full-length protein or specific domains:

• For Full-Length Protein:

  • Membrane fraction isolation via ultracentrifugation after cell disruption

  • Solubilization using mild detergents (0.5-1% n-dodecyl β-D-maltoside or digitonin)

  • Affinity chromatography using Ni-NTA for His-tagged constructs

  • Size exclusion chromatography for final purification and detergent exchange

• For Soluble Domains (e.g., residues 30-156):

  • Direct purification from soluble fraction after cell lysis

  • Initial capture via affinity chromatography

  • Ion-exchange chromatography to remove contaminants

  • Size exclusion chromatography for final polishing

• Quality Control Metrics:

  • SDS-PAGE should show >95% purity

  • Circular dichroism to confirm secondary structure content (high α-helical content expected)

  • Dynamic light scattering to verify monodispersity and absence of aggregation

• Stability Considerations:

  • The addition of 5-10% glycerol and maintaining pH 7.5-8.0 significantly enhances protein stability

  • Storage at -80°C after flash-freezing in liquid nitrogen preserves structure and function

Nuclear magnetic resonance studies have successfully utilized this approach to determine the solution structure of the b30-82 region, revealing its α-helical arrangement and distinctive surface charge patterns .

What are the most effective experimental approaches for studying subunit b interactions with other ATP synthase components?

To investigate the interactions between E. fergusonii ATP synthase subunit b and other components of the ATP synthase complex, researchers can employ several advanced techniques:

• Site-Directed Crosslinking: Introducing cysteine residues at strategic positions in subunit b (as demonstrated with positions 61, 68, 70, and 72) allows for disulfide bond formation studies to map interaction surfaces . This approach has successfully identified interaction sites between subunit b and the a, α, β, and δ subunits .

• Co-Immunoprecipitation Combined with Mass Spectrometry: This approach can identify native interaction partners of subunit b without introducing modifications to the protein structure. When coupled with chemical crosslinking, it can capture transient interactions.

• Surface Plasmon Resonance (SPR): For quantitative binding kinetics measurements between purified subunit b (or its domains) and potential interaction partners. SPR provides real-time interaction data and affinity constants.

• Isothermal Titration Calorimetry (ITC): For thermodynamic characterization of binding events, providing both binding constants and thermodynamic parameters (ΔH, ΔS, and ΔG).

• Fluorescence Resonance Energy Transfer (FRET): For monitoring interactions in real-time and potentially in living cells when using fluorescent protein fusions of subunit b and its partners.

The peripheral stalk formed by subunit b has been shown to interact with specific regions of the ATP synthase complex, acting both as a structural support and potentially playing a more active role in the coupling mechanism during ATP synthesis .

How can site-directed mutagenesis be applied to investigate critical residues in E. fergusonii ATP synthase subunit b?

Site-directed mutagenesis provides a powerful approach for investigating structure-function relationships in E. fergusonii ATP synthase subunit b:

• Target Selection Strategy:

  • Alanine scanning of the dimerization domain (residues 60-122) to identify critical positions for dimer stability

  • Substitution of charged residues to neutralize potential electrostatic interactions with other subunits

  • Introduction of cysteine pairs to test proximity through disulfide bridge formation, as successfully demonstrated with positions 61, 68, and 72

  • Mutation of conserved residues identified through comparative sequence analysis across bacterial species

• Functional Assessment Methods:

  • ATP synthesis/hydrolysis assays using reconstituted proteoliposomes

  • Proton pumping assays utilizing pH-sensitive fluorescent dyes

  • Growth complementation studies in ATP synthase-deficient bacterial strains

  • Protein-protein interaction assays to evaluate effects on complex assembly

• Structural Impact Analysis:

  • Circular dichroism spectroscopy to assess changes in secondary structure

  • Thermal stability assays to evaluate effects on protein folding

  • Size exclusion chromatography to detect alterations in oligomeric state

Previous studies have successfully used cysteine substitution at positions 61, 68, 70, and 72, revealing that positions 61, 68, and 72 form disulfide bonds while position 70 does not. This pattern provides critical structural information about the arrangement of α-helices in the b22-156 segment .

