Recombinant Escherichia coli O45:K1 Cardiolipin synthase (cls)

What is cardiolipin synthase and what role does it play in Escherichia coli?

Cardiolipin synthase (Cls) is an enzyme that catalyzes the formation of cardiolipin, a key phospholipid component of bacterial membranes. In Escherichia coli, cardiolipin synthase primarily catalyzes the conversion of two phosphatidylglycerol (PG) molecules to form cardiolipin and glycerol. Cardiolipin typically constitutes approximately 5-15% of phospholipids in E. coli, with the remainder being primarily phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), though this proportion varies depending on growth phase and culture conditions. Cardiolipin plays critical roles in membrane structure, energy metabolism, and stress responses in bacterial cells. The enzyme is crucial for bacterial adaptation to environmental stressors, particularly during stationary phase and under osmotic stress conditions .

How many cardiolipin synthase variants exist in Escherichia coli and how do they differ?

Escherichia coli possesses three distinct cardiolipin synthase variants, each encoded by different genes:

• ClsA (encoded by cls gene) - The primary cardiolipin synthase that uses two PG molecules as substrates

• ClsB (encoded by ybhO gene) - A secondary cardiolipin synthase that also uses PG as substrate

• ClsC (encoded by ymdC gene) - A third cardiolipin synthase that operates with a different mechanism

All three cardiolipin synthases belong to the phospholipase D superfamily and share sequence homology, though they function differently. ClsA contributes detectible levels of cardiolipin at low osmolarity during logarithmic growth, while all three synthases increase cardiolipin production with increasing medium osmolarity during both logarithmic growth and stationary phase. The triple mutant (ΔclsABC) lacks detectable cardiolipin regardless of growth phase or conditions, demonstrating the complete set of cardiolipin synthases in E. coli .

What is the unique feature of ClsC compared to other cardiolipin synthases?

Unlike ClsA and ClsB which condense two phosphatidylglycerol (PG) molecules to form cardiolipin, ClsC employs a fundamentally different mechanism. ClsC, particularly when co-expressed with its neighboring gene ymdB, uses phosphatidylethanolamine (PE) as the phosphatidyl donor to PG to form cardiolipin. This represents a third and unique mode for cardiolipin synthesis distinct from both the other E. coli synthases and eukaryotic cardiolipin synthases (which use PG and CDP-diacylglycerol as substrates). This mechanism was demonstrated through in vitro assays using synthetic phospholipids with specific acyl chains and multiple reaction monitoring (MRM) mass spectrometry to confirm the incorporation of phosphatidyl moieties from PE into cardiolipin .

What is the significance of Escherichia coli O45:K1 strains in human disease?

Escherichia coli O45:K1 represents a virulent clonal group belonging to the highly virulent subgroup B2 that has significant clinical importance. This strain has been identified as a major causative agent of neonatal meningitis in France, accounting for approximately one-third of E. coli neonatal meningitis cases. The K1 capsular antigen is a key virulence factor associated with invasive disease, particularly meningitis, as it allows the bacteria to evade host immune responses. The pathogenicity of this strain is further enhanced by various virulence factors carried on plasmids, including those that may affect membrane composition and function. Understanding cardiolipin synthesis in these strains is relevant for comprehending their adaptation to host environments and potential contribution to pathogenesis .

How does the plasmid content of Escherichia coli O45:K1 contribute to its virulence?

The plasmid pS88, found in representative O45:K1 strain S88, significantly contributes to the virulence of these bacteria. This large plasmid (133,853 bp) contains 144 protein-coding genes, including multiple iron uptake systems (aerobactin, salmochelin, and the sitABCD genes) and other putative virulence genes (iss, etsABC, ompTP, and hlyF). The pS88 sequence consists of several gene blocks homologous to avian pathogenic E. coli plasmids. PCR analysis has demonstrated that pS88-like plasmids are present in multiple meningitis-causing clonal groups (O18:K1, O1:K1, and O83:K1), as well as in avian pathogenic strains and human urosepsis strains belonging to subgroup B2. This plasmid distribution pattern suggests horizontal gene transfer has spread these virulence factors among pathogenic E. coli strains .

