Recombinant Staphylococcus aureus Cardiolipin synthase 2 (cls2)

What is cardiolipin synthase 2 (cls2) in Staphylococcus aureus?

Cardiolipin synthase 2 (cls2) is the primary enzyme responsible for cardiolipin synthesis in Staphylococcus aureus, encoded by the gene SA1891 . It functions by converting two phosphatidylglycerol (PG) molecules into cardiolipin and glycerol . S. aureus possesses two cardiolipin synthase genes, cls1 (SA1155) and cls2 (SA1891), but cls2 is the dominant synthase under standard growth conditions . Biochemical studies have confirmed that cls2 is responsible for the majority of cardiolipin production during normal bacterial growth, particularly in stationary phase where cardiolipin becomes the major phospholipid in the bacterial membrane .

How does cardiolipin production via cls2 vary throughout S. aureus growth phases?

Cardiolipin production in S. aureus shows distinct patterns across growth phases. In actively growing cells (exponential phase), phosphatidylglycerol (PG) is the predominant phospholipid in the membrane . As cells transition to stationary phase, cardiolipin becomes the major phospholipid, with levels increasing significantly . This transition occurs primarily through cls2 activity, which converts two PG molecules into cardiolipin and glycerol . Research shows that cls2 is constitutively expressed and functions as the primary cardiolipin synthase throughout normal growth conditions, with its activity becoming particularly important during stationary phase . The accumulation of cardiolipin in stationary phase suggests its role in adaptation to nutrient limitation and other stresses encountered during this growth phase.

What is the difference between cls1 and cls2 in S. aureus?

While S. aureus possesses two cardiolipin synthase genes (cls1 and cls2), they differ significantly in their expression patterns and functional roles:

Studies have demonstrated that deletion of cls1 does not noticeably affect cardiolipin levels under normal conditions, whereas deletion of cls2 significantly reduces cardiolipin production . Interestingly, in a cls2 mutant, cls1 can be activated under stress conditions such as high salinity, low pH, high temperature, or anaerobic environments, suggesting a backup role for cls1 during environmental stress .

How does cardiolipin produced by cls2 regulate the SaeRS two-component system in S. aureus?

Cardiolipin produced by cls2 has been demonstrated to play a critical role in regulating the SaeRS two-component system (TCS), which controls virulence factor expression in S. aureus. The mechanism involves direct interaction between cardiolipin and the SaeS sensor histidine kinase . Research has shown that deletion of cls2 significantly reduces SaeRS activity, as measured by the expression of SaeR-regulated genes . Specifically, elimination of cardiolipin from the membrane results in decreased kinase activity of the SaeS protein, which in turn reduces phosphorylation of the response regulator SaeR and subsequent activation of virulence genes .

The direct binding of SaeS to cardiolipin has been confirmed through protein-lipid binding assays using purified maltose-binding protein (MBP)-SaeS fusion proteins . This interaction is specific, as SaeS binds directly to both cardiolipin and phosphatidylglycerol, whereas control proteins do not show this binding capacity . The association of cardiolipin with SaeS appears to enhance its kinase activity, demonstrating how membrane phospholipid composition directly influences bacterial virulence regulation .

What other bacterial two-component systems are affected by cardiolipin levels controlled by cls2?

Beyond the SaeRS system, research has identified several other two-component systems (TCSs) in S. aureus that are influenced by cardiolipin levels regulated by cls2. Analysis of transcription patterns has revealed that at least eight of the sixteen TCSs in S. aureus are affected by cardiolipin content . These TCSs can be categorized into two groups based on their response to cls mutations:

The remaining TCSs (AgrC, AirS, HssS, KdpD, NreB, PhoR, SsrB, and WalR) were not significantly affected by cls mutations . This differential impact suggests that cardiolipin may interact with or influence various sensor kinases through distinct mechanisms. Most sensor kinases in S. aureus are membrane proteins with at least two transmembrane domains, making them potentially responsive to changes in membrane phospholipid composition . This broad influence of cardiolipin on multiple regulatory systems highlights its importance in coordinating bacterial responses to environmental changes.

What is the role of cls2-produced cardiolipin in S. aureus virulence and host-pathogen interactions?

