Recombinant Xanthobacter autotrophicus ATP synthase subunit a (atpB)

What is the structural composition of Xanthobacter autotrophicus ATP synthase?

Xanthobacter autotrophicus ATP synthase is a multi-subunit enzyme complex belonging to the F-type ATP synthase family. Like other bacterial F-ATP synthases, it consists of two major domains: the membrane-embedded F₀ sector and the catalytic F₁ sector. The complete holoenzyme typically follows an α₃β₃γδεab₂c₁₀-₁₅ stoichiometry, where subunit a (atpB) is part of the membrane-integrated F₀ sector. The subunit a plays a crucial role in proton translocation across the membrane, working in conjunction with the c-ring to convert the proton motive force into mechanical energy that drives ATP synthesis .

What is the amino acid sequence and structural characteristics of the atpB protein in X. autotrophicus?

The ATP synthase subunit a (atpB) in Xanthobacter autotrophicus (strain ATCC BAA-1158/Py2) is characterized by a 250-amino acid sequence. The full amino acid sequence is:

MTVDPIHQFEIKRYVDLLNFGGVQFSFTNAALFMFGIVAIIFFFLTFATRGRTLVPGRAQ SAAEMSYEFIAKMVRDSAGSEGMVFFPLVFSLFTFVLVSNVVGLIPYTFTVTAHLIVTAA MALLVIGTVIVYGFVRHGTHFLHLFVPSGVPAFLLPFLVVIEVVSFLSRPISLSLRLFAN mLAGHIALKVFAFFVVGLASAGVVGWFGATLPFFMIVALYALELLVAmLQAYVFAVLTSI YLNDAIHPGH

The protein contains multiple hydrophobic transmembrane helices as expected for a membrane-embedded component. These transmembrane domains are critical for creating the proton channel necessary for ATP synthesis.

How does recombinant atpB differ from its native form in X. autotrophicus?

Recombinant Xanthobacter autotrophicus ATP synthase subunit a (atpB) is produced in heterologous expression systems using molecular cloning techniques. While the amino acid sequence remains identical to the native protein, several differences may exist:

• The recombinant protein often includes affinity tags (such as His-tag) to facilitate purification

• Post-translational modifications present in the native protein may be absent or different in the recombinant version depending on the expression host

• The recombinant protein may have altered folding characteristics due to differences in the cellular environment of the expression host

• Stability and solubility can differ between recombinant and native forms due to the absence of natural binding partners

These differences must be considered when designing experiments with recombinant atpB, particularly for structural and functional studies.

What expression systems are most effective for producing recombinant X. autotrophicus atpB?

Based on similar membrane protein expression studies, the following expression systems can be effective for producing recombinant Xanthobacter autotrophicus atpB:

For membrane proteins like atpB, optimization of detergent conditions is critical during purification to maintain native-like structure and function. Screening multiple detergents (DDM, LMNG, etc.) is recommended to determine optimal solubilization conditions .

How does X. autotrophicus atpB function in relation to the ATP synthesis mechanism?

The atpB subunit in X. autotrophicus ATP synthase functions as an essential component of the proton-conducting pathway in the F₀ sector. Like in other bacteria, it works by:

• Creating a half-channel structure at the interface with the c-ring

• Facilitating proton movement from the periplasmic space to the c-ring binding sites

• Enabling the rotation of the c-ring through proton translocation

• Coordinating with other F₀ components to convert proton motive force into rotational energy

The unique amino acid composition of X. autotrophicus atpB may contribute to specific functional characteristics adapted to the bacterium's ecological niche. While no specific functional studies of X. autotrophicus atpB are directly available from the search results, insights can be drawn from related bacterial ATP synthases, where the a subunit contains conserved charged residues critical for proton translocation .

What are the recommended approaches for studying atpB-protein interactions in X. autotrophicus?

For investigating atpB-protein interactions in X. autotrophicus ATP synthase, the following methodological approaches are recommended:

• Cross-linking studies: Using chemical cross-linkers with various spacer lengths to identify interaction partners. This approach was successfully used in studies of mycobacterial ATP synthase to establish proximity relationships between subunits .

• Co-immunoprecipitation with recombinant components: Expressing tagged versions of atpB and potential binding partners, followed by pull-down assays.

