Recombinant Salmonella paratyphi B Cardiolipin synthase (cls)

What is Cardiolipin synthase in Salmonella paratyphi B and what is its primary function?

Cardiolipin synthase (cls) in Salmonella paratyphi B is an enzyme originally annotated for its role in cardiolipin biosynthesis. The protein contains characteristic phospholipase-D motifs and catalyzes the formation of cardiolipin, a key phospholipid in bacterial membranes. In Salmonella species, three cls enzymes (ClsA, ClsB, and ClsC) have been identified, each with distinct substrate preferences and activation conditions .

How should recombinant Salmonella paratyphi B Cardiolipin synthase be stored and handled to maintain optimal activity?

For optimal stability and activity retention of recombinant Salmonella paratyphi B Cardiolipin synthase, follow these research-validated protocols:

Storage conditions:

• Store at -20°C for routine use or -80°C for extended storage

• The protein is typically supplied in a Tris-based buffer with 50% glycerol

• Shelf life: approximately 6 months at -20°C/-80°C in liquid form; 12 months in lyophilized form

Handling recommendations:

• Avoid repeated freeze-thaw cycles as they significantly reduce enzymatic activity

• For working aliquots, store at 4°C for no more than one week

• When reconstituting lyophilized protein, use deionized sterile water to a concentration of 0.1-1.0 mg/mL

• For long-term storage after reconstitution, add glycerol to a final concentration of 5-50% and aliquot before storing at -20°C/-80°C

Following these guidelines will help maintain the structural integrity and enzymatic activity of the recombinant protein for experimental applications.

What are the recommended methods for assessing the enzymatic activity of recombinant Cardiolipin synthase?

Several validated methods can be employed to assess the enzymatic activity of recombinant Cardiolipin synthase from Salmonella paratyphi B:

Thin-Layer Chromatography (TLC) Analysis:

• Extract lipids from reaction mixtures using chloroform:methanol (2:1 v/v)

• Separate lipids on silica TLC plates using appropriate solvent systems

• Visualize with iodine vapor, phosphomolybdic acid, or other lipid-specific stains

• Quantify CL production by densitometry of the TLC plates

Catalytic Activity Assays:

• Measure the consumption of substrate phospholipids (phosphatidylglycerol)

• Quantify cardiolipin production using mass spectrometry

• Monitor changes in fluorescently labeled phospholipid substrates

Functional Complementation:

• Express the cls gene in cls-deficient bacteria

• Verify restoration of cardiolipin production by lipid extraction and analysis

• Compare with known catalytically inactive mutants (e.g., clsBH290A has been shown to be catalytically inert)

For enzymes suspected of having dual functions (like ClsB's role in trehalose phospholipid synthesis), specialized assays may be necessary to detect alternative reaction products.

How can researchers generate cardiolipin synthase mutants for functional studies in Salmonella?

Generating cardiolipin synthase mutants in Salmonella requires careful genetic manipulation to ensure specific gene disruption without polar effects on adjacent genes. Based on published methodologies, the following approach is recommended:

Single Gene Deletion Method:

• Use lambda Red recombination system for precise gene replacement

• Design primers with 40-bp homology to regions flanking the target cls gene

• Replace the target gene with an antibiotic resistance cassette

• Verify deletion by PCR and sequencing

• Remove the antibiotic cassette using FLP recombinase if needed for multiple mutations

Multiple Gene Deletion Strategy:
For generating double (ΔclsAB, ΔclsBC, ΔclsAC) and triple mutants (ΔclsABC):

• Use sequential deletion with different antibiotic markers

• Alternatively, use marker-less deletion methods with counterselection

• Verify each deletion step by PCR and phenotypic analysis

• Confirm lipid profile alterations by thin-layer chromatography

Complementation Testing:

• Clone wild-type cls genes into low or medium-copy plasmids

• Transform cls-deficient mutants with complementation plasmids

• Assess restoration of cardiolipin production

• Generate site-directed mutants (e.g., ClsBH290A) to test catalytic activity requirements

This approach has been successfully employed to create a complete set of cls gene mutants in S. Typhimurium, enabling detailed functional analysis.

