Recombinant Pseudomonas syringae pv. tomato ATP synthase subunit b (atpF)

What is the structural role of ATP synthase subunit b (atpF) in Pseudomonas syringae pv. tomato?

ATP synthase subunit b (atpF) in Pseudomonas syringae pv. tomato is a critical component of the ATP synthase complex, functioning as part of the peripheral stalk that connects the F₀ membrane domain with the F₁ catalytic domain. Similar to what has been observed in other organisms like S. cerevisiae, the subunit b (also referred to as Atp4 in some nomenclature systems) contains both a membrane-embedded portion and a matrix part . The membrane part contributes to the F₀ domain structure alongside other subunits such as the c-ring (Atp9), subunit a (Atp6), and small subunits like Atp8, i/Atp18, and f/Atp17 . The matrix part of subunit b forms part of the external stalk, working in conjunction with other subunits to connect the F₀ domain to the F₁ domain . This structural arrangement is crucial for maintaining the proper orientation and function of the ATP synthase complex during ATP synthesis.

What expression systems are most effective for producing recombinant P. syringae pv. tomato atpF protein?

For recombinant expression of P. syringae pv. tomato atpF, E. coli-based expression systems are commonly employed due to their established protocols and high yield potential. When designing expression experiments, researchers should consider implementing Design of Experiments (DoE) methodology to optimize expression conditions efficiently rather than using the one-factor-at-a-time (OFAT) approach . Critical factors to evaluate include:

• Expression temperature (typically 16-37°C)

• Induction parameters (IPTG concentration)

• Media composition

• Post-induction time

A fractionate factorial design allows researchers to test multiple factors simultaneously while reducing the number of required experiments . For instance, a 2³⁻¹ fractionate factorial design can be used to study three factors with just four experiments instead of eight . For more precise optimization, three-level designs such as Box-Behnken or central composite designs should be employed to model complex response surfaces and identify optimal expression conditions .

How can I verify the purity and integrity of isolated recombinant atpF protein?

Verification of recombinant atpF protein purity and integrity requires a multi-technique approach:

Purity Assessment:

• SDS-PAGE analysis with Coomassie or silver staining

• Western blotting with anti-His tag antibodies (if His-tagged construct is used)

• Size exclusion chromatography to confirm monomeric state

Integrity Verification:

• Mass spectrometry analysis to confirm molecular weight and sequence coverage

• Circular dichroism (CD) to assess secondary structure

For hydrophobic membrane proteins like atpF, special consideration should be given to sample preparation methods. The hydrophobic nature of small membrane proteins can make them difficult to identify by mass spectrometry due to poor fragmentation and low abundance . Consider specialized mass spectrometry protocols developed for hydrophobic proteins, including alternative digestion methods and optimized ionization parameters.

What DoE approach is most suitable for optimizing recombinant atpF purification protocols?

When optimizing purification protocols for recombinant atpF, a systematic DoE approach is significantly more effective than traditional OFAT methods. Based on the complexity of membrane protein purification, the following DoE implementation is recommended:

Initial Screening Phase:
Implement a fractionate factorial design to identify significant factors affecting purification yield and purity. For atpF purification, consider these critical factors:

• Detergent type and concentration

• Salt concentration

• pH conditions

• Imidazole concentration (for His-tagged constructs)

• Temperature

A Plackett-Burman design would be appropriate for this initial screening as it allows evaluation of up to N-1 factors (where N is the number of experiments) . This design can identify the most influential factors with minimal experimental runs.

Optimization Phase:
After identifying significant factors, employ a response surface methodology using Box-Behnken design or central composite design to determine optimal conditions. These designs use 3-5 levels for each factor, allowing modeling of quadratic response surfaces that can identify optimal conditions more precisely .

Where k is the number of factors, p is the fraction size, and cp is the number of center points .

How should phosphorylation studies of atpF be designed to identify potential regulatory sites?

