Recombinant Salmonella dublin NADH-quinone oxidoreductase subunit K (nuoK)

What is NADH-quinone oxidoreductase subunit K (nuoK) and what is its function in Salmonella dublin?

NADH-quinone oxidoreductase subunit K (nuoK) is a critical component of the NADH:quinone oxidoreductase-1 complex (also known as NDH-1 or Complex I) in the Salmonella dublin respiratory chain . This protein plays a fundamental role in energy metabolism by participating in electron transfer from NADH to quinones (such as ubiquinone, menaquinone, or demethylmenaquinone), coupling this process to proton translocation across the bacterial membrane . The nuoK subunit specifically contributes to the hydrophobic, membrane-embedded domain of the complex, facilitating proton transport . As part of the electron transport chain, this protein enables Salmonella to generate energy through both aerobic and anaerobic respiration, which is crucial for bacterial survival and pathogenicity under varied environmental conditions .

How does nuoK interact with different quinones in the Salmonella respiratory chain?

The NADH:quinone oxidoreductase complex containing nuoK can interact with multiple types of quinones depending on the respiratory conditions. Under aerobic conditions, ubiquinone serves as the primary electron acceptor, while under anaerobic conditions, demethylmenaquinone and menaquinone function as alternative electron carriers . Research using HPLC analysis has shown that wild-type Salmonella cells produce both ubiquinone and menaquinone . In strains with ubiquinone biosynthesis disruptions (such as ubiA or ubiE deletion mutants), the bacteria shift to utilizing alternative quinones, and the NADH:quinone oxidoreductase complex adapts to interact with these different electron carriers . Functional studies demonstrate that the complex can transfer electrons from NADH to demethylmenaquinone or menaquinone when ubiquinone is unavailable, albeit with potential changes in efficiency . This metabolic flexibility highlights the adaptability of the respiratory chain in Salmonella for survival under varying oxygen conditions.

What expression systems are commonly used for producing recombinant nuoK protein?

Recombinant nuoK protein production typically employs bacterial expression systems optimized for membrane proteins. While the search results don't specify exact expression protocols for nuoK, common approaches for related membrane proteins include:

• Expression Region Selection: For nuoK, the full expression region (1-100) has been utilized in recombinant protein production, covering the complete amino acid sequence of the native protein .

• Tag Selection: Affinity tags are commonly employed to facilitate purification, with the specific tag type determined during the production process based on the protein's characteristics and intended use .

• Buffer Optimization: For stable storage of recombinant nuoK, specialized buffers containing Tris-based components and 50% glycerol are utilized, specifically optimized for this hydrophobic membrane protein .

For research applications, commercially available recombinant nuoK is typically supplied in 50 μg quantities, with other sizes available upon request . Proper storage at -20°C or -80°C for extended periods is recommended, with working aliquots kept at 4°C for up to one week to maintain protein integrity .

How do suppressor mutations in other NADH:quinone oxidoreductase subunits affect the function of the complex when nuoK operates in quinone-limited environments?

Specifically, genome sequence analysis identified several key mutations:

• nuoG(Q297K) mutation in the hydrophilic domain (found in four independent suppressor mutants)

• nuoM(A254S) mutation in the hydrophobic domain

• nuoN(A444E) mutation in the hydrophobic domain

These suppressor mutations significantly improve electron flow activity from NADH to alternative quinones (demethylmenaquinone and menaquinone), rescuing respiration and growth capabilities under specific conditions . Interestingly, these adaptations also allow the mutant strains to utilize L-malate as a sole carbon source, suggesting broader metabolic adjustments . The increased levels of NADH:quinone oxidoreductase-1 observed in ubiquinone-biosynthesis mutant strains through immunoblotting further indicate compensatory upregulation of the respiratory complex . These findings suggest that while nuoK itself was not found to harbor suppressor mutations in these specific experiments, its function within the complex is maintained and potentially enhanced through adaptive changes in interacting subunits.

