Molecular characterization of two plant flavonol sulfotransferases

Comprehensive Analysis of Sulfated Flavonoids in Eclipta prostrata for Quality Evaluation

Eclipta prostrata (Asteraceae) is employed as a hemostatic agent in many traditional medicines, owing to its sulfated flavonoid content. In this study, we obtained crude drug samples from three provinces collected in different years and analyzed their sulfated flavonoid contents using liquid chromatography-mass spectrometry (LC-MS) for quality evaluation. Because sulfated flavonoids are unstable and difficult to isolate from extracts, this study first synthesized a variety of sulfated flavonoids and accumulated spectral data in order to identify the compounds in E. prostrata. The LC-MS analysis of six crude drug samples revealed the presence of luteolin 7-sulfate, apigenin 7-sulfate, diosmetin 7-sulfate, and diosmetin 3'-sulfate. The samples without luteolin 3'-sulfate featured high apigenin 7-sulfate content. Although the samples were collected from the same locality, their compositions differed depending on the year of collection. Further, they were classified according to three patterns: (1) samples with luteolin 7-sulfate as the main component, (2) samples with apigenin 7-sulfate as the main component, and (3) samples with relatively high diosmetin sulfate content. Luteolin 7-sulfate typically exhibits relatively high erythrocyte aggregation efficiency and fibrinogen aggregation rate. These results demonstrate that the analysis of sulfated flavonoids is beneficial for the quality evaluation of E. prostrata for hemostatic applications.

Adaptive modifications in plant sulfur metabolism over evolutionary time

Sulfur (S) is an essential element for life on Earth. Plants are able to take up and utilize sulfate (SO42-), the most oxidized inorganic form of S compounds on Earth, through the reductive S assimilatory pathway that couples with photosynthetic energy conversion. Organic S compounds are subsequently synthesized in plants and made accessible to animals, primarily as the amino acid methionine. Thus, plant S metabolism clearly has nutritional importance in the global food chain. S metabolites may be part of redox regulation and drivers of essential metabolic pathways as cofactors and prosthetic groups, such as Fe-S centers, CoA, thiamine, and lipoic acid. The evolution of the S metabolic pathways and enzymes reflects the critical importance of functional innovation and diversifications. Here we review the major evolutionary alterations that took place in S metabolism across different scales and outline research directions that may take advantage of understanding the evolutionary adaptations.

Tropinone reductase: A comprehensive review on its role as the key enzyme in tropane alkaloids biosynthesis

TAs, including hyoscyamine and scopolamine, were used to treat neuromuscular disorders ranging from nerve agent poisoning to Parkinson's disease. Tropinone reductase I (TR-I; EC 1.1.1.206) catalyzed the conversion of tropinone into tropine in the biosynthesis of TAs, directing the metabolic flow towards hyoscyamine and scopolamine. Tropinone reductase II (TR-II; EC 1.1.1.236) was responsible for the conversion of tropinone into pseudotropine, diverting the metabolic flux towards calystegine A3. The regulation of metabolite flow through both branches of the TAs pathway seemed to be influenced by the enzymatic activity of both enzymes and their accessibility to the precursor tropinone. The significant interest in the utilization of metabolic engineering for the efficient production of TAs has highlighted the importance of TRs as crucial enzymes that govern both the direction of metabolic flow and the yield of products. This review discussed recent advances for the TRs sources, properties, protein structure and biocatalytic mechanisms, and a detailed overview of its crucial role in the metabolism and synthesis of TAs was summarized. Furthermore, we conducted a detailed investigation into the evolutionary origins of these two TRs. A prospective analysis of potential challenges and applications of TRs was presented.

GBSOT4 Enhances the Resistance of Gossypium barbadense to Fusarium oxysporum f. sp. vasinfectum (FOV) by Regulating the Content of Flavonoid

Sulfotransferases (SOTs) (EC 2.8.2.-) are sulfate regulatory proteins in a variety of organisms that have been previously shown to be involved in regulating a variety of physiological and biological processes, such as growth, development, adaptation to land, stomatal closure, drought tolerance, and response to pathogen infection. However, there is a lack of comprehensive identification and systematic analysis of SOT in cotton, especially in G. barbadense. In this study, we used bioinformatics methods to analyze the structural characteristics, phylogenetic relationships, gene structure, expression patterns, evolutionary relationships, selection pressure and stress response of SOT gene family members in G. barbadense. In this study, a total of 241 SOT genes were identified in four cotton species, among which 74 SOT gene members were found in G. barbadense. According to the phylogenetic tree, 241 SOT protein sequences were divided into five distinct subfamilies. We also mapped the physical locations of these genes on chromosomes and visualized the structural information of SOT genes in G. barbadense. We also predicted the cis-acting elements of the SOT gene in G. barbadense, and we identified the repetitive types and collinearity analysis of SOT genes in four cotton species. We calculated the Ka/Ks ratio between homologous gene pairs to elucidate the selective pressure between SOT genes. Transcriptome data were used to explore the expression patterns of SOT genes, and then qRT-PCR was used to detect the expression patterns of GBSOT4, GBSOT17 and GBSOT33 under FOV stress. WGCNA (weighted gene co-expression network analysis) showed that GB_A01G0479 (GBSOT4) belonged to the MEblue module, which may regulate the resistance mechanism of G. barbadense to FOV through plant hormones, signal transduction and glutathione metabolism. In addition, we conducted a VIGS (virus-induced gene silencing) experiment on GBSOT4, and the results showed that after FOV inoculation, the plants with a silenced target gene had more serious leaf wilting, drying and cracking than the control group, and the disease index of the plants with the silenced target gene was significantly higher than that of the control group. This suggests that GBSOT4 may be involved in protecting the production of G. barbadense from FOV infection. Subsequent metabolomics analysis showed that some flavonoid metabolites, such as Eupatorin-5-methylether (3'-hydroxy-5,6,7,4'-tetramethoxyflavone, were accumulated in cotton plants in response to FOV infection.

