Recombinant Tolumonas auensis Na (+)-translocating NADH-quinone reductase subunit E

What is the structure and function of Na+-NQR in Tolumonas auensis?

Na+-translocating NADH:quinone oxidoreductase (Na+-NQR) is a six-subunit membrane protein complex encoded by the consecutive structural genes nqrABCDEF . This enzyme complex functions as a respiratory sodium ion pump that couples the oxidation of NADH to ubiquinone with the translocation of sodium ions across the membrane . The complex contains multiple redox centers including flavins and Fe-S clusters, which facilitate electron transfer through the complex . In Tolumonas auensis, a gram-negative bacterium isolated from anoxic sediments of a freshwater lake, this enzyme likely serves as the main generator of the sodium motive force (SMF) that drives energy-dependent processes such as flagellar rotation, substrate uptake, and ATP synthesis .

What is the specific role of NqrE subunit within the Na+-NQR complex?

NqrE is a membrane-bound subunit that, together with NqrD, ligates an Fe center deeply embedded within the membrane part of the Na+-NQR complex . This Fe center is crucial for the electron transfer pathway within the complex . As a membrane-embedded component, NqrE likely participates in the mechanism of sodium ion translocation across the membrane, contributing to the generation of the sodium motive force . The membrane location of NqrE suggests its involvement in forming the channel or pathway through which sodium ions are transported.

What expression systems are suitable for producing recombinant NqrE?

Based on studies with other Na+-NQR subunits, appropriate expression systems for recombinant NqrE include both homologous and heterologous hosts. Successful expression of soluble variants of NqrF and its individual domains has been achieved using Vibrio cholerae or Escherichia coli as expression hosts . For membrane proteins like NqrE, expression strategies might include:

• Using E. coli strains optimized for membrane protein expression

• Expression in the native organism (T. auensis) for proper folding

• Inclusion of appropriate affinity tags for purification

• Co-expression with chaperones to enhance proper folding

The choice of expression host should consider the requirements for proper incorporation of the Fe center that NqrE coordinates.

What cofactors are associated with Na+-NQR and how do they relate to NqrE function?

The Na+-NQR complex contains multiple redox cofactors distributed among its subunits:

The Fe center coordinated jointly by NqrD and NqrE is positioned within the membrane domain and likely participates in both electron transfer and the sodium ion translocation mechanism . Understanding the properties of this Fe center is crucial for elucidating the function of recombinant NqrE.

What methodological approaches are optimal for expressing and purifying functional NqrE?

Expression and purification of functional recombinant NqrE presents several challenges due to its membrane-bound nature and involvement in coordinating an Fe center. Based on approaches used for other membrane proteins and Na+-NQR subunits, the following methodological strategies are recommended:

• Expression optimization:

  • Test multiple expression vectors with different promoter strengths

  • Evaluate various fusion tags (His-tag, MBP, SUMO) for improving solubility

  • Explore expression at reduced temperatures (16-25°C) to enhance proper folding

  • Consider co-expression with NqrD, as they coordinate the Fe center together

• Membrane extraction and purification:

  • Utilize gentle detergents for membrane solubilization (DDM, LMNG, or digitonin)

  • Implement affinity chromatography followed by size exclusion chromatography

  • Consider using native electrophoresis to verify oligomeric state

  • Employ ion exchange chromatography for removing contaminants

• Functional validation:

  • Spectroscopic analysis to confirm Fe center incorporation

  • Reconstitution into proteoliposomes to assess functionality

  • Co-purification with NqrD to maintain the intact Fe-binding domain

The success of these approaches should be measured by protein yield, purity, stability, and retention of native structural properties.

How does the Fe center in NqrD-NqrE contribute to electron transfer and Na+ translocation?

The Fe center jointly coordinated by NqrD and NqrE subunits is a critical component in the electron transfer pathway of Na+-NQR . Based on studies of the Na+-NQR complex, the electron transfer likely follows this pathway: NADH → FAD → [2Fe-2S] in NqrF → Fe center in NqrD/NqrE → FMN cofactors in NqrB/NqrC → ubiquinone .

To investigate the specific role of this Fe center, researchers could:

• Generate site-directed mutants of potential Fe-coordinating residues in NqrE

• Perform EPR, Mössbauer, and resonance Raman spectroscopy to characterize the Fe center

• Conduct time-resolved spectroscopy to measure electron transfer rates through the complex

• Perform electrochemical measurements to determine redox potentials

• Use inhibitors that specifically target Fe centers to assess functional impact

The connection between electron transfer through this Fe center and Na+ translocation could be studied using Na+-sensitive fluorescent probes in reconstituted systems or by measuring Na+ uptake in proteoliposomes containing the reconstituted complex.

What is the role of NqrE in reactive oxygen species (ROS) production by Na+-NQR?

Studies have identified Na+-NQR as a producer of reactive oxygen species (ROS) in vivo, with the FAD cofactor in NqrF subunit recognized as the site for intracellular superoxide formation in Vibrio cholerae . Membranes from wild-type V. cholerae showed significantly higher superoxide production (9.8 ± 0.6 μmol superoxide min−1 mg−1 membrane protein) compared to membranes from the mutant lacking Na+-NQR (0.18 ± 0.01 μmol min−1 mg−1) .

