Conservation between human and fungal squalene synthetases: similarities in structure, function, and regulation

Structural and Molecular Characterization of Squalene Synthase Belonging to the Marine Thraustochytrid Species Aurantiochytrium limacinum Using Bioinformatics Approach

The marine microorganisms thraustochytrids have been explored for their potential in the production of various bioactive compounds, such as DHA, carotenoids, and squalene. Squalene is a secondary metabolite of the triterpenoid class and is known for its importance in various industrial applications. The bioinformatic analysis for squalene synthase (SQS) gene (the first key enzyme in the tri-terpenoid synthesis pathway), that is prevailing among thraustochytrids, is poorly investigated. In-silico studies combining sequence alignments and bioinformatic tools helped in the preliminary characterization of squalene synthases found in Aurantiochytrium limacinum. The sequence contained highly conserved regions for SQS found among different species indicated the enzyme had all the regions for its functionality. The signal peptide sequence and transmembrane regions were absent, indicating an important aspect of the subcellular localization. Secondary and 3-D models generated using appropriate templates demonstrated the similarities with SQS of the other species. The 3-D model also provided important insights into possible active, binding, phosphorylation, and glycosylation sites.

Molecular cloning and functional characterization of two squalene synthase genes in Atractylodes lancea

Two squalene synthase genes AlSQS1 and AlSQS2 were isolated from Atractylodes lancea and functionally characterized using in vitro enzymatic reactions. Atractylodes lancea is a traditional herb used for the treatment of rheumatic diseases, gastric disorders, and influenza. Its major active ingredients include sesquiterpenoids and triterpenes. Squalene synthase (SQS; EC 2.5.1.21) catalyzes the first enzymatic step in the central isoprenoid pathway towards sterol and triterpenoid biosynthesis. In this study, we aimed to investigate two SQSs from A. lancea using cloning and in vitro enzymatic characterization. Bioinformatics and phylogenetic analyses revealed that the AlSQSs exhibited high homology with other plant SQSs. Furthermore, AlSQS1 was observed to be localized in both the nucleus and cytoplasm, whereas AlSQS2 was localized in the cytoplasm and endoplasmic reticulum. To obtain soluble recombinant enzymes, AlSQS1 and AlSQS2 were successfully expressed as glutathione S-transferase (GST)-tagged fusion proteins in Escherichia coli Transetta (DE3). Approximately 68 kDa recombinant proteins were obtained using GST-tag affinity chromatography and Western blot analysis. Results of the in vitro enzymatic reactions established that both AlSQS1 and AlSQS2 were functional, which verifies their catalytic ability in converting two farnesyl pyrophosphates to squalene. The expression patterns of AlSQS and selected terpenoid genes were also investigated in two A. lancea chemotypes using available RNA sequencing data. AlSQS1 and AlSQS2, which showed relatively similar expression in the three tissues, were more highly expressed in the stems than in the leaves and rhizomes. Methyl jasmonate (MeJA) was used as an elicitor to analyze the expression profiles of AlSQSs. The results of qRT-PCR analysis revealed that the gene expression of AlSQS1 and AlSQS2 plummeted at lowest value at 12 h and reached its peak at 24 h. This study is the first report on the cloning, characterization, and expression of SQSs in A. lancea. Therefore, our findings contribute novel insights that may be useful for future studies regarding terpenoid biosynthesis in A. lancea.

Recent advances in topical carriers of anti-fungal agents

Fungal skin infections are the most common global issue for skin health. Fungal infections are often treated by topical or systemic anti-fungal therapy. Topical fungal therapy is usually preferred because of their targeted therapy and fewer side effects. Advanced topical carriers because of their distinct structural and functional features, overcome biopharmaceutical challenges associated with conventional drug delivery systems like poor retention and low bioavailability. Literature evidence indicated topical nanocarriers loaded with anti-fungal agents display superior therapeutic response with minimum toxicity. Nanocarriers often used for topical anti-fungal medication includes Solid-Lipid nanoparticles, Microemulsions, Liposomes, Niosomes, Microsponge, Nanogel, Nanoemulsion, Micelles etc. This review summarizes recent advances in novel strategies employed in topical carriers to improve the therapeutic performance of anti-fungal drugs.

Molecular Cloning and Functional Identification of a Squalene Synthase Encoding Gene from Alfalfa (Medicago sativa L.)

The quality of alfalfa, a main forage legume worldwide, is of great importance for the dairy industry and is affected by the content of triterpene saponins. These natural terpenoid products of triterpene aglycones are catalyzed by squalene synthase (SQS), a highly conserved enzyme present in eukaryotes. However, there is scare information on alfalfa SQS. Here, an open reading frame (ORF) of SQS was cloned from alfalfa. Sequence analysis showed MsSQS had the same exon/intron composition and shared high homology with its orthologs. Bioinformatic analysis revealed the deduced MsSQS had two transmembrane domains. When transiently expressed, GFP-MsSQS fusion protein was localized on the plasma membrane of onion epidermal cells. Removal of the C-terminal transmembrane domain of MsSQS improved solubility in Escherichia coli. MsSQS was preferably expressed in roots, followed by leaves and stems. MeJA treatment induced MsSQS expression and increased the content of total saponins. Overexpression of MsSQS in alfalfa led to the accumulation of total saponins, suggesting a correlation between MsSQS expression level with saponins content. Therefore, MsSQS is a canonical squalene synthase and contributes to saponin synthesis in alfalfa. This study provides a key candidate gene for genetic manipulation of the synthesis of triterpene saponins, which impact both plant and animal health.