What techniques are most effective for purifying recombinant Cardiolipin synthase from expression systems?

Purification of recombinant Cardiolipin synthase presents challenges due to its membrane-associated properties. Based on successful approaches in the literature, the following purification strategy is recommended:

Expression System Selection:

• E. coli is the preferred heterologous expression system for Salmonella paratyphi B Cardiolipin synthase

• BL21(DE3) or similar strains are recommended for high-level expression

• Use expression vectors with inducible promoters (T7, tac) for controlled expression

Solubilization and Extraction:

• Harvest cells and disrupt by sonication or pressure homogenization

• Isolate membrane fractions by ultracentrifugation

• Solubilize membrane proteins using mild detergents (n-dodecyl-β-D-maltoside, CHAPS, or Triton X-100)

• Optimize detergent concentration to maintain enzymatic activity

Affinity Purification:

• Express the protein with affinity tags (His-tag, GST-tag, etc.)

• Purify using appropriate affinity chromatography

• Include detergents in all buffers to maintain protein solubility

• Perform on-column washing with detergent-containing buffers

• Elute with appropriate competitors (imidazole for His-tagged proteins)

• Verify purity by SDS-PAGE (>85% purity is typically achievable)

Additional Purification Steps:

• Size exclusion chromatography to remove aggregates

• Ion exchange chromatography for further purification

• Concentrate using ultrafiltration with detergent above critical micelle concentration

Following this protocol can yield functional recombinant Cardiolipin synthase suitable for enzymatic and structural studies.

How do the three cardiolipin synthases (ClsA, ClsB, ClsC) differ in Salmonella species?

Salmonella species possess three distinct cardiolipin synthases that differ in substrate specificity, regulation, and physiological roles:

ClsA (Primary Cardiolipin Synthase):

• Predominant synthase during logarithmic growth

• Uses two phosphatidylglycerol (PGl) molecules as substrates to produce one cardiolipin molecule

• Expressed constitutively during normal growth conditions

• Has the highest activity for cardiolipin synthesis under standard conditions

ClsB (Dual-Function Enzyme):

• Initially annotated as a cardiolipin synthase but has broader substrate specificity

• Can synthesize cardiolipin from two PGl molecules

• Uniquely capable of synthesizing phosphatidyltrehalose (PT) and diphosphatidyltrehalose (diPT)

• Activated during stationary phase and stress conditions

• Essential for trehalose phospholipid biosynthesis

• Demonstrates promiscuous enzymatic activity that can synthesize phosphatidylalcohols

ClsC (Stress-Activated Synthase):

• Utilizes phosphatidylethanolamine (PE) and phosphatidylglycerol (PGl) as substrates

• Contributes to the cardiolipin pool primarily during stress conditions

• Activated during stationary phase and high osmolarity conditions

• Functions in a complementary manner with the other synthases

These enzymes exhibit functional redundancy while also providing biochemical flexibility that allows Salmonella to modulate membrane composition under different environmental conditions, which may be crucial for adaptation during infection.

What is the relationship between cardiolipin synthase activity and Salmonella pathogenesis?

The relationship between cardiolipin synthase activity and Salmonella pathogenesis reveals surprising complexity:

Intracellular Survival and Inflammasome Activation:

• Single mutants (ΔclsA, ΔclsB, ΔclsC) and some double mutants (ΔclsAC) behave like wild-type in macrophage infection models

• Other double mutants (ΔclsAB, ΔclsBC) and triple mutants (ΔclsABC) show attenuated intracellular survival

• Mutants lacking specific cls gene combinations elicit reduced secretion of inflammatory cytokines IL-1β and IL-18, suggesting altered inflammasome activation

• When both clsA and clsC are deleted, clsB becomes necessary and sufficient for intracellular survival

• Similarly, when clsB is deleted, clsA and clsC together become necessary and sufficient for proper function

Virulence in Mouse Models:

Functional Redundancy vs. Distinct Roles:

• The three cls genes show cooperative and redundant influence on inflammasome activation

• Expressing clsA or clsB in trans in the triple mutant restores cardiolipin production but does not rescue macrophage phenotypes

• This indicates that the observed phenotypes are not directly caused by alterations in cardiolipin content but may involve other functions of these enzymes

This complex relationship highlights the multifunctional nature of cardiolipin synthases and suggests they may have roles beyond their enzymatic activity in phospholipid biosynthesis.

