Recombinant Mycobacterium sp. ATP synthase subunit b (atpF)

What is the structure and function of ATP synthase subunit b (atpF) in Mycobacterium species?

Mycobacterial ATP synthase subunit b is part of the peripheral stalk (b:b':δ) that connects the F1 catalytic domain to the FO membrane domain. The complete mycobacterial F-ATP synthase contains the F1 subunits α3:β3:γ:ε, the H+-translocating FO domain subunits a:c9, and the peripheral stalk subunits b:b':δ . The peripheral stalk holds both domains together and plays a crucial role in preventing rotation of the α3:β3 hexamer during ATP synthesis. Unlike in some other bacteria, the mycobacterial F-ATP synthase exhibits unique characteristics including latent ATPase activity, meaning it cannot establish a significant H+-gradient during ATP hydrolysis . This feature is regulated by the mycobacterial-specific extended C-terminal domain of subunit α.

What expression systems are commonly used for recombinant Mycobacterium ATP synthase subunit b?

Escherichia coli is the predominant expression system for recombinant mycobacterial proteins, including ATP synthase components. For high-throughput recombinant protein expression, several approaches are employed:

Recombination-based cloning systems:

• Gateway cloning (Thermo Fisher Scientific)

• Echo Cloning (Thermo Fisher Scientific)

• Creator (Clontech)

• Cold Fusion (System Biosciences)

• CloneEZ (GenScript)

The Gateway system is particularly popular for high-throughput approaches, exploiting the site-specific recombination system of bacteriophage λ to shuttle sequences between plasmids with flanking-compatible recombination attachment (att) sites. The main advantage of this method is that once an entry clone has been created, the gene of interest can be easily subcloned into various destination vectors using the LR reaction .

How do researchers distinguish between different mycobacterial species when studying ATP synthase?

Researchers distinguish between mycobacterial species by focusing on species-specific structural elements that have been identified through comparative genomics and structural biology. The cryo-EM structures of Mycobacterium smegmatis F1-ATPase and F1FO-ATP synthase have revealed critical species-specific elements including:

• The extended C-terminal domain (αCTD) of subunit α

• The unique mycobacterial γ-loop

• Specific structures in subunit δ

These elements are not only taxonomically significant but also functionally important, as they are critical for ATP formation and the self-inhibition mechanism of ATP hydrolysis that is characteristic of TB and NTM bacteria . Experimental designs therefore often include comparative analyses of these structural elements across different mycobacterial species and against non-mycobacterial reference organisms.

What are the optimal conditions for expressing and purifying recombinant Mycobacterium ATP synthase subunit b?

The purification of recombinant mycobacterial ATP synthase components requires careful optimization of several parameters. Affinity-based methods are commonly employed, with the following considerations for optimal results:

Researchers should note that while IMAC (Immobilized Metal Affinity Chromatography) is effective for initial purification of HIS-tagged recombinant proteins, there are limitations: "Naturally occurring metal-binding proteins and the presence of histidine and cysteine-rich spots in superfluous proteins compete with tagged protein to bind to the column and interfere with IMAC often resulting in contamination of the final product" . Additionally, "the possibility of heavy metal leaching from the column during purification can be of concern" .

How can researchers reconstitute and measure ATP synthesis activity of recombinant mycobacterial F-ATP synthase?

Reconstitution and activity measurement of recombinant mycobacterial F-ATP synthase follows a specific protocol:

• Purification: Purify the recombinant enzyme following established protocols .

• Reconstitution into proteoliposomes:

  • Generate small unilamellar vesicles from Phosphatidylcholine type II S soybeans

  • Incorporate the purified enzyme into these vesicles

  • Collect proteoliposomes by centrifugation (150,000× g, 30 min)

  • Resuspend in ATP synthesis buffer (100 mM Tris, 100 mM maleic acid, 5 mM MgCl2, 150 mM NaCl, 200 mM KCl, 5 mM KH2PO4, pH 7.5)

• ATP synthesis measurement:

  • Use a continuous luciferase assay to monitor emitted light in a luminometer

  • Mix 375 µL proteoliposomes with 20 µL ATP Bioluminescence Assay Kit

  • Record baseline for 3 min at 37°C

  • Initiate ATP synthesis by adding 2 µM valinomycin (to induce ΔΨ) and 5 mM ADP

• Inhibitor studies:

  • Preincubate proteoliposomes with the inhibitor (e.g., 50-100 μM) for 10 min at 4°C

  • Perform ATP synthesis measurements as described above

This methodology allows for accurate quantification of ATP synthesis activity and evaluation of potential inhibitors.

What experimental approaches resolve the apparent contradiction between recombination data and evidence of erasure/re-establishment in ATP-dependent processes?

The apparent contradictions between recombination data and evidence for erasure/re-establishment in ATP-dependent processes require sophisticated experimental designs to resolve. Researchers employ several complementary approaches:

• High-resolution mapping: Employing both LD-based statistical methods and direct sperm-typing studies to validate hot spot locations. Studies have shown that "hot spots in the HLA region identified by sperm typing were also detected by population studies" .

• Comparative analysis of maps: Analyzing discrepancies between "recombination maps of human chromosomes in LD units per Mb pair (which reflects historical recombination) and centimorgans (cM) per Mb pair (which reflects one-generation recombination)" .

