Recombinant Escherichia coli O139:H28 ATP synthase subunit b (atpF)

What is the structure and function of ATP synthase subunit b in E. coli?

The 156-residue b subunit of E. coli ATP synthase is a major component of the peripheral stalk (also called the "stator stalk") that connects the F₁ and F₀ sectors of the enzyme. The b subunit exists as an extended helical dimer, with two copies extending from the membrane to the top of F₁, where they interact with the delta subunit. Structurally, the sequence has been divided into four domains:

• N-terminal membrane-spanning domain

• Tether domain

• Dimerization domain (contained within residues 60-122)

• C-terminal delta-binding domain

The dimerization domain has properties of a coiled-coil structure, while the delta-binding domain is more globular. Functionally, the b dimer serves as an elastic element during rotational catalysis and also directly influences the catalytic sites, suggesting an active role in coupling energy transfer within the ATP synthase complex .

How does the b subunit contribute to ATP synthase assembly and function?

The b subunit is essential for normal assembly and function of the F₁F₀ ATP synthase complex. It forms a homodimer that constitutes the peripheral stalk, providing structural stability to the entire complex. Sites of crosslinking between b and other subunits (a, alpha, beta, and delta) have been identified, highlighting its importance in maintaining proper assembly and functional coupling.

The b subunit plays a crucial role in:

• Connecting the membrane-embedded F₀ sector to the catalytic F₁ sector

• Providing structural stability during the rotational catalysis

• Participating in the energy coupling mechanism between proton translocation and ATP synthesis

• Allowing proper assembly of the ATP synthase complex

What are the optimal conditions for expressing recombinant ATP synthase subunit b in E. coli?

Based on studies with E. coli ATP synthase components, the optimal expression conditions should consider:

• Host strain selection:

  • For high-level expression, mutant host strains E. coli C41(DE3) and C43(DE3) have shown superior performance for membrane proteins

  • C43(DE3) cells demonstrated maximal levels of intracellular membrane proliferation when overexpressing ATP synthase subunit b

• Expression vectors and promoters:

  • pET-based expression systems with T7 promoter are commonly used

  • The uncF gene (encoding the b subunit) should be cloned into an appropriate expression vector with an inducible promoter

• Growth conditions:

  • Temperature: Lower temperatures (16-25°C) often improve soluble expression

  • Induction: Use moderate inducer concentrations (0.1-0.5 mM IPTG)

  • Medium: Rich media (such as LB or TB) supplemented with appropriate antibiotics

  • Growth phase: Induction at mid-log phase (OD₆₀₀ of 0.6-0.8)

How can I overcome inclusion body formation when expressing ATP synthase subunit b?

ATP synthase subunit b, like many membrane and membrane-associated proteins, can form inclusion bodies when overexpressed. Several strategies can be employed to enhance soluble expression:

• Strain engineering approaches:

  • Use E. coli C41(DE3) or C43(DE3) strains, which have been specifically developed for membrane protein expression

  • These strains allow for the proliferation of intracellular membranes that can accommodate overexpressed membrane proteins without inclusion body formation

• Expression optimization:

  • Lower the temperature during induction (16-20°C)

  • Use lower inducer concentrations

  • Consider auto-induction media

  • Optimize growth conditions to reduce metabolic burden

• Construct design:

  • Express soluble domains separately (e.g., the dimerization domain or delta-binding domain)

  • Use solubility-enhancing fusion partners (MBP, SUMO, Trx)

  • Consider expressing fragments of the protein (like b30-82 or b22-156) that have been successfully produced in soluble form

What purification strategy is recommended for obtaining high-quality recombinant ATP synthase subunit b?

Based on successful purification strategies for ATP synthase components:

• For soluble domains or fragments:

  • The b30-82 fragment (including the tether region and part of the dimerization domain) has been successfully purified for NMR studies

  • Include appropriate protease inhibitors during lysis

  • Consider adding reducing agents if the construct contains cysteines

  • Purify using a combination of affinity chromatography and size exclusion

What analytical techniques are most effective for studying the structure of ATP synthase subunit b?

