Recombinant Psychrobacter sp. Na (+)-translocating NADH-quinone reductase subunit E

What is Na(+)-translocating NADH-quinone reductase and what is the function of its E subunit?

Na(+)-translocating NADH-quinone reductase (NQR) is a respiratory complex that couples the oxidation of NADH to quinone with the translocation of sodium ions across the membrane. The E subunit (NqrE) is a transmembrane protein component of this complex that plays a crucial role in the ion translocation process. NqrE typically contains multiple transmembrane domains and forms part of the channel through which sodium ions are transported. Based on homologous proteins, NqrE contains approximately 202 amino acid residues and contributes to the membrane-embedded portion of the NQR complex .

How does Psychrobacter sp. NqrE differ from homologous proteins in other bacterial species?

While the search results don't provide specific information about Psychrobacter sp. NqrE, comparative analysis with homologous proteins such as those from Pseudoalteromonas atlantica suggests potential adaptations to cold environments. The NqrE from Pseudoalteromonas atlantica consists of 202 amino acid residues and contains multiple transmembrane domains, as evidenced by its highly hydrophobic amino acid sequence rich in leucine, isoleucine, and valine residues . Psychrophilic adaptations might include differences in amino acid composition that enhance protein flexibility at low temperatures, modified substrate binding regions, or alterations in ion channel properties to maintain function in cold environments .

What expression systems are recommended for recombinant production of Psychrobacter sp. NqrE?

For recombinant production of membrane proteins like NqrE, E. coli expression systems are commonly employed. Based on protocols for similar proteins, the following approach is recommended:

• Clone the nqrE gene from Psychrobacter sp. into an expression vector with an appropriate tag (His-tag is commonly used for purification)

• Transform into an E. coli expression strain optimized for membrane protein production

• Express at lower temperatures (16-25°C) to facilitate proper folding

• Include appropriate detergents during membrane protein extraction and purification

For instance, the homologous NqrE protein from Pseudoalteromonas atlantica was successfully expressed in E. coli as a full-length protein (1-202aa) fused to an N-terminal His tag .

How can researchers optimize purification protocols for NqrE to maintain structural integrity and function?

Purification of membrane proteins like NqrE requires specialized approaches to maintain their native structure. Based on protocols for similar proteins, the following methodology is recommended:

• Cell lysis should be performed using methods that minimize protein denaturation, such as gentle sonication or enzymatic lysis.

• Membrane fraction isolation through differential centrifugation.

• Solubilization using appropriate detergents (commonly DDM, LDAO, or C12E8) at concentrations above their critical micelle concentration.

• Affinity chromatography using the protein's His-tag, with detergent present in all buffers.

• Size exclusion chromatography as a polishing step.

• Storage in a buffer containing 6% trehalose to maintain stability, as used for similar proteins .

For long-term storage, the protein can be lyophilized or stored in buffer with 50% glycerol at -20°C/-80°C. Avoid repeated freeze-thaw cycles as they can disrupt protein structure .

What are the implications of NqrE dysfunction for bacterial metabolism and survival in extreme environments?

NqrE, as part of the Na(+)-translocating NADH-quinone reductase complex, plays a critical role in respiratory metabolism and bioenergetics. Dysfunction of this complex would likely have several consequences:

• Disrupted sodium ion gradient maintenance, affecting numerous secondary transport processes

• Altered NADH/NAD+ ratios, impacting central metabolic pathways

• Compromised respiratory efficiency, particularly under energy-limited conditions

• Reduced ability to adapt to environmental stressors, including temperature extremes

In psychrophilic organisms like Psychrobacter sp., NQR may be particularly important for maintaining energy metabolism at low temperatures where enzyme kinetics are slower. The Na(+) motive force generated by this complex could be crucial for substrate uptake and other cellular processes under cold conditions .

Additionally, studies on related respiratory complexes in other bacteria suggest that NADH dehydrogenases play important roles in adaptation to variable oxygen concentrations. The presence of alternative respiratory complexes in many bacteria indicates the importance of metabolic flexibility in extreme environments .