How do Escherichia coli O45:H2 and O45:H16 strains differ genetically and in virulence potential?

Genome-based phylogenetic analysis reveals significant differences between E. coli O45:H2 and O45:H16 strains despite sharing the O45 antigen:

E. coli O45:H2 strains possess the locus of enterocyte effacement pathogenicity island and share high homology with E. coli O103:H2 strains in terms of virulence factors such as Stx prophages. This suggests E. coli O45:H2 may be as virulent as E. coli O103:H2, which is frequently associated with severe illness. These findings provide genomic evidence to facilitate Shiga toxin-producing E. coli (STEC) surveillance and differentiate between potentially more pathogenic variants within the O45 serogroup .

What are effective methods for amplifying and purifying cardiolipin synthase from Escherichia coli?

Amplification and purification of cardiolipin synthase can be achieved through recombinant protein expression systems. One effective approach is to insert the cls gene downstream from a strong promoter, such as the T7 RNA promoter, in an expression vector. For example, strain BL21(DE3) bearing recombinant plasmid pLR3 (with cls gene inserted downstream from a T7 RNA promoter) can produce membranes with over 1200 times more cardiolipin synthase activity than wild-type cells.

For purification to homogeneity, the following protocol has been effective:

• Membrane extraction with Triton X-114 detergent

• Chromatography on DEAE-cellulose

• Monitoring of purity by SDS-PAGE (purified enzyme appears as a single band at 46 kDa)

This approach supports the identification of cls as the structural gene for cardiolipin synthase. Activity of the purified enzyme can be measured using a mixed micelle assay with phosphatidyl[2-3H]glycerol as substrate .

How can one measure cardiolipin synthase activity in vitro?

Cardiolipin synthase activity can be measured using a mixed micelle assay with the following components:

• Substrate preparation: Phosphatidyl[2-3H]glycerol serves as an effective radiolabeled substrate that allows sensitive detection of enzymatic activity.

• Reaction conditions: The assay should include appropriate buffer conditions with consideration for factors affecting enzyme activity:

  • Magnesium chloride concentration (affects inhibition patterns)

  • Presence of other phospholipids that may modulate activity

  • pH optimization typically around neutral pH

• Product detection methods:

  • Thin-layer chromatography (TLC) with subsequent charring for visualization

  • Mass spectrometry (MS) analysis for more sensitive detection

  • Liquid chromatography coupled with tandem mass spectrometry (LC/MS/MS) for precise determination of cardiolipin species

For studying substrate specificity, particularly for ClsC, one can use synthetic phospholipids with specific acyl chains. For example: synthetic PG (12:0/13:0) combined with either synthetic PE or PA with 14:1/17:0 acyl chains allows determination of which phospholipid serves as the phosphatidyl donor through multiple reaction monitoring (MRM) mass spectrometry .

What techniques are most effective for detecting cardiolipin in bacterial samples?

Several complementary techniques can be employed for reliable detection and quantification of cardiolipin in bacterial samples:

• Thin-layer chromatography (TLC):

  • Basic detection through charring of TLC plates

  • Advantage: Simple and accessible method

  • Limitation: Less sensitive for low cardiolipin levels

• Mass spectrometry (MS):

  • Can detect small amounts of cardiolipin not visible by TLC charring

  • Particularly useful for measuring low levels of cardiolipin as seen with ClsC expression

  • Provides information about cardiolipin molecular species

• Liquid chromatography-tandem mass spectrometry (LC/MS/MS):

  • Multiple reaction monitoring (MRM) method using triple quadrupole instruments

  • First mass filter (Q1) selects specific cardiolipin species (e.g., doubly charged [M-2H]2– ion)

  • Collision cell fragments the molecules

  • Second mass filter (Q3) isolates specific fragment ions (e.g., m/z 225.2 for 14:1 and 269.2 for 17:0 acyl chains)

  • Advantage: High specificity without interference from endogenous lipids

• Radioactive labeling:

  • Using phosphatidyl[2-3H]glycerol as substrate allows tracking of newly synthesized cardiolipin

  • Useful for studying enzyme kinetics and mechanisms

How is cardiolipin synthase activity regulated in Escherichia coli?

