Recombinant Cucumis sativus NAD (P)H-quinone oxidoreductase subunit 4L, chloroplastic (ndhE)

What is Recombinant Cucumis sativus NAD(P)H-quinone oxidoreductase subunit 4L, chloroplastic (ndhE) and what is its biological significance?

Recombinant Cucumis sativus NAD(P)H-quinone oxidoreductase subunit 4L, chloroplastic (ndhE) is a protein component of the chloroplastic NAD(P)H dehydrogenase complex involved in cyclic electron flow around photosystem I in plants. This 100 amino acid protein plays a critical role in photosynthetic efficiency, particularly under stress conditions.

The protein is part of the NAD(P)H dehydrogenase complex that catalyzes the transfer of electrons from NAD(P)H to plastoquinone, contributing to ATP production without net NADPH formation. This process is particularly important when plants are exposed to environmental stresses such as high light intensity, drought, or temperature extremes .

Understanding this protein's structure and function provides insights into photosynthetic regulation mechanisms and potential applications in improving plant stress tolerance and productivity. In research contexts, recombinant forms of this protein, often tagged with histidine residues, are produced to facilitate detailed biochemical and structural studies .

What expression systems are typically used for producing recombinant Cucumis sativus ndhE protein?

The production of recombinant Cucumis sativus ndhE protein commonly employs bacterial expression systems, with E. coli being the predominant host organism due to its rapid growth, high protein yields, and well-established genetic manipulation protocols .

Methodology for E. coli-based expression:

• Vector selection: pET-series vectors are frequently used, incorporating a His-tag (typically N-terminal) to facilitate purification.

• E. coli strain optimization: BL21(DE3) and its derivatives are commonly employed due to their reduced protease activity and tight expression control.

• Expression conditions: Growth at 18-25°C after induction often improves solubility of chloroplastic proteins.

• Induction parameters: IPTG concentration (typically 0.1-1.0 mM) and induction time (4-16 hours) require optimization for each construct.

The full-length protein (amino acids 1-100) with an N-terminal His-tag has been successfully expressed in E. coli systems, resulting in functional protein suitable for downstream applications .

For researchers requiring alternative expression platforms, virus-based expression systems in plants offer promising approaches. For instance, cucumber green mottle mosaic virus (CGMMV) has been used successfully for expressing foreign proteins in cucumber plants, achieving yields up to 35.84 mg/kg of fresh leaf weight .

What are the recommended storage and handling conditions for recombinant ndhE protein?

Proper storage and handling of recombinant Cucumis sativus ndhE protein is critical for maintaining its structural integrity and functional activity. Based on standard protocols for similar membrane-associated proteins, the following guidelines are recommended:

Storage recommendations:

• Store lyophilized protein powder at -20°C to -80°C upon receipt.

• After reconstitution, store at -80°C as single-use aliquots to avoid repeated freeze-thaw cycles, which significantly diminish protein activity.

• For short-term use (up to one week), working aliquots may be stored at 4°C .

Reconstitution methodology:

• Briefly centrifuge the vial prior to opening to bring contents to the bottom.

• Reconstitute in deionized sterile water to a concentration of 0.1-1.0 mg/mL.

• Add glycerol to a final concentration of 5-50% (with 50% being optimal for long-term storage) to prevent freeze-damage.

• Create small aliquots to minimize freeze-thaw cycles .

Buffer considerations:

• The protein is typically supplied in Tris/PBS-based buffer with 6% trehalose at pH 8.0.

• This formulation helps maintain protein stability during lyophilization and reconstitution.

• When changing buffers for specific applications, gradual dialysis is recommended to prevent protein aggregation .

What experimental designs are most effective for studying the functional properties of ndhE within the NAD(P)H dehydrogenase complex?

Investigating the functional properties of ndhE requires carefully designed experiments that account for its membrane association and role within a multiprotein complex. A comprehensive experimental approach should include:

Screening experimental design (initial phase):

• Factorial designs: Use 2^k factorial designs to efficiently screen multiple factors affecting ndhE function with minimal experimental runs. This approach is particularly valuable in early-stage research when many variables must be considered .

• Variables to consider:

  • Buffer composition (pH, salt concentration)

  • Detergent types and concentrations

  • Temperature and time conditions

  • Presence of specific substrates or inhibitors

Optimization phase experimental design:

• Response surface methodology (RSM): Once key factors are identified, employ central composite designs or Box-Behnken designs to map the response surface and identify optimal conditions for ndhE activity.

• Key metrics to measure:

  • Electron transfer rates

  • NADPH oxidation kinetics

  • Protein-protein interaction stability

  • Complex assembly efficiency

In vitro reconstitution studies:

• Co-expression of multiple subunits: Express ndhE along with interacting partners to study complex formation.

• Systematic mutation analysis: Introduce specific mutations to identify functional residues and domains.

• Cross-linking experiments: Use chemical cross-linkers followed by mass spectrometry to identify interacting regions.

Research with the oxidative stress model using cumene hydroperoxide and the carbonyl stress model with glyoxal could provide valuable insights into ndhE's potential protective functions, particularly given that Cucumis sativus extracts have shown protective effects against these stressors in hepatocyte models .

What are the challenges in expressing and purifying functional full-length ndhE protein and how can they be overcome?

The expression and purification of functional full-length ndhE protein presents several challenges due to its hydrophobic nature and normally membrane-embedded state. The following methodological approaches address these challenges:

Challenge 1: Low expression yields

• Solution: Optimize codon usage for E. coli expression by synthesizing a codon-optimized gene.

• Methodology: Analyze the Codon Adaptation Index (CAI) of the native sequence and redesign to match E. coli codon preferences while maintaining the amino acid sequence.

• Expected improvement: 2-5 fold increase in expression levels.

Challenge 2: Protein insolubility

• Solution: Employ specialized expression protocols designed for membrane proteins.

• Methodology:

  • Use E. coli strains like C41(DE3) or C43(DE3) specifically developed for membrane protein expression.

  • Reduce expression temperature to 16-18°C and IPTG concentration to 0.1 mM.

  • Include solubilizing agents such as mild detergents (n-Dodecyl-β-D-maltoside or CHAPS) in lysis buffers.

  • Consider fusion partners such as MBP (maltose-binding protein) to enhance solubility.

Challenge 3: Maintaining protein stability during purification

• Solution: Develop a specialized purification protocol.

• Methodology:

  • Perform immobilized metal affinity chromatography (IMAC) using the N-terminal His-tag in the presence of stabilizing agents.

  • Include 6% trehalose in purification buffers to enhance stability .

  • Maintain glycerol at 10-20% throughout purification to prevent aggregation.

  • Consider purification under reducing conditions with 1-5 mM DTT or TCEP to prevent oxidation of cysteine residues.

Challenge 4: Verifying proper folding and functionality

• Solution: Implement multiple quality control assessments.

• Methodology:

  • Circular dichroism (CD) spectroscopy to confirm secondary structure content.

  • Size-exclusion chromatography to assess oligomeric state.

  • Functional assays measuring NADPH oxidation activity using artificial electron acceptors like ferricyanide or quinone analogs.

  • Thermal shift assays to evaluate protein stability under various buffer conditions.

These methodologies have successfully yielded functional recombinant ndhE protein with purity greater than 90% as determined by SDS-PAGE analysis .

How can protein engineering be applied to enhance the stability or modulate the function of ndhE protein?

Protein engineering offers powerful approaches to enhance ndhE stability and modulate its functional properties for both research and potential biotechnological applications. The following methodological framework outlines key strategies:

Rational design approaches:

• Stability enhancement via hydrophobic core optimization:

  • Identify buried hydrophobic residues using structural models or homology-based predictions.

  • Introduce mutations that optimize packing interactions (e.g., Leu→Ile or Val→Ile substitutions).

  • Validate improvements using thermal shift assays and long-term stability measurements.

• Surface engineering for improved solubility:

  • Identify surface-exposed hydrophobic patches.

  • Introduce charged or polar residues at these positions to enhance water interactions.

  • Reduce surface entropy by replacing flexible residues (Lys, Glu) with Ala or other order-promoting residues.

• Disulfide bond introduction:

  • Analyze the protein structure to identify residue pairs with appropriate geometry for disulfide formation.

  • Introduce Cys pairs at these positions to create stabilizing crosslinks.

  • Verify disulfide formation using non-reducing SDS-PAGE and mass spectrometry.

Semi-rational approaches:

• Consensus design methodology:

  • Align ndhE sequences from multiple plant species.

  • Identify positions where the Cucumis sativus sequence differs from the consensus.

  • Mutate these positions to the consensus amino acid to potentially enhance stability.

• Ancestral sequence reconstruction:

  • Infer ancestral sequences of ndhE using phylogenetic methods.

  • Express and characterize these reconstructed proteins, which often exhibit enhanced stability.

  • Use insights from ancestral proteins to guide engineering of the modern ndhE.

Function modulation strategies:

• Substrate specificity engineering:

  • Identify residues in the active site or NADPH binding pocket.

  • Introduce mutations to alter cofactor preference (NADH vs. NADPH).

  • Validate using enzymatic assays with different cofactors.

• Electron transfer efficiency enhancement:

  • Target residues at interfaces with electron transfer partners.

  • Optimize electrostatic interactions to enhance complex formation.

  • Measure electron transfer rates using stopped-flow spectroscopy.

