Recombinant Shigella dysenteriae serotype 1 Protein psiE (psiE)

What expression systems are recommended for producing recombinant Shigella dysenteriae psiE protein?

For recombinant expression of Shigella dysenteriae psiE protein, several expression systems have proven effective, each with distinct advantages depending on research objectives:

• E. coli expression system: Most commonly used due to high yield, cost-effectiveness, and rapid growth. The psiE protein has been successfully expressed with N-terminal His-tags in E. coli systems with yields sufficient for structural and functional studies .

• Yeast expression systems: May provide better folding for complex proteins and some post-translational modifications. Recommended when E. coli-expressed protein shows poor solubility or activity .

• Baculovirus expression system: Offers superior folding and post-translational modifications compared to prokaryotic systems, though at higher cost and complexity .

• Mammalian cell expression: While available as an option, this is typically unnecessary for psiE expression unless studying specific mammalian interactions .

For most applications, E. coli expression with optimization of induction conditions (temperature, IPTG concentration, and induction time) is sufficient to obtain functional psiE protein. Recommended purification strategies include immobilized metal affinity chromatography (IMAC) followed by size exclusion chromatography to achieve >90% purity .

What are the optimal storage and handling conditions for recombinant psiE protein?

For optimal stability and activity of recombinant psiE protein, the following storage and handling conditions are recommended based on empirical data:

• Short-term storage (up to one week): Store at 4°C in appropriate buffer conditions (typically Tris-based buffer at pH 8.0) .

• Long-term storage: Store at -20°C or -80°C, with -80°C preferred for extended periods. Addition of glycerol (final concentration 50%) significantly improves stability during freeze-thaw cycles .

• Buffer composition: Tris/PBS-based buffer with 6% trehalose at pH 8.0 has been demonstrated to maintain protein stability .

• Aliquoting: Divide purified protein into single-use aliquots before freezing to avoid repeated freeze-thaw cycles, which significantly reduce protein activity .

• Reconstitution of lyophilized protein: For lyophilized preparations, reconstitute in deionized sterile water to a concentration of 0.1-1.0 mg/mL, then add glycerol to 5-50% final concentration before storage .

• Centrifugation before use: Briefly centrifuge vials before opening to collect all liquid at the bottom, especially after shipping or long storage .

Research indicates that following these guidelines can maintain >90% of protein activity for at least 6 months in liquid form and 12 months in lyophilized form .

How can recombinant psiE protein be used in vaccine development against Shigella dysenteriae?

Recombinant psiE protein from Shigella dysenteriae presents several potential applications in vaccine development strategies:

• Subunit vaccine candidate: While not among the primary virulence antigens traditionally targeted (like Ipa proteins), psiE could serve as a complement to these antigens in subunit vaccine formulations due to its membrane association and conservation across Shigella species .

• Carrier protein for O-antigen conjugates: The O-specific polysaccharide of S. dysenteriae 1 is a primary target for protective immunity. Recombinant psiE could potentially be used as a carrier protein for conjugate vaccines linking O-antigens to enhance immunogenicity .

• Component in outer membrane vesicle (OMV) vaccines: Recent advanced vaccine platforms utilize OMVs, where membrane-associated proteins like psiE are naturally incorporated. Recombinant approaches have successfully incorporated heterologous antigens into Shigella OMVs, suggesting a similar approach might be viable for enriching psiE in OMV-based vaccines .

• Diagnostic marker for vaccine studies: Recombinant psiE can be used in ELISA-based assays to evaluate antibody responses in animal models following vaccination with Shigella-based vaccines, helping assess immunological memory and cross-reactive potential .

What analytical methods are recommended for characterizing the structural and functional properties of recombinant psiE?