Cardiolipin synthase activity in Escherichia coli is subject to sophisticated regulatory mechanisms:

• Product inhibition: Cardiolipin itself acts as an inhibitor of cardiolipin synthase, creating a negative feedback loop that likely plays an important role in regulating cardiolipin synthesis in E. coli. This self-regulatory mechanism helps maintain appropriate membrane phospholipid composition .

• Inhibition by other phospholipids: Phosphatidate also inhibits cardiolipin synthase activity. Notably, not all anionic phosphoglycerides exhibit inhibitory effects - phosphatidylinositol and bis-phosphatidate do not inhibit the enzyme, suggesting specificity in the regulatory mechanisms .

• Counteracting factors: Phosphatidylethanolamine partially offsets inhibition by cardiolipin but not by phosphatidate. Conversely, magnesium chloride has the opposite effect. These interactions reveal a complex regulatory network involving multiple phospholipid species and divalent cations .

• Growth phase-dependent expression: The three cardiolipin synthases show differential expression patterns depending on growth phase. While ClsA contributes detectible levels of cardiolipin at low osmolarity during logarithmic growth, all three Cls enzymes increase cardiolipin production with increasing medium osmolarity during both logarithmic growth and stationary phase .

What factors affect cardiolipin levels in Escherichia coli cells?

Multiple factors influence cardiolipin levels in E. coli cells:

• Growth phase: Cardiolipin content increases during stationary phase, with the triple cls mutant (ΔclsABC) showing complete absence of detectible cardiolipin regardless of growth phase.

• Medium osmolarity: Higher osmolarity induces increased cardiolipin synthesis during both logarithmic growth and stationary phase for all cardiolipin synthases.

• Cls enzyme expression patterns: ClsA is the predominant synthase during logarithmic growth at low osmolarity, while all three synthases contribute to cardiolipin production under other conditions.

• Co-expression of accessory proteins: For ClsC, co-expression with its neighboring gene ymdB significantly enhances cardiolipin production. When expressed alone, ClsC produces only small amounts of cardiolipin, but co-expression with ymdB results in cardiolipin levels comparable to those achieved with ClsA or ClsB expression.

• Substrate availability: The unique substrate requirements of different Cls enzymes (PG for ClsA/ClsB, PE and PG for ClsC) mean that alterations in membrane phospholipid composition can affect cardiolipin synthesis capacity .

What is the role of YmdB in ClsC-mediated cardiolipin synthesis?

YmdB plays a critical enhancing role in ClsC-mediated cardiolipin synthesis:

• Operon structure: The clsC gene (ymdC) is separated by only one base pair from the preceding ymdB gene in the same operon, suggesting functional relatedness.

• Protein structure: YmdB contains a macro domain with a predicted adenosine diphosphate (ADP) ribose-binding potential, suggesting a possible regulatory function.

• Functional enhancement: Co-expression of YmdB with ClsC in a ΔclsABC mutant results in cardiolipin levels comparable to those achieved by expression of ClsA or ClsB alone. In contrast, expression of ClsC without YmdB produces only minimal cardiolipin.

• Independent expression verification: Even when clsC and ymdB are expressed independently from different but compatible plasmids, they can function together to produce cardiolipin, though optimal production is achieved when expressed from the same operon.

• YmdB dependency: A strain lacking all three cls genes and ymdB (ΔclsABC, ΔymdB::KanR) shows restored cardiolipin production only when both clsC and ymdB are present, confirming that YmdB is necessary for efficient ClsC function .

What are effective strategies for cloning and expressing cardiolipin synthase genes?

Effective strategies for cloning and expressing cardiolipin synthase genes include:

• Vector selection: Arabinose-inducible expression vectors such as pBAD30 have proven effective for controlled expression of cls genes. This system allows for tunable expression through arabinose concentration adjustment.

• Inclusion of regulatory elements: Ensure ribosome-binding sites are included when cloning cls genes to optimize translation efficiency.

• Co-expression considerations: For ClsC, co-expression with YmdB significantly enhances activity. When cloning clsC, include the entire two-gene operon (ymdB-clsC) to achieve optimal cardiolipin production.

• Growth conditions for expression:

  • Medium: LB with appropriate antibiotics for plasmid selection

  • Inducer: 0.2% arabinose for pBAD vectors

  • Growth phase: Expression can be evaluated in both logarithmic and stationary phases

  • Temperature: Standard growth at 37°C is typically effective

• Expression verification: Monitor cardiolipin production through lipid extraction and analysis by TLC or mass spectrometry to confirm functional expression of the recombinant enzyme .

How can researchers construct and validate knockout mutants for cardiolipin synthase genes?

Construction and validation of cardiolipin synthase knockout mutants can be accomplished through these steps:

• Gene replacement strategy: Replace the target cls gene with an antibiotic resistance marker (e.g., chloramphenicol resistance cat gene) through homologous recombination.

• PCR confirmation: Verify correct gene replacement using primers homologous to the cat gene and the flanking region of the target. This confirms proper insertion at the intended genomic location.

• Phenotypic validation: For each mutant, verify:

  • Capsule antigen expression (e.g., K1 capsule) remains intact

  • Bacteriocin production status (if relevant)

  • Presence of non-deleted plasmid genes

  • Presence of chromosomal virulence genes using multiplex PCRs

• Lipid profile analysis: Confirm the absence of cardiolipin or altered cardiolipin profiles through:

  • Lipid extraction and TLC analysis

  • MS analysis for more sensitive detection of residual cardiolipin

  • LC/MS/MS for detailed phospholipid composition analysis

• Complementation testing: Restore cardiolipin production by introducing plasmids expressing the deleted cls genes, confirming the phenotype is specifically due to the targeted deletion .

What methods can be used to cure plasmids from Escherichia coli strains for studying plasmid-encoded functions?

For studying plasmid-encoded functions in E. coli strains, particularly those related to virulence or membrane lipid metabolism, the following methods can be used to cure plasmids:

• Sodium dodecyl sulfate (SDS) treatment:

  • Grow bacterial cultures to early exponential phase (~5×10^5 CFU/ml)

  • Add serial concentrations of SDS (2.5%, 5%, and 10%)

  • Continue growth with shaking for 18 hours

  • Plate cultures on LB agar and screen colonies for plasmid loss

• Screening for plasmid loss:

  • Functional screening: For ColV plasmids, test colonies for colicin production by pricking them onto LB agar plates overlaid with a sensitive E. coli strain (e.g., K-12)

  • Loss of colicin production indicates potential plasmid curing

• Confirmation of plasmid loss:

  • PCR verification: Use PCR to confirm the absence of plasmid-related genes

  • Physical confirmation: Perform plasmid preparation and agarose gel electrophoresis to verify the absence of extrachromosomal DNA

  • Functional verification: Test for the loss of plasmid-encoded phenotypes

• Verification of strain integrity:

  • Confirm that chromosomal features remain intact (e.g., K1 capsule expression)

  • Verify presence of chromosomal virulence genes using multiplex PCR

  • Ensure no unintended mutations have occurred during the curing process

How can mass spectrometry be used to study cardiolipin structure and metabolism?

Mass spectrometry offers powerful approaches for studying cardiolipin structure and metabolism:

• Basic MS analysis:

  • Can detect cardiolipin species not visible by TLC charring

  • Effective for analyzing the complex mixture of cardiolipin molecular species

  • Particularly valuable for detecting low-abundance cardiolipin in strains with limited synthesis capability

• Multiple Reaction Monitoring (MRM):

  • Performed on triple quadrupole instruments

  • First mass filter (Q1) selects specific doubly charged cardiolipin ions (e.g., [M-2H]^2- at m/z 618.4)

  • After collision-induced dissociation, the last mass filter (Q3) isolates specific fragment ions (e.g., m/z 225.2 for 14:1 and 269.2 for 17:0 acyl chains)