What approaches can be used to study the role of E. fergusonii ATP synthase subunit b in antimicrobial resistance?

E. fergusonii has emerged as a significant reservoir for antimicrobial resistance genes, with implications for ATP synthase structure and function:

• Genetic Approaches:

  • Generation of atpF knockout strains to assess changes in antimicrobial susceptibility profiles

  • Introduction of mutated atpF variants to test specific hypotheses about structure-function relationships

  • Complementation studies using wildtype and mutant atpF genes in knockout backgrounds

• Phenotypic Characterization:

  • Comprehensive antimicrobial susceptibility testing using broth microdilution and agar dilution methods

  • Determination of minimum inhibitory concentrations (MICs) for various antimicrobial agents

  • Growth kinetics analysis under antimicrobial stress conditions

• Molecular Mechanism Studies:

  • Transcriptomic analysis to identify changes in gene expression patterns

  • Proteomic approaches to detect alterations in protein abundance and post-translational modifications

  • Metabolomic profiling to assess changes in cellular energy metabolism

Recent research has shown that E. fergusonii isolates exhibit high levels of antimicrobial resistance, with 97.74% resistant to sulfafurazole and 94.74% resistant to tetracycline . Additionally, 51.88% of E. fergusonii isolates were found to be extended spectrum beta-lactamase (ESBL)-positive, highlighting the significant role this organism plays in antimicrobial resistance transmission .

How can researchers investigate the role of subunit b in the proton-to-ATP coupling mechanism?

The coupling efficiency between proton translocation and ATP synthesis is a fundamental aspect of ATP synthase function that can be investigated using several approaches:

• Engineering Approaches:

  • Modification of the peripheral stalk architecture to alter the H⁺/ATP ratio

  • Introduction of multiple peripheral stalks, each bound to a proton-conducting a-subunit, as demonstrated in recent engineering experiments that achieved an H⁺/ATP ratio of 5.9

  • Creation of chimeric constructs combining elements from species with different coupling efficiencies

• Biophysical Measurement Methods:

  • Single-molecule FRET to monitor conformational changes during the catalytic cycle

  • High-resolution cryo-electron microscopy to visualize structural states during proton translocation

  • Patch-clamp electrophysiology to directly measure proton currents through the F₀ sector

• Computational Approaches:

  • Molecular dynamics simulations to model proton movement through the a-c interface

  • Elastic network models to predict mechanical coupling between the peripheral stalk and catalytic sites

  • Quantum mechanical calculations to assess energetics of proton transfer events

The recent engineering of ATP synthase to form multiple peripheral stalks has demonstrated the possibility of enhancing the H⁺/ATP ratio beyond what is typically observed in nature (2.7 to 5), achieving a ratio of 5.9 . This suggests that the peripheral stalk architecture, including subunit b, plays a critical role in determining coupling efficiency.

How does E. fergusonii ATP synthase subunit b compare structurally and functionally to homologs in other bacterial species?

Comparative analysis of ATP synthase subunit b across different bacterial species reveals important evolutionary patterns and functional constraints:

E. coli ATP synthase subunit b, which has been extensively characterized, provides a useful reference point for understanding the E. fergusonii homolog. The E. coli b subunit is 156 residues in length and forms a highly extended helical dimer , a structural arrangement that is likely conserved in E. fergusonii given the close phylogenetic relationship between these species.

What computational methods are most effective for analyzing the evolutionary conservation of E. fergusonii ATP synthase subunit b?