How do the structural features of psychrophilic NqrE compare with mesophilic homologs in terms of cold adaptation?

While specific structural comparisons between psychrophilic and mesophilic NqrE are not provided in the search results, general principles of cold adaptation in membrane proteins can be applied:

The amino acid sequence of Pseudoalteromonas atlantica NqrE (MEQYLSLFIRSIFLENMALFYFLGMCTFLAVSKKVKTAMGLGVAVIVVLTISVPVNQLVYANILAPGALGWAGFPDTDLSFLSFLTFIGVIAALVQILEMTLDKFFPALYNALGIFLPLITVNCAIFGGVAFAVQRDYTFTESIFYGAGSGAGWALAITLLAAVREKLKYADMPEGVRGLGSVFMIAGLMALGFQSFSGVSI) suggests a highly hydrophobic protein with multiple membrane-spanning regions . In a psychrophilic version, we might expect modifications to this sequence that maintain essential structural features while enhancing flexibility at low temperatures.

What expression vectors and conditions optimize the yield of functional recombinant Psychrobacter sp. NqrE?

For optimal expression of functional NqrE, researchers should consider:

• Expression vector selection:

  • Vectors with tightly regulated promoters (such as pET series) to control expression

  • Inclusion of appropriate fusion tags (His-tag for purification, potentially MBP for solubility)

  • Consideration of codon optimization for E. coli expression

• Expression conditions:

  • Lower induction temperatures (16-20°C) to slow expression and improve folding

  • Extended expression time (overnight to 24h) at reduced temperatures

  • Induction at lower OD600 values (0.4-0.6) to avoid excessive protein production

  • Supplementation with additional cofactors if required for proper folding

• Host strain considerations:

  • C41(DE3) or C43(DE3) strains optimized for membrane protein expression

  • Rosetta strains if rare codons are present in the Psychrobacter sp. sequence

  • BL21(DE3)pLysS for tighter expression control

For recombinant production similar to that achieved with Pseudoalteromonas atlantica NqrE, expression in E. coli with an N-terminal His tag allows for effective purification and functional studies .

What methods can accurately assess the sodium-translocating activity of purified NqrE?

Assessing sodium-translocating activity requires specialized techniques:

• Reconstitution into liposomes:

  • Purified NqrE should be reconstituted into liposomes containing appropriate lipids

  • A sodium-sensitive fluorescent dye (e.g., SBFI) can be entrapped in the liposomes

  • Addition of NADH and appropriate quinones initiates electron transport

• Direct measurement approaches:

  • 22Na+ uptake assays with reconstituted proteoliposomes

  • pH changes monitored with pH-sensitive dyes to assess proton/sodium antiport

  • Membrane potential measurements using voltage-sensitive dyes

• Enzyme-coupled assays:

  • NADH oxidation can be monitored spectrophotometrically at 340 nm

  • Quinone reduction can be monitored by absorbance changes

  • Correlation of these activities with sodium movement confirms coupling

For functional characterization, researchers should test the enzyme's sensitivity to known inhibitors of Na(+)-translocating NADH-quinone reductases and verify substrate specificity with different electron donors and acceptors .

How can researchers effectively use immunoprecipitation techniques to study NqrE interactions with other subunits?

For studying protein-protein interactions involving NqrE, the following immunoprecipitation methodology is recommended:

• Sample preparation:

  • Extract membrane proteins using appropriate detergents that maintain protein-protein interactions

  • Use approximately 600 μg of protein extract per immunoprecipitation reaction

• Antibody selection and incubation:

  • Generate or obtain specific antibodies against NqrE

  • Incubate protein extracts with 3 μl (1 mg/ml) of antibody for 4 hours at 4°C with rotation

  • Use an immunoprecipitation buffer containing: Tris HCl (500 mM, pH 7.5), NaCl (200 mM), 0.2% Triton X-100, 0.2% NP-40, EDTA (5 mM, pH 8), and protease inhibitors

• Protein G-Sepharose addition:

  • Add 30 μl of washed Protein G-Sepharose (50% slurry)

  • Incubate for an additional hour at 4°C with rotation

• Analysis of co-precipitated proteins:

  • Wash the beads thoroughly to remove non-specific binding

  • Elute bound proteins and analyze by Western blotting or mass spectrometry

This protocol, adapted from methods used in proteomic studies of psychrophilic organisms, provides stringent conditions to minimize non-specific complexes while preserving genuine protein interactions .

