Recombinant Salmonella arizonae ATP synthase subunit b (atpF)

What is the ATP synthase subunit b (atpF) in Salmonella arizonae and what is its primary function?

ATP synthase subunit b, encoded by the atpF gene in Salmonella arizonae, is a critical component of the F1Fo ATP synthase complex. This subunit forms part of the peripheral stalk (also called the stator) of the enzyme complex, connecting the membrane-embedded Fo sector to the catalytic F1 sector. The primary function of subunit b is structural - it helps maintain the integrity of the ATP synthase complex during rotational catalysis.

In Salmonella, the ATP synthase complex couples proton translocation across the bacterial membrane to ATP synthesis/hydrolysis. Research has shown that the F1Fo ATP synthase is required for virulence in Salmonella, as it helps maintain physiological ATP levels and cytoplasmic pH . The b subunit specifically contributes to the stability of this complex, allowing efficient energy conversion during bacterial metabolism and infection processes.

How does ATP synthase subunit b (atpF) structurally and functionally differ from other subunits in the F1Fo complex?

The ATP synthase subunit b (atpF) differs from other subunits in several important structural and functional aspects. Structurally, atpF forms an elongated, predominantly alpha-helical protein that spans from the membrane to the top of the F1 sector. Unlike the catalytic subunits (α, β) in the F1 sector that directly participate in ATP synthesis, or the c-ring subunits that facilitate proton translocation in the Fo sector, the b subunit primarily serves as a structural component.

Functionally, while subunit a (atpB) in the Fo sector forms part of the proton channel and interacts with regulatory proteins like MgtC , subunit b does not directly participate in proton translocation or ATP synthesis. Instead, it transmits conformational changes between the Fo and F1 sectors and prevents the F1 sector from rotating with the c-ring during catalysis, which is essential for the conversion of the proton gradient energy into chemical energy in the form of ATP.

What is the significance of ATP synthase in Salmonella pathogenicity?

The F1Fo ATP synthase plays a critical role in Salmonella pathogenicity through multiple mechanisms that support bacterial survival and replication within host environments:

• Energy metabolism: ATP synthase is essential for energy generation, particularly under the nutrient-limited conditions encountered within host cells.

• pH homeostasis: Research demonstrates that F1Fo ATP synthase helps maintain cytoplasmic pH in Salmonella, which is crucial for survival in the acidic environment of phagosomes .

• Interaction with virulence factors: Evidence shows that virulence proteins such as MgtC interact with the ATP synthase complex (specifically with subunit a) to modulate its activity. This interaction inhibits ATP-driven proton translocation and NADH-driven ATP synthesis, helping maintain physiological ATP levels during infection .

• Survival in macrophages: The regulation of ATP synthase activity is critical for Salmonella survival within macrophages, a key step in establishing systemic infection .

Mutations affecting ATP synthase function can significantly attenuate Salmonella virulence, highlighting its importance in pathogenicity and potential as a target for therapeutic intervention.

How do interactions between virulence factors and ATP synthase components modify Salmonella pathogenicity?

Research has revealed sophisticated interactions between bacterial virulence factors and ATP synthase components that significantly impact Salmonella pathogenicity. The virulence protein MgtC, required for survival within macrophages and lethal infection in mice, directly interacts with the a subunit (atpB) of the F1Fo ATP synthase . This interaction inhibits ATP-driven proton translocation and NADH-driven ATP synthesis.

The functional consequences of this interaction include:

• Reduced ATP hydrolysis: Vesicles from wild-type Salmonella release less phosphate than those from mgtC mutants, indicating inhibited ATP hydrolysis .

• Maintained physiological ATP levels: Wild-type Salmonella maintains lower ATP levels compared to mgtC mutants, which display ~2.2-fold higher ATP levels .

• Cytoplasmic pH regulation: The interaction helps maintain appropriate cytoplasmic pH, which is crucial within the acidic phagosomal environment .

A single amino acid substitution (N92T) in MgtC prevents interaction with the F1Fo ATP synthase and compromises these phenotypes, demonstrating the specificity and importance of this interaction . While these interactions have been documented for subunit a, further research is needed to determine if similar interactions occur with the b subunit (atpF) or if modifying atpF could affect these interactions indirectly by altering complex structure or function.

What are the key considerations when designing experiments to study recombinant Salmonella arizonae ATP synthase subunit b (atpF)?

