Recombinant Brucella ovis NADH-quinone oxidoreductase subunit K (nuoK)

What is NADH-quinone oxidoreductase subunit K (nuoK) in Brucella ovis?

NADH-quinone oxidoreductase subunit K (nuoK) is a membrane protein component of the NADH dehydrogenase I complex in Brucella ovis. This enzyme (EC 1.6.99.5) participates in electron transport chain processes, converting NADH to NAD+ and transferring electrons to quinones. The protein consists of 102 amino acids with a sequence of "MEIGIAHYLTVSAILFTLGVFGIFLNRKNVIVILMSIELILLSVNLNFVAFSSQLGDLVGQVFALFVLTVAAAEAAIGLAILVVFFRNRGSIAVEDVNVMKG" and is encoded by the nuoK gene (locus BOV_0807) .

What is the genomic context of nuoK in B. ovis?

The nuoK gene in B. ovis is part of the nuo operon encoding for NADH dehydrogenase I components. Genome analysis reveals that unlike some other genes that have undergone degradation in B. ovis compared to zoonotic Brucella species, the electron transport chain components remain largely conserved. This conservation occurs despite B. ovis having undergone genome degradation in other areas, which may have contributed to its narrower host range and specific tissue tropism .

What expression systems are most effective for recombinant nuoK production?

For optimal recombinant B. ovis nuoK expression, E. coli-based systems with tightly controlled inducible promoters (such as T7 or trc promoters) are recommended, as membrane proteins can be toxic when overexpressed. To enhance proper folding, consider using low induction temperatures (16-20°C) and specialized E. coli strains like C41(DE3) or C43(DE3) designed for membrane protein expression. For purification, construct a fusion with a C-terminal affinity tag rather than N-terminal, as the latter may interfere with membrane insertion. The protein should be maintained in appropriate detergent micelles such as n-dodecyl-β-D-maltoside (DDM) or lauryl maltose neopentyl glycol (LMNG) throughout purification to preserve native conformation .

What are effective methods for assessing nuoK function in vitro?

NADH dehydrogenase activity can be evaluated through several complementary approaches:

• Spectrophotometric assays: Monitor NADH oxidation at 340 nm in the presence of appropriate quinone acceptors

• Artificial electron acceptors: Use 2,6-dichlorophenolindophenol (DCPIP) reduction measured at 600 nm

• Membrane potential measurements: Employ fluorescent probes like Rhodamine 123 or DiSC3(5)

• Oxygen consumption assays: Use Clark-type electrodes to measure respiratory capacity

For B. ovis nuoK specifically, reconstitution into proteoliposomes may be necessary to evaluate its native function within the complete NADH dehydrogenase complex, as isolated subunits often lack activity outside their native complex .

How can nuoK-specific antibodies be developed and validated?

To develop specific antibodies against B. ovis nuoK:

• Select antigenic peptides from hydrophilic regions (preferably loops exposed to cytoplasm)

• Synthesize KLH-conjugated peptides for immunization

• Implement a validation protocol including:

  • ELISA against synthetic peptides and recombinant nuoK

  • Western blotting against both recombinant protein and B. ovis lysates

  • Immunoprecipitation to verify native protein recognition

  • Negative controls using lysates from ΔnuoK mutants

Cross-reactivity testing against B. abortus and other Brucella species is essential due to sequence conservation. If antibody specificity cannot be achieved due to high homology, epitope tagging of nuoK in B. ovis through genetic manipulation may be an alternative approach .

How does nuoK contribute to B. ovis energy metabolism and stress adaptation?

NADH-quinone oxidoreductase (Complex I) containing nuoK plays a crucial role in B. ovis energy metabolism by coupling NADH oxidation to proton translocation across the membrane, contributing to the proton motive force required for ATP synthesis. Research indicates that B. ovis may rely more heavily on oxidative phosphorylation than other Brucella species under certain conditions.

When B. ovis enters host cells, it faces oxidative stress from reactive oxygen species (ROS). Complex I activity influences the redox state of the bacterial cell, potentially affecting its ability to detoxify ROS. Though not directly demonstrated for nuoK, research on cysteine biosynthesis mutants shows that redox balance is critical for B. ovis survival during oxidative stress and intracellular growth. The transmembrane nature of nuoK suggests it could participate in sensing or responding to environmental changes, particularly membrane stress .

How does nuoK expression change during different growth phases and intracellular infection?

While specific data on nuoK expression regulation in B. ovis is limited, research on bacterial physiology during growth phases and infection suggests potential patterns:

During stationary phase, B. ovis experiences nutrient limitation and stress conditions similar to those in intracellular environments. Genes involved in energy metabolism typically show expression changes during this transition. TnSeq studies of B. ovis stationary phase fitness identified multiple metabolic pathways as important for survival, although nuoK was not specifically highlighted.

Inside host cells, B. ovis must adapt to low pH, nutrient limitation, and oxidative stress. Expression studies in related Brucella species suggest respiratory chain components may be regulated in response to these conditions. The function of nuoK likely becomes particularly important when B. ovis establishes itself in the replicative Brucella-containing vacuole (rBCV), which supports bacterial replication from approximately 12 hours post-infection .

What strategies can be used to generate nuoK deletion mutants in B. ovis?

Creating nuoK deletion mutants in B. ovis requires specialized approaches:

• Non-polar deletion strategy:

  • Design primers targeting ~500 bp upstream and downstream of nuoK

  • Create overlap extension PCR products for homologous recombination

  • Clone into suicide plasmids (e.g., pJQ200KS or pEX18Ap)

  • Introduce into B. ovis via electroporation

  • Select for double crossover events using sucrose sensitivity (sacB counterselection)

• Critical considerations:

  • Verify non-polar effects by examining expression of downstream genes in the nuo operon

  • Include complementation with wild-type nuoK on a stable plasmid

  • Create a merodiploid strain expressing nuoK before attempting deletion if the gene proves essential

This approach has been successfully used for creating multiple mutants in B. ovis for cell envelope-related genes. If nuoK proves essential, conditional expression systems or partial deletions may be necessary .

How can CRISPR-Cas9 technologies be adapted for nuoK manipulation in B. ovis?

CRISPR-Cas9 adaptation for B. ovis nuoK manipulation:

• Vector selection:

  • Choose broad-host-range plasmids with appropriate antibiotic resistance markers

  • Consider temperature-sensitive replicons for transient expression

• sgRNA design:

  • Target unique sequences within nuoK to minimize off-target effects

  • Design multiple sgRNAs targeting different regions for efficiency

  • Verify specificity using whole-genome BLAST against B. ovis genome

• Homology-directed repair:

  • Provide repair templates with at least 500 bp homology arms

  • Include selectable markers flanked by FRT sites for subsequent removal

  • Consider adding epitope tags for tracking protein expression

• Delivery method:

  • Optimize electroporation parameters specifically for B. ovis

  • Use methylation-deficient E. coli strains for plasmid preparation to avoid restriction

  • Consider conjugation as an alternative delivery method

• Screening strategy:

  • PCR verification of target modification

  • Whole-genome sequencing to verify absence of off-target effects

  • Functional complementation to validate phenotypes

While CRISPR-Cas9 methods are less documented in Brucella compared to model organisms, studies in related α-proteobacteria provide a foundation for protocol adaptation .

What phenotypes would be expected in B. ovis nuoK mutants based on related research?

Based on studies of respiratory chain components in Brucella and related bacteria, B. ovis nuoK mutants would likely exhibit:

• Growth defects:

  • Slower growth in nutrient-rich media

  • More pronounced defects under nutrient limitation

  • Potential requirement for alternative carbon sources that don't require NADH oxidation via Complex I

• Stress sensitivity:

  • Increased sensitivity to oxidative stress (H₂O₂, superoxide)

  • Altered sensitivity to pH stress

  • Potential membrane integrity issues

• Virulence attenuation:

  • Reduced survival within macrophages, particularly after the initial entry phase

  • Defects in establishment of the replicative niche

  • Attenuated infection in animal models

• Metabolic alterations:

  • Shifts in NAD⁺/NADH ratios

  • Compensatory upregulation of alternative respiratory pathways

  • Changes in central carbon metabolism

Research on cysteine biosynthesis mutants of B. ovis showing defects in intracellular survival between 2-24 hours post-infection suggests that metabolic capacity is critical during this period, which would likely also apply to respiratory chain components like nuoK .

How can recombinant nuoK be used to develop attenuated vaccine candidates against B. ovis?

