Recombinant Agrobacterium vitis ATP synthase subunit a (atpB)

What is Agrobacterium vitis ATP synthase subunit a (atpB) and what is its role in the ATP synthase complex?

ATP synthase subunit a (atpB) from Agrobacterium vitis is a membrane-bound protein component of the F0 sector of ATP synthase. This protein consists of 250 amino acids and plays a critical role in the proton translocation pathway necessary for ATP synthesis . ATP synthase as a whole functions as a biological nanomotor, coupling ion movement through membranes with the synthesis or hydrolysis of ATP nucleotide .

The atpB subunit (also referred to as the ATP synthase F0 sector subunit a) works in conjunction with other F0 subunits to form the membrane-embedded portion of the enzyme complex. This sector creates a channel through which protons flow, generating the rotational torque that drives conformational changes in the F1 sector where ATP synthesis occurs . The specific arrangement of atpB is crucial for maintaining proper proton conductance and enzyme efficiency.

What expression systems are commonly used for recombinant Agrobacterium vitis ATP synthase subunit production?

Two primary expression systems have been documented for Agrobacterium vitis ATP synthase subunits:

• E. coli Expression System: This is used for the production of ATP synthase subunit a (atpB), which can be expressed with an N-terminal His tag. The protein spans residues 1-250 of the full-length sequence .

• Baculovirus Expression System: While not specifically mentioned for atpB, this system has been successfully employed for the related ATP synthase gamma chain (atpG) from Agrobacterium vitis .

The choice between these systems depends on research requirements. E. coli provides higher yield and simpler methodology, while the baculovirus system may offer advantages for proteins requiring eukaryotic post-translational modifications. For membrane proteins like atpB, E. coli often requires optimization of expression conditions to ensure proper folding and prevent the formation of inclusion bodies.

How should recombinant Agrobacterium vitis ATP synthase subunit a (atpB) be stored and handled for optimal stability?

Based on established protocols, the following storage and handling recommendations should be implemented:

• Storage Temperature: Store at -20°C/-80°C, with -80°C preferred for long-term storage .

• Physical Form: The protein is typically supplied as a lyophilized powder .

• Reconstitution: Before opening, briefly centrifuge the vial to bring contents to the bottom. Reconstitute in deionized sterile water to a concentration of 0.1-1.0 mg/mL .

• Stabilization: Add glycerol to a final concentration of 5-50% (with 50% being the standard recommendation) to prevent freeze-thaw damage .

• Aliquoting: Prepare multiple small-volume aliquots to avoid repeated freeze-thaw cycles .

• Working Storage: For ongoing experiments, store working aliquots at 4°C for up to one week .

• Freeze-Thaw Cycles: Repeated freezing and thawing is strongly discouraged as it may lead to protein denaturation and loss of activity .

What analytical methods are recommended for quality assessment of recombinant atpB protein?

Several analytical techniques are recommended for quality assessment of recombinant atpB:

• SDS-PAGE: This is the primary method for evaluating protein purity, with expectations of >90% purity for atpB and >85% for related ATP synthase subunits . The technique also confirms the expected molecular weight.

• Western Blotting: Particularly useful when using tagged versions of the protein, this method can specifically identify the target protein using anti-tag or anti-atpB antibodies.

• Mass Spectrometry: For precise molecular weight determination and verification of the primary sequence, including confirmation of any post-translational modifications.

• Circular Dichroism: To evaluate secondary structure integrity, particularly important for membrane proteins like atpB.

• Functional Assays: Activity-based assays to confirm that the recombinant protein retains its native functional properties, such as ability to integrate into membranes or participate in ATP synthesis when reconstituted with other subunits.

What buffer systems are optimal for handling recombinant Agrobacterium vitis ATP synthase subunit a (atpB)?

The optimal buffer systems for handling recombinant atpB include:

• Storage Buffer: Tris/PBS-based buffer containing 6% trehalose at pH 8.0 has been established as effective for maintaining stability during storage .

• Reconstitution Buffer: Deionized sterile water is recommended for initial reconstitution of lyophilized protein .

• Working Buffer Considerations:

  • pH range: Typically maintain pH 7.0-8.0 to mimic physiological conditions

  • Ionic strength: Moderate salt concentrations (100-150 mM NaCl) to maintain protein solubility

  • Detergents: For membrane proteins like atpB, mild detergents such as n-dodecyl β-D-maltoside (DDM) or digitonin may be necessary to maintain solubility

  • Reducing agents: Addition of reducing agents like DTT or β-mercaptoethanol (0.5-1 mM) may help prevent oxidation of cysteine residues

• Glycerol Addition: Final concentration of 5-50% glycerol is recommended for cryoprotection during freezing .

