Recombinant Proteus mirabilis Na (+)-translocating NADH-quinone reductase subunit E

How does subunit E integrate with other components of the Na(+)-NQR complex?

Subunit E functions as part of the six-subunit Na(+)-NQR complex (NqrA-F), where it primarily contributes to the membrane domain and sodium ion translocation pathway. Integration studies employing crosslinking techniques and co-purification experiments suggest that subunit E forms direct interactions with subunits B, D, and F within the membrane, creating part of the ion channel structure.

The assembly process of the complex follows a sequential pattern where subunit E incorporation occurs after subunits B and D but precedes the addition of subunit A. This ordered assembly process is critical for proper complex formation and function.

Research methodologies to investigate these interactions include:

These approaches collectively enable researchers to build a comprehensive model of how subunit E contributes to the structural integrity and functional capabilities of the complete Na(+)-NQR complex.

What conserved domains and residues are essential for subunit E function?

Sequence analysis of Na(+)-translocating NADH-quinone reductase subunit E across multiple bacterial species reveals several highly conserved domains critical for function:

• Transmembrane helices: Multiple hydrophobic regions that anchor the protein within the membrane bilayer, with particularly high conservation in the predicted membrane-spanning regions.

• Ion-coordinating residues: Negatively charged aspartate and glutamate residues at positions 121, 133, and 172 (Proteus mirabilis numbering) that likely participate in sodium ion coordination during transport.

• Subunit interface regions: Conserved motifs at positions 45-67 and 151-178 that mediate interactions with other NQR complex components, particularly subunits B and D.

To experimentally validate the functional importance of these domains, researchers should employ site-directed mutagenesis targeting:

• Charge neutralization (D→N or E→Q) of potential ion-coordinating residues

• Alanine scanning of conserved hydrophobic residues in subunit interfaces

• Conservative substitutions that alter side chain properties while maintaining general chemical characteristics

Following mutagenesis, functional assessment through complementation of nqrE-deficient strains, membrane potential measurements, and sodium transport assays can determine the impact of these modifications on protein function.

What expression systems yield optimal production of functional recombinant Proteus mirabilis Na(+)-translocating NADH-quinone reductase subunit E?

Successful expression of functional recombinant Proteus mirabilis Na(+)-translocating NADH-quinone reductase subunit E requires careful consideration of expression systems that accommodate membrane protein characteristics. Based on empirical research, the following approaches yield optimal results:

• E. coli expression system optimization:

  • Recommended strains: C41(DE3) or C43(DE3), specialized for membrane protein expression

  • Vector selection: pET series vectors with T7 promoter and N-terminal His-tag for purification

  • Growth conditions: Initial growth at 37°C to OD600 0.6-0.8, followed by temperature reduction to 18°C prior to induction

  • Induction parameters: 0.1-0.3 mM IPTG for 16-18 hours at 18°C

• Co-expression strategies:

  • Simultaneous expression with chaperone proteins (GroEL/ES, DnaK/J)

  • Co-expression with other Na(+)-NQR complex subunits to promote proper folding and assembly

  • Addition of membrane-stabilizing compounds (glycerol 5-10%) to culture media

• Alternative expression systems:

  • Cell-free expression systems with supplied lipids or detergents

  • Bacillus subtilis expression for a closer native membrane environment

  • Insect cell expression systems for complex eukaryotic modifications

The expression yield and functionality can be monitored through Western blotting with anti-His antibodies, membrane fractionation to confirm proper localization, and preliminary activity assays using membrane preparations before proceeding to purification.

What purification protocol maximizes recovery of properly folded subunit E protein?

Purification of properly folded Na(+)-translocating NADH-quinone reductase subunit E requires specialized techniques to maintain the native structure of this integral membrane protein. The following protocol maximizes recovery of functional protein:

• Membrane preparation:

  • Harvest cells by centrifugation (5,000 × g, 10 min, 4°C)

  • Resuspend in buffer containing 50 mM Tris-HCl pH 8.0, 300 mM NaCl, and protease inhibitors

  • Disrupt cells using sonication or cell disruption systems

  • Remove unbroken cells by centrifugation (10,000 × g, 20 min, 4°C)

  • Isolate membranes by ultracentrifugation (150,000 × g, 1 hour, 4°C)

• Solubilization optimization:

  • Resuspend membrane fraction in solubilization buffer containing appropriate detergent

  • Screen detergents including n-dodecyl-β-D-maltoside (DDM, 1%), digitonin (1-2%), or CHAPS (0.5%)

  • Incubate with gentle rotation (4°C, 1-2 hours)

  • Remove insoluble material by ultracentrifugation (150,000 × g, 30 min, 4°C)

• Affinity chromatography:

  • Load solubilized protein onto Ni-NTA resin equilibrated with buffer containing detergent

  • Wash extensively with buffer containing 20-30 mM imidazole

  • Elute protein with buffer containing 250-300 mM imidazole

  • Immediately exchange into buffer with lower imidazole concentration

• Size exclusion chromatography:

  • Further purify protein using Superdex 200 column

  • Use buffer containing 50 mM Tris-HCl pH 8.0, 150 mM NaCl, and detergent at CMC

• Storage conditions:

  • Store purified protein in buffer containing 50% glycerol at -20°C or -80°C

  • Avoid repeated freeze-thaw cycles which can destabilize the protein

  • For extended storage, aliquot in small volumes to minimize freeze-thaw damage

This protocol typically yields protein with >90% purity as determined by SDS-PAGE, making it suitable for structural and functional studies.

What methods can verify proper folding and functionality of the purified protein?

Verifying proper folding and functionality of purified Na(+)-translocating NADH-quinone reductase subunit E requires a multi-faceted approach. The following complementary methods provide comprehensive assessment:

• Structural integrity assessment:

  • Circular dichroism (CD) spectroscopy: Confirms secondary structure composition, particularly the expected high α-helical content

  • Fluorescence spectroscopy: Monitors tertiary structure through intrinsic tryptophan fluorescence

  • Limited proteolysis: Properly folded proteins show distinctive digestion patterns compared to misfolded proteins

  • Thermal shift assays: Measures protein stability through denaturation profiles

• Membrane insertion verification:

  • Liposome reconstitution efficiency: Quantifies protein incorporation into artificial membranes

  • Sucrose gradient ultracentrifugation: Separates properly folded (membrane-associated) from misfolded (aggregated) protein

• Functional assays:

  • Na+ binding studies: Using 22Na+ isotopes or sodium-sensitive fluorescent dyes

  • Complex assembly: Blue native PAGE to assess incorporation into the full Na(+)-NQR complex

  • Electron paramagnetic resonance (EPR): Detects structural changes upon substrate addition

• Interaction studies:

  • Surface plasmon resonance (SPR): Measures binding to other subunits or known ligands

  • Isothermal titration calorimetry (ITC): Quantifies binding thermodynamics with interaction partners

Collectively, these methods provide a comprehensive evaluation of protein quality. Researchers should establish baseline measurements using positive controls (such as native membrane preparations) and negative controls (heat-denatured samples) to properly interpret results from recombinant protein preparations.

How can researchers accurately measure electron transfer activity coupled to Na+ translocation?

Accurate measurement of electron transfer activity coupled to Na+ translocation in the Na(+)-NQR complex containing subunit E requires specialized techniques that simultaneously monitor both processes. The following methodological approaches provide comprehensive insights:

• Reconstituted systems preparation:

  • Proteoliposome reconstitution: Purified Na(+)-NQR complex incorporation into phospholipid vesicles with controlled internal composition

  • Inverted membrane vesicle preparation: Right-side-out or inside-out vesicles from cells expressing the complex

  • Planar lipid bilayer systems: For single-complex electrophysiological measurements

• Electron transfer measurements:

  • Spectrophotometric NADH oxidation: Real-time monitoring at 340 nm (ε = 6,220 M⁻¹cm⁻¹)

  • Ubiquinone reduction: Using ubiquinone analogs like decylubiquinone (absorption at 275 nm)

  • Oxygen consumption: Clark-type electrode measurements in the presence of terminal oxidases

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

• Na+ translocation detection:

  • 22Na+ flux measurements: Quantification of radioactive sodium accumulation in vesicles

  • Sodium-sensitive fluorescent probes: SBFI (Sodium-binding benzofuran isophthalate) fluorescence changes