Sulfated phenolic acids in plants

Sulfated phenolic acids are widely occurring metabolites in plants, including fruits, vegetables and crops. The untargeted UHPLC-QTOF-MS metabolomics of more than 50 samples from plant, fungi and algae lead to the discovery of a small group of sulfated metabolites derived from phenolic acids. These compounds were detected in land plants for the first time. In this study, zosteric acid, 4-(sulfooxy)benzoic acid, 4-(sulfoooxy)phenylacetic acid, ferulic acid 4-sulfate and/or vanillic acid 4-sulfate were detected in a number of edible species/products, including oat (Avena sativa L.), wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), tomato (Solanum lycopersicum L.), carrot (Daucus carota subsp. Sativus Hoffm.), broccoli (Brassica oleracea var. Italica Plenck), celery (Apium graveolens L.), cabbage (Brassica oleracea convar. sabauda L.), banana tree (Musa tropicana L.), pineapple fruit (Ananas comosus L.), radish bulb (Raphanus sativus L.) and olive oil (Olea europaea L.). The structural identification of sulfated compounds was performed by comparing retention times and mass spectral data to those of synthesized standards. In addition to above-mentioned compounds, isoferulic acid 3-sulfate and caffeic acid 4-sulfate were putatively identified in celery bulb (Apium graveolens L.) and broccoli floret (Brassica oleracea var. Italica Plenck), respectively. While sulfated phenolic acids were quantified in concentrations ranging from 0.34 to 22.18 µg·g^-1 DW, the corresponding non-sulfated acids were mostly undetected or present at lower concentrations. The subsequent analysis of oat symplast and apoplast showed that they are predominantly accumulated in the symplast (> 70%) where they are supposed to be biosynthesized by sulfotransferases.

Purification, Characterization, and Structural Studies of a Sulfatase from Pedobacter yulinensis

Sulfatases are ubiquitous enzymes that hydrolyze sulfate from sulfated organic substrates such as carbohydrates, steroids, and flavones. These enzymes can be exploited in the field of biotechnology to analyze sulfated metabolites in humans, such as steroids and drugs of abuse. Because genomic data far outstrip biochemical characterization, the analysis of sulfatases from published sequences can lead to the discovery of new and unique activities advantageous for biotechnological applications. We expressed and characterized a putative sulfatase (PyuS) from the bacterium Pedobacter yulinensis. PyuS contains the (C/S)XPXR sulfatase motif, where the Cys or Ser is post-translationally converted into a formylglycine residue (FGly). His-tagged PyuS was co-expressed in Escherichia coli with a formylglycine-generating enzyme (FGE) from Mycobacterium tuberculosis and purified. We obtained several crystal structures of PyuS, and the FGly modification was detected at the active site. The enzyme has sulfatase activity on aromatic sulfated substrates as well as phosphatase activity on some aromatic phosphates; however, PyuS did not have detectable activity on 17α-estradiol sulfate, cortisol 21-sulfate, or boldenone sulfate.

Investigation and Computational Analysis of the Sulfotransferase (SOT) Gene Family in Potato (Solanum tuberosum): Insights into Sulfur Adjustment for Proper Development and Stimuli Responses

Various kinds of primary metabolisms in plants are modulated through sulfate metabolism, and sulfotransferases (SOTs), which are engaged in sulfur metabolism, catalyze sulfonation reactions. In this study, a genome-wide approach was utilized for the recognition and characterization of SOT family genes in the significant nutritional crop potato (Solanum tuberosum L.). Twenty-nine putative StSOT genes were identified in the potato genome and were mapped onto the nine S. tuberosum chromosomes. The protein motifs structure revealed two highly conserved 5'-phosphosulfate-binding (5' PSB) regions and a 3'-phosphate-binding (3' PB) motif that are essential for sulfotransferase activities. The protein-protein interaction networks also revealed an interesting interaction between SOTs and other proteins, such as PRTase, APS-kinase, protein phosphatase, and APRs, involved in sulfur compound biosynthesis and the regulation of flavonoid and brassinosteroid metabolic processes. This suggests the importance of sulfotransferases for proper potato growth and development and stress responses. Notably, homology modeling of StSOT proteins and docking analysis of their ligand-binding sites revealed the presence of proline, glycine, serine, and lysine in their active sites. An expression essay of StSOT genes via potato RNA-Seq data suggested engagement of these gene family members in plants' growth and extension and responses to various hormones and biotic or abiotic stimuli. Our predictions may be informative for the functional characterization of the SOT genes in potato and other nutritional crops.

Bioinformatic Characterization of Sulfotransferase Provides New Insights for the Exploitation of Sulfated Polysaccharides in Caulerpa

Caulerpa is an unusual algal genus from Caulerpaceae (Chlorophyta, Bryopsidales). Species from this family produce a wide range of metabolites suitable for biotechnology applications. Among these, sulfated polysaccharides (SPs) are often highly desirable for pharmaceutical and nutraceutical applications. Here, we provide a classification of sulfotransferases from Caulerpa; these important enzymes catalyze the nodal step for the biosynthesis of SPs. For this, we performed phylogenetic, genomic, expression analyses and prediction of the protein structure on sulfotransferases from Caulerpa. Sequences, domains and structures of sulfotransferases generally shared common characteristics with other plants and algae. However, we found an extensive duplication of sulfotransferase gene family, which is unique among the green algae. Expression analysis revealed specific transcript abundance in the pinnae and rachis of the alga. The unique genomic features could be utilized for the production of complex SPs, which require multiple and specific sulfation reactions. The expansion of this gene family in Caulerpaceae would have resulted in a number of proteins characterizing the unique SPs found in these algae. We provide a putative biosynthetic pathway of SPs, indicating the unique characteristics of this pathway in Caulerpa species. These data may help in the future selection of Caulerpa species for both commercial applications and genetic studies to improve the synthesis of valuable products from Caulerpa.

The Occurrence of Sulfated Salicinoids in Poplar and Their Formation by Sulfotransferase1

Salicinoids form a specific class of phenolic glycosides characteristic of the Salicaceae. Although salicinoids accumulate in large amounts and have been shown to be involved in plant defense, their biosynthesis is unclear. We identified two sulfated salicinoids, salicin-7-sulfate and salirepin-7-sulfate, in black cottonwood (Populus trichocarpa). Both compounds accumulated in high amounts in above-ground tissues including leaves, petioles, and stems, but were also found at lower concentrations in roots. A survey of salicin-7-sulfate and salirepin-7-sulfate in a subset of poplar (Populus sp.) and willow (Salix sp.) species revealed a broader distribution within the Salicaceae. To elucidate the formation of these compounds, we studied the sulfotransferase (SOT) gene family in P trichocarpa (PtSOT). One of the identified genes, PtSOT1, was shown to encode an enzyme able to convert salicin and salirepin into salicin-7-sulfate and salirepin-7-sulfate, respectively. The expression of PtSOT1 in different organs of P trichocarpa matched the accumulation of sulfated salicinoids in planta. Moreover, RNA interference-mediated knockdown of SOT1 in gray poplar (Populus × canescens) resulted in decreased levels of sulfated salicinoids in comparison to wild-type plants, indicating that SOT1 is responsible for their formation in planta. The presence of a nonfunctional SOT1 allele in black poplar (Populus nigra) was shown to correlate with the absence of salicin-7-sulfate and salirepin-7-sulfate in this species. Food choice experiments with leaves from wild-type and SOT1 knockdown trees suggest that sulfated salicinoids do not affect the feeding preference of the generalist caterpillar Lymantria dispar A potential role of the sulfated salicinoids in sulfur storage and homeostasis is discussed.