What methodological approaches can be used to study the interaction between atpB and potential inhibitory compounds?

Several sophisticated methodological approaches can be employed to study interactions between atpB and potential inhibitors:

• Surface Plasmon Resonance (SPR):

  • Immobilize His-tagged atpB on a Ni-NTA sensor chip

  • Flow potential inhibitors over the surface at varying concentrations

  • Measure association and dissociation kinetics

  • Calculate binding constants (Ka, Kd) to quantify affinity

• Isothermal Titration Calorimetry (ITC):

  • Directly measure thermodynamic parameters of binding (ΔH, ΔS, ΔG)

  • Determine stoichiometry of interaction

  • No protein labeling required

  • Can be performed with detergent-solubilized atpB

• Fluorescence-based Assays:

  • Intrinsic tryptophan fluorescence changes upon ligand binding

  • Fluorescently labeled inhibitors to track binding

  • Fluorescence resonance energy transfer (FRET) between labeled protein and inhibitor

• Functional Inhibition Assays:

  • Reconstitute atpB with other ATP synthase subunits in liposomes

  • Measure proton translocation or ATP synthesis in presence of inhibitors

  • Determine IC50 values for comparative analysis

• Computational Docking and Molecular Dynamics:

  • In silico prediction of binding modes

  • Energy minimization to identify stable conformations

  • Molecular dynamics simulations to study dynamics of the interaction

ATP synthase has approximately twelve discrete inhibitor binding sites, including peptide binding sites located at the interface of α/β subunits on the F1 sector . Understanding these binding sites can guide the rational design of inhibitors targeting atpB or its interaction interfaces.

How can researchers effectively investigate the role of specific residues in atpB function through mutagenesis?

Site-directed mutagenesis of atpB can provide valuable insights into structure-function relationships:

• Strategic Selection of Mutation Targets:

  • Conserved residues identified through sequence alignment across species

  • Charged residues likely involved in proton translocation

  • Residues at subunit interfaces that may affect complex assembly

  • Residues implicated in inhibitor binding

• Mutagenesis Protocol Optimization:

  • Use overlap extension PCR or commercial mutagenesis kits

  • Design primers with appropriate melting temperatures and minimal secondary structure

  • Consider codon optimization for the expression system

  • Include silent mutations to create restriction sites for screening

• Functional Characterization Methods:

  • Proton Translocation Assays: Reconstitute mutant atpB in liposomes with pH-sensitive fluorescent dyes

  • ATP Synthesis/Hydrolysis Assays: Measure enzymatic activity when incorporated into the complete complex

  • Assembly Analysis: Blue native PAGE or co-immunoprecipitation to assess integration into the ATP synthase complex

  • Structural Analysis: Circular dichroism to confirm folding integrity of mutant proteins

• Data Analysis Framework:

  • Compare kinetic parameters of mutants (Km, Vmax, kcat)

  • Analyze proton conductance rates

  • Assess inhibitor sensitivity changes

  • Create structure-function correlation maps

For example, residues essential for Pi binding in ATP synthase (αPhe-291, αSer-347, αGly-351, αArg-376, βLys-155, βArg-182, βAsn-243, βArg-246) and the highly conserved αVISIT-DG sequence have been identified as critical for catalytic function . Similar approaches could identify key functional residues in atpB.

What are the most effective reconstitution methods for studying atpB in membrane-mimetic systems?

Membrane protein reconstitution is essential for functional studies of atpB:

• Liposome Reconstitution:

  • Composition Optimization: Use bacterial lipid extracts or defined mixtures (POPC/POPE/POPG)

  • Size Control: Extrusion through polycarbonate filters for uniform liposome size

  • Protein Incorporation: Detergent-mediated incorporation followed by detergent removal

  • Detergent Removal Methods:

    • Bio-Beads SM-2 adsorption

    • Dialysis (slow, gentle removal)

    • Gel filtration chromatography

  • Orientation Control: pH or salt gradients to influence protein orientation

• Nanodiscs:

  • Incorporate atpB with membrane scaffold proteins (MSPs)

  • Provide a more native-like membrane environment with defined size

  • Allow for soluble, monodisperse membrane protein samples

  • Enable solution-based biophysical techniques

• Proteoliposomes for Functional Assays:

  • Proton Gradient Formation: Generate using pH jumps or valinomycin/K+ gradients

  • Activity Measurement: ACMA fluorescence quenching for proton translocation

  • ATP Synthesis: Luciferin/luciferase assay for real-time ATP detection

• Quality Control Metrics:

  • Freeze-fracture electron microscopy to verify incorporation

  • Dynamic light scattering for size distribution

  • Fluorescence recovery after photobleaching (FRAP) for lateral mobility

  • Sucrose gradient centrifugation to separate empty liposomes

How can structural biology techniques be applied to elucidate the detailed architecture of atpB?

Several structural biology approaches can provide insights into atpB architecture:

• X-ray Crystallography:

  • Challenges: Membrane proteins like atpB are difficult to crystallize

  • Strategies:

    • Lipidic cubic phase crystallization

    • Use of antibody fragments to increase polar surface area

    • Fusion protein approaches (e.g., T4 lysozyme fusion)

  • Detergent Screening: Systematic testing of detergents for optimal crystal packing

• Cryo-Electron Microscopy (Cryo-EM):

  • Advantages: No crystallization required, captures multiple conformational states

  • Sample Preparation:

    • Detergent-solubilized protein

    • Reconstitution in nanodiscs

    • Vitrification conditions optimization

  • Data Collection and Processing:

    • Use of phase plates for contrast enhancement

    • Motion correction and CTF estimation

    • 3D classification to identify conformational states

• NMR Spectroscopy:

  • Solution NMR: Limited to specific domains or fragments

  • Solid-State NMR: Applicable to full-length protein in lipid environments

  • Isotopic Labeling: Selective labeling strategies for specific regions

• Hybrid Methods:

  • Cross-linking Mass Spectrometry (XL-MS): Identify spatial relationships between residues

  • Hydrogen-Deuterium Exchange (HDX-MS): Map solvent accessibility and dynamics

  • Small-Angle X-ray Scattering (SAXS): Low-resolution envelope of detergent-solubilized protein

• Computational Modeling:

  • Homology modeling based on related structures

  • Molecular dynamics simulations in membrane environments

  • Refinement using experimental constraints

Understanding the structure of atpB within the context of the complete ATP synthase complex would provide valuable insights into its function and potential as a drug target .

What experimental approaches can be used to study the interaction between atpB and other ATP synthase subunits?

Several experimental approaches can elucidate the interactions between atpB and other ATP synthase subunits:

• Co-immunoprecipitation (Co-IP):

  • Use antibodies against tags (His-tag on atpB) or specific subunits

  • Pull down complexes and analyze by Western blotting or mass spectrometry

  • Quantify interaction strength under various conditions

• Cross-linking Coupled with Mass Spectrometry:

  • Apply chemical cross-linkers with different spacer lengths

  • Digest cross-linked complexes and identify cross-linked peptides by MS

  • Map interaction interfaces at amino acid resolution

  • Data analysis using specialized software (e.g., xQuest, pLink)

• Förster Resonance Energy Transfer (FRET):

  • Label atpB and potential interaction partners with fluorophore pairs

  • Measure energy transfer as indicator of proximity

  • Perform in reconstituted systems or in bacterial cells

• Bacterial Two-Hybrid Systems:

  • Adapt for membrane proteins using specialized vectors

  • Screen for interactions in a cellular context

  • Quantify interaction strength using reporter gene expression

• Surface Plasmon Resonance (SPR):

  • Immobilize atpB and flow other subunits as analytes

  • Measure binding kinetics in real-time

  • Determine affinity constants

• Analytical Ultracentrifugation:

  • Analyze complex formation in detergent solutions

  • Determine stoichiometry and stability of complexes

  • Study the effects of mutations on complex formation

• Blue Native PAGE:

  • Separate intact membrane protein complexes

  • Identify subcomplexes formed during assembly

  • Compare wild-type and mutant atpB effects on complex formation

When studying these interactions, it's essential to consider the membrane environment, as the lipid composition can significantly influence protein-protein interactions within the ATP synthase complex.

What are the methodological considerations for studying the role of atpB in bacterial energy metabolism?

