Recombinant Salmonella schwarzengrund Cardiolipin synthase (cls)

What is Salmonella schwarzengrund Cardiolipin synthase and what is its function?

Cardiolipin synthase (cls) in Salmonella schwarzengrund is an enzyme that catalyzes the formation of cardiolipin (CL), an acidic glycerophospholipid found in bacterial membranes. The protein belongs to the phospholipase D superfamily and is responsible for condensing phospholipid molecules to form cardiolipin, which plays critical roles in membrane structure and function . The cls enzyme from S. schwarzengrund strain CVM19633 is identified by UniProt accession number B4TX53 and consists of 486 amino acids . Similar to other Enterobacteriaceae, Salmonella species encode multiple cardiolipin synthases that function under different growth conditions, with the primary role of maintaining appropriate phospholipid composition in the bacterial membrane, particularly during environmental stress conditions .

How many types of Cardiolipin synthases exist in Salmonella species and how do they differ?

Salmonella species, like other Enterobacteriaceae, possess three distinct cardiolipin synthases, designated as ClsA, ClsB, and ClsC. These enzymes display differential expression and activity depending on growth phase and environmental conditions . ClsA (encoded by the clsA gene) is the primary cardiolipin synthase during logarithmic growth phase, while ClsB and ClsC contribute more significantly to cardiolipin production during stationary phase . The three enzymes also differ in their substrate specificity and catalytic mechanisms. ClsA and ClsB catalyze the condensation of two phosphatidylglycerol (PG) molecules to form cardiolipin and glycerol. In contrast, ClsC functions in conjunction with YmdB protein and utilizes phosphatidylethanolamine (PE) as the phosphatidyl donor to phosphatidylglycerol (PG) for cardiolipin synthesis, demonstrating a unique mode of action compared to other cardiolipin synthases .

What are the optimal conditions for expressing recombinant Salmonella schwarzengrund Cardiolipin synthase?

For optimal expression of recombinant Salmonella schwarzengrund Cardiolipin synthase, the E. coli heterologous expression system has been successfully employed . When expressing cls proteins, researchers should consider the following methodological approach:

• Host strain selection: BL21(DE3) or similar E. coli strains designed for membrane protein expression are recommended due to the transmembrane domains present in cls proteins.

• Vector design: Include an N-terminal or C-terminal affinity tag (commonly His-tag) to facilitate purification, as demonstrated with other Salmonella cls proteins .

• Induction conditions: Induction with arabinose (0.2%) has been successful for cls expression when using arabinose-inducible vectors like pBAD30 . Alternatively, IPTG induction (0.1-0.5 mM) can be used with T7 promoter-based systems.

• Growth temperature: Reduce temperature to 16-25°C post-induction to enhance proper folding of membrane proteins.

• Growth phase: Harvest cells during late logarithmic or early stationary phase to maximize protein yield.

The expression of membrane proteins like cls often requires optimization of these parameters to balance protein yield with proper folding and activity . It is also important to note that co-expression with chaperones may improve solubility and proper folding of recombinant cls proteins.

What are the recommended methods for purifying recombinant Salmonella Cardiolipin synthase?

Purification of recombinant Salmonella Cardiolipin synthase requires specific methodologies due to its membrane-associated nature. Based on successful approaches with similar proteins, the following purification protocol is recommended:

• Cell lysis: Use gentle lysis methods such as enzymatic lysis with lysozyme (100 μg/ml) followed by sonication in buffer containing 50 mM Tris-HCl (pH 8.0), 300 mM NaCl, and 10% glycerol.

• Membrane extraction: Solubilize membranes using detergents such as n-dodecyl-β-D-maltoside (DDM) at 1% concentration or other mild detergents suitable for membrane proteins.

• Affinity chromatography: For His-tagged cls proteins, use Ni-NTA affinity chromatography with imidazole gradient elution (20-250 mM) .

• Further purification: Size exclusion chromatography can be employed as a polishing step to achieve >90% purity, using buffers containing reduced detergent concentrations (0.05-0.1% DDM) to maintain protein stability.

• Storage: Store the purified protein in Tris-based buffer with 50% glycerol at -20°C for extended storage, or at -80°C for long-term preservation .