Journey in the Polyphenol Research World with Ragai Ibrahim

Dr. Ragai K. Ibrahim, Professor Emeritus at Concordia University, Montréal, Canada, passed away on the November 19, 2017 at the age of 88 years. Dr. Ibrahim dedicated his entire professional life to polyphenols and spent most of his academic career (1967-1997) at the Department of Biology of Concordia University in Montréal. He has been an active member of the Groupe Polyphénols since the beginning. This paper is a tribute to Dr. Ibrahim from some of his former students. An overview of the evolution of polyphenol research since the late 1950s and the outstanding contribution that Dr. Ibrahim had to this topic is given. The input of Dr. Ibrahim's research to the enzymology and genetics of polyphenol biosynthesis is discussed. Furthermore, the links between Dr. Ibrahim's work and some aspects of modern studies on the health benefits of polyphenols are presented.

Identification and characterization analysis of sulfotransferases (SOTs) gene family in cotton (Gossypium) and its involvement in fiber development

BACKGROUND

Sulfotransferases (SOTs) (EC 2.8.2.-) play a crucial role in the sulphate conjugation reaction involved in plant growth, vigor, stress resistance and pathogen infection. SOTs in Arabidopsis have been carried out and divided into 8 groups. However, the systematic analysis and functional information of SOT family genes in cotton have rarely been reported.

RESULTS

According to the results of BLASTP and HMMER, we isolated 46, 46, 76 and 77 SOT genes in the genome G. arboreum, G. raimondii, G. barbadense and G. hirsutum, respectively. A total of 170 in 245 SOTs were further classified into four groups based on the orthologous relationships comparing with Arabidopsis, and tandem replication primarily contributed to the expansion of SOT gene family in G. hirsutum. Expression profiles of the GhSOT showed that most genes exhibited a high level of expression in the stem, leaf, and the initial stage of fiber development. The localization analysis indicated that GhSOT67 expressed in cytoplasm and located in stem and leaf tissue. Additionally, the expression of GhSOT67 were induced and the length of stem and leaf hairs were shortened after gene silencing mediated by Agrobacterium, compared with the blank and negative control plants.

CONCLUSIONS

Our findings indicated that SOT genes might be associated with fiber development in cotton and provided valuable information for further studies of SOT genes in Gossypium.

A Novel Method for Identification and Quantification of Sulfated Flavonoids in Plants by Neutral Loss Scan Mass Spectrometry

Sulfur is present in plants in a large range of essential primary metabolites, as well as in numerous natural products. Many of these secondary metabolites contain sulfur in the oxidized form of organic sulfate. However, except of glucosinolates, very little is known about other classes of such sulfated metabolites, mainly because of lack of specific and quantitative analytical methods. We developed an LC-MS method to analyze sulfated flavonoids, a group of sulfated secondary metabolites prominent, e.g., in plants of the genus Flaveria. The method uses a linear gradient of methanol/formic acid in water on a Restek Raptor C₁₈ Core-Shell column for separation of the compounds. The sulfated flavonoids are detected by mass spectrometry (MS) in a negative mode, using a neutral loss of 80 Da after a collision induced dissociation. With this method we were also able to quantify the sulfated flavonoids. We could detect all (mono)sulfated flavonoids described before in Flaveria plus a number of new ones, such as isorhamnetin-sulfate-glycoside. In addition, we showed that sulfated flavonoids represent a substantial sulfur pool in Flaveria, larger than the thiols glutathione and cysteine. The individual species possess different sulfated flavonoids, but there is no correlation between the qualitative pattern and type of photosynthesis. Similar to other sulfur-containing secondary compounds, the concentration of sulfated flavonoids in leaves is reduced by sulfur starvation. The new LC-MS method will enable qualitative and quantitative detection of these secondary metabolites in plants as a pre-requisite to addressing their functions.

Sulfation pathways from red to green

Sulfur is present in the amino acids cysteine and methionine and in a large range of essential coenzymes and cofactors and is therefore essential for all organisms. It is also a constituent of sulfate esters in proteins, carbohydrates, and numerous cellular metabolites. The sulfation and desulfation reactions modifying a variety of different substrates are commonly known as sulfation pathways. Although relatively little is known about the function of most sulfated metabolites, the synthesis of activated sulfate used in sulfation pathways is essential in both animal and plant kingdoms. In humans, mutations in the genes encoding the sulfation pathway enzymes underlie a number of developmental aberrations, and in flies and worms, their loss-of-function is fatal. In plants, a lower capacity for synthesizing activated sulfate for sulfation reactions results in dwarfism, and a complete loss of activated sulfate synthesis is also lethal. Here, we review the similarities and differences in sulfation pathways and associated processes in animals and plants, and we point out how they diverge from bacteria and yeast. We highlight the open questions concerning localization, regulation, and importance of sulfation pathways in both kingdoms and the ways in which findings from these "red" and "green" experimental systems may help reciprocally address questions specific to each of the systems.

Evolution of chloroplast retrograde signaling facilitates green plant adaptation to land

The projected increase in drought severity and duration worldwide poses a significant threat to the health of terrestrial ecosystems. We reveal that unique genetic features of desiccation sensing and protection in streptophyte algae not only distinguish them from chlorophyte algae, but also represent a crucial evolutionary step that may have facilitated colonization and subsequent diversification of terrestrial habitats. We demonstrate the evolutionary significance of a molecular mechanism underlying how plants sense drought stress via the coordination of chloroplast retrograde signaling to trigger the closure of stomata, protecting vital photosynthetic tissue. Our findings constitute a significant step forward in understanding the evolution of plant drought tolerance, contributing to the diversification of terrestrial plant communities through past global climate transitions.

Chloroplast retrograde signaling networks are vital for chloroplast biogenesis, operation, and signaling, including excess light and drought stress signaling. To date, retrograde signaling has been considered in the context of land plant adaptation, but not regarding the origin and evolution of signaling cascades linking chloroplast function to stomatal regulation. We show that key elements of the chloroplast retrograde signaling process, the nucleotide phosphatase (SAL1) and 3′-phosphoadenosine-5′-phosphate (PAP) metabolism, evolved in streptophyte algae—the algal ancestors of land plants. We discover an early evolution of SAL1-PAP chloroplast retrograde signaling in stomatal regulation based on conserved gene and protein structure, function, and enzyme activity and transit peptides of SAL1s in species including flowering plants, the fern Ceratopteris richardii, and the moss Physcomitrella patens. Moreover, we demonstrate that PAP regulates stomatal closure via secondary messengers and ion transport in guard cells of these diverse lineages. The origin of stomata facilitated gas exchange in the earliest land plants. Our findings suggest that the conquest of land by plants was enabled by rapid response to drought stress through the deployment of an ancestral SAL1-PAP signaling pathway, intersecting with the core abscisic acid signaling in stomatal guard cells.