For comprehensive characterization of recombinant psiE protein, multiple complementary analytical approaches are recommended:

• Structural characterization:

  • SDS-PAGE: Primary method for assessing purity and molecular weight (approximately 15.6 kDa for the native protein, with tag sizes varying)

  • Western blotting: For confirmation of identity using anti-His antibodies or specific anti-psiE antibodies

  • Circular dichroism (CD) spectroscopy: For secondary structure analysis, particularly important for membrane-associated proteins like psiE

  • Mass spectrometry: For precise molecular weight determination and identification of any post-translational modifications

  • NMR or X-ray crystallography: For detailed tertiary structure determination, though challenging for membrane-associated proteins

• Functional characterization:

  • Lipid binding assays: To evaluate membrane interactions

  • Phosphate stress response assays: To assess if recombinant psiE complements psiE-deficient bacterial strains under phosphate limitation

  • Protein-protein interaction studies: Co-immunoprecipitation or pull-down assays to identify binding partners

  • Immunological assays: ELISA and flow cytometry to assess antibody recognition and binding properties

• Quality control metrics:

  • Endotoxin testing: Critical for proteins intended for immunological studies; limulus amebocyte lysate (LAL) assay recommended with acceptance threshold typically <1 EU/μg protein

  • Thermal stability analysis: Differential scanning fluorimetry to determine melting temperature and stability under different buffer conditions

  • Size exclusion chromatography: To assess aggregation state and homogeneity

These methodologies provide complementary data for full characterization of recombinant psiE protein and ensure consistency between production batches for research applications.

What is the relationship between psiE expression and Shigella virulence in host infection models?

The relationship between psiE expression and Shigella virulence remains incompletely characterized, but several lines of evidence suggest potential connections:

• Stress response regulation: As a phosphate-starvation-inducible protein, psiE expression increases during phosphate limitation, a condition Shigella likely encounters within host cells. This suggests psiE may contribute to bacterial adaptation within the intracellular environment .

• Correlation with virulence gene expression: Studies of environmental Shigella isolates have shown variable patterns of virulence gene retention. While plasmid-encoded invasion genes (like ipaB, ipaC, ipaD) are frequently lost in environmental isolates, chromosomal genes (including potential stress response genes like psiE) show greater conservation, suggesting selective pressure to maintain these functions .

• Infection model observations: In cellular and animal infection models, Shigella undergoes significant transcriptional reprogramming upon host cell entry. While direct evidence for psiE upregulation is limited, related phosphate-responsive genes show altered expression during infection, particularly during the intracellular growth phase .

• Comparative virulence studies: When comparing clinical vs. environmental isolates, the presence of core chromosomal genes (which would include psiE) remains relatively constant, while virulence plasmid genes show greater variability. This suggests that while psiE may not be a primary virulence determinant, it may contribute to bacterial fitness during infection .

To definitively establish psiE's role in virulence, targeted gene deletion studies coupled with infection models would be necessary, but such specific studies for psiE have not been prominently reported in the literature.

How does recombinant psiE compare to other Shigella proteins as a target for diagnostic or immunological assays?

When evaluating recombinant psiE as a target for diagnostic or immunological assays compared to other Shigella proteins, several factors must be considered:

• Specificity vs. conservation:

  • psiE shows high conservation across Shigella species and related enterobacteria (95% identity with E. coli homolog), limiting its utility for species-specific diagnostics

  • In contrast, virulence factors like IpaB, IpaC, IpaD (invasion plasmid antigens) or Shigella enterotoxins (ShET-1, ShET-2) offer greater specificity for Shigella

• Expression levels during infection:

  • Classic virulence factors (Ipa proteins, Type III secretion system components) are abundantly expressed during infection and directly interact with host cells, making them prominent targets for the immune system

  • As a stress-response protein, psiE expression patterns during human infection are less characterized, potentially limiting its utility as a diagnostic marker

• Immunogenicity comparison:

  • LPS O-antigens remain the dominant target of protective antibody responses in natural infection and are serotype-specific

  • Invasion plasmid antigens (Ipa proteins) are recognized by convalescent sera and are targets for vaccine development

  • psiE's immunogenicity during natural infection is not well-documented in the literature

• Diagnostic application considerations:

  • For molecular diagnostics, chromosomal multicopy genes like ipaH provide superior sensitivity (present in all Shigella isolates, including environmental strains that have lost the virulence plasmid)

  • For serological diagnostics, LPS and major outer membrane proteins typically provide better sensitivity and specificity than accessory proteins like psiE

While psiE represents a stable target due to its conservation, this same conservation limits its specificity for Shigella diagnostics. For immunological assays, traditional virulence factors (particularly Ipa proteins) and serotype-specific LPS remain more established targets with better-characterized immunological properties.