• Two-hybrid systems adapted for membrane proteins: Modified bacterial or yeast two-hybrid systems designed specifically for membrane protein interactions.

• Surface Plasmon Resonance (SPR): For quantitative measurement of binding kinetics between purified atpB and other subunits.

• Hydrogen-deuterium exchange mass spectrometry (HDX-MS): To identify regions involved in protein-protein interactions by measuring changes in deuterium uptake upon complex formation.

When designing these experiments, it's important to consider the membrane-embedded nature of atpB and to use appropriate detergents or nanodiscs to maintain proper protein folding and accessibility .

How can researchers effectively purify functional recombinant X. autotrophicus atpB protein?

Purification of membrane proteins like atpB presents unique challenges. A recommended workflow for obtaining functional recombinant X. autotrophicus atpB includes:

• Membrane fraction isolation:

  • Culture cells expressing recombinant atpB

  • Disrupt cells by sonication or French press

  • Separate membrane fraction by ultracentrifugation

• Solubilization optimization:

  • Screen detergents (DDM, LMNG, UDM) at various concentrations

  • Add cardiolipin (0.1-0.5 mg/ml) to stabilize the protein

  • Incubate with gentle rotation at 4°C for 1-2 hours

• Affinity chromatography:

  • Use immobilized metal affinity chromatography (IMAC) for His-tagged protein

  • Include detergent at CMC level in all buffers

  • Add glycerol (10-20%) to enhance stability

• Size exclusion chromatography:

  • Remove aggregates and further purify the protein

  • Assess oligomeric state and homogeneity

• Verification of functionality:

  • Reconstitute into liposomes or nanodiscs

  • Perform proton translocation assays using pH-sensitive fluorescent dyes

This protocol draws on approaches used for other bacterial ATP synthase components, adapting them to the specific characteristics of X. autotrophicus atpB .

What methods are available for analyzing the membrane topology of atpB in X. autotrophicus?

Several complementary methods can be used to analyze the membrane topology of atpB in X. autotrophicus:

• In silico prediction: Using topology prediction algorithms (TMHMM, Phobius, TOPCONS) to generate initial models based on the amino acid sequence.

• Cysteine scanning mutagenesis: Introducing cysteine residues at various positions and testing their accessibility to membrane-impermeable thiol-reactive reagents.

• Fluorescence quenching: Labeling specific sites with fluorophores and measuring quenching to determine exposure to the aqueous environment.

• Protease protection assays: Exposing membrane vesicles to proteases and identifying protected fragments by mass spectrometry.

• Electron microscopy with gold labeling: Using antibodies against specific epitopes coupled to gold particles to visualize the positioning of domains.

• EPR spectroscopy: Incorporating spin labels at specific sites to determine their microenvironment and accessibility.

By combining these approaches, researchers can develop a comprehensive model of how atpB spans the membrane and identify critical functional domains .

How does the regulatory mechanism of ATP hydrolysis in X. autotrophicus compare to other bacterial species like Mycobacterium tuberculosis and Acinetobacter baumannii?

Regulation of ATP hydrolysis in F-ATP synthases varies significantly across bacterial species, representing evolutionary adaptations to different energetic requirements. Comparative analysis reveals:

• Mycobacterium tuberculosis: Contains a unique 36-amino acid C-terminal extension in subunit α (Mtα(514–549)) that suppresses ATP hydrolysis activity. Removal of this extension increases ATP hydrolysis by 63% and H⁺-pumping by 10%. This C-terminal domain interacts with subunit γ residues 104-109, influencing the rotation of this camshaft-like subunit and suppressing reverse ATP hydrolysis function .

• Acinetobacter baumannii: Exhibits latent ATPase activity primarily regulated by subunit ε. An ε-free AbF₁-αβγ complex showed a 21.5-fold increase in ATP hydrolysis. The C-terminal domain of subunit ε in an extended position plays a crucial role in this inhibition .

• Xanthobacter autotrophicus: While specific regulatory mechanisms for X. autotrophicus are not directly described in the search results, based on its phylogenetic positioning, it likely employs regulatory strategies that may combine elements from other α-proteobacteria. Given the importance of energy conservation in nitrogen-fixing bacteria like X. autotrophicus, it likely has evolved specific regulatory mechanisms to prevent futile ATP hydrolysis.

