Recombinant Salmonella dublin ATP synthase subunit b (atpF)

What is the function of ATP synthase subunit b (atpF) in Salmonella dublin?

ATP synthase subunit b, encoded by the atpF gene, serves as a critical component of the peripheral stalk in the F₁F₀ ATP synthase complex. It forms part of the stator that anchors the catalytic F₁ portion to the membrane-embedded F₀ portion, preventing rotation of the F₁ segment during proton translocation. The peripheral stalk, composed of subunits b and b', is essential for the coupling of proton movement through the membrane to ATP synthesis or hydrolysis .

In Salmonella, this structural role is crucial for maintaining proper ATP synthesis function, which directly impacts cellular energy homeostasis. Studies have demonstrated that frame-shift mutations in atpF prevent ATP synthase function and accumulation, highlighting its essential nature in the biogenesis and functionality of the complete complex .

How does atpF contribute to Salmonella dublin pathogenicity?

The ATP synthase complex, including the atpF subunit, plays an indirect but significant role in Salmonella pathogenicity. The complex is targeted by virulence factors such as MgtC, which binds to the F₁F₀ ATP synthase and inhibits proton translocation and ATP synthesis . This interaction helps Salmonella maintain physiological ATP levels and cytoplasmic pH within macrophages, which contributes to bacterial survival in the hostile host environment.

Research indicates that proper ATP synthase function is required for virulence. Mutations affecting ATP synthase components disrupt bacterial energy metabolism, impacting the pathogen's ability to adapt to the intracellular environment. The importance of intact ATP synthase is underscored by observations that virulence proteins specifically target this complex to modulate its activity during infection .

What is the structural relationship between atpF and other ATP synthase components?

ATP synthase subunit b (atpF) functions as part of a heterodimeric peripheral stalk along with subunit b' (encoded by ATPG in some organisms). These subunits form an extended structure that connects the membrane-embedded F₀ portion to the catalytic F₁ portion of ATP synthase.

The peripheral stalk prevents the rotation of the F₁ complex during proton translocation through F₀. Research in various systems, including Chlamydomonas reinhardtii, has shown that both peripheral stalk subunits are essential for proper ATP synthase biogenesis and function . Structural studies indicate that atpF has an N-terminal membrane-anchoring domain and an extended α-helical domain that reaches up to the F₁ sector.

Cross-linking and proteomic studies have revealed interactions between atpF and other ATP synthase subunits, particularly the a and α subunits, forming a rigid connection that maintains the structural integrity of the complex during the rotational catalysis mechanism.

What are the most effective expression systems for producing recombinant Salmonella dublin atpF?

The most effective expression systems for recombinant atpF production utilize E. coli-based platforms, particularly those designed for membrane protein expression. Researchers commonly employ the following methodological approaches:

• E. coli C41(DE3) or C43(DE3) strains: These strains, derived from BL21(DE3), are engineered to tolerate membrane protein overexpression.

• Expression vectors: pET-based vectors with T7 promoters provide controlled, high-level expression. For atpF specifically, including a cleavable N-terminal tag (His₆, GST, or MBP) facilitates purification without disrupting function.

• Induction conditions: Low-temperature induction (16-20°C) with reduced IPTG concentrations (0.1-0.5 mM) for 16-20 hours generally yields optimal results for maintaining protein solubility.

Similar experimental approaches have been successfully applied to other ATP synthase components, as demonstrated in studies of the SopB protein from Salmonella dublin, where glutathione S-transferase fusion constructs were created using the pGEX-2T plasmid vector .

What purification strategies yield the highest purity and activity of recombinant atpF?

Purifying recombinant atpF requires specialized approaches due to its membrane-associated nature. The following protocol has proven effective:

• Membrane isolation: Harvest cells and disrupt by French press or sonication, followed by differential centrifugation to isolate membrane fractions (typically 100,000 × g for 1 hour).

• Detergent solubilization: Solubilize membranes using mild detergents such as n-dodecyl-β-D-maltoside (DDM) or digitonin (0.5-1%) at 4°C for 1-2 hours with gentle agitation.

• Affinity chromatography: For His-tagged constructs, use Ni-NTA resin with imidazole gradients (20-250 mM) for elution in the presence of low detergent concentrations.

• Size exclusion chromatography: As a final purification step, gel filtration using Superdex 200 columns separates monomeric atpF from aggregates and contaminants.