Development of nuoK-based attenuated vaccines would follow these methodological steps:

• Mutant construction and characterization:

  • Create defined mutations in nuoK that reduce function without completely eliminating it

  • Alternatively, develop regulated expression systems where nuoK expression is sufficient for growth but reduced during infection

  • Characterize growth kinetics in vitro under various conditions

• Safety profiling:

  • Verify genetic stability through multiple passages

  • Demonstrate inability to revert to virulence

  • Test attenuation in animal models with increasing doses

• Immunogenicity assessment:

  • Evaluate antibody responses against multiple B. ovis antigens

  • Measure cell-mediated immunity (CMI) through T-cell proliferation assays

  • Assess cytokine profiles (particularly IFN-γ, IL-17, and TNF-α)

• Protection studies:

  • Challenge with virulent B. ovis after immunization

  • Compare with existing vaccine approaches (B. melitensis Rev1)

  • Evaluate bacterial clearance from tissues

• Delivery formulation:

  • Develop stabilization methods for field use

  • Test different routes of administration

This approach aligns with successful attenuated vaccine development strategies for other Brucella species, where defined metabolic mutations have produced protective immunity without causing disease .

What methods are effective for studying nuoK protein-protein interactions within the bacterial membrane?

For studying membrane protein interactions involving B. ovis nuoK:

• In vivo crosslinking approaches:

  • Use membrane-permeable crosslinkers like DSP or formaldehyde

  • Apply photoactivatable crosslinkers for higher specificity

  • Analyze crosslinked complexes by LC-MS/MS

• Genetic approaches:

  • Split-protein complementation assays (e.g., BACTH system adapted for membrane proteins)

  • Suppressor mutation analysis to identify functional interactions

  • In vivo site-specific incorporation of photoreactive amino acids

• Biophysical methods:

  • Single-molecule FRET with fluorescently labeled proteins

  • Surface plasmon resonance with nanodiscs containing reconstituted proteins

  • Native mass spectrometry of membrane protein complexes

• Structural approaches:

  • Cryo-electron microscopy of purified complexes

  • X-ray crystallography of co-purified interaction partners

  • Hydrogen-deuterium exchange mass spectrometry

These approaches have been successfully applied to study respiratory complexes in other bacteria and could be adapted for B. ovis nuoK to understand its integration into the NADH dehydrogenase complex and potential interactions with other cell envelope components .

How does the conservation of nuoK across Brucella species inform evolution of host specificity?

The high conservation of nuoK amino acid sequence between B. ovis and B. abortus (and likely other Brucella species) provides insights into Brucella evolution:

• Core metabolism versus adaptive elements:

  • Conservation of nuoK suggests respiratory functions represent core metabolism essential across host ranges

  • This contrasts with significant genome degradation observed in other B. ovis genes, particularly those involved in cell envelope structure and nutrient acquisition

• Regulatory differences in conserved genes:

  • Despite sequence conservation, expression regulation of nuoK may differ between species

  • Comparative transcriptomics during infection could reveal how conserved metabolic genes are differentially regulated

• Evolutionary implications:

  • B. ovis genome analysis reveals increased pseudogenes and insertion sequences compared to zoonotic Brucella

  • This genomic reduction pattern suggests B. ovis evolved from ancestors with broader host range

  • Essential metabolic components like nuoK remained intact while host-interaction factors diverged

• Functional conservation despite niche adaptation:

  • Analysis of membrane proteins shows substantial differences between rough (B. ovis) and smooth (B. abortus, B. melitensis) strains

  • Yet core bioenergetic functions represented by nuoK remain conserved

  • This suggests adaptation occurs primarily through changes in surface structures rather than central metabolism

Understanding the balance between conservation and adaptation provides insight into how pathogens can specialize for particular hosts while maintaining essential metabolic functions .

What are the optimal storage and handling conditions for recombinant nuoK protein?

For maximum stability and activity of recombinant B. ovis nuoK:

When handling the protein:

• Avoid repeated freeze-thaw cycles which can lead to protein aggregation

• Maintain appropriate detergent concentrations above critical micelle concentration

• Use low-protein-binding tubes and pipette tips

• Consider addition of reducing agents if the protein contains cysteine residues

• Filter-sterilize preparations to prevent microbial contamination during storage

What analytical methods are most suitable for characterizing recombinant nuoK quality?

To ensure high-quality recombinant nuoK preparations:

• Purity assessment:

  • SDS-PAGE with Coomassie or silver staining (expect >90% purity)

  • Western blotting with anti-His or anti-nuoK antibodies

  • Analytical size exclusion chromatography

• Structural integrity:

  • Circular dichroism spectroscopy to verify secondary structure content

  • Thermal shift assays to assess protein stability

  • Limited proteolysis to evaluate folding quality

• Functional verification:

  • Reconstitution into proteoliposomes for activity assays

  • NADH oxidation activity in presence of appropriate quinones

  • Membrane integration assays using fluorescent probes

• Aggregation analysis:

  • Dynamic light scattering to detect protein aggregates

  • Native PAGE to assess oligomeric state

  • Analytical ultracentrifugation for detailed analysis of species distribution

Quality benchmarks should include monodispersity in solution, appropriate secondary structure content (high alpha-helical content expected for membrane proteins), and specific activity in functional assays .

How can researchers troubleshoot expression and purification challenges specific to nuoK?

Common challenges with recombinant nuoK and their solutions:

• Low expression yields:

  • Optimize codon usage for expression host

  • Test different promoter strengths and induction conditions

  • Use specialized strains like C41(DE3) designed for membrane proteins

  • Consider fusion partners that enhance folding (e.g., MBP, SUMO)

• Protein aggregation:

  • Screen detergent panel (DDM, LMNG, DMNG, C12E8)

  • Add stabilizing agents (glycerol, specific lipids, cholesteryl hemisuccinate)

  • Reduce expression temperature to 16-20°C

  • Add chemical chaperones during expression

• Poor purity:

  • Implement multi-step purification (IMAC followed by size exclusion)

  • Use stringent washing steps with low concentrations of imidazole

  • Consider on-column detergent exchange

  • Test orthogonal purification approaches

• Inactive protein:

  • Verify membrane integration during expression

  • Include E. coli lipids during purification

  • Reconstitute into nanodiscs or liposomes with defined lipid composition

  • Optimize buffer conditions (pH, salt, additives)

• Verification approaches:

  • Mass spectrometry to confirm protein identity

  • N-terminal sequencing to verify absence of degradation

  • Functional complementation of bacterial mutants

These approaches address the specific challenges of membrane protein biochemistry relevant to nuoK research .

How might high-resolution structural studies of nuoK contribute to understanding B. ovis physiology?

High-resolution structural studies of B. ovis nuoK would advance understanding in several ways:

• Structure-function insights:

  • Identification of critical residues in proton translocation

  • Understanding of quinone binding sites

  • Mapping of protein-protein interfaces within Complex I

• Comparative structural biology:

  • Structural differences between Brucella and mitochondrial Complex I

  • Identification of bacteria-specific features as potential drug targets

  • Evolutionary insights through structural comparison with other alpha-proteobacteria

• Methodological approaches:

  • Cryo-electron microscopy of intact Complex I

  • X-ray crystallography of nuoK within minimal complexes

  • Molecular dynamics simulations to analyze proton transfer pathways

• Applications to pathogenesis:

  • Structural basis for adaptation to intracellular environment

  • Potential identification of regions involved in stress response

  • Rational design of attenuating mutations for vaccine development

Structural studies would complement existing genetic and biochemical approaches to provide a more complete understanding of B. ovis bioenergetics and its role in pathogenesis .

What role might nuoK play in B. ovis adaptation to oxidative stress during infection?

NADH dehydrogenase I containing nuoK likely contributes to oxidative stress adaptation through several mechanisms:

Research on B. ovis cysteine biosynthesis mutants demonstrated increased sensitivity to hydrogen peroxide and reduced intracellular fitness between 2-24 hours post-infection. Given the central role of respiratory complexes in cellular redox status, nuoK likely contributes to similar adaptive processes during oxidative stress, particularly in macrophages where respiratory burst produces high levels of ROS .

How could computational modeling advance understanding of nuoK function in the context of B. ovis metabolism?

Computational approaches offer powerful tools for understanding nuoK in the broader context of B. ovis metabolism:

• Genome-scale metabolic modeling:

  • Integration of nuoK function within complete metabolic networks

  • Flux balance analysis to predict effects of nuoK perturbation

  • Identification of compensatory pathways activated during respiratory chain dysfunction

• Protein structure prediction and analysis:

  • AlphaFold2 or RoseTTAFold predictions of nuoK structure

  • Molecular docking to identify potential inhibitor binding sites

  • Molecular dynamics simulations of proton transfer through the complex

• Systems biology approaches:

  • Multi-omics data integration (transcriptomics, proteomics, metabolomics)

  • Prediction of nuoK-dependent adaptations during host infection

  • Network analysis to identify critical nodes in stress response pathways

• Machine learning applications:

  • Pattern recognition in experimental data to identify nuoK-dependent signatures

  • Prediction of environmental conditions affecting nuoK function

  • Design of optimal experimental conditions for nuoK characterization

These computational approaches would complement experimental studies by generating testable hypotheses about nuoK function in contexts difficult to address experimentally, such as in vivo infection dynamics and metabolic adaptations over time .