Recombinant Campylobacter curvus ATP synthase subunit c (atpE)

Basic Research Questions

• What is Recombinant Campylobacter curvus ATP synthase subunit c (atpE)?

Recombinant Campylobacter curvus ATP synthase subunit c (atpE) is a protein component of the c-ring structure within the F0 portion of ATP synthase, expressed through recombinant technology. The c-ring forms a critical part of the rotor assembly in bacterial ATP synthase and contains the proton-binding sites essential for energy conversion. Similar to other Campylobacter species, C. curvus atpE is part of the ATP synthase complex that couples the proton-motive force across the bacterial membrane to ATP synthesis. As a recombinant protein, it is typically produced in heterologous expression systems for research applications in structural biology, enzyme characterization, and antimicrobial development .

• How is ATP synthase subunit c involved in energy production in Campylobacter species?

ATP synthase subunit c functions as a critical component in the energy production pathway of Campylobacter species. In ATP synthase, the c-ring contains H+-binding sites that facilitate proton translocation across the bacterial membrane. This proton movement drives the rotation of the c-ring, which is mechanically coupled to the central stalk of the F1 catalytic domain, ultimately leading to ATP synthesis.

In Campylobacter species, this energy production mechanism is essential for survival and pathogenicity. Studies examining ATP contents of different Campylobacter morphologies have demonstrated that ATP levels correlate with viability and metabolic activity. For instance, filamentous forms of Campylobacter jejuni have been shown to contain significantly higher ATP levels (up to 17.4 fg ATP/CFU in strain 12661) compared to spiral forms (approximately 1-2 fg ATP/CFU), while coccoid forms show extremely low or undetectable ATP levels, suggesting they are metabolically inactive or non-viable .

• What expression systems are commonly used to produce recombinant Campylobacter ATP synthase proteins?

Multiple expression systems can be employed for the recombinant production of Campylobacter ATP synthase proteins, with selection dependent on research objectives:

For Campylobacter proteins specifically, E. coli expression systems have been successfully employed for ATP synthase components, as evidenced by the development of whole-operon expression plasmids for Pseudomonas aeruginosa ATP synthase in E. coli, which facilitates structure-function studies that could be adapted for Campylobacter ATP synthase research . The choice of expression system should be guided by the specific requirements of the downstream applications, such as structural analysis, functional studies, or inhibitor screening .

• What structural features distinguish the ATP synthase c-ring in Campylobacter from other bacterial species?

The ATP synthase c-ring in Campylobacter possesses distinct structural features compared to other bacterial species, particularly in the H+-binding region. While specific structural data for C. curvus is limited, insights can be drawn from related bacterial ATP synthases:

The c-subunit's H+-binding site in Campylobacter likely contains specific amino acid residues that determine proton affinity and inhibitor binding properties. Comparative analysis with other gram-negative bacteria suggests that Campylobacter ATP synthase c-rings may have unique features in their proton-binding sites. For example, in Pseudomonas aeruginosa, the c-subunit's binding site is less sterically hindered and more hydrophobic compared to Mycobacterium tuberculosis, with key substitutions including Asp→Phe27, Tyr→Thr63, Phe→Met64, and Leu→Val67 .

These structural differences significantly impact inhibitor binding efficacy and specificity, which has direct implications for antimicrobial development targeting Campylobacter ATP synthase.

Advanced Research Questions

• What methodologies are most effective for assessing inhibition of Campylobacter ATP synthase activity?

The evaluation of inhibitors targeting Campylobacter ATP synthase requires robust methodological approaches. Based on established protocols for related bacterial ATP synthases, the following methods are recommended:

In vitro ATP synthesis assay using inverted membrane vesicles:

• Prepare inverted membrane vesicles from Campylobacter cultures by differential centrifugation and disruption techniques.

• Measure NADH-driven ATP synthesis using a luciferin/luciferase-based luminescence assay.

• Determine IC50 values by testing inhibitor compounds at increasing concentrations (typically 0.1-100 μg/mL).

• Apply a dose-response model to calculate accurate IC50 values.

This approach has been successfully employed with ATP synthase inhibitors in Pseudomonas aeruginosa, where compounds targeting the c-ring showed IC50 values ranging from 0.7 to 11.1 μg/mL .

Mutational analysis to confirm binding sites:

• Introduce site-directed mutations in the atpE gene using synthetic gene fragments.

• Express the mutant ATP synthase in a suitable host system.

• Prepare membrane vesicles from mutant strains and wild-type controls.

• Compare inhibition profiles between mutant and wild-type ATP synthases.

This approach can provide strong evidence for the binding site of inhibitors, as demonstrated by the altered inhibition profiles of quinoline compounds against P. aeruginosa ATP synthase with mutations at Ile65 in the c-subunit .

• How can site-directed mutagenesis be leveraged to study the H+ binding site in ATP synthase subunit c?

Site-directed mutagenesis provides a powerful approach to investigate the H+ binding site in ATP synthase subunit c, offering insights into both function and inhibitor interactions:

Methodological approach:

• Identify conserved and variant residues in the c-subunit H+ binding region through sequence alignment of Campylobacter ATP synthase with well-characterized bacterial homologs.

• Design mutations targeting:

  • The essential acidic residue involved in H+ binding

  • Neighboring residues that may affect inhibitor binding

  • Residues that differ between Campylobacter and other bacterial species

• Introduce mutations to the atpE gene in an expression plasmid using PCR-based site-directed mutagenesis or synthetic gene fragments.

• Express wild-type and mutant proteins in a suitable host system.

• Evaluate functional consequences using:

  • ATP synthesis assays with inverted membrane vesicles

  • Inhibitor binding studies with various ATP synthase inhibitors

Key considerations based on previous research:

• Mutations near the H+ binding site can significantly alter inhibitor efficacy without abolishing ATP synthesis function, as demonstrated with Ile65Ala and Ile65Phe mutations in P. aeruginosa ATP synthase.

• The Ile65Phe mutation increased the potency of compound 4 (a quinoline derivative) by more than 6-fold, likely by enhancing π-stacking interactions.

• Conversely, the Ile65Ala mutation decreased the potency of compound 5, potentially by reducing van der Waals interactions .

This mutational approach can reveal the specific binding mode of inhibitors and provide insights into the structural determinants of c-ring function in Campylobacter ATP synthase.

• What are the optimal methods for measuring ATP content in different morphological forms of Campylobacter?

Accurate measurement of ATP content in different morphological forms of Campylobacter requires specialized techniques to separate and analyze distinct cell populations:

Recommended methodology:

• Culture preparation and morphotype separation:

  • Culture Campylobacter in appropriate media (e.g., MEM with sodium pyruvate) under microaerobic conditions at 37°C.

  • Monitor growth phases through viable counting and microscopic examination.

  • Harvest cells at specific time points corresponding to different morphological states:

    • Exponential phase (12-14h): predominantly spiral forms

    • Decline phase (96h): mixture of spiral and filamentous forms

    • Late decline phase (>168h): predominantly coccoid forms

  • Separate morphotypes through differential centrifugation or filtration techniques.

• ATP quantification:

  • Lyse cells using an appropriate buffer system.

  • Determine ATP content using a luciferase/luciferin luminescence assay.

  • Calculate ATP content per CFU by correlating luminescence readings with viable counts.

  • For mixed populations, correct for the proportion of each morphotype.

Expected findings based on C. jejuni research:

• Spiral cells in exponential phase typically contain 0.99-1.7 fg ATP per CFU.

• Filamentous forms may contain significantly higher ATP levels (2.66-17.4 fg ATP per CFU), with longer filaments correlating with higher ATP content.

• Coccoid forms generally show extremely low or undetectable ATP levels, suggesting metabolic inactivity .

This methodology enables precise characterization of the metabolic state of different Campylobacter morphologies, providing insights into survival strategies and potential antimicrobial targets.

• What approaches can be used to investigate the stoichiometry and assembly of the c-ring in Campylobacter ATP synthase?

Investigating the c-ring stoichiometry and assembly in Campylobacter ATP synthase requires a multi-faceted approach combining structural, biochemical, and genetic techniques:

Structural determination methods:

• Cryo-electron microscopy (cryo-EM):

  • Purify intact ATP synthase or isolated c-rings using detergent solubilization and chromatography.

  • Prepare samples for cryo-EM using appropriate grid preparation protocols.

  • Collect high-resolution images and perform single-particle analysis.