How does psiE function within the broader context of Shigella stress response systems?

Within the broader context of Shigella stress response systems, psiE functions as part of an integrated network that helps the bacterium adapt to changing environmental conditions:

• Phosphate starvation response network:

  • psiE is induced during phosphate limitation as part of the Pho regulon, which includes multiple genes controlled by the PhoR/PhoB two-component regulatory system

  • Other components of this regulatory network include high-affinity phosphate transporters (e.g., PstSCAB), phosphatases, and enzymes involved in phosphate storage and mobilization

  • This system allows Shigella to sense and respond to environments with limited phosphate availability, such as within host cells or certain environmental niches

• Integration with virulence regulation:

  • Stress response systems in Shigella, including nutrient limitation responses, are interconnected with virulence gene regulation

  • Environmental signals like osmolarity, pH, temperature, and nutrient availability modulate expression of virulence factors through global regulators including H-NS, VirF, and VirB

  • While direct links between psiE and virulence regulation are not well-established, other stress response systems show clear integration with virulence control mechanisms

• Membrane integrity and adaptation:

  • Based on its predicted transmembrane structure, psiE likely contributes to membrane adaptation during stress conditions

  • Membrane composition and integrity are critical for Shigella survival during exposure to host defense mechanisms, including antimicrobial peptides and bile salts

  • Studies have shown that exposure to bile salts triggers changes in Shigella outer membrane proteins and LPS, enhancing adhesion and invasion capabilities

• Comparative analysis with other enteric pathogens:

  • The conservation of psiE across enteric bacteria suggests a fundamental role in bacterial physiology

  • In E. coli, similar phosphate starvation response systems regulate transitions between environmental persistence and host colonization

  • Understanding how psiE contributes to these adaptive responses could provide insights into Shigella's ability to transition between host and environmental reservoirs

A systems biology approach integrating transcriptomics, proteomics, and metabolomics would be valuable for fully elucidating psiE's role within these interconnected stress response networks.

What advanced bioinformatic approaches can be applied to predict potential interactions of psiE with host cell components?

Advanced bioinformatic approaches for predicting potential interactions between psiE and host cell components include:

• Structural prediction and molecular docking:

  • Homology modeling using AlphaFold2 or RoseTTAFold to generate accurate 3D structural models of psiE

  • Molecular docking simulations using tools like HADDOCK, AutoDock Vina, or ClusPro to predict potential interactions with host membrane components or proteins

  • Molecular dynamics simulations to assess the stability of predicted interactions and conformational changes under physiological conditions

• Sequence-based interaction prediction:

  • Machine learning approaches trained on known bacterial-host protein interactions

  • Identification of short linear motifs (SLiMs) that might mediate host protein interactions

  • Analysis of surface-exposed regions and electrostatic properties to identify potential interaction interfaces

• Network-based approaches:

  • Guilt-by-association analysis using known Shigella-host interaction networks

  • Interolog mapping, which transfers known interactions from well-studied proteins to less-characterized homologs

  • Context-based methods that incorporate co-expression, phylogenetic profiles, and genomic context

• Comparative analysis across pathogenic species:

  • Analysis of selective pressure on specific amino acid residues using dN/dS ratios across multiple bacterial species

  • Identification of conserved motifs that might indicate functional importance in host interaction

  • Cross-referencing with known virulence factors from related pathogens

• Integration with experimental data:

  • Incorporation of bacterial transcriptomics data during infection to identify co-expressed gene clusters

  • Using available proteomics data from Shigella-infected cells to validate predictions

  • Implementation of protein-protein interaction screens (like yeast two-hybrid or proximity labeling) guided by bioinformatic predictions

These computational approaches can generate testable hypotheses about psiE's potential interactions with host components, guiding focused experimental studies to elucidate its role in Shigella pathogenesis or stress response.

What are the major technical challenges in studying membrane-associated proteins like psiE, and how can they be addressed?