To analyze evolutionary patterns in E. fergusonii ATP synthase subunit b, researchers can employ several computational approaches:

• Multiple Sequence Alignment (MSA) Techniques:

  • Profile-based methods (MUSCLE, MAFFT) for accurate alignment of divergent sequences

  • Structure-informed alignments incorporating known structural elements of the b subunit

  • Domain-specific alignments to account for differential evolutionary rates across functional regions

• Conservation Analysis Methods:

  • Calculation of position-specific conservation scores using information theory approaches

  • Identification of co-evolving residue networks through statistical coupling analysis

  • Mapping conservation patterns onto structural models to identify functionally important surfaces

• Phylogenetic Analysis:

  • Maximum likelihood or Bayesian approaches for tree construction

  • Reconciliation of gene trees with species trees to identify horizontal gene transfer events

  • Tests for positive or purifying selection across different lineages and functional domains

• Structural Prediction Integration:

  • Threading-based structure prediction using known b subunit structures as templates

  • AlphaFold2 or RoseTTAFold predictions to generate high-confidence structural models

  • Molecular dynamics simulations to assess structural stability of predicted models

These computational approaches can provide insights into the evolutionary history of E. fergusonii ATP synthase subunit b and identify structurally and functionally important residues that have been conserved throughout evolution, which can then guide experimental design.

How might engineering of E. fergusonii ATP synthase subunit b enhance energy conversion efficiency?

Engineering the E. fergusonii ATP synthase subunit b presents several opportunities for enhancing energy conversion efficiency:

• Peripheral Stalk Modification Strategies:

  • Engineering multiple peripheral stalks to increase the H⁺/ATP ratio, as demonstrated in recent studies that achieved a ratio of 5.9

  • Altering the rigidity of the dimerization domain to optimize elastic energy storage during rotational catalysis

  • Modifying the length of the peripheral stalk to optimize the structural coupling between F₀ and F₁ domains

• Interface Optimization Approaches:

  • Enhancing interactions between subunit b and subunit δ to improve mechanical coupling

  • Modifying the interface between subunit b and subunit a to optimize proton translocation

  • Engineering the membrane-spanning domain to improve stability in the lipid bilayer

• Experimental Validation Methods:

  • Measurement of ATP synthesis rates in reconstituted proteoliposomes

  • Determination of proton translocation efficiency using pH-sensitive fluorescent dyes

  • Single-molecule studies to directly observe rotational dynamics and stall forces

Recent engineering efforts have demonstrated that modifying ATP synthase to form multiple peripheral stalks, each bound to a proton-conducting a-subunit, can significantly enhance the H⁺/ATP ratio beyond what is observed in natural systems . This approach represents a novel strategy that differs fundamentally from natural evolutionary solutions, which typically vary the number of H⁺-binding c-subunits to achieve different H⁺/ATP ratios.

What role might E. fergusonii ATP synthase subunit b play in antimicrobial resistance mechanisms?

E. fergusonii has emerged as a significant reservoir for antimicrobial resistance genes, with potential implications for ATP synthase function:

• Potential Resistance Mechanisms Involving ATP Synthase:

  • Alterations in ATP synthase structure affecting binding of ATP synthase inhibitors

  • Changes in energy metabolism to compensate for antibiotic-induced stress

  • Modifications in membrane properties affecting proton gradient maintenance

• Research Findings on E. fergusonii Antimicrobial Resistance:

  • E. fergusonii isolates show high levels of resistance to multiple antibiotics, with 97.74% resistant to sulfafurazole and 94.74% resistant to tetracycline

  • 51.88% of E. fergusonii isolates are extended spectrum beta-lactamase (ESBL)-positive

  • E. fergusonii appears to play an important role in the transmission and development of antibiotic resistance

• Experimental Approaches to Investigate ATP Synthase Involvement:

  • Comparative genomics of atpF sequences from resistant and susceptible isolates

  • Functional characterization of ATP synthase activity in resistant strains

  • Creation of chimeric ATP synthases to identify regions contributing to resistance phenotypes

The prevalence of multidrug-resistant E. fergusonii in both clinical cases and food animals suggests that antimicrobial resistance is widespread in this species . Given that E. fergusonii has been found to be prevalent in foods and food animals, it potentially poses a risk to food safety and public health, with ATP synthase potentially playing a role in the organism's survival under antimicrobial stress.

What are the most promising directions for structural studies of E. fergusonii ATP synthase subunit b?

