The recombinant Morus indica ATP synthase subunit c, chloroplastic (atpH), is a genetically engineered version of a membrane-bound protein critical to photosynthetic ATP production in chloroplasts. This protein is part of the F₀ sector of the ATP synthase complex, which translocates protons across the thylakoid membrane to drive ATP synthesis via rotational catalysis .
The subunit c (atpH) assembles into a cylindrical cₙ ring in the F₀ sector, where n denotes the number of c-subunits per ring. This ring rotates during proton translocation, coupling mechanical energy to ATP synthesis in the F₁ sector . In plants, n typically ranges from 10–15, influencing the ATP/proton ratio (3.3–5.0) .
The atpH gene resides in the chloroplast genome of Morus indica. A comparative study of Morus species (M. atropurpurea, M. multicaulis) revealed conserved chloroplast genome structures, including genes like atpH, which encode subunit c .
Recombinant atpH is typically expressed using codon-optimized sequences in E. coli, leveraging fusion tags (e.g., maltose-binding protein) to enhance solubility . After cleavage of the tag, the protein is purified via reversed-phase chromatography, yielding high-purity (>90%) material .
Structural Studies: Enables investigation of c-ring assembly and stoichiometric variability (n) .
Functional Assays: Used to study proton translocation efficiency and ATP synthesis coupling .
In spinach, recombinant subunit c₁ was expressed as a soluble MBP-c₁ fusion, purified, and confirmed to retain native α-helical structure . This approach informs production methods for Morus indica subunit c.
Mammals express isoforms (P1, P2, P3) with distinct mitochondrial targeting peptides, influencing respiratory chain maintenance . While Morus indica subunit c lacks such isoforms, its targeting peptides may similarly modulate chloroplast functionality.
ATP synthase subunit c forms the critical c-ring structure within the F0 sector of the ATP synthase complex. In chloroplasts, this transmembrane component creates a rotary motor driven by proton flow across the thylakoid membrane. The c-ring rotation mechanically couples to the F1 sector to catalyze ATP synthesis from ADP and inorganic phosphate.
In Morus indica, the chloroplastic ATP synthase subunit c (atpH) is an 81 amino acid protein with the sequence: MNPLISAASVIAAGLAVGLASIGPGVGQGTAAGQAVEGIARQPEAEGKIRGTLLLSLAFMEALTIYGLVVALALLFANPFV . This highly hydrophobic protein contains transmembrane domains that assemble into an oligomeric ring structure, with each c-subunit capable of binding and transporting one proton during rotation.
The protein functions as part of the ATP synthase complex, which serves as the main enzyme in the ATP biosynthetic pathway and photosynthesis . Its structure is critical for maintaining the proper proton-to-ATP ratio, which in plant chloroplasts is typically 4.6 (14 protons for 3 ATP molecules) .
The recombinant version of Morus indica ATP synthase subunit c is produced with an N-terminal His-tag in E. coli expression systems . This creates several key differences from the native protein:
Additional N-terminal sequence: The His-tag adds approximately 6-10 histidine residues and potentially linker amino acids to the N-terminus of the protein.
Altered solubility properties: The His-tag can change the protein's solubility characteristics, potentially making it more soluble in certain buffer conditions.
Absence of post-translational modifications: Since E. coli lacks the chloroplastic post-translational machinery, any native modifications present in plant-derived atpH would be absent.
Purification characteristics: The His-tag facilitates protein purification via metal affinity chromatography, allowing for isolation at ≥90% purity as determined by SDS-PAGE .
Potential structural impacts: While the core function is preserved, the tag may slightly alter the protein's folding dynamics or oligomerization properties.
These differences must be considered when using the recombinant protein for structural or functional studies, as they may impact experimental outcomes.
For optimal stability and activity maintenance of recombinant Morus indica atpH protein, researchers should follow these evidence-based protocols:
Storage temperature: Store at -20°C/-80°C upon receipt. Long-term storage at -80°C is recommended for maintaining protein integrity beyond 6 months .