While NqrE has not been directly implicated in ROS formation, its role in the electron transfer chain suggests it could influence this process. To investigate this:

• Compare ROS production in wild-type vs. NqrE-deficient or mutant complexes

• Assess how alterations in the Fe center affect superoxide formation rates

• Examine ROS production under varying sodium concentrations to understand the relationship between ion translocation and ROS generation

• Investigate potential redox cycling of the Fe center under different conditions

Understanding NqrE's influence on ROS production could provide insights into how Na+-NQR affects virulence in bacterial pathogens, as suggested by studies in V. cholerae .

How can recombinant NqrE be incorporated into functional Na+-NQR for structural studies?

Reconstitution of a functional Na+-NQR complex containing recombinant NqrE presents a significant challenge due to the complex nature of this multi-subunit enzyme with various cofactors. A methodological approach would include:

• Individual subunit preparation:

  • Express and purify all six Nqr subunits with appropriate tags

  • Verify cofactor incorporation for each subunit

  • Ensure proper folding using circular dichroism and fluorescence spectroscopy

• Complex assembly:

  • Combine purified subunits in appropriate detergent micelles

  • Add necessary cofactors (FAD, FMN, riboflavin) if not already incorporated

  • Verify complex formation using blue native PAGE, size exclusion chromatography, or negative-stain electron microscopy

• Functional validation:

  • Measure NADH:quinone oxidoreductase activity

  • Assess Na+ translocation using fluorescent probes

  • Compare properties with native complex isolated from membranes

• Structural studies:

  • Reconstitute into nanodiscs or amphipols for cryo-EM studies

  • Attempt crystallization for X-ray crystallography

  • Perform cross-linking mass spectrometry to identify subunit interactions

This methodological framework would enable researchers to investigate structural details of the NqrE subunit within the context of the entire complex.

How does iron availability affect NqrE expression and function?

Studies on V. cholerae have shown that the Na+-NQR influences iron metabolism, with a comparative proteome study revealing a 2.7-fold increase in abundance of the predicted Fe2+ transporter (FeoB) in an nqr deletion strain . This suggests a relationship between Na+-NQR function and iron homeostasis.

For investigating how iron availability affects NqrE specifically:

• Conduct qRT-PCR to measure nqrE transcript levels under varying iron concentrations

• Perform western blot analysis using NqrE-specific antibodies to quantify protein levels

• Assess Fe center incorporation efficiency under iron-limited vs. iron-replete conditions

• Compare growth rates of wild-type vs. nqrE mutant strains under different iron regimes

• Investigate whether iron regulatory proteins interact with nqrE mRNA

These approaches would help elucidate how iron availability affects NqrE expression and function, potentially revealing regulatory mechanisms controlling Na+-NQR assembly and activity.

What is the relationship between NqrE function and bacterial virulence?

The Na+-NQR has been implicated in virulence in several bacterial pathogens, including V. cholerae, though the exact mechanism remains unclear . Research methodologies to investigate NqrE's potential role in virulence could include:

• Generate nqrE deletion or point mutants and assess virulence in appropriate model systems

• Examine gene expression profiles of virulence factors in wild-type vs. nqrE mutant strains

• Investigate whether NqrE-dependent ROS production affects virulence gene expression

• Study the impact of nqrE mutations on bacterial colonization and persistence

• Analyze host immune responses to bacteria with functional vs. non-functional NqrE

Understanding this relationship could provide insights into how Na+-NQR-dependent processes, such as ROS production and maintenance of the sodium motive force, contribute to bacterial pathogenesis in environments like the human intestine.

How does T. auensis NqrE compare to homologs in other bacterial species?

Tolumonas auensis, identified as a member of the gamma subclass of Proteobacteria, possesses distinctive metabolic capabilities including toluene production from aromatic amino acids . Comparative analysis of its NqrE subunit with homologs from other bacterial species would provide evolutionary insights and functional information.

Methodological approaches include:

• Perform sequence alignments to identify conserved residues, particularly those involved in Fe center coordination

• Conduct phylogenetic analysis to trace the evolutionary history of NqrE

• Compare predicted transmembrane topologies across species

• Identify species-specific insertions or deletions that might confer unique properties

• Construct chimeric proteins to investigate domain-specific functions

This comparative analysis would help identify core functional elements of NqrE conserved across species, as well as adaptations that might reflect specific ecological niches or metabolic requirements.

What experimental approaches can determine the Na+ translocation mechanism involving NqrE?

Understanding how NqrE participates in Na+ translocation requires sophisticated biophysical and biochemical approaches:

• Site-directed mutagenesis:

  • Identify and mutate conserved charged residues in transmembrane segments

  • Create cysteine mutants for accessibility studies using sulfhydryl reagents

• Biophysical measurements:

  • Perform solid-state NMR to study conformational changes upon Na+ binding

  • Use EPR spectroscopy with spin labels to detect structural rearrangements

  • Implement electrophysiological studies in reconstituted systems

• Ion binding studies:

  • Utilize isothermal titration calorimetry to measure Na+ binding parameters

  • Perform Na+ competition assays with other cations

  • Use fluorescent Na+ indicators to track ion movement in real-time

These experimental approaches would help elucidate the structural basis for Na+ recognition, binding, and translocation involving the NqrE subunit, contributing to our understanding of the Na+-NQR as an ion pump.