Future structural studies of E. fergusonii ATP synthase subunit b could explore several promising directions:

• High-Resolution Structure Determination:

  • Cryo-electron microscopy of the intact ATP synthase complex to visualize subunit b in its native context

  • X-ray crystallography of the soluble domains in complex with interacting partners

  • Integrative structural biology approaches combining multiple experimental techniques with computational modeling

• Dynamic Structural Studies:

  • Time-resolved FRET to monitor conformational changes during the catalytic cycle

  • Hydrogen-deuterium exchange mass spectrometry to identify regions with different conformational flexibility

  • Single-molecule force spectroscopy to characterize the mechanical properties of the peripheral stalk

• Structure-Based Drug Design Opportunities:

  • Identification of potential binding pockets in subunit b for selective inhibitors

  • Virtual screening against identified pockets to discover novel antimicrobial compounds

  • Structure-activity relationship studies of identified hits through iterative optimization

Nuclear magnetic resonance studies have already provided valuable insights into the structure of the b30-82 region, revealing an α-helix between residues 39 and 72 with a length of 48.07 Å . Building on this foundation, future studies can aim to determine the complete structure of the full-length protein and characterize its dynamic behavior during ATP synthesis.

What strategies can address solubility issues when expressing recombinant E. fergusonii ATP synthase subunit b?

Researchers working with recombinant E. fergusonii ATP synthase subunit b often encounter solubility challenges. The following strategies can help address these issues:

• Domain-Based Expression Approaches:

  • Express soluble domains separately (e.g., residues 30-156) to avoid membrane domain aggregation

  • Focus on specific regions like b30-82, which has been successfully expressed and characterized by NMR

  • Use overlapping constructs to ensure complete coverage of the protein sequence

• Fusion Partner Optimization:

  • Test multiple solubility-enhancing tags (MBP, SUMO, TRX, GST) to identify optimal expression conditions

  • Position tags at either N- or C-terminus to determine optimal orientation

  • Include protease cleavage sites that can be efficiently processed without affecting protein stability

• Expression Condition Modifications:

  • Reduce expression temperature to 16-18°C to slow folding and prevent aggregation

  • Test different induction conditions (IPTG concentration, induction timing)

  • Screen various media formulations, including auto-induction media

• Detergent and Buffer Optimization:

  • For full-length protein, screen detergents with different micelle properties

  • Test various buffer compositions, including those with stabilizing additives (glycerol, arginine, trehalose)

  • Optimize ionic strength and pH conditions based on protein theoretical properties

When working with specific segments like b30-82, researchers have successfully used nuclear magnetic resonance to determine solution structures, revealing important details about the α-helical arrangement between residues 39 and 72 .

How can researchers resolve conflicting experimental data regarding E. fergusonii ATP synthase subunit b structure?

When encountering conflicting experimental data regarding E. fergusonii ATP synthase subunit b structure, researchers should consider the following approaches:

• Critical Assessment of Methodological Differences:

  • Evaluate protein construct boundaries used in different studies

  • Compare expression systems and purification protocols

  • Assess differences in experimental conditions (buffer, pH, temperature, ionic strength)

  • Consider the resolution limits of different structural techniques

• Integrative Structural Biology Approaches:

  • Combine data from multiple experimental techniques (NMR, X-ray crystallography, cryo-EM, SAXS)

  • Use computational modeling to reconcile apparent discrepancies

  • Employ cross-linking mass spectrometry to validate proposed structural models

  • Perform molecular dynamics simulations to explore conformational flexibility

• Targeted Validation Experiments:

  • Design site-directed mutagenesis experiments to test specific structural hypotheses

  • Use disulfide crosslinking to verify proximity relationships, as demonstrated with positions 61, 68, and 72

  • Employ deuterium exchange mass spectrometry to map solvent-accessible regions

  • Conduct FRET experiments to measure distances between key residues

• Contextual Interpretation:

  • Consider whether discrepancies might reflect genuine conformational flexibility

  • Evaluate whether differences result from distinct functional states of the protein

  • Assess whether interactions with other subunits might alter the observed structure

Previous research has demonstrated the value of cysteine substitution experiments, where alanine residues at positions 61, 68, 70, and 72 were replaced with cysteines. The pattern of disulfide formation provided crucial structural insights about the arrangement of α-helices in the b22-156 segment .