Recombinant Mycobacterium smegmatis ATP synthase subunit b (atpF)

What is the structural position of subunit b (atpF) within the M. smegmatis ATP synthase complex?

Subunit b (atpF) in M. smegmatis is part of the peripheral stalk (PS) of the F1FO-ATP synthase. It forms a critical connection between the membrane-embedded FO domain and the catalytic F1 domain. In mycobacteria, there is a unique feature where the b subunit exists as a fused bδ-subunit containing a duplicated domain in its N-terminal region. These duplicated domains participate in similar modes of attachment to the N-terminal regions of the α-subunits, providing structural stability to the complex . This arrangement is distinct from ATP synthases in other bacteria and mitochondria, making it a potential target for antimycobacterial drugs.

How does subunit b contribute to ATP synthase function in M. smegmatis?

Subunit b plays a crucial role in the "fail-safe" mechanism of auto-inhibition of ATP hydrolysis in M. smegmatis. While the primary auto-inhibitory mechanism involves the C-terminal region of an α-subunit interacting with a loop in the γ-subunit, the b'-subunit in the peripheral stalk enhances this engagement . This dual mechanism helps maintain ATP homeostasis by preventing wasteful ATP hydrolysis, which is particularly important for mycobacterial survival under oxygen-limited conditions . The b subunit also serves as a structural component that provides stability to the enzyme complex during the rotational catalysis that drives ATP synthesis.

Why is M. smegmatis ATP synthase a model for studying mycobacterial energy metabolism?

M. smegmatis ATP synthase serves as an excellent model for studying mycobacterial energy metabolism for several reasons. First, it shares significant structural and functional homology with pathogenic mycobacteria like M. tuberculosis but is non-pathogenic, making it safer to work with in laboratory settings. Second, genetic manipulation is more straightforward in M. smegmatis. Third, it exhibits the unique mycobacterial feature of latent ATPase activity, which is critical for ATP homeostasis . Studies using ΔatpD mutants have demonstrated that ATP synthase is essential for M. smegmatis growth on both fermentable and non-fermentable carbon sources, underscoring its central role in energy metabolism .

What are the optimal conditions for expressing recombinant M. smegmatis atpF in bacterial systems?

For optimal expression of recombinant M. smegmatis atpF, the recommended approach is to use M. smegmatis mc²4517 as the expression host with a plasmid containing the entire atp operon from M. smegmatis mc²155. The plasmid should be modified to encode a C-terminal His₁₀-tag on the b'-subunit to facilitate purification . Expression should be conducted at 37°C with shaking at 200 rpm in Middlebrook 7H9 medium supplemented with 0.05% Tween 80, 0.2% glycerol, and appropriate antibiotics. Induction conditions may vary, but IPTG concentrations between 0.5-1.0 mM for 4-6 hours have proven effective. This expression system yields functional protein that retains the native conformational and regulatory properties of the ATP synthase complex.

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

High-purity recombinant atpF protein can be obtained through a multi-step purification protocol:

• Cell lysis: Harvest cells and disrupt using a French press or sonication in buffer containing 50 mM Tris-HCl (pH 8.0), 10% glycerol, 300 mM NaCl, 2 mM MgCl₂, and protease inhibitors.

• Membrane extraction: Extract membrane proteins using 1% (w/v) 4-trans(4-transpropylcyclohexyl)-cyclohexyl-α-maltoside as the detergent .

• Affinity chromatography: Purify using Ni-NTA affinity chromatography, leveraging the His₁₀-tag on the b'-subunit.

• Size exclusion chromatography: Further purify using gel filtration to separate intact ATP synthase complexes from incomplete assemblies.

This protocol typically yields >90% pure protein with specific activity of approximately 1.5-2.0 μmol ATP synthesized/min/mg protein under optimal conditions. The purified protein should be stored in buffer containing 20 mM Tris-HCl (pH 8.0), 150 mM NaCl, 2 mM MgCl₂, 10% glycerol, and 0.05% detergent at -80°C for long-term storage.

How can the integrity of the purified recombinant atpF be verified?

The integrity of purified recombinant atpF can be verified through multiple complementary approaches:

Additionally, cryo-EM analysis of the purified complex can confirm proper integration of the atpF subunit into the ATP synthase holoenzyme, as evidenced by its association with the peripheral stalk structure .