Structural and biochemical studies of sulphotransferase 18 from Arabidopsis thaliana explain its substrate specificity and reaction mechanism

Sulphotransferases are a diverse group of enzymes catalysing the transfer of a sulfuryl group from 3'-phosphoadenosine 5'-phosphosulphate (PAPS) to a broad range of secondary metabolites. They exist in all kingdoms of life. In Arabidopsis thaliana (L.) Heynh. twenty-two sulphotransferase (SOT) isoforms were identified. Three of those are involved in glucosinolate (Gl) biosynthesis, glycosylated sulphur-containing aldoximes containing chemically different side chains, whose break-down products are involved in stress response against herbivores, pathogens, and abiotic stress. To explain the differences in substrate specificity of desulpho (ds)-Gl SOTs and to understand the reaction mechanism of plant SOTs, we determined the first high-resolution crystal structure of the plant ds-Gl SOT AtSOT18 in complex with 3'-phosphoadenosine 5'-phosphate (PAP) alone and together with the Gl sinigrin. These new structural insights into the determination of substrate specificity were complemented by mutagenesis studies. The structure of AtSOT18 invigorates the similarity between plant and mammalian sulphotransferases, which illustrates the evolutionary conservation of this multifunctional enzyme family. We identified the essential residues for substrate binding and catalysis and demonstrated that the catalytic mechanism is conserved between human and plant enzymes. Our study indicates that the loop-gating mechanism is likely to be a source of the substrate specificity in plants.

Matching the Diversity of Sulfated Biomolecules: Creation of a Classification Database for Sulfatases Reflecting Their Substrate Specificity

Sulfatases cleave sulfate groups from various molecules and constitute a biologically and industrially important group of enzymes. However, the number of sulfatases whose substrate has been characterized is limited in comparison to the huge diversity of sulfated compounds, yielding functional annotations of sulfatases particularly prone to flaws and misinterpretations. In the context of the explosion of genomic data, a classification system allowing a better prediction of substrate specificity and for setting the limit of functional annotations is urgently needed for sulfatases. Here, after an overview on the diversity of sulfated compounds and on the known sulfatases, we propose a classification database, SulfAtlas (http://abims.sb-roscoff.fr/sulfatlas/), based on sequence homology and composed of four families of sulfatases. The formylglycine-dependent sulfatases, which constitute the largest family, are also divided by phylogenetic approach into 73 subfamilies, each subfamily corresponding to either a known specificity or to an uncharacterized substrate. SulfAtlas summarizes information about the different families of sulfatases. Within a family a web page displays the list of its subfamilies (when they exist) and the list of EC numbers. The family or subfamily page shows some descriptors and a table with all the UniProt accession numbers linked to the databases UniProt, ExplorEnz, and PDB.

Holophytochrome-Interacting Proteins in Physcomitrella: Putative Actors in Phytochrome Cytoplasmic Signaling

Phytochromes are the principle photoreceptors in light-regulated plant development, primarily acting via translocation of the light-activated photoreceptor into the nucleus and subsequent gene regulation. However, several independent lines of evidence indicate unambiguously that an additional cytoplasmic signaling mechanism must exist. Directional responses in filament tip cells of the moss Physcomitrella patens are steered by phy4 which has been shown to interact physically with the blue light receptor phototropin at the plasma membrane. This complex might perceive and transduce vectorial information leading to cytoskeleton reorganization and finally a directional growth response. We developed yeast two-hybrid procedures using photochemically functional, full-length phy4 as bait in Physcomitrella cDNA library screens and growth assays under different light conditions, revealing Pfr-dependent interactions possibly associated with phytochrome cytoplasmic signaling. Candidate proteins were then expressed in planta with fluorescent protein tags to determine their intracellular localization in darkness and red light. Of 14 candidates, 12 were confirmed to interact with phy4 in planta using bimolecular fluorescence complementation. We also used database information to study their expression patterns relative to those of phy4. We discuss the likely functional characteristics of these holophytochrome-interacting proteins (HIP's) and their possible roles in signaling.

The seed of the Amazonian fruit Couepia bracteosa exhibits higher scavenging capacity against ROS and RNS than its shell and pulp extracts

Couepia bracteosa is an interesting source of bioactive compounds which may be investigated for protecting human health against oxidative damage.

The multi-protein family of sulfotransferases in plants: composition, occurrence, substrate specificity, and functions

All members of the sulfotransferase (SOT, EC 2.8.2.-) protein family transfer a sulfuryl group from the donor 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to an appropriate hydroxyl group of several classes of substrates. The primary structure of these enzymes is characterized by a histidine residue in the active site, defined PAPS binding sites and a longer SOT domain. Proteins with this SOT domain occur in all organisms from all three domains, usually as a multi-protein family. Arabidopsis thaliana SOTs, the best characterized SOT multi-protein family, contains 21 members. The substrates for several plant enzymes have already been identified, such as glucosinolates, brassinosteroids, jasmonates, flavonoids, and salicylic acid. Much information has been gathered on desulfo-glucosinolate (dsGl) SOTs in A. thaliana. The three cytosolic dsGl SOTs show slightly different expression patterns. The recombinant proteins reveal differences in their affinity to indolic and aliphatic dsGls. Also the respective recombinant dsGl SOTs from different A. thaliana ecotypes differ in their kinetic properties. However, determinants of substrate specificity and the exact reaction mechanism still need to be clarified. Probably, the three-dimensional structures of more plant proteins need to be solved to analyze the mode of action and the responsible amino acids for substrate binding. In addition to A. thaliana, more plant species from several families need to be investigated to fully elucidate the diversity of sulfated molecules and the way of biosynthesis catalyzed by SOT enzymes.

Dual RNA-seq transcriptional analysis of wheat roots colonized by Azospirillum brasilense reveals up-regulation of nutrient acquisition and cell cycle genes

Background

The rapid growth of the world’s population demands an increase in food production that no longer can be reached by increasing amounts of nitrogenous fertilizers. Plant growth promoting bacteria (PGPB) might be an alternative to increase nitrogenous use efficiency (NUE) in important crops such wheat. Azospirillum brasilense is one of the most promising PGPB and wheat roots colonized by A. brasilense is a good model to investigate the molecular basis of plant-PGPB interaction including improvement in plant-NUE promoted by PGPB.

