Recombinant Pseudomonas aeruginosa ATP synthase subunit a (atpB)

What is the structural and functional role of ATP synthase subunit a (atpB) in P. aeruginosa?

ATP synthase subunit a (atpB) is a critical component of the F₀ domain of the ATP synthase complex in P. aeruginosa. It is embedded in the bacterial plasma membrane and forms part of the proton channel necessary for ATP synthesis. The subunit consists of 289 amino acids and contains multiple transmembrane segments that facilitate proton translocation across the membrane . Functionally, atpB uses the H⁺ electrochemical gradient present across the cell membrane to drive rotation of the oligomeric c-ring, which subsequently drives rotation within the F₁ motor to catalyze the phosphorylation of ADP to produce ATP . This energy conversion process is essential for bacterial survival, making atpB an attractive target for antimicrobial development.

How does P. aeruginosa ATP synthase differ from ATP synthases in other bacterial species?

P. aeruginosa ATP synthase shares the fundamental structural organization with other bacterial ATP synthases but exhibits distinct features that may contribute to its role in antibiotic resistance. Unlike some bacteria where ATP synthase primarily functions in ATP synthesis, P. aeruginosa ATP synthase can also operate in reverse as an ATP-driven proton pump to maintain membrane potential under certain conditions . Additionally, studies investigating inhibitors of ATP synthase have demonstrated that compounds effective against Mycobacterium tuberculosis ATP synthase (such as bedaquiline) show different efficacy profiles against P. aeruginosa, suggesting structural or functional differences between species . These differences are particularly evident in the binding sites for inhibitors, with P. aeruginosa ATP synthase showing preferences for bulky/hydrophobic C1/C2 substitutions in quinoline-based inhibitors .

What is the significance of ectopic ATP synthase expression in P. aeruginosa interactions with host cells?

Interestingly, ATP synthase or its subunit ATP5B has been found on the surface of several cell types, including hepatocytes, cancer cells, and vascular endothelial cells . Research has demonstrated that P. aeruginosa flagellar hook protein FlgE can interact with these ectopic ATP synthases or subunits to modulate host cell properties . This interaction plays a role in bacterial adhesion to host cells and contributes to pathogenic effects. For example, when FlgE binds to ectopic ATP5B on endothelial cells, it can decrease surface ATP production, increase apoptosis, and enhance permeability of endothelial layers both in vitro and in vivo . This interaction between bacterial components and ectopic ATP synthase represents a novel mechanism in P. aeruginosa pathogenesis that extends beyond its traditional role in energy metabolism.

How do recombinant antimicrobial peptides affect P. aeruginosa ATP synthase function, and what proteomic changes occur as a result?

Recombinant antimicrobial peptides (AMPs) such as defensin-d2 and actifensin induce complex proteomic changes in P. aeruginosa within 1 hour of treatment . These changes affect various cellular processes, including DNA replication, repair, translation, and membrane transport. In P. aeruginosa specifically, the differentially expressed proteins (DEPs) identified after AMP treatment are primarily cytoplasmic proteins related to ion transport and homeostasis, nucleic and amino acid metabolism, and structural biogenesis .

ATP synthase has been highlighted as particularly significant among these affected proteins. AMPs disrupt the function of ATP synthase by interfering with the proton motive force (PMF), which is essential for ATP production. For instance, the synthetic helper peptide D-11 can dissipate the PMF, thereby reducing ATP production and inhibiting the activity of efflux pumps . This disruption contributes to the antimicrobial effect by facilitating the uptake and accumulation of antibiotics inside bacterial cells.

What mechanisms underlie the synergistic effects between ATP synthase inhibitors and conventional antibiotics against multidrug-resistant P. aeruginosa?

The synergistic effects between ATP synthase inhibitors and conventional antibiotics against multidrug-resistant P. aeruginosa operate through multiple complementary mechanisms:

• Increased membrane permeability: Compounds targeting ATP synthase, such as the peptide D-11, can attach to lipopolysaccharide (LPS) and membrane phospholipids, thereby increasing membrane permeability and facilitating antibiotic uptake .

