Recombinant Pseudotsuga menziesii Probable serine acetyltransferase

What is serine acetyltransferase and what reaction does it catalyze in plant metabolism?

Serine acetyltransferase (SAT or SERAT, EC 2.3.1.30) is the first enzyme involved in the two-step enzymatic pathway of L-cysteine biosynthesis in bacteria and plants, though notably not in humans. This enzyme catalyzes the critical reaction in which the acetyl group from acetyl-CoA is transferred to L-serine, resulting in the formation of O-acetyl-L-serine (OAS) and coenzyme A (CoA) . This reaction represents the rate-limiting step in cysteine biosynthesis, making SERAT enzymes crucial control points in sulfur assimilation pathways in plants . The reaction specifically involves:

L-serine + Acetyl-CoA → O-acetyl-L-serine + CoA

The functional importance of these enzymes has been confirmed through complementation studies, where plant SERAT genes successfully restored growth in E. coli cysteine-auxotrophic mutants lacking endogenous SAT activity .

What are the optimal storage conditions for recombinant Pseudotsuga menziesii serine acetyltransferase?

For optimal preservation of enzymatic activity, recombinant Pseudotsuga menziesii probable serine acetyltransferase should be stored at -20°C for regular usage, with extended storage recommended at either -20°C or -80°C . The manufacturer specifically advises against repeated freezing and thawing cycles, which can lead to protein denaturation and activity loss. For working solutions, aliquots can be maintained at 4°C for up to one week .

Prior to opening, the manufacturer recommends briefly centrifuging the vial to ensure all contents are collected at the bottom. For reconstitution, the protein should be dissolved in deionized sterile water to a concentration range of 0.1-1.0 mg/mL. To extend shelf life, adding glycerol to a final concentration of 5-50% is recommended before making aliquots for long-term storage .

How is serine acetyltransferase activity measured in laboratory settings?

Serine acetyltransferase activity can be measured through several established methodologies, each with specific advantages depending on research objectives:

For kinetic parameter determination, researchers typically measure initial reaction velocities across various substrate concentrations. For example, in M. tuberculosis CysE, the Michaelis constants for acetyl-CoA and L-serine were determined to be 0.0513±0.0050 mM and 0.0264±0.0006 mM respectively, with a maximum velocity (Vmax) of 0.0073±0.0005 mM/min .

What is the protein structure and sequence information for Pseudotsuga menziesii serine acetyltransferase?

The recombinant Pseudotsuga menziesii probable serine acetyltransferase is cataloged with UniProt accession number P85931 . The expression region used for the recombinant protein covers amino acids 1-14 of the native sequence, with the peptide sequence "IANVFAVDIH PAAR" . This represents a fragment of the full-length protein.

The recombinant protein exhibits high purity (>85% by SDS-PAGE) . While the specific tag information is determined during the manufacturing process and may vary between production batches, the protein is expressed in a mammalian cell system, which can provide mammalian-type post-translational modifications if present in the native protein .

How do different isoforms of plant serine acetyltransferases differ in their biochemical properties?

Plant serine acetyltransferase isoforms display significant diversity in biochemical properties, subcellular localization, and regulatory mechanisms:

Table 1: Comparative Properties of SERAT Isoforms in Plants

The cytosolic SERAT3 family shows particularly interesting characteristics. In Arabidopsis, Serat3;1 is notably resistant to feedback inhibition by L-cysteine, while Serat3;2 is strongly feedback-inhibited . The kinetic parameters also vary significantly between isoforms - SERAT3 enzymes typically showing much higher Km values for both L-serine and acetyl-CoA compared to other isoforms, indicating lower catalytic efficiencies for these substrates .

In tomato (Solanum lycopersicum), despite containing a long C-terminus, SlSERAT3;1 demonstrates a high ability to catalyze OAS formation. An interesting structural feature is the retention of the essential C-terminal isoleucine residue, which appears to be characteristic of SERAT3 subfamily members specifically in Solanaceae .