What is the role of ClsB in trehalose phospholipid biosynthesis and how does this relate to its annotated function?

The role of ClsB in trehalose phospholipid biosynthesis represents a significant case of functional misannotation in bacterial genomics:

Discovery of Trehalose Phospholipids:

• Research comparing lipidomes of Salmonella Paratyphi and S. Typhi led to the discovery of previously unknown trehalose phospholipids: 6-phosphatidyltrehalose (PT) and 6,6′-diphosphatidyltrehalose (diPT)

• These compounds were found to be enriched in S. Typhi, which causes typhoid fever

Functional Reannotation of ClsB:

• Systematic gene knockout experiments revealed that clsB, previously annotated as one of three cardiolipin synthases, was essential for trehalose phospholipid biosynthesis

• This led to the proposal that ClsB is actually a PT and diPT synthase rather than simply a cardiolipin synthase

• This highlights the issue of poor annotation and propagation of errors in bacterial genome databases

Structural and Functional Analysis:

• ClsB shares the phospholipase-D motif characteristic of cardiolipin synthases

• Despite this structural similarity, it demonstrates substrate promiscuity, being able to catalyze the formation of cardiolipin, phosphatidylalcohols, PT, and diPT

• The enzyme can still produce cardiolipin from two phosphatidylglycerol molecules but has evolved additional functionalities

Phylogenetic Distribution:

• DiPT production is restricted to a subset of Gram-negative bacteria

• Large amounts are produced by S. Typhi, with lower amounts by other pathogens

• Variable amounts are synthesized by Escherichia coli strains

• "Chemotyping" (direct measurement of diPT production) proved more accurate than clsB homology analysis for predicting this function

Immunological Significance:

• DiPT activates Mincle, a macrophage activating receptor that also recognizes mycobacterial cord factor (6,6′-trehalose dimycolate)

• This represents convergent function between Gram-negative bacteria and mycobacteria

• These compounds may play important roles in host-pathogen interactions

This case illustrates how experimental validation of enzyme function can reveal unexpected roles that significantly impact our understanding of bacterial pathogenesis and immune interactions.

How do mutations in cardiolipin synthase impact membrane composition and bacterial stress responses?

The impact of cardiolipin synthase mutations on membrane composition and bacterial stress responses involves complex compensatory mechanisms:

Membrane Lipid Remodeling:

• Deletion of individual cls genes leads to compensatory increases in activity of remaining synthases

• Total cardiolipin levels may be maintained despite loss of one or two synthases

• Triple mutants (ΔclsABC) show complete absence of cardiolipin with corresponding increases in precursor phospholipids (phosphatidylglycerol)

• Changes in membrane phospholipid composition can alter membrane fluidity, curvature, and protein organization

Stress Response Modulation:

• Cardiolipin is enriched at cell poles and division sites, suggesting roles in cell division

• Loss of cardiolipin may affect bacterial response to:

  • Osmotic stress

  • pH fluctuations

  • Antimicrobial peptides

  • Temperature variations

• Defects in cls genes may alter expression of stress response regulons through membrane sensing mechanisms

Redox Balance and Energy Metabolism:

• Cardiolipin interacts with respiratory chain complexes in the membrane

• Alterations in cardiolipin content may affect electron transport and ATP generation

• This can influence bacterial growth rates under nutrient limitation or oxidative stress conditions

Experimental approaches to study these effects include:

• Membrane fluidity measurements using fluorescence anisotropy

• Lipidomic profiling under various stress conditions

• Transcriptomic analysis to identify compensatory gene expression changes

• Electron microscopy to examine membrane ultrastructure

These findings highlight the remarkable adaptability of bacterial membrane systems and the complex interplay between lipid composition and stress responses.

What techniques can resolve the apparent contradiction between in vitro attenuation and in vivo virulence of cardiolipin synthase mutants?