  • Eliminates interference from endogenous lipids

  • Ideal for tracing specific phosphatidyl donors in cardiolipin synthesis

• Tracking substrate incorporation:

  • Use synthetic phospholipids with defined acyl chain compositions

  • For example, combining synthetic PG (12:0/13:0) with synthetic PE (14:1/17:0)

  • Monitor incorporation of specific acyl chains into cardiolipin products

  • This approach conclusively demonstrated that ClsC uses PE as a phosphatidyl donor to PG

• Quantitative analysis:

  • LC/MS/MS can provide quantitative data on cardiolipin content

  • Allows comparison of cardiolipin levels between different strains or growth conditions

  • Enables monitoring of changes in cardiolipin molecular species distribution

What are the key considerations when designing experiments to analyze the catalytic mechanism of cardiolipin synthase?

When designing experiments to analyze the catalytic mechanism of cardiolipin synthase, researchers should consider:

• Putative catalytic motifs:

  • All three Cls enzymes belong to the phospholipase D superfamily

  • Mutation of the putative catalytic motif of ClsC prevents cardiolipin formation

  • Site-directed mutagenesis of conserved residues can identify catalytic sites

• Substrate specificity:

  • ClsA and ClsB use two PG molecules as substrates

  • ClsC uses PE as the phosphatidyl donor to PG

  • Synthetic lipids with defined acyl chains allow precise tracking of substrate incorporation

• Cofactor requirements:

  • Assess the role of divalent cations (e.g., Mg^2+)

  • Evaluate the influence of other phospholipids on activity

• Kinetic parameters:

  • Determine Km values for different substrates

  • Measure reaction rates under varying substrate concentrations

  • Analyze product inhibition mechanisms

• Structural considerations:

  • The relationship between protein structure and function

  • Role of transmembrane domains in substrate access

  • Potential oligomerization effects on activity

• Interaction with accessory proteins:

  • The mechanism by which YmdB enhances ClsC activity

  • Potential protein-protein interactions affecting catalysis

  • The role of the macro domain in YmdB (with predicted ADP ribose-binding potential)

How does cardiolipin composition change under different growth conditions in Escherichia coli?

Cardiolipin composition in Escherichia coli exhibits notable changes under different growth conditions:

• Growth phase effects:

  • Cardiolipin content typically increases during stationary phase

  • ClsA contributes detectible levels during logarithmic growth at low osmolarity

  • ClsC with YmdB contributes primarily during stationary phase

  • The ΔclsAB mutant still makes cardiolipin in stationary phase through ClsC activity

• Osmolarity influence:

  • All three Cls enzymes increase cardiolipin production with increasing medium osmolarity

  • This occurs during both logarithmic growth and stationary phase

  • Suggests a role for cardiolipin in osmotic stress adaptation

• Strain-specific variations:

  • Virulent strains like E. coli O45:K1 may have different baseline cardiolipin compositions

  • Plasmid presence can affect membrane composition through various mechanisms

  • Virulence-associated features may correlate with specific cardiolipin profiles

• Environmental stress responses:

  • pH stress likely triggers changes in cardiolipin content and composition

  • Temperature shifts can alter cardiolipin acyl chain composition

  • Nutrient limitation affects cardiolipin synthesis and turnover

• Contribution of different synthases:

  • ClsA or ClsB expression in the ΔclsABC mutant can restore cardiolipin to near wild-type levels in stationary phase

  • ClsC alone results in low levels of cardiolipin in stationary phase

  • Co-expression of ClsC with YmdB increases cardiolipin to near wild-type levels

How might understanding cardiolipin synthesis in pathogenic Escherichia coli strains contribute to new antimicrobial strategies?