The success of these approaches can be evaluated through comparative functional assays, similar to those that demonstrated the protective effects of Cucumis sativus against oxidative and carbonyl stress in cellular models .

What methodologies can be used to investigate protein-protein interactions involving ndhE within the NAD(P)H dehydrogenase complex?

Investigating protein-protein interactions involving ndhE is crucial for understanding its function within the NAD(P)H dehydrogenase complex. The following comprehensive methodological approaches are recommended for this specialized membrane protein:

In vitro interaction analysis methods:

• Co-immunoprecipitation (Co-IP) with tagged ndhE:

  • Express His-tagged ndhE protein in E. coli .

  • Solubilize with appropriate detergents (DDM, CHAPS, or digitonin).

  • Use His-tag affinity capture followed by Western blotting to identify interacting partners.

  • Control experiments should include non-tagged proteins and irrelevant antibodies.

• Surface Plasmon Resonance (SPR) analysis:

  • Immobilize purified ndhE on a sensor chip.

  • Flow potential interaction partners over the surface.

  • Measure binding kinetics (kon and koff rates) and calculate dissociation constants (KD).

  • Validate specificity through competition experiments with unlabeled proteins.

• Microscale Thermophoresis (MST):

  • Label ndhE with fluorescent dye.

  • Titrate with potential binding partners.

  • Measure changes in thermophoretic mobility to determine binding affinities.

  • This technique requires minimal protein amounts and tolerates detergents.

Structural analysis approaches:

• Cross-linking coupled with mass spectrometry (XL-MS):

  • Treat purified complexes with chemical cross-linkers (e.g., DSS, BS3, or EDC).

  • Digest cross-linked samples with proteases.

  • Analyze by LC-MS/MS to identify cross-linked peptides.

  • Map interaction interfaces using specialized software (e.g., xQuest, pLink).

• Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS):

  • Compare hydrogen-deuterium exchange rates of ndhE alone versus in complex.

  • Identify regions with altered solvent accessibility upon complex formation.

  • This technique is particularly valuable for membrane proteins as it can be performed in detergent solutions.

In vivo interaction studies:

• Split-GFP complementation in plant chloroplasts:

  • Fuse ndhE to one fragment of GFP and potential partners to complementary fragments.

  • Transform into plant cells.

  • Visualize interaction through reconstituted GFP fluorescence.

  • This approach allows visualization of interactions in their native compartment.

• Förster Resonance Energy Transfer (FRET):

  • Create fusion proteins with appropriate fluorophore pairs (e.g., CFP/YFP).

  • Express in plant systems.

  • Measure energy transfer as evidence of protein proximity.

  • Negative controls should include non-interacting protein pairs with similar localization.

These methodologies could be adapted from approaches used to study similar plant protein complexes, providing valuable insights into the structural organization and function of the NAD(P)H dehydrogenase complex in cucumber chloroplasts.

How does the expression and function of ndhE in Cucumis sativus relate to stress response mechanisms?

The relationship between ndhE expression/function and stress response mechanisms in Cucumis sativus involves complex cellular signaling and metabolic adaptations. Understanding this relationship requires specialized experimental approaches:

Stress-responsive expression analysis:

• Quantitative transcriptomics methodology:

  • Subject Cucumis sativus plants to various stressors (drought, high light, temperature extremes, oxidative stress).

  • Harvest tissue at multiple time points (0, 1, 3, 6, 12, 24, 48 hours).

  • Perform RNA-seq or qRT-PCR focusing on ndhE and related genes.

  • Analyze co-expression networks to identify stress-responsive regulatory patterns.

• Promoter analysis approach:

  • Clone the ndhE promoter region (1-2 kb upstream of start codon).

  • Create reporter gene constructs (GUS or luciferase).

  • Analyze reporter expression under various stress conditions.

  • Identify stress-responsive elements through deletion analysis.

Functional role during stress conditions:

• ROS measurement methodology:

  • Generate ndhE overexpression and knockdown/knockout lines in model plants.

  • Expose to oxidative stress inducers like cumene hydroperoxide .

  • Measure ROS formation using fluorescent probes (DCF-DA, MitoSOX).

  • Compare lipid peroxidation levels between wild-type and modified lines.

• Redox state analysis:

  • Monitor NADPH/NADP+ and GSH/GSSG ratios under stress conditions.

  • Compare wild-type plants with ndhE-modified lines.

  • Correlate changes with stress tolerance phenotypes.

  • This approach builds on findings of Cucumis sativus protective effects against glutathione depletion .

Stress tolerance phenotyping:

• Photosynthetic efficiency measurement:

  • Subject plants to combined stressors (e.g., high light + drought).

  • Measure chlorophyll fluorescence parameters (Fv/Fm, NPQ, ETR).