When designing phosphorylation studies for atpF, incorporate methodologies similar to those used for studying other bacterial effector proteins like AvrPtoB. Based on approaches used for AvrPtoB phosphorylation studies in P. syringae, consider the following experimental design:

• In vivo phosphorylation detection:

  • Express recombinant atpF with a C-terminal epitope tag (e.g., HA tag) in plant or bacterial cells

  • Conduct metabolic labeling with [³²P]orthophosphate

  • Immunoprecipitate the tagged protein and analyze by SDS-PAGE and autoradiography to detect phosphorylation

• Mass spectrometry for phosphorylation site identification:

  • Purify recombinant atpF protein after expression in an appropriate system

  • Perform tryptic digestion and phosphopeptide enrichment (using TiO₂ or IMAC)

  • Analyze by LC-MS/MS with collision-induced dissociation (CID) or electron transfer dissociation (ETD)

  • Use targeted mass spectrometry approaches (MRM/PRM) for low-abundance phosphopeptides

• Functional validation of identified sites:

  • Generate site-directed mutants (Ser/Thr/Tyr to Ala or Asp/Glu)

  • Assess functional consequences through ATP synthase activity assays

  • Examine protein-protein interactions with other ATP synthase subunits

This comprehensive approach provides both identification of phosphorylation sites and insights into their functional significance within the ATP synthase complex.

What controls should be included when studying atpF protein-protein interactions?

When investigating protein-protein interactions involving atpF, appropriate controls are essential for experimental validity. Based on ATP synthase interactome analysis methodologies, implement the following controls:

Positive Controls:

• Known interaction partners from the ATP synthase complex (e.g., other peripheral stalk subunits)

• Tagged version of a confirmed interaction partner

Negative Controls:

• Empty vector/tag-only expression

• Unrelated membrane protein of similar size and topology

• Non-specific IgG for immunoprecipitation experiments

Technical Validation Controls:

• Input sample (pre-immunoprecipitation) to confirm starting protein expression

• Reciprocal co-immunoprecipitation to verify interactions

• Competition assays with untagged protein to confirm specificity

For identifying novel interaction partners, include appropriate sample processing controls to account for the challenges associated with hydrophobic membrane proteins, as these proteins often present difficulties in mass spectrometry identification due to their low abundance, poor fragmentation, and hydrophobic nature .

How does the structure of P. syringae pv. tomato atpF compare with homologous proteins in other bacterial species?

The structural comparison of P. syringae pv. tomato atpF with homologous proteins in other bacterial species reveals important evolutionary and functional insights. While specific structural data for P. syringae atpF is limited, comparative analysis with better-characterized homologs provides valuable information.

ATP synthase subunit b serves as part of the peripheral stalk connecting the F₀ and F₁ domains across various species. In S. cerevisiae and mammals, subunit b (Atp4) consists of both membrane-embedded and matrix components . The membrane component contributes to the F₀ domain alongside subunits c/Atp9, a/Atp6, and small regulatory subunits .

Structural Conservation Analysis:

The primary sequence alignment typically shows higher conservation in the C-terminal domain that interacts with F₁ components, while the membrane-spanning regions show greater variability. This pattern suggests evolutionary constraints related to functional interactions within the ATP synthase complex while allowing adaptations to different membrane environments.

What are the methodological challenges in determining the ATP-dependent conformational changes in atpF?

Studying ATP-dependent conformational changes in atpF presents several methodological challenges requiring specialized approaches. Drawing from studies of ATP-dependent conformational changes in other ATP-binding proteins , researchers should consider:

Challenge 1: Capturing transient states

• Solution: Implement time-resolved techniques such as stopped-flow spectroscopy or temperature-jump methods

• Approach: Use ATP analogs (AMP-PNP, ATP-γ-S) to trap specific conformational states

• Validation: Combine with mutagenesis of key residues in ATP-binding domains

Challenge 2: Membrane protein structural analysis

• Solution: Employ a combination of structural biology techniques

• Approaches:

  • X-ray crystallography with lipidic cubic phase crystallization

  • Cryo-electron microscopy of intact ATP synthase complexes

  • Site-directed spin labeling coupled with electron paramagnetic resonance (EPR)

  • Hydrogen-deuterium exchange mass spectrometry (HDX-MS)

Challenge 3: Conformational dynamics in native-like environments

• Solution: Reconstitute purified atpF in nanodiscs or liposomes

• Approach: Implement single-molecule FRET to detect distance changes between labeled domains

• Validation: Correlate structural changes with functional assays measuring ATP synthase activity

The combination of these complementary approaches allows researchers to build a comprehensive model of ATP-dependent conformational changes in atpF and their functional significance in ATP synthase operation.

How do mutations in the atpF gene affect ATP synthase assembly and function in P. syringae pv. tomato?

Mutations in the atpF gene can have profound effects on ATP synthase assembly and function, with significant consequences for P. syringae pv. tomato physiology. Based on studies of ATP synthase subunits in other organisms, the following methodological approach is recommended for characterizing atpF mutations:

Systematic Mutation Analysis:

• Generate a library of site-directed mutations targeting:

  • Membrane-spanning domains

  • Stalk region

  • Interface regions that interact with other subunits

  • Conserved residues identified through multi-species alignment

• Express mutant proteins in appropriate expression systems and assess:

  • Protein stability and expression levels

  • Membrane integration

  • Complex assembly

Assembly Analysis Methodology:

• Blue native PAGE to assess intact complex formation

• Sucrose gradient ultracentrifugation to separate assembled complexes

• Crosslinking mass spectrometry to identify altered subunit interactions

• Immunoprecipitation followed by Western blotting to detect changes in subunit associations

Functional Assessment:

• ATP synthesis/hydrolysis assays in membrane vesicles or reconstituted systems

• Proton translocation measurements using pH-sensitive fluorescent dyes

• Growth complementation assays in atpF-deficient strains

Predicted Impact of Domain-Specific Mutations:

This systematic approach allows researchers to establish structure-function relationships for atpF and identify critical residues for ATP synthase assembly and function.

How can we design experiments to study the interaction between atpF and other ATP synthase subunits?

Designing experiments to study interactions between atpF and other ATP synthase subunits requires a comprehensive approach incorporating multiple complementary techniques. Based on established ATP synthase interactome analysis methods, the following experimental design is recommended:

In vivo Interaction Studies:

• Co-immunoprecipitation (Co-IP): Express epitope-tagged atpF in P. syringae and perform pulldown experiments followed by mass spectrometry or Western blotting to identify interacting partners

• Bacterial two-hybrid assays: Construct fusion proteins with split reporter domains to assess binary interactions between atpF and other subunits

• in vivo crosslinking: Utilize cell-permeable crosslinkers followed by tandem mass spectrometry (MS/MS) to capture transient or weak interactions

In vitro Interaction Analysis:

• Surface plasmon resonance (SPR): Measure binding kinetics between purified atpF and other subunits

• Isothermal titration calorimetry (ITC): Determine thermodynamic parameters of binding

• Microscale thermophoresis (MST): Assess interactions using minimal sample amounts with label or label-free approaches

Structural Studies:

• Cryo-electron microscopy: Visualize the intact ATP synthase complex with focus on the peripheral stalk region containing atpF

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS): Identify regions with altered solvent accessibility when atpF interacts with partner subunits

• Crosslinking mass spectrometry (XL-MS): Determine distance constraints between atpF and interacting subunits

When designing these experiments, implement DoE methodology to systematically optimize experimental conditions rather than using OFAT approaches . For instance, use fractionate factorial designs to evaluate multiple factors affecting protein interaction detection, followed by response surface methodology to fine-tune the most significant parameters .

What post-translational modifications (PTMs) occur on atpF and how do they affect its function?

Post-translational modifications of atpF likely play important regulatory roles in ATP synthase function in P. syringae pv. tomato. While specific PTMs on atpF in P. syringae have not been directly characterized in the provided search results, a methodological framework for their investigation can be developed based on approaches used for studying other bacterial proteins:

Identification of PTMs:

• Phosphorylation analysis:
  Similar to the approach used for AvrPtoB in P. syringae , express tagged atpF in the presence of [³²P]orthophosphate, immunoprecipitate, and analyze by SDS-PAGE and autoradiography to detect phosphorylation events .