How can researchers interpret differences in kinetic parameters between psychrophilic and mesophilic NqrE proteins?

When analyzing kinetic parameters of psychrophilic versus mesophilic NqrE proteins, consider the following interpretative framework:

• Temperature-dependent parameters:

  • Psychrophilic enzymes typically show higher catalytic efficiency (kcat/Km) at low temperatures

  • Lower activation energy (Ea) values in psychrophilic enzymes indicate less temperature-dependent activity

  • Broader temperature activity profiles often indicate cold adaptation

• Substrate affinity considerations:

  • Changes in Km values reflect adaptations in substrate binding sites

  • Lower Km values at cold temperatures indicate improved substrate binding under cold conditions

  • Different temperature-dependencies of Km between psychrophilic and mesophilic variants

• Structural stability trade-offs:

  • Higher thermolability of psychrophilic enzymes reflects reduced structural rigidity

  • Different denaturation profiles between variants provide insight into structural adaptations

  • Potential for conformational differences that facilitate activity at low temperatures

Researchers should analyze these parameters across a temperature range (0-40°C) to fully characterize the temperature-dependent behavior of both enzyme variants. The trade-off between structural flexibility needed for cold activity and stability required for function should be carefully evaluated .

What bioinformatic approaches are most effective for identifying cold-adaptation signatures in NqrE sequences?

For identifying cold-adaptation signatures in NqrE sequences, the following bioinformatic approaches are recommended:

• Sequence-based analyses:

  • Multiple sequence alignment of NqrE homologs from psychrophilic, mesophilic, and thermophilic organisms

  • Calculation of amino acid composition bias, particularly focusing on reduced proline content, increased glycine, and reduced arginine in psychrophilic variants

  • Analysis of charged versus uncharged residue distribution

  • Identification of regions with altered hydrophobicity profiles

• Structure-based predictions:

  • Homology modeling based on available crystal structures of related proteins

  • Analysis of predicted hydrogen bonding networks and salt bridge formation

  • Evaluation of loop regions for increased flexibility

  • Identification of amino acid substitutions near catalytic or ion-transporting regions

• Evolutionary analyses:

  • Construction of phylogenetic trees to identify convergent evolution patterns in cold adaptation

  • Calculation of dN/dS ratios to identify sites under positive selection

  • Identification of co-evolving residues that may contribute to cold adaptation

  • Comparison with adaptation patterns in other membrane proteins from the same organisms

These approaches should be integrated to identify statistically significant patterns that distinguish psychrophilic NqrE from mesophilic homologs, focusing particularly on regions involved in sodium translocation and subunit interactions .

How might recombinant NqrE be utilized in studies of bioenergetics and respiratory metabolism?

Recombinant NqrE can serve as a valuable tool in bioenergetic research through several applications:

• Comparative bioenergetic studies:

  • Investigation of ion-coupled electron transport in different bacterial species

  • Analysis of energy conservation mechanisms in extremophiles

  • Comparison of sodium-motive force versus proton-motive force in bacterial energetics

• Functional reconstitution systems:

  • Creation of minimal electron transport systems in liposomes

  • Study of ion selectivity in respiratory complexes

  • Investigation of quinone specificity in respiratory chains

• Structural and mechanistic investigations:

  • Component for structural studies of complete NQR complexes

  • Analysis of conformational changes during ion translocation

  • Investigation of subunit interactions during electron transfer

• Biotechnological applications:

  • Development of biosensors for respiratory inhibitors

  • Creation of minimal systems for testing electron transport inhibitors

  • Platform for screening novel antibacterial compounds targeting bacterial respiration

These applications can provide crucial insights into bacterial energy metabolism and adaptation to extreme environments, particularly for psychrophilic organisms with specialized respiratory adaptations .

