Recombinant Acidovorax citrulli NADH-quinone oxidoreductase subunit K (nuoK)

What is the structure and function of Acidovorax citrulli NADH-quinone oxidoreductase subunit K?

The NADH-quinone oxidoreductase subunit K (nuoK) is a membrane protein consisting of 102 amino acids with the sequence: MTLTLGHFLSLGAMLFALSVIGIFLNRKNLIVLLMAIELMLLAVNMNFVAFSHYLGDMHGQVFVFFILTVAAAESAIGLAILVLLFRNKSSIDAEDLNTLKG . This protein functions as part of Complex I in the respiratory chain, which couples electron transfer from NADH to quinone with proton translocation across the membrane. In Acidovorax citrulli, nuoK appears to be encoded within the bacterial chromosome, along with other genes involved in energy metabolism.

nuoK is also referred to as "NADH dehydrogenase I subunit K" or "NDH-1 subunit K" in scientific literature . As a membrane-embedded component, nuoK contains multiple transmembrane domains that contribute to the proton-pumping mechanism of Complex I. The hydrophobic nature of the protein is evident from its amino acid composition, which includes numerous non-polar residues arranged in transmembrane helices.

What role does nuoK play in Acidovorax citrulli virulence and metabolism?

While the search results don't directly address nuoK's role in virulence, we can infer its importance based on the critical function of the respiratory chain in bacterial metabolism. As a component of Complex I, nuoK contributes to energy generation through oxidative phosphorylation, which is essential for various virulence-associated processes including bacterial growth, motility, and secretion systems.

Research on related proteins in Acidovorax citrulli, such as OxyR, demonstrates that metabolic and oxidative stress response proteins significantly impact virulence factors including swimming motility, twitching motility, biofilm formation, and bacterial growth in planta . Disruptions in energy metabolism through nuoK mutation would likely affect these virulence-associated traits.

The relationship between energy metabolism and pathogenicity is particularly relevant in plant-pathogen interactions, where bacteria must adapt to different host environments. For instance, studies on OxyR show that this regulator contributes to Acidovorax citrulli's ability to induce hypersensitive response in Nicotiana benthamiana and cause bacterial fruit blotch symptoms in melon seedlings . Similar investigations of nuoK's role in pathogenicity would be valuable for understanding Acidovorax citrulli's infection mechanisms.

How can researchers effectively express and purify recombinant Acidovorax citrulli nuoK for structural studies?

Expressing and purifying membrane proteins like nuoK presents significant challenges due to their hydrophobic nature. Based on the available information about the recombinant nuoK protein, the following methodological approach is recommended:

For crystallography or cryo-EM studies, detergent exchange or reconstitution into nanodiscs or lipid cubic phase might be necessary to obtain high-resolution structural data of this challenging membrane protein.

What experimental approaches can be used to study the interaction between nuoK and other subunits of the NADH-quinone oxidoreductase complex?

Studying protein-protein interactions within the NADH-quinone oxidoreductase complex requires multiple complementary approaches:

• Co-immunoprecipitation (Co-IP): Using antibodies against tagged nuoK to pull down interacting complex components. This technique can be enhanced by chemical crosslinking to stabilize transient interactions.

• Bacterial two-hybrid (B2H) assays: Similar to the approach used in the OxyR study , B2H assays can identify direct protein-protein interactions between nuoK and other subunits. This would require cloning nuoK and potential interacting partners into appropriate vectors.

• Blue native PAGE: This technique allows for the separation of intact protein complexes and can determine whether recombinant nuoK properly incorporates into the native complex structure.

• Surface Plasmon Resonance (SPR) or Microscale Thermophoresis (MST): These techniques can provide quantitative measurements of binding affinities between nuoK and other complex components.

• Cryogenic electron microscopy (cryo-EM): For structural analysis of the entire complex, cryo-EM can reveal the spatial arrangement of nuoK relative to other subunits.

Data from these experiments should be complemented with computational predictions of interaction interfaces based on the amino acid sequence of nuoK and homology modeling using related structures from other bacterial species.

How does oxidative stress affect the expression and function of nuoK in Acidovorax citrulli?