Studying membrane-associated proteins like psiE presents several technical challenges that require specialized approaches:

• Expression and purification challenges:

  • Problem: Membrane proteins often form inclusion bodies when overexpressed in traditional systems

  • Solutions:

    • Use lower induction temperatures (16-25°C) with reduced inducer concentrations

    • Explore specialized E. coli strains designed for membrane protein expression (C41/C43)

    • Consider cell-free expression systems that allow direct incorporation into artificial liposomes

    • Engineer fusion proteins with solubility-enhancing tags (SUMO, thioredoxin, MBP) at the N-terminus

• Solubilization and stability issues:

  • Problem: Maintaining native conformation while extracting from membranes

  • Solutions:

    • Screen multiple detergents (DDM, LDAO, OG) at varying concentrations for optimal extraction

    • Employ nanodisc technology to maintain a lipid environment around the protein

    • Use styrene maleic acid copolymer (SMA) for native nanodiscs that preserve the lipid environment

    • Incorporate stabilizing additives like glycerol (10-20%) and specific lipids during purification

• Structural characterization limitations:

  • Problem: Traditional structural biology methods are challenging for membrane proteins

  • Solutions:

    • Cryo-electron microscopy for structure determination without crystallization

    • Solid-state NMR for membrane-embedded structural analysis

    • Hydrogen-deuterium exchange mass spectrometry for conformational dynamics

    • Molecular dynamics simulations to complement experimental structural data

• Functional assay development:

  • Problem: Difficulty in assessing function outside native membrane environment

  • Solutions:

    • Reconstitution into proteoliposomes for functional assays

    • Development of cell-based reporter systems for indirect functional assessment

    • Targeted mutagenesis of conserved residues followed by complementation studies

    • Protein-lipid interaction assays using labeled lipids or surface plasmon resonance

• Limited availability of specific antibodies:

  • Problem: Generating antibodies against membrane proteins is challenging

  • Solutions:

    • Design peptide antigens from predicted extracellular loops for antibody production

    • Develop recombinant antibodies using phage display against purified protein

    • Use epitope tags strategically positioned in extracellular domains

    • Consider nanobody development for improved recognition of membrane proteins

These technical approaches, combined with careful experimental design, can overcome many of the challenges associated with studying membrane proteins like psiE, advancing our understanding of their structure-function relationships in bacterial physiology and pathogenesis.

What emerging technologies could advance our understanding of psiE's role in Shigella pathogenesis?

Several cutting-edge technologies hold promise for elucidating psiE's role in Shigella pathogenesis:

• CRISPR-Cas9 genome editing and interference:

  • Precise genomic manipulation to create clean deletions, point mutations, or regulatable expression of psiE

  • CRISPRi approaches for temporal control of psiE expression during different stages of infection

  • CRISPR-based screening to identify genetic interactions between psiE and other bacterial factors

• Single-cell technologies:

  • Single-cell RNA-seq of infected host cells to track transcriptional responses to Shigella strains with/without psiE

  • Single-bacterium transcriptomics to capture heterogeneity in psiE expression during infection

  • Spatial transcriptomics to map psiE expression patterns in tissue infection models

• Advanced imaging techniques:

  • Super-resolution microscopy (STORM, PALM) for precise localization of psiE in bacterial cells

  • Live-cell imaging using split fluorescent proteins to track psiE interactions in real-time

  • Correlative light and electron microscopy (CLEM) to visualize psiE in the context of host-pathogen interfaces

  • Cryo-electron tomography to visualize membrane organization and potential structural changes mediated by psiE

• Protein-centric approaches:

  • Proximity labeling methods (BioID, APEX) to identify proteins that interact with psiE in living bacteria

  • Hydrogen-deuterium exchange mass spectrometry to map conformational changes under different conditions

  • Protein correlation profiling to identify membrane microdomains containing psiE

  • Native mass spectrometry to characterize psiE complexes in their native state

• Systems biology integration:

  • Multi-omics approaches combining transcriptomics, proteomics, and metabolomics to place psiE in global regulatory networks

  • Network analysis to identify central regulators connecting psiE to virulence pathways

  • Flux analysis to determine if psiE affects metabolic adaptations during infection

  • Machine learning approaches to predict phenotypic outcomes of psiE modulation

• Advanced infection models:

  • Intestinal organoids to study Shigella-host interactions in more physiologically relevant systems

  • Humanized mouse models incorporating human immune components

  • Microfluidic organ-on-chip systems to model tissue-specific interactions

  • In situ sequencing approaches to map bacterial gene expression directly in infected tissues

Integration of these technologies would provide unprecedented insights into psiE's functional role within the complex host-pathogen interaction landscape of Shigella infection.

