Recombinant Haemophilus parasuis serovar 5 Na (+)-translocating NADH-quinone reductase subunit E

What is the structure and function of Na(+)-translocating NADH-quinone reductase subunit E in Haemophilus parasuis?

Na(+)-translocating NADH-quinone reductase subunit E (nqrE) is one of six subunits (NqrA to NqrF) of the Na+-NQR complex found in the respiratory chain of Haemophilus parasuis. This protein consists of 198 amino acids with the sequence beginning with "MEHYLSLFVKSIFIEN..." and contains multiple transmembrane domains. The nqrE subunit is integral to the membrane-embedded portion of the Na+-NQR complex that catalyzes electron transfer from NADH to ubiquinone, coupled with Na+ translocation across the bacterial membrane .

Functionally, nqrE participates in:

• Formation of the (Cys)4[Fe] center with NqrD, which is critical for electron transport

• Binding riboflavin between nqrE and nqrB subunits

• Contributing to the Na+ translocation pathway through its transmembrane regions

• Maintaining structural integrity of the Na+-NQR complex

Researchers typically work with recombinant nqrE expressed in E. coli with affinity tags for purification and characterization studies.

How should recombinant nqrE protein be stored and handled in laboratory settings?

Proper storage and handling of recombinant nqrE protein is essential for maintaining its structural integrity and activity :

Important handling considerations:

• Briefly centrifuge vials before opening to bring contents to the bottom

• Minimize repeated freeze-thaw cycles which can lead to protein degradation

• When thawing, keep the protein on ice and use immediately or store at 4°C

• For experimental work, maintain the protein at appropriate temperature conditions specific to the assay being performed

What is the significance of Haemophilus parasuis serovar 5 in veterinary research?

Haemophilus parasuis is the etiological agent of Glässer's disease, a significant infectious disease in swine characterized by fibrinous polyserositis, polyarthritis, and meningitis . Serovar 5 has garnered particular research attention for several reasons:

• Economic impact: Glässer's disease causes substantial financial losses in the global pig industry

• Virulence: Serovar 5 includes highly virulent strains such as H46, which are commonly used in challenge models

• Genomic resources: The complete genome sequence of H. parasuis serovar 5 (strain SH0165) is available, facilitating comprehensive genomic and proteomic studies

• Vaccine development: Serovar 5 has been extensively studied for identification of immunoprotective antigens that could contribute to subunit vaccine development

Understanding the molecular components of H. parasuis serovar 5, including the Na+-NQR complex, contributes to our knowledge of bacterial energy metabolism and potential targets for therapeutic intervention.

What expression systems are typically used for producing recombinant nqrE?

Based on the search results, several expression systems and strategies are employed for producing recombinant nqrE :

Expression vectors:

• pET-based vectors (e.g., pET28a+) for IPTG-inducible expression

• pBAD vectors for arabinose-inducible expression when co-expressing multiple components

Host organisms:

• Escherichia coli (typically BL21(DE3) or similar strains)

• Expression in the original host (H. parasuis) is less common due to challenging culture conditions

Fusion tags:

• N-terminal or C-terminal His-tag for affinity purification

• The His-tag does not appear to interfere with protein structure or function

Expression conditions:

• Induction parameters must be optimized (temperature, inducer concentration, time)

• Growth media often supplemented with specific nutrients to enhance protein yield

When expressing nqrE for functional studies of the complete Na+-NQR complex, co-expression of all six nqr genes (nqrA-F) along with the maturation factors apbE and nqrM is necessary to obtain a functional complex .

What are the basic assays used to characterize nqrE activity?

Characterization of nqrE activity is typically performed in the context of the complete Na+-NQR complex, as the individual subunit does not exhibit independent enzymatic activity. The following assays are commonly used :

NADH/dNADH oxidation assay:

• Measures the decrease in absorbance at 340 nm as NADH or dNADH is oxidized

• Conducted at 30°C using a spectrophotometer

• Quantified using an extinction coefficient (ε340) of 6.22 mM−1 cm−1

Na+-dependent quinone reductase activity:

• Reaction buffer: 20 mM HEPES-Tris, 5 mM MgSO4, and 50 mM KCl (pH 8.0)

• Activity is measured with and without Na+ to determine the Na+-dependent component

• HQNO (2-heptyl-4-hydroxyquinoline N-oxide) sensitivity confirms specificity

dNADH:menadione oxidoreductase activity:

• Measures the FAD-dependent activity using menadione as electron acceptor

• Does not require the intact complex and is Na+-independent

• Reaction medium is supplemented with 50 μM menadione

The activity data from these assays provide insights into both the electron transfer function and the Na+-coupling mechanism of the Na+-NQR complex containing nqrE.