What role could NqrE studies play in understanding neurodegeneration mechanisms?

Studies of bacterial Na(+)-translocating NADH-quinone reductase subunit E may have indirect relevance to neurodegenerative research through several connections:

• Mitochondrial complex I dysfunction model:

  • Bacterial NQR complexes share some functional similarities with mitochondrial complex I

  • Understanding bacterial sodium/proton translocation mechanisms may inform studies of mitochondrial ion transport

  • Bacterial systems provide simplified models for studying electron transport coupling

• Therapeutic development framework:

  • Single-subunit NADH dehydrogenases have been explored as potential therapeutic tools for neurodegenerative disorders

  • The yeast Ndi1 enzyme has been studied as a replacement for dysfunctional complex I in mammalian cells

  • Understanding ion-translocating mechanisms could inform development of novel therapeutic approaches

• Oxidative stress connections:

  • Electron transport chain dysfunction is linked to oxidative stress in neurodegenerative disorders

  • Bacterial models help elucidate fundamental principles of electron leakage and reactive oxygen species generation

  • Mechanisms of respiratory complex inhibition by toxins can inform understanding of environmental factors in neurodegeneration

Research has shown that expression of alternative NADH dehydrogenases can protect against complex I inhibitors like rotenone and pyridaben, which are associated with Parkinson's disease models. Similarly, understanding fundamental mechanisms of ion-coupled electron transport in bacterial systems may provide insights applicable to mitochondrial dysfunction in neurodegenerative conditions .

What techniques can be used to study the interaction between NqrE and specific inhibitors?

To investigate interactions between NqrE and potential inhibitors, researchers can employ several complementary techniques:

• Enzymatic activity assays:

  • NADH oxidation assays in the presence of varying inhibitor concentrations

  • Determination of IC50 values for different inhibitors

  • Analysis of inhibition mechanisms (competitive, non-competitive, uncompetitive)

  • Evaluation of sodium transport inhibition using ion-selective electrodes or fluorescent indicators

• Binding studies:

  • Isothermal titration calorimetry (ITC) to determine binding affinity and thermodynamic parameters

  • Surface plasmon resonance (SPR) for real-time binding kinetics

  • Fluorescence-based binding assays if appropriate fluorescent probes are available

  • Thermal shift assays to assess protein stabilization/destabilization upon inhibitor binding

• Structural approaches:

  • X-ray crystallography of NqrE in complex with inhibitors

  • Cryo-EM analysis of the NQR complex with bound inhibitors

  • Hydrogen-deuterium exchange mass spectrometry to identify regions affected by inhibitor binding

  • Site-directed mutagenesis to identify key residues involved in inhibitor interactions

These methods can help identify specific binding sites and inhibition mechanisms, which is particularly valuable for developing targeted antimicrobial compounds against pathogenic bacteria that rely on NQR complexes for energy metabolism .

What are the recommended storage and handling procedures for maintaining recombinant NqrE stability?

For optimal stability of recombinant NqrE, the following storage and handling procedures are recommended:

• Short-term storage (up to one week):

  • Store working aliquots at 4°C

  • Maintain in appropriate buffer containing detergent above critical micelle concentration

  • Avoid repeated freeze-thaw cycles

• Long-term storage:

  • Store at -20°C/-80°C in small aliquots

  • Include cryoprotectants such as 50% glycerol or 6% trehalose

  • Consider lyophilization for extended storage periods

• Handling recommendations:

  • Reconstitute lyophilized protein in deionized sterile water to a concentration of 0.1-1.0 mg/mL

  • Centrifuge vials briefly before opening to bring contents to the bottom

  • Add glycerol to 5-50% final concentration for aliquots intended for freezing

  • Use appropriate detergents at all times to maintain protein solubility

• Buffer considerations:

  • Use Tris/PBS-based buffer, pH 8.0

  • Include stabilizing agents such as trehalose (6%)

  • For membrane protein stability, ensure detergent concentration remains above CMC

These recommendations are based on protocols for similar membrane proteins and should be optimized for specific research applications .

