Recombinant Acinetobacter sp. Disulfide bond formation protein B (dsbB)

What is the role of Disulfide bond formation protein B (dsbB) in Acinetobacter species?

Disulfide bond formation protein B (dsbB) in Acinetobacter species functions as a membrane protein that reoxidizes DsbA after each catalytic cycle of disulfide bond formation. In the bacterial periplasm, this oxidative folding system is crucial for the proper folding of many secreted proteins, including virulence factors. In Acinetobacter baumannii, which constitutes approximately 80% of all Acinetobacter hospital-acquired infections, dsbB likely contributes to the pathogen's ability to maintain structural integrity of its virulence factors . The protein works by transferring electrons from reduced DsbA to ubiquinone in the respiratory chain, maintaining the oxidative environment necessary for disulfide bond formation in newly synthesized proteins.

How does dsbB differ between Acinetobacter baumannii and other Acinetobacter species?

While A. baumannii has received the most research attention due to its clinical significance, dsbB proteins across different Acinetobacter species show varying degrees of sequence homology. Based on phylogenetic analyses using rpoB gene sequencing, which has been demonstrated as the most accurate identification method for Acinetobacter species, closely related species such as A. pittii, A. calcoaceticus, and A. nosocomialis (which together account for 85.5% of non-A. baumannii isolates) likely have similar dsbB structures with subtle amino acid variations that might affect substrate specificity or regulatory mechanisms . These differences may correlate with the significantly lower antimicrobial resistance rates observed in non-A. baumannii isolates (2.6% resistance to carbapenems compared to much higher rates in A. baumannii) .

What expression systems are most effective for producing recombinant Acinetobacter sp. dsbB?

• Expression Vector Selection: pET vector systems with T7 promoters provide controlled, high-level expression

• Host Strain Optimization: C41(DE3) or C43(DE3) strains are preferred for membrane proteins like dsbB

• Induction Conditions: Lower temperatures (16-25°C) and reduced IPTG concentrations (0.1-0.5 mM) typically yield properly folded protein

• Detergent Screening: A panel of detergents should be evaluated for optimal solubilization:

The choice of expression system should be tailored to downstream applications, with considerations for maintaining the native conformation of this integral membrane protein.

How can I confirm the proper folding and activity of recombinant Acinetobacter dsbB?

Confirming proper folding and activity of recombinant Acinetobacter dsbB requires multiple complementary approaches:

• Functional Assays: The oxidoreductase activity can be measured using a ubiquinone-coupled electron transfer assay that monitors the reduction of artificial electron acceptors like DCPIP (2,6-dichlorophenolindophenol).

• Structural Analysis: Circular dichroism (CD) spectroscopy can verify the expected secondary structure composition (predominantly α-helical for membrane-spanning regions).

• Thermal Stability Assessment: Differential scanning fluorimetry with appropriate membrane protein-compatible dyes can evaluate thermal stability under various buffer conditions.

• Reconstitution Studies: Functional reconstitution into liposomes followed by activity measurements can confirm native-like behavior in a membrane environment.

• Complex Formation: Co-purification or crosslinking studies with DsbA can verify proper interaction with its physiological partner.

When interpreting results, it's essential to compare with control proteins and consider that Acinetobacter sp. dsbB may have specific requirements for optimal activity that differ from better-characterized homologs in E. coli or other model organisms.

What is the relationship between dsbB expression and antibiotic resistance in Acinetobacter species?

The relationship between dsbB expression and antibiotic resistance in Acinetobacter species represents a complex interplay between protein folding and resistance mechanisms. Several key observations suggest important connections:

• Virulence Factor Maturation: Properly folded outer membrane proteins, including efflux pumps that contribute to antibiotic resistance, rely on the disulfide bond formation pathway. When dsbB function is compromised, these proteins may fold incorrectly, potentially reducing resistance .