The observed contradiction between in vitro attenuation of cardiolipin synthase mutants in macrophages and their retained virulence in mouse models presents an intriguing research puzzle that can be approached with several sophisticated methodologies:

Advanced In Vivo Imaging Techniques:

• Utilize bioluminescent or fluorescent reporter strains of cls mutants

• Track bacterial distribution and replication in real-time in live animals

• Compare tissue tropism and growth kinetics between wild-type and mutant strains

• Correlate with inflammatory responses using dual-reporter systems

Cell Type-Specific Host Interactions:

• Examine cls mutant interactions with different host cell types beyond macrophages (epithelial cells, neutrophils, dendritic cells)

• Use ex vivo tissue cultures or organoids to better mimic in vivo complexity

• Employ flow cytometry-based approaches to quantify bacterial association with specific cell populations from infected tissues

Compensatory Mechanisms Analysis:

• Perform transcriptomic analysis of cls mutants during in vivo infection versus in vitro macrophage culture

• Identify differentially expressed genes that might compensate for cls deficiency in vivo

• Use transposon mutagenesis screens to identify genetic suppressors that restore in vitro fitness

Host Response Profiling:

• Compare host immune responses to wild-type and cls mutants using cytokine profiling

• Examine inflammasome activation in various tissues, not just cultured macrophages

• Use immunodeficient mouse models to determine if specific immune pathways mask the importance of cls genes in vivo

Alternative Infection Routes:

• Test different infection routes (oral, intravenous, intraperitoneal) to determine if mode of entry affects the requirement for cls genes

• Examine competitive indices between wild-type and mutant strains in mixed infections

• Extend the timeline of infection studies to capture potential differences in persistence

Metabolic Environment Consideration:

• Compare growth media used in vitro with in vivo metabolite availability

• Supplement in vitro conditions with host-derived factors

• Conduct metabolomic analysis of infected tissues versus culture media

By systematically applying these approaches, researchers can identify the contextual factors that explain why cardiolipin synthase activity appears critical in isolated macrophages but dispensable in the complex in vivo environment.

How can structural biology approaches be used to elucidate the substrate promiscuity of ClsB?

Understanding the structural basis for ClsB's substrate promiscuity requires sophisticated structural biology approaches. The following methodologies can provide critical insights into how this enzyme accommodates different substrates:

X-ray Crystallography of ClsB:

• Express and purify ClsB with appropriate detergents to maintain native conformation

• Attempt co-crystallization with various substrates (phosphatidylglycerol, trehalose)

• Analyze substrate binding pockets and catalytic sites

• Compare with structures of ClsA and ClsC to identify unique features

• Focus on resolving the active site architecture to understand the molecular basis of promiscuity

Cryo-Electron Microscopy:

• Visualize ClsB in different conformational states

• Study the enzyme in membrane-mimetic environments (nanodiscs, liposomes)

• Capture enzyme-substrate complexes in near-native conditions

• Generate 3D reconstructions to analyze conformational changes upon substrate binding

Molecular Dynamics Simulations:

• Use the amino acid sequence to model ClsB structure based on homologous proteins

• Simulate substrate docking and enzyme flexibility

• Predict binding energy differences between various substrates

• Model transition states to understand catalytic mechanism variations

Site-Directed Mutagenesis and Structure-Function Analysis:

• Identify conserved residues across cardiolipin synthases

• Create point mutations at predicted substrate-binding sites

• Test activity against different substrates to map specificity-determining residues

• The catalytically inactive ClsBH290A mutant provides a starting point for such analyses

Hydrogen-Deuterium Exchange Mass Spectrometry:

• Analyze conformational dynamics of ClsB with different substrates

• Identify regions with differential solvent accessibility

• Map substrate-induced conformational changes

• Compare dynamics between wild-type and mutant variants

Small-Angle X-ray Scattering (SAXS):

• Obtain low-resolution envelopes of ClsB in solution

• Compare conformational states with different substrates

• Analyze oligomerization state changes upon substrate binding

These approaches, used in combination, can provide a comprehensive understanding of the structural features that enable ClsB to catalyze multiple reactions with different substrates, explaining its dual role in cardiolipin and trehalose phospholipid biosynthesis.

What are the implications of trehalose phospholipids in host-pathogen interactions and vaccine development?

The discovery of trehalose phospholipids synthesized by ClsB opens new avenues in understanding host-pathogen interactions and potential vaccine development:

Immunomodulatory Properties:

• Diphosphatidyltrehalose (diPT) activates Mincle, a C-type lectin receptor on macrophages

• This represents convergent evolution with mycobacterial cord factor (trehalose dimycolate), a potent immunostimulant

• Activation of Mincle can trigger pro-inflammatory responses, potentially influencing infection outcomes

• These compounds may serve as pathogen-associated molecular patterns (PAMPs)

Differential Expression Across Pathogens:

• S. Typhi produces large amounts of diPT, while other pathogens produce lower amounts

• This differential expression may contribute to pathogen-specific immune responses

• Expression patterns correlate with particular disease manifestations (e.g., typhoid fever)

• Understanding this variation could help explain serovar-specific pathogenicity

Potential as Vaccine Adjuvants:

• Given their immunostimulatory properties, synthetic trehalose phospholipids could be developed as adjuvants

• Their structural similarity to established adjuvants like trehalose dimycolate suggests potential utility

• Chemical synthesis methods have been established, allowing for structure-activity relationship studies

• Modified versions could be engineered for enhanced adjuvant properties with reduced toxicity

Biomarker Applications:

• The presence of trehalose phospholipids could serve as biomarkers for specific infections

• Diagnostic tests could be developed to detect these compounds in patient samples

• This may allow rapid differentiation between Salmonella serovars with different clinical implications

Therapeutic Targeting:

• Inhibitors of ClsB could potentially attenuate virulence

• Such inhibitors might have selective activity against pathogens that rely on trehalose phospholipids

• Structure-based drug design could utilize the unique features of ClsB for targeted therapeutics

These findings highlight how the reannotation of a single enzyme can open multiple new research directions with significant translational potential.

How can lipidomic approaches be improved to better characterize bacterial membrane adaptations during infection?

Advanced lipidomic approaches are crucial for fully understanding bacterial membrane adaptations during infection. The following methodological improvements can enhance this research area:

In Vivo Extraction and Analysis:

• Develop methods to extract bacterial lipids directly from infected tissues

• Employ stable isotope labeling to distinguish bacterial from host lipids

• Implement tissue-clearing techniques compatible with lipid preservation

• Combine with spatial transcriptomics to correlate lipid profiles with gene expression patterns

Single-Cell Lipidomics:

• Adapt mass spectrometry imaging for single bacterial cell analysis

• Develop fluorescent probes for specific lipid classes to enable live-cell imaging

• Use microfluidic approaches to analyze lipids from individual bacteria isolated from infection sites

• Correlate single-cell lipid profiles with bacterial transcriptomes and proteomes

Temporal Resolution of Lipid Dynamics:

• Implement pulse-chase labeling with isotope-labeled precursors to track lipid turnover

• Develop time-resolved sampling methods for infection models

• Use biosensors to monitor membrane properties in real-time during infection

• Correlate lipid changes with stages of pathogenesis (adherence, invasion, replication)

Functional Lipidomics:

• Couple lipid profiling with phenotypic assays to determine functional consequences

• Use synthetic biology approaches to engineer bacteria with defined lipid compositions

• Develop CRISPR-based screens targeting lipid biosynthesis genes

• Create reporter systems that respond to membrane property changes

Advanced Analytical Platforms:

• Implement ion mobility mass spectrometry for improved separation of lipid isomers

• Utilize high-resolution mass spectrometry for comprehensive lipidome characterization

• Develop machine learning algorithms for automated lipid identification and quantification

• Create standardized workflows and databases specific for bacterial lipids

Host-Pathogen Lipid Interactions:

• Analyze lipid exchange between bacteria and host membranes

• Investigate how bacterial lipids modulate host membrane properties and signaling

• Study the role of lipid rafts in bacteria-host interactions

• Examine how host lipids are incorporated into or modify bacterial membranes

These methodological advances would significantly enhance our ability to understand the complex lipid adaptations that occur during bacterial infections and could reveal new targets for therapeutic intervention.