Multiple complementary techniques have been successfully applied to study ATP synthase subunit b structure:

• Nuclear Magnetic Resonance (NMR) Spectroscopy:

  • Successfully used to determine the solution structure of b30-82 fragment

  • Revealed an α-helical structure between residues 39 and 72

  • Provided detailed information about surface charge distribution and hydrophobic surface patterns

• Cross-linking Studies:

  • Cysteine substitution at specific positions (e.g., A61C, A68C, A70C, A72C) followed by disulfide formation analysis

  • Revealed proximity relationships between residues in the dimeric structure

  • Helped determine the orientation of helices in the dimer

• Circular Dichroism (CD) Spectroscopy:

  • Used to analyze secondary structure content

  • Helped determine the alpha-helical nature of various domains

  • Useful for assessing coiled-coil formation

• Mass Spectrometry:

  • Laser-induced liquid bead ion desorption (LILBID) mass spectrometry has been used to study ATP synthase assembly

  • Helps identify assembly intermediates under non-denaturing conditions

How can I investigate the assembly pathway of ATP synthase incorporating the b subunit?

To study the assembly pathway of ATP synthase:

• In vitro assembly studies:

  • Purify individual subunits (including subunit b) under non-denaturing conditions

  • Monitor self-assembly in different environments

  • Use LILBID mass spectrometry to identify important assembly intermediates

  • Correlate findings with activity measurements

• Nucleotide dependence analysis:

  • Study the role of nucleotide binding in assembly

  • Evidence suggests nucleotide binding is crucial for F₁ assembly, whereas ATP hydrolysis appears less critical

• Molecular chaperone studies:

  • Investigate the role of chaperones like Hsp70 in promoting ATP synthase assembly

  • Recent findings show Hsp70 not only acts as a folding helper but also promotes ATP synthase assembly

  • Specifically, Hsp70 is involved with partner proteins in the assembly of the catalytic head and monitors the linkage of the catalytic head to the stator

• Genetic approaches:

  • Create fusion proteins (e.g., b/δ fusion) to analyze the roles of individual b subunits

  • Generate mutant variants with specific deletions or substitutions

  • Study genetic complementation between mutant b subunits

What experimental approaches can determine the interaction between subunit b and other ATP synthase components?

Several experimental approaches can be employed:

• Crosslinking studies:

  • Site-directed mutagenesis to introduce cysteine residues at specific positions

  • Disulfide crosslinking analysis to identify proximity relationships

  • Chemical crosslinking with reagents like succinimidyl 4,4'-azipentanoate (SDA)

• Affinity purification coupled with mass spectrometry (AP-MS):

  • Use epitope-tagged subunit b to isolate interacting partners

  • Quantitative comparison to control samples to identify specific interactions

  • In-cell photo-crosslinking to capture transient interactions

• Genetic complementation studies:

  • Express two different b subunits with distinct mutations

  • Assess functional complementation through activity assays

  • Studies have shown that defective b subunits can complement each other, indicating unique contributions of each b subunit to the functions of the peripheral stalk

• Biophysical interaction analysis:

  • Surface plasmon resonance (SPR) to measure binding kinetics

  • Isothermal titration calorimetry (ITC) for thermodynamic parameters

  • Förster resonance energy transfer (FRET) to study conformational changes during ATP synthesis/hydrolysis

How can recombinant ATP synthase or its subunits be used for ATP regeneration systems?