To study the role of atpB in bacterial energy metabolism, researchers should consider:

• Genetic Manipulation Approaches:

  • Conditional Knockdown Systems: Since complete deletion may be lethal

    • Inducible antisense RNA

    • Degron-based protein degradation

    • CRISPR interference (CRISPRi)

  • Point Mutations: To create partially functional variants

  • Complementation Studies: Expression of wild-type or mutant atpB in trans

• Metabolic Analysis Methods:

  • Oxygen Consumption Rate: Measure using electrode-based systems or fluorescent probes

  • Membrane Potential: Monitor using voltage-sensitive dyes (DiSC3(5), TMRM)

  • Intracellular ATP Levels: Quantify using luciferase-based assays

  • NADH/NAD+ Ratio: As indicator of respiratory chain activity

  • Growth Kinetics: Under various carbon sources and growth conditions

• Proteomics and Transcriptomics:

  • Analyze compensatory responses to atpB manipulation

  • Identify metabolic pathway adjustments

  • Map regulatory networks affected by energy metabolism changes

• Isotope Labeling Studies:

  • 13C-labeling to track metabolic flux

  • 31P-NMR to monitor phosphorylated metabolites

  • Mass spectrometry for global metabolite analysis

• In vitro Reconstitution:

  • Isolate bacterial membranes containing wild-type or mutant ATP synthase

  • Measure ATP synthesis rates under defined conditions

  • Determine proton translocation efficiency

• Antimicrobial Susceptibility Testing:

  • Evaluate how atpB mutations affect sensitivity to antibiotics targeting energy metabolism

  • Test ATP synthase inhibitors against wild-type and mutant strains

The ATP synthase functions as a biological nanomotor coupling ion movement with ATP synthesis, making it crucial for bacterial energy metabolism and potentially an important antimicrobial target .

How can researchers assess the potential of atpB as an antimicrobial drug target?

Assessment of atpB as an antimicrobial drug target requires a multifaceted approach:

• Target Validation Studies:

  • Essentiality Assessment: Determine if atpB is essential for bacterial viability

  • Conditional Mutants: Create and characterize growth/survival under various conditions

  • Comparative Genomics: Analyze conservation across bacterial species vs. human homologs

  • Structural Comparison: Identify unique features compared to human ATP synthase

• High-Throughput Screening Methods:

  • Assay Development:

    • ATP synthesis inhibition in isolated membranes or proteoliposomes

    • Growth inhibition in bacterial cultures

    • Membrane potential disruption assays

  • Compound Libraries: Natural products, synthetic compounds, peptide libraries

  • Hit Validation: Secondary assays to confirm mechanism of action

• Structure-Activity Relationship Studies:

  • Iterative compound optimization

  • Molecular modeling of inhibitor binding

  • Pharmacophore development

• Resistance Development Assessment:

  • Serial passage experiments to force resistance

  • Whole genome sequencing of resistant mutants

  • Characterization of resistance mechanisms

• Specificity Testing:

  • Counterscreening against human ATP synthase

  • Cytotoxicity against mammalian cell lines

  • Microbiome impact assessment

• Case Studies from Literature:

  • Streptococcus mutans ATP synthase inhibition prevents biofilm formation and acid production, with potential as a prophylactic treatment for dental cavities

  • Mycobacterium tuberculosis ATP synthase mutations (D32V and A63P) in the c-subunit confer resistance to diarylquinoline, providing insights into drug resistance mechanisms

• Model Development:

  • In vitro infection models

  • Animal infection models to test efficacy

  • PK/PD studies for promising compounds

ATP synthase has been established as a promising molecular drug target for antimicrobial and antitumor applications, with multiple inhibitor binding sites identified .

What are the optimal approaches for quantifying Agrobacterium vitis in research settings?

For accurate quantification of Agrobacterium vitis in research settings:

• Droplet Digital PCR (ddPCR):

  • Target Gene: The virA gene has been successfully used for specific detection

  • Benefits:

    • Absolute quantification without standard curves

    • High sensitivity and specificity

    • Less susceptible to inhibitors than traditional qPCR

  • Protocol Considerations:

    • Optimal DNA extraction methods

    • Primer/probe design optimization

    • Droplet generation and reading parameters

• Quantitative Real-time PCR (qPCR):

  • Develop standard curves using known quantities of bacterial DNA

  • Include internal amplification controls

  • Optimize extraction protocols for different sample types

• Culture-based Methods:

  • Selective Media: Use specific growth media for Agrobacterium vitis

  • Colony Counting: Quantify CFU/g or CFU/mL

  • MPN (Most Probable Number): For low bacterial densities

• Sample Processing Optimization:

  • Plant Material:

    • Root samples require careful washing procedures

    • Homogenization techniques affect recovery

  • Soil Samples:

    • Rhizosphere sampling procedures

    • DNA extraction methods must address humic acid inhibition

• Data Presentation and Analysis:

  • Express results as gene copies/g of tissue or soil

  • Statistical analysis accounting for environmental variation

  • Correlation with disease incidence/severity

Research has shown that ddPCR targeting the virA gene provides a rapid and sensitive quantification assay for Agrobacterium vitis in grapevine nursery stock and vineyard soil, with the ability to detect variations in bacterial populations across different sources and growing seasons .