It is crucial to maintain detergent concentrations above the critical micelle concentration throughout the purification process to prevent protein aggregation. Additionally, inclusion of stabilizers like glycerol (6-50%) or trehalose (6%) in storage buffers significantly enhances protein stability .

How should recombinant Cardiolipin synthase be stored to maintain optimal activity?

Proper storage of recombinant Cardiolipin synthase is critical for maintaining enzymatic activity. Based on empirical data and manufacturer recommendations, the following storage guidelines should be followed:

• Short-term storage: For working aliquots, store at 4°C for up to one week in appropriate buffer systems .

• Medium-term storage: Store at -20°C in buffer containing cryoprotectants such as 50% glycerol or 6% trehalose to prevent freezing damage .

• Long-term storage: For extended preservation, aliquot the protein in small volumes and store at -80°C in Tris/PBS-based buffer (pH 8.0) with 6% trehalose or 50% glycerol .

• Avoid freeze-thaw cycles: Repeated freezing and thawing significantly reduces enzyme activity; therefore, single-use aliquots are strongly recommended .

• Reconstitution protocol: When using lyophilized protein, reconstitute in deionized sterile water to a concentration of 0.1-1.0 mg/mL. Add glycerol to a final concentration of 5-50% before aliquoting for storage .

When planning experiments, researchers should consider that freshly purified enzyme typically exhibits higher activity than stored samples. For critical activity assays, verification of enzyme activity using established assays is recommended before proceeding with experimental procedures.

How can one design an in vitro assay to measure Cardiolipin synthase activity of recombinant S. schwarzengrund cls?

Designing a robust in vitro assay for Cardiolipin synthase activity requires careful consideration of substrates, reaction conditions, and detection methods. For S. schwarzengrund cls, the following methodological approach is recommended:

• Substrate preparation: Prepare phosphatidylglycerol (PG) liposomes as the primary substrate for ClsA and ClsB, or a mixture of phosphatidylethanolamine (PE) and PG for ClsC activity assays. Radiolabeled substrates (³²P-labeled phospholipids) can be used for enhanced sensitivity .

• Reaction conditions:

  • Buffer composition: 50 mM Tris-HCl (pH 7.5), 50 mM KCl, 10 mM MgCl₂

  • Temperature: 30-37°C

  • Incubation time: 30-60 minutes

  • Cofactors: Divalent cations (Mg²⁺ or Mn²⁺) at 10 mM concentration

  • For ClsC activity, include purified YmdB protein in the reaction mixture

• Product detection methods:

  • Thin-layer chromatography (TLC) with phospholipid standards, followed by charring or autoradiography for radiolabeled substrates

  • Mass spectrometry: LC/MS/MS for precise quantification of cardiolipin production and characterization of acyl chain compositions

  • Fluorescent substrate analogs coupled with HPLC separation for continuous monitoring of activity

• Controls and standardization:

  • Include negative controls (heat-inactivated enzyme, reaction without substrate)

  • Use purified cardiolipin as a positive control for product detection

  • Include assays with characterized cls enzymes (e.g., E. coli ClsA) as reference standards

When investigating the catalytic properties of different cls isoforms, researchers should note that ClsA is primarily active during logarithmic growth conditions, while ClsB and ClsC show increased activity during stationary phase and under osmotic stress conditions .

What experimental approaches can be used to study the role of Cardiolipin synthase in Salmonella virulence?

Investigating the role of Cardiolipin synthase in Salmonella virulence requires multidisciplinary approaches spanning genetics, biochemistry, and infection models. The following methodological framework is recommended:

• Genetic manipulation strategies:

  • Generate single, double, and triple deletion mutants of cls genes (ΔclsA, ΔclsB, ΔclsC, ΔclsAB, ΔclsBC, ΔclsAC, and ΔclsABC) using lambda Red recombination or CRISPR-Cas9 systems

  • Complement mutants with plasmid-encoded cls genes to confirm phenotype specificity

  • Create site-directed mutants in catalytic domains to study structure-function relationships

• Membrane composition analysis:

  • Quantify cardiolipin levels in different mutants using thin-layer chromatography and mass spectrometry

  • Analyze membrane physical properties (fluidity, permeability) in wildtype versus cls mutants

