Recombinant Sulfurihydrogenibium sp. ATP synthase subunit b (atpF)

What is ATP synthase subunit b (atpF) from Sulfurihydrogenibium sp. and what is its significance in research?

ATP synthase subunit b (atpF) from Sulfurihydrogenibium sp. is a component of the membrane-embedded F₀ region of ATP synthase, specifically part of the peripheral stalk (PS). This protein plays a crucial role in energy metabolism by helping to anchor the catalytic F₁ domain to the membrane, contributing to the stability of the ATP synthase complex.

The significance of studying this protein stems from several factors:

• It comes from extremophile bacteria (Sulfurihydrogenibium sp.) that thrive in high-temperature environments

• Its thermostable properties make it valuable for understanding protein stability mechanisms

• It serves as a model for investigating evolutionary adaptations in energy-generating systems

• The peripheral stalk components like subunit b are essential for maintaining proper ATP synthase structure and function

How does Sulfurihydrogenibium sp. ATP synthase subunit b compare to ATP synthase components from other organisms?

Comparative analysis reveals several notable differences:

Unlike human mitochondrial ATP synthase, which contains 29 proteins of 18 kinds organized into various modules , the bacterial ATP synthase from Sulfurihydrogenibium has a simpler subunit composition while maintaining the core functional elements. This makes it valuable for studying the minimal requirements for ATP synthase function.

Interestingly, phylogenetic analysis based on RNase P RNA places Sulfurihydrogenibium with green sulfur, cyanobacterial, and δ/ε proteobacterial branches rather than as the deepest bacterial lineage as suggested by 16S rRNA phylogeny .

What are the recommended protocols for expression and purification of recombinant Sulfurihydrogenibium sp. ATP synthase subunit b?

Based on established methodologies for similar proteins, the following protocol is recommended:

• Expression System Setup:

  • Clone the atpF gene into an expression vector such as pET-based systems

  • Transform into a suitable E. coli strain (BL21(DE3) or similar)

  • Culture in LB medium with appropriate antibiotic selection (typically 50 μg/mL kanamycin)

• Protein Expression:

  • Grow cultures at 37°C until OD₆₀₀ reaches 0.6

  • Induce with IPTG at a final concentration of 0.5 mM

  • Continue incubation at a lower temperature (30°C) for 6 hours to optimize protein folding

• Cell Harvest and Lysis:

  • Harvest cells by centrifugation (12,000 × g)

  • Resuspend in a suitable buffer containing protease inhibitors (e.g., 50 mM Tris-HCl, pH 8.0, 0.3 M NaCl, 0.1% Triton X-100, 1 M NH₄Cl, 40 μg/mL PMSF)

  • Lyse cells by sonication (e.g., 15 minutes on ice using a Branson Sonifier or equivalent)

• Purification Strategy:

  • For His-tagged protein, use Ni-NTA agarose affinity chromatography

  • Wash with buffer containing 30 mM imidazole

  • Elute with buffer containing 0.3 M imidazole

  • Dialyze against a storage buffer (e.g., 50 mM Tris-HCl, pH 7.0, 0.1 M NaCl, 10% glycerol)

• Quality Control:

  • Verify purity by SDS-PAGE (expect a band at approximately 20 kDa)

  • Confirm identity by Western blot or mass spectrometry

  • Assess functional integrity through appropriate activity assays

For optimal results, storage in Tris-based buffer with 50% glycerol at -20°C or -80°C is recommended, avoiding repeated freeze-thaw cycles .

What experimental approaches can be used to study ATP synthase assembly involving subunit b?

Several complementary experimental approaches can be employed:

• Genetic Knockout and Complementation Studies:

  • Generate knockout cell lines lacking individual subunits (Δb, Δe, Δf, Δg, Δδ)

  • Analyze the formation of subcomplexes using BN-PAGE and immunoblotting

  • Complement with wild-type or mutated versions to identify critical regions

• Protein-Protein Interaction Analysis:

  • Use co-immunoprecipitation to identify interacting partners

  • Apply crosslinking mass spectrometry to map interaction interfaces

  • Employ FRET-based approaches to study dynamic interactions in live cells

• Structural Biology Techniques:

  • Cryo-EM analysis of intact ATP synthase complexes

  • X-ray crystallography of subcomplexes (e.g., b-e-g-f)

  • NMR studies of isolated domains to determine atomic-level details

• Assembly Pathway Elucidation:

  • Pulse-chase labeling with metabolic incorporation of radioactive amino acids

  • Time-course analysis of complex formation using native PAGE

  • Investigation of assembly intermediates using density gradient centrifugation

Research has shown that ATP synthase assembly can proceed through multiple pathways. For example, studies have identified at least three alternative paths for introducing the peripheral stalk (PS) module into the ATP synthase complex:

• Formation of a b-e-g complex that binds subunit f to form a b-e-g-f subcomplex

• Association of an e-g subcomplex with an F₁-c₈ subcomplex already containing OSCP, b, d, and F₆

• Association of the F₁ domain with a fully assembled PS followed by addition of the c-ring

What methods are recommended for assessing the thermostability of Sulfurihydrogenibium sp. ATP synthase subunit b?

To evaluate the thermostability of recombinant Sulfurihydrogenibium sp. ATP synthase subunit b, researchers should employ a multi-technique approach:

• Differential Scanning Calorimetry (DSC):

  • Determine the melting temperature (Tm) by monitoring heat absorption during protein unfolding

  • Compare thermal transition profiles at different pH values and salt concentrations

  • Analyze the impact of ligands or interacting proteins on thermal stability

• Circular Dichroism (CD) Spectroscopy:

  • Monitor secondary structure changes as a function of temperature

  • Perform thermal melting curves from 25°C to 95°C

  • Determine the temperature at which 50% of the protein is unfolded (T₅₀)

• Intrinsic Fluorescence Spectroscopy:

  • Exploit the natural fluorescence of tryptophan residues

  • Track structural changes during thermal denaturation

  • Calculate thermodynamic parameters of unfolding

• Activity-Based Assays:

  • Assess functional integrity at different temperatures

  • Measure enzymatic activity or binding capacity after thermal challenge

  • Determine the temperature optimum for protein function

• Thermal Shift Assays:

  • Use fluorescent dyes (e.g., SYPRO Orange) that bind to hydrophobic regions exposed during unfolding

  • Generate melting curves using real-time PCR instruments

  • Compare stability in different buffer conditions

For context, RNase P RNAs from related Aquificales (S. azorense and P. marina) have been shown to be more thermostable than E. coli P RNA and require higher temperatures for proper folding, demonstrating the thermophilic nature of proteins from these organisms .

How does the b subunit of ATP synthase from Sulfurihydrogenibium sp. contribute to the assembly and stability of the ATP synthase complex?

The b subunit of ATP synthase from Sulfurihydrogenibium sp. plays multiple crucial roles in the assembly and stability of the complete ATP synthase complex:

Studies with vestigial ATPase complexes formed by disruption of genes for individual subunits have revealed that when the b subunit is deleted, the entire peripheral stalk fails to form, demonstrating its indispensable role in complex assembly. Additionally, the b-e-g-f subcomplex has been identified as an important assembly intermediate that can form even in the absence of an assembled F₁ domain .

What are the molecular adaptations in Sulfurihydrogenibium sp. ATP synthase subunit b that contribute to its thermostability?

The thermostability of Sulfurihydrogenibium sp. ATP synthase subunit b likely results from several molecular adaptations common to proteins from thermophilic organisms:

• Primary Sequence Characteristics:

  • Higher proportion of hydrophobic and charged amino acids

  • Increased number of ionic interactions (salt bridges)

  • Reduced number of thermolabile residues (Asn, Gln, Cys, Met)

  • Strategic placement of proline residues to enhance rigidity

• Secondary Structure Elements:

  • Enhanced α-helical content, particularly in the soluble domain

  • More compact folding with shorter loop regions

  • Specialized structural motifs that provide thermal resistance

• Tertiary Structure Stabilization:

  • Additional intramolecular hydrogen bonding networks

  • Increased hydrophobic packing in the protein core

  • Optimized electrostatic interactions on the protein surface

• Quaternary Structure Contributions:

  • Enhanced subunit interface interactions

  • Formation of more extensive contact surfaces with partner proteins

  • Stabilizing interactions within the b-e-g-f subcomplex

Research on related thermostable proteins from Aquificales has identified structural idiosyncrasies that determine folding properties. For instance, RNase P RNAs from S. azorense and P. marina lack helix P18 (one of three major interdomain tertiary contacts) yet demonstrate greater thermostability than E. coli P RNA . Similar specialized structural adaptations likely exist in the ATP synthase subunit b that enable function at elevated temperatures characteristic of the Sulfurihydrogenibium natural habitat.