ATP regeneration systems using recombinant components offer significant advantages for biocatalytic applications:

• Heat-treated E. coli producing thermostable enzymes:

  • E. coli producing thermostable polyphosphate kinase (PPK) can be heat-treated to increase membrane permeability

  • These cells can synthesize ATP from external ADP and polyphosphate

  • More than 60% of activity is retained even after a 1-week incubation at 70°C

  • This approach provides an inexpensive ATP regeneration system

• Advantages over direct ATP addition:

  • Overcomes inhibitory effects of accumulated ADP or AMP

  • Avoids enzyme inhibition by high ATP concentrations

  • Significantly reduces costs (polyP costs $9/lb vs. ATP equivalents at $2,000/lb)

  • Sustains ATP regeneration over extended periods

This system can be combined with other thermostable enzymes for various applications, such as the production of fructose 1,6-diphosphate from fructose and polyphosphate .

What are the approaches to create functional heterodimeric b subunits in E. coli ATP synthase?

Creating functional heterodimeric b subunits provides insights into the distinct roles of each b subunit in the ATP synthase complex:

• Expression system development:

  • Develop a system to express two different b subunits simultaneously in E. coli

  • This overcomes limitations of traditional mutagenesis where mutations affect both b subunits

  • Enables study of heterodimeric F₁F₀ ATP synthase complexes

• Chimeric b subunits construction:

  • Replace segments of the tether and dimerization domains with homologous regions from other organisms

  • For example, successful chimeras have been created by replacing E. coli b residues 39-86 with Thermosynechococcus elongatus b and b' sequences

  • These chimeric subunits form heterodimeric peripheral stalks incorporated into functional F₁F₀ ATP synthase complexes

• b/δ fusion protein approach:

  • Generate a fusion protein between one b subunit and the δ subunit

  • This mimics the arrangement found in mycobacteria, where one b subunit and the δ subunit are replaced by a b/δ fusion protein

  • Studies show that in such constructs, the full-length b subunit (linked to δ) connects the stalk to F₁, while the other b subunit (which can be shortened to b' type) attaches to F₀

• Complementation analysis:

  • Express two defective b subunits together (e.g., one with mutation at conserved R36 and another with C-terminal deletion)

  • Assess assembly and function of the resulting ATP synthase complexes

  • Successful complementation between defective b subunits demonstrates their unique functional contributions

How can mutations in subunit b be designed to investigate its role in energy coupling and enzyme regulation?

Strategic mutation design in subunit b can reveal critical aspects of ATP synthase function:

• Targeting the membrane-spanning domain:

  • Mutations in the transmembrane region can affect proton translocation and interaction with other F₀ components

  • The conserved R36 residue is crucial for function and can be targeted for mutagenesis

• Tether domain modifications:

  • Alterations in the tether domain (b30-82) can affect the flexibility of the peripheral stalk

  • Alanine residues (A32, A45, A50, A57, A61, A68, A72) form a hydrophobic surface that can be targeted

  • Cysteine substitutions at positions 61, 68, and 72 result in disulfide formation, while position 70 does not - providing insight into helix orientation

• Dimerization domain engineering:

  • Modifications in the dimerization domain (residues 60-122) can affect the stability of the b dimer

  • Changes in the coiled-coil nature of this region may impact the elasticity of the peripheral stalk during rotational catalysis

• Delta-binding domain alterations:

  • C-terminal modifications affect interaction with the delta subunit and F₁ attachment

  • Deletion of the last four C-terminal amino acids significantly impacts enzyme assembly

  • These mutations can be used to investigate the coupling mechanism between F₁ and F₀

What experimental strategies can reveal the influence of lipid composition on ATP synthase function?

Lipid composition significantly affects ATP synthase structure and function:

• Cardiolipin interaction studies:

  • Intracellular membranes containing overexpressed subunit b are particularly rich in cardiolipin

  • Antimicrobial peptides like EcDBS1R4 can sequester cardiolipin and modulate ATP synthase activity

  • Molecular dynamics simulations suggest that alterations in cardiolipin distribution affect the membrane environment of the F₀ motor

• Membrane environment manipulation:

  • Compare native versus artificial membrane environments

  • Investigate the effects of specific lipids (phosphatidylethanolamine, phosphatidylglycerol, cardiolipin)

  • Study how lipid:protein ratios and phospholipid compositions affect enzyme activity

• Structural analysis of lipid-protein interactions:

  • Crosslinking studies to identify specific lipid binding sites

  • Cryo-electron microscopy to visualize lipid-protein interactions

  • Molecular dynamics simulations to model how lipids affect protein conformations and hydrophobic matching between ATP synthase and the bilayer

How can I diagnose and resolve issues with recombinant ATP synthase subunit b expression?