  • Monitor changes in membrane composition under infection-relevant conditions (acidic pH, oxidative stress, antimicrobial peptides)

• In vitro infection models:

  • Macrophage infection assays to assess intracellular survival and replication

  • Inflammasome activation assays measuring IL-1β production and pyroptosis in infected macrophages

  • Evaluation of bacterial resistance to antimicrobial peptides and oxidative stress

• In vivo virulence assessment:

  • Oral infection of mice to evaluate intestinal colonization and systemic dissemination

  • Competitive index assays comparing wild-type and cls mutants in mixed infections

  • Measurement of inflammatory markers and tissue damage in infected organs

Interestingly, research with S. Typhimurium has shown that despite the regulation of cardiolipin levels during infection, cls triple mutants (ΔclsABC) remain highly virulent during both oral and systemic infection in C57BL/6J mice . This suggests that cardiolipin synthesis may be dispensable for virulence in certain contexts, highlighting the complex and potentially redundant roles of membrane phospholipids in bacterial pathogenesis.

How does osmotic stress affect the expression and activity of different Cardiolipin synthases in Salmonella?

Osmotic stress significantly impacts the expression and activity of Cardiolipin synthases in Salmonella, with differential effects on the three cls isoforms. To investigate these effects, researchers should employ the following methodological approaches:

• Expression analysis under varying osmolarity:

  • Cultivate bacteria in media with different NaCl concentrations (0.1-0.5 M) or other osmolytes

  • Quantify transcriptional changes using qRT-PCR or RNA-seq for each cls gene

  • Monitor protein expression using Western blot with isoform-specific antibodies or tagged constructs

• Membrane lipid composition analysis:

  • Extract total membrane lipids and analyze cardiolipin content using TLC and mass spectrometry

  • Compare wild-type and single/multiple cls mutants to determine the contribution of each synthase

  • Track changes in CL/PG/PE ratios across growth phases at different osmolarities

• Enzyme activity measurements:

  • Perform in vitro activity assays with membranes isolated from cells grown under different osmotic conditions

  • Determine kinetic parameters (Km, Vmax) for each cls isoform at varying ionic strengths

  • Assess the effect of osmolytes on enzyme stability and substrate specificity

Table 1: Comparative contribution of Cardiolipin synthases to CL synthesis under different conditions

How do S. schwarzengrund Cardiolipin synthases compare with orthologs from other Salmonella serovars?

Comparative analysis of Cardiolipin synthases across Salmonella serovars reveals important insights into evolutionary conservation and functional divergence. To effectively study these differences, researchers should implement the following approaches:

• Sequence alignment and phylogenetic analysis:

  • Align cls sequences from multiple Salmonella serovars (S. schwarzengrund, S. Typhimurium, S. heidelberg, etc.)

  • Identify conserved catalytic domains and strain-specific variations

  • Construct phylogenetic trees to understand evolutionary relationships

S. schwarzengrund cls (UniProt: B4TX53) shares high sequence identity with S. heidelberg cls (UniProt: B4TJM2), both maintaining the critical catalytic motifs of the phospholipase D superfamily . The comparative analysis of the full-length protein sequences from these serovars shows conservation in the functional domains but subtle variations in non-catalytic regions that may influence substrate specificity or regulation.

• Functional complementation studies:

  • Express cls genes from different serovars in an E. coli or Salmonella ΔclsABC background

  • Measure cardiolipin production and membrane composition

  • Assess restoration of phenotypes related to membrane function

• Structural biology approaches:

  • Generate homology models based on crystallized phospholipase D family members

  • Identify potential structural differences affecting substrate binding or catalysis

  • Use molecular dynamics simulations to predict functional implications of sequence variations

Interestingly, while the primary sequence and catalytic mechanism are largely conserved, the regulatory patterns of cls genes may differ between serovars, potentially contributing to niche-specific adaptations and host preferences. This suggests that comparative studies of cls regulation across serovars could provide valuable insights into Salmonella evolution and host adaptation strategies .

What are the challenges in distinguishing the specific roles of ClsA, ClsB, and ClsC in membrane remodeling during infection?

Distinguishing the specific roles of the three cardiolipin synthases in membrane remodeling during infection presents several methodological challenges that researchers must address through sophisticated experimental designs:

• Functional redundancy challenges:

  • Single mutations often show minimal phenotypes due to compensation by other cls genes

  • Triple mutants (ΔclsABC) completely lacking cardiolipin may exhibit pleiotropic effects unrelated to specific cls functions

  • Solution: Generate conditional expression systems to modulate individual cls activity during specific infection stages

• Temporal and spatial regulation complexities:

  • Different cls enzymes are active at different growth phases and microenvironments

  • Intracellular infection involves transitions through multiple compartments with varying conditions

  • Solution: Develop fluorescent reporters for real-time monitoring of cls expression during infection

• Substrate availability variations:

  • ClsA/B use PG as substrate while ClsC/YmdB use PE and PG

  • Changes in precursor availability during infection may influence which enzyme predominates

  • Solution: Label phospholipid precursors to track flux through different synthetic pathways during infection

• Technical limitations in membrane analysis:

  • Direct measurement of membrane composition in intracellular bacteria is challenging

  • Solution: Develop techniques for isolation of bacteria from infected cells with minimal membrane perturbation, coupled with sensitive lipidomic analysis

Research has revealed that despite the regulation of cardiolipin levels within the outer membrane during infection, S. Typhimurium strains lacking all three cls genes (ΔclsABC) remain highly virulent during both oral and systemic infection in mice . This surprising finding challenges assumptions about the essentiality of cardiolipin for pathogenesis and underscores the need for more nuanced approaches to understand the specific contributions of each cls enzyme to membrane remodeling during host-pathogen interactions.

How can researchers address the conflicting data regarding the requirement of Cardiolipin synthases for Salmonella pathogenesis?

Addressing conflicting data regarding the requirement of Cardiolipin synthases for Salmonella pathogenesis requires systematic investigation using complementary approaches. Researchers should consider the following methodological framework:

• Standardization of experimental systems:

  • Use consistent bacterial strains, growth conditions, and infection models

  • Standardize genetic engineering approaches for creating cls mutants

  • Establish clear phenotypic readouts for virulence assessment

• Context-dependent analysis:

  • Compare cls requirement across different infection models (cell types, animal hosts)

  • Evaluate cls contribution under varying host immune statuses

  • Assess the role of cls genes in competition with gut microbiota versus systemic infection

• Redundancy and compensation investigation:

  • Analyze membrane composition changes in cls mutants for possible compensatory mechanisms

  • Examine alterations in other membrane components (LPS, outer membrane proteins) in cls mutants

  • Investigate potential cross-talk between phospholipid synthesis pathways

• Resolution of temporal dynamics:

  • Develop inducible cls expression systems for stage-specific analysis during infection

  • Compare requirements during initial invasion versus persistent infection

  • Examine cls roles during transition between host environments

The conflicting data indicating that S. Typhimurium remains virulent despite complete depletion of cardiolipin contradicts previous assumptions about membrane phospholipid requirements for pathogenesis . This suggests either compensatory mechanisms or context-dependent roles for cardiolipin. Furthermore, while mitochondrial cardiolipin can activate host inflammasomes, the contribution of bacterial cardiolipin to this process appears more complex than initially hypothesized . These contradictions highlight the need for nuanced experimental designs that can disentangle direct cls effects from compensatory responses and distinguish between roles in bacterial physiology versus host-pathogen interactions.

What novel applications might recombinant Cardiolipin synthase have in synthetic biology and membrane engineering?

Recombinant Cardiolipin synthase offers numerous potential applications in synthetic biology and membrane engineering that researchers can explore using the following methodological approaches:

• Designer membrane construction:

  • Express different cls isoforms in liposomes or synthetic membrane systems

  • Modulate membrane curvature and physical properties through controlled cardiolipin synthesis

  • Engineer membranes with defined phospholipid compositions for drug delivery vehicles

• Bioenergy applications:

  • Develop bacterial strains with enhanced cls expression for increased membrane surface area

  • Optimize electron transport chain efficiency through cardiolipin-enriched membranes

  • Create cardiolipin-rich membranes mimicking mitochondrial inner membranes for bioenergetic studies

• Biosensor development:

  • Utilize cls enzymes for detection of specific phospholipids in biological samples

  • Create cls-based biosensors for monitoring osmotic stress in industrial fermentations