How can recombinant Sulfurihydrogenibium sp. ATP synthase subunit b be used as a model for studying pathogenic mutations in human ATP synthase?

Recombinant Sulfurihydrogenibium sp. ATP synthase subunit b can serve as a valuable model system for studying pathogenic mutations in human ATP synthase through several research approaches:

• Comparative Structural Analysis:

  • Identify conserved functional domains between bacterial and human homologs

  • Map pathogenic mutations from human ATP synthase onto corresponding regions of the bacterial protein

  • Use the simplified bacterial system to isolate effects of specific mutations

• Structure-Function Relationship Studies:

  • Introduce equivalent human disease-associated mutations into the bacterial subunit

  • Assess the impact on protein stability, assembly, and function

  • Determine molecular mechanisms underlying pathogenicity

• Suppressor Mutation Screening:

  • Identify secondary mutations that can rescue function of primary pathogenic mutations

  • Develop potential therapeutic strategies based on compensatory mechanisms

  • Test hypotheses about structural constraints in a simplified system

• Thermostability as a Diagnostic Tool:

  • Use the inherent thermostability of the bacterial protein to develop sensitivity assays

  • Evaluate the destabilizing effects of mutations on protein folding and complex assembly

  • Correlate thermal stability changes with disease severity

This approach is supported by previous work with human mitochondrial ATP synthase, where mutations in subunit 6 associated with neurological muscle weakness, ataxia, and retinitis pigmentosa (NARP) syndrome have been studied. These mutations led to decreased ATP synthesis capacity, abnormal levels of ATP synthase sub-complexes, altered assembly, and instability of the holoenzyme . Similarly, ATP synthase subunit-β has been implicated in diabetic nephropathy, with ATP5b potentially playing a protective role against renal fibrosis .

The bacterial system offers advantages of simplified genetics, easier protein production, and better experimental tractability while still providing insights into fundamental mechanisms that can be translated to human disease contexts.

What are common challenges in expressing and purifying Sulfurihydrogenibium sp. ATP synthase subunit b and how can they be addressed?

Researchers frequently encounter several challenges when working with recombinant Sulfurihydrogenibium sp. ATP synthase subunit b:

• Protein Solubility Issues:

  • Challenge: Formation of inclusion bodies due to hydrophobic regions

  • Solution: Express at lower temperatures (18-25°C), use solubility-enhancing tags (SUMO, MBP), or optimize buffer conditions with mild detergents

• Protein Stability Concerns:

  • Challenge: Degradation during purification

  • Solution: Include protease inhibitors (e.g., PMSF at 40 μg/mL), maintain samples at 4°C, add stabilizing agents like glycerol (10-50%)

• Purification Efficiency:

  • Challenge: Co-purification of contaminants

  • Solution: Implement multi-step purification strategies combining affinity chromatography with ion exchange or size exclusion chromatography

• Proper Folding Verification:

  • Challenge: Ensuring native conformation after recombinant expression

  • Solution: Perform circular dichroism analysis, verify activity through functional assays, or assess binding to known interaction partners

• Tag Interference:

  • Challenge: Fusion tags affecting protein function or structure

  • Solution: Include a cleavable linker between the tag and protein, compare activity before and after tag removal, or try different tag positions (N- vs C-terminal)

For example, in experiments with similar proteins, researchers have successfully used a strategy involving initial sonication in buffer containing 50 mM Tris-HCl (pH 8.0), 0.3 M NaCl, 0.1% Triton X-100, and 1 M NH₄Cl, followed by Ni-NTA agarose purification and dialysis against 50 mM Tris-HCl (pH 7.0), 0.1 M NaCl, and 10% glycerol to obtain pure, functional protein .

How can researchers effectively design experiments to study the functional interaction between ATP synthase subunit b and other components of the ATP synthase complex?

To investigate functional interactions between ATP synthase subunit b and other components of the complex, researchers should consider the following experimental design strategies:

• Site-Directed Mutagenesis Approaches:

  • Systematically mutate conserved residues at predicted interface regions

  • Create truncation variants to identify minimal binding domains

  • Introduce cysteine residues for crosslinking studies at hypothesized interaction sites

• Reconstitution Experiments:

  • Develop in vitro assembly systems using purified components

  • Reconstitute partial complexes (e.g., b-e-g-f) to study assembly steps

  • Assess functional properties of reconstituted complexes through ATP synthesis/hydrolysis assays

• Real-Time Binding Assays:

  • Employ surface plasmon resonance (SPR) to determine binding kinetics

  • Use microscale thermophoresis (MST) to measure affinities in solution

  • Apply isothermal titration calorimetry (ITC) for thermodynamic characterization

• Co-Evolutionary Analysis:

  • Identify co-evolving residue pairs between subunit b and interaction partners

  • Design mutations that disrupt or restore predicted co-evolutionary networks

  • Validate interaction models through functional recovery experiments

• Experimental Controls and Validation:

  • Include appropriate negative controls (non-interacting proteins)

  • Use both gain-of-function and loss-of-function approaches

  • Validate key findings with multiple independent techniques

Research has identified three alternative pathways for introducing the peripheral stalk module (including subunit b) into ATP synthase assembly. These pathways involve different sequences of interactions between components, suggesting flexibility in the assembly process . Experimental designs should account for this complexity by examining multiple potential assembly routes.

What are the most appropriate statistical approaches and experimental designs for comparative studies of wildtype versus mutant forms of ATP synthase subunit b?

When conducting comparative studies between wildtype and mutant forms of ATP synthase subunit b, researchers should implement robust statistical approaches and experimental designs:

• Recommended Experimental Designs:

  • Randomized Complete Block Design: To control for day-to-day or batch-to-batch variation

  • Factorial Design: To evaluate multiple factors simultaneously (e.g., temperature, pH, mutation type)

  • Time-Series Experiment: For studying dynamic processes like assembly or stability over time

• Control Strategies:

  • Include positive controls (known functional variants)

  • Incorporate negative controls (known non-functional variants)

  • Use reference proteins (unrelated proteins subjected to identical conditions)

  • Implement internal standards for normalization

• Statistical Analysis Methods:

  • ANOVA: For comparing multiple groups with post-hoc tests (Tukey's HSD, Bonferroni)

  • Repeated Measures ANOVA: For time-course experiments

  • Non-parametric tests: When normality assumptions aren't met (Mann-Whitney, Kruskal-Wallis)

  • Regression analysis: For dose-response or continuous variable relationships

• Sample Size Considerations:

  • Conduct power analysis to determine appropriate sample sizes

  • Use biological replicates (independent protein preparations)

  • Include technical replicates (repeated measurements of the same sample)

• Data Presentation Guidelines:

  • Report both effect sizes and statistical significance

  • Use appropriate data visualization (box plots, scatter plots with error bars)

  • Clearly indicate variability (standard deviation, standard error, confidence intervals)

For example, when studying thermostability differences, temperatures should be systematically varied and the resulting data may be analyzed using nonlinear regression to determine melting temperatures (Tm). Statistical comparison of Tm values between wildtype and mutant proteins can then be performed using appropriate t-tests or ANOVA, depending on the number of variants being compared .

What are emerging research areas and unanswered questions regarding Sulfurihydrogenibium sp. ATP synthase subunit b?

Several promising research directions remain to be explored:

• Structural Adaptations for Thermostability:

  • Determining the atomic-level basis for thermostability through high-resolution structures

  • Identifying unique structural features compared to mesophilic homologs

  • Mapping the network of stabilizing interactions within the protein

• Evolutionary Considerations:

  • Resolving the phylogenetic positioning of Aquificales based on ATP synthase sequences

  • Investigating horizontal gene transfer events that might have shaped ATP synthase evolution

  • Comparing Sulfurihydrogenibium sp. ATP synthase with other extremophiles from diverse environments

• Assembly Pathway Dynamics:

  • Characterizing the kinetics of assembly pathway alternatives

  • Identifying conditions that favor particular assembly routes

  • Determining whether assembly chaperones exist for thermophilic ATP synthases

  • Establishing whether assembly factors are required, as none have been found for human peripheral stalk assembly

• Functional Specializations:

  • Investigating whether thermophilic ATP synthases possess unique regulatory mechanisms

  • Determining ion specificity and transport characteristics under extreme conditions

  • Exploring potential additional functions beyond ATP synthesis

• Biotechnological Applications:

  • Developing thermostable hybrid ATP synthases for biotechnological applications

  • Creating sensor systems based on the thermostability properties

  • Utilizing structure-informed protein engineering to enhance desired properties

Notably, while the RNase P RNA and protein (rnpA) genes have been identified in Sulfurihydrogenibium azorense and Persephonella marina, neither of these genes has been found in the sequenced genome of their close relative, Aquifex aeolicus . Similar comparative genomic approaches may reveal interesting patterns in ATP synthase gene organization and regulation across the Aquificales order.

How might the study of Sulfurihydrogenibium sp. ATP synthase subunit b contribute to understanding human mitochondrial diseases?

Research on Sulfurihydrogenibium sp. ATP synthase subunit b has significant potential to enhance our understanding of human mitochondrial diseases through multiple avenues:

• Model System for Pathogenic Mutations:

  • Provides a simplified experimental platform for studying equivalent human mutations

  • Allows rapid screening of mutation effects on protein structure and function

  • Facilitates mechanistic studies divorced from the complexity of human cells

• Assembly Pathway Insights:

  • Elucidates fundamental principles of ATP synthase assembly relevant to human disease

  • Identifies critical checkpoints where disease mutations might disrupt assembly

  • Reveals alternative assembly pathways that might be therapeutic targets

  • Advances understanding of how assembly defects lead to clinical manifestations

• Structure-Function Relationships:

  • Defines essential structural elements conserved from bacteria to humans

  • Maps functional domains susceptible to pathogenic mutations

  • Identifies structural motifs critical for ATP synthase stability and function

• Potential Therapeutic Strategies:

  • Supports development of small molecules that could stabilize mutant proteins

  • Enables identification of critical interactions that could be therapeutically targeted

  • Provides insights for gene therapy approaches to correct ATP synthase deficiencies

Human mitochondrial diseases associated with ATP synthase defects include severe conditions such as NARP syndrome (Neurogenic muscle weakness, Ataxia, and Retinitis Pigmentosa), MILS (Maternally Inherited Leigh Syndrome), and various forms of mitochondrial encephalomyopathy. These conditions often involve mutations in ATP synthase subunits that alter assembly and stability . The bacterial system offers a tractable model to understand how such mutations disrupt the complex's structure and function.

What novel experimental techniques could advance research on thermostable ATP synthase components like those from Sulfurihydrogenibium sp.?

Several cutting-edge experimental techniques hold promise for advancing research on thermostable ATP synthase components:

• Cryo-Electron Tomography:

  • Enables visualization of ATP synthase in native membrane environments

  • Reveals structural organization and supramolecular arrangements

  • Provides insights into how thermostability manifests in cellular contexts

• Single-Molecule Techniques:

  • Single-molecule FRET to monitor conformational dynamics

  • Optical tweezers to measure mechanical properties and force generation

  • Nanodiscs technology for studying membrane proteins in defined lipid environments

• Advanced Computational Methods:

  • Molecular dynamics simulations at extended timescales to capture thermal adaptations

  • Machine learning approaches to predict thermostabilizing mutations

  • Quantum mechanics/molecular mechanics (QM/MM) to study catalytic mechanisms

• Synthetic Biology Approaches:

  • Creation of minimal ATP synthase complexes to define essential components

  • Design of chimeric ATP synthases combining thermophilic and mesophilic elements

  • Directed evolution to engineer enhanced thermostability or novel functions

• Integrated Structural Biology:

  • Combination of X-ray crystallography, cryo-EM, and NMR for complete structural characterization

  • Hydrogen-deuterium exchange mass spectrometry (HDX-MS) to probe protein dynamics

  • Cross-linking mass spectrometry to map interaction interfaces at amino acid resolution

• In Situ Methods:

  • Proximity labeling techniques (BioID, APEX) to identify interaction partners in vivo

  • Live-cell imaging with genetically encoded sensors to monitor ATP synthesis

  • In-cell NMR to study protein structure in cellular environments

Recent advances have improved our understanding of bacterial ATP synthase structure, as demonstrated by cryo-EM studies that have allowed building atomic models of complexes in different rotational states . Similar techniques applied to thermophilic ATP synthases could reveal unique structural adaptations that enable function at high temperatures.