What expression systems and purification strategies are most effective for recombinant serine acetyltransferase?

Based on the research literature, several expression and purification approaches have proven effective for recombinant serine acetyltransferases:

Bacterial Expression Systems:

• E. coli BL21(DE3): Successfully used for the expression of M. tuberculosis CysE (Rv2335) . This strain is deficient in lon and ompT proteases, reducing degradation of the recombinant protein.

• Complementation-based expression: Functional expression can be verified using E. coli cysteine-auxotrophic mutants (like JM39/5) that lack endogenous SAT activity. Successful complementation, allowing growth on minimal medium without cysteine supplementation, confirms the functionality of the expressed enzyme .

Expression Vectors and Tags:

• pTV118N: Used effectively for SERAT expression under lacZ promoter control .

• Maltose-binding protein fusion: The pMAL system has been successfully employed for SERAT expression, with the added advantage that the maltose-binding protein tag can be cleaved using factor Xa to obtain the native protein .

• His-tag purification: Ni²⁺ affinity chromatography has proven effective for the purification of His-tagged SERAT proteins .

Purification Challenges:
A common challenge in recombinant protein expression is contamination with E. coli chaperone proteins such as DnaK and GroEL, which may persist even after affinity chromatography. Additionally, truncated versions of the target protein may be present in samples . These issues might necessitate additional purification steps, such as size exclusion chromatography or ion exchange chromatography.

For Pseudotsuga menziesii serine acetyltransferase specifically, expression in mammalian cell systems has been successfully implemented , though the detailed methodology is not described in the provided search results.

How does serine acetyltransferase function within the cysteine biosynthesis pathway under stress conditions?

Serine acetyltransferase plays a critical regulatory role in plant responses to various environmental stresses through modulation of cysteine biosynthesis:

Sulfur Deficiency Response:
Under sulfur limitation, plants often upregulate specific SERAT isoforms to maintain cysteine synthesis. In Arabidopsis, SERAT3;2 expression is significantly induced under sulfur deficiency . This selective upregulation suggests specialized roles for certain isoforms when plants are subjected to nutritional stress.

Heavy Metal Stress Response:
Cadmium exposure triggers increased expression of SERAT genes, particularly SERAT3;2 in Arabidopsis . This response likely supports increased synthesis of glutathione and phytochelatins, sulfur-containing compounds that are essential for heavy metal detoxification.

Developmental Stage-Specific Regulation:
Expression patterns of SERAT isoforms change during plant development, with SERAT3;2 showing significantly increased expression during generative developmental stages . This indicates developmental stage-specific regulation of cysteine biosynthesis.

Osmotic and Ionic Stress Responses:
In tomato, chloroplast-localized SERAT isoforms appear to be the primary responders to abiotic stresses, with different SERAT genes employing distinct strategies to cope with osmotic and ion toxicity stresses .

These diverse responses highlight the sophisticated regulatory mechanisms governing SERAT expression and activity in plants, allowing them to adjust cysteine and downstream metabolite production according to specific environmental challenges.

What are the potential applications of serine acetyltransferase in antimicrobial drug discovery?

Serine acetyltransferase represents a promising target for antimicrobial drug discovery because it catalyzes an essential step in cysteine biosynthesis in bacteria and plants, but not in humans . This pathway divergence creates an opportunity for selective inhibition:

Targeting Mycobacterium tuberculosis:
M. tuberculosis CysE (Rv2335) has been characterized biochemically, with established assays for inhibitor screening. The optimal conditions for enzyme activity (pH 7.5, 37°C) and kinetic parameters (Km for acetyl-CoA: 0.0513±0.0050 mM; Km for L-Ser: 0.0264±0.0006 mM) provide a foundation for rational inhibitor design .

Targeting Salmonella:
Researchers have conducted virtual screening campaigns to identify inhibitors of Salmonella serine acetyltransferase (StSAT) . The high sequence identity between the Salmonella and E. coli enzymes (particularly in active site residues) allows for cross-species inhibitor development strategies.