  • Determine the number of c-subunits by analyzing symmetry and structural features.

• Cross-linking mass spectrometry:

  • Apply chemical cross-linkers to stabilize c-ring structure.

  • Digest cross-linked complexes and analyze by mass spectrometry.

  • Identify cross-linked peptides to map spatial relationships between subunits.

  • Determine stoichiometry from the pattern of cross-links.

Functional approaches:

• Site-specific labeling and quantification:

  • Introduce unique labeling sites in the c-subunit.

  • Quantify the ratio of labeled sites to ATP synthase complexes.

  • Calculate stoichiometry based on labeling efficiency.

• Genetic manipulation:

  • Create fusion constructs with defined numbers of covalently linked c-subunits.

  • Express these constructs and assess functionality.

  • Determine the minimum number of subunits required for function.

Based on related bacterial systems, Campylobacter ATP synthase c-rings likely contain 10-15 c-subunits, with the exact number potentially influencing bioenergetic properties and inhibitor binding. As observed in Acinetobacter baumannii, c-rings typically have a defined stoichiometry with multiple potential binding sites for inhibitors around the ring .

• How do inhibitors targeting ATP synthase subunit c exert their antimicrobial effects against Campylobacter?

Inhibitors targeting ATP synthase subunit c exert antimicrobial effects against Campylobacter through multiple mechanisms that disrupt energy metabolism:

Primary mechanism of action:

• Disruption of proton translocation:

  • Inhibitors bind to the H+ binding sites on the c-ring, blocking the proton channel.

  • This prevents proton flow through the F0 domain, disrupting the proton-motive force.

  • Without proton translocation, c-ring rotation is inhibited, halting ATP synthesis.

• Impact on cellular bioenergetics:

  • ATP depletion leads to energy starvation in bacterial cells.

  • Disruption of the proton gradient affects other membrane-dependent processes.

  • Metabolic collapse ultimately results in cell death.

Structure-activity relationships of inhibitors:
Based on studies with quinoline derivatives targeting ATP synthase in P. aeruginosa, effective inhibitors typically share these structural features:

• A hydrophobic core structure (e.g., quinoline) that interacts with the c-ring.

• Specific substituents that enhance binding to the H+ binding site region.

• Structural elements that promote interaction with specific residues in the c-subunit.

For example, compounds with a C1 methyl sulfide and a bulky, hydrophobic group at C2 show enhanced inhibitory effects, with IC50 values as low as 0.7 μg/mL for compound 5 against P. aeruginosa ATP synthase .

Challenges in Campylobacter inhibition:

• Bacterial efflux systems may reduce inhibitor effectiveness by pumping compounds out of the cell.

• Differences in c-ring structure between bacterial species affect inhibitor binding affinity.

• The outer membrane of gram-negative bacteria like Campylobacter presents a permeability barrier for some inhibitors.

These insights provide a foundation for developing targeted antimicrobials against Campylobacter ATP synthase, potentially addressing the growing concern of antimicrobial resistance in this pathogen.

• What correlation exists between ATP synthase activity and morphological transitions in Campylobacter species?

The relationship between ATP synthase activity and morphological transitions in Campylobacter species reveals important insights into bacterial adaptation strategies:

ATP content across morphological states:
Research with C. jejuni has demonstrated significant variations in ATP content across different cell morphologies:

Physiological significance:

• Filamentation as a survival strategy:

  • Filamentous forms appear during nutrient limitation and stress conditions.

  • The higher ATP content in filaments (up to 17.4 fg ATP/CFU in C. jejuni 12661) suggests they maintain metabolic activity during adverse conditions.

  • Filament length correlates with ATP content, with longer filaments showing higher energy reserves.

• Transition to coccoid form:

  • The transformation to coccoid morphology occurs in late decline phase (>168h).

  • Coccoid forms show extremely low or undetectable ATP levels, indicating they are metabolically inactive or non-viable.

  • This transformation may represent a degenerative process rather than an active adaptation strategy.

• Metabolic activity and viability:

  • The ATP content serves as a reliable indicator of metabolic activity and potential viability.

  • Cultures transitioning to predominantly coccoid forms (58-61% in C. jejuni PT14 and 12661 at 216h) maintain a small subpopulation of viable cells, likely representing remaining spiral and filamentous forms .