This comparative analysis suggests that bacterial F-ATP synthases have evolved diverse mechanisms to regulate ATP hydrolysis, with specific adaptations reflecting the metabolic requirements of each organism. Future research should focus on identifying the specific regulatory elements in X. autotrophicus atpB and their interactions with other subunits .

What is the relationship between atpB structure/function and the nitrogen-fixing capabilities of X. autotrophicus?

Xanthobacter autotrophicus is known for its ability to fix atmospheric nitrogen, a highly energy-intensive process. The ATP synthase, including the atpB subunit, plays a critical role in this process through several mechanisms:

• Energy provision for nitrogenase: Nitrogen fixation by X. autotrophicus requires substantial ATP input, with approximately 16 ATP molecules consumed per N₂ molecule reduced. The ATP synthase complex, with atpB as an essential component, is responsible for generating this ATP supply .

• Integration with hydrogen metabolism: X. autotrophicus can couple hydrogen oxidation to nitrogen fixation. The ATP synthase works in concert with hydrogenases to maximize energy conservation during this process. The search results indicate that X. autotrophicus can utilize hydrogen from catalytic water splitting to drive nitrogen fixation, producing ammonia that can support plant growth (increasing radish storage root mass by up to 1,440%) .

• Adaptation to microaerobic conditions: During nitrogen fixation, X. autotrophicus often operates under microaerobic conditions to protect the oxygen-sensitive nitrogenase. The ATP synthase must be optimized to function efficiently under these conditions, potentially requiring specific adaptations in the proton-conducting pathway involving atpB.

• Potential regulatory connections: The ATP/ADP ratio regulated by ATP synthase may serve as a signal for nitrogen fixation activity, linking energy status to nitrogen metabolism.

While direct evidence of atpB-specific adaptations for nitrogen fixation is not presented in the search results, the protein's fundamental role in energy generation positions it as a key player in supporting this metabolically demanding process .

How can site-directed mutagenesis of X. autotrophicus atpB be designed to investigate critical functional residues?

Site-directed mutagenesis of X. autotrophicus atpB represents a powerful approach to probe structure-function relationships. Based on studies of related ATP synthases, the following experimental design is recommended:

• Target residue selection strategy:

  a) Conserved charged residues in predicted transmembrane regions: These often participate directly in proton translocation

  b) Residues at the interface with c-ring: Critical for proton transfer from the a subunit to the c-ring

  c) Regions homologous to regulatory domains identified in M. tuberculosis and A. baumannii

• Mutation design principles:

  a) Conservative substitutions (e.g., Arg→Lys) to assess importance of specific chemical properties

  b) Non-conservative substitutions (e.g., Arg→Ala) to completely abolish function

  c) Cysteine substitutions compatible with subsequent labeling studies

• Functional analysis workflow:

  a) Expression validation by Western blotting

  b) Assembly verification by blue native PAGE

  c) ATP synthesis activity measurements in membrane vesicles

  d) Proton translocation assays using pH-sensitive fluorophores

• Specific mutation targets:

  Region	Target Residues	Mutation Strategy	Expected Outcome
  TM2-4 interface	Arg76, Glu219	R76K, R76A, E219Q, E219A	Altered proton translocation
  c-ring interface	Gln252, Asn255	Q252L, N255A	Disrupted a-c subunit interaction
  C-terminal domain	230-250 region	Truncations	Modified regulatory properties

This systematic mutagenesis approach, combined with functional assays, can provide detailed insights into the proton translocation mechanism and regulatory features specific to X. autotrophicus ATP synthase .

What are the methodological considerations for using cryo-EM to determine the structure of X. autotrophicus ATP synthase with focus on atpB?

Cryo-electron microscopy (cryo-EM) has emerged as a powerful technique for structural determination of membrane protein complexes like ATP synthases. For X. autotrophicus ATP synthase with focus on atpB, the following methodological considerations are critical:

• Sample preparation optimization:

  • Utilize amphipol A8-35 or LMNG as detergents, which have proven successful for other bacterial ATP synthases

  • Consider nanodisc reconstitution to better mimic the lipid bilayer environment

  • Implement GraFix (gradient fixation) methodology to stabilize the complex

  • Screen multiple buffer conditions with emphasis on pH 7.5-8.0 and presence of 2-5 mM MgCl₂