Assay Development for High-throughput Screening:
The DTNB-based colorimetric assay has been optimized for screening compound libraries, overcoming the limitations of the traditional spectrophotometric method that measures thioester bond hydrolysis at 232 nm .

Therapeutic Potential:
Inhibition of serine acetyltransferase could disrupt cysteine biosynthesis, potentially affecting:

• DNA synthesis

• Serine and methionine formation

• Cell wall integrity

• Bacterial survival under oxidative stress conditions

These characteristics make serine acetyltransferase inhibitors promising candidates for novel antimicrobial agents that may help address the growing challenge of antimicrobial resistance.

What techniques are available for studying the structure-function relationships of serine acetyltransferase?

Several complementary approaches can be employed to investigate structure-function relationships in serine acetyltransferase:

Site-Directed Mutagenesis:
Targeted modification of specific amino acid residues can help identify catalytically important sites. For example, studies on the C-terminal region of SERAT3 family members have revealed the importance of the C-terminal isoleucine for enzymatic activity . Site-directed mutagenesis could be used to:

• Alter putative active site residues

• Modify substrate binding sites

• Investigate the basis of feedback inhibition by cysteine

• Examine the importance of protein-protein interaction domains

X-ray Crystallography and Structural Analysis:
While the crystal structure of Pseudotsuga menziesii SERAT is not reported in the provided search results, structural studies of related enzymes can provide valuable insights. This approach would allow visualization of:

• Substrate binding pockets

• Protein-protein interaction interfaces

• Conformational changes upon substrate binding

• Structural basis for feedback inhibition

Protein-Protein Interaction Studies:
In plants, SERAT interacts with OAS-(thiol)-lyase to form the cysteine synthase complex. This interaction is functionally important, as demonstrated by the observation that SlSERAT1;1 and SlSERAT2;2 catalytic abilities are significantly improved by the addition of an OAS-(thiol)-lyase protein . Techniques to study these interactions include:

• Pull-down assays

• Yeast two-hybrid screens

• Surface plasmon resonance

• Isothermal titration calorimetry

Kinetic Analysis and Inhibitor Studies:
Detailed kinetic characterization, including determination of Km and Vmax values for substrates under various conditions, can provide insights into the catalytic mechanism. For Arabidopsis SERAT3 isoforms, HPLC-based detection of OAS formation revealed much higher Km values compared to other isoforms , indicating different catalytic efficiencies.

How can researchers optimize recombinant protein reconstitution for functional studies?

Optimizing recombinant protein reconstitution is critical for maintaining enzymatic activity in functional studies. For Pseudotsuga menziesii serine acetyltransferase, the following protocol is recommended:

• Initial Preparation:
  Begin with brief centrifugation of the protein vial prior to opening to ensure all contents settle at the bottom .

• Reconstitution Medium:
  Use deionized sterile water for initial reconstitution, targeting a protein concentration of 0.1-1.0 mg/mL .

• Stabilizing Additives:
  Add glycerol to a final concentration of 5-50% (50% being commonly used) to enhance protein stability during storage .

• Aliquoting Strategy:
  Prepare small working aliquots to avoid repeated freeze-thaw cycles, which can significantly reduce enzymatic activity .

• Storage Temperature Hierarchy:

  • Short-term working solutions: Store at 4°C for maximum of one week

  • Medium-term storage: Use -20°C

  • Long-term archival storage: Use -80°C

• Handling Inclusion Bodies:
  When recombinant proteins form inclusion bodies (as often observed with heterologous expression), refolding protocols using detergents or KCl-based methods can be employed to recover active protein. In studies with Plasmodium falciparum proteins expressed in E. coli, both detergent and KCl refolding methods have been tested .

• Activity Assessment:
  Following reconstitution, it's essential to verify enzyme activity using established assays such as the DTNB-based colorimetric method or direct measurement of OAS formation via HPLC .

• Consideration of Salt and Buffer Conditions:
  The choice of buffer system and salt concentration can significantly impact enzyme stability and activity. For M. tuberculosis CysE, optimal activity was observed at pH 7.5 , suggesting that phosphate or HEPES buffer systems at physiological pH may be suitable for many serine acetyltransferases.

How can researchers address issues with protein stability and aggregation during serine acetyltransferase studies?

Protein stability and aggregation challenges are common when working with recombinant enzymes like serine acetyltransferase. Several strategies can mitigate these issues:

Preventing Aggregation During Storage:

• Add glycerol to a final concentration of 5-50% before freezing aliquots .

• Avoid repeated freeze-thaw cycles by preparing single-use aliquots .

• Consider the addition of reducing agents like DTT or β-mercaptoethanol if the protein contains critical cysteine residues.

• Store protein solutions at appropriate pH (typically near physiological pH for most SATases).

Addressing Expression-Related Aggregation:

• Co-expression with chaperones: When expressing in E. coli, co-expression with chaperone proteins like GroEL/GroES can improve folding.

• Expression temperature optimization: Lower induction temperatures (16-25°C) often reduce inclusion body formation.

• Solubility tags: Fusion with solubility-enhancing tags such as MBP (maltose-binding protein), as used with Arabidopsis SERAT3 proteins , can improve soluble expression.

Recovering Activity from Aggregated Protein:
For proteins expressed as inclusion bodies, refolding protocols can be employed:

• Detergent-based refolding: Using mild detergents to solubilize and gradually remove denaturants.

• KCl refolding method: As described for other recombinant proteins expressed in E. coli .

• Dialysis-based refolding: Gradual removal of denaturants through serial dialysis.

Analytical Approaches for Monitoring Aggregation:

• Native-PAGE: To assess the oligomeric state and presence of high-molecular-weight aggregates.

• Size exclusion chromatography: For quantitative assessment of aggregate content.

• Dynamic light scattering: To monitor particle size distribution in solution.

What strategies can resolve expression challenges for recombinant Pseudotsuga menziesii serine acetyltransferase?

Expression of recombinant proteins from conifers like Pseudotsuga menziesii can present unique challenges. Based on the search results and general recombinant protein expression principles, the following strategies may help overcome these challenges:

Codon Optimization Approaches:
While codon harmonization did not improve expression levels or purity for some Plasmodium falciparum proteins , codon optimization remains an important strategy for plant proteins expressed in heterologous systems. This can involve:

• Adapting the codon usage to match the expression host

• Removing rare codons that might cause ribosomal pausing

• Optimizing GC content and avoiding secondary structures in mRNA

Host System Selection:

• Mammalian cell expression: As used for the commercial Pseudotsuga menziesii serine acetyltransferase , which can provide appropriate folding environment and post-translational modifications.

• E. coli expression: Different strains can be tested, such as BL21(DE3), BL21(DE3)pLysS, or Rosetta strains that supply tRNAs for rare codons.

• Alternative hosts: Consider yeast (Pichia pastoris) or insect cell (baculovirus) expression systems for difficult-to-express plant proteins.

Expression Construct Design:

• Fusion tags: Strategic use of solubility-enhancing tags such as MBP, SUMO, or TrxA.

• Removal of problematic sequences: Consider removing signal peptides or transmembrane domains if not essential for activity.

• Expression of functional domains: If full-length expression proves challenging, expressing the catalytic domain alone might be feasible.

Induction and Growth Conditions:

• Temperature optimization: Testing expression at 18°C, 25°C, and 37°C can significantly impact soluble protein yield.

• Inducer concentration: Titrating IPTG concentration (for lac-based systems) to find optimal expression conditions.

• Media composition: Rich media (like TB or 2xYT) often provide better yields than standard LB.

• Growth phase at induction: Inducing at different optical densities can affect soluble protein yield.

Contamination Management:
As observed with other recombinantly expressed proteins, contamination with E. coli chaperone proteins like DnaK and GroEL can persist even after affinity chromatography . Additional purification steps such as ion exchange or size exclusion chromatography may be necessary to obtain highly pure protein.

What emerging research questions surround serine acetyltransferase in conifer metabolism and stress response?

Several promising research directions emerge from current understanding of serine acetyltransferase in plant systems, particularly regarding conifer-specific aspects:

Conifer-Specific Metabolic Adaptations:

• How does serine acetyltransferase function in the unique sulfur metabolism of conifers like Pseudotsuga menziesii, particularly in relation to specialized metabolites like terpenoids and phenolics?

• What role does SERAT play in the extraordinary environmental stress tolerance exhibited by conifers, including drought, cold, and pathogen resistance?

• How do serine acetyltransferase isoforms differ between angiosperms and gymnosperms in terms of regulation, localization, and biochemical properties?

Climate Change Response Mechanisms:

• How does serine acetyltransferase activity in conifers respond to elevated CO2 levels and increasing global temperatures?

• What is the role of cysteine biosynthesis in conifer adaptation to extreme weather events like prolonged drought or unseasonable frost?

• How might genetic variation in SERAT genes contribute to adaptation potential in conifer populations facing changing climatic conditions?

Developmental Regulation:

• How does serine acetyltransferase expression change throughout the extremely long lifespan of conifers?

• What is the role of SERAT in conifer reproductive development, particularly in cone formation and seed maturation?

• How is SERAT activity regulated during seasonal dormancy cycles unique to temperate conifers?

Interspecies Comparative Studies:

• How do SERAT isoforms compare between Pseudotsuga menziesii and other economically important conifers like Pinus and Picea species?

• What convergent or divergent evolutionary patterns emerge when comparing conifer SERATases with those of angiosperms?

• How might differences in SERAT regulation contribute to the distinct ecological niches occupied by different conifer species?

How might multi-omics approaches advance our understanding of serine acetyltransferase function?

Integrative multi-omics approaches offer powerful tools to comprehensively understand serine acetyltransferase function within the broader metabolic network:

Genomics and Comparative Genomics:

• Whole genome sequencing and annotation can identify the complete SERAT gene family in Pseudotsuga menziesii and related conifers.

• Comparative genomics across plant species can reveal evolutionary patterns and selection pressures on SERAT genes.

• Analysis of genomic variation within populations can identify naturally occurring variants with potentially altered activity or regulation.

Transcriptomics:

• RNA-seq analysis under various environmental conditions can reveal differential expression patterns of SERAT isoforms, similar to the tissue-specific and stress-responsive expression observed in Arabidopsis .

• Single-cell transcriptomics can provide unprecedented resolution of SERAT expression at the cellular level within complex conifer tissues.

• Time-course expression studies can capture the dynamic regulation of SERAT genes during development and stress responses.

Proteomics:

• Quantitative proteomics can verify whether transcript-level changes translate to protein abundance changes.

• Post-translational modification analysis can identify regulatory modifications affecting SERAT activity.

• Protein-protein interaction studies using co-immunoprecipitation coupled with mass spectrometry can identify novel interaction partners beyond the known association with OAS-(thiol)-lyase.

Metabolomics:

• Targeted metabolomics focusing on sulfur-containing compounds can directly assess the impact of SERAT activity on downstream metabolism.

• Untargeted approaches may reveal unexpected connections between cysteine biosynthesis and other metabolic pathways.

• Stable isotope labeling experiments can trace the flux through the cysteine biosynthetic pathway under various conditions.

Integrative Analysis:

• Network analysis integrating transcriptomics, proteomics, and metabolomics data can position SERAT within the broader context of conifer metabolism.

• Machine learning approaches applied to multi-omics datasets can identify previously unrecognized patterns of regulation and metabolic connections.

• Genome-scale metabolic models incorporating SERAT can predict the system-wide effects of altered enzyme activity or expression.

These multi-omics approaches would provide a comprehensive understanding of SERAT function beyond what conventional biochemical approaches alone can achieve, potentially revealing novel applications in forestry, conservation, and biotechnology.