Common expression issues and solutions:

For membrane protein expression specifically:

• Monitor proliferation of intracellular membranes (can be visualized by electron microscopy)

• Use C43(DE3) strain which shows maximal levels of intracellular membrane proliferation when overproducing subunit b

• In successful cases, recombinant subunit b can represent up to 80% of the protein content of proliferated membranes

What factors should be optimized for functional reconstitution of ATP synthase containing recombinant subunit b?

Key factors for successful reconstitution:

• Detergent selection and concentration:

  • Dodecylmaltoside (DDM) at 1-2% (w/v) is commonly used for extraction

  • Detergent concentration must be optimized to maintain protein stability while allowing efficient reconstitution

• Lipid composition of proteoliposomes:

  • Include cardiolipin (10-20%) which is particularly important for ATP synthase function

  • Optimize the phospholipid composition to mimic native E. coli membranes

  • Lipid:protein ratio affects enzyme activity (typically 50,000:1 molar ratio)

• Reconstitution method:

  • Rapid dilution method: Mix protein-detergent micelles with lipids and quickly dilute

  • Dialysis method: Slowly remove detergent by dialysis

  • Both approaches require optimization for specific protein-lipid combinations

• Buffer conditions:

  • pH: Typically 7.4-8.0 for optimal stability

  • Salt concentration: 100-150 mM for physiological conditions

  • Divalent cations: Include Mg²⁺ (2-5 mM) for stabilization

  • Glycerol (10-20%): Enhances protein stability

How can I verify the proper folding and assembly of recombinant ATP synthase subunit b?

Multiple analytical approaches can verify proper folding and assembly:

• Circular Dichroism (CD) spectroscopy:

  • Assess secondary structure content (α-helical content should be high)

  • Monitor thermal stability through temperature scans

  • Compare spectra with reference data for properly folded subunit b

• Disulfide crosslinking analysis:

  • Introduce cysteine residues at specific positions (A61C, A68C, A72C)

  • Analyze disulfide formation pattern through non-reducing SDS-PAGE

  • Specific crosslinking patterns indicate proper dimerization and alignment of helices

• Functional assays:

  • ATP hydrolysis activity measurements

  • Proton pumping assays using pH-sensitive fluorescent dyes

  • ATP synthesis assays in reconstituted proteoliposomes

• Size-exclusion chromatography:

  • Analyze oligomeric state and homogeneity

  • Compare elution profiles with properly assembled reference samples

  • Combination with multi-angle light scattering (SEC-MALS) provides absolute molecular weight determination

What controls should be included when studying ATP synthesis/hydrolysis in systems with recombinant subunit b?

Essential controls for rigorous experimental design:

• Negative controls:

  • Heat-inactivated enzyme preparation

  • Preparations lacking essential components (e.g., F₁ or F₀ sectors)

  • Systems without proton gradient for ATP synthesis assays

  • Inclusion of specific inhibitors (oligomycin, venturicidin, DCCD)

• Positive controls:

  • Wild-type ATP synthase preparation

  • Commercially available F₁-ATPase

  • Well-characterized mutants with known activity levels

• Validation experiments:

  • Proton gradient verification using pH-sensitive dyes

  • ATP detection method validation (luciferase assay, NADH-coupled assay)

  • Membrane integrity assessment for reconstituted systems

• Additional recommended controls:

  • Test protein-free liposomes for background signals

  • Include uncouplers (FCCP, CCCP) to dissipate proton gradients

  • Apply Mg²⁺ chelators (EDTA) to inhibit ATPase activity

  • Use membrane potential indicators to verify energization state

How can ATP synthase subunit b be leveraged for synthetic biology applications?