  • Sodium-selective electrodes: Direct measurement of Na+ concentration changes

• Coupling efficiency determination:

  • Simultaneous recording: Parallel measurement of electron transfer and ion movement

  • Inhibitor studies: Selective inhibition of electron transfer (with rotenone) or Na+ movement (with monensin)

  • Stoichiometry calculations: Ratio of Na+ ions transported per electron transferred

A typical experimental protocol would involve:

• Preparing proteoliposomes with controlled internal Na+ concentration and pH

• Loading vesicles with appropriate indicators for Na+ detection

• Initiating electron transfer by NADH addition

• Simultaneous recording of NADH oxidation and Na+ movement

• Calibration with known Na+ concentrations or standard additions

This approach allows determination of Na+/e⁻ stoichiometry and provides insight into the mechanism coupling electron transfer to ion translocation.

What mutagenesis strategies best elucidate the Na+ translocation pathway through subunit E?

To elucidate the Na+ translocation pathway through Na(+)-translocating NADH-quinone reductase subunit E, researchers should implement systematic mutagenesis strategies targeting key functional domains. The following approaches provide comprehensive insights:

• Alanine-scanning mutagenesis:

  • Systematic replacement of consecutive residues with alanine

  • Focus on predicted transmembrane regions and conserved charged residues

  • Quantitative assessment of Na+ transport activity in each mutant

  • Identification of residues whose mutation causes complete loss of function versus partial impairment

• Charge manipulation strategies:

  • Charge neutralization: Replace negatively charged residues (Asp, Glu) with neutral counterparts (Asn, Gln)

  • Charge reversal: Replace negatively charged residues with positively charged ones (Lys, Arg)

  • Charge introduction: Add new charged residues at strategic positions

  • Systematic mapping of charge-sensitive regions in the putative ion channel

• Cysteine accessibility methods:

  • Engineer cysteine-free variant as background

  • Introduce single cysteines at positions of interest

  • Probe accessibility with membrane-impermeable sulfhydryl reagents

  • Map accessible versus inaccessible regions to define the translocation pathway

• Conservative substitution approach:

  • Replace residues with others of similar chemistry but altered properties

  • Examples: Asp→Glu (longer side chain), Ser→Thr (bulkier side chain)

  • Determine effects on ion selectivity and transport rates

  • Identify precise spatial requirements for Na+ coordination

A comprehensive mutagenesis study should include control mutations outside predicted functional regions and assessment of protein expression and stability for each mutant. Data analysis should correlate structural position with functional impact to map the complete translocation pathway and identify critical residues involved in Na+ binding and movement through subunit E.

How do researchers distinguish between Na+ and H+ selectivity in the NQR complex?

Distinguishing between Na+ and H+ selectivity in the Na(+)-NQR complex requires specialized techniques that can differentiate between these physiologically important cations. Researchers should employ the following methodological approaches:

• Ion competition experiments:

  • Varying ion concentrations: Systematically alter Na+ and H+ concentrations while measuring complex activity

  • Kinetic analysis: Determine Km values for both ions

  • Competition plots: Establish whether inhibition is competitive, non-competitive, or uncompetitive

  • Hill coefficient determination: Assess cooperativity in ion binding

• pH-dependent measurements:

  • pH titration curves: Measure activity across pH range 6.0-9.0

  • pH jump experiments: Rapid pH changes during steady-state measurements

  • pD versus pH activity: Compare activity in H2O versus D2O to exploit isotope effects

  • pH indicators: Use pH-sensitive dyes to detect proton movements during catalysis

• Ionic substitution studies:

  • Sodium replacement: Substitute Na+ with Li+, K+, or Rb+

  • Sodium-free conditions: Special buffers containing no sodium

  • Choline chloride substitution: Replace NaCl with choline chloride to maintain ionic strength

  • isotope tracer studies: Compare 22Na+ versus tritiated water transport

• Electrophysiological approaches:

  • Patch-clamp of reconstituted systems: Direct measurement of ion currents

  • Ion selectivity measurements: Reversal potential determination under bi-ionic conditions

  • Single-channel recordings: Analysis of conductance and selectivity at the single-molecule level

A standardized experimental protocol would include:

• Preparation of proteoliposomes with defined internal composition

• Precise control of external pH and sodium concentration

• Initiation of transport with NADH addition

• Parallel monitoring of electron transfer, sodium movement, and pH changes

• Systematic data analysis to quantify relative contributions of Na+ and H+ to the observed activity

This multi-parameter approach allows researchers to establish the primary coupling ion and quantify any secondary ion movements or pH dependencies in the Na(+)-NQR complex function.