What factors are critical for the maturation of functional Na+-NQR complex containing nqrE?

The maturation of a functional Na+-NQR complex requires specific factors beyond the expression of the six structural subunits (NqrA-F) :

1. Flavin transferase (ApbE):

• Catalyzes covalent attachment of FMN to threonine residues in NqrB and NqrC

• This post-translational modification occurs on the periplasmic side of the bacterial membrane

• Essential for quinone reductase activity

2. NqrM protein (formerly DUF539):

• Contains a single transmembrane α-helix and four conserved cysteine residues

• Presumed to be involved in Fe delivery for the (Cys)4[Fe] center between NqrD and NqrE

• Only present in bacteria containing Na+-NQR

Expression studies in E. coli demonstrate that co-expression of the nqr operon with apbE alone is insufficient for producing functional Na+-NQR. The complex only becomes capable of Na+-dependent quinone reduction when nqrM is also co-expressed. Na+-NQR isolated from nqrM-deficient strains lacks several subunits, indicating nqrM's essential role in complex assembly .

This intricate maturation process highlights the complexity of expressing fully functional membrane protein complexes in heterologous systems and provides insights into the assembly mechanisms of multisubunit respiratory enzymes.

How do mutations in conserved cysteine residues of NqrM affect Na+-NQR maturation and nqrE incorporation?

The NqrM protein, essential for Na+-NQR maturation, contains four conserved cysteine residues that play differential roles in complex assembly :

Mutation analysis results:

• Cys33 is critical: Mutation of Cys33 to Ser completely prevents Na+-NQR maturation

• Other Cys residues (Cys42, Cys45, Cys53): Mutations decrease the yield of mature Na+-NQR but do not completely prevent maturation

The differential impacts suggest a hierarchy of importance among these cysteine residues:

These findings are significant because:

• They suggest Cys33 may be directly involved in coordinating iron delivery to form the (Cys)4[Fe] center between NqrD and NqrE

• The other cysteines may play supporting roles in the NqrM structure or function

• Understanding these residues provides insight into the mechanism of iron-sulfur center assembly in membrane proteins

Researchers investigating nqrE incorporation into the Na+-NQR complex should consider the critical role of NqrM and particularly its Cys33 residue in the proper assembly process.

What methodological approaches can be used to study the interaction between nqrE and other subunits of the Na+-NQR complex?

Several methodological approaches can be employed to study the interactions between nqrE and other subunits of the Na+-NQR complex:

Co-purification and complex isolation:

• Expression of His-tagged nqrE with other untagged subunits

• Metal chelate chromatography purification to identify co-purifying partners

• Analysis by SDS-PAGE and MALDI-TOF/TOF MS to identify interacting subunits

Crosslinking studies:

• Chemical crosslinking with agents that have defined spacer arms

• Identification of crosslinked products by mass spectrometry

• Mapping of interaction interfaces between nqrE and its partners

Mutational analysis:

• Site-directed mutagenesis of conserved residues in nqrE

• Analysis of effects on complex assembly and function

• Special focus on residues involved in the (Cys)4[Fe] center formation with NqrD

Structural studies:

• Cryo-electron microscopy of the intact complex

• X-ray crystallography (as mentioned in search result , a crystal structure of Na+-NQR exists)

• Computational modeling of subunit interactions

Functional reconstitution:

• Reconstitution of purified subunits into proteoliposomes

• Measurement of Na+ transport and electron transfer activities

• Correlation of structure with function

These approaches can provide comprehensive insights into how nqrE interacts with other subunits to form a functional Na+-NQR complex and contribute to our understanding of membrane protein complex assembly mechanisms.

What is the role of the (Cys)4[Fe] center between nqrD and nqrE in Na+-NQR function?