When designing experiments to study recombinant atpF, researchers should consider several critical factors:

• Expression system selection:

  • Homologous vs. heterologous expression systems

  • Impact of expression tags on structure and function

  • Membrane protein expression challenges

• Purification strategy:

  • Detergent selection for membrane protein extraction

  • Maintaining structural integrity during purification

  • Assessing protein folding and oligomerization state

• Functional assays:

  • ATP synthesis/hydrolysis measurements

  • Proton translocation assays

  • Protein-protein interaction studies

• Mutation analysis:

  • Selection of residues for site-directed mutagenesis

  • Assessing impact on complex assembly

  • Evaluating effects on function and interactions

• Physiological relevance:

  • Correlation between in vitro and in vivo findings

  • Assessment in bacterial infection models

  • Validation in different Salmonella strains

The experimental design should account for the membrane-associated nature of atpF and its role within a multi-subunit complex. Researchers should also consider how modifications to atpF might affect interactions with other ATP synthase subunits and potentially with virulence factors like MgtC that are known to target the ATP synthase complex .

How can structural studies of ATP synthase subunit b contribute to understanding Salmonella virulence mechanisms?

Structural studies of ATP synthase subunit b can provide critical insights into Salmonella virulence mechanisms through several approaches:

• Identification of interaction interfaces: Determining the precise structural regions of atpF that interact with other ATP synthase subunits and potential virulence factors can reveal mechanisms of regulatory control during infection.

• Conformational dynamics: Analyzing structural changes in atpF during different functional states of ATP synthase can elucidate how energy generation is modulated during infection.

• Comparative structural biology: Comparing atpF structures across different bacterial species can identify Salmonella-specific features that might contribute to its unique pathogenicity.

• Structure-guided drug design: Detailed structural information can facilitate the development of inhibitors that specifically target Salmonella ATP synthase, potentially leading to new antimicrobial strategies.

• Understanding complex assembly: Structural studies can reveal how atpF contributes to the assembly and stability of the ATP synthase complex, which is essential for bacterial energy metabolism during infection.

What are the optimal protocols for expressing and purifying recombinant Salmonella arizonae ATP synthase subunit b (atpF)?

Expressing and purifying recombinant Salmonella atpF requires specialized protocols due to its membrane-associated nature. Based on successful approaches with ATP synthase components, the following methodological framework is recommended:

Expression System Selection:

• E. coli C41(DE3) or C43(DE3) strains are preferred for membrane protein expression

• pET-based vectors with T7 promoter provide controllable, high-level expression

• C-terminal His6-tag facilitates purification while minimizing functional interference

Expression Protocol:

• Transform expression plasmid into host strain

• Culture in LB medium at 37°C until OD600 reaches 0.6-0.8

• Induce with 0.1-0.5 mM IPTG

• Reduce temperature to 18-25°C post-induction

• Continue expression for 12-16 hours

Membrane Preparation:

• Harvest cells by centrifugation (5,000×g, 15 min, 4°C)

• Resuspend in buffer (50 mM Tris-HCl pH 8.0, 100 mM NaCl, 5 mM MgCl2)

• Disrupt cells using French press or sonication

• Remove cell debris by centrifugation (10,000×g, 20 min, 4°C)

• Collect membranes by ultracentrifugation (150,000×g, 1 hour, 4°C)

Purification Strategy:

• Solubilize membranes with appropriate detergent (n-dodecyl-β-D-maltoside at 1% is often effective)

• Perform immobilized metal affinity chromatography using Ni-NTA

• Include size exclusion chromatography as a polishing step

• Maintain protein in buffer containing 0.05% detergent to prevent aggregation

Quality Assessment:

• SDS-PAGE for purity evaluation

• Western blotting for identity confirmation

• Circular dichroism to assess secondary structure

• Dynamic light scattering for homogeneity analysis

This methodology can be adapted for co-expression with other ATP synthase subunits if studying the assembled complex is desired.

What techniques are most effective for studying interactions between ATP synthase subunit b and other bacterial proteins?

Multiple complementary techniques can effectively characterize interactions between ATP synthase subunit b and other bacterial proteins:

In vitro Interaction Assays:

• Co-immunoprecipitation (Co-IP):

  • Particularly effective for detecting stable protein complexes

  • Can be performed with antibodies against native proteins or epitope tags

  • Has successfully identified interactions between MgtC and ATP synthase subunit a

• Pull-down Assays:

  • Using recombinant His-tagged atpF as bait

  • Useful for confirming direct interactions

  • Can be coupled with mass spectrometry for unbiased identification of binding partners

• Surface Plasmon Resonance (SPR):

  • Provides real-time binding kinetics

  • Requires immobilization of one protein partner

  • Can determine association/dissociation constants

In vivo Interaction Methods:

• Bacterial Two-Hybrid System:

  • Adapted for membrane proteins using split-ubiquitin approach

  • Allows screening for interactions in a cellular context

• Förster Resonance Energy Transfer (FRET):

  • Detects interactions between fluorescently labeled proteins

  • Particularly valuable for dynamic interaction studies in living cells

• Cross-linking Coupled with Mass Spectrometry:

  • Captures transient interactions

  • Identifies specific interaction interfaces

  • Has been effective for mapping ATP synthase subunit interactions

Structural Approaches:

• Cryo-electron Microscopy:

  • Visualizes protein complexes in near-native state

  • Increasingly powerful for membrane protein complexes

• X-ray Crystallography:

  • Provides atomic-level details of interaction interfaces

  • Challenging for membrane proteins but possible with advanced techniques

When applying these methods, researchers should consider controls to verify specificity, such as using point mutations that disrupt interactions (as demonstrated with the MgtC N92T variant that failed to interact with the F1Fo ATP synthase ).

How can one effectively design mutation studies to analyze functional domains in ATP synthase subunit b?

Designing effective mutation studies for ATP synthase subunit b requires a systematic approach that integrates structural information, sequence conservation analysis, and functional considerations:

Strategic Planning for Mutation Studies:

• Sequence-based Selection:

  • Perform multiple sequence alignment across bacterial species

  • Identify highly conserved residues as potential functional hotspots

  • Analyze Salmonella-specific residues that may contribute to unique functions

• Structure-guided Approaches:

  • Target residues at interfaces with other subunits

  • Focus on regions predicted to be important for stator function

  • Consider the transmembrane domain, which anchors the protein

• Mutation Design Principles:

  • Conservative substitutions (e.g., Leu→Ile) to test structural roles

  • Non-conservative substitutions (e.g., charged→uncharged) to test functional roles

  • Alanine-scanning for systematic functional mapping

  • Cysteine substitutions for accessibility and cross-linking studies

• Experimental Verification Workflow:

• Controls and Validation:

  • Include wild-type controls in all experiments

  • Verify expression levels of mutant proteins

  • Assess protein stability and complex assembly

  • Consider genomic replacements to maintain native expression levels

This strategic approach has proven effective in other ATP synthase subunits, as demonstrated by the N92T mutation in MgtC that abolished interaction with ATP synthase and compromised Salmonella pathogenicity .

How should researchers interpret changes in ATP synthase activity when studying atpF mutants?

Interpreting changes in ATP synthase activity in atpF mutants requires careful consideration of multiple parameters and potential indirect effects:

Primary Activity Parameters:

• ATP Synthesis Rate:

  • Decreased synthesis rates may indicate structural destabilization of the complex

  • Complete loss of activity suggests critical functional role or complex assembly failure

  • Increased rates might indicate relief from regulatory constraints

• ATP Hydrolysis Activity:

  • Can be measured through phosphate release assays as demonstrated in previous studies

  • May not correlate directly with synthesis activity

  • Important for reverse function during certain physiological conditions

• Proton Translocation Efficiency:

  • Critical for understanding energy coupling

  • Can be measured using fluorescent probes or vesicle-based assays

  • Uncoupling of ATP synthesis from proton translocation suggests stator function disruption

Interpretation Framework:

Contextual Considerations:

• Expression Level Effects:

  • Ensure mutant protein is expressed at levels comparable to wild-type

  • Consider using genomic replacements rather than plasmid-based expression

• Complex Assembly:

  • Assess whether detected phenotypes result from impaired assembly rather than direct functional effects

  • Blue Native PAGE can evaluate intact complex formation

• Physiological Context:

  • Interpret biochemical findings in context of bacterial growth phenotypes

  • Consider phenotypes under different energy source conditions

  • Evaluate impacts on virulence in infection models

This methodical approach to interpretation helps distinguish direct functional effects from secondary consequences and provides mechanistic insights into atpF function.

What are the key challenges in correlating in vitro and in vivo findings for ATP synthase studies in Salmonella?