  • Analyze PSI cyclic electron flow rates.

  • Correlate with ndhE expression/activity levels.

• Metabolic adaptation analysis:

  • Perform targeted metabolomics focusing on stress-related compounds.

  • Compare metabolic profiles between wild-type and ndhE-modified plants.

  • Identify key metabolites associated with altered ndhE function.

  • Integrate with transcriptomic data to map metabolic networks.

These approaches would help elucidate how ndhE contributes to the documented protective effects of Cucumis sativus against oxidative and carbonyl stress, potentially through mechanisms involving ROS regulation and glutathione metabolism .

What biotechnological applications could be developed based on the properties of recombinant ndhE protein?

Recombinant ndhE protein presents several promising opportunities for biotechnological applications based on its structural characteristics and functional properties. Current research directions include:

Bioenergy applications:

• Enhanced photosynthetic efficiency systems:

  • Engineer optimized ndhE variants with improved electron transfer capabilities.

  • Incorporate into synthetic electron transport chains for bioenergy applications.

  • Potential to increase biomass production by 10-15% through enhanced cyclic electron flow.

  • Methodology: Use directed evolution coupled with high-throughput screening for photosynthetic performance.

• Bioelectrochemical devices:

  • Immobilize engineered ndhE on electrodes to create bio-hybrid electron transfer systems.

  • Develop biosensors for detection of electron transport inhibitors.

  • Create biocatalytic systems for NAD(P)H regeneration in enzymatic synthesis reactions.

  • Experimental approach: Test various immobilization techniques (covalent attachment, entrapment in conductive polymers) to optimize electrode performance.

Stress tolerance engineering:

• Development of stress-resistant crops:

  • Create transgenic plants with optimized ndhE expression.

  • Target improved tolerance to drought, high light, and temperature stresses.

  • Building on findings that Cucumis sativus components provide protection against oxidative and carbonyl stress .

  • Methodology: Using CRISPR/Cas9 for precise genome editing of ndhE regulatory elements.

• Molecular farming platforms:

  • Utilize ndhE promoter elements responsive to specific stresses.

  • Develop inducible expression systems for valuable proteins in plants.

  • Integrate with virus-based expression systems like CGMMV for enhanced protein production .

  • Experimental design: Test chimeric promoters combining ndhE stress-responsive elements with strong core promoters.

Protein engineering platforms:

• Membrane protein expression systems:

  • Optimize ndhE expression systems as models for difficult-to-express membrane proteins.

  • Develop standardized protocols for chloroplast membrane protein production.

  • Create fusion tag systems based on ndhE membrane-integration domains.

  • Approach: Systematic testing of expression conditions, solubilization methods, and purification strategies.

These applications represent the intersection of fundamental research on ndhE with practical biotechnological development, potentially contributing to sustainable agriculture and bioenergy production systems.

What computational approaches are most effective for modeling ndhE structure and predicting functional domains?

Effective computational modeling of ndhE structure and function requires specialized approaches that address the challenges of membrane protein prediction. The following methodological framework outlines state-of-the-art computational strategies:

Structure prediction approaches:

• AlphaFold2 and related deep learning methods:

  • Input: Primary amino acid sequence of Cucumis sativus ndhE .

  • Processing: Utilize multiple sequence alignments from diverse plant species.

  • Output: Predicted 3D structures with confidence scores (pLDDT) for each residue.

  • Validation: Compare predictions from multiple algorithms (AlphaFold2, RoseTTAFold, trRosetta).

  • Limitations: Reduced accuracy for membrane proteins requires careful interpretation.

• Membrane-specific modeling refinement:

  • Apply membrane protein-specific force fields (e.g., CHARMM-GUI Membrane Builder).

  • Perform molecular dynamics simulations in explicit lipid bilayers.

  • Analyze stability through RMSD calculations and hydrogen bond networks.

  • Methodology: 100-500 ns simulations with lipid compositions mimicking chloroplast membranes.

Functional domain prediction:

Integration with experimental data:

• Hybrid modeling approach:

  • Incorporate low-resolution experimental data (e.g., cross-linking constraints, HDX-MS results).

  • Refine computational models using experimental restraints.

  • Generate ensemble models that satisfy both computational predictions and experimental observations.

  • This integrated approach significantly improves model accuracy and biological relevance.

• Functional site prediction validation:

  • Use computational predictions to guide site-directed mutagenesis experiments.

  • Test effects of mutations on electron transfer activity.

  • Iteratively refine models based on experimental results.

  • This cycle of prediction-validation-refinement maximizes the utility of computational approaches.

These computational methodologies provide a foundation for understanding ndhE structure-function relationships and guide experimental designs for further characterization of this important protein in plant energy metabolism.