The (Cys)4[Fe] center formed between nqrD and nqrE subunits plays critical roles in Na+-NQR function :

1. Electron transfer pathway:

• Serves as an essential component in the electron transport chain from NADH to ubiquinone

• Functions alongside other cofactors (FAD in NqrF, riboflavin between NqrB and NqrE, FMN in NqrB and NqrC)

• May participate in the redox-driven conformational changes required for ion transport

2. Structural integrity:

3. Na+ translocation mechanism:

• Likely contributes to the coupling mechanism between electron transfer and Na+ movement

• Changes in the redox state of the iron center may trigger conformational changes in the transmembrane domains

• These conformational changes could alter Na+ binding sites, facilitating ion transport across the membrane

The importance of this center is emphasized by the absolute requirement for NqrM in delivering iron for its formation. Without proper assembly of the (Cys)4[Fe] center, the Na+-NQR complex lacks Na+-dependent quinone reductase activity, even though NADH dehydrogenase activity may still be present due to the FAD domain in NqrF .

How can recombinant nqrE be evaluated as a potential vaccine candidate against H. parasuis infections?

Evaluation of recombinant nqrE as a potential vaccine candidate against H. parasuis would follow a systematic approach based on methodologies described in the search results :

1. Immunogenicity assessment:

2. Functional immune assays:

• Bactericidal activity of whole blood from immunized animals

• Opsonophagocytic assays with immune sera

• Complement-mediated killing

3. Protection studies:

• Challenge with virulent H. parasuis serovar 5 (e.g., strain H46)

• Monitoring survival rates and clinical signs

• Bacterial load determination in tissues (liver, spleen, lung)

• PCR confirmation of H. parasuis in tissues

4. Combination strategies:
Research indicates that combining multiple antigens often provides better protection than individual proteins. Testing nqrE alone and in combination with other identified immunogenic proteins would be valuable .

While the search results don't specifically identify nqrE among previously tested vaccine candidates for H. parasuis, the systematic approach described for other proteins can be applied to evaluate its potential as a vaccine component.

What are the challenges in expressing and purifying functional Na+-NQR complex containing nqrE in heterologous systems?

Expressing and purifying a functional Na+-NQR complex containing nqrE in heterologous systems presents several significant challenges :

1. Requirement for multiple maturation factors:

• Expression of the nqr operon alone is insufficient

• Both ApbE (flavin transferase) and NqrM are essential co-factors

• These factors must be from compatible species with the expressed nqr genes

2. Complex post-translational modifications:

• Covalent FMN attachment to NqrB and NqrC requires functional ApbE

• Formation of the (Cys)4[Fe] center between NqrD and NqrE requires NqrM

• These modifications must occur correctly in the heterologous system

3. Membrane protein expression issues:

• Proper membrane insertion of multiple hydrophobic subunits

• Potential toxicity to host cells when overexpressed

• Different lipid composition in heterologous membranes

4. Assembly of multiple subunits:

• All six subunits must be correctly folded and assembled

• Stoichiometric expression may be difficult to achieve

• Incomplete complexes lack full functionality

5. Species compatibility:

• Factors from the native species are often required

• For example, V. harveyi Na+-NQR requires V. harveyi NqrM for functionality in E. coli

6. Purification challenges:

• Maintaining the intact complex during solubilization and purification

• Preserving activity through the purification process

• Keeping all cofactors associated with the complex

These challenges must be addressed through careful experimental design, including co-expression strategies with compatible plasmids, optimization of expression conditions, and gentle purification methods that maintain the structural and functional integrity of the complex.

How does the Na+-NQR complex containing nqrE differ from other bacterial NADH dehydrogenases?

The Na+-NQR complex containing nqrE differs substantially from other bacterial NADH dehydrogenases in several key aspects :

The Na+-NQR complex represents a unique energy-converting enzyme that couples Na+ transport to the respiratory chain, rather than H+ transport as in Complex I. This difference is significant for bacterial bioenergetics, particularly in alkaliphilic or marine bacteria where Na+ cycling may be advantageous.

The unique subunit composition and cofactor arrangement in Na+-NQR, including the essential role of nqrE in forming the (Cys)4[Fe] center, highlight the diverse evolutionary solutions to the challenge of coupling electron transfer to ion translocation in biological systems.

What techniques can be used to study the topology and membrane insertion of nqrE?