Correlating in vitro and in vivo findings in ATP synthase studies presents significant challenges that researchers must address through careful experimental design and interpretation:

Major Correlation Challenges:

• Environmental Differences:

  • In vitro systems lack the complex physiological environment of living bacteria

  • Intracellular conditions during infection (pH, ion concentrations) are difficult to replicate

  • Membrane composition affects ATP synthase function but differs between in vitro and in vivo settings

• Regulatory Network Complexity:

  • ATP synthase is regulated by numerous factors in vivo

  • Virulence factors like MgtC modulate ATP synthase activity during infection

  • These regulatory networks are often absent in purified systems

• Technical Limitations:

  • In vitro measurements often require non-physiological conditions

  • ATP synthase modifications for in vitro studies may alter natural function

  • Signal-to-noise ratio challenges in measuring activities in whole cells

Bridging Strategies:

• Graduated Complexity Approach:

  • Start with purified components

  • Progress to membrane vesicles (as used in studies of MgtC effects on ATP synthase )

  • Move to whole cells and infection models

  • Compare results across this complexity gradient

• Parallel Measurements:

  • Develop compatible assays for both settings

  • Measure same parameters (e.g., ATP levels, proton translocation)

  • Use consistent environmental conditions where possible

• Genetic Validation Approaches:

  • Create precise genetic modifications based on in vitro findings

  • Test phenotypic effects in bacterial cultures and infection models

  • Use complementation studies to confirm specificity

Previous research demonstrated how a systematic approach can bridge in vitro and in vivo findings. MgtC was shown to inhibit ATP synthase activity in inverted membrane vesicles, and corresponding phenotypes (altered ATP levels and cytoplasmic pH) were observed in whole bacteria. The N92T mutation in MgtC disrupted both the in vitro interaction and in vivo phenotypes, providing strong correlative evidence .

How can researchers distinguish between direct effects on ATP synthase function versus indirect metabolic consequences when studying atpF?

Distinguishing direct effects on ATP synthase function from indirect metabolic consequences when studying atpF requires a multi-faceted approach:

Diagnostic Experimental Designs:

• Immediate vs. Delayed Effects:

  • Direct effects manifest immediately upon perturbation

  • Indirect effects appear after metabolic adjustments

  • Time-course studies can differentiate these patterns

• Isolated Systems Analysis:

  • Study effects in purified ATP synthase complexes

  • Test in membrane vesicles where metabolic networks are disrupted

  • Compare to whole-cell phenomena to identify discrepancies

• Genetic Approach:

  • Create specific point mutations in atpF

  • Engineer compensatory mutations in interaction partners

  • Suppressor mutation analysis can identify functional relationships

• Metabolic Network Control:

  • Measure effects under different carbon sources

  • Control electron transport chain activity

  • Use metabolic inhibitors to block specific pathways

Analytical Framework:

Specialized Techniques:

• Direct Interaction Assessment:

  • Site-specific cross-linking to identify precise interaction points

  • FRET-based approaches to measure conformational changes

  • Hydrogen-deuterium exchange mass spectrometry to map interaction surfaces

• Real-time Activity Monitoring:

  • Luciferase-based ATP sensors to track ATP levels

  • pH-sensitive fluorescent proteins to monitor cytoplasmic pH

  • Membrane potential dyes to assess proton motive force

This systematic approach helps researchers trace the causal chain from atpF modifications to observed phenotypes, distinguishing primary effects from secondary adaptations.

What therapeutic potential exists in targeting ATP synthase subunit b in Salmonella?

ATP synthase subunit b represents a promising target for novel antimicrobial development through several strategic approaches:

Therapeutic Targeting Rationales:

• Essential Function:

  • ATP synthase is vital for energy generation in Salmonella

  • Inhibition leads to energy depletion and growth arrest

  • F1Fo ATP synthase is required for virulence

• Structural Vulnerability:

  • Subunit b forms a critical part of the stator that stabilizes the complex

  • Targeting could disrupt the conformational coupling mechanism

  • May be more accessible than catalytic sites within the membrane

• Specificity Potential:

  • Structural differences between bacterial and human ATP synthases

  • Potential for selective targeting to minimize host toxicity

  • Salmonella-specific features could enable narrow-spectrum activity

Intervention Approaches:

• Small Molecule Inhibitors:

  • Design molecules that disrupt subunit b interactions with other complex components

  • Target the interface between subunit b and the F1 sector

  • Focus on regions unique to bacterial ATP synthases

• Peptide-based Therapeutics:

  • Develop peptides mimicking interaction domains

  • Create cell-penetrating peptides targeting specific subunit b regions

  • Engineer peptides that compete with natural binding partners

• Allosteric Modulation:

  • Identify allosteric sites that affect conformational dynamics

  • Design molecules that lock subunit b in non-functional conformations

  • Target sites that prevent proper energy coupling

Drug Development Considerations:

This approach leverages the critical role of ATP synthase in bacterial bioenergetics while exploiting structural and functional features for selective targeting.