• Data collection strategy:

  • Use a Titan Krios microscope with energy filter and K3 direct electron detector

  • Employ beam-tilt data collection for aberration correction

  • Implement image shift methodology for increased throughput

  • Collect data in counting mode with dose fractionation (40-50 frames, total dose ~40-50 e⁻/Ų)

• Processing pipeline recommendations:

  • Use motion correction with dose weighting (MotionCor2)

  • Perform CTF estimation with CTFFIND4 or GCTF

  • Implement 2D classification to eliminate damaged particles

  • Use 3D variability analysis to address conformational heterogeneity

  • Apply focused refinement on the F₀ region containing atpB

• Validation approaches:

  • Cross-validate with biophysical data from cross-linking experiments

  • Perform tilt-pair analysis

  • Assess local resolution distribution across the structure

  • Compare with homologous structures from related bacteria

Based on the success with A. baumannii F₁-ATPase (3.0 Å resolution), similar approaches should be applicable to X. autotrophicus ATP synthase, though additional stabilization may be required to overcome preferential orientation issues commonly encountered with membrane proteins .

How can researchers investigate the potential role of X. autotrophicus atpB as a drug target for developing new antimicrobials?

While X. autotrophicus itself is not pathogenic, investigating its ATP synthase as a model system can provide valuable insights for developing antimicrobials targeting homologous proteins in pathogenic bacteria. A systematic approach would include:

• Comparative analysis with pathogenic species:

  • Identify structural and functional differences between X. autotrophicus atpB and homologs in pathogenic bacteria

  • Focus on unique features that could be exploited for selective targeting

  • Determine conservation patterns across species to predict spectrum of activity

• Essential binding pocket identification:

  • Use structural data combined with computational approaches to identify potential drug-binding sites

  • Focus particularly on regions involved in proton translocation

  • Consider the unique C-terminal extensions identified in other bacterial ATP synthases as potential targeting sites

• High-throughput screening approach:

  • Develop activity assays suitable for screening compound libraries

  • Implement membrane potential-sensitive fluorescent reporters

  • Design ATP synthesis/hydrolysis coupled assays in reconstituted systems

• Structure-guided drug design:

  • Utilize homology models based on solved structures of related ATP synthases

  • Implement molecular dynamics simulations to identify transient binding pockets

  • Develop fragments targeting the interface between atpB and other subunits

• Validation in pathogenic models:

  • Test compounds against ATP synthases from pathogenic species

  • Assess specificity relative to human mitochondrial ATP synthase

  • Evaluate potential for resistance development

The Mycobacterium-specific C-terminal domain (Mtα(514–549)) has been identified as a promising drug epitope, and similar unique regions in X. autotrophicus atpB could represent valuable targets for antimicrobial development. The advantage of targeting such regions is that they may provide species selectivity while affecting the essential function of ATP synthesis .

How can X. autotrophicus atpB be utilized in bioenergetic engineering for enhanced nitrogen fixation applications?

X. autotrophicus has significant potential for bioenergetic engineering applications, particularly in enhancing nitrogen fixation capabilities. Several approaches targeting atpB can be considered:

• Optimizing ATP synthesis efficiency:

  • Engineering atpB to improve proton translocation efficiency

  • Modifying regulatory elements to increase ATP output under nitrogen-fixing conditions

  • Adjusting c-ring/a-subunit interactions to optimize the H⁺/ATP ratio

• Hybrid systems development:

  • The search results describe a hybrid inorganic-biological system where X. autotrophicus couples hydrogen generation from catalytic water splitting to nitrogen and CO₂ reduction

  • Engineering atpB to better interface with artificial electron donors could enhance this system

  • Optimizing ATP synthase to function under the specific conditions of such hybrid systems

• Agricultural applications:

  • Direct application of engineered X. autotrophicus as a biofertilizer

  • The search results indicate significant improvements in plant growth (up to 1,440% increase in radish storage root mass)

  • Enhancing ATP synthesis through atpB modifications could further improve nitrogen fixation capacity

• Multi-enzyme pathway coordination:

  • Coordinating ATP synthase activity with nitrogenase and hydrogenase functions

  • Engineering regulatory interfaces between these enzyme systems

  • Developing synthetic regulatory circuits linking energy production to nitrogen fixation

These applications represent promising directions for sustainable agriculture and bioenergy production, leveraging the natural capabilities of X. autotrophicus enhanced through targeted engineering of its bioenergetic machinery .