Cardiolipin produced by cls2 plays a significant role in S. aureus virulence and host-pathogen interactions through multiple mechanisms. Studies have demonstrated that strains lacking cls2 or both cls1 and cls2 exhibit reduced cytotoxicity to human neutrophils and decreased virulence in mouse infection models . This attenuation in virulence can be attributed to several factors:

• Reduced activation of the SaeRS two-component system, which controls the expression of numerous virulence factors

• Impaired activity of at least seven other two-component systems that regulate various aspects of bacterial physiology and virulence

• Decreased survival under stress conditions encountered during infection, including high salinity and interaction with host defense peptides

What are the optimal conditions for expressing recombinant S. aureus cls2 in heterologous systems?

For optimal expression of recombinant S. aureus cls2 in heterologous systems, researchers should consider several key factors based on the protein's membrane-associated nature and enzymatic function:

• Expression System Selection: E. coli BL21(DE3) strains typically yield good expression of S. aureus membrane proteins. For higher eukaryotic systems, consider using Pichia pastoris for improved protein folding .

• Vector Design: The cls2 gene (SA1891) should be cloned with its native ribosome binding site for optimal expression. Including affinity tags (His6 or MBP) facilitates purification, with N-terminal tags generally preserving function better than C-terminal tags .

• Induction Parameters: For E. coli systems, IPTG induction at concentrations of 0.1-0.5 mM when cultures reach OD600 of 0.6-0.8, followed by incubation at lower temperatures (16-25°C) for 16-20 hours, typically yields functional enzyme .

• Membrane Fraction Isolation: Since cls2 is a membrane-associated enzyme, proper isolation of membrane fractions is critical for activity assessment. Differential centrifugation protocols with initial low-speed centrifugation (8,000 × g) followed by high-speed ultracentrifugation (100,000 × g) effectively isolate membrane fractions containing the enzyme .

The functionality of recombinant cls2 can be verified through cardiolipin synthase activity assays using fluorescently labeled phosphatidylglycerol substrates and thin-layer chromatography for product analysis .

How can cardiolipin synthase activity be accurately measured in recombinant cls2 preparations?

Accurate measurement of cardiolipin synthase activity in recombinant cls2 preparations can be achieved through several complementary approaches:

• In vitro Enzymatic Assays: The most direct method involves measuring the conversion of phosphatidylglycerol (PG) to cardiolipin. This can be achieved by incubating purified recombinant cls2 or membrane fractions containing the enzyme with radiolabeled or fluorescently labeled PG substrates . The reaction products can then be separated by thin-layer chromatography and quantified by densitometry or scintillation counting.

• SaeS Phosphorylation Assays: Since cls2-produced cardiolipin directly affects SaeS kinase activity, SaeS autophosphorylation assays can serve as an indirect measure of functional cls2 activity . This involves:

  • Isolation of membrane fractions from wild-type or mutant S. aureus strains

  • Addition of recombinant cls2 or purified cardiolipin

  • Measurement of SaeS autophosphorylation using [γ-32P]ATP

  • Quantification of phosphorylated SaeS by autoradiography or phosphorimaging

• Reporter Gene Assays: The activity of recombinant cls2 can be assessed in vivo using reporter gene constructs such as the P1-gfp reporter plasmid (pYJ-saeP1-gfp) that measures SaeRS activity . Complementation of cls2-deficient strains with recombinant cls2 should restore reporter gene expression if the recombinant enzyme is functional.

For all methods, positive controls (native membrane preparations) and negative controls (heat-inactivated enzyme preparations) should be included to validate assay specificity .

What purification strategies yield the highest activity for recombinant cls2?

Purification of recombinant cls2 with high enzymatic activity requires specialized approaches due to its membrane-associated nature:

• Detergent Solubilization: The critical first step involves careful selection of detergents. Mild non-ionic detergents such as n-dodecyl-β-D-maltoside (DDM) at 1-2% have shown effectiveness in solubilizing membrane proteins while preserving activity . A sequential solubilization approach starting with lower detergent concentrations can help separate cls2 from other membrane components.

• Affinity Chromatography: For His-tagged constructs, immobilized metal affinity chromatography (IMAC) using Ni-NTA resin with detergent-containing buffers (typically 0.05-0.1% DDM) provides good initial purification . For MBP-fusion constructs, amylose resin can be used for affinity purification.

