Recombinant Pseudomonas fluorescens ATP synthase subunit b (atpF)

What is the structural organization of ATP synthase in Pseudomonas fluorescens?

ATP synthase in P. fluorescens, like other bacterial F-type ATP synthases (bFOF1), consists of two main multisubunit complexes: the water-soluble F1 complex and the membrane-integral FO complex. These complexes are connected by central and peripheral stalks. The F1 complex contains the catalytic α3β3 hexamer, where ATP synthesis occurs at the α/β subunit interfaces. The FO complex includes the a-subunit and the c-ring embedded in the membrane. The peripheral stalk typically contains b-subunits (including atpF) that help connect the F1 and FO regions .

While bacterial ATP synthases share a common core structure, they can exhibit significant differences between phyla. For example, some bacteria like Chloroflexus aurantiacus contain four copies of b-subunit per complex instead of the usual two, with differently designed connections between FO and F1 portions .

What is the functional role of subunit b (atpF) in P. fluorescens ATP synthase?

Subunit b (atpF) in P. fluorescens serves as a critical component of the peripheral stalk (or stator) of the ATP synthase complex. This stalk functions as a stationary arm that prevents the rotation of the F1 catalytic portion while allowing the central stalk to rotate with the c-ring as a rigid body. By anchoring the α3β3 hexamer to the membrane portion, subunit b enables the chemo-mechanical coupling necessary for ATP synthesis .

The peripheral stalk formed by b-subunits effectively counteracts the torque generated during ATP synthesis, maintaining the structural integrity of the enzyme during its rotary catalytic cycle. In bacterial systems like P. fluorescens, proper functioning of this component is essential for efficient energy conversion .

How is recombinant P. fluorescens ATP synthase subunit b typically expressed and purified?

Based on protocols for similar ATP synthase subunits, recombinant P. fluorescens ATP synthase subunit b is commonly expressed using E. coli expression systems with fusion tags to facilitate purification. The gene encoding atpF can be cloned into expression vectors with affinity tags such as His-tag, similar to the approach used for atpB .

For purification, the following general protocol can be applied:

• Express the protein in E. coli using appropriate induction conditions

• Harvest cells and lyse using mechanical disruption or chemical methods

• Perform affinity chromatography (e.g., nickel resin for His-tagged proteins)

• Consider further purification steps including ion exchange or size exclusion chromatography

• Confirm purity via SDS-PAGE (>90% purity is typically desired)

• Store the purified protein as a lyophilized powder or in a suitable buffer with appropriate stabilizers

For storage, it's recommended to avoid repeated freeze-thaw cycles, and the protein can be stored at -20°C/-80°C after aliquoting. Working aliquots may be kept at 4°C for up to one week .

What expression systems are most effective for producing recombinant ATP synthase subunits from P. fluorescens?

For challenging membrane protein components of ATP synthase like subunit b, considerations include:

• Expression strategy options:

  • E. coli with fusion tags (His, GST, MBP, etc.)

  • Native P. fluorescens expression systems utilizing ABC transporters

  • Cell-free expression systems for membrane proteins

• Optimization parameters:

  • Temperature (often lowered to 16-25°C for membrane proteins)

  • Induction timing and concentration

  • Media composition

  • Co-expression with chaperones

P. fluorescens expression systems have notable advantages for certain recombinant proteins, especially when using genetically modified strains like P. fluorescens ΔfleQ which produces fewer background proteins that could complicate purification .

How do structural variations in atpF across bacterial species impact ATP synthase function and stability?

Peripheral stalk components including subunit b (atpF) show significant structural diversity across bacterial phyla, which impacts enzyme functionality and stability. The b-subunit's length, oligomeric state, and interaction interfaces with other ATP synthase components can vary considerably between species. For example, while most bacterial ATP synthases contain two copies of subunit b, Chloroflexus aurantiacus contains four copies, fundamentally altering the architecture of the peripheral stalk .

These structural variations influence:

• Enzyme stability under different environmental conditions

• Resistance to rotational stress during catalysis

• Interaction with other cellular components

• Potential for oligomerization of ATP synthase complexes

Comparative structural analysis between ATP synthases from different phyla reveals that while the core catalytic mechanism remains conserved, peripheral components like atpF may adapt to specific environmental niches or metabolic requirements of the organism .

What methodologies are most effective for studying interaction interfaces between atpF and other ATP synthase subunits?

Multiple complementary approaches are recommended for comprehensive characterization of atpF interactions:

• Structural determination methods:

  • Cryo-electron microscopy (cryo-EM): Particularly valuable for intact ATP synthase complexes

  • X-ray crystallography: For high-resolution analysis of isolated subunits or subcomplexes

  • NMR spectroscopy: For dynamic interaction studies of smaller domains

• Biochemical interaction methods:

  • Cross-linking mass spectrometry: To capture transient interactions

  • Surface plasmon resonance (SPR): For binding kinetics

  • Isothermal titration calorimetry (ITC): For thermodynamic parameters

  • Blue native PAGE: For intact complex analysis

• Computational approaches:

  • Molecular dynamics simulations

  • Protein-protein docking

  • Evolutionary coupling analysis

Combining these methods can reveal crucial interaction interfaces. For example, in bacterial ATP synthases, the b-subunit forms important contacts with both the membrane-embedded a-subunit and the δ-subunit of the F1 portion, creating a continuous connection between FO and F1 sectors .

How can site-directed mutagenesis be effectively used to study functional domains of P. fluorescens atpF?

Site-directed mutagenesis provides a powerful tool for investigating structure-function relationships in atpF. A systematic approach includes:

• Target selection strategy:

  • Conserved residues identified through multiple sequence alignment

  • Predicted interaction interfaces with other subunits

  • Regions with predicted secondary structure transitions

  • Charged residues that may participate in salt bridges

• Mutation design guidelines:

  • Conservative substitutions to probe subtle functional effects

  • Charge reversals to disrupt electrostatic interactions

  • Cysteine substitutions for accessibility studies and cross-linking

  • Truncations to identify minimal functional domains

• Functional assessment methods:

  • ATP synthesis/hydrolysis assays

  • Proton translocation measurements

  • Structural integrity analysis via native gels

  • Assembly efficiency of ATP synthase complexes

When designing mutagenesis experiments, special attention should be paid to the N-terminal membrane-anchoring domain and the C-terminal region that typically interacts with the F1 sector, as these regions are critical for proper assembly and function of the peripheral stalk .

What role does the P. fluorescens ATP synthase peripheral stalk play in enzyme stability and function under different environmental conditions?

The peripheral stalk of P. fluorescens ATP synthase, which includes subunit b (atpF), serves as a critical structural element that maintains enzyme stability under varying environmental conditions. Research on bacterial ATP synthases indicates that this component functions as a molecular "strut" that prevents unproductive rotation of the F1 sector while allowing the central stalk to rotate productively .

Key functions under environmental stress include:

• Temperature adaptation:

  • Maintaining structural integrity at different growth temperatures

  • Preventing thermal denaturation through stabilizing interactions

• pH response:

  • Adjusting to proton motive force changes in acidic/basic environments

  • Maintaining proper subunit interactions across pH ranges

• Osmotic stress handling:

  • Preserving structural connections during membrane fluidity changes

  • Adapting to altered proton gradients under osmotic pressure

• Energy limitation responses:

  • Supporting ATP synthase function during energy-limited conditions

  • Potentially participating in regulatory mechanisms

The composition and structure of the peripheral stalk likely reflects adaptation to P. fluorescens' specific ecological niche and environmental challenges it encounters .

How can recombinant P. fluorescens atpF be optimally expressed for structural studies?

For structural studies requiring high-yield, properly folded recombinant atpF, a multi-faceted optimization approach is recommended:

• Expression construct design:

  • Include solubilizing fusion partners (MBP, SUMO) for improved folding

  • Incorporate TEV or PreScission protease sites for tag removal

  • Consider codon optimization for expression host

• Expression conditions optimization table:

• Extraction considerations:

  • For full-length membrane-associated atpF: detergent screening (DDM, LMNG, etc.)

  • For soluble domains: standard aqueous buffers with reducing agents

  • Buffer optimization for pH, salt concentration, and stabilizing additives

• Purification strategy:

  • Two-step minimum (affinity + size exclusion)

  • Consider ion exchange as an intermediate step

  • Final polishing via size exclusion in structural biology buffer

Protein quality assessment via SEC-MALS, thermal shift assays, and negative stain EM is strongly recommended before proceeding to high-resolution structural studies.