How does sequence variation in psiE correlate with geographical distribution and virulence of Shigella dysenteriae isolates?

Analyzing the correlation between psiE sequence variation, geographical distribution, and virulence of Shigella dysenteriae isolates reveals several important patterns:

The available evidence suggests that while psiE is highly conserved across S. dysenteriae isolates worldwide, its minor sequence variations reflect population structure rather than adaptation to specific virulence niches or geographical environments. This pattern is consistent with a core physiological function rather than a specialized virulence role.

What insights can comparative genomic analysis of psiE across pathogenic and non-pathogenic enterobacteria provide?

Comparative genomic analysis of psiE across pathogenic and non-pathogenic enterobacteria reveals several important insights into bacterial evolution and function:

• Evolutionary conservation and selective pressure:

  • psiE shows remarkable conservation across both pathogenic Shigella species and non-pathogenic commensal E. coli strains, with sequence identity typically exceeding 90%

  • This high degree of conservation suggests psiE performs a fundamental function under strong selective pressure, rather than a specialized virulence role

  • Analysis of dN/dS ratios (nonsynonymous to synonymous substitution rates) across enterobacterial psiE sequences indicates purifying selection, further supporting a conserved functional role

• Genomic context and operon structure:

  • The genomic neighborhood of psiE is generally conserved across enterobacteria, with similar upstream and downstream genes

  • In most species, psiE appears to be monocistronic rather than part of a larger operon structure

  • Promoter region analysis shows conservation of potential PhoB binding sites across species, suggesting maintained regulation by phosphate limitation across diverse enterobacteria

• Correlation with ecological niches:

  • psiE is present in enterobacteria occupying diverse ecological niches, from obligate pathogens (Shigella) to environmental bacteria and commensals

  • This distribution suggests psiE's function relates to adaptation to conditions encountered across these varied lifestyles

  • The conservation of psiE in both host-adapted and free-living bacteria indicates it likely helps bacteria respond to nutrient limitation, a challenge common across diverse environments

• Relationship to virulence evolution:

  • While psiE itself shows little evidence of pathoadaptive evolution, its genomic context in pathogenic species sometimes shows integration of mobile genetic elements or pathogenicity islands nearby

  • This pattern suggests that while psiE itself is part of the ancestral genome, pathogenic adaptation has occurred in its vicinity

  • The maintenance of psiE during the reductive evolution characteristic of host-adapted pathogens like Shigella (which have lost many ancestral genes) further supports its importance for in-host survival

• Implications for bacterial physiology:

  • The universal presence of psiE across diverse enterobacteria suggests it mediates responses to a fundamental stress condition encountered by these bacteria

  • Phosphate limitation occurs in both host and environmental settings, consistent with psiE's conservation across species with different lifestyles

  • The transmembrane structure of psiE, conserved across species, implies a potential role in membrane adaptation during phosphate stress

These comparative genomic insights suggest that psiE likely performs a fundamental role in bacterial stress response that predates the divergence of pathogenic and non-pathogenic enterobacteria, rather than representing a specific virulence adaptation.

How might recombinant psiE be utilized in novel antimicrobial development strategies?

Recombinant psiE protein could contribute to novel antimicrobial development strategies through several innovative approaches:

• Target-based drug discovery:

  • If psiE plays an essential role in bacterial stress response or membrane integrity, it could serve as a novel drug target

  • High-throughput screening of compound libraries against recombinant psiE could identify small molecules that inhibit its function

  • Structure-based drug design using solved psiE structures could enable rational development of inhibitors targeting functional domains

  • The conservation of psiE across enterobacteria suggests inhibitors might have broad-spectrum activity against multiple pathogens

• Antibody-antibiotic conjugates:

  • If psiE has surface-exposed domains, antibodies developed against recombinant psiE could be conjugated to existing antibiotics

  • Such conjugates could enhance antibiotic targeting to bacteria, potentially reducing required doses and minimizing disruption to commensal flora

  • This approach could be particularly valuable against multi-drug resistant Shigella strains, which are increasingly common

• Antimicrobial peptide development:

  • Analysis of psiE interactions with membranes could inform the design of antimicrobial peptides targeting similar membrane regions

  • Recombinant psiE could be used in membrane model systems to screen peptide libraries for those disrupting bacterial membrane integrity

  • The specificity of such peptides could be enhanced by incorporating motifs that interact with bacterial-specific membrane components

• Anti-virulence approaches:

  • If psiE contributes to stress adaptation during infection, inhibiting its function could reduce bacterial fitness within the host

  • Unlike traditional antibiotics that kill bacteria and select for resistance, anti-virulence approaches may exert less selective pressure

  • Compounds that interfere with psiE function could potentially be used as adjuncts to conventional antibiotics in combination therapy

• Diagnostic applications for targeted therapy:

  • Antibiotic resistance is increasingly common in Shigella, with extensively drug-resistant (XDR) strains emerging globally

  • Rapid detection of specific Shigella strains using recombinant psiE-based diagnostics could guide more targeted antimicrobial therapy

  • This precision medicine approach could help preserve the effectiveness of remaining treatment options for severe Shigella infections

While these approaches show promise, it's important to note that the development of any psiE-targeted antimicrobial strategy would require detailed validation of psiE's essentiality or contribution to virulence, which is currently not fully characterized in the literature.

What are the potential applications of psiE in synthetic biology approaches for Shigella research?

Recombinant psiE protein offers several innovative applications in synthetic biology approaches for Shigella research:

• Reporter systems for stress response pathways:

  • The psiE promoter region, responsive to phosphate limitation, can be fused to reporter genes (GFP, luciferase) to create biosensors

  • These constructs could monitor phosphate stress in real-time during infection processes

  • By integrating multiple stress-responsive promoters (including psiE) with different fluorescent reporters, multiplexed stress monitoring systems could be developed

  • Such systems would allow simultaneous tracking of multiple environmental conditions encountered by Shigella during host colonization

• Engineered bacterial chassis for vaccine development:

  • Building on recent advances in Shigella vaccine development, psiE could be utilized as a stable chromosomal locus for heterologous antigen expression

  • The conservation of psiE across Shigella strains makes it a reliable integration target for synthetic constructs

  • Recombinant Shigella strains expressing foreign antigens from the psiE locus could serve as multivalent vaccine candidates, similar to the approach used for ETEC heat-labile toxin B integration

  • The membrane association of psiE could potentially facilitate incorporation of fused antigens into outer membrane vesicles (OMVs), enhancing their immunogenicity

• Controllable gene expression systems:

  • Synthetic biology tools incorporating the psiE regulatory elements could create phosphate-responsive gene circuits

  • These circuits could enable conditional expression of virulence factors, toxin genes, or antibiotic resistance genes specifically under phosphate limitation

  • Such tools would allow precise temporal control of gene expression during infection studies

  • The natural responsiveness of psiE to host-relevant conditions makes it valuable for designing environmentally-triggered synthetic circuits

• Protein scaffold for membrane engineering:

  • As a membrane-associated protein, psiE could serve as a scaffold for displaying functional domains at the bacterial surface

  • Fusion proteins combining psiE with adhesins, enzymes, or binding domains could create bacteria with novel surface properties

  • These engineered surface properties could enable new applications in bacterial targeting, diagnostic development, or biofilm studies

  • The integration of heterologous domains into psiE scaffolds could be optimized using the recombinant protein for structure-function studies

• Minimal genome approaches:

  • In efforts to create minimal Shigella genomes for synthetic biology applications, understanding which genes (including psiE) are essential under various conditions is critical

  • Recombinant psiE could complement deletion strains to assess functional requirements under defined conditions

  • This information would guide rational genome reduction strategies while maintaining bacterial fitness in research applications

  • Synthetic Shigella strains with streamlined genomes could serve as optimized platforms for vaccine development or heterologous protein expression