During all purification steps, maintaining a pH of 7.5-8.0 and including glycerol (10%) and salt (150-300 mM NaCl) in buffers enhances protein stability. Purification yields of 1-2 mg pure protein per liter of culture are typically achievable when optimal expression conditions are employed.

How can researchers verify the proper folding and functionality of recombinant atpF?

Verifying proper folding and functionality of recombinant atpF requires multiple complementary approaches:

• Circular dichroism (CD) spectroscopy: Analysis of secondary structure content, particularly α-helical content which should be high for properly folded atpF.

• Limited proteolysis: Properly folded protein shows distinct fragmentation patterns compared to misfolded variants.

• Functional reconstitution:

  • Reconstitution into liposomes with other ATP synthase subunits

  • ATP-driven proton translocation assays using pH-sensitive fluorescent dyes like ACMA (9-amino-6-chloro-2-methoxyacridine)

  • NADH-driven ATP synthesis monitoring using luciferase-based ATP detection systems

• Binding assays: Co-immunoprecipitation or pull-down assays to verify interactions with other ATP synthase subunits, particularly the F₁ sector components.

A combination of these approaches provides comprehensive validation. Research on related ATP synthase components has demonstrated the utility of fluorescence quenching assays to monitor proton translocation and ATP hydrolysis/synthesis activity measurements to confirm functional reconstitution .

How do mutations in atpF affect ATP synthase assembly and function in Salmonella?

Mutations in atpF have profound effects on ATP synthase assembly and function. Frame-shift mutations completely prevent ATP synthase function and accumulation, similar to the effects observed with knockout mutations of the ATPG gene (encoding subunit b') . The impacts can be characterized across several dimensions:

• Complex assembly: Without functional atpF, the peripheral stalk fails to form, preventing proper assembly of the complete F₁F₀ complex. This leads to degradation of unassembled subunits, as observed with AtpH (subunit c) becoming a substrate for the FTSH protease in the absence of proper complex formation .

• Proton translocation: Defective atpF prevents proper coupling between the F₀ and F₁ sectors, abolishing ATP-driven proton translocation. This can be measured using fluorescence quenching assays with membrane vesicles and pH-sensitive dyes like ACMA .

• ATP synthesis/hydrolysis: Without functional atpF, both ATP synthesis driven by proton gradients and ATP hydrolysis coupled to proton pumping are severely impaired. This results in altered cellular ATP levels and disrupted energy homeostasis .

• Physiological consequences: Strains with atpF mutations show significant phenotypic changes, including growth defects in energy-limited conditions, altered cytoplasmic pH regulation, and potentially reduced virulence due to inability to adapt to host environments .

Research methodologies to study these effects include comparative proteomic analysis, membrane potential measurements, and ATP synthesis/hydrolysis assays comparing wild-type and mutant strains.

What is the relationship between virulence factor MgtC and the ATP synthase complex containing atpF?

The relationship between the virulence factor MgtC and the ATP synthase complex (including atpF) represents a fascinating example of pathogen adaptation to host environments. Research has revealed several key aspects of this interaction:

• Direct physical interaction: MgtC specifically binds to the a subunit of the F₁F₀ ATP synthase, as demonstrated through co-immunoprecipitation experiments. While MgtC doesn't directly interact with atpF, its binding to the a subunit affects the function of the entire complex .

• Functional consequences:

  • MgtC inhibits ATP-driven proton translocation in inside-out membrane vesicles

  • MgtC inhibits NADH-driven ATP synthesis

  • MgtC reduces ATP hydrolysis activity of the F₁F₀ complex

• Physiological implications:

  • Wild-type Salmonella maintains lower intracellular ATP levels compared to mgtC mutants

  • MgtC activity helps maintain cytoplasmic pH during exposure to acidic environments

  • The MgtC-ATP synthase interaction contributes to bacterial survival in macrophages

• Structural basis: The N92T mutation in MgtC prevents its interaction with the F₁F₀ ATP synthase and abolishes its inhibitory effects on ATP synthase function, suggesting this residue is critical for the interaction interface .

This interaction exemplifies how pathogens evolved mechanisms to modulate their own core metabolic machinery to optimize survival in hostile host environments.

How do post-translational modifications affect atpF function in the ATP synthase complex?