While there's no direct information about nuoK response to oxidative stress in the search results, the data on OxyR provides a framework for designing experiments to address this question:

The OxyR study demonstrated that oxidative stress significantly impacts the expression of virulence-related genes in Acidovorax citrulli . To investigate nuoK's response to oxidative stress, researchers could:

• Transcriptional analysis: Use qRT-PCR to measure nuoK expression levels under different H₂O₂ concentrations (similar to the approach used for OxyR ). This would determine whether nuoK is transcriptionally regulated under oxidative stress conditions.

• Protein expression analysis: Employ western blotting with anti-nuoK antibodies to examine protein levels in response to oxidative stress, as performed for OxyR in the referenced study .

• Enzymatic activity assays: Measure NADH dehydrogenase activity in membrane preparations from Acidovorax citrulli cultures exposed to varying levels of oxidative stress to determine functional impacts.

• Mutant analysis: Generate nuoK deletion mutants and assess their sensitivity to H₂O₂ compared to wild-type strains, following methodology similar to the OxyR mutant studies .

• Complementation studies: Introduce recombinant nuoK into deletion mutants to confirm that observed phenotypes are specifically attributed to nuoK function.

Given that the respiratory chain can be both a source and target of reactive oxygen species, understanding nuoK's response to oxidative stress would provide valuable insights into Acidovorax citrulli's pathogenicity mechanisms.

What are the optimal conditions for reconstituting lyophilized recombinant nuoK protein?

Based on the product information provided , the following methodological protocol is recommended for optimal reconstitution of lyophilized recombinant nuoK:

• Initial preparation: Briefly centrifuge the vial containing lyophilized protein to ensure all material is at the bottom before opening.

• Reconstitution buffer: Use deionized sterile water to reconstitute the protein to a concentration of 0.1-1.0 mg/mL. For membrane proteins like nuoK, adding appropriate detergents (0.05-0.1% DDM or LMNG) to the reconstitution buffer may improve solubility.

• Glycerol addition: Add glycerol to a final concentration of 5-50% for long-term storage stability. The reference material suggests 50% as a default concentration .

• Aliquoting: Divide the reconstituted protein into small aliquots to avoid repeated freeze-thaw cycles, which can denature membrane proteins.

• Storage: Store working aliquots at 4°C for up to one week and long-term storage aliquots at -20°C/-80°C .

For functional studies, researchers should consider buffer optimization beyond the basic reconstitution guidelines. Testing various pH conditions (pH 6.5-8.0), salt concentrations (100-500 mM NaCl), and stabilizing agents (glycerol, sucrose, or specific lipids) may improve protein stability and activity for specific experimental applications.

How can researchers design effective functional assays for recombinant nuoK in vitro?

Designing functional assays for nuoK presents unique challenges due to its role as part of a multi-subunit membrane complex. Consider these methodological approaches:

• Reconstitution into proteoliposomes: Incorporate purified recombinant nuoK into artificial liposomes to create a membrane environment that more closely mimics its native context.

• NADH:ubiquinone oxidoreductase activity assay: While nuoK alone won't show full complex activity, researchers can measure activity in membrane preparations from expression systems or in reconstituted systems containing multiple complex subunits. This typically involves spectrophotometric monitoring of NADH oxidation at 340 nm.

• Proton translocation assays: Using pH-sensitive fluorescent dyes (like ACMA or pyranine) entrapped in proteoliposomes to measure proton pumping activity when nuoK is incorporated with other essential subunits.

• Binding assays: Develop assays to measure the binding of nuoK to other complex subunits using fluorescence-based techniques or surface plasmon resonance.

• Structural integrity assessment: Circular dichroism (CD) spectroscopy can verify that recombinant nuoK maintains proper secondary structure after reconstitution.

For more robust functional characterization, researchers should consider creating a partial or complete reconstituted Complex I system by co-expressing multiple subunits or combining individually purified components. This approach, though technically challenging, would provide more physiologically relevant insights into nuoK function.

What are the best approaches for generating and characterizing nuoK mutants in Acidovorax citrulli?