Innovative applications in synthetic biology:

• Engineered ATP production systems:

  • Creation of heat-stable ATP regeneration systems using thermostable components

  • Development of cell-free energy production modules

  • Design of artificial organelles with enhanced ATP synthesis capabilities

• Membrane protein scaffolds:

  • Using the membrane-spanning domain as a scaffold for designing novel membrane proteins

  • Creating chimeric proteins with specific membrane localization properties

  • Engineering membrane protein expression systems with improved yields

• Bionanotechnology applications:

  • Harnessing the rotary motor properties of ATP synthase for nanomachines

  • Developing molecular sensors based on conformational changes in subunit b

  • Creating energy-harvesting devices inspired by ATP synthase architecture

• Metabolic engineering:

  • Modification of ATP synthase efficiency to enhance cellular energy production

  • Creation of strains with altered ATP/ADP ratios for specific biochemical productions

  • Development of cells with improved energy conversion capabilities for biotechnological applications

What is the relationship between ATP synthase subunit b and bacterial stress responses or antimicrobial resistance?

Emerging connections to stress response and antimicrobial resistance:

• Membrane integrity and antimicrobial peptides:

  • Antimicrobial peptides like EcDBS1R4 can modulate ATP synthase activity by sequestering cardiolipin

  • This alters the membrane environment of the transmembrane F₀ motor

  • Impairs cardiolipin interactions with the cytoplasmic face of the peripheral stalk

  • Represents a novel mechanism of action for antimicrobial compounds targeting energy production

• Stress adaptation mechanisms:

  • ATP synthase regulation is linked to bacterial responses to pH, temperature, and osmotic stress

  • Subunit b may play a role in sensing membrane perturbations during stress conditions

  • Post-translational modifications of subunit b might modulate ATP synthase activity under stress

• Metabolic state sensing:

  • Recent research reveals interactions between mitochondrial apoptosis-inducing factor 1 (AIFM1) and adenylate kinase 2 (AK2) as gatekeepers of ATP synthase

  • These interactions are NADH-dependent and influenced by metabolic state

  • Similar mechanisms might exist in bacterial systems to adjust ATP synthesis to metabolic demands

• Therapeutic targeting:

  • ATP synthase inhibitors represent potential antimicrobial compounds

  • Understanding the structure and function of subunit b can lead to the development of specific inhibitors

  • Over 300 natural and synthetic ATP synthase inhibitors have been identified with different modes of action

How might structural information about ATP synthase subunit b inform drug development strategies?

Structural insights for drug development:

• Target site identification:

  • The interface between subunit b and other ATP synthase components represents potential binding sites

  • The hydrophobic surface formed by alanine residues in the tether domain offers a specific target region

  • The dimerization domain contains many potential protein-protein interaction surfaces that could be disrupted

• Mechanism-based inhibitor design:

  • Understanding the conformational changes during ATP synthesis/hydrolysis can inform the design of mechanism-based inhibitors

  • Compounds that lock subunit b in specific conformations might inhibit energy coupling

  • Small molecules targeting the elastic properties of the b dimer could affect rotational catalysis

• Peptide-based inhibitors:

  • Antimicrobial peptides that modulate ATP synthase activity by altering lipid organization represent novel therapeutic approaches

  • Peptides designed to mimic subunit b regions could compete for binding to other ATP synthase components

  • These approaches offer alternatives to traditional antibiotics with potentially different resistance mechanisms

• Structure-guided screening:

  • Molecular docking against identified binding sites on subunit b

  • Fragment-based approaches targeting specific functional domains

  • Virtual screening campaigns focused on the interface between subunit b and other components