Cloning and functional analysis of squalene synthase gene from Dryopteris fragrans (L.) Schott

Dryopteris fragrans (L.) Schott is a traditional herbal medicine containing medicinal sterols and triterpenoids. Squalene synthase (SQS) is the first crucial enzyme in the biosynthesis pathway of sterols and triterpenoids. The full-length cDNA named DfSQS1 was isolated by RACE. It was predicted that DfSQS1 contained an open reading frame (ORF) of 1239 bp coding 412 amino acid residues with molecular weight of 46.6 kDa. It had 18 potential phosphorylation sites, 1 potential N-glycosylation site and 2 transmembrane domains. In neighbor-joining (NJ) phylogenetic tree, DfSQS1 was away from branch of gymnosperms and angiosperms. One hydrophobic domain at the C-terminal of DfSQS1 was deleted to express soluble recombinant enzyme. The truncated DfSQS1 (tDfSQS1) was expressed in Escherichia coli BL21 (DE3). Then, tDfSQS1 was obtained and incubated with farnesyl diphosphate (FPP) to identify its enzymatic activity. The result demontrated that squalene, the product of enzyme catalyzed reaction, was detected by HPLC. Quantitative real-time PCR (qRT-PCR) analysis revealed that the transcription level of DfSQS1 in D. fragrans was the highest in roots, followed by leaves and rhizomes. This work is the first report on cloning, characteration and expression of SQS from D. fragrans. It will be helpful to understand the regulatory role of SQS on the biosynthesis of triterpenoids in the fern.

Enhanced expression of ginsenoside biosynthetic genes and in vitro ginsenoside production in elicited Panax sikkimensis (Ban) cell suspensions

Dual metabolite, i.e., ginsenoside and anthocyanin, co-accumulating cell suspensions of Panax sikkimensis were subjected to elicitation with culture filtrates of Serratia marcescens (SD 21), Bacillus subtilis (FL11), Trichoderma atroviridae (TA), and T. harzianum (TH) at 1.25% and 2.5% v/v for 1- and 3-week duration. The fungal-derived elicitors (TA and TH) did not significantly affect biomass accumulation; however, bacterial elicitors (SD 21 and FL11), especially SD 21, led to comparable loss in biomass growth. In terms of ginsenoside content, differential responses were observed. A maximum of 3.2-fold increase (222.2 mg/L) in total ginsenoside content was observed with the use of 2.5% v/v TH culture filtrate for 1 week. Similar ginsenoside accumulation was observed with the use of 1-week treatment with 2.5% v/v SD 21 culture filtrate (189.3 mg/L) with a 10-fold increase in intracellular Rg2 biosynthesis (31 mg/L). Real-time PCR analysis of key ginsenoside biosynthesis genes, i.e., FPS, SQS, DDS, PPDS, and PPTS, revealed prominent upregulation of particularly PPTS expression (20-23-fold), accounting for the observed enhancement in protopanaxatriol ginsenosides. However, none of the elicitors led to successful enhancement in in vitro anthocyanin accumulation as compared to control values.

Functional characterization of squalene synthase and squalene epoxidase in Taraxacum koksaghyz

The Russian dandelion Taraxacum koksaghyz produces high-value isoprenoids such as pentacyclic triterpenes and natural rubber in the latex of specialized cells known as laticifers. Squalene synthase (SQS) and squalene epoxidase (SQE) catalyze key steps in the biosynthesis of cyclic terpenoids, but neither enzyme has yet been characterized in T. koksaghyz. Genomic analysis revealed the presence of two genes (TkSQS1 and TkSQS2) encoding isoforms of SQS, and four genes (TkSQE1-4) encoding isoforms of SQE. Spatial expression analysis in different T. koksaghyz tissues confirmed that TkSQS1 and TkSQE1 are the latex-predominant isoforms, with highly similar mRNA expression profiles. The TkSQS1 and TkSQE1 proteins colocalized in the endoplasmic reticulum membrane and their enzymatic functions were confirmed by in vitro activity assays and yeast complementation studies, respectively. The functions of TkSQS1 and TkSQE1 were further characterized in the latex of T. koksaghyz plants with depleted TkSQS1 or TkSQE1 mRNA levels, produced by RNA interference. Comprehensive expression analysis revealed the coregulation of TkSQS1 and TkSQE1, along with a downstream gene in the triterpene biosynthesis pathway encoding the oxidosqualene cyclase TkOSC1. This indicates that the coregulation of TkSQS1, TkSQE1, and TkOSC1 could be used to optimize the flux toward specific terpenoids during development.

Cloning, Expression Analysis and Functional Characterization of Squalene Synthase (SQS) from Tripterygium wilfordii

Celastrol is an active triterpenoid compound derived from Tripterygium wilfordii which is well-known as a traditional Chinese medicinal plant. Squalene synthase has a vital role in condensing two molecules of farnesyl diphosphate to form squalene, a key precursor of triterpenoid biosynthesis. In the present study, T. wilfordii squalene synthase (TwSQS) was cloned followed by prokaryotic expression and functional verification. The open reading frame cDNA of TwSQS was 1242 bp encoding 413 amino acids. Bioinformatic and phylogenetic analysis showed that TwSQS had high homology with other plant SQSs. To obtain soluble protein, the truncated TwSQS without the last 28 amino acids of the carboxy terminus was inductively expressed in Escherichia coliTransetta (DE3). The purified protein was detected by SDS-PAGE and Western blot analysis. Squalene was detected in the product of in vitro reactions by gas chromatograph-mass spectrometry, which meant that TwSQS did have catalytic activity. Organ-specific and inducible expression levels of TwSQS were detected by quantitative real-time PCR. The results indicated that TwSQS was highly expressed in roots, followed by the stems and leaves, and was significantly up-regulated upon MeJA treatment. The identification of TwSQS is important for further studies of celastrol biosynthesis in T. wilfordii.

Tetraterpene Synthase Substrate and Product Specificity in the Green Microalga Botryococcus braunii Race L

Recently, the biosynthetic pathway for lycopadiene, a C₄₀ tetraterpenoid hydrocarbon, was deciphered from the L race of Botryococcus braunii, an alga that produces hydrocarbon oils capable of being converted into combustible fuels. The lycopadiene pathway is initiated by the squalene synthase (SS)-like enzyme lycopaoctaene synthase (LOS), which catalyzes the head-to-head condensation of two C₂₀ geranylgeranyl diphosphate (GGPP) molecules to produce C₄₀ lycopaoctaene. LOS shows unusual substrate promiscuity for SS or SS-like enzymes by utilizing C₁₅ farnesyl diphosphate (FPP) and C₂₀ phytyl diphosphate in addition to GGPP as substrates. These three substrates can be combined by LOS individually or in combinations to produce six different hydrocarbons of C₃₀, C₃₅, and C₄₀ chain lengths. To understand LOS substrate and product specificity, rational mutagenesis experiments were conducted based on sequence alignment with several SS proteins as well as a structural comparison with the human SS (HSS) crystal structure. Characterization of the LOS mutants in vitro identified Ser276 and Ala288 in the LOS active site as key amino acids responsible for controlling substrate binding, and thus the promiscuity of this enzyme. Mutating these residues to those found in HSS largely converted LOS from lycopaoctaene production to C₃₀ squalene production. Furthermore, these studies were confirmed in vivo by expressing LOS in E. coli cells metabolically engineered to produce high FPP and GGPP levels. These studies also offer insights into tetraterpene hydrocarbon metabolism in B. braunii and provide a foundation for engineering LOS for robust production of specific hydrocarbons of a desired chain length.

Virus-induced gene silencing of the two squalene synthase isoforms of apple tree (Malus × domestica L.) negatively impacts phytosterol biosynthesis, plastid pigmentation and leaf growth

The use of a VIGS approach to silence the newly characterized apple tree SQS isoforms points out the biological function of phytosterols in plastid pigmentation and leaf development. Triterpenoids are beneficial health compounds highly accumulated in apple; however, their metabolic regulation is poorly understood. Squalene synthase (SQS) is a key branch point enzyme involved in both phytosterol and triterpene biosynthesis. In this study, two SQS isoforms were identified in apple tree genome. Both isoforms are located at the endoplasmic reticulum surface and were demonstrated to be functional SQS enzymes using an in vitro activity assay. MdSQS1 and MdSQS2 display specificities in their expression profiles with respect to plant organs and environmental constraints. This indicates a possible preferential involvement of each isoform in phytosterol and/or triterpene metabolic pathways as further argued using RNAseq meta-transcriptomic analyses. Finally, a virus-induced gene silencing (VIGS) approach was used to silence MdSQS1 and MdSQS2. The concomitant down-regulation of both MdSQS isoforms strongly affected phytosterol synthesis without alteration in triterpene accumulation, since triterpene-specific oxidosqualene synthases were found to be up-regulated to compensate metabolic flux reduction. Phytosterol deficiencies in silenced plants clearly disturbed chloroplast pigmentation and led to abnormal development impacting leaf division rather than elongation or differentiation. In conclusion, beyond the characterization of two SQS isoforms in apple tree, this work brings clues for a specific involvement of each isoform in phytosterol and triterpene pathways and emphasizes the biological function of phytosterols in development and chloroplast integrity. Our report also opens the door to metabolism studies in Malus domestica using the apple latent spherical virus-based VIGS method.

Molecular cloning and promoter analysis of squalene synthase and squalene epoxidase genes from Betula platyphylla

Betula platyphylla is a rich repository of pharmacologically active secondary metabolites known as birch triterpenoids (TBP). Here, we cloned the squalene synthase (SS) and squalene epoxidase genetic (SE) sequences from B. platyphylla that encode the key enzymes that are involved in triterpenoid biosynthesis and analyzed the conserved domains and phylogenetics of their corresponding proteins. The full-length sequence of BpSS is 1588 bp with a poly-A tail, which contained an open reading frame (ORF) of 1241 bp that encoded a protein of 413 amino acids. Additionally, the BpSE full-length sequence of 2040 bp with a poly-A tail was also obtained, which contained an ORF of 1581 bp encoding a protein of 526 amino acids. Their organ-specific expression patterns in 4-week-old tissue culture seedlings of B. platyphylla were detected by real-time PCR and showed that they were all highly expressed in leaves, as compared to stem and root tissues. Additionaly, both BpSS and BpSE were enhanced following stimulation with ethephon and MeJA. The expression of BpSS was enhanced by ABA, whereas BpSE was not. The SA treatment did not affect the BpSS and BpSE transcripts notably. Using a genome walking approach, promoter sequences of 965 and 1193 bp, respectively, for BpSS and BpSE were isolated, and they revealed several key cis-regulatory elements known to be involved in the response to phytohormone and abiotic plant stress. We also found that the BpSS protein is localized in the cytoplasm. Opening reading frames of BpSS and BpSE were ligated into yeast expression plasmid pYES2 under control of GAL1 promoter and introduced into the yeast INVScl1 strain. The transformants were cultured for 12 h, the squalene content of galactose-induced BpSS expression yeast cells was 13.2 times of control (empty vector control yeast cells) by high-performance liquid chromatography (HPLC) test method. And, the squalene epoxidase activity of induced BpSE expression yeast cell was about 11.8 times of control. These indicated that we cloned birch BpSS and BpSE that were indeed involved in the synthesis of triteropenoids. This is the first report wherein SS and SE from B. platyphylla were cloned and may be of significant interest to understand the regulatory role of SS and SE in the triterpenoids biosynthesis of B. platyphylla. This is the first report wherein SS and SE from B. platyphylla were cloned and may be of significant interest to understand the regulatory role of SS and SE in the biosynthesis of birch triterpenoids.

Mapping a kingdom-specific functional domain of squalene synthase

Squalene synthase catalyzes the first committed step in sterol biosynthesis and consists of both an amino-terminal catalytic domain and a carboxy-terminal domain tethering the enzyme to the ER membrane. While the overall architecture of this enzyme is identical in eukaryotes, it was previously shown that plant and animal genes cannot complement a squalene synthase knockout mutation in yeast unless the carboxy-terminal domain is swapped for one of fungal origin. This implied a unique component of the fungal carboxy-terminal domain was responsible for the complementation phenotype. To identify this motif, we used Saccharomyces cerevisiae with a squalene synthase knockout mutation, and expressed intact and chimeric squalene synthases originating from fungi, plants, and animals. In contrast to previous observations, all enzymes tested could partially complement the knockout mutation when the genes were weakly expressed. However, when highly expressed, non-fungal squalene synthases could not complement the yeast mutation and instead led to the accumulation of a toxic intermediate(s) as defined by mutations of genes downstream in the ergosterol pathway. Restoration of the complete complementation phenotype was mapped to a 26-amino acid hinge region linking the catalytic and membrane-spanning domains specific to fungal squalene synthases. Over-expression of the C-terminal domain containing a hinge domain from fungi, not from animals or plants, led to growth inhibition of wild-type yeast. Because this hinge region is unique to and highly conserved within each kingdom of life, the data suggests that the hinge domain plays an essential functional role, such as assembly of ergosterol multi-enzyme complexes in fungi.

Genetic engineering and molecular characterization of yeast strain expressing hybrid human-yeast squalene synthase as a tool for anti-cholesterol drug assessment

AIMS

The main objective of the study is molecular and biological characterization of the human-yeast hybrid squalene synthase (SQS), as a promising target for treatment of hypercholesterolaemia.

METHODS AND RESULTS

The human-yeast hybrid SQS, with 67% amino acids, including the catalytic site derived from human enzyme, was expressed in Saccharomyces cerevisiae strain deleted of its own SQS gene. The constructed strain has a decreased level of sterols compared to the control strain. The mevalonate pathway and sterol biosynthesis genes are induced and the level of triacylglycerols is increased. Treatment of the strain with rosuvastatin or zaragozic acid, two mevalonate pathway inhibitors, decreased the amounts of squalene, lanosterol and ergosterol, and up-regulated expression of several genes encoding enzymes responsible for biosynthesis of ergosterol precursors. Conversely, expression of the majority genes implicated in the biosynthesis of other mevalonate pathway end products, ubiquinone and dolichol, was down-regulated.

CONCLUSIONS

The S. cerevisiae strain constructed in this study enables to investigate the physiological and molecular effects of inhibitors on cell functioning.

SIGNIFICANCE AND IMPACT OF THE STUDY

The yeast strain expressing hybrid SQS with the catalytic core of human enzyme is a convenient tool for efficient screening for novel inhibitors of cholesterol-lowering properties.

Molecular Cloning and Functional Analysis of Squalene Synthase 2(SQS2) in Salvia miltiorrhiza Bunge

Salvia miltiorrhiza Bunge, which is also known as a traditional Chinese herbal medicine, is widely studied for its ability to accumulate the diterpene quinone Tanshinones. In addition to producing a variety of diterpene quinone, S. miltiorrhiza Bunge also accumulates sterol, brassinosteroid and triterpenoids. During their biosynthesis, squalene synthase (SQS, EC 2.5.1.21) converts two molecules of the hydrophilic substrate farnesyl diphosphate (FPP) into a hydrophobic product, squalene. In the present study, cloning and characterization of S. miltiorrhiza Bunge squalene synthase 2 (SmSQS2, Genbank Accession Number: KM408605) cDNA was investigated subsequently followed by its recombinant expression and preliminary enzyme activity. The full-length cDNA of SmSQS2 was 1 597 bp in length, with an open reading frame of 1 245 bp encoding 414 amino acids. The deduced amino acid sequence of SmSQS2 shared high similarity with those of SQSs from other plants. To obtain soluble recombinant enzymes, the truncated SmSQS2 in which 28 amino acids were deleted from the carboxy terminus was expressed as GST-Tag fusion protein in Escherichia coli BL21 (DE3) and confirmed by SDS-PAGE and Western Blot analysis, and the resultant bacterial crude extract was incubated with FPP and NADPH. Gas chromatograph-mass spectrometer analysis showed that squalene was detected in the in vitro reaction mixture. The gene expression level was analyzed through Quantitative real-time PCR, and was found to be higher in roots as compared to the leaves, and was up-regulated upon YE+ Ag(+) treatment. These results could serve as an important to understand the function of the SQS family. In addition, the identification of SmSQS2 is important for further studies of terpenoid and sterol biosynthesis in S. miltiorrhiza Bunge.

A Decade of Molecular Understanding of Withanolide Biosynthesis and In vitro Studies in Withania somnifera (L.) Dunal: Prospects and Perspectives for Pathway Engineering

Withania somnifera, a multipurpose medicinal plant is a rich reservoir of pharmaceutically active triterpenoids that are steroidal lactones known as withanolides. Though the plant has been well-characterized in terms of phytochemical profiles as well as pharmaceutical activities, limited attempts have been made to decipher the biosynthetic route and identification of key regulatory genes involved in withanolide biosynthesis. This scenario limits biotechnological interventions for enhanced production of bioactive compounds. Nevertheless, recent emergent trends vis-à-vis, the exploration of genomic, transcriptomic, proteomic, metabolomics, and in vitro studies have opened new vistas regarding pathway engineering of withanolide production. During recent years, various strategic pathway genes have been characterized with significant amount of regulatory studies which allude toward development of molecular circuitries for production of key intermediates or end products in heterologous hosts. Another pivotal aspect covering redirection of metabolic flux for channelizing the precursor pool toward enhanced withanolide production has also been attained by deciphering decisive branch point(s) as robust targets for pathway modulation. With these perspectives, the current review provides a detailed overview of various studies undertaken by the authors and collated literature related to molecular and in vitro approaches employed in W. somnifera for understanding various molecular network interactions in entirety.

Accumulation of squalene in a microalga Chlamydomonas reinhardtii by genetic modification of squalene synthase and squalene epoxidase genes

Several microalgae accumulate high levels of squalene, and as such provide a potentially valuable source of this useful compound. However, the molecular mechanism of squalene biosynthesis in microalgae is still largely unknown. We obtained the sequences of two enzymes involved in squalene synthesis and metabolism, squalene synthase (CrSQS) and squalene epoxidase (CrSQE), from the model green alga Chlamydomonas reinhardtii. CrSQS was functionally characterized by expression in Escherichia coli and CrSQE by complementation of a budding yeast erg1 mutant. Transient expression of CrSQS and CrSQE fused with fluorescent proteins in onion epidermal tissue suggested that both proteins were co-localized in the endoplasmic reticulum. CrSQS-overexpression increased the rate of conversion of 14C-labeled farnesylpyrophosphate into squalene but did not lead to over-accumulation of squalene. Addition of terbinafine caused the accumulation of squalene and suppression of cell survival. On the other hand, in CrSQE-knockdown lines, the expression level of CrSQE was reduced by 59-76% of that in wild-type cells, and significant levels of squalene (0.9-1.1 μg mg-1 cell dry weight) accumulated without any growth inhibition. In co-transformation lines with CrSQS-overexpression and CrSQE-knockdown, the level of squalene was not increased significantly compared with that in solitary CrSQE-knockdown lines. These results indicated that partial knockdown of CrSQE is an effective strategy to increase squalene production in C. reinhardtii cells.

Biosynthesis and biotechnological production of ginsenosides

Medicinal plants are essential for improving human health, and around 75% of the population in developing countries relies mainly on herb-based medicines for health care. As the king of herb plants, ginseng has been used for nearly 5,000 years in the oriental and recently in western medicines. Among the compounds studied in ginseng plants, ginsenosides have been shown to have multiple medical effects such as anti-oxidative, anti-aging, anti-cancer, adaptogenic and other health-improving activities. Ginsenosides belong to a group of triterpene saponins (also called ginseng saponins) that are found almost exclusively in Panax species and accumulated especially in the plant roots. In this review, we update the conserved and diversified pathway/enzyme biosynthesizing ginsenosides which have been presented. Particularly, we highlight recent milestone works on functional characterization of key genes dedicated to the production of ginsenosides, and their application in engineering plants and yeast cells for large-scale production of ginsenosides.

Molecular cloning and differential expression analysis of a squalene synthase gene from Dioscorea zingiberensis, an important pharmaceutical plant

Diosgenin is a steroid derived from cholesterol in plants and used as a typical initial intermediate for synthesis of numerous steroidal drugs in the world. Commercially, this compound is extracted mainly from the rhizomes or tubers of some Dioscorea species. Squalene synthase (SQS: EC 2.5.1.21) catalyzes the condensation of two molecules of farnesyl diphosphate to form squalene, the first committed step for biosynthesis of plant sterols including cholesterol, and is thought to play an important role in diosgenin biosynthesis. A full-length cDNA of a putative squalene synthase gene was cloned from D. zingiberensis and designated as DzSQS (Genbank Accession Number KC960673). DzSQS was contained an open reading frame of 1,230 bp encoding a polypeptide of 409 amino acids with a predicted molecular weight of 46 kDa and an isoelectric point of 6.2. The deduced amino acid sequence of DzSQS shared over 70 % sequence identity with those of SQSs from other plants. The truncated DzSQS in which 24 amino acids were deleted from the carboxy terminus was expressed in Escherichia coli, and the resultant bacterial crude extract was incubated with farnesyl diphosphate and NADPH. GC-MS analysis showed that squalene was detected in the in vitro reaction mixture. Quantitative real-time PCR analysis revealed that DzSQS was expressed from highest to lowest order in mature leaves, newly-formed rhizomes, young leaves, young stems, and two-year-old rhizomes of D. zingiberensis.

Molecular characterization of Glycine max squalene synthase genes in seed phytosterol biosynthesis

The reaction catalyzed by squalene synthase (EC.2.5.1.21) that converts two molecules of farnesyl pyrophosphate to squalene represents a crucial branch point of the isoprenoid pathway in diverting carbon flux towards the biosynthesis of sterols. In the present study two soybean squalene synthase genes, GmSQS1 and GmSQS2, were identified in the soybean genome and functionally characterized for their roles in sterol biosynthesis. Both genes encode a deduced protein of 413 amino acids. Complementation assays showed that the two genes were able to convert yeast sterol auxotrophy erg9 mutant to sterol prototrophy. Expression of GmSQS1 and GmSQS2 was ubiquitous in roots, stem, leaves, flower and young seeds of soybean, however GmSQS1 transcript was preferential in roots while GmSQS2 transcript was more in leaves. Their expression was lower in response to dehydration treatments suggesting they might be negative regulators of water stress adaptation. Transgenic Arabidopsis plants overexpressing GmSQS1 driven by either constitutive or seed-specific promoters showed increases in the major end product sterols: campesterol, sitosterol and stigmasterol, which resulted in up to 50% increase in total sterol content in the seeds. The increase in the end product sterols by GmSQS1 overexpression was at the level achievable by previously reported overexpression of individual or combination of other key enzymes in the sterol pathway. Together the data demonstrate that soybean SQS genes play an important role in diverting carbon flux to the biosynthesis of the end product sterols in the seeds.

Characterization of Squalene synthase gene from Chlorophytum borivilianum (Sant. and Fernand.)

Saponins are important group of secondary metabolites known for their pharmacological properties. Chlorophytum borivilianum contains high amount of saponins and is thus, recognized as an important medicinal plant with aphrodisiac properties. Though the plant is well known for its pharmaceutical properties, there is meager information available about the genes and enzymes responsible for biosynthesis of saponins from this plant. Squalene synthase (SqS) is the key enzyme of saponin biosynthesis pathway and here, we report cloning and characterization of SqS gene from C. borivilianum. A full-length CbSqS cDNA consisting of 1,760 bp was cloned which contained an open reading frame (ORF) of 1,233 bp, encoding a protein of 411 amino acids. Analysis of deduced amino acid sequence of CbSqS predicted the presence of conserved isoprenoid family domain and catalytic sites. Phylogenetic analysis revealed that CbSqS is closer to Glycine max and monocotyledonous plants. 3D structure prediction using various programs showed CbSqS structure to be similar to SqS from other species. C-terminus truncated recombinant squalene synthase (TruncCbSqS) was expressed in E. coli M15 cells with optimum expression induced with 1 mM IPTG at 37 °C. The gene expression level was analyzed through semi-quantitative RT-PCR and was found to be higher in leaves as compared to the roots.

The C8ORF38 homologue Sicily is a cytosolic chaperone for a mitochondrial complex I subunit

Sicily, which was identified in a screen for proteins involved in neurodegeneration, interacts with cytosolic Hsp90 to chaperone the complex I subunit ND42, before its mitochondrial import.

Mitochondrial complex I (CI) is an essential component in energy production through oxidative phosphorylation. Most CI subunits are encoded by nuclear genes, translated in the cytoplasm, and imported into mitochondria. Upon entry, they are embedded into the mitochondrial inner membrane. How these membrane-associated proteins cope with the hydrophilic cytoplasmic environment before import is unknown. In a forward genetic screen to identify genes that cause neurodegeneration, we identified sicily, the Drosophila melanogaster homologue of human C8ORF38, the loss of which causes Leigh syndrome. We show that in the cytoplasm, Sicily preprotein interacts with cytosolic Hsp90 to chaperone the CI subunit, ND42, before mitochondrial import. Loss of Sicily leads to loss of CI proteins and preproteins in both mitochondria and cytoplasm, respectively, and causes a CI deficiency and neurodegeneration. Our data indicate that cytosolic chaperones are required for the subcellular transport of ND42.

Functional characterization and differential expression studies of squalene synthase from Withania somnifera

Squalene synthase (SQS: EC 2.5.1.21) is a potential branch point regulatory enzyme and represents the first committed step to diverge the carbon flux from the main isoprenoid pathway towards sterol biosynthesis. In the present study, cloning and characterization of Withania somnifera squalene synthase (WsSQS) cDNA was investigated subsequently followed by its heterologous expression and preliminary enzyme activity. Two different types of WsSQS cDNA clones (WsSQS1and WsSQS2) were identified that contained an open reading frames of 1,236 and 1,242 bp encoding polypeptides of 412 and 414 amino acids respectively. Both WsSQS isoforms share 99 % similarity and identity with each other. WsSQS deduced amino acids sequences, when compared with SQS of other plant species, showed maximum similarity and identity with Capsicum annuum followed by Solanum tuberosum and Nicotiana tabacum. To obtain soluble recombinant enzymes, 24 hydrophobic amino acids were deleted from the carboxy terminus and expressed as 6X His-Tag fusion protein in Escherichia coli. Approximately 43 kDa recombinant protein was purified using Ni-NTA affinity chromatography and checked on SDS-PAGE. Preliminary activity of the purified enzymes was determined and the products were analyzed by gas chromatograph-mass spectrometer (GC-MS). Quantitative real-time PCR (qRT-PCR) analysis showed that WsSQS expresses more in young leaves than mature leaves, stem and root.

Molecular cloning, bacterial expression and promoter analysis of squalene synthase from Withania somnifera (L.) Dunal

Withania somnifera (ashwagandha) is a rich repository of large number of pharmacologically active secondary metabolites known as withanolides. Though the plant has been well characterized in terms of phytochemical profiles as well as pharmaceutical activities, but there is sparse information about the genes responsible for biosynthesis of these compounds. In this study, we have cloned and characterized a gene encoding squalene synthase (EC 2.5.1.21) from a withaferin A rich variety of W. somnifera, a key enzyme in the biosynthesis of isoprenoids. Squalene synthase catalyses dimerization of two farnesyl diphosphate (FPP) molecules into squalene, a key precursor for sterols and triterpenes. A full-length cDNA consisting of 1765 bp was isolated and contained a 1236 bp open reading frame (ORF) encoding a polypeptide of 411 amino acids. Recombinant C-terminus truncated squalene synthase (WsSQS) was expressed in BL21 cells (Escherichia coli) with optimum expression induced with 1mM IPTG at 37°C after 1h. Quantitative RT-PCR analysis showed that squalene synthase (WsSQS) expressed in all tested tissues including roots, stem and leaves with the highest level of expression in leaves. The promoter region of WsSQS isolated by genome walking presented several cis-acting elements in the promoter region. Biosynthesis of withanolides was up-regulated by different signalling components including methyl-jasmonate, salicylic acid and 2, 4-D, which was consistent with the predicted results of WsSQS promoter region. This work is the first report of cloning and expression of squalene synthase from W. somnifera and will be useful to understand the regulatory role of squalene synthase in the biosynthesis of withanolides.

A new approach to the molecular analysis of docking, priming, and regulated membrane fusion

Studies using isolated sea urchin cortical vesicles have proven invaluable in dissecting mechanisms of Ca(2+)-triggered membrane fusion. However, only acute molecular manipulations are possible in vitro. Here, using selective pharmacological manipulations of sea urchin eggs ex vivo, we test the hypothesis that specific lipidic components of the membrane matrix selectively affect defined late stages of exocytosis, particularly the Ca(2+)-triggered steps of fast membrane fusion. Egg treatments with cholesterol-lowering drugs resulted in the inhibition of vesicle fusion. Exogenous cholesterol recovered fusion extent and efficiency in cholesterol-depleted membranes; α-tocopherol, a structurally dissimilar curvature analogue, selectively restored fusion extent. Inhibition of phospholipase C reduced vesicle phosphatidylethanolamine and suppressed both the extent and kinetics of fusion. Although phosphatidylinositol-3-kinase inhibition altered levels of polyphosphoinositide species and reduced all fusion parameters, sequestering polyphosphoinositides selectively inhibited fusion kinetics. Thus, cholesterol and phosphatidylethanolamine play direct roles in the fusion pathway, contributing negative curvature. Cholesterol also organizes the physiological fusion site, defining fusion efficiency. A selective influence of phosphatidylethanolamine on fusion kinetics sheds light on the local microdomain structure at the site of docking/fusion. Polyphosphoinositides have modulatory upstream roles in priming: alterations in specific polyphosphoinositides likely represent the terminal priming steps defining fully docked, release-ready vesicles. Thus, this pharmacological approach has the potential to be a robust high-throughput platform to identify molecular components of the physiological fusion machine critical to docking, priming, and triggered fusion.

Gene regulation patterns in triterpene biosynthetic pathway driven by overexpression of squalene synthase and methyl jasmonate elicitation in Bupleurum falcatum

The root of Bupleurum falcatum L. (Apiaceae) has long been one of the most important traditional herbal medicines in Asian countries. A group of triterpene saponins (saikosaponins) are the major constituents of this plant. Squalene synthase (SS) may play a regulatory role in directing triterpene intermediates and sterol pathways. Here, we investigated the regulatory role of the squalene synthase (BfSS1) gene in the biosynthesis of phytosterol and triterpene in B. falcatum. BfSS1 mRNA accumulated ubiquitously in plant organs and markedly increased in roots after treatment with methyl jasmonate (MeJA), ABA and ethephon. Transgenic B. falcatum constructs overexpressing BfSS1 in the sense and antisense orientations were assembled using the Agrobacterium-mediated method. Transgenic roots overexpressing BfSS1 in the sense orientation resulted in enhanced production of both phytosterol and saikosaponins. Overexpression of the BfSS1 gene in the sense orientation increased the mRNA accumulation of downstream genes such as squalene epoxidase and cycloartenol synthase but unexpectedly decreased the mRNA levels of β-amyrin synthase (β-AS), a triterpene synthase mRNA. MeJA treatment of wild-type roots strongly stimulated β-AS mRNA accumulation and saikosaponin production but suppressed phytosterol production. MeJA treatment of transgenic roots overexpressing BfSS1 in the sense orientation failed to stimulate β-AS mRNA accumulation but still enhanced saikosaponin and phytosterol production. These results indicate that overexpression of BfSS1 in B. falcatum regulates more powerfully the downstream genes than elicitor (MeJA) treatment in triterpene and phytosterol biosynthesis.

Design of more potent squalene synthase inhibitors with multiple activities

With the increasing realization that modulating a multiplicity of targets can be an asset in the treatment of multifactorial disorders, we hereby report the synthesis and evaluation of the first compounds in which antioxidant, anti-inflammatory as well as squalene synthase (SQS) inhibitory activities are combined by design, in a series of simple molecules, extending their potential range of activities against the multifactorial disease of atherosclerosis. The activity of the initially synthesized antihyperlipidemic morpholine derivatives (1-6), in which we combined several pharmacophore moieties, was evaluated in vitro (antioxidant, inhibition of SQS and lipoxygenase) and in vivo (anti-dyslipidemic and anti-inflammatory effect). We further compared the in vitro SQS inhibitory action of these derivatives with theoretically derived molecular interactions by performing an in silico docking study using the X-ray crystal structure of human SQS. Based on low energy preferred binding modes, we designed potentially more potent SQS ligands. We proceeded with synthesizing and evaluating these new structures (7-12) in vitro and in vivo, to show that the new derivatives were significantly more active than formerly developed congeners, both as SQS inhibitors (20-70-fold increase in activity) and antioxidants (4-30-fold increase in activity). A significant correlation between experimental activity [Log(1/IC(50))] and the corresponding binding free energy (ΔG(b)) of the docked compounds was shown. These results, taken together, show a promising alternative and novel approach for the design and development of multifunctional antiatherosclerosis agents.

Sorting defects of the tryptophan permease Tat2 in an erg2 yeast mutant

Cholesterol (ergosterol in yeast) in conjunction with sphingolipids forms tight-packing microdomains, 'lipid rafts,' which are thought to be critical for intracellular protein sorting in eukaryotic cells. When the activity of Erg9 involved in the first step of ergosterol biogenesis, but not that of Erg6 involved in a late step, is compromised, vacuolar degradation of the tryptophan permease Tat2 is promoted. It is unknown whether this difference simply reflects the difference between the inhibition of early and late steps. Here, it is shown that the deletion in ERG2, which encodes sterol C8-C7 isomerase (the next enzymatic step after Erg6), promotes the vacuolar degradation of Tat2. It suggests that the accumulation of specific sterol intermediates may alter lipid raft structures, promoting Tat2 degradation. The erg2Delta-mediated Tat2 degradation required Tat2 ubiquitination. Lipid raft association of Tat2 is compromised in erg2Delta cells. The erg2Delta mutation showed a synthetic growth defect with the trp1 mutation, indicating that Tat2 sorting is preferentially compromised in these mutants. Consistent with this notion, the raft-associated protein Pma1 was associated with detergent-resistant membranes and sorted to the plasma membrane. This study suggests the potential for the pharmacological control of cellular nutrient uptake in humans by regulating enzymes involved in cholesterol biogenesis.

Arabidopsis thaliana contains a single gene encoding squalene synthase

Squalene synthase (SQS) catalyzes the condensation of two molecules of farnesyl diphosphate (FPP) to produce squalene (SQ), the first committed precursor for sterol, brassinosteroid, and triterpene biosynthesis. Arabidopsis thaliana contains two SQS-annotated genomic sequences, At4g34640 (SQS1) and At4g34650 (SQS2), organized in a tandem array. Here we report that the SQS1 gene is widely expressed in all tissues throughout plant development, whereas SQS2 is primarily expressed in the vascular tissue of leaf and cotyledon petioles, and the hypocotyl of seedlings. Neither the complete A. thaliana SQS2 protein nor the chimeric SQS resulting from the replacement of the 69 C-terminal residues of SQS2 by the 111 C-terminal residues of the Schizosaccharomyces pombe SQS were able to confer ergosterol prototrophy to a Saccharomyces cerevisiae erg9 mutant strain lacking SQS activity. A soluble form of SQS2 expressed in Escherichia coli and purified was unable to synthesize SQ from FPP in the presence of NADPH and either Mg2+ or Mn2+. These results demonstrated that SQS2 has no SQS activity, so that SQS1 is the only functional SQS in A. thaliana. Mutational studies revealed that the lack of SQS activity of SQS2 cannot be exclusively attributed to the presence of an unusual Ser replacing the highly conserved Phe at position 287. Expression of green fluorescent protein (GFP)-tagged versions of SQS1 in onion epidermal cells demonstrated that SQS1 is targeted to the endoplasmic reticulum (ER) membrane and that this location is exclusively dependent on the presence of the SQS1 C-terminal hydrophobic trans-membrane domain.

The ergosterol biosynthesis inhibitor zaragozic acid promotes vacuolar degradation of the tryptophan permease Tat2p in yeast

Ergosterol is the yeast functional equivalent of cholesterol in mammalian cells. Deletion of the ERG6 gene, which encodes an enzyme catalyzing a late step of ergosterol biosynthesis, impedes targeting of the tryptophan permease Tat2p to the plasma membrane, but does not promote vacuolar degradation. It is unknown whether similar features appear when other steps of ergosterol biogenesis are inhibited. We show herein that the ergosterol biosynthesis inhibitor zaragozic acid (ZA) evoked massive vacuolar degradation of Tat2p, accompanied by a decrease in tryptophan uptake. ZA inhibits squalene synthetase (SQS, EC 2.5.1.21), which catalyzes the first committed step in the formation of cholesterol/ergosterol. The degradation of Tat2p was dependent on the Rsp5p-mediated ubiquitination of Tat2p and was not suppressed by deletions of VPS1, VPS27, VPS45 or PEP12. We will discuss ZA-mediated Tat2p degradation in the context of lipid rafts.

Repression of ergosterol level during oxidative stress by fission yeast F-box protein Pof14 independently of SCF

We describe a new member of the F-box family, Pof14, which forms a canonical, F-box dependent SCF (Skp1, Cullin, F-box protein) ubiquitin ligase complex. The Pof14 protein has intrinsic instability that is abolished by inactivation of its Skp1 interaction motif (the F-box), Skp1 or the proteasome, indicating that Pof14 stability is controlled by an autocatalytic mechanism. Pof14 interacts with the squalene synthase Erg9, a key enzyme in ergosterol metabolism, in a membrane-bound complex that does not contain the core SCF components. pof14 transcription is induced by hydrogen peroxide and requires the Pap1 transcription factor and the Sty1 MAP kinase. Pof14 binds to and decreases Erg9 activity in vitro and a pof14 deletion strain quickly loses viability in the presence of hydrogen peroxide due to its inability to repress ergosterol synthesis. A pof14 mutant lacking the F-box and an skp1-3 ts mutant behave as wild type in the presence of oxidant showing that Pof14 function is independent of SCF. This indicates that modulation of ergosterol level plays a key role in adaptation to oxidative stress.

Overexpression of squalene synthase in Eleutherococcus senticosus increases phytosterol and triterpene accumulation

Squalene synthase (SS) catalyzes the first committed step in sterol and triterpenoid biosynthesis. Transgenic Eleutherococcus senticosus Rupr. and Maxim. plants were generated by introducing an SS-encoding gene derived from Panax ginseng (PgSS1) together with genes expressing hygromycin phosphotransferase and green fluorescent protein (GFP) through Agrobacterium-mediated transformation. Early globular embryo clusters developing from the embryogenic callus were used for Agrobacterium-mediated transformation. Transformants were selected on Murashige Skoog medium containing 25 mg/L hygromycin. Hygromycin-resistant somatic embryos developed into plants after the cotyledonary embryos were treated with 14.4 microM gibberellic acid. Transformation was confirmed by polymerase chain reaction, Southern, and GFP analyses. The SS enzyme activity of the transgenic plants was up to 3-fold higher than that of wild-type plants. In addition, GC-MS and HPLC analysis revealed that phytosterols (beta-sitosterol and stigmasterol) as well as triterpene saponins (ciwujianosides B (1), C(1) (2), C(2) (3), C(3) (4), C(4) (5), D(1) (6) and D(2) (7)) levels in transgenic E. senticosus were increased by 2- to 2.5-fold. These results suggest that the metabolic engineering of E. senticosus to enhance production of phytosterols and triterpenoids by introducing the PgSS1 gene was successfully achieved by Agrobacterium-mediated genetic transformation.

Characterization of botryococcene synthase enzyme activity, a squalene synthase-like activity from the green microalga Botryococcus braunii, Race B

The extracellular matrix of the alga Botryococcus braunii, Race B, consists mainly of botryococcenes, which have potential as a hydrocarbon fuel. Botryococcenes are structurally similar to squalene raising the possibility of a common enzyme for the biosynthesis of both. While B. braunii squalene synthase (SS) enzyme activity has been documented, botryococcene synthase (BS) enzyme activity has not been. In the current study, an assay for BS activity has been developed and used to show that many of the assay conditions for BS enzyme activity are similar to those of SS. However, SS enzyme activity is stimulated by Tween 80 while BS enzyme activity is inhibited. Moreover, BS enzyme activity was correlated with the accumulation of botryococcenes during a B. braunii culture growth cycle, which was distinctly different from the profile of SS enzyme activity. While the current results indicate a conservation of enzymological features amongst the BS and SS enzymes, raising the possibility of one enzyme capable of catalyzing both activities, they are also consistent with these two activities arising from separate and distinct enzymes.

Model systems in drug discovery: chemical genetics meets genomics

Animal model systems are an intricate part of the discovery and development of new medicines. The sequencing of not only the human genome but also those of the various pathogenic bacteria, the nematode Caenorhabditis elegans, the fruitfly Drosophila, and the mouse has enabled the discovery of new drug targets to push forward at an unprecedented pace. The knowledge and tools in these "model" systems are allowing researchers to carry out experiments more efficiently and are uncovering previously hidden biological connections. While the history of bacteria, yeast, and mice in drug discovery are long, their roles are ever evolving. In contrast, the history of Drosophila and C. elegans at pharmaceutical companies is short. We will briefly review the historic role of each model organism in drug discovery and then update the readers as to the abilities and liabilities of each model within the context of drug development.