How can recombinant Proteus mirabilis Na(+)-translocating NADH-quinone reductase subunit E be used for inhibitor development?

Recombinant Proteus mirabilis Na(+)-translocating NADH-quinone reductase subunit E provides an excellent platform for identifying and developing selective inhibitors with potential antimicrobial applications. This protein is particularly valuable because Na(+)-NQR is absent in mammals but critical for energy metabolism in many bacteria, including P. mirabilis which is associated with urinary tract infections . Researchers can utilize recombinant subunit E in inhibitor development through these methodological approaches:

• High-throughput screening systems:

  • Activity-based assays: Measuring Na(+)-NQR activity inhibition in multi-well format

  • Binding assays: Thermal shift assays to detect compound binding

  • Fragment-based screening: Identification of initial chemical scaffolds

  • Virtual screening: In silico docking against structural models

• Structure-activity relationship studies:

  • Combinatorial chemistry approaches: Systematic modification of lead compounds

  • Photoaffinity labeling: Identification of precise binding sites within subunit E

  • Competition assays: Determination of binding mode (competitive, non-competitive)

  • Residence time measurements: Evaluation of inhibitor-protein complex stability

• Selectivity profiling:

  • Cross-species activity assessment: Testing against Na(+)-NQR from different bacterial pathogens

  • Mammalian toxicity screening: Testing against human cell lines and mitochondrial preparations

  • Respiratory chain component panels: Testing against other respiratory enzymes to confirm selectivity

• Mechanistic characterization:

  • Enzyme kinetic analyses: Determining inhibition constants and mechanisms

  • Conformational impact assessment: Evaluating effects on protein structure and dynamics

  • Resistance development studies: In vitro evolution of resistant strains to identify binding sites

• Translational development:

  • Membrane permeability optimization: Chemical modifications to enhance bacterial penetration

  • Synergy testing: Combination with conventional antibiotics

  • In vivo efficacy evaluation: Testing in appropriate infection models

This systematic approach can lead to the identification of novel antimicrobial compounds targeting a system that is essential for P. mirabilis energy metabolism, particularly in the alkaline environments created by this organism's urease activity during urinary tract infections .

What techniques can map conformational changes in subunit E during the catalytic cycle?

Mapping conformational changes in Na(+)-translocating NADH-quinone reductase subunit E during the catalytic cycle requires sophisticated biophysical techniques capable of capturing dynamic structural rearrangements. Researchers should employ the following advanced methodologies:

Experimental design should include:

• Strategic selection of labeling positions based on predicted functional regions

• Time-resolved measurements synchronized with substrate addition

• Parallel application of complementary techniques to corroborate findings

• Correlation of structural changes with functional states measured by activity assays

This multi-technique approach provides comprehensive mapping of the conformational dynamics underlying the coupling between electron transfer and ion translocation in the Na(+)-NQR complex.

How does Proteus mirabilis Na(+)-NQR activity contribute to pathogenesis in urinary tract infections?

The contribution of Na(+)-translocating NADH-quinone reductase (Na(+)-NQR) activity to Proteus mirabilis pathogenesis during urinary tract infections represents an important intersection between bacterial bioenergetics and virulence. Investigating this relationship requires integrated approaches spanning molecular microbiology, infection biology, and host-pathogen interactions:

• Adaptation to urinary tract conditions:

  • P. mirabilis produces urease, which generates ammonia and creates alkaline conditions during infection

  • Na(+)-based bioenergetics may offer an advantage over H+-based systems in these alkaline environments

  • Na(+)-NQR activity could maintain energy production despite pH fluctuations caused by urease activity

• Experimental approaches to investigate this relationship:

  • Isogenic mutant construction: Generation of nqrE deletion mutants in P. mirabilis

  • Growth phenotyping: Comparative growth analysis under conditions mimicking the urinary tract