What computational approaches can improve functional annotation of lipid-modifying enzymes in bacterial genomes?

Improving functional annotation of lipid-modifying enzymes in bacterial genomes requires sophisticated computational approaches to overcome current limitations, as highlighted by the ClsB misannotation case:

Advanced Homology Detection Methods:

• Implement profile hidden Markov models (HMMs) specific for lipid-modifying enzyme families

• Develop position-specific scoring matrices that account for catalytic residues

• Use deep learning approaches to detect remote homologies missed by conventional methods

• Incorporate evolutionary coupling analysis to identify co-evolving residues critical for function

Substrate Specificity Prediction:

• Develop machine learning algorithms trained on experimentally validated enzyme-substrate pairs

• Identify substrate-determining regions through comparative analysis of related enzymes

• Implement active site architecture analysis for substrate compatibility

• Create metabolic context-aware annotation that considers available substrates in a given organism

Integration of Structural Information:

• Apply AlphaFold or RoseTTAFold to predict structures of putative lipid-modifying enzymes

• Conduct virtual screening of potential substrates against predicted structures

• Identify key structural motifs that differentiate between related enzyme functions

• Develop structure-based functional classification systems

Metabolic Context Analysis:

• Consider genomic context and operon structure in functional predictions

• Implement metabolic pathway gap analysis to identify missing enzymatic functions

• Use correlated gene presence/absence patterns across species to infer functional relationships

• Apply flux balance analysis to test feasibility of predicted lipid biosynthesis pathways

Automated Literature Mining:

• Develop natural language processing tools to extract experimental evidence for enzyme functions

• Create knowledge graphs connecting genes, proteins, substrates, and phenotypes

• Implement systems for propagating experimental corrections to existing annotations

• Develop confidence scoring for functional annotations based on available evidence

Community Curation and Validation:

• Create platforms for expert curation of lipid-modifying enzyme annotations

• Implement systems for flagging potentially misannotated enzymes

• Develop metrics for annotation quality assessment

• Establish standardized experimental validation protocols for computational predictions

By implementing these approaches, researchers could significantly improve the accuracy of functional annotations for lipid-modifying enzymes like cardiolipin synthases, reducing the propagation of annotation errors that currently hinder research progress.

What are the most promising directions for cardiolipin synthase research in bacterial pathogens?

Based on current findings and knowledge gaps, several promising research directions for cardiolipin synthase in bacterial pathogens emerge:

Clarification of Specialized Functions:

• Further investigation of the differentiated roles of ClsA, ClsB, and ClsC in various bacterial species

• Comprehensive examination of substrate ranges for each enzyme under different physiological conditions

• Investigation of potential moonlighting functions beyond lipid synthesis

• Exploration of potential roles in prokaryotic organelle organization and function

Host-Pathogen Interface Exploration:

• Further characterization of how cardiolipin and trehalose phospholipids interact with host immune receptors

• Investigation of temporal changes in lipid composition during different infection stages

• Examination of cardiolipin-dependent membrane domains in bacterial adaptation to host environments

• Study of cardiolipin's role in antibiotic resistance mechanisms

Therapeutic Target Development:

• Design of specific inhibitors targeting ClsB's trehalose phospholipid synthesis capability

• Exploration of cardiolipin synthesis inhibition as an antivirulence strategy

• Development of compounds that disrupt cardiolipin-dependent membrane organization

• Creation of diagnostic tools based on lipid profiles to identify specific pathogens

Structural Biology Advances:

• Resolution of crystal structures for all three cardiolipin synthases

• Comparative analysis of enzyme-substrate interactions across different bacterial species

• Investigation of potential protein-protein interactions involving cardiolipin synthases

• Examination of regulatory mechanisms controlling enzyme activity and expression

System-Level Understanding:

• Integration of lipidomics, transcriptomics, and proteomics to develop comprehensive models of membrane adaptation

• Exploration of cardiolipin's role in bacterial stress response networks

• Investigation of evolutionary trajectories of cardiolipin synthase diversification

• Development of synthetic biology approaches to engineer membrane composition