• Mass spectrometry-based PTM profiling:

  • Employ enrichment strategies specific to different PTMs (e.g., TiO₂ for phosphopeptides)

  • Use electron transfer dissociation (ETD) fragmentation to preserve labile modifications

  • Implement targeted approaches for low-abundance modified peptides

PTM Mapping and Functional Analysis:

Contextual Analysis:

• Compare PTM profiles under different growth conditions (e.g., plant apoplast vs. laboratory media)

• Assess PTM changes during different growth phases

• Evaluate PTM alterations in response to environmental stressors (pH, temperature, oxidative stress)

For validation of identified PTMs and their functional significance, implement approaches that combine site-directed mutagenesis with functional assays measuring ATP synthase assembly, stability, and catalytic activity. Particular attention should be given to potential regulatory mechanisms that may coordinate ATP synthase activity with bacterial metabolism and virulence processes.

What approaches can be used to target atpF for antimicrobial development against P. syringae pv. tomato?

Targeting atpF for antimicrobial development against P. syringae pv. tomato represents a promising strategy due to the essential nature of ATP synthase in bacterial energy metabolism. A systematic approach for such development includes:

Target Validation Methodology:

• Demonstrate essentiality of atpF through:

  • Conditional knockdown experiments

  • Chemical genetic approaches

  • Growth inhibition studies with known ATP synthase inhibitors

• Assess target druggability by:

  • Homology modeling of P. syringae atpF

  • Identification of potential binding pockets

  • Comparison with ATP synthase inhibitor binding sites in other bacteria

Inhibitor Discovery Strategies:

• Structure-based approaches:

  • Virtual screening against predicted binding sites

  • Fragment-based drug discovery targeting the atpF-subunit a interface

  • Rational design based on known ATP synthase inhibitors

• High-throughput screening:

  • Develop biochemical assays measuring ATP synthase activity

  • Design cell-based assays with reporter systems linked to ATP synthase function

  • Employ DoE principles to optimize screening conditions

Selectivity Considerations:

• Conduct comparative structural analysis of atpF between P. syringae, beneficial bacteria, and plant ATP synthases

• Identify unique structural features or sequences in P. syringae atpF

• Target P. syringae-specific regions to minimize off-target effects

Validation and Optimization Framework:
For hit compounds, implement a systematic validation pipeline with these key steps:

• Confirm target engagement using thermal shift assays or competition binding studies

• Assess effects on ATP synthase assembly and function

• Determine antimicrobial activity against P. syringae under various conditions

• Evaluate efficacy in plant infection models

• Assess potential for resistance development

This comprehensive approach maximizes the potential for developing effective and selective antimicrobials targeting P. syringae ATP synthase.

How can recombinant atpF be utilized as a tool for studying plant-pathogen interactions?

Recombinant atpF can serve as a valuable tool for investigating various aspects of plant-pathogen interactions. The following methodological approaches illustrate its potential applications:

1. As a molecular probe for ATP synthase assembly and function:

• Generate fluorescently tagged recombinant atpF to visualize ATP synthase localization during infection

• Develop antibodies against P. syringae atpF to track protein levels during different infection phases

• Create affinity-tagged versions to capture intact ATP synthase complexes from infected tissues

2. For studying host immune responses:

• Assess whether plant hosts recognize bacterial ATP synthase components as microbe-associated molecular patterns (MAMPs)

• Determine if atpF elicits pattern-triggered immunity (PTI) responses when introduced into plant tissues

• Investigate potential interactions between atpF and plant immune receptors

3. In comparative studies with effector proteins:
Similar to studies with AvrPtoB , recombinant atpF can be used to:

• Determine if ATP synthase components undergo post-translational modifications in planta

• Investigate potential moonlighting functions beyond ATP synthesis

• Assess whether atpF interacts with plant cellular components

Experimental Design Considerations:
When designing experiments with recombinant atpF, implement DoE approaches to systematically optimize experimental conditions . For example:

• Use fractionate factorial designs to identify key factors affecting protein-protein interactions

• Apply response surface methodology to optimize conditions for in vitro assays

• Employ Plackett-Burman designs for initial screening of multiple variables affecting recombinant protein activity

This systematic approach enables researchers to maximize the utility of recombinant atpF as a tool for understanding the complex dynamics of plant-pathogen interactions.