Post-translational modifications (PTMs) of atpF can significantly impact ATP synthase assembly, stability, and activity. While specific data on Salmonella dublin atpF modifications is limited, research in related bacterial systems has identified several relevant PTMs:

• Phosphorylation: Phosphorylation of peripheral stalk components can alter the structural dynamics of the stator, affecting the coupling efficiency between proton translocation and ATP synthesis.

• Oxidative modifications: Cysteine residues in atpF can undergo oxidation under stress conditions, potentially forming disulfide bonds that impact protein conformation and interaction with other subunits.

• Acetylation: Lysine acetylation has been observed in multiple ATP synthase components and may represent a regulatory mechanism affecting complex assembly or activity.

Research approaches to investigate these PTMs include:

These modifications may be particularly relevant under stress conditions, including those encountered during infection, potentially linking PTMs of atpF to virulence and stress adaptation.

What are common challenges when expressing recombinant atpF and how can they be addressed?

Researchers frequently encounter several challenges when working with recombinant atpF:

• Poor expression levels:

  • Problem: atpF often expresses poorly in standard E. coli systems

  • Solution: Use specialized strains (C41/C43), reduce induction temperature (16-20°C), and optimize codon usage for the expression host. Consider using fusion partners like MBP that enhance solubility.

• Inclusion body formation:

  • Problem: Overexpressed atpF tends to aggregate

  • Solution: Reduce induction levels, optimize growth media (TB rather than LB), and add solubility enhancers like sorbitol (0.5 M) and glycylbetaine (2.5 mM) to growth media.

• Proteolytic degradation:

  • Problem: atpF is susceptible to proteolysis during expression/purification

  • Solution: Use protease-deficient strains (BL21), include multiple protease inhibitors, and maintain samples at 4°C throughout purification.

• Detergent-related issues:

  • Problem: Improper detergent selection can denature or destabilize atpF

  • Solution: Screen multiple detergents (DDM, LMNG, digitonin) at varying concentrations; consider fluorescence-based thermal stability assays to identify optimal detergent conditions.

• Loss of interacting partners:

  • Problem: Isolated atpF may be unstable without its natural binding partners

  • Solution: Consider co-expression with other peripheral stalk components or purify the entire peripheral stalk subcomplex.

The field has addressed similar challenges with other ATP synthase components by optimizing expression conditions, as demonstrated in studies of glutathione S-transferase fusion proteins of Salmonella virulence factors .

How can researchers distinguish between direct and indirect effects when studying atpF mutations?

Distinguishing between direct and indirect effects of atpF mutations requires a systematic approach:

• Complementation experiments:

  • Express wild-type atpF in trans from a plasmid in the mutant background

  • Quantify restoration of ATP synthase function, ATP levels, and growth phenotypes

  • Partial complementation may indicate indirect effects or dominant-negative properties

• Double mutant analysis:

  • Generate double mutants with related pathways

  • For example, combining atpF mutations with mutations in other ATP synthase components can reveal epistatic relationships

  • Studies have shown that combining mgtC and atpB (F₀ a subunit) mutations eliminates the ATP level differences seen in mgtC single mutants, supporting direct interaction

• In vitro reconstitution:

  • Purify components and reconstitute minimal functional systems

  • Directly measure biochemical activities with defined components

  • Compare wild-type and mutant atpF in otherwise identical conditions

• Time-course analyses:

  • Monitor changes in multiple parameters over time following induction of mutations

  • Primary effects typically manifest earlier than secondary consequences

  • Parameters to monitor include protein levels, complex assembly, ATP levels, and membrane potential

This multi-faceted approach helps establish causality and distinguish primary effects from downstream consequences of atpF mutations.

What controls are essential for validating results in atpF functional studies?

Rigorous validation of atpF functional studies requires comprehensive controls:

• Genetic controls:

  • Wild-type strain (positive control)

  • Complete deletion mutant (negative control)

  • Point mutants affecting specific functions

  • Complemented strains expressing wild-type atpF

  • Mutations in other ATP synthase components (e.g., atpB mutants)

• Biochemical controls:

  • ATP synthesis assays: Include controls without substrate (Pi) or with ATP synthase inhibitors

  • Proton translocation: Include ionophores (CCCP) to dissipate proton gradients as negative controls

  • ATP hydrolysis: Include specific inhibitors (oligomycin, DCCD) to confirm specificity

• Experimental validation approaches:

  • Ensure protein expression by Western blot with specific antibodies

  • Verify complex assembly through Blue Native PAGE or co-immunoprecipitation

  • Confirm membrane potential using appropriate fluorescent dyes

Research on the MgtC virulence factor's interaction with ATP synthase demonstrates the importance of these controls, as double mutant analyses and specific biochemical controls were crucial for establishing the direct nature of the interaction .