Understanding cardiolipin synthesis in pathogenic E. coli strains could lead to novel antimicrobial approaches:

• Targeting virulence-specific membrane adaptations:

  • Pathogenic strains like E. coli O45:K1 may rely on specific membrane compositions for virulence

  • Cardiolipin distribution affects membrane curvature and protein localization

  • Inhibitors targeting pathogen-specific cardiolipin synthesis patterns could reduce virulence

• Exploiting unique aspects of ClsC-mediated synthesis:

  • The ClsC/YmdB system represents a unique cardiolipin synthesis pathway

  • This pathway uses PE as a phosphatidyl donor, unlike other known mechanisms

  • Small molecules targeting this interaction could provide selective inhibition

• Disturbing stress adaptation:

  • Cardiolipin synthesis increases under stress conditions including stationary phase and high osmolarity

  • Preventing this adaptation could make pathogens more susceptible to host defenses

  • Combination therapies targeting stress response systems could enhance existing antimicrobials

• Evolution-informed targeting:

  • E. coli O45:H2 strains are evolutionarily close to E. coli O103:H2 strains

  • They share high homology in virulence factors and may share membrane composition patterns

  • Broad-spectrum approaches targeting conserved features across pathogenic lineages could be developed

• Plasmid-centric approaches:

  • Plasmids like pS88 in E. coli O45:K1 encode numerous virulence factors

  • Targeting plasmid maintenance systems could reduce virulence

  • Understanding how plasmid presence affects membrane composition could reveal new intervention points

What are the most promising future research directions for understanding the relationship between cardiolipin synthesis and pathogenicity?

Several promising research directions could further elucidate the relationship between cardiolipin synthesis and pathogenicity:

• Membrane microdomains in host-pathogen interactions:

  • Investigate how cardiolipin-rich domains affect localization of virulence factors

  • Study the impact of cardiolipin on bacterial adhesion, invasion, and intracellular survival

  • Examine co-localization of virulence factors with specific lipid compositions

• Stress-responsive membrane remodeling:

  • Characterize how pathogenic strains remodel membranes during infection

  • Compare cardiolipin dynamics between commensal and pathogenic strains

  • Identify infection-specific signals that trigger membrane composition changes

• Systems biology approaches:

  • Integrate transcriptomics, proteomics, and lipidomics to create comprehensive models

  • Map regulatory networks connecting virulence gene expression and membrane composition

  • Use computational modeling to predict membrane adaptations during infection

• Host-derived signals affecting cardiolipin synthesis:

  • Investigate how host-derived molecules affect bacterial cardiolipin synthesis

  • Study the impact of antimicrobial peptides on bacterial membrane composition

  • Examine how immune cell contact triggers membrane remodeling

• Evolutionary aspects of cardiolipin synthesis:

  • Compare cardiolipin synthesis mechanisms across related pathogenic strains

  • Investigate horizontal gene transfer of membrane-related genes

  • Study how membrane composition contributes to adaptation to specific host niches

How can genetic engineering of cardiolipin synthase be used to create research tools for membrane biology?

Genetic engineering of cardiolipin synthase offers opportunities to develop sophisticated research tools for membrane biology:

• Tunable membrane composition systems:

  • Engineer strains with inducible expression of different cls genes

  • Create systems with precisely controlled cardiolipin content and composition

  • Develop conditional mutants for studying essentiality under specific conditions

• Fluorescent reporter systems:

  • Generate fusion proteins between Cls enzymes and fluorescent proteins

  • Create cardiolipin-binding protein domains linked to fluorescent markers

  • Enable real-time visualization of enzyme localization and lipid distribution

• Synthetic biology approaches:

  • Design orthogonal cardiolipin synthesis pathways with non-native substrates

  • Create synthetic regulatory circuits controlling membrane composition

  • Engineer membrane properties for biotechnology applications

• Heterologous expression systems:

  • Express recombinant cardiolipin synthases in different bacterial hosts

  • Study the impact of foreign cardiolipin synthesis on membrane properties

  • Develop platforms for studying membrane protein function in defined lipid environments

• Domain-swapping experiments:

  • Create chimeric enzymes between different Cls variants

  • Identify functional domains responsible for substrate specificity

  • Engineer enzymes with novel or enhanced activities for membrane engineering

• Site-specific incorporation of modified lipids:

  • Engineer Cls enzymes to incorporate modified or labeled lipid precursors

  • Enable tracking of cardiolipin dynamics with minimal perturbation

  • Create tools for studying membrane domain formation and function