Aliquoting strategy: Prepare multiple small-volume aliquots immediately after reconstitution to prevent repeated freeze-thaw cycles. Working aliquots can be stored at 4°C for up to one week .
Reconstitution procedure:
Buffer composition: The recombinant protein is supplied in Tris/PBS-based buffer with 6% Trehalose at pH 8.0 . This formulation stabilizes the protein during lyophilization and storage.
Avoiding degradation factors: Minimize exposure to:
Repeated freeze-thaw cycles
Prolonged storage at room temperature
Extreme pH conditions
Proteases and oxidizing agents
Adhering to these storage guidelines ensures maximum protein stability and consistency in experimental results.
Comprehensive functional assessment of recombinant Morus indica atpH requires multiple complementary approaches:
Proton translocation assays: Reconstitute the protein in liposomes containing pH-sensitive fluorescent dyes (e.g., ACMA or pyranine) to measure proton pumping activity across the membrane upon addition of an artificial proton gradient.
ATP synthase activity reconstitution: Integrate the recombinant c-subunit with other ATP synthase components to assess if the c-ring forms properly and contributes to ATP synthesis. This requires:
Isolation of other ATP synthase subunits
Reconstitution in proteoliposomes
Measurement of ATP synthesis under proton gradient conditions
Oligomerization assessment: Analyze c-ring formation using:
Blue native PAGE
Size exclusion chromatography
Electron microscopy techniques
Binding studies: Validate functional attributes through:
Interaction measurements with other ATP synthase subunits
Proton binding/release kinetics using pH-jump techniques
Inhibitor binding studies (e.g., with oligomycin)
Structural integrity analysis:
Circular dichroism spectroscopy to verify secondary structure
Limited proteolysis to assess proper folding
Thermal shift assays to determine stability
For comparative analysis, include control experiments with native chloroplast ATP synthase complexes isolated from Morus indica or related species to benchmark the recombinant protein performance against native standards.
When investigating atpH's role in plant stress responses, researchers should implement the following experimental design elements:
Stress condition selection and standardization:
Define precise stress parameters (duration, intensity)
Apply multiple stress types (drought, salt, temperature) separately and in combination
Include recovery phases to assess reversibility
Temporal expression profiling:
Monitor atpH expression at multiple timepoints (early, mid, late stress)
Correlate expression changes with physiological responses
Compare with known stress marker genes
Tissue-specific analysis:
Protein-level validation:
Perform western blotting to confirm proteomics results
Use appropriate antibodies against atpH or the His-tag
Include loading controls and quantification methods
Functional correlation:
Measure ATP production rates under stress conditions
Assess proton gradient formation across thylakoid membranes
Correlate ATP synthase activity with photosynthetic efficiency
Control experiments:
Include wild-type plants and appropriate negative controls
Compare with other ATP synthase subunits
Use plants with altered expression of atpH (if available)
Studies have shown that ATP synthase is upregulated in stressed mulberry leaves as confirmed by western blotting analysis, similar to observations in wheat and cucumber . This suggests that plants require increased energy production under stress conditions, making ATP synthase a critical component of stress response mechanisms.
The c-ring stoichiometry directly determines the ion-to-ATP ratio in ATP synthases, fundamentally influencing bioenergetic efficiency. Research approaches to investigate this relationship include:
Manipulation of c-ring composition:
Biophysical characterization techniques:
Atomic force microscopy to directly visualize and count c-subunits
Cryo-electron microscopy for high-resolution structural analysis
Cross-linking mass spectrometry to determine subunit arrangement
Functional consequences assessment:
Measure ATP/H⁺ ratios in isolated systems
Analyze proton flux requirements for ATP synthesis
Quantify ATP synthesis rates under varying pmf conditions
Physiological impact analysis:
Monitor photosynthetic parameters (electron transport rates, NPQ)
Measure growth rates under different light conditions
Assess stress tolerance with altered ATP synthase stoichiometry
The research by Yamamoto et al. demonstrated that increasing the c-ring from 14 to 15 subunits in tobacco chloroplasts maintained normal growth despite reduced ATP synthase abundance (25% of wild-type levels) . This was achieved through enhancement of the membrane potential component of the proton motive force, ensuring sufficient proton flux without triggering low pH-induced feedback inhibition .