• Disruption of proton motive force: ATP synthase inhibitors dissipate the proton motive force, which:

  • Reduces ATP production necessary for cellular processes

  • Inhibits the activity of energy-dependent efflux pumps that normally expel antibiotics

• Impairment of respiratory chain: Inhibiting ATP synthase can impair the respiratory chain function, creating metabolic stress within the bacterial cell .

• Enhanced reactive oxygen species (ROS) production: ATP synthase inhibition promotes the production of reactive oxygen species, which causes oxidative damage to bacterial cells .

• Increased intracellular antibiotic accumulation: The combined effects of increased membrane permeability and decreased efflux pump activity lead to higher intracellular antibiotic concentrations .

These mechanisms collectively enhance the efficacy of conventional antibiotics against otherwise resistant P. aeruginosa, making ATP synthase an attractive target for combination therapy approaches.

What structural characteristics of quinoline-based inhibitors determine their efficacy against P. aeruginosa ATP synthase?

Structure-activity relationship studies of quinoline-based inhibitors against P. aeruginosa ATP synthase have revealed several key determinants of efficacy:

• C1/C2 substitutions: Bulky and hydrophobic substitutions at the C1 and C2 positions of the quinoline ring are preferred for effective inhibition of P. aeruginosa ATP synthase . This contrasts with some other bacterial species, highlighting the structural uniqueness of P. aeruginosa ATP synthase.

• Hydrophilicity correlation: For some quinoline derivatives (particularly C3-substituted quinolines), there is a correlation between hydrophilicity (as measured by cLog P) and antibacterial activity against gram-negative bacteria .

• Lateral chain modifications: While bedaquiline (BDQ) derivatives with novel lateral chains have shown effectiveness against some bacteria like Streptococcus pneumoniae, these modifications need to be specifically tailored for activity against P. aeruginosa .

• Binding site interactions: The most effective inhibitors demonstrate specific interactions with the binding pocket of P. aeruginosa ATP synthase, which appears to accommodate larger hydrophobic groups differently than ATP synthases from other bacterial species .

These structure-activity insights are crucial for rational drug design approaches targeting P. aeruginosa ATP synthase specifically.

What are the optimal conditions for expressing and purifying recombinant P. aeruginosa atpB?

Based on the available research data, the optimal conditions for expressing and purifying recombinant P. aeruginosa atpB include:

Expression System:

• While not explicitly stated in the search results for atpB specifically, recombinant bacterial membrane proteins are typically expressed in E. coli systems with controlled induction protocols.

• For membrane proteins like atpB, E. coli strains optimized for membrane protein expression such as C41(DE3) or C43(DE3) are recommended.

Purification Approach:

• Initial isolation via His-tag affinity chromatography using Ni-NTA beads, similar to the approach used for FlgE protein purification .

• Buffer conditions typically include Tris-based buffer with 50% glycerol for stability .

• For membrane proteins like atpB, detergent solubilization (using detergents like n-dodecyl-β-D-maltoside or CHAPS) is crucial during purification.

Storage Conditions:

• Store at -20°C for short-term storage

• For extended storage, conserve at -20°C or -80°C

• Avoid repeated freezing and thawing

• Working aliquots can be stored at 4°C for up to one week

It's important to note that the full protein sequence (amino acids 1-289) has been used for recombinant expression , with the complete amino acid sequence available for reference in designing expression constructs.

What assays can be used to evaluate the inhibition of P. aeruginosa ATP synthase activity in vitro and in bacterial cultures?

Several complementary assays can be employed to evaluate the inhibition of P. aeruginosa ATP synthase:

In Vitro Biochemical Assays:

• ATP synthesis/hydrolysis assays: Measuring the rate of ATP synthesis or hydrolysis in isolated membrane vesicles or purified enzyme using luciferase-based ATP detection systems.