What chaperone systems might facilitate proper folding of recombinant NqrE during expression?

Proper folding of membrane proteins like NqrE often requires assistance from chaperone systems. The following approaches may enhance functional protein production:

• Co-expression with molecular chaperones:

  • DnaK-DnaJ-GrpE system (70 kDa, 41 kDa, and 20 kDa respectively)

  • GroEL-GroES system (60 kDa and 10 kDa respectively)

  • Consider using commercially available chaperone plasmids like pG-KJE8 or pG-Tf2

• Chaperone expression optimization:

  • Induce chaperone expression prior to target protein induction

  • Maintain appropriate chaperone levels through regulated expression systems

  • Consider temperature downshift to increase chaperone availability

• Detection and monitoring of chaperone interactions:

  • Use immunoprecipitation with antibodies against specific chaperones (e.g., anti-DnaK or anti-GroEL)

  • Apply Western blotting techniques to detect chaperone associations

  • Consider the following antibody dilutions based on experimental protocols:

• Experimental workflow for chaperone interaction analysis:

  • Prepare protein extracts (approximately 600 μg)

  • Incubate with antibodies (3 μl at 1 mg/ml) for 4 hours at 4°C

  • Add 30 μl of Protein G-Sepharose (50% slurry)

  • Continue incubation for 1 hour under identical conditions

  • Wash and analyze co-precipitated proteins

These approaches can significantly improve the yield of properly folded recombinant NqrE protein for downstream applications .

What are the key research frontiers in understanding psychrophilic Na(+)-translocating NADH-quinone reductases?

Current research frontiers in understanding psychrophilic Na(+)-translocating NADH-quinone reductases include:

• Structural biology perspectives:

  • Determination of high-resolution structures of complete NQR complexes from psychrophilic organisms

  • Elucidation of cold-adapted mechanisms for ion translocation

  • Investigation of conformational flexibility at low temperatures

• Bioenergetic aspects:

  • Quantification of sodium pumping efficiency at different temperatures

  • Understanding the energetic trade-offs in cold adaptation

  • Comparison of ATP yield in psychrophilic versus mesophilic NQR systems

• Ecological and evolutionary considerations:

  • Role of NQR in niche adaptation for psychrophilic bacteria

  • Evolutionary trajectories leading to cold-adapted respiratory complexes

  • Comparative genomic analysis across diverse psychrophilic species

• Biotechnological applications:

  • Development of cold-active biocatalysts based on psychrophilic respiratory components

  • Exploration of energy-efficient bioprocesses utilizing psychrophilic electron transport systems

  • Potential applications in biosensors functioning at low temperatures

These research directions represent important areas for future investigation to advance our understanding of bacterial adaptation to cold environments and the fundamental principles of ion-coupled electron transport .

How might advances in structural biology techniques impact our understanding of NqrE function?

Recent and emerging advances in structural biology techniques are poised to significantly enhance our understanding of NqrE function through several approaches:

• Cryo-electron microscopy advancements:

  • Single-particle cryo-EM now achieves near-atomic resolution for membrane protein complexes

  • Ability to capture multiple conformational states provides insights into the mechanism of ion translocation

  • Visualization of the complete NQR complex architecture in native-like lipid environments

• Integrative structural approaches:

  • Combination of X-ray crystallography, NMR, and computational modeling

  • Cross-linking mass spectrometry to identify subunit interaction interfaces

  • Hydrogen-deuterium exchange mass spectrometry to map conformational changes

• Time-resolved structural methods:

  • Time-resolved cryo-EM to capture transient states during the catalytic cycle

  • Serial crystallography using X-ray free-electron lasers for dynamic studies

  • Correlation of structural changes with functional measurements

• In situ structural biology:

  • Cryo-electron tomography of intact bacterial cells

  • Visualization of respiratory complexes in their native membrane environment

  • Correlative light and electron microscopy to link structural and functional information