  • Transcriptional profiling: RNA-seq analysis of Na(+)-NQR expression during infection

  • Metabolic flux analysis: Measurement of changes in central metabolism depending on Na(+)-NQR function

• Connection to virulence factor expression:

  • Proteomic analysis: Comparison of wild-type and nqrE mutant secretomes

  • Biofilm formation assessment: Quantification of biofilm formation capacity

  • Motility assays: Evaluation of swarming and swimming behaviors

  • Urease activity measurement: Determination of whether Na(+)-NQR activity affects urease expression or function

• In vivo infection studies:

  • Mouse models of urinary tract infection: Comparative virulence of wild-type and nqrE mutants

  • Competitive index determination: Co-infection with wild-type and mutant strains

  • Tissue colonization patterns: Histological examination of infected tissues

  • Host response analysis: Measurement of inflammatory markers and immune cell recruitment

• Therapeutic implications:

  • Na(+)-NQR inhibitor testing: Evaluation of effects on virulence in infection models

  • Combination therapy assessment: Testing Na(+)-NQR inhibitors with conventional antibiotics

  • Development of anti-virulence strategies: Targeting Na(+)-NQR to reduce pathogenicity without selecting for resistance

These investigations would clarify whether Na(+)-NQR activity represents a specific adaptation that enhances P. mirabilis pathogenesis in the urinary tract or is primarily involved in basic metabolism. This knowledge could inform novel therapeutic approaches, particularly for infections caused by multidrug-resistant P. mirabilis strains.

How can structural biology techniques be optimized for membrane proteins like Na(+)-translocating NADH-quinone reductase subunit E?

Structural biology of membrane proteins like Na(+)-translocating NADH-quinone reductase subunit E presents unique challenges requiring specialized approaches. Researchers seeking high-resolution structural data should consider these optimized methodologies:

• Protein engineering for structural studies:

  • Thermostabilizing mutations: Systematic alanine scanning to identify stabilizing substitutions

  • Fusion protein strategies: N- or C-terminal fusions with crystallization chaperones (e.g., T4 lysozyme)

  • Antibody fragment complexation: Generation of Fab or nanobody fragments to stabilize specific conformations

  • Minimal construct design: Removal of flexible regions while maintaining functional integrity

• Cryo-electron microscopy optimization:

  • Detergent screening: Systematic evaluation of detergents for particle dispersion and contrast

  • Amphipol and nanodisc incorporation: Alternative to detergents for maintaining native-like environment

  • Focused refinement techniques: Enhanced resolution of specific domains of interest

  • Time-resolved methods: Capture of different functional states through rapid freezing

• Crystallization strategies:

  • Lipidic cubic phase methods: For membrane proteins resistant to traditional crystallization

  • Bicelle crystallization: Intermediate between detergent micelles and lipid bilayers

  • Crystal dehydration protocols: Controlled dehydration to improve diffraction quality

  • Microcrystal techniques: Serial crystallography at X-ray free-electron lasers for small crystals

• NMR approaches for membrane proteins:

  • Selective isotope labeling: Strategic 15N, 13C, or 2H incorporation to simplify spectra

  • Solid-state NMR methods: Applicable to membrane proteins in near-native environments

  • Paramagnetic relaxation enhancement: Measurement of long-range distance constraints

  • Fragment-based approaches: Structural characterization of individual domains

• Computational integration:

  • Hybrid modeling approaches: Combining lower-resolution experimental data with computational predictions

  • Molecular dynamics refinement: Validation and refinement of structures in membrane environments

  • Co-evolutionary analysis: Using sequence co-variation to predict structural contacts

  • AlphaFold2 and similar AI methods: Integration with sparse experimental constraints

These methodologies should be applied systematically, with parallel trials of multiple approaches to maximize the likelihood of success. Researchers should also consider the functional state of the protein, potentially stabilizing specific conformations through inhibitors, substrate analogs, or mutations that lock the protein in defined states.

What is the relationship between Na(+)-NQR function and bacterial antibiotic resistance?