• Surface ATP production assay: This technique measures ATP production catalyzed by ectopic ATP synthase. Cells are treated with the potential inhibitor, then ADP is added (typically 100 μM) with 2 mM Mg²⁺ and 10 mM Pi. Medium is harvested after a short incubation (15 seconds), and ATP content is measured using an ATP assay kit, with results expressed as relative luminescence units (RLU) .

Cellular and Functional Assays:

• Membrane potential measurements: Using fluorescent dyes like DiSC3(5) to assess the effect of inhibitors on membrane potential, which is directly related to ATP synthase function.

• Bacterial growth inhibition assays: Determining minimum inhibitory concentrations (MICs) against wild-type P. aeruginosa to assess whole-cell activity of ATP synthase inhibitors .

• Synergistic activity assays: Testing combinations of ATP synthase inhibitors with conventional antibiotics to assess potential synergistic effects, using methods like checkerboard assays .

• Reactive oxygen species (ROS) detection: Measuring ROS production following ATP synthase inhibition using fluorescent probes like DCFH-DA .

• Efflux pump activity assays: Assessing the impact of ATP synthase inhibition on efflux pump function by measuring the accumulation of fluorescent substrates like ethidium bromide .

How can protein-protein interactions between atpB and potential binding partners be effectively studied?

Based on the research methodologies described in the search results, the following techniques have proven effective for studying protein-protein interactions involving ATP synthase components:

Pull-Down Assays:
Pull-down analysis has been successfully used to demonstrate affinity between recombinant proteins and ATP synthase components. For example, membrane proteins from vascular endothelial cells were incubated with His-tagged protein-conjugated Ni-NTA beads, and recovered proteins were subjected to immunoblotting with anti-ATP5B antibodies . This approach could be adapted to study interactions between atpB and potential binding partners.

Immunofluorescence Colocalization:
Colocalization studies using fluorescence microscopy can visualize interactions between atpB and binding partners. In previous studies, potential binding partners were incubated with cells expressing ATP synthase components, fixed with 4% paraformaldehyde, and blocked with 3% BSA. Proteins were then stained with appropriate primary and fluorescently-labeled secondary antibodies to visualize colocalization .

Flow Cytometry:
Flow cytometry has been used to confirm binding interactions with ATP synthase components on cell surfaces. This technique allows quantitative assessment of binding under various conditions and can be used to study competition between different potential binding partners .

ELISA-Based Binding Assays:
Direct binding can be confirmed using ELISA techniques, where one protein is immobilized and the binding of potential partners is detected using specific antibodies. This approach allows for quantitative assessment of binding affinity and can be used to screen multiple potential interactions .

Functional Interference Assays:
The biological relevance of interactions can be assessed by using antibodies or competing peptides to block specific binding sites and measuring the effect on functional outcomes. For example, anti-ATP5B antibodies have been used to determine if observed effects are specifically due to interactions with ATP synthase .

How does targeting P. aeruginosa ATP synthase compare to other novel antibiotic approaches for multidrug-resistant infections?

Targeting P. aeruginosa ATP synthase offers several distinctive advantages compared to other novel antibiotic approaches:

• Essential Target with Limited Resistance Potential: ATP synthase is essential for bacterial energy metabolism and survival. Unlike targets involved in cell wall synthesis or protein translation where single mutations can confer resistance, ATP synthase's critical role in energy production means mutations that significantly reduce inhibitor binding often also reduce its essential function . This creates a higher barrier to resistance development.

• Multi-level Impact: ATP synthase inhibitors affect bacterial cells at multiple levels simultaneously:

  • Direct inhibition of ATP production

  • Disruption of membrane potential

  • Impairment of efflux pump function

  • Enhancement of oxidative stress via ROS production
    This multi-target effect contrasts with single-target approaches that bacteria can more easily circumvent.

• Synergistic Potential: Unlike many novel antibiotics that function independently, ATP synthase inhibitors show remarkable synergistic effects with existing antibiotics. For example, peptides targeting ATP synthase can restore sensitivity to antibiotics to which P. aeruginosa is normally resistant . This synergistic approach allows for repurposing existing antibiotics rather than requiring completely novel compounds.