Based on methodologies used for other Acidovorax citrulli genes , the following approaches are recommended for generating and characterizing nuoK mutants:

• Mutant construction:

  • Homologous recombination: Design constructs with antibiotic resistance markers flanked by sequences homologous to regions surrounding the nuoK gene

  • CRISPR-Cas9 system: Design guide RNAs targeting the nuoK gene to create precise deletions or modifications

  • Site-directed mutagenesis: For studying specific amino acid residues within nuoK

• Phenotypic characterization:

  • Growth curve analysis in different media and under various stress conditions

  • Membrane potential measurements using fluorescent dyes

  • Respiratory activity assays measuring oxygen consumption rates

  • Swimming and twitching motility assays as performed in the OxyR study

  • Biofilm formation assays using crystal violet staining

  • Virulence assessment through plant inoculation experiments similar to those used for OxyR mutants

• Complementation studies:

  • Introduce wild-type or modified nuoK genes into the mutant strains to validate phenotypes

  • Use inducible expression systems to control nuoK expression levels

  • Create point mutations to identify critical residues for nuoK function

• Transcriptomic and proteomic analyses:

  • RNA-seq to identify genes differentially expressed in nuoK mutants

  • Comparative proteomics to assess changes in protein expression patterns

  • qRT-PCR validation of key differentially expressed genes

This multi-faceted approach would provide comprehensive insights into nuoK's role in Acidovorax citrulli physiology and pathogenicity.

How can nuoK research contribute to developing strategies for controlling bacterial fruit blotch?

Understanding nuoK's role in Acidovorax citrulli metabolism could inform novel control strategies for bacterial fruit blotch:

• Target-based inhibitor development: If nuoK is confirmed to be essential for Acidovorax citrulli virulence, it could serve as a target for developing specific inhibitors that disrupt respiratory function in the pathogen.

• Vaccine development: Recombinant nuoK protein could potentially be used to develop immunization strategies for plants, stimulating defense responses against Acidovorax citrulli infection.

• Diagnostic applications: Knowledge of nuoK conservation and variation across Acidovorax citrulli strains could enable the development of nuoK-targeted molecular diagnostics for early detection of the pathogen.

• Host resistance breeding: Identification of plant proteins that interact with bacterial nuoK could reveal potential resistance mechanisms that could be enhanced through breeding programs.

The research on OxyR demonstrated its significant impact on virulence and bacterial growth in planta . Similar investigations for nuoK would elucidate whether energy metabolism components like nuoK could be effective targets for controlling bacterial fruit blotch.

What experimental techniques could be employed to study the interaction between host plant proteins and bacterial nuoK?

Investigating host-pathogen protein interactions involving nuoK requires specialized approaches due to nuoK's membrane localization:

• Yeast two-hybrid membrane system: Modified Y2H systems designed specifically for membrane proteins could identify potential host interactors.

• Split-ubiquitin assays: This technique is particularly suitable for studying membrane protein interactions and could be applied to screen plant protein libraries for nuoK interactors.

• Co-immunoprecipitation from infected plant tissue: Using antibodies against nuoK to pull down complexes from infected plant tissue, followed by mass spectrometry identification of associated host proteins.

• Bimolecular Fluorescence Complementation (BiFC): By fusing complementary fragments of fluorescent proteins to nuoK and candidate host proteins, interactions can be visualized in planta.

• Proximity-dependent biotin labeling (BioID or TurboID): Fusing biotin ligase to nuoK would allow biotinylation of proximal proteins in the host, identifying potential interaction partners.

• In vitro binding assays: Purified recombinant nuoK could be used in binding studies with candidate host proteins identified through other screening methods.

These approaches would help determine whether nuoK directly interacts with host components during infection, potentially revealing new mechanisms of Acidovorax citrulli pathogenicity.

How does Acidovorax citrulli nuoK compare to homologs in other bacterial pathogens?

A comparative analysis of nuoK across bacterial species provides context for Acidovorax citrulli research:

*Estimated values based on typical conservation patterns of respiratory complex components

What is the current state of research on respiratory chain components in Acidovorax citrulli?

Research on Acidovorax citrulli's respiratory chain remains relatively limited compared to studies on its virulence factors and secretion systems. The complete genome assembly of Acidovorax citrulli M6 provides a foundation for identifying all respiratory chain components , but functional characterization of these components is still emerging.

Oxidative stress response mechanisms, which are closely linked to respiratory function, have received more attention. The study on OxyR demonstrates how oxidative stress response regulators contribute to virulence through multiple pathways . This research provides a framework for investigating connections between respiratory metabolism and pathogenicity.

Future research directions should include:

• Comprehensive characterization of all respiratory chain complexes in Acidovorax citrulli

• Investigation of metabolic adaptation during host infection

• Comparative analysis of respiratory components across diverse Acidovorax citrulli strains

• Exploration of potential antimicrobial targets within the respiratory chain