  • Develop high-throughput screening platforms for cardiolipin-binding compounds

• Therapeutic target exploration:

  • Use recombinant cls to screen for selective inhibitors as potential antimicrobials

  • Develop assays to identify compounds that modulate cardiolipin content without affecting bacterial viability

  • Explore immunomodulatory applications based on cardiolipin's known interactions with host immune systems

ClsC's unique ability to use PE as a phosphatidyl donor, especially when co-expressed with YmdB, offers particularly interesting applications in synthetic biology where alternative substrate utilization might be advantageous . Furthermore, the differential activity profiles of ClsA, ClsB, and ClsC under varying growth conditions provide flexible tools for designing responsive membrane systems that change composition based on environmental cues.

How might advanced structural studies of Cardiolipin synthase inform the development of novel antimicrobials?

Advanced structural studies of Cardiolipin synthase can significantly inform antimicrobial development through comprehensive understanding of enzyme structure-function relationships. Researchers should pursue the following methodological approaches:

• High-resolution structure determination:

  • Utilize X-ray crystallography or cryo-electron microscopy to resolve cls structures

  • Focus on substrate binding sites and catalytic domains

  • Compare structures of different cls isoforms to identify unique features

• Molecular dynamics simulations:

  • Model cls interactions with membrane environments

  • Simulate substrate binding and catalytic mechanisms

  • Identify transient binding pockets for potential inhibitor design

• Structure-guided inhibitor design:

  • Perform virtual screening against resolved cls structures

  • Design transition-state analogs targeting the phospholipase D-like catalytic mechanism

  • Develop allosteric inhibitors targeting cls-specific regulatory domains

• Fragment-based drug discovery:

  • Screen chemical libraries for fragments binding to cls active sites

  • Use NMR or thermal shift assays to identify binding fragments

  • Elaborate hit fragments into lead compounds with improved potency and selectivity

The unique catalytic mechanism of ClsC when functioning with YmdB, which uses PE as a substrate rather than the PG-PG condensation employed by ClsA and ClsB, represents a potentially selective target for antimicrobial development . Additionally, because cardiolipin plays crucial roles in membrane organization, especially at cell division sites, targeting cls function could disrupt multiple essential processes simultaneously, potentially reducing the emergence of resistance.

What research approaches could clarify the relationship between bacterial Cardiolipin synthases and host inflammasome activation?

Investigating the complex relationship between bacterial Cardiolipin synthases and host inflammasome activation requires integrated approaches spanning microbiology, immunology, and structural biology. Researchers should implement the following methodological strategies:

• Purified component systems:

  • Isolate cardiolipin from wild-type and mutant Salmonella strains

  • Test purified cardiolipin molecules for direct inflammasome activation in macrophage cultures

  • Compare bacterial cardiolipin with mitochondrial cardiolipin for structural differences affecting immune recognition

• Advanced infection models:

  • Use fluorescent cardiolipin probes to track cardiolipin distribution during infection

  • Employ inflammasome reporter systems in infected cells to correlate cardiolipin exposure with activation

  • Compare responses to isogenic Salmonella strains expressing different levels of cardiolipin

• Mechanistic dissection of inflammasome activation:

  • Identify host cardiolipin-binding proteins involved in immune recognition

  • Use proximity labeling to capture transient interactions between bacterial cardiolipin and host proteins

  • Employ CRISPR screens to identify host factors required for cardiolipin-mediated inflammasome activation

• Structure-function analysis of cardiolipin-host interactions:

  • Determine the specific cardiolipin structural features required for immunostimulatory activity

  • Compare cardiolipin species with varying acyl chain compositions for differential immune activation

  • Develop synthetic cardiolipin analogs with modified immunomodulatory properties

Research has shown that mitochondrial cardiolipin can prime and activate host inflammasomes, yet somewhat surprisingly, Salmonella lacking all three cls genes (and thus completely devoid of cardiolipin) remain capable of activating inflammasomes during infection . This apparent contradiction suggests that bacterial cardiolipin may play more complex roles in host-pathogen interactions than simply acting as a direct inflammasome activator. Elucidating these mechanisms could reveal new insights into host innate immune recognition of bacterial pathogens and potentially lead to novel immunomodulatory strategies.