Understanding the topology and membrane insertion of nqrE requires specialized techniques for membrane protein analysis:

1. Computational prediction methods:

• Hydropathy analysis of the amino acid sequence reveals potential transmembrane segments

• The nqrE sequence (MEHYLSLFVKSIFIEN...) suggests multiple hydrophobic regions

• Topology prediction algorithms can identify cytoplasmic, transmembrane, and periplasmic domains

2. Experimental topology mapping:

• Cysteine scanning mutagenesis with membrane-permeable and impermeable thiol reagents

• Reporter fusion techniques (PhoA, LacZ, or GFP fusions at various positions)

• Protease accessibility studies with membrane vesicles of defined orientation

3. Structural studies:

• X-ray crystallography of the Na+-NQR complex has been reported

• Cryo-electron microscopy to determine the orientation of nqrE within the complex

• Molecular dynamics simulations to model membrane insertion and lipid interactions

4. Biochemical approaches:

• Limited proteolysis of the purified complex followed by mass spectrometry

• Crosslinking with membrane-restricted reagents

• Antibody accessibility studies with epitope-tagged variants

5. Functional approaches:

• Site-directed mutagenesis of residues predicted to be important for membrane insertion

• Functional complementation studies with chimeric proteins

• Analysis of membrane extraction properties with different detergents

These techniques can provide comprehensive information about how nqrE inserts into the membrane, which regions interact with the lipid bilayer, and how it orients relative to other subunits in the Na+-NQR complex.

How might nqrE contribute to potential antimicrobial resistance mechanisms in H. parasuis?

While the search results don't directly address nqrE's role in antimicrobial resistance, its function in the Na+-NQR complex suggests potential mechanisms:

1. Alternative respiratory pathways:

• Na+-NQR provides an alternative electron transport pathway distinct from the H+-dependent Complex I

• This metabolic flexibility could allow adaptation to conditions where certain respiratory inhibitors are present

• Pathogens with multiple respiratory options may better survive antimicrobial challenges

2. Energy metabolism adaptations:

• Alterations in nqrE or other Na+-NQR components could modify the efficiency of energy generation

• Such adaptations might compensate for antimicrobial effects that target cellular energetics

• Metabolic adaptability is a known factor in antibiotic tolerance

3. Membrane potential modulation:

• Na+-NQR contributes to the creation of a sodium motive force across the membrane

• Changes in this ion gradient could affect the uptake of certain antimicrobials

• Many antibiotics require specific membrane potential for uptake or activity

4. Potential role in persistence:

• Alterations in energy metabolism can contribute to bacterial persistence

• Persistent states are often associated with tolerance to antimicrobials

• The Na+-NQR system might be involved in metabolic states that favor persistence

5. Target for resistance development:

• If the Na+-NQR complex were targeted by new antimicrobials, mutations in nqrE could contribute to resistance

• Such mutations would need to maintain function while reducing antimicrobial binding

Research investigating these possibilities would require:

• Comparative studies of nqrE sequences from susceptible and resistant isolates

• Functional analysis of Na+-NQR activity in the presence of antimicrobials

• Generation of nqrE mutants and assessment of their impact on antimicrobial susceptibility

What advanced techniques can be employed to study the electron transfer mechanism involving nqrE in the Na+-NQR complex?

Advanced techniques for studying the electron transfer mechanism involving nqrE in the Na+-NQR complex include:

1. Fast kinetic methods:

• Stopped-flow spectroscopy to measure rates of electron transfer between cofactors

• Rapid freeze-quench EPR to trap intermediates in the electron transfer process

• Flash photolysis with time-resolved spectroscopy

2. Spectroscopic techniques:

• Electron paramagnetic resonance (EPR) to characterize the (Cys)4[Fe] center between nqrD and nqrE

• Resonance Raman spectroscopy to probe the properties of flavin and iron centers

• Fluorescence spectroscopy to monitor conformational changes during electron transfer

3. Site-directed mutagenesis approaches:

• Systematic mutation of residues near the (Cys)4[Fe] center and other cofactors

• Altered amino acids that might participate in proton-coupled electron transfer

• Analysis of effects on electron transfer rates and coupling to Na+ translocation

4. Electrochemical methods:

• Protein film voltammetry to determine redox potentials of cofactors

• Direct electrochemistry of the purified complex on modified electrodes

• Correlation of electrochemical properties with functional activity

5. Advanced structural approaches:

• Time-resolved structural techniques to capture conformational changes

• Hydrogen/deuterium exchange mass spectrometry to identify dynamic regions

• Molecular dynamics simulations based on structural data

6. Single-molecule techniques:

• Single-molecule FRET to observe conformational dynamics during turnover

• Nanoscale electrochemistry of individual complexes

• Correlation of electron transfer events with conformational changes

These sophisticated techniques can provide detailed insights into how electrons move through the Na+-NQR complex, the role of nqrE and the (Cys)4[Fe] center in this process, and how electron transfer is coupled to Na+ translocation across the membrane.