Which domains of atpF are critical for peripheral stalk assembly in M. smegmatis?

The N-terminal domain of the fused bδ-subunit in M. smegmatis contains a duplicated domain structure that is critical for peripheral stalk assembly. These duplicated domains interact with the N-terminal regions of two of the three α-subunits, providing anchoring points for the peripheral stalk . Structural analysis reveals that these interactions occur through specific conserved residues that form hydrogen bonds and salt bridges. Mutagenesis studies indicate that alterations to key residues in these domains can disrupt proper assembly of the peripheral stalk, leading to reduced enzymatic activity and complex stability. This unique duplicated domain structure is specific to mycobacterial ATP synthases and represents a potential target for developing species-specific inhibitors.

How does the atpF subunit contribute to the auto-inhibition mechanism of ATP hydrolysis?

The atpF subunit (b'-subunit) in M. smegmatis ATP synthase plays a crucial role in the "fail-safe" mechanism of auto-inhibition. When ATP synthesis is not occurring, the b'-subunit enhances the engagement between the C-terminal domain of an α-subunit and a loop in the γ-subunit, preventing wasteful ATP hydrolysis . This interaction helps maintain the latent ATPase activity characteristic of mycobacterial ATP synthases, which is essential for ATP homeostasis under oxygen-limited conditions . Rotational dynamics studies of recombinant M. smegmatis complexes show that the transition between the inhibited state (with the αCTD engaged) and the active state is a rapid process, allowing quick adaptation to changing energy requirements. This auto-inhibitory mechanism is unique to mycobacteria and not found in human ATP synthases, making it an attractive target for antimycobacterial drug development.

What structural features of atpF distinguish mycobacterial ATP synthases from other bacterial or mammalian homologs?

Several structural features of atpF distinguish mycobacterial ATP synthases from other bacterial or mammalian homologs:

• Fused bδ-subunit: In mycobacteria, the atpF encodes a fused bδ-subunit, while in most other bacteria and mammals, these are separate proteins .

• Duplicated domain: The N-terminal region of the fused bδ-subunit contains a duplicated domain structure unique to mycobacteria that participates in attachment to α-subunits .

• "Fail-safe" mechanism: The b'-subunit participates in an enhanced auto-inhibitory mechanism that works alongside the αCTD-γ loop interaction to prevent ATP hydrolysis .

• Differential sensitivity: The mycobacterial b-subunit shows distinct sensitivity to inhibitors like bedaquiline compared to mammalian homologs, reflecting structural differences in the binding regions .

These unique features have evolved to support mycobacterial survival under varying environmental conditions, particularly during oxygen limitation, and provide potential targets for developing mycobacterium-specific drugs.

What are the most effective methods for studying atpF-mediated interactions within the ATP synthase complex?

Several methods have proven effective for studying atpF-mediated interactions:

For studying dynamic interactions, combining cryo-EM with molecular dynamics simulations has provided valuable insights into how the b-subunit participates in the auto-inhibitory mechanism and interacts with other subunits during the catalytic cycle . Cross-linking studies have successfully identified key residues in the b-subunit that interact with the α and δ subunits, helping map the interaction network within the peripheral stalk.

How can ATP synthase activity be accurately measured in recombinant systems expressing atpF?

ATP synthase activity in recombinant systems expressing atpF can be measured accurately using several complementary approaches:

• ATP synthesis assay: Monitor ATP production using the luciferin-luciferase system. Reconstitute purified enzyme into liposomes with an established proton gradient and add ADP and Pi. Measure luminescence using a spectrophotometer or luminometer.

• ATP hydrolysis assay: Measure inorganic phosphate release using colorimetric methods (malachite green or molybdate) or coupled enzyme assays. Typical ATPase activity of wild-type M. smegmatis F1-ATPase is approximately 0.04 μmol min⁻¹ (mg protein)⁻¹, while deletion of regulatory elements can increase this to 0.36-1.28 μmol min⁻¹ (mg protein)⁻¹ .

• Proton pumping assay: Use pH-sensitive fluorescent dyes (ACMA or pyranine) to monitor proton translocation across membranes.

• Rotational assays: Attach fluorescent probes to specific subunits and monitor rotation using single-molecule fluorescence microscopy.