• Size Exclusion Chromatography: Further purification by size exclusion chromatography helps remove protein aggregates and provides information about the oligomeric state of cls2 . Running buffers should contain detergent at concentrations above the critical micelle concentration to maintain protein solubility.

• Lipid Incorporation: Addition of phospholipids, particularly phosphatidylglycerol, during purification can stabilize cls2 and preserve enzymatic activity . Typically, a phospholipid:protein ratio of 10:1 to 100:1 (w/w) is effective.

• Activity Preservation: Throughout purification, maintaining low temperatures (4°C), including protease inhibitors, and minimizing exposure to air can help preserve enzyme activity. Additionally, inclusion of 10-15% glycerol in buffers helps stabilize the protein .

The purified cls2 should be assessed for activity using the assays described in section 3.2, with specific activity measurements (activity per mg protein) at each purification step to track purification efficiency and activity recovery .

How should experiments be designed to study the role of cls2 in S. aureus under different stress conditions?

When designing experiments to study cls2 function under different stress conditions, researchers should consider a comprehensive approach that integrates genetic, biochemical, and physiological analyses:

• Genetic System Setup:

  • Create a panel of strains: wild-type, Δcls1, Δcls2, and Δcls1Δcls2 double mutant

  • Develop complementation strains with cls2 under native or inducible promoters

  • Include reporter constructs (e.g., pYJ-saeP1-gfp) to monitor downstream effects on TCS activity

• Stress Condition Parameters:

  • High salinity: 0.5-2.5 M NaCl concentrations (simulate wound environments)

  • pH stress: pH range 4.5-8.0 (relevant to different infection sites)

  • Temperature stress: 30-45°C (normal growth to fever temperatures)

  • Antimicrobial peptides: 0-20 μg/ml HNP1 or other relevant peptides

  • Oxygen limitation: aerobic vs. anaerobic growth conditions

• Key Measurements:

  • Membrane phospholipid composition via thin-layer chromatography

  • Growth kinetics under each stress condition

  • Survival rates after prolonged exposure to stress

  • Expression of cls1 and cls2 using qRT-PCR

  • Activity of SaeRS and other TCSs using reporter constructs

  • Virulence factor production

• Time-course Analysis:

  • Short-term adaptive responses (minutes to hours)

  • Long-term survival effects (hours to days)

  • Growth phase-dependent effects (exponential vs. stationary)

A particularly informative experimental design would be to conduct parallel analyses of all strains across multiple stress conditions, followed by detailed transcriptomic and lipidomic analyses to identify condition-specific regulatory networks and membrane composition changes .

What controls and variables should be considered when investigating cls2-dependent regulation of two-component systems?

When investigating cls2-dependent regulation of two-component systems (TCSs), several critical controls and variables must be considered to ensure robust and interpretable results:

• Genetic Controls:

  • Wild-type strain (positive control)

  • Δcls2 mutant (primary experimental strain)

  • Δcls1 mutant (control for cls2 specificity)

  • Δcls1Δcls2 double mutant (complete CL-deficient control)

  • Complementation strains (pOS1 vector with cls2 under native promoter)

  • TCS deletion mutants (e.g., ΔsaeRS) to confirm specificity of observed effects

• Growth Phase Variables:

  • Exponential phase (4h growth) - when PG is the predominant phospholipid

  • Stationary phase (24h growth) - when CL becomes the major phospholipid

  • This is particularly important as the requirement for cardiolipin in TCS activation varies by growth phase

• Environmental Variables:

  • Media composition (nutrient-rich vs. nutrient-limited)

  • Temperature (standard growth vs. heat stress)

  • pH (neutral vs. acidic conditions)

  • Osmolarity (standard vs. high-salt conditions)

  • Presence/absence of host factors (e.g., HNP1 at 5 μg/ml)

• Measurement Parameters:

  • TCS gene expression (qRT-PCR for all 16 sensor kinases)

  • TCS protein activity (phosphorylation assays)

  • TCS target gene expression (reporter constructs or qRT-PCR)

  • Membrane phospholipid composition (to correlate with TCS activity)

  • Protein-lipid binding (for direct interaction assessment)

The research by Yeo et al. demonstrated that measurements should include both basal activity and induced activity (e.g., with HNP1 stimulation) of TCSs to fully understand the role of cls2-produced cardiolipin in different contexts .

How can researchers effectively investigate the interaction between recombinant cls2, cardiolipin, and membrane proteins like SaeS?

Investigating the interactions between recombinant cls2, cardiolipin, and membrane proteins like SaeS requires specialized approaches that address the complexities of membrane protein interactions:

• Protein-Lipid Overlay Assays:

  • Immobilize purified lipids (CL, PG, PE, etc.) on nitrocellulose membranes

  • Incubate with purified membrane proteins (e.g., MBP-SaeS)

  • Detect binding using antibodies against the protein or fusion tag

  • This approach demonstrated direct binding of SaeS to cardiolipin and phosphatidylglycerol

• Reconstitution Systems:

  • Prepare liposomes with defined lipid compositions (varying CL percentages)

  • Incorporate purified membrane proteins (SaeS or other sensor kinases)

  • Measure protein activity (kinase activity for SaeS) in these controlled environments

  • Compare activity across different lipid compositions to determine optimal CL requirements

• Fluorescence-Based Interaction Studies:

  • Label recombinant cls2 and SaeS with different fluorophores

  • Utilize Förster resonance energy transfer (FRET) to detect protein proximity in membranes

  • Monitor changes in FRET efficiency upon cardiolipin production to assess dynamic interactions

• Surface Plasmon Resonance (SPR) Analysis:

  • Immobilize one protein component on a sensor chip

  • Flow the other protein or lipid component over the surface

  • Measure binding kinetics and affinity constants

  • This approach can determine the strength and specificity of interactions

• In vivo Crosslinking:

  • Utilize photo-activatable or chemical crosslinkers in living bacteria

  • Identify protein-protein or protein-lipid complexes involving cls2, cardiolipin, and membrane proteins

  • Analyze crosslinked products by mass spectrometry to identify interaction partners

For all these approaches, researchers should include appropriate controls (heat-inactivated proteins, irrelevant lipids or proteins) and consider the effect of detergents, which may interfere with natural membrane protein-lipid interactions .

How should researchers interpret changes in cls2 expression and cardiolipin levels in relation to bacterial adaptation mechanisms?

When interpreting changes in cls2 expression and cardiolipin levels, researchers should consider several key perspectives to understand bacterial adaptation mechanisms comprehensively:

• Growth Phase Context:

  • The transition from exponential to stationary phase normally shows increased cardiolipin accumulation via cls2 activity

  • This represents a programmed adaptation to nutrient limitation rather than a stress response

  • Baseline cardiolipin levels should always be interpreted relative to growth phase

• Stress Response Framework:

  • Under stress conditions (high salinity, acidic pH, oxygen limitation), changes in cls1 and cls2 expression should be evaluated as part of the broader stress response

  • Compare with other stress response genes to determine if cls2 regulation is part of a coordinated adaptive program

  • In the cls2 mutant, stress conditions induce cls1 expression, suggesting a compensatory mechanism that should be considered when interpreting data

• Regulatory Network Integration:

  • Changes in cardiolipin levels affect multiple TCSs simultaneously

  • The differential impacts on various TCSs (e.g., complete dependence vs. partial dependence on cardiolipin) suggest that changes should be interpreted within a network context rather than as isolated events

  • The temporal sequence of TCS activation or inhibition following cardiolipin level changes provides insights into the hierarchy of bacterial adaptation mechanisms

• Physiological Outcome Correlation:

  • Changes in cls2 expression and cardiolipin levels should be correlated with functional outcomes:

    • Membrane permeability alterations

    • Resistance to antimicrobial compounds

    • Survival under stress conditions

    • Virulence capacity

  • This correlation helps distinguish between adaptive vs. incidental changes in cardiolipin metabolism

The research indicates that while cls2 is constitutively expressed under normal conditions, its role in bacterial adaptation becomes particularly significant under stress conditions and in stationary phase, where cardiolipin helps maintain membrane integrity and function .

What statistical approaches are most appropriate for analyzing the impact of cls2 deletion on virulence factor expression?

When analyzing the impact of cls2 deletion on virulence factor expression, several statistical approaches are recommended to ensure robust and meaningful interpretations:

• Multifactorial Analysis of Variance (ANOVA):

  • Appropriate for experimental designs comparing multiple strains (WT, Δcls1, Δcls2, Δcls1Δcls2) under various conditions

  • Allows assessment of interaction effects between genotype and environmental factors

  • Post-hoc tests (Tukey's HSD or Bonferroni) should be used to identify specific significant differences between groups

  • This approach was effectively used to analyze SaeRS activity across different cls mutants under various conditions

• Time-Series Analysis:

  • For growth-phase dependent expression studies

  • Repeated measures ANOVA or mixed-effects models account for correlation between measurements at different time points

  • This is particularly relevant as cls2's impact on SaeRS activity varies between exponential and stationary phases

• Correlation Analysis:

  • Pearson or Spearman correlation between cardiolipin levels and virulence factor expression

  • Multiple regression models to assess the relative contribution of different phospholipids to virulence expression

  • This approach helps identify whether the relationship between cardiolipin and virulence is linear, threshold-dependent, or follows another pattern

• Principal Component Analysis (PCA) or Hierarchical Clustering:

  • For analyzing transcriptomic data from virulence factor profiling

  • Helps identify patterns of co-regulated genes affected by cls2 deletion

  • Can reveal whether cls2 deletion affects specific virulence pathways or causes global changes

  • This approach is valuable for interpreting the broader impact of cls2 on the virulence regulon beyond SaeRS-controlled genes

• Biological Replication and Power Analysis:

  • At least three biological replicates should be used for all experiments

  • Power analysis should be conducted to determine adequate sample sizes

  • Non-parametric tests should be considered when data do not meet normality assumptions

  • For RNA-seq or other high-throughput data, appropriate corrections for multiple testing (e.g., Benjamini-Hochberg) must be applied

The statistical significance threshold should typically be set at p < 0.05, with presentation of exact p-values rather than threshold statements to allow readers to interpret the strength of evidence .

How can contradictory data regarding cls2 function be reconciled across different experimental systems?

Reconciling contradictory data regarding cls2 function across different experimental systems requires a systematic approach that considers multiple factors that could contribute to variability:

• Strain Background Effects:

  • S. aureus has significant strain-to-strain variation that can affect cls2 function

  • When contradictory results emerge, directly compare cls2 function in different genetic backgrounds (e.g., USA300, N315, 8325-4, SH1000) under identical conditions

  • The phospholipid synthesis profiles observed in cls mutants were consistent across N315, 8325-4, and SH1000 backgrounds, suggesting some aspects of cls2 function are conserved across strains

• Growth Condition Standardization:

  • Variations in media composition, oxygen availability, and temperature can significantly alter phospholipid metabolism

  • Contradictory results may be reconciled by carefully controlling:

    • Media composition (especially osmolarity and pH)

    • Growth phase sampling points

    • Temperature and aeration conditions

  • For instance, the requirement for cardiolipin in SaeRS activation is growth phase-dependent, which could explain contradictory findings if growth phases differ between studies

• Methodological Differences:

  • Different techniques for measuring cardiolipin (e.g., TLC vs. mass spectrometry)

  • Various approaches to measuring cls2 activity or expression

  • Different reporter systems for downstream effects

  • Create a comparative analysis framework that normalizes results across methodologies or directly compares methods within a single experimental system

• Contextual Integration:

  • Apparent contradictions may reflect context-dependent functions rather than truly contradictory data

  • For example, the dual role of cardiolipin in both exponential and stationary phases, but with different mechanisms and requirements

  • Construct comprehensive models that incorporate growth phase, stress conditions, and genetic background to resolve seemingly contradictory observations

• Meta-Analysis Approach:

  • When multiple studies present contradictory findings, a formal meta-analysis can help identify patterns

  • Weight studies by methodological rigor, sample size, and consistency of controls

  • Identify moderating variables that explain divergent results across studies

This approach has successfully reconciled apparently contradictory observations regarding cls2 function, such as its differential requirement for SaeRS activation under different growth conditions or in response to different stimuli .