• Activity Against Biofilm and Persistent Forms: ATP synthase inhibitors have demonstrated efficacy against P. aeruginosa in complex environments including biofilms, which are notoriously resistant to conventional antibiotics . This activity against persistent forms addresses a significant limitation of many existing and novel antibiotics.

What evidence supports the use of atpB as a therapeutic target in multidrug-resistant P. aeruginosa infections?

Multiple lines of evidence support atpB as a promising therapeutic target:

• Essential Function: ATP synthase plays a vital role in bacterial energy metabolism that is essential for survival. Studies have confirmed that ATP synthase inhibition leads to bacterial cell death in P. aeruginosa .

• Synergistic Enhancement of Antibiotic Efficacy: Compounds targeting ATP synthase have demonstrated remarkable ability to restore antibiotic sensitivity in resistant P. aeruginosa. For example, the peptide D-11 can make P. aeruginosa susceptible to a broad range of antibiotics to which it is normally resistant .

• Efficacy in Complex Environments: ATP synthase inhibitors have shown effectiveness in clinically relevant complex environments, including:

  • Pseudomonas biofilms

  • Human blood

  • In vivo infection models (mouse models)

• Novel Mechanism Distinct from Existing Antibiotics: ATP synthase represents a mechanism of action distinct from conventional antibiotics, making cross-resistance less likely. This is particularly valuable given that P. aeruginosa has developed resistance mechanisms for most commonly used antibiotics .

• Precedent from Other Bacterial Infections: The success of the ATP synthase inhibitor bedaquiline (BDQ) against Mycobacterium tuberculosis provides precedent for this approach . While the specific inhibitors effective against M. tuberculosis differ from those effective against P. aeruginosa, the general principle of ATP synthase inhibition as an antibiotic strategy has been clinically validated.

• Identification of Effective Chemical Scaffolds: Research has already identified chemical scaffolds (particularly quinoline derivatives with specific C1/C2 substitutions) capable of inhibiting P. aeruginosa ATP synthase in vitro , providing promising starting points for drug development.

What challenges exist in developing selective inhibitors of P. aeruginosa ATP synthase that don't affect human mitochondrial ATP synthase?

Developing selective inhibitors presents several significant challenges:

• Structural Conservation: ATP synthase is evolutionarily conserved across species, with significant structural similarities between bacterial and human mitochondrial versions. This conservation makes selective targeting challenging, as compounds that bind to bacterial ATP synthase may also interact with human ATP synthase.

• Differential Binding Site Characteristics: While structural studies have identified differences in the binding pockets between bacterial and human ATP synthases, these differences are subtle. Successful selective inhibitors must exploit these subtle differences without compromising antimicrobial potency .

• Membrane Permeability Barriers: P. aeruginosa possesses an intrinsically less permeable outer membrane compared to other bacteria, restricting the entry of many compounds. Additionally, compounds must be able to access bacterial ATP synthase without accumulating in human mitochondria .

• Efflux Mechanisms: P. aeruginosa has robust efflux mechanisms that can expel potential inhibitors before they reach effective concentrations. While ATP synthase inhibition itself can compromise efflux function, the initial penetration and retention of inhibitors remains challenging .

• Balancing Physicochemical Properties: Developing compounds with the appropriate balance of hydrophilicity/hydrophobicity is crucial. Studies have shown that bulky/hydrophobic C1/C2 substitutions on quinoline scaffolds are preferred for P. aeruginosa ATP synthase inhibition , but these properties must be carefully balanced to avoid non-specific membrane disruption and human toxicity.

• In vivo Efficacy Translation: Compounds showing promising in vitro activity against isolated ATP synthase often fail to demonstrate comparable efficacy in whole-cell assays or in vivo models due to physiological barriers, protein binding, or metabolic inactivation.

Research approaches to address these challenges include rational drug design based on structural differences between bacterial and human ATP synthases, development of prodrugs that are selectively activated in bacterial environments, and combination approaches that enhance the selective delivery of ATP synthase inhibitors to bacterial cells.