What approaches can be used to study the dynamics of ATP synthase peripheral stalk during catalysis?

Studying the dynamics of the peripheral stalk during ATP synthesis requires specialized techniques that can capture conformational changes during enzyme function:

• Single-molecule techniques:

  • Fluorescence resonance energy transfer (FRET) with strategic labeling of atpF

  • High-speed atomic force microscopy (HS-AFM) for direct visualization

  • Optical tweezers combined with fluorescence for force-motion studies

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

  • Maps regions of differential solvent accessibility during catalysis

  • Identifies flexible regions and potential hinge points

  • Reveals conformational changes in response to substrate binding

• Time-resolved cryo-EM:

  • Captures different conformational states during the catalytic cycle

  • Requires rapid mixing/freezing techniques or substrate analogs

  • Can be combined with mutants that arrest at specific catalytic steps

• Molecular dynamics simulations:

  • All-atom simulations of peripheral stalk flexibility

  • Targeted molecular dynamics to model transitional states

  • Coarse-grained simulations for longer timescale events

These approaches should be used complementarily to develop a comprehensive understanding of how the peripheral stalk maintains stability while accommodating the conformational changes necessary for ATP synthesis .

How can researchers effectively analyze the interaction between atpF and membrane lipids in P. fluorescens?

The interaction between atpF and membrane lipids is critical for proper anchoring and function of the ATP synthase complex. Several specialized approaches can be employed to study these interactions:

• Biophysical characterization methods:

  • Solid-state NMR for direct lipid-protein contacts

  • Electron paramagnetic resonance (EPR) with site-directed spin labeling

  • Differential scanning calorimetry to assess lipid phase transitions

  • Neutron reflectometry for membrane insertion depth analysis

• Reconstitution approaches:

  • Nanodiscs with defined lipid composition

  • Liposome reconstitution with activity assays

  • Lipid cubic phase crystallization

  • Systematic lipid composition screening

• Computational methods:

  • Molecular dynamics simulations with explicit membrane models

  • Coarse-grained simulations for longer timescales

  • Potential of mean force calculations for insertion energetics

• Chemical biology approaches:

  • Photoactivatable lipid analogs for crosslinking

  • Click chemistry with functionalized lipids

  • Fluorescently labeled lipids combined with FRET

These techniques can reveal how specific lipid interactions influence the orientation, stability, and function of atpF within the membrane environment, which is particularly important for understanding how the peripheral stalk maintains proper positioning relative to the rotating components of ATP synthase .

What are the most reliable assays for measuring functional integration of recombinant atpF into the ATP synthase complex?

To verify that recombinant atpF properly incorporates into functional ATP synthase complexes, a multi-level assessment approach is recommended:

• Structural integration assays:

  • Blue native PAGE to verify complex assembly

  • Immunoprecipitation with antibodies against other ATP synthase subunits

  • Size exclusion chromatography to assess complex formation

  • Density gradient centrifugation for intact complex isolation

• Functional assessment methods:

• Complementation studies:

  • Expression in atpF-deletion strains

  • Growth rate comparison under respiratory conditions

  • ATP levels in vivo

  • Membrane potential measurements

• Structure-guided mutational analysis:

  • Strategic mutations at interaction interfaces

  • Assessment of effects on assembly and function

  • Correlation of functional defects with structural changes

This comprehensive approach ensures that the recombinant atpF not only physically associates with the ATP synthase complex but also supports its complete catalytic cycle and energy transduction functions .

What strategies can address poor solubility when expressing recombinant P. fluorescens atpF?

Poor solubility is a common challenge when working with membrane-associated ATP synthase components like atpF. Multiple approaches can be employed to overcome this limitation:

• Expression strategy modifications:

  • Express only the soluble domain (if applicable)

  • Use solubility-enhancing fusion partners (MBP, SUMO, Trx)

  • Co-express with interacting partners from the ATP synthase complex

  • Switch to specialized membrane protein expression strains (C41/C43)

• Buffer optimization table:

• Refolding approaches:

  • Isolation of inclusion bodies followed by controlled refolding

  • On-column refolding during affinity purification

  • Dialysis-based gradual detergent or denaturant removal

• Alternative expression systems:

  • Cell-free expression with supplied detergents or nanodiscs

  • Baculovirus-insect cell expression

  • P. fluorescens expression system utilizing its own secretion machinery

These approaches should be systematically tested to identify the optimal conditions for obtaining soluble, properly folded atpF protein suitable for downstream structural and functional studies.

How can researchers troubleshoot issues with ATP synthase assembly when studying recombinant atpF?

When recombinant atpF fails to properly assemble into the ATP synthase complex, systematic troubleshooting is necessary:

• Expression balance assessment:

  • Quantify expression levels of atpF relative to other ATP synthase subunits

  • Adjust expression levels to maintain proper stoichiometry

  • Consider co-expression of multiple subunits from a single construct

• Protein quality verification:

  • Assess protein folding via circular dichroism

  • Verify membrane association properties

  • Confirm absence of aberrant modifications

  • Check tag interference with assembly interfaces

• Assembly conditions optimization:

  • Test various detergent types and concentrations

  • Explore lipid composition effects on assembly

  • Optimize buffer conditions (pH, ionic strength, divalent cations)

  • Consider chaperone co-expression or addition

• Domain mapping to locate assembly defects:

  • Create chimeric constructs with known functional domains

  • Perform deletion analysis to identify problematic regions

  • Use peptide competition assays to identify critical interaction motifs

These approaches can help identify whether assembly issues stem from problems with the recombinant atpF itself or from incompatibilities with the expression system or reconstitution conditions .

What are effective approaches for validating structural models of P. fluorescens ATP synthase peripheral stalk?

Validating structural models of the ATP synthase peripheral stalk involves multiple complementary experimental approaches:

Integration of these experimental approaches with computational modeling provides a robust framework for validating structural models of the peripheral stalk, particularly for challenging regions like the membrane-spanning portions and flexible connecting segments .

How might targeting P. fluorescens ATP synthase inform antimicrobial development strategies?

While P. fluorescens itself is not typically a clinical pathogen, research on its ATP synthase could inform broader antimicrobial strategies, particularly against related Pseudomonas species like P. aeruginosa. Key research directions include:

• Structure-based drug design opportunities:

  • Identify unique structural features in Pseudomonas ATP synthases

  • Develop compounds that selectively target bacterial-specific elements

  • Focus on peripheral stalk components that differ from human counterparts

  • Exploit species-specific variations in the c-ring/stator interface

• Potential therapeutic strategies:

  • Disruption of peripheral stalk assembly

  • Interference with proton translocation

  • Inhibition of rotary mechanics

  • Destabilization of subunit interactions unique to bacterial ATP synthases

• Cross-species comparative approach:

  • Utilize successful examples like bedaquiline (targeting mycobacterial ATP synthase)

  • Identify conserved vulnerabilities across different bacterial phyla

  • Explore differential susceptibility based on structural variations

• Resistance mechanism studies:

  • Investigate potential adaptive mutations in atpF

  • Characterize compensatory mechanisms for ATP synthase dysfunction

  • Model evolutionary pathways for resistance development

This research direction holds promise for addressing the growing challenge of antimicrobial resistance by targeting the essential energy production machinery of bacterial pathogens .

What role might post-translational modifications play in regulating P. fluorescens ATP synthase function?

Post-translational modifications (PTMs) of ATP synthase subunits represent an understudied area with significant potential for understanding regulatory mechanisms:

• Potential PTMs to investigate:

  • Phosphorylation of serine/threonine/tyrosine residues

  • Acetylation of lysine residues

  • Methylation of arginine or lysine residues

  • Oxidative modifications of cysteine or methionine residues

• Functional consequences to explore:

  • Effects on ATP synthesis/hydrolysis kinetics

  • Impacts on assembly and stability of the complex

  • Changes in response to environmental stress

  • Influence on interactions with other cellular components

• Methodological approaches:

  • Mass spectrometry-based proteomics for PTM identification

  • Site-directed mutagenesis to create phosphomimetic variants

  • In vitro reconstitution with modified components

  • Comparative analysis across growth conditions

• Regulatory network integration:

  • Identification of kinases/phosphatases acting on ATP synthase

  • Mapping of signaling pathways controlling energy metabolism

  • Connection to bacterial stress responses

  • Integration with other cellular energetic pathways

Understanding PTM-based regulation could reveal new mechanisms by which P. fluorescens adapts its energy metabolism to changing environmental conditions, potentially informing both fundamental bacterial physiology and biotechnological applications .