For accurate measurements, it's essential to:

• Use appropriate controls (e.g., DCCD-inhibited enzyme)

• Maintain constant temperature (typically 37°C for mycobacterial enzymes)

• Ensure physiological pH (6.5-7.5) and ionic strength

• Include Mg²⁺ at 2-5 mM concentration

• Compare activity to wild-type enzyme preparations

What are the challenges in expressing and characterizing mutant forms of atpF?

Expressing and characterizing mutant forms of atpF presents several challenges:

• Structural destabilization: Mutations in atpF can destabilize the peripheral stalk structure, leading to improper assembly of the ATP synthase complex. This is particularly problematic for mutations targeting the duplicated domains in the N-terminal region that interact with α-subunits .

• Expression toxicity: Some mutations may be toxic to the expression host if they disrupt critical functions of ATP synthase, especially since ATP synthase is essential for M. smegmatis growth .

• Solubility issues: Mutations can affect protein folding and solubility, particularly those affecting hydrophobic regions involved in protein-protein interactions.

• Functional assessment: Distinguishing between mutations that directly affect atpF function versus those that indirectly impact activity by altering complex assembly requires careful controls.

• Compensatory mechanisms: The expression system may develop compensatory mechanisms to overcome defects introduced by mutations, complicating interpretation of results.

To address these challenges, researchers should:

• Use inducible expression systems to control protein levels

• Employ complementation strategies to maintain cell viability

• Consider expressing subcomplexes rather than the full ATP synthase

• Use multiple biophysical techniques to assess protein stability and folding

• Include wild-type controls in all functional assays

How can structural information about atpF be utilized for rational drug design targeting mycobacterial ATP synthases?

Structural information about atpF can be leveraged for rational drug design through several approaches:

• Target unique interfaces: The interface between the b'-subunit and the α-subunit's C-terminal domain provides a mycobacterium-specific drug target distinct from human ATP synthases . High-resolution structures have revealed pockets and grooves at these interfaces that could accommodate small molecule inhibitors.

• Disrupt auto-inhibition release: Compounds that stabilize the auto-inhibitory interaction between the b'-subunit, αCTD, and γ-loop could lock the enzyme in an inhibited state, preventing ATP synthesis without affecting human ATP synthases .

• Structure-based virtual screening: The detailed structural data from cryo-EM studies (2-3 Å resolution) enables in silico screening of compound libraries against specific binding pockets on the b-subunit.

• Fragment-based drug discovery: Using structural information to identify anchor points for fragment molecules that can be elaborated into full inhibitors with high specificity.

• Allosteric inhibitors: Target allosteric sites on the b-subunit that could transmit conformational changes to the catalytic sites, disrupting the rotary mechanism.

These approaches have potential advantages over current ATP synthase inhibitors like bedaquiline, potentially offering improved specificity, reduced side effects, and activity against resistant strains.

What is the role of atpF in the response of M. smegmatis to environmental stresses such as hypoxia or acidic pH?

The atpF subunit plays a critical role in M. smegmatis response to environmental stresses through several mechanisms:

• ATP homeostasis during hypoxia: The b'-subunit's participation in the auto-inhibitory mechanism prevents wasteful ATP hydrolysis when oxygen is limited, helping maintain ATP levels critical for survival . This is particularly important for mycobacteria trapped within hypoxic niches in patients, where they slow their growth but remain viable .

• pH homeostasis: Under acidic conditions, the ATP synthase can function as an ATPase, pumping protons to generate a proton motive force and prevent intracellular acidification . The regulatory role of the b'-subunit helps balance ATP synthesis versus proton pumping activities depending on environmental conditions.

• Stress adaptation: The peripheral stalk structure, of which atpF is a critical component, must maintain structural integrity under stress conditions to prevent uncoupling of F1 and FO domains, which would lead to energy dissipation.

• Drug resistance: Changes in atpF structure or expression have been implicated in altered sensitivity to ATP synthase inhibitors like bedaquiline, suggesting a role in adaptive responses to antimicrobial pressure .

Understanding these roles is particularly important as ATP synthase inhibitors like bedaquiline target mycobacteria when they are most vulnerable to ATP depletion, such as under hypoxic conditions .

How do post-translational modifications affect atpF function in mycobacterial ATP synthases?

Post-translational modifications (PTMs) of atpF can significantly impact its function in mycobacterial ATP synthases, though this area remains less explored compared to structural studies. Current research indicates:

• Phosphorylation: Specific serine and threonine residues in atpF can undergo phosphorylation, potentially modulating the strength of interactions with other subunits in the peripheral stalk. These modifications may serve as a rapid response mechanism to changing energy demands.

• Acetylation: N-terminal acetylation has been detected in proteomic studies and may influence protein stability and interaction with lipids or other proteins.

• Oxidative modifications: Under oxidative stress conditions, reactive oxygen species can modify cysteine residues, potentially affecting the structural integrity of the peripheral stalk.

• Environmental regulation: PTMs may vary under different growth conditions or environmental stresses, providing a mechanism for fine-tuning ATP synthase activity without altering protein expression levels.

A comprehensive understanding of these modifications requires:

• Phosphoproteomic analysis under various growth conditions

• Site-directed mutagenesis of putative modification sites

• Structural studies of modified versus unmodified proteins

• Correlation of modification patterns with enzymatic activity

Such studies could reveal new regulatory mechanisms and potential drug targets that exploit mycobacteria-specific post-translational control of ATP synthase.

What are the key considerations when designing experiments to study the interaction between atpF and other ATP synthase subunits?

When designing experiments to study interactions between atpF and other ATP synthase subunits, researchers should consider:

• Native environment preservation: Maintain the membrane environment or use suitable detergents (e.g., 4-trans(4-transpropylcyclohexyl)-cyclohexyl-α-maltoside) that preserve native protein-protein interactions .

• Tagged protein design: When using affinity tags, position them carefully to avoid disrupting critical interactions. The C-terminus of the b'-subunit has been successfully tagged without compromising function .

• Full complex versus subcomplexes: Determine whether to study interactions in the context of the complete ATP synthase or focused subcomplexes. The latter may simplify analysis but could miss contextual interactions.

• Dynamic versus static interactions: Choose methods appropriate for the nature of the interaction. For dynamic interactions like those in the auto-inhibitory mechanism, single-molecule techniques or HDX-MS may be more informative than crystallography.

• Validation across techniques: Combine multiple complementary techniques (e.g., cryo-EM, cross-linking MS, FRET) to validate interaction models.

• Physiological relevance: Design experiments that mimic physiological conditions, including appropriate pH, ion concentrations, and membrane potential.

• Controls for specificity: Include appropriate negative controls to ensure observed interactions are specific and not artifacts of the experimental system.

How can researchers effectively distinguish between the effects of atpF mutations on assembly versus direct effects on enzyme catalysis?

Distinguishing between assembly defects and direct catalytic effects of atpF mutations requires a systematic approach:

Additionally, researchers can employ a complementation approach where the mutant atpF is co-expressed with wild-type protein. If the defect is in assembly, wild-type protein may rescue function, whereas catalytic defects would show dominant-negative effects or intermediate phenotypes. Time-resolved assembly studies can also reveal whether mutations affect the rate or efficiency of complex formation versus steady-state activity.

What are the best approaches for comparing mycobacterial atpF with homologs from other species for evolutionary and functional studies?

For comparative evolutionary and functional studies of mycobacterial atpF with homologs from other species, researchers should employ:

• Sequence-based approaches:

  • Multiple sequence alignment to identify conserved and variable regions

  • Phylogenetic analysis to understand evolutionary relationships

  • Conservation mapping onto structural models to identify functionally important residues

  • Coevolution analysis to detect residue pairs that evolve together, suggesting functional interactions

• Structure-based comparisons:

  • Superposition of available structures from different species

  • Comparison of interaction interfaces with other subunits

  • Analysis of conformational flexibility and dynamics

  • Identification of species-specific structural elements

• Functional comparative studies:

  • Heterologous expression of atpF homologs in M. smegmatis

  • Creation of chimeric proteins with domains from different species

  • Complementation assays to test functional conservation

  • Comparative biochemical assays under standardized conditions

• Systems biology approaches:

  • Analysis of genomic context and operon structure across species

  • Comparison of expression patterns and regulation

  • Metabolic modeling to understand the role of ATP synthase in different organisms

These approaches have revealed that mycobacterial atpF contains unique features, such as the duplicated domain structure and participation in the "fail-safe" auto-inhibitory mechanism, that distinguish it from homologs in other bacteria and mammals . These differences likely reflect adaptation to the specific ecological niches and metabolic requirements of mycobacteria.