What methodologies can be employed to investigate nuoK's role in Salmonella dublin virulence and pathogenicity?

Investigating nuoK's potential role in Salmonella dublin virulence requires a multi-faceted approach combining genetic, biochemical, and infection models. Several methodological strategies can be employed:

• Gene Deletion and Complementation Studies:

  • Creating nuoK knockout mutants using precise gene deletion techniques

  • Complementing with wild-type and mutant variants to establish causality

  • Evaluating resulting phenotypes in growth, motility, and virulence assays

• Infection Models:

  • In vitro cell invasion assays using bovine intestinal epithelial cells

  • Ex vivo tissue models to study dissemination

  • In vivo models assessing colonization, systemic spread, and persistence

• Transcriptomic and Proteomic Analysis:

  • RNA-Seq to identify gene expression changes in response to nuoK deletion

  • Comparative proteomics between wild-type and mutant strains

  • Analysis under conditions mimicking host environments (pH changes, oxygen limitation, nutrient restriction)

• Metabolic Profiling:

  • Assessment of intracellular ATP levels and redox states

  • Monitoring of quinone pool composition using HPLC

  • Evaluation of metabolic flexibility in various carbon sources

These approaches should consider S. dublin's host adaptation mechanisms and virulence factors, including its Salmonella Pathogenicity Islands (SPIs), Type III and Type VI Secretion Systems, and virulence plasmids . The demonstrated ability of S. dublin to survive as a facultative intracellular pathogen and evade immune responses through specific adaptations suggests that respiratory chain components like nuoK may contribute to its persistence within hosts .

How can protein-protein interactions between nuoK and other subunits of the NADH:quinone oxidoreductase complex be characterized?

Characterizing the protein-protein interactions between nuoK and other subunits of the NADH:quinone oxidoreductase complex requires specialized techniques for membrane protein analysis:

• Crosslinking Studies:

  • Chemical crosslinking followed by mass spectrometry to identify interaction interfaces

  • Site-directed photo-crosslinking to capture transient interactions

  • Analysis of crosslinked products using SDS-PAGE and Western blotting

• Co-immunoprecipitation and Pull-down Assays:

  • Generation of antibodies against nuoK or epitope tagging

  • Pull-down experiments to identify interacting partners

  • Analysis of complex assembly in wild-type versus mutant backgrounds

• Structural Biology Approaches:

  • Cryo-electron microscopy of the intact complex

  • X-ray crystallography of subcomplexes

  • NMR studies of labeled subunits for dynamic interactions

• Genetic Interaction Screens:

  • Suppressor mutation analysis as demonstrated in studies of ubiA mutants

  • Synthetic lethality/sickness screens with other respiratory chain components

  • Bacterial two-hybrid systems adapted for membrane proteins

• Functional Reconstitution:

  • Purification and reconstitution of subcomplexes with defined subunit composition

  • Activity measurements with different quinone substrates

  • Assessment of proton pumping efficiency using liposome-reconstituted systems

When implementing these methods, researchers should consider the highly hydrophobic nature of nuoK and its embedding within the membrane domain of the complex . The spontaneous emergence of suppressor mutations in nuoG, nuoM, and nuoN subunits in response to ubiquinone deficiency provides valuable insights into functional interactions within the complex and potential experimental approaches for further characterization .

What is the relationship between nuoK function and antibiotic resistance in Salmonella dublin?

The relationship between nuoK function and antibiotic resistance presents an important area for investigation, particularly given Salmonella dublin's characterization as a multi-drug resistant (MDR) pathogen . While direct evidence linking nuoK specifically to antibiotic resistance is not explicitly provided in the search results, several conceptual frameworks can guide research in this area:

• Membrane Potential and Drug Efflux:

  • The NADH:quinone oxidoreductase complex contributes to establishing the proton motive force across the bacterial membrane

  • This electrochemical gradient powers numerous processes including active efflux pumps that expel antibiotics

  • Alterations in nuoK function could potentially impact membrane energetics and therefore drug efflux efficiency

• Metabolic Adaptation and Persistence:

  • S. dublin can survive as a facultative intracellular pathogen in numerous organs and lymph nodes, evading adaptive immune responses

  • Respiratory flexibility enabled by complexes containing nuoK may contribute to persistence under antibiotic pressure

  • Metabolic adaptations involving alternative electron transport pathways might influence susceptibility to certain antibiotics

• Genetic Linkage with Resistance Determinants:

  • S. dublin harbors the pSDV plasmid that contributes to virulence and encodes antimicrobial resistance genes

  • Potential genetic linkages between respiratory chain adaptations and resistance determinants could be investigated

• Stress Response Coordination:

  • Respiratory chain function intersects with bacterial stress responses

  • Drug exposure often triggers stress responses that may involve altered respiratory chain component expression

Research methodologies to investigate these relationships could include:

• Comparative genomics of resistant versus susceptible S. dublin isolates

• Transcriptional analysis under antibiotic exposure

• Minimum inhibitory concentration (MIC) testing in nuoK mutant strains

• Assessment of membrane potential and drug accumulation in strains with altered nuoK function

What are the optimal conditions for expressing and purifying recombinant Salmonella dublin nuoK protein?

The expression and purification of recombinant Salmonella dublin nuoK protein requires careful optimization due to its hydrophobic nature and membrane localization. Based on available information and standard protocols for similar membrane proteins, the following methodological considerations are recommended:

Expression System Optimization:

• Selection of E. coli strains designed for membrane protein expression (e.g., C41(DE3), C43(DE3))

• Use of tightly controlled inducible promoters to prevent toxicity

• Low-temperature induction (16-18°C) to facilitate proper membrane integration

• Supplementation with specific lipids to enhance membrane protein folding

Purification Strategy:

• Membrane Preparation:

  • Gentle cell lysis methods to preserve membrane integrity

  • Differential centrifugation to isolate membrane fractions

  • Washing steps to remove peripheral proteins

• Solubilization:

  • Screening of detergents compatible with nuoK stability (e.g., DDM, LMNG)

  • Optimization of detergent:protein ratios

  • Inclusion of stabilizing agents (glycerol, specific lipids)

• Affinity Purification:

  • Utilizing appropriate tag systems determined during production process

  • Inclusion of detergents throughout purification steps

  • Careful buffer optimization to maintain stability

• Storage Conditions:

  • Use of Tris-based buffer with 50% glycerol as optimized for this protein

  • Storage at -20°C for standard use or -80°C for extended storage

  • Avoidance of repeated freeze-thaw cycles

  • Preparation of working aliquots for short-term (4°C) storage up to one week

Quality Control Assessments:

• SDS-PAGE and Western blotting to confirm identity and purity

• Size-exclusion chromatography to assess oligomeric state

• Functional assays to verify activity after purification

For researchers working with commercially available recombinant nuoK, storage recommendations include maintaining the protein at -20°C for regular use and -80°C for long-term storage, with working aliquots kept at 4°C for no more than one week to preserve functionality .

What techniques are most effective for studying electron transfer activity in complexes containing nuoK?

Studying electron transfer activity in complexes containing nuoK requires specialized techniques that can assess both the intact complex function and specific electron transfer steps. The following methodologies have proven valuable:

Research has demonstrated that NADH:quinone oxidoreductase levels increase in ubiquinone-biosynthesis mutant strains, as determined by immunoblotting . Enzyme assays have successfully measured electron transfer from NADH to alternative quinones (demethylmenaquinone or menaquinone) when ubiquinone is unavailable . These methodologies have revealed that suppressor mutations in other complex subunits can improve electron flow activity under certain growth conditions, particularly in cells bearing ubiquinone biosynthesis defects .

How can researchers effectively generate and characterize nuoK mutants to study its functional domains?

Generating and characterizing nuoK mutants requires strategic approaches to ensure meaningful insights into functional domains while overcoming challenges associated with membrane protein manipulation:

• Mutation Design Strategies:

  • Alanine-scanning mutagenesis of conserved residues

  • Targeted mutations based on homology modeling and evolutionary conservation

  • Chimeric constructs with related nuoK proteins from other species

  • Domain swapping to identify functional regions

• Mutation Generation Methods:

  • Site-directed mutagenesis using PCR-based approaches

  • CRISPR-Cas9 genome editing for chromosomal mutations

  • Lambda Red recombineering for scarless genomic modifications

  • Construction of complementation plasmids with mutant variants

• Phenotypic Characterization:

  • Growth curve analysis under various carbon sources and oxygen conditions

  • Motility assays in soft agar media

  • Respiration measurements with oxygen electrodes

  • Membrane potential assessment using fluorescent probes

• Biochemical Characterization:

  • NADH:quinone oxidoreductase activity assays with various quinone substrates

  • Complex assembly analysis via Blue Native PAGE

  • Crosslinking studies to assess subunit interactions

  • Proton pumping efficiency measurements

• Structural Characterization:

  • Cryo-EM analysis of wild-type versus mutant complexes

  • Accessibility studies using membrane-impermeable reagents

  • Protein stability assessments via thermal shift assays

Research has demonstrated the value of studying spontaneous suppressor mutations that emerge under selective pressure, such as the nuoG(Q297K), nuoM(A254S), and nuoN(A444E) mutations that arose in ubiquinone biosynthesis mutants . These natural genetic adaptations provided insights into functional relationships between subunits and domain interactions within the complex . Similar approaches could be applied to nuoK, potentially by creating conditions where compensatory mutations in nuoK would emerge in response to defects in other complex components.

How might understanding nuoK function contribute to developing new antimicrobial strategies against Salmonella dublin?

Understanding nuoK function could open novel avenues for antimicrobial development against Salmonella dublin, particularly given the increasing challenge of multi-drug resistance in this pathogen . Several strategic approaches emerge from current knowledge:

• Targeting Respiratory Chain Flexibility:

  • S. dublin can utilize different quinones (ubiquinone, menaquinone, demethylmenaquinone) depending on environmental conditions

  • Compounds that interfere with the nuoK-containing complex's ability to interact with alternative quinones could limit metabolic adaptability

  • Such inhibitors might be particularly effective during host colonization when the bacterium faces changing oxygen environments

• Exploiting Host Adaptation Mechanisms:

  • S. dublin has evolved specific adaptations to cattle, including mechanisms that evade innate immune responses

  • If nuoK contributes to host-specific adaptation, targeting unique features of this subunit might disrupt host colonization

  • Comparative analysis with non-host-adapted Salmonella strains could identify potential targets

• Disrupting Persistence Mechanisms:

  • S. dublin can persist as a facultative intracellular pathogen in various organs

  • If respiratory chain components like nuoK are essential for intracellular survival, targeting them could reduce persistence

  • Combination therapies affecting both active growth and persistent states might be more effective

• Addressing Multi-Drug Resistance:

  • S. dublin has become increasingly characterized by multi-drug resistance

  • Novel targets in essential metabolic pathways, such as the respiratory chain, offer alternatives to conventional antibiotics

  • Targeting nuoK or its interactions might circumvent existing resistance mechanisms

Research approaches could include:

• Screening for small molecules that specifically inhibit the nuoK-containing complex

• Testing combination approaches targeting both ubiquinone and alternative quinone utilization

• Evaluating effectiveness in both active growth and persistence models

• Assessing impact on virulence in infection models

What role might nuoK play in Salmonella dublin adaptation to different host environments during infection?

The role of nuoK in Salmonella dublin adaptation to various host environments presents a fascinating area for investigation, particularly considering this pathogen's host adaptation to cattle and its ability to cause systemic disease :

• Adaptation to Oxygen Fluctuations:

  • Gastrointestinal environments present varying oxygen levels

  • The NADH:quinone oxidoreductase complex containing nuoK can operate with different quinones under aerobic and anaerobic conditions

  • This respiratory flexibility likely facilitates adaptation as S. dublin transitions from intestinal colonization to systemic spread

• Intracellular Survival Mechanisms:

  • S. dublin can survive as a facultative intracellular pathogen in various organs and lymph nodes

  • The ability to function under the metabolic constraints of the intracellular environment may depend on respiratory chain adaptations

  • nuoK's contribution to maintaining energy production under these conditions warrants investigation

• Host-Specific Immune Evasion:

  • S. dublin has evolved to evade bovine innate immune responses

  • Metabolic adaptations involving respiratory chain components might contribute to reducing inflammatory responses in the intestinal mucosa

  • This potentially facilitates dissemination throughout the host

• Stress Response Integration:

  • Host environments impose various stresses (nutrient limitation, pH fluctuations, antimicrobial peptides)

  • Respiratory chain function likely intersects with stress response pathways

  • nuoK's potential role in coordinating metabolic adjustments during stress warrants exploration

Research methodologies to investigate these aspects could include:

• Transcriptomic analysis of nuoK expression during different stages of infection

• Infection models comparing wild-type and nuoK mutant strains

• In vitro systems mimicking specific host microenvironments

• Monitoring respiratory chain function during host cell interactions

The demonstrated ability of S. dublin to acquire virulence factors through genetic elements and to adapt through gene loss or modification suggests that respiratory chain components like nuoK may have undergone specific evolutionary adaptations to facilitate host colonization and persistence .

How do the electron transport properties of nuoK-containing complexes differ between Salmonella dublin and other bacterial pathogens?

Comparative analysis of nuoK-containing NADH:quinone oxidoreductase complexes across bacterial pathogens offers valuable insights into evolutionary adaptations and potential species-specific therapeutic targets:

• Sequence and Structural Variations:

  • nuoK from Salmonella dublin has a specific amino acid sequence that may contain unique features compared to other pathogens

  • Comparative sequence analysis could identify conserved residues essential for function across species versus Salmonella-specific adaptations

  • Structural modeling based on available complex I structures from other organisms could highlight distinctive features

• Quinone Utilization Patterns:

  • Salmonella dublin can utilize multiple quinones (ubiquinone, menaquinone, demethylmenaquinone) with its respiratory complex

  • The efficiency and regulation of alternative quinone usage may differ between pathogens

  • This flexibility could contribute to Salmonella dublin's ability to colonize various host environments

• Regulatory Mechanisms:

  • Expression and activity regulation of the NADH:quinone oxidoreductase complex may show species-specific patterns

  • Salmonella dublin shows increased levels of the complex in ubiquinone-biosynthesis mutant strains

  • Comparative studies could reveal whether this compensatory response is conserved or unique

• Integration with Virulence Mechanisms:

  • S. dublin possesses specific virulence factors like Salmonella Pathogenicity Islands and Type III/VI Secretion Systems

  • The coordination between respiratory chain function and virulence factor expression might show pathogen-specific patterns

  • Energy requirements for different virulence mechanisms may drive adaptive changes in respiratory complexes

• Host Adaptation Influences:

  • S. dublin is host-adapted to cattle, which may have selected for specific respiratory chain adaptations

  • Comparison with broad-host-range Salmonella serovars could reveal adaptation signatures

  • Analysis across host-restricted pathogens might identify convergent evolutionary patterns

Research approaches could include:

• Phylogenetic analysis of nuoK across bacterial pathogens

• Functional complementation studies exchanging nuoK between species

• Comparative biochemical characterization of purified complexes

• Bioinformatic analysis of co-evolution between respiratory chain components and virulence factors