What are the most promising techniques for studying the rotary mechanism of ATP synthase with focus on atpB's role in X. autotrophicus?

Investigating the rotary mechanism of X. autotrophicus ATP synthase requires sophisticated methodologies focused on capturing dynamic processes. The most promising techniques include:

• Single-molecule rotation assays:

  • Attach fluorescent beads or gold nanoparticles to the rotating γ subunit

  • Immobilize the α₃β₃ hexamer on a surface

  • Track rotation using high-speed cameras or dark-field microscopy

  • These techniques have successfully revealed that the C-terminal extension of subunit α in M. tuberculosis decreases the angular velocity of the power-stroke after ATP binding

• Fluorescence resonance energy transfer (FRET):

  • Label specific sites on atpB and interacting subunits with FRET pairs

  • Monitor distance changes during catalysis in real-time

  • Develop a FRET sensor system to detect conformational changes associated with proton translocation

• Time-resolved cryo-EM:

  • Use microfluidic mixing devices to capture different states of the rotary cycle

  • Apply 3D classification to separate distinct conformational states

  • Develop movie-mode analysis to visualize the trajectory of the rotary motion

• Molecular dynamics simulations:

  • Develop atomistic models of the complete X. autotrophicus ATP synthase

  • Simulate proton movement through the a/c interface

  • Calculate energy profiles for the complete rotary cycle

• Site-specific cross-linking with engineered photosensitive amino acids:

  • Incorporate photoreactive amino acids at key positions in atpB

  • Trigger cross-linking at defined time points during catalysis

  • Identify cross-linked products to map dynamic interactions

By combining these approaches, researchers can develop a comprehensive understanding of how atpB contributes to the conversion of proton flow into rotary motion and ultimately ATP synthesis in X. autotrophicus .

How does the proton-conducting pathway in X. autotrophicus atpB compare with other bacterial and mitochondrial ATP synthases?

The proton-conducting pathway is a critical functional element in all F-type ATP synthases, with evolutionary adaptations across different organisms. A comparative analysis reveals:

The search results highlight that bacterial ATP synthases have evolved diverse mechanisms to regulate proton conduction and ATP hydrolysis/synthesis, with pathogenic bacteria often featuring additional regulatory elements that prevent wasteful ATP hydrolysis. While specific information about X. autotrophicus is limited in the search results, its environmental niche suggests potential adaptations for energy efficiency during nitrogen fixation .

What is the potential impact of studying X. autotrophicus atpB on understanding bacterial adaptation to environmental stresses?

Studying X. autotrophicus atpB can provide significant insights into bacterial adaptation mechanisms for several reasons:

• Adaptations to energy fluctuations:

  • X. autotrophicus thrives in environments with variable nutrient availability

  • ATP synthase regulation through atpB likely plays a key role in energy conservation during stress

  • Understanding these adaptations can reveal fundamental principles of bacterial energy management

• Environmental remediation capabilities:

  • X. autotrophicus can degrade halogenated compounds as shown in search result

  • The search results mention "degradation of halogenated aliphatic compounds by Xanthobacter autotrophicus GJ10" and its ability to utilize "halogenated short-chain hydrocarbons" as carbon sources

  • ATP synthesis efficiency during xenobiotic metabolism may involve specialized adaptations in atpB

• Nitrogen fixation under changing conditions:

  • As a nitrogen-fixing bacterium, X. autotrophicus must balance the high energy demands of nitrogenase with variable environmental conditions

  • ATP synthase function, particularly the proton-conducting mechanism involving atpB, likely reflects adaptations to optimize this balance

  • The search results describe X. autotrophicus in a hybrid inorganic-biological system for ambient nitrogen reduction

• Comparative insights across species:

  • Comparing atpB from X. autotrophicus with homologs from other bacteria adapted to different stresses can reveal convergent and divergent evolutionary strategies

  • The unique regulatory elements found in M. tuberculosis and A. baumannii ATP synthases suggest that similar adaptations might exist in X. autotrophicus atpB

These studies can inform broader questions about bacterial evolution and adaptation while potentially yielding applications in bioremediation, sustainable agriculture, and industrial biotechnology .