| C-ring Stoichiometry | H⁺ Required per Rotation | ATP Produced per Rotation | H⁺/ATP Ratio | Physiological Impacts |
|---|---|---|---|---|
| 14 subunits (wild-type) | 14 | 3 | 4.67 | Standard ATP production rate |
| 15 subunits (engineered) | 15 | 3 | 5.00 | Requires higher proton flux, compensated by increased membrane potential contribution to pmf |
This table illustrates how c-ring modifications directly impact the fundamental bioenergetic parameters of ATP synthesis in chloroplasts.
Selecting the appropriate expression system for recombinant Morus indica atpH requires balancing multiple factors. Below are evidence-based methodological approaches with their respective advantages and challenges:
E. coli expression systems (most common approach):
Methodology: The documented approach uses E. coli to express the full-length protein (1-81aa) with an N-terminal His-tag .
Advantages: High yield, simple culture conditions, rapid growth, well-established protocols.
Challenges: Membrane protein expression may lead to inclusion bodies, potential misfolding, lack of post-translational modifications.
Optimization strategies:
Use specialized E. coli strains (C41, C43) designed for membrane protein expression
Employ lower induction temperatures (16-20°C)
Add solubilizing agents or fusion partners to improve solubility
Plant-based expression systems:
Methodology: Express in tobacco or other plant chloroplasts via chloroplast transformation.
Advantages: Native-like folding environment, appropriate post-translational processing, formation of proper membrane associations.
Challenges: Lower yields, slower production timeline, more complex purification.
Application: Particularly valuable for functional studies requiring authentic protein structure.
Cell-free expression systems:
Methodology: Use wheat germ or E. coli extracts supplemented with lipids/detergents.
Advantages: Avoids toxicity issues, allows incorporation into nanodiscs or liposomes during synthesis.
Challenges: Higher cost, potential scalability limitations.
Best for: Initial screening or producing small amounts of highly pure protein.
For functional studies, the expression system should be selected based on the specific experimental requirements, with E. coli being suitable for structural studies and plant-based systems preferred when authentic function is critical.
A systematic multi-stage approach is required for effective purification and characterization of recombinant atpH:
Purification workflow:
Initial extraction: Membrane solubilization using mild detergents (DDM, LDAO, or C12E8)
Primary purification: Immobilized metal affinity chromatography (IMAC) using the N-terminal His-tag
Secondary purification: Size exclusion chromatography to remove aggregates
Detergent exchange: Replacement with detergents suitable for downstream applications
Purity assessment:
Structural characterization:
Circular dichroism (CD): To assess secondary structure content
Dynamic light scattering: To evaluate size distribution and potential aggregation
Analytical ultracentrifugation: For oligomeric state determination
Cryo-electron microscopy: For high-resolution structural analysis of c-ring assembly
Functional characterization:
Reconstitution into liposomes: For proton translocation studies
Patch-clamp experiments: To measure ion conductance
Assembly assays: To evaluate integration into larger ATP synthase complex
Biophysical parameter determination:
Thermal stability: Using differential scanning calorimetry
Ligand binding: Using isothermal titration calorimetry
Conformational dynamics: Using hydrogen-deuterium exchange mass spectrometry
This methodological pipeline ensures thorough characterization from primary sequence verification to functional validation, providing a complete profile of the recombinant protein.
Reconstituting functional ATP synthase complexes containing recombinant atpH requires a strategic approach:
Component preparation strategies:
Hybrid complex assembly: Combine recombinant atpH with isolated native components from chloroplasts
Full recombinant assembly: Express and purify all ATP synthase components individually
Partial complex reconstitution: Focus on F0 sector assembly to study proton translocation
Reconstitution methodology:
Sequential addition protocol:
Solubilize purified atpH in appropriate detergent
Mix with other F0 components at optimal stoichiometric ratios
Add F1 components under ATP/Mg²⁺ conditions
Remove detergent using biobeads or dialysis
Co-expression approach: Express multiple components simultaneously in compatible systems
Membrane incorporation techniques:
Proteoliposome formation:
Prepare liposomes with defined lipid composition
Add detergent-solubilized proteins
Remove detergent for spontaneous incorporation
Nanodisc assembly:
Incorporate protein into lipid bilayers surrounded by scaffold proteins
Provides defined membrane environment and size control
Functional validation methods:
ATP synthesis assays:
Generate proton gradient using acid-base transition or light-driven systems
Measure ATP production using luciferase-based detection
Proton pumping measurements:
Monitor fluorescence changes of pH-sensitive dyes
Quantify proton:ATP stoichiometry under varying conditions
Analytical approaches:
Single-molecule techniques: Observe rotational motion using attached fluorescent probes
Structural analysis: Verify complex integrity using negative-stain electron microscopy
Inhibitor studies: Confirm specificity using oligomycin and other known inhibitors
These methodological approaches enable detailed investigation of how the Morus indica atpH contributes to ATP synthase function, proton translocation mechanics, and energy coupling efficiency.
The regulation of ATP synthase subunit c in mulberry species exhibits distinct patterns under environmental stress conditions:
These findings highlight the central role of ATP synthase regulation in plant energy homeostasis under stress conditions, making atpH an important target for understanding stress adaptation mechanisms in Morus species.
The relationship between atpH function and photosynthetic efficiency is multifaceted and critically important for plant energy metabolism:
Bioenergetic coupling parameters:
In plant chloroplasts with a c₁₄-ring, 14 H⁺ are required for one complete rotation of the c-ring
This rotation produces 3 molecules of ATP, resulting in an ion-to-ATP ratio of 4.67
Linear photosynthetic electron transport translocates 12 H⁺ across the thylakoid membrane per 2 NADPH produced
This generates an ATP/NADPH ratio of approximately 1.29, insufficient for Calvin-Benson cycle requirements
ATP synthase and pmf regulation:
Light intensity adaptation mechanisms:
Engineering impacts on photosynthesis:
Manipulating c-ring stoichiometry affects the H⁺/ATP ratio and photosynthetic efficiency
In tobacco chloroplast mutants with increased c-ring size (14 to 15 subunits), plants maintained normal growth despite reduced ATP synthase abundance
This was achieved through enhancement of membrane potential contribution to pmf
| Parameter | Wild-type (c₁₄-ring) | Engineered (c₁₅-ring) | Physiological Impact |
|---|---|---|---|
| H⁺/ATP ratio | 4.67 | 5.00 | Increased proton requirement per ATP |
| ATP synthase abundance | 100% | 25% | Reduced enzyme concentration |
| Photosynthetic electron transport | Normal | Maintained | Unaffected despite lower enzyme levels |
| pmf composition | Standard ΔpH:Δψ ratio | Enhanced Δψ contribution | Prevents low pH inhibition |
These relationships demonstrate the complex interplay between ATP synthase function, photosynthetic electron transport, and plant energy metabolism, highlighting atpH's critical role in photosynthetic efficiency.
Research on recombinant atpH protein provides valuable insights connecting energy metabolism to the medicinal properties of Morus species:
Energy metabolism and diabetes connection:
Morus species demonstrate significant hypoglycemic effects beneficial for type 2 diabetes mellitus (DM2) treatment
ATP synthesis regulation is fundamentally linked to glucose metabolism through AMPK activation
Understanding atpH function helps elucidate how Morus extracts affect cellular energy sensing
Mechanistic pathway interactions:
Stress response and medicinal properties correlation:
Inflammation and energy metabolism:
Translation to therapeutic applications:
Detailed understanding of atpH structure and function aids identification of bioactive compounds targeting energy metabolism
Recombinant protein studies facilitate screening of Morus-derived compounds that may modulate ATP synthase activity
This research bridges fundamental plant biochemistry with medicinal applications
The following table summarizes connections between atpH research insights and observed medicinal properties of Morus species:
This research area demonstrates how fundamental studies on chloroplast ATP synthase components can inform understanding of traditional medicinal plants' therapeutic properties.
Comparative analysis of ATP synthase subunit c across plant species reveals important insights about conservation and specialization:
The Morus indica atpH represents an important comparative model for understanding both the conserved aspects of ATP synthase function and the species-specific adaptations that contribute to unique physiological properties of mulberry trees.
Recombinant protein studies have revealed several distinctive characteristics of Morus indica atpH that contribute to our understanding of this specific ATP synthase component:
Structural features:
The 81-amino acid sequence of Morus indica atpH contains the characteristic hydrophobic regions required for membrane integration
The protein exhibits successful expression and folding when produced with an N-terminal His-tag in E. coli systems
The recombinant protein maintains >90% purity after purification, indicating stable structural properties
Expression optimization:
Stress-responsive regulation:
Stability characteristics:
Potential biotechnological applications:
The successful recombinant expression and purification of Morus indica atpH enables its use in:
Structural studies to understand mulberry-specific ATP synthase properties
Screening for compounds that may affect ATP synthesis in relation to Morus medicinal properties
Comparative studies with other plant species to identify specialized adaptations
These findings from recombinant protein studies provide a foundation for deeper investigation into the specific properties of ATP synthase in Morus indica and how they relate to the unique physiological and medicinal characteristics of mulberry trees.
Evolutionary analysis of atpH sequences provides valuable insights into ATP synthase adaptation and specialization:
Phylogenetic analysis approaches:
Sequence alignment methodology:
Multiple sequence alignment of atpH from diverse plant species
Identification of conserved versus variable regions
Analysis of selection pressure across different domains
Tree construction techniques:
Maximum likelihood methods to infer evolutionary relationships
Bayesian approaches to estimate divergence times
Reconciliation with species trees to identify lineage-specific events
Functional domain conservation:
Proton-binding site analysis:
Universal conservation of the essential glutamate residue
Varying conservation of surrounding residues affecting proton affinity
Interface residues:
Higher conservation in regions contacting other ATP synthase subunits
Variable regions potentially reflecting co-evolution with partner subunits
Environmental adaptation signatures:
Climate correlation analysis:
Identification of sequence variations correlating with environmental conditions
Species from similar habitats showing convergent adaptations
Stress adaptation markers:
Variations in regulatory regions correlating with stress tolerance
Specific amino acid changes associated with thermal or drought adaptation
C-ring stoichiometry evolution:
Residue packing determinants:
Bioenergetic implications:
Analysis of how stoichiometry changes affect H⁺/ATP ratios
Adaptive significance of different ratios in various ecological contexts
Methodological framework for comparative analysis:
| Analysis Level | Methods | Outcomes | Research Applications |
|---|---|---|---|
| Sequence | Conservation scoring, Selection analysis (dN/dS) | Identification of functional constraints | Structure-function relationships |
| Structure | Homology modeling, Molecular dynamics | Prediction of structural consequences of variations | Engineering improved proteins |
| Expression | Transcriptome data meta-analysis | Regulatory pattern differences | Understanding stress adaptations |
| Function | Ancestral sequence reconstruction | Testing evolutionary hypotheses | Revealing adaptation mechanisms |
This evolutionary perspective enables researchers to understand how ATP synthase has adapted to diverse environmental conditions across plant lineages, providing insights that can inform both basic science understanding and potential applications in crop improvement or biomimetic energy systems design.
The study of recombinant Morus indica ATP synthase subunit c presents several compelling future research directions that could significantly advance our understanding of plant bioenergetics and stress adaptation:
Structure-function relationship investigation:
High-resolution structural determination of the Morus indica c-ring
Comparative analysis with c-rings from other plant species
Investigation of how specific amino acid variations affect proton binding and translocation
Stress adaptation mechanisms:
Detailed temporal analysis of atpH expression under various stress conditions
Investigation of post-translational modifications during stress response
Engineering of stress-responsive ATP synthase variants to improve plant resilience
Integration with medicinal applications:
Exploration of connections between ATP synthase function and bioactive compound production
Screening of Morus-derived compounds for effects on ATP synthase activity
Development of ATP synthase-based assays for bioactive discovery
Engineering for improved photosynthetic efficiency:
Modification of c-ring stoichiometry in crop plants based on insights from natural variation
Fine-tuning of H⁺/ATP ratios for specific agricultural conditions
Integration with other photosynthetic enhancement approaches
Evolutionary bioenergetics:
Comprehensive phylogenetic analysis of atpH across plant lineages
Correlation of sequence variations with ecological adaptations
Ancestral sequence reconstruction to test evolutionary hypotheses
These research directions build upon the current understanding of Morus indica atpH while expanding into new frontiers that connect fundamental biophysics with applied aspects of plant biology, agriculture, and medicinal research.
Despite significant progress, several technical challenges persist in atpH research that require innovative approaches:
Expression and purification optimization:
Challenge: Achieving high yields of properly folded membrane protein
Solutions:
Testing novel fusion partners to enhance solubility
Exploring cell-free expression systems with membrane mimetics
Developing improved detergent screening platforms
Functional reconstitution barriers:
Challenge: Creating fully functional ATP synthase complexes with recombinant components
Solutions:
Developing co-expression systems for multiple subunits
Optimizing lipid compositions for proteoliposome formation
Utilizing nanodiscs or other membrane mimetics for stabilization
Structural analysis limitations:
Challenge: Obtaining high-resolution structures of membrane-embedded c-rings
Solutions:
Applying advanced cryo-EM techniques optimized for membrane proteins
Using new detergents or amphipols that better preserve native structure
Developing improved crystallization approaches for membrane proteins
In vivo relevance validation:
Challenge: Connecting in vitro findings to physiological roles in planta
Solutions:
Developing chloroplast transformation systems for Morus species
Creating plant expression systems with tagged versions for in vivo tracking
Using rapid transient expression systems for functional testing
Biophysical measurement precision:
Challenge: Accurately measuring proton translocation and ATP synthesis in reconstituted systems
Solutions:
Implementing single-molecule techniques for direct observation
Developing more sensitive fluorescent probes for proton movement
Utilizing microfluidic approaches for precise control of conditions
Addressing these technical challenges will require interdisciplinary approaches combining advances in membrane protein biochemistry, structural biology techniques, and synthetic biology tools to fully exploit the potential of recombinant atpH for fundamental and applied research.
Research on Morus indica atpH provides valuable insights that extend to broader plant science domains:
Fundamental bioenergetic principles:
Understanding the molecular basis of proton-to-ATP energy conversion
Elucidating how plants balance energy production against photodamage risks
Revealing evolutionary solutions to the bioenergetic challenges of photosynthesis
Stress adaptation frameworks:
Demonstrating how energy metabolism reconfiguration supports stress response
Identifying common patterns of ATP synthase regulation across stress types
Providing comparative context for how different plant lineages optimize energy use under stress
Agricultural applications:
Informing approaches to engineer crops with improved photosynthetic efficiency
Identifying potential targets for enhancing stress tolerance in agriculture
Contributing to sustainable crop production under changing climate conditions
Medicinal plant research integration:
Connecting traditional medicinal applications to molecular mechanisms
Bridging ethnobotanical knowledge with modern molecular understanding
Supporting evidence-based approaches to natural product research
Evolutionary insights:
Revealing how fundamental energy conversion machinery adapts to diverse environments
Providing case studies in the evolution of multi-protein complexes
Demonstrating the balance between conservation and adaptation in essential cellular machinery
The specialized knowledge gained from Morus indica atpH research thus contributes to a comprehensive understanding of plant bioenergetics, with implications ranging from fundamental science to practical applications in agriculture, medicine, and biotechnology.