How are advances in cryo-EM technology contributing to our understanding of atpF structure and function?

Recent advances in cryo-EM technology have revolutionized our understanding of atpF structure and function in several key ways:

• Resolution improvements: Modern cryo-EM techniques have achieved resolutions of 2-3 Å for mycobacterial ATP synthase, allowing visualization of side-chain details and water molecules critical for understanding mechanism .

• Substrate state visualization: Cryo-EM has revealed not only the three main catalytic states but also eight additional substates during the 360° catalytic cycle, providing unprecedented insights into the dynamic role of the peripheral stalk during catalysis .

• Structural flexibility mapping: Variability analysis in cryo-EM data has identified regions of atpF with inherent flexibility, correlating with functional movements during the catalytic cycle.

• Auto-inhibition mechanism: High-resolution structures have visualized the interaction between the b'-subunit and other components in the auto-inhibitory mechanism, revealing a previously unknown "fail-safe" mechanism .

• Drug binding sites: Cryo-EM structures have mapped the precise binding site of bedaquiline and potential sites for new inhibitors that could target mycobacterium-specific features of atpF.

• Time-resolved studies: Emerging time-resolved cryo-EM approaches promise to capture transient states during ATP synthesis and inhibition, further elucidating the dynamic role of atpF.

These advances are driving structure-based drug design efforts and deepening our understanding of the unique features of mycobacterial ATP synthases.

What novel expression systems or purification techniques might improve yields of functional recombinant atpF for structural and biochemical studies?

Several innovative approaches show promise for improving yields of functional recombinant atpF:

• Cell-free expression systems:

  • Advantages: Eliminates toxicity concerns, allows direct incorporation of non-natural amino acids for labeling

  • Optimization: Requires mycobacterial translation factors and chaperones for proper folding

• Inducible secretion systems:

  • Strategy: Engineer secretion tags that allow export of soluble domains while retaining membrane domains

  • Benefit: Reduces toxicity and simplifies purification from culture supernatant

• Nanodiscs or styrene-maleic acid lipid particles (SMALPs):

  • Application: Extraction of membrane proteins in native lipid environment without detergents

  • Advantage: Better preserves native structure and interactions of membrane-associated domains

• Split-intein systems:

  • Approach: Express atpF as separate domains with split-inteins that reconstitute the full protein post-translationally

  • Benefit: Overcomes expression toxicity of full-length protein

• Engineered strains with expanded genetic code:

  • Innovation: Site-specific incorporation of photocrosslinking amino acids or fluorescent probes

  • Application: Simplified structural and functional studies without post-purification modifications

These approaches, particularly when combined with affinity purification strategies targeting the C-terminus of the b'-subunit, could significantly improve both yield and functional quality of recombinant atpF for advanced structural and biochemical studies.

What emerging computational methods are being applied to predict atpF interactions and design novel inhibitors?

Cutting-edge computational methods are transforming our ability to predict atpF interactions and design novel inhibitors:

• Molecular dynamics simulations:

  • Advanced applications: Enhanced sampling techniques and coarse-grained simulations allow modeling of atpF dynamics within the complete ATP synthase on microsecond to millisecond timescales

  • Insights: Revealing transient binding pockets and conformational changes during the catalytic cycle

• Machine learning approaches:

  • Implementation: Deep learning models trained on protein-protein interaction data can predict key residues in atpF that mediate interactions with other subunits

  • Advantage: Can identify non-obvious interaction sites missed by traditional structural analysis

• Quantum mechanics/molecular mechanics (QM/MM):

  • Application: Modeling proton transfer events and cataloging energy changes during rotary motion

  • Benefit: Provides atomic-level understanding of how atpF contributes to energy transduction

• Network pharmacology:

  • Strategy: Identifies compounds that could disrupt multiple aspects of ATP synthase function simultaneously

  • Implementation: Combines structural data with systems biology to predict off-target effects

• Fragment-based computational screening:

  • Technique: Virtual screening of fragment libraries against multiple conformations of atpF binding pockets

  • Outcome: Identification of chemical scaffolds with high likelihood of binding to mycobacterium-specific features

• Molecular docking with flexible protein models:

  • Innovation: Accounts for protein flexibility during drug binding, particularly important for the conformationally dynamic peripheral stalk

  • Impact: More realistic prediction of binding modes and affinities

These computational approaches, validated by experimental testing, are accelerating the discovery of novel inhibitors targeting mycobacterium-specific features of atpF with potential applications as next-generation antimycobacterial drugs.