• Genome-wide association studies: Large-scale studies of recombination events have found "that only 60% of detected recombination events coincided with hot spots inferred from LD analysis" . This suggests multiple mechanisms may be at play.

• Statistical modeling: Using "coalescent-based statistical methods" to "infer probabilities that haplotype boundaries represent historical hot spots" .

Researchers should recognize the limitations of individual approaches. For example, "statistical analyses based on coalescent approaches are powerful in providing broad-scale, high-resolution recombination maps" but "they describe sex-averaged, historical recombination among genetically heterogeneous populations" , which may not fully capture the complexity of recombination processes.

How are within-subject experimental designs applied to study ATP synthase function across different mycobacterial strains?

Within-subject experimental designs are valuable for studying ATP synthase function across different mycobacterial strains as they reduce variability and increase statistical power. Unlike between-subjects designs, within-subject designs allow for the comparison of multiple conditions using the same experimental units.

Key considerations for implementing within-subject designs for ATP synthase studies:

• Control for carryover effects: When testing multiple strains sequentially, ensure complete washing or regeneration of testing apparatus between measurements .

• Counterbalancing: Systematically vary the order of testing different strains to distribute potential order effects .

• Statistical analysis: Use repeated measures ANOVA or mixed-effects models that account for within-subject correlation structure .

What structural elements contribute to the latent ATPase activity in mycobacterial ATP synthase, and how can researchers experimentally verify their function?

Mycobacterial ATP synthase exhibits unique latent ATPase activity that distinguishes it from other bacterial ATP synthases. Recent research has identified specific structural elements responsible for this characteristic:

• The extended C-terminal domain (αCTD) of subunit α: Cryo-EM structures have revealed that "α533-545 was trapped inside the γ subunit, forming a lock to stall the rotation of rotary elements in the M. smegmatis F-ATP synthase" .

• The mycobacterial γ-loop: This unique structure is a critical element required for ATP formation .

• Subunit δ: This component has been identified as essential for ATP synthesis .

To experimentally verify the function of these elements, researchers employ several methodologies:

These approaches have confirmed that "the αCTD of subunit α is the main element for the self-inhibition mechanism of ATP hydrolysis for TB and NTM bacteria" , providing potential targets for species-specific inhibitors.

How can researchers design inhibitors specifically targeting mycobacterial-specific elements of the ATP synthase complex?

The design of mycobacterial-specific ATP synthase inhibitors requires targeting unique structural features not present in human ATP synthase. Recent research has established a platform for discovering such inhibitors:

• Receptor-peptide-based pharmacophore development:

  • Focus on "the unique interactions of mycobacterial α's C-terminus and γ"

  • Map at least six features in the peptide-based pharmacophore

• Database screening process:

  • Create a focused library based on pharmacophore matching

  • Perform ADMET (absorption, distribution, metabolism, excretion, and toxicity) property calculations

  • Conduct molecular docking with both standard precision (SP) and extra precision (XP) scoring methods

• Selection criteria for experimental validation:

  • Base selection on XP docking scores

  • Prioritize compounds with molecular interaction patterns matching α533-545 to subunit γ

This approach has led to the discovery of novel inhibitors: "Our ATP synthesis assays on M. smegmatis IMVs and recombinant F-ATP synthase reconstituted into proteo-liposomes led to the discovery of the novel mycobacterial F-ATP synthase inhibitor, AlMF1, which potently inhibited ATP synthesis with a 72% inhibition at 50 µM in recombinant MsF-ATP synthase mediated ATP synthesis" .

Researchers must validate candidate inhibitors against both wild-type and mutant forms of the enzyme, and assess specificity against human ATP synthase to avoid potential toxicity issues.

How do researchers assess the correlation between ATP synthase inhibition and mycobacterial viability for drug development?

Establishing the relationship between ATP synthase inhibition and mycobacterial viability is crucial for drug development. Researchers employ a multi-layered approach:

• Validation of ATP synthase as an essential enzyme:

  • Evidence has confirmed that "F1FO-ATP synthase is required for the viability of tuberculosis (TB) and nontuberculous mycobacteria (NTM) and has been validated as a drug target"

  • This establishes the theoretical basis for targeting this enzyme

• In vitro enzyme inhibition studies:

  • Measure ATP synthesis inhibition in reconstituted systems

  • Quantify dose-response relationships for candidate inhibitors

• Cellular viability assays:

  • Correlate enzyme inhibition with mycobacterial growth inhibition

  • Determine minimum inhibitory concentrations (MICs)

• Resistance mechanism studies:

  • Investigate potential resistance pathways

  • Consider that "Besides mutations in drug targets, resistance is caused by low permeability of the cell wall, biofilm formation, deficient drug-activating enzymes, target modifications, drug metabolism, or induction of drug efflux pumps"

  • Recognize that such efflux pumps are "either ATP- or proton-motive force (PMF) driven energy forms, generated by the electron transport chain (ETC) and the ATP forming F1FO-ATP synthase"

These methodologies provide comprehensive data on the relationship between target engagement and antimicrobial activity, essential for advancing candidates through the drug development pipeline.