How might targeting atpF contribute to new antimicrobial strategies against Salmonella?

ATP synthase represents a promising antimicrobial target, with atpF offering several unique advantages:

• Essentiality and conservation: ATP synthase is essential for energy metabolism in most growth conditions, and while atpF is conserved in bacteria, it differs structurally from human homologs, providing potential selectivity.

• Targeting approaches:

  • Small molecules that disrupt the atpF-atpA interaction, destabilizing the peripheral stalk

  • Peptides that mimic interaction interfaces, preventing proper complex assembly

  • Compounds that lock atpF in non-functional conformations

• Synergistic potential:

  • Combined targeting of ATP synthase and virulence factors like MgtC

  • Dual inhibition could prevent compensatory adaptations

  • Research shows MgtC already targets ATP synthase naturally, suggesting evolutionary validation of this approach

• Resistance considerations:

  • Mutations in atpF that confer resistance may compromise fitness

  • The essential nature of ATP synthase limits viable resistance mechanisms

  • Multiple binding sites could be targeted simultaneously to reduce resistance development

Future directions include high-throughput screening for atpF binders, structure-based drug design targeting the peripheral stalk interface, and development of peptide mimetics based on interaction domains of natural ATP synthase regulators like MgtC.

What new methodologies are advancing our understanding of atpF dynamics in living cells?

Several cutting-edge methodologies are transforming our understanding of atpF dynamics:

• Super-resolution microscopy:

  • PALM/STORM imaging reveals nanoscale organization of ATP synthase complexes

  • Single-molecule tracking of fluorescently labeled atpF provides insights into diffusion and clustering behavior

  • Multi-color imaging allows visualization of interactions with other components in real-time

• Cryo-electron tomography:

  • Direct visualization of ATP synthase in native membrane environments

  • Structural analysis of different conformational states in situ

  • Mapping of peripheral stalk position relative to the complete complex

• Genetic code expansion:

  • Site-specific incorporation of photo-crosslinkable amino acids in atpF

  • UV-inducible crosslinking captures transient interaction partners

  • Identification of novel regulatory proteins that interact with atpF

• FRET-based sensors:

  • Engineered atpF variants with fluorescent protein fusions

  • Real-time monitoring of conformational changes during ATP synthesis/hydrolysis

  • Observation of how virulence factors like MgtC alter atpF dynamics

These approaches complement traditional biochemical methods and provide unprecedented insights into how atpF functions within the dynamic context of living bacterial cells, potentially revealing new regulatory mechanisms and interaction partners.

How does the interplay between translation factors and atpF expression influence bacterial fitness?

Research has revealed intriguing connections between translation factors and ATP synthase expression:

• Elongation Factor P (EF-P) effects:

  • The absence of EF-P in Salmonella causes a 20-fold reduction in the abundance of the ATP synthase catalytic subunit AtpD

  • This suggests that efficient translation of ATP synthase components depends on specialized translation factors

  • EF-P is particularly important for efficient translation of proteins containing polyproline motifs

• Translation efficiency consequences:

  • Reduced translation efficiency of ATP synthase components leads to imbalanced subunit stoichiometry

  • This imbalance can trigger quality control mechanisms and degradation of unassembled subunits

  • The FTSH protease has been identified as degrading unassembled ATP synthase components

• Metabolic implications:

  • Deletion of efp in Salmonella results in hyperactive metabolism and reduced stress resistance

  • These phenotypes may be partially explained by altered ATP synthase levels and activity

  • The connection provides insight into how translation regulation feeds into energy metabolism control

• Evolutionary considerations:

  • Conservation of regulatory mechanisms suggests fundamental importance

  • Differential translation efficiency may allow rapid adaptation to environmental changes

  • Specialization of translation factors may represent an evolved regulatory mechanism

This emerging field connects two fundamental cellular processes—translation and energy metabolism—and suggests that their coordination is crucial for bacterial fitness and adaptation to changing environments.