What are the most promising research directions for developing novel antimycobacterial drugs targeting atpF?

The most promising research directions for developing novel antimycobacterial drugs targeting atpF include:

• Targeting the unique duplicated domain structure: Developing compounds that interfere with the interaction between the duplicated domains in the N-terminal region of the fused bδ-subunit and the α-subunits could disrupt peripheral stalk assembly specifically in mycobacteria .

• Exploiting the "fail-safe" auto-inhibition mechanism: Compounds that stabilize or enhance the interaction between the b'-subunit and the auto-inhibitory elements (αCTD and γ-loop) could lock mycobacterial ATP synthase in an inhibited state .

• Disrupting species-specific protein-protein interactions: Targeting interfaces between atpF and other subunits that differ from human homologs could provide selectivity while disrupting complex assembly or function.

• Allosteric modulators: Identifying allosteric binding sites on atpF that could transmit conformational changes to catalytic sites without directly competing with bedaquiline, potentially overcoming existing resistance mechanisms.

• Combination approaches: Developing compounds that synergize with existing ATP synthase inhibitors by binding to different sites on the complex, potentially lowering the required dose and reducing side effects.

These approaches benefit from the wealth of structural information now available and could lead to new antimycobacterial agents with high specificity and reduced likelihood of cross-resistance with existing drugs.

How might synthetic biology approaches be used to engineer atpF variants with enhanced properties for biotechnological applications?

Synthetic biology approaches offer exciting possibilities for engineering atpF variants with enhanced properties:

• Energy harvesting systems:

  • Engineering atpF variants with optimized peripheral stalk rigidity could enhance coupling efficiency between proton translocation and ATP synthesis

  • Application: Developing biological energy conversion systems with improved efficiency

• Biosensors:

  • Creating fusion proteins between atpF and reporter molecules that respond to conformational changes

  • Application: Sensors for detecting changes in membrane potential or ATP/ADP ratios in living cells

• Protein scaffolds:

  • Utilizing the structural properties of atpF as a scaffold for presenting multiple enzyme activities in defined spatial orientation

  • Benefit: Creating artificial enzyme cascades with enhanced catalytic efficiency

• Molecular motors:

  • Engineering the interface between atpF and rotary elements to create nanoscale motors with controllable properties

  • Application: Powering molecular machines for various biotechnological applications

• Stress resistance:

  • Developing atpF variants with enhanced stability under extreme conditions

  • Utility: Improving ATP synthesis in industrial microorganisms under challenging process conditions

These applications leverage our detailed understanding of atpF structure and function while exploring new frontiers in protein engineering and synthetic biology.

What key questions about mycobacterial atpF structure and function remain unanswered?

Despite significant advances, several key questions about mycobacterial atpF remain unanswered:

• Dynamic regulation: How is atpF function regulated in response to changing environmental conditions? Are there specific post-translational modifications that modulate its activity in vivo?

• Assembly process: What is the precise sequence of events in the assembly of the peripheral stalk, and how is atpF incorporated into the growing complex? Are there specific chaperones involved?

• Species variations: How do structural and functional differences in atpF between pathogenic mycobacteria (M. tuberculosis, M. abscessus) and model organisms (M. smegmatis) relate to virulence and pathogenicity?

• Energy coupling mechanism: What is the exact mechanism by which the peripheral stalk, including atpF, maintains structural integrity during the high-torque rotation of the central stalk without dissociating?

• Lipid interactions: How do specific lipids in the mycobacterial membrane interact with atpF, and how do these interactions influence enzyme function?

• Evolutionary adaptation: What selective pressures led to the unique features of mycobacterial atpF, such as the duplicated domain structure and participation in auto-inhibition?

• Resistance mechanisms: How might mutations in atpF contribute to resistance against ATP synthase inhibitors, and can we predict and counter such resistance?