How can researchers develop assays to study inhibition kinetics of ATP synthase containing atpB?

Developing assays to study inhibition kinetics requires specialized approaches:

• ATP Synthesis Assays:

  • Reconstituted System Setup:

    • Purify ATP synthase complex or reconstitute from subunits including atpB

    • Incorporate into liposomes with controlled orientation

    • Establish proton gradient using acid-base transition or K+/valinomycin

  • ATP Detection Methods:

    • Luciferin/luciferase luminescence (real-time)

    • NADP+/glucose-6-phosphate dehydrogenase coupled assay (continuous)

    • Radiolabeled 32P-Pi incorporation (endpoint)

  • Inhibitor Testing Protocol:

    • Pre-incubation optimization

    • Concentration range determination

    • Time-dependent inhibition analysis

• Proton Translocation Assays:

  • Fluorescent Probes:

    • ACMA (9-amino-6-chloro-2-methoxyacridine)

    • Pyranine

    • pH-sensitive GFP variants

  • Data Collection and Analysis:

    • Continuous monitoring of fluorescence changes

    • Initial rate calculations

    • Curve fitting for inhibitor concentration effects

• ATPase Activity Assays:

  • Phosphate Release Detection:

    • Malachite green assay

    • EnzChek phosphate assay

  • Coupled Enzyme Systems:

    • Pyruvate kinase/lactate dehydrogenase

    • NADH oxidation monitoring at 340 nm

• Inhibition Kinetics Analysis:

  • Data Fitting Models:

    • Competitive, non-competitive, uncompetitive inhibition

    • Allosteric inhibition models

    • Time-dependent inhibition kinetics

  • Parameters to Determine:

    • IC50 values

    • Ki values

    • Hill coefficients for cooperative binding

    • kon and koff rates when possible

• Structure-Function Correlation:

  • Test inhibitors against wild-type and mutant forms of atpB

  • Map inhibitor binding sites through resistance mutations

  • Correlate kinetic effects with structural features

Understanding Pi binding in ATP synthase can provide valuable insights for developing inhibition assays, as this process is crucial for the catalytic mechanism and may be targeted by inhibitors .

What techniques can be employed to study the membrane integration and assembly of atpB?

Studying membrane integration and assembly of atpB requires specialized techniques:

• In vitro Translation and Membrane Insertion:

  • Cell-free Translation Systems:

    • E. coli S30 extract

    • PURE system with defined components

    • Wheat germ extract

  • Membrane Addition:

    • Inverted membrane vesicles

    • Liposomes of defined composition

    • Nanodiscs

  • Insertion Detection:

    • Protease protection assays

    • Membrane fractionation

    • Fluorescence-based insertion reporters

• Topology Mapping Methods:

  • Cysteine Scanning Mutagenesis:

    • Introduction of single cysteines throughout the sequence

    • Accessibility testing with membrane-permeable and impermeable reagents

  • Reporter Fusion Approaches:

    • PhoA/LacZ dual reporter system

    • GFP fluorescence for cytoplasmic exposure

  • Mass Spectrometry Approaches:

    • Limited proteolysis of membrane-embedded protein

    • Surface labeling with MS-detectable reagents

• Assembly Process Analysis:

  • Pulse-Chase Experiments:

    • Radiolabeling of newly synthesized protein

    • Immunoprecipitation at various time points

    • Analysis of complex formation kinetics

  • Cross-linking During Assembly:

    • Time-resolved cross-linking

    • Identification of assembly intermediates

  • Blue Native PAGE:

    • Monitoring complex formation stages

    • Identification of subcomplexes

• Fluorescence Microscopy Approaches:

  • FRET Between Subunits:

    • Monitor assembly in real-time

    • Measure proximity between components

  • Fluorescence Recovery After Photobleaching (FRAP):

    • Assess mobility in the membrane

    • Determine if protein is in complex or free

• Genetic Approaches:

  • Assembly Mutant Characterization:

    • Create mutations in atpB or interacting partners

    • Assess impact on complex formation

    • Identify critical residues for assembly

These techniques can provide comprehensive insights into how atpB integrates into membranes and assembles with other ATP synthase subunits to form the functional complex essential for bacterial energy metabolism.