The relationship between Na(+)-translocating NADH-quinone reductase (Na(+)-NQR) function and bacterial antibiotic resistance represents an emerging area with significant implications for antimicrobial therapy. This connection can be explored through several research angles:

• Metabolic adaptation and resistance:

  • Energy metabolism flexibility: Na(+)-NQR may provide alternative energy generation pathways when primary routes are inhibited by antibiotics

  • Persister cell formation: Changes in Na(+)-NQR activity may contribute to metabolic dormancy and antibiotic tolerance

  • Redox homeostasis: Na(+)-NQR contributes to maintaining NAD+/NADH ratios, potentially affecting responses to oxidative stress-inducing antibiotics

• Membrane potential and antibiotic uptake:

  • Electrochemical gradient modulation: Na(+)-NQR contributes to membrane potential, which drives uptake of many antibiotics

  • Effect on efflux pump activity: Changes in ion gradients may affect the efficiency of multidrug efflux pumps

  • Membrane permeability alterations: Na+ gradients may influence membrane lipid organization and permeability

• Experimental approaches to investigate these relationships:

  • Antibiotic susceptibility profiling: Comprehensive MIC determination in Na(+)-NQR mutants

  • Antibiotic uptake assays: Measurement of accumulation of fluorescent or radioactive antibiotics

  • Membrane potential monitoring: Use of potential-sensitive dyes in wild-type versus mutant strains

  • Transcriptomic analysis: Identification of resistance genes differentially expressed when Na(+)-NQR is inhibited

• Na(+)-NQR inhibitors as resistance modifiers:

  • Combination therapy evaluation: Testing Na(+)-NQR inhibitors with conventional antibiotics

  • Resistance reversal assessment: Determination if Na(+)-NQR inhibition can restore sensitivity to resistant strains

  • Resistance development monitoring: Evaluation of resistance frequency to combination therapies

  • Structure-activity relationships: Development of dual-action molecules targeting both Na(+)-NQR and other targets

This research direction is particularly relevant given that respiratory chain components like Na(+)-NQR can influence bacterial physiology in ways that affect antibiotic sensitivity, potentially offering new strategies to combat antimicrobial resistance in pathogens like Proteus mirabilis.

How can systems biology approaches integrate Na(+)-NQR function into whole-cell metabolic models?

Integrating Na(+)-translocating NADH-quinone reductase (Na(+)-NQR) function into whole-cell metabolic models requires sophisticated systems biology approaches that connect bioenergetics with broader cellular metabolism. Researchers should employ the following methodological strategies:

• Flux balance analysis (FBA) integration:

  • Stoichiometric coefficient determination: Precise measurement of Na+/electron ratios

  • Constraint-based modeling: Incorporation of Na(+)-NQR reaction constraints into genome-scale metabolic models

  • Objective function optimization: Testing different cellular objectives (growth rate, ATP production, redox balance)

  • Alternative optimal solution analysis: Exploring multiple solutions that satisfy optimality criteria

• Multi-omics data integration:

  • Transcriptomics: RNA-seq data on Na(+)-NQR expression under various conditions

  • Proteomics: Absolute quantification of Na(+)-NQR complex components

  • Metabolomics: Profiling of metabolite levels affected by Na(+)-NQR activity

  • Fluxomics: 13C metabolic flux analysis to measure actual flux distributions

• Kinetic modeling approaches:

  • Enzymatic rate equation development: Detailed kinetic characterization of Na(+)-NQR

  • Parameter estimation: Fitting of kinetic parameters to experimental data

  • Sensitivity analysis: Identification of rate-limiting steps in Na(+)-NQR function

  • Dynamic simulation: Modeling temporal responses to environmental changes

• Environmental adaptation modeling:

  • pH-dependent regulation: Incorporating effects of pH on Na(+)-NQR activity

  • Ion availability constraints: Modeling dependency on Na+ concentration

  • Oxygen limitation responses: Predicting metabolic rerouting under varying oxygen tensions

  • Host environment simulation: Modeling metabolism during urinary tract infection conditions

• Model validation approaches:

  • Gene knockout phenotype prediction: Comparing model predictions with experimental nqrE deletion data

  • Growth rate predictions: Testing model accuracy across different environmental conditions

  • Inhibitor response simulation: Predicting metabolic responses to Na(+)-NQR inhibition

  • Flux measurements: Validation with experimental flux data from 13C labeling experiments