• Plasmid-Mediated Resistance: A. baumannii is known to harbor multiple plasmids carrying antibiotic resistance markers. Some virulence factors encoded on these plasmids may require disulfide bonds for proper function .

• Genetic Context: Recent research has identified dif-like recombination sites (pdif) flanking antibiotic resistance genes, which may facilitate the transfer of resistance determinants via Xer site-specific recombination. This suggests potential involvement of recombination near dsbB or disulfide-dependent proteins in horizontal gene transfer events .

The variability in resistance profiles between A. baumannii and non-A. baumannii isolates (with resistance rates to carbapenems at 2.6% for non-A. baumannii species) may partially reflect differences in their respective dsbB activities or regulation .

What purification strategies yield the highest purity and activity for recombinant Acinetobacter dsbB?

Purification of recombinant Acinetobacter dsbB presents challenges typical of membrane proteins but can be optimized using the following strategy:

• Affinity Chromatography: Immobilized metal affinity chromatography (IMAC) using Ni-NTA resin with His-tagged dsbB provides effective initial capture. Buffer optimization is critical:

• Size Exclusion Chromatography: Secondary purification using SEC separates monomeric protein from aggregates and removes remaining contaminants.

• Anion Exchange Chromatography: Optional tertiary purification step to achieve >95% purity if required.

• Quality Control Checkpoints:

  • SDS-PAGE analysis after each step

  • Western blotting against His-tag or dsbB-specific antibodies

  • Activity assays to track functional protein yield

Critical factors for success include maintaining appropriate detergent concentrations throughout all steps and including reducing agents only when specifically investigating reduced states of the protein.

How can I design experiments to investigate the interplay between dsbB and plasmid-borne resistance genes in Acinetobacter baumannii?

Investigating the relationship between dsbB and plasmid-borne resistance genes requires a multifaceted approach:

• Genetic Knockout and Complementation:

  • Generate dsbB deletion mutants in A. baumannii using CRISPR-Cas9 or homologous recombination

  • Complement with wild-type and mutant versions of dsbB

  • Measure changes in minimum inhibitory concentrations (MICs) for various antibiotics

  • Assess plasmid stability and copy number

• Transcriptomic and Proteomic Analysis:

  • Compare RNA-seq profiles between wild-type and ΔdsbB strains

  • Use quantitative proteomics to identify proteins with altered abundance or redox state

  • Focus on membrane proteins and known resistance determinants

• Plasmid Mobilization Studies:

  • Design reporter plasmids containing resistance genes flanked by dif-like recombination sites (pdif)

  • Measure transfer efficiency in the presence and absence of functional dsbB

  • Analyze the role of Xer site-specific recombination in mobilizing resistance genes

• Protein-DNA Interaction Studies:

  • Use chromatin immunoprecipitation (ChIP) to identify dsbB-associated DNA regions

  • Assess if dsbB physically interacts with components of the Xer recombination system

This experimental framework should provide insights into whether dsbB plays a direct or indirect role in the dissemination of resistance genes in A. baumannii, which is particularly important given this organism's status as a reservoir of multiple plasmids carrying antibiotic resistance markers .

What structural features of Acinetobacter dsbB influence substrate specificity compared to other bacterial species?

The structural features of Acinetobacter dsbB that influence substrate specificity remain incompletely characterized but can be investigated through comparative structural biology approaches:

• Homology Modeling and Molecular Dynamics:

  • Generate structural models based on known DsbB structures (e.g., E. coli DsbB, PDB: 2HI7)

  • Identify key differences in the transmembrane helices and periplasmic loops

  • Use molecular dynamics simulations to predict substrate-binding sites and conformational flexibility

• Critical Residue Analysis:

  • Conserved CXXC motifs are essential for redox activity

  • Acinetobacter-specific residues may confer different redox potential compared to E. coli homologs

  • Surface charge distribution likely influences interaction with DsbA and other partners

• Redox Potential Measurements:

  • Determine the standard redox potential of Acinetobacter dsbB using electrochemical methods

  • Compare with values from other species to identify thermodynamic differences

• Chimeric Protein Studies:

  • Create domain-swapped variants between Acinetobacter and E. coli dsbB

  • Assess which regions confer species-specific activities

Acinetobacter baumannii's remarkable ability to acquire and disseminate resistance genes through plasmid transfer suggests its dsbB may have evolved unique features to support the folding of diverse acquired proteins, potentially contributing to its success as a multidrug-resistant pathogen.

How does the Xer recombination system interact with disulfide bond formation in the mobilization of resistance genes in Acinetobacter species?

The interaction between the Xer recombination system and disulfide bond formation in Acinetobacter species represents a frontier in understanding antimicrobial resistance transfer mechanisms:

• Mechanistic Connection:

  • The Xer site-specific recombination system utilizes XerC and XerD recombinases to bind to dif sites, catalyzing DNA strand cleavage, exchange, and ligation

  • Recent discovery of dif-like recombination sites (pdif) flanking antibiotic resistance genes suggests a novel gene transfer system

  • The orientation of pdif sites determines whether the internal DNA segment undergoes excision (same orientation) or inversion (opposite orientation)

• Experimental Approaches to Study Interactions:

  • Co-immunoprecipitation to identify potential physical interactions between dsbB and Xer recombination components

  • Redox proteomics to determine if Xer proteins contain disulfide bonds that might be substrates for the DSB system

  • Site-directed mutagenesis of cysteine residues in XerC/D to assess the importance of disulfide bonds for function

• Functional Analysis:

  • Compare recombination efficiency between wild-type and dsbB mutant strains

  • Measure formation of circular intermediates (resistance gene cassettes) using PCR

  • Assess whether oxidative stress conditions affect recombination frequency

• Clinical Relevance:

  • Analyze correlation between dsbB expression levels and frequency of resistance gene mobilization in clinical isolates

  • Evaluate potential of dsbB inhibitors as adjuvants to reduce resistance gene transfer

This research area is particularly significant given A. baumannii's reputation as a "reservoir of multiple plasmids carrying antibiotic resistance markers" and the concerning trend of increasing multidrug resistance in this pathogen.

What bioinformatic approaches are recommended for identifying and characterizing dsbB homologs across Acinetobacter species?

Identifying and characterizing dsbB homologs across Acinetobacter species requires sophisticated bioinformatic approaches:

• Sequence-Based Identification:

  • BLASTP searches using established dsbB sequences as queries against Acinetobacter genomes

  • Profile hidden Markov models (HMMs) constructed from known dsbB sequences to identify distant homologs

  • Transmembrane topology prediction to confirm expected membrane protein architecture (4 transmembrane segments)

• Phylogenetic Analysis:

  • Multiple sequence alignment using MAFFT with specific parameters for membrane proteins

  • Maximum-likelihood or Bayesian phylogenetic tree construction

  • Comparison with species phylogeny based on rpoB gene sequences, which has been demonstrated as the most accurate method for Acinetobacter species identification

• Genomic Context Analysis:

  • Examination of conserved gene neighborhoods to identify synteny

  • Investigation of proximity to mobile genetic elements or resistance determinants

  • Identification of potential regulatory elements in promoter regions

• Structural Prediction:

  • Secondary structure prediction to identify conserved α-helical transmembrane domains

  • Homology modeling based on available crystal structures

  • Prediction of critical functional residues using evolutionary conservation mapping

When applying these approaches, special attention should be given to distinguishing between A. baumannii and closely related species within the Acb complex (A. pittii, A. calcoaceticus, and A. nosocomialis), as phenotypic identification methods often fail to accurately differentiate these species .

What are the optimal conditions for studying dsbB-mediated disulfide bond formation in vitro?

Studying dsbB-mediated disulfide bond formation in vitro requires careful attention to redox conditions and protein stability:

• Buffer Optimization:

• Redox Cofactor Requirements:

  • Ubiquinone (Q1 or Q8): 10-50 μM

  • Optional molecular oxygen as terminal electron acceptor

  • NADPH/thioredoxin reductase system for recycling

• Activity Assay Conditions:

  • Temperature: 25-30°C (optimal for Acinetobacter enzymes)

  • Monitor reaction by following:

    • Fluorescence of labeled substrate proteins

    • Oxygen consumption using oxygen electrode

    • Ubiquinone reduction spectrophotometrically

  • Use purified Acinetobacter DsbA as the physiological partner

• Controls and Validation:

  • Site-directed mutagenesis of active site CXXC motifs as negative controls

  • Competition assays with known DsbB inhibitors

  • Mass spectrometry to confirm disulfide bond formation in substrate proteins

Particular attention should be paid to the redox state of the system throughout experiments, as atmospheric oxygen can cause non-enzymatic oxidation that may confound results.

How can I design a high-throughput screen for inhibitors of Acinetobacter dsbB as potential adjuvants to antibiotic therapy?

Designing a high-throughput screen for Acinetobacter dsbB inhibitors requires balancing throughput with physiological relevance:

• Primary Assay Design:

  • Enzymatic Activity Screen:

    • Measure reduction of artificial electron acceptors (DCPIP) coupled to dsbB activity

    • Fluorescence-based assay using self-quenching fluorescent peptides that increase signal upon reduction

    • Plate format: 384-well plates with 30-50 μL reaction volume

  • Assay Parameters:

    • Z' factor optimization (aim for >0.7)

    • Signal-to-background ratio >5:1

    • DMSO tolerance testing (typically up to 2%)

• Compound Library Selection:

  • Diversity-oriented collections for initial screening

  • Focused libraries targeting redox-active compounds

  • Natural product extracts, particularly from soil microorganisms competing with Acinetobacter

• Secondary Validation Assays:

  • Whole-cell assays measuring disulfide bond formation in vivo

  • Synergy testing with antibiotics against A. baumannii

  • Counter-screens against mammalian disulfide isomerases to ensure selectivity

• Tertiary Characterization:

  • Mechanism of inhibition (competitive, non-competitive, irreversible)

  • Binding site identification using hydrogen-deuterium exchange mass spectrometry

  • Crystal structure of inhibitor-bound dsbB (if achievable)

• Physiological Relevance Testing:

  • Evaluation of inhibitor effect on virulence factor maturation

  • Assessment of impact on resistance gene transfer frequency

  • Testing effectiveness against clinical isolates with different resistance profiles

This screening approach could yield valuable adjuvants that sensitize multidrug-resistant A. baumannii to existing antibiotics by compromising the folding of resistance determinants that require disulfide bonds for proper function .

What is the potential of targeting dsbB in Acinetobacter species for developing novel antimicrobial strategies?

Targeting dsbB in Acinetobacter species presents a promising approach for novel antimicrobial development, with several advantages over conventional antibiotics:

• Pathogen-Specific Vulnerability:

  • DsbB inhibition would primarily affect the folding of secreted virulence factors

  • The disulfide bond formation pathway is critical for bacterial pathogenesis but absent in mammalian cytoplasm

  • Selective pressure on resistance mechanisms may be reduced compared to direct bactericidal agents

• Anti-Virulence Approach:

  • Rather than killing bacteria directly, dsbB inhibitors would reduce pathogenicity

  • This strategy may reduce the selective pressure driving resistance development

  • Potential for preservation of beneficial microbiota compared to broad-spectrum antibiotics

• Synergistic Potential:

  • DsbB inhibition could increase susceptibility to existing antibiotics

  • Particularly relevant for A. baumannii, which has developed resistance to most available antibiotics

  • May restore effectiveness of older, less toxic antibiotics

• Resistance Transfer Disruption:

  • If dsbB function is linked to the Xer recombination system's ability to mobilize resistance genes, inhibitors might reduce horizontal gene transfer

  • This would address A. baumannii's concerning capacity as a "known reservoir of multiple plasmids carrying antibiotic resistance markers"

• Challenges and Considerations:

  • Membrane protein drug targets typically have lower druggability

  • Penetration of inhibitors into the periplasmic space requires careful medicinal chemistry

  • Potential for bypass mechanisms through alternative folding pathways

The alarming rise of multidrug-resistant A. baumannii, particularly in critical care units, underscores the urgent need for novel therapeutic approaches that this strategy could address .

How can structural data on Acinetobacter dsbB inform structure-based drug design efforts?

Structural data on Acinetobacter dsbB can significantly accelerate structure-based drug design through multiple avenues:

• Identification of Druggable Pockets:

  • Active site containing CXXC motifs represents the primary target

  • Hydrophobic quinone-binding site offers opportunities for competitive inhibitors

  • Interface with DsbA provides potential for disrupting protein-protein interactions

• Structure-Activity Relationship Development:

  • Homology models based on E. coli DsbB crystal structures provide starting points

  • Molecular dynamics simulations can reveal transient pockets not visible in static structures

  • Fragment-based screening against structural models can identify initial chemical matter

• Virtual Screening Approaches:

  • Structure-based virtual screening using docking against defined binding sites

  • Pharmacophore modeling based on known inhibitors or natural substrates

  • Machine learning models trained on structural data to predict binding affinity

• Rational Design Principles:

  • Design of covalent inhibitors targeting active site cysteines

  • Development of peptidomimetics that compete with DsbA binding

  • Exploration of allosteric inhibitors that lock dsbB in inactive conformations

• Species Selectivity Considerations:

  • Comparative analysis of dsbB structures across bacterial species can identify Acinetobacter-specific features

  • Focus on regions that differ between A. baumannii and commensal bacteria

  • Potential to design narrow-spectrum agents that minimize disruption of beneficial microbiota

This structure-guided approach is particularly valuable given A. baumannii's status as a serious public health concern worldwide with high mortality rates in hospital settings , highlighting the need for novel therapeutic strategies.

What are the most promising approaches for assessing the in vivo efficacy of dsbB inhibitors against Acinetobacter infections?

Assessing the in vivo efficacy of dsbB inhibitors against Acinetobacter infections requires a comprehensive evaluation strategy:

• Animal Model Selection and Optimization:

  • Murine Pneumonia Model: Reflects common clinical presentation of A. baumannii infections

  • Wound Infection Models: Relevant for military and trauma settings where A. baumannii is prevalent

  • Galleria mellonella (Wax Moth) Larva Model: Cost-effective preliminary screening tool

• Efficacy Parameters:

  • Primary Endpoints:

    • Bacterial burden in tissues

    • Survival rates

    • Inflammatory markers (cytokines, neutrophil recruitment)

  • Secondary Endpoints:

    • Antibiotic synergy evaluation

    • Resistance development monitoring

    • Microbiome impact assessment

• Pharmacokinetic/Pharmacodynamic (PK/PD) Considerations:

  • Determination of minimum effective concentration in relevant tissues

  • Assessment of protein binding effects on efficacy

  • Establishment of PK/PD indices that correlate with efficacy

• Resistance Development Assessment:

  • Serial passage experiments under inhibitor pressure

  • Whole genome sequencing of resistant isolates

  • Biochemical characterization of resistant dsbB variants

• Translational Biomarkers:

  • Identification of disulfide-dependent proteins that can serve as pharmacodynamic markers

  • Development of assays to measure inhibitor target engagement in vivo

  • Correlation of biochemical effects with clinical outcomes

This multifaceted approach aligns with the critical need for new therapeutic options against A. baumannii, which has been identified as a serious public health concern worldwide responsible for high mortality, particularly in intensive care units .