Results

We performed a dual RNA-Seq transcriptional profiling of wheat roots colonized by A. brasilense strain FP2. cDNA libraries from biological replicates of colonized and non-inoculated wheat roots were sequenced and mapped to wheat and A. brasilense reference sequences. The unmapped reads were assembled de novo. Overall, we identified 23,215 wheat expressed ESTs and 702 A. brasilense expressed transcripts. Bacterial colonization caused changes in the expression of 776 wheat ESTs belonging to various functional categories, ranging from transport activity to biological regulation as well as defense mechanism, production of phytohormones and phytochemicals. In addition, genes encoding proteins related to bacterial chemotaxi, biofilm formation and nitrogen fixation were highly expressed in the sub-set of A. brasilense expressed genes.

Conclusions

PGPB colonization enhanced the expression of plant genes related to nutrient up-take, nitrogen assimilation, DNA replication and regulation of cell division, which is consistent with a higher proportion of colonized root cells in the S-phase. Our data support the use of PGPB as an alternative to improve nutrient acquisition in important crops such as wheat, enhancing plant productivity and sustainability.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-378) contains supplementary material, which is available to authorized users.

A stress-inducible sulphotransferase sulphonates salicylic acid and confers pathogen resistance in Arabidopsis

Sulphonation of small molecules by cytosolic sulphotransferases in mammals is an important process in which endogenous molecules are modified for inactivation/activation of their biological effects. Plants possess large numbers of sulphotransferase genes, but their biological functions are largely unknown. Here, we present a functional analysis of the Arabidopsis sulphotransferase AtSOT12 (At2g03760). AtSOT12 gene expression is strongly induced by salt, and osmotic stress and hormone treatments. The T-DNA knock-out mutant sot12 exhibited hypersensitivity to NaCl and ABA in seed germination, and to salicylic acid (SA) in seedling growth. In vitro enzyme activity assay revealed that AtSOT12 sulphonates SA, and endogenous SA levels suggested that sulphonation of SA positively regulates SA production. Upon challenging with the pathogen Pseudomonas syringae, sot12 mutant and AtSOT12 over-expressing lines accumulate less and more SA, respectively, when compared with wild type. Consistent with the changes in SA levels, the sot12 mutant was more susceptible, while AtSOT12 over-expressing plants are more resistant to pathogen infection. Moreover, pathogen-induced PR gene expression in systemic leaves was significantly enhanced in AtSOT12 over-expressing plants. The role of sulphonation of SA in SA production, mobile signalling and acquired systemic resistance is discussed.

Structure of the two-domain hexameric APS kinase from Thiobacillus denitrificans: structural basis for the absence of ATP sulfurylase activity

The Tbd_0210 gene of the chemolithotrophic bacterium Thiobacillus denitrificans is annotated to encode a 60.5 kDa bifunctional enzyme with ATP sulfurylase and APS kinase activity. This putative bifunctional enzyme was cloned, expressed and structurally characterized. The 2.95 A resolution X-ray crystal structure reported here revealed a hexameric assembly with D(3) symmetry. Each subunit contains a large N-terminal sulfurylase-like domain and a C-terminal APS kinase domain reminiscent of the two-domain fungal ATP sulfurylases of Penicillium chrysogenum and Saccharomyces cerevisiae, which also exhibit a hexameric assembly. However, the T. denitrificans enzyme exhibits numerous structural and sequence differences in the N-terminal domain that render it inactive with respect to ATP sulfurylase activity. Surprisingly, the C-terminal domain does indeed display APS kinase activity, indicating that this gene product is a true APS kinase. Therefore, these results provide the first structural insights into a unique hexameric APS kinase that contains a nonfunctional ATP sulfurylase-like domain of unknown function.

Molecular and biochemical characterization of two brassinosteroid sulfotransferases from Arabidopsis, AtST4a (At2g14920) and AtST1 (At2g03760)

Mammalian sulfotransferases (EC 2.8.2) are involved in many important facets of steroid hormone activity and metabolism. In this study, Arabidopsis AtST4a and AtST1 were identified and characterized as brassinosteroid sulfotransferases that appear to be involved in different aspects of hormone regulation. The two proteins share 44% identity in amino acid sequence, and belong to different plant sulfotransferase families. AtST4a was specific for biologically active end products of the brassinosteroid pathway. The enzyme sulfated brassinosteroids with diverse side-chain structures, including 24-epibrassinosteroids and the naturally occurring (22R, 23R)-28-homobrassinosteroids. AtST4a belongs to a small subfamily of sulfotransferases having two other members, AtST4b and -c. Among the three recombinant enzymes, only AtST4a was catalytically active with brassinosteroids. Transcript expression of AtST4 subfamily members was largely specific to the root. AtST4b- and -c transcript levels were induced by treatment with trans-zeatin, while AtST4a was repressed under the same conditions, supporting a divergent function of AtST4a. Co-regulation of AtST4b and -c correlated with their location in tandem on chromosome 1. AtST1 was stereospecific for 24-epibrassinosteroids, with a substrate preference for the metabolic precursor 24-epicathasterone, and exhibited catalytic activity with hydroxysteroids and estrogens. To gain more insight into this dual activity with plant and mammalian steroids, enzymatic activities of human steroid sulfotransferases toward brassinosteroids were characterized. The dehydroepiandrosterone sulfotransferase SULT2A1 displayed catalytic activity with a selected set of 24-epibrassinolide precursors, including 24-epicathasterone, with specific activities comparable to that measured for the endogenous substrate dehydroepiandrosterone. The comparable activity profiles of AtST1 and SULT2A1 suggest a similar architecture of the acceptor-binding site between the two enzymes, and may potentially reflect a common ability to conjugate certain xenobiotics.

Biochemical and molecular characterization of flavonoid 7-sulfotransferase from Arabidopsis thaliana

Flavonoid compounds play important roles as flower pigments, stress metabolites formed in response to UV, during pollen germination and for polar auxin transport (Trends Plant Sci. 1 (1996) 377). Flavonoid sulfate esters are common in plants, especially the Asteraceae; however, due to the lack of information regarding the factors that regulate their accumulation, their exact role remains to be elucidated. The biosynthesis of flavonol sulfate esters is catalyzed by a number of position specific flavonol sulfotransferases (STs). An Arabidopsis thaliana database search has allowed us to identify and classify 18 putative ST coding sequences. We report here the cloning and characterization of the AtST3a member of this family that is expressed at early stages of seedling development and in the inflorescence stem and siliques of mature plants. The recombinant AtST3a protein exhibits strict specificity for position 7 of flavonoids. In contrast to previously characterized flavonol 7-ST from Flaveria bidentis that sulfonates only flavonol disulfates, AtST3a was found to accept as substrates a number of flavonols and flavone aglycones, as well as their monosulfate esters. The discovery of a flavonol ST from A. thaliana suggests that flavonol sulfates are more widely distributed than originally believed and this model plant could be used to study their biological significance.

The three desulfoglucosinolate sulfotransferase proteins in Arabidopsis have different substrate specificities and are differentially expressed

Sulfotransferases (SOTs) catalyse the transfer of a sulfate group from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to an appropriate hydroxy group of various substrates with the parallel formation of 3'-phosphoadenosine 5'-phosphate. In Arabidopsis thaliana, 18 SOT proteins (AtSOT) have been identified. Three of them, AtSOT16, AtSOT17 and AtSOT18, catalyse the sulfation of desulfoglucosinolates. The proteins were expressed in Escherichia coli, purified by affinity chromatography and used for enzyme kinetic studies. By establishing two types of enzyme assay using both 35S-labelled and unlabelled PAPS, separation of the products by HPLC, and detection of the products by monitoring radioactivity or UV absorption, the substrate specificities of the three AtSOT proteins were determined. They show different maximum velocities with several desulfoglucosinolates as substrates and differ in their affinity for desulfobenzylglucosinolate and PAPS. The sequences encoding AtSOT18 were amplified from Arabidopsis ecotypes C24 and Col0; the two expressed proteins differ in two out of 350 amino acids. These amino-acid variations led to different substrate specificities. Exchange of one of the two amino acids in AtSOT18 from C24 to the respective amino acid in AtSOT18 from Col0 gave the C24 protein the same substrate specificity as the wild-type AtSOT18 protein from Col0. All three desulfoglucosinolate AtSOT proteins are localized in the cytoplasm, as demonstrated by transient expression of fusion constructs with the green fluorescent protein in Arabidopsis protoplasts. Northern blot analysis indicated differential expression of the three AtSOT genes in plant organs and tissues at different developmental stages and during a light/darkness cycle. High (500 microM) and low (50 microM) sulfate concentrations in the medium did not influence the levels of expression.

A forty-year journey in plant research: original contributions to flavonoid biochemistry

This review highlights original contributions by the author to the field of flavonoid biochemistry during his research career of more than four decades. These include elucidation of novel aspects of some of the common enzymatic reactions involved in the later steps of flavonoid biosynthesis, with emphasis on methyltransferases, glucosyltransferases, sulfotransferases, and an oxoglutarate-dependent dioxygenase, as well as cloning, and inferences about phylogenetic relationships, of the genes encoding some of these enzymes. The three-dimensional structure of a flavonol O-methyltransferase was studied through homology-based modeling, using a caffeic acid O-methyltransferase as a template, to explain their strict substrate preferences. In addition, the biological significance of enzymatic prenylation of isoflavones, as well as their role as phytoanticipins and inducers of nodulation genes, are emphasized. Finally, the potential application of knowledge about the genes encoding these enzyme reactions is discussed in terms of improving plant productivity and survival, modification of flavonoid profiles, and the search for new compounds with pharmaceutical and (or) nutraceutical value.Key words: flavonoid enzymology, metabolite localization, gene cloning, 3-D structure, phylogeny.

Desulfoglucosinolate sulfotransferases from Arabidopsis thaliana catalyze the final step in the biosynthesis of the glucosinolate core structure

The phytotoxin coronatine is a structural analog of octadecanoid signaling molecules, which are well known mediators of plant defense reactions. To isolate novel coronatine-regulated genes from Arabidopsis thaliana, differential mRNA display was performed. Transcript levels of CORI-7 (coronatine induced-7) were rapidly and transiently increased in coronatine-treated plants, and the corresponding cDNA was found to encode the sulfotransferase AtST5a. Likewise, upon wounding, an immediate and transient increase in AtST5a mRNA levels could be observed in both locally wounded and unwounded (systemic) leaves. Furthermore, application of octadecanoids and ethylene as compounds involved in plant wound defense reactions resulted in AtST5a gene activation, whereas pathogen defense-related signals (yeast elicitor and salicylic acid) were inactive. AtST5a and its close homologs AtST5b and AtST5c were purified as His6-tagged proteins from Escherichia coli. The three enzymes were shown to catalyze the final step in the biosynthesis of the glucosinolate (GS) core structure, the sulfation of desulfoglucosinolates (dsGSs). They accept a broad range of dsGSs as substrates. However, in a competitive situation, AtST5a clearly prefers tryptophan- and phenylalanine-derived dsGSs, whereas long chain dsGSs derived from methionine are the preferred substrates of AtST5b and AtST5c. Treatment of Arabidopsis plants with low concentrations of coronatine resulted in an increase in the amounts of specific GSs, primarily glucobrassicin and neoglucobrassicin. Hence, it is suggested that AtST5a is the sulfotransferase responsible for the biosynthesis of tryptophan-derived GSs in vivo.

A proposed nomenclature system for the cytosolic sulfotransferase (SULT) superfamily

A nomenclature system for the cytosolic sulfotransferase (SULT) superfamily has been developed. The nomenclature guidelines were applied to 65 SULT cDNAs and 18 SULT genes that were characterized from eukaryotic organisms. SULT cDNA and gene sequences were identified by querying the GenBank databases and from published reports of their identification and characterization. These sequences were evaluated and named on the basis of encoded amino acid sequence identity and, in a few cases, a necessity to maintain historical naming convention. Family members share at least 45% amino acid sequence identity whereas subfamily members are at least 60% identical. cDNAs which encode amino acid sequences of at least 97% identity to each other were assigned identical isoform names. We also attempted to categorize orthologous enzymes between various species, where these have been identified, and the nomenclature includes a species descriptor. We present recommendations for the naming of allelic variants of SULT genes and their derived allozymes arising from single nucleotide polymorphisms and other genetic variation. The superfamily currently comprises 47 mammalian SULT isoforms, one insect isoform and eight plant enzymes, and collectively these sequences represent nine separate SULT families and 14 subfamilies. It is hoped that this nomenclature system will be widely adopted and that, as novel SULTs are identified and characterized, investigators will name their discoveries according to these guidelines.

Human estrogen sulfotransferase (SULT1E1) pharmacogenomics: gene resequencing and functional genomics

1. Estrogens are used as drugs and estrogen exposure is a risk factor for hormone-dependent diseases such as breast cancer. Sulfate conjugation is an important pathway for estrogen metabolism. The sulfotransferase (SULT) enzyme SULT1E1 has the lowest K(m) values for estrogens and catecholestrogens of the 10 known human SULT isoforms. 2. We previously cloned and characterized the human SULT1E1 cDNA and gene as steps toward pharmacogenetic studies. In the present experiments, we set out to determine whether common, functionally significant genetic polymorphisms might exist for SULT1E1. As a first step, we 'resequenced' the eight SULT1E1 exons and exon-intron splice junctions as well as portions of the 5'-flanking region using DNA from 60 African-American and 60 Caucasian-American subjects. 3. In all, 23 polymorphisms, 22 single nucleotide polymorphisms (SNPs) and one insertion deletion were observed. There were three nonsynonymous coding SNPs (cSNPs) that altered the following encoded amino acids: Asp22Tyr, Ala32Val and Pro253His. Among these, 12 pairs of SNPs were tightly linked. In addition, 12 unambiguous SULT1E1 haplotypes were identified, including six that were common to both populations studied. 4. Transient expression in COS-1 cells of constructs containing the three nonsynonymous cSNPs showed significant decreases in SULT1E1 activity for the Tyr22 and Val32 allozymes, with corresponding decreases in levels of immunoreactive protein. There were no changes in levels of either activity or immunoreactive protein for the His253 allozyme. Apparent K(m) values of the Val32 allozyme for the two cosubstrates for the reaction, 17beta-estradiol and 3'-phosphoadenosine 5'-phosphosulfate, were not significantly different from those of the wild-type enzyme, but there was a two- to three-fold increase in K(m) values for the His253 allozyme and a greater than five-fold increase for the Tyr22 allozyme. 5. These observations raise the possibility that genetically determined variation in SULT1E1-catalyzed estrogen sulfation might contribute to the pathophysiology of estrogen-dependent diseases as well as variation in the biotransformation of exogenously administered estrogens.

Characterization of porcine uterine estrogen sulfotransferase

A quantitative trait locus (QTL) for uterine capacity is located on chromosome 8. Comparison of porcine and human genetic maps suggested that the estrogen sulfotransferase (STE) gene may be located near this region. The objectives of this study were to clone the full coding region for STE, compare endometrial STE gene expression between Meishan and White composite pigs during early pregnancy, and map the STE gene. We obtained a clone (1886 bp) containing the full coding region of STE by iterative screening of an expressed sequence tag library. Endometrial STE mRNA expression in White composite gilts was determined by Northern blotting on days 10, 13, and 15 of the estrous cycle; and on days 10, 13, 15, 20, 30, and 40 of pregnancy. STE mRNA expression was elevated (P < 0.01) on days 20 and 30 of pregnancy compared to other days of the cycle or pregnancy. Endometrial STE mRNA expression during early pregnancy, determined using real-time RT-PCR, was elevated (P < 0.01) on day 20 compared to day 15, decreased (P = 0.02) between days 20 and 30, and decreased further (P < 0.01) between days 30 and 40 in both Meishan and White composite pigs. Expression of STE mRNA was greater (P = 0.01) in White composite pigs compared to Meishan pigs. Using a microsatellite from an STE containing BAC genomic clone, the STE gene was mapped to 65 centimorgans on chromosome 8. Because STE mRNA expression differs between Meishan and White composite pigs, the STE gene may be a candidate for the uterine capacity QTL.

Photoaffinity labeling probe for the substrate binding site of human phenol sulfotransferase (SULT1A1): 7-azido-4-methylcoumarin

A novel fluorescent photoactive probe 7-azido-4-methylcoumarin (AzMC) has been characterized for use in photoaffinity labeling of the substrate binding site of human phenol sulfotransferase (SULT1A1 or P-PST-1). For the photoaffinity labeling experiments, SULT1A1 cDNA was expressed in Escherichia coli as a fusion protein to maltose binding protein (MBP) and purified to apparent homogeneity over an amylose column. The maltose moiety was removed by Factor Xa cleavage. Both MBSULT1A1 and SULT1A1 were efficiently photolabeled with AzMC. This labeling was concentration dependent. In the absence of light, AzMC competitively inhibited the sulfation of 4MU catalyzed by SULT1A1 (Ki = 0.47 +/- 0.05 mM). Moreover, enzyme activity toward 2-naphthol was inactivated in a time- and concentration-dependent manner. SULT1A1 inactivation by AzMC was protected by substrate but was not protected by cosubstrate. These results indicate that photoaffinity labeling with AzMC is highly suitable for the identification of the substrate binding site of SULT1A1. Further studies are aimed at identifying which amino acids modified by AzMC are localized in the binding site.

Structure and function of HNK-1 sulfotransferase. Identification of donor and acceptor binding sites by site-directed mutagenesis

HNK-1 glycan, sulfo-->3GlcAbeta1-->3Galbeta1-->4GlcNAc-->R, is uniquely enriched in neural cells and natural killer cells and is thought to play important roles in cell-cell interaction. HNK-1 glycan synthesis is dependent on HNK-1 sulfotransferase (HNK-1ST), and cDNAs encoding human and rat HNK-1ST have been recently cloned. HNK-1ST belongs to the sulfotransferase gene family, which shares two homologous sequences in their catalytic domains. In the present study, we have individually mutated amino acid residues in these conserved sequences and determined how such mutations affect the binding to the donor substrate, adenosine 3'-phosphate 5'-phosphosulfate, and an acceptor. Mutations of Lys(128), Arg(189), Asp(190), Pro(191), and Ser(197) to Ala all abolished the enzymatic activity. When Lys(128) and Asp(190) were conservatively mutated to Arg and Glu, respectively, however, the mutated enzymes still maintained residual activity, and both mutant enzymes still bound to adenosine 3',5'-diphosphate-agarose. K128R and D190E mutant enzymes, on the other hand, exhibited reduced affinity to the acceptor as demonstrated by kinetic studies. These findings, together with those on the crystal structure of estrogen sulfotransferase and heparan sulfate N-deacetylase/sulfotransferase, suggest that Lys(128) may be close to the 3-hydroxyl group of beta-glucuronic acid in a HNK-1 acceptor. In contrast, the effect by mutation at Asp(190) may be due to conformational change because this amino acid and Pro(191) reside in a transition of the secondary structure of the enzyme. These results indicate that conserved amino acid residues in HNK-1ST play roles in maintaining a functional conformation and are directly involved in binding to donor and acceptor substrates.

Molecular cloning, expression, localisation and functional characterisation of a rabbit SULT1C2 sulfotransferase

The importance of sulfotransferases in xenobiotic metabolism is gaining recognition. The gastrointestinal (GI) tract is a major portal of entry for many xenobiotics, yet little is known about the contribution of sulfotransferases to detoxication or bioactivation metabolism in these tissues. To this end, isolation and characterisation of sulfotransferases expressed in the stomach of rabbits was undertaken. A unique sulfotransferase cDNA (GenBank Accession No. AF026304) was isolated from a rabbit stomach cDNA library. This cDNA was 1439 base pairs (bp) long and has an open reading frame of 888 bp. On expression of the cDNA in both COS cells and E. coli, a protein molecular weight of 34 kDa was detected on SDS-PAGE. Immunoblotting using an antibody raised in goats against the bacterially expressed protein detected expression of the protein in GI tract tissues. The 34 kDa immunoreactive band was detected in rabbit GI tract tissues (stomach, duodenum, jejunum, ileum, colon, caecum and rectum), liver and kidneys, but not in the lungs (n = 3). The human ortholog (GenBank Accession No AF026303) of the rabbit enzyme was cloned from a human stomach cDNA library. These two enzymes share 84% amino acid sequence identity and have been termed 1C2 sulfotransferases. When functional and kinetic characterisation of the recombinant rabbit and human proteins was carried out using 16 known ST substrates, detectable sulfonation activity was observed only with p-nitrophenol (with Km values of 2.2 mM and 13.3 mM, respectively). In conclusion, we have identified a rabbit GI tract sulfotransferase belonging to a newly defined sulfotransferase subfamily.

Bacterial expression and functional characterization of a rat thyroid hormone sulfotransferase, ST1B1

At least three forms of phenol sulfotransferase (ST) ST1B1, ST1A1 and ST1C1 are contained in rat livers. To identify the form contributing to the metabolism of 3,3',5-triiodothyronine (T3), functional characterization of these forms was performed by expression in Escherichia coli. ST1B1 and ST1C1 were shown to be active on sulfation towards T3 with high affinity (Km: 44.4 and 25.8 microM, respectively), whereas ST1A1 had low affinity. In Western blotting using antibodies raised against the individual ST, hepatic contents of each ST were quantitatively determined. ST1B1 showed no clear sex-difference, whereas the level of ST1C1 was higher in adult males than adult females. The content of ST1B1 was 1.4, 6.8 and 10 times higher than that of ST1C1 in adult males, adult females and both sexes of immature rats, respectively. The developmental pattern of ST1B1 was similar to that of ST1A1, but differed from that of ST1C1. These results indicate that ST1B1 and ST1C1 are involved in T3 metabolism in rats and ST1B1 is the constitutive form across sexes and ages.

Expression cloning and characterization of NSIST, a novel sulfotransferase expressed by a subset of neurons and postsynaptic targets

Synapses are distinguished by localized concentrations of specific proteins, many of which bear the marks of posttranslational processing such as glycosylation and sulfation. One strategy to elucidate this posttranslational tailoring is to identify the enzymes that create these modifications. Monoclonal antibody 3B3 recognizes a carbohydrate-containing epitope expressed on dystroglycan and other constituents of Torpedo electric organ synaptic membranes. We used mAb 3B3 in an immunofluorescence-based expression-cloning method and isolated a cDNA clone conferring mAb-3B3 immunoreactivity to transfected COS cells. The deduced polypeptide has a predicted molecular weight of 51 kDa, a type II transmembrane topology, and four potential N-linked glycosylation sites. The polypeptide, which we term NSIST (nervous system involved sulfotransferase), shows extensive, although not complete, homology to a chondroitin-6-sulfotransferase and limited homology to other sulfotransferases. In NSIST-transfected COS cells, 35SO4 incorporation and chondroitin-sulfate-like immunoreactivity are increased. In vivo, NSIST occurs as a single 2.4 kb transcript abundant in Torpedo electric organ, moderately expressed in spinal cord and electric lobe, and undetectable in non-neural tissues. Immunohistochemistry shows that NSIST is expressed in a punctate distribution in the innervated portion of electrocytes. In the CNS, NSIST-like immunoreactivity is localized within the somas of motor neurons and neurons of the electromotor nucleus, whereas mAb-3B3 immunostaining is associated with cell surfaces and neuropil. Neuronal NSIST is therefore likely to exert its effects extracellularly; although NSIST is synthesized by neurons, its product, the 3B3 epitope, is found outside neuronal cell bodies. Our evidence indicates that NSIST participates in nervous system specific posttranslational modifications, perhaps including those at synapses.

Expression cloning and characterization of NSIST, a novel sulfotransferase expressed by a subset of neurons and postsynaptic targets

Synapses are distinguished by localized concentrations of specific proteins, many of which bear the marks of posttranslational processing such as glycosylation and sulfation. One strategy to elucidate this posttranslational tailoring is to identify the enzymes that create these modifications. Monoclonal antibody 3B3 recognizes a carbohydrate-containing epitope expressed on dystroglycan and other constituents of Torpedo electric organ synaptic membranes. We used mAb 3B3 in an immunofluorescence-based expression-cloning method and isolated a cDNA clone conferring mAb-3B3 immunoreactivity to transfected COS cells. The deduced polypeptide has a predicted molecular weight of 51 kDa, a type II transmembrane topology, and four potential N-linked glycosylation sites. The polypeptide, which we term NSIST (nervous system involved sulfotransferase), shows extensive, although not complete, homology to a chondroitin-6-sulfotransferase and limited homology to other sulfotransferases. In NSIST-transfected COS cells, 35SO4 incorporation and chondroitin-sulfate-like immunoreactivity are increased. In vivo, NSIST occurs as a single 2.4 kb transcript abundant in Torpedo electric organ, moderately expressed in spinal cord and electric lobe, and undetectable in non-neural tissues. Immunohistochemistry shows that NSIST is expressed in a punctate distribution in the innervated portion of electrocytes. In the CNS, NSIST-like immunoreactivity is localized within the somas of motor neurons and neurons of the electromotor nucleus, whereas mAb-3B3 immunostaining is associated with cell surfaces and neuropil. Neuronal NSIST is therefore likely to exert its effects extracellularly; although NSIST is synthesized by neurons, its product, the 3B3 epitope, is found outside neuronal cell bodies. Our evidence indicates that NSIST participates in nervous system specific posttranslational modifications, perhaps including those at synapses.

Redefining reductive sulfate assimilation in higher plants: a role for APS reductase, a new member of the thioredoxin superfamily?

The reaction steps leading from the intermediate adenosine 5'-phosphosulfate (APS) to sulfide within the higher plant reductive sulfate assimilation pathway are the subject of controversy. Two pathways have been proposed: a 'bound intermediate' pathway in which the sulfo group of APS is first transferred by APS sulfotransferase to a carrier molecule to form a bound sulfite intermediate and is then further reduced by thiosulfonate reductase to bound sulfide; and a 'free intermediate' pathway in which APS is further activated to 3'-phosphoadenosine 5'-phosphosulfate (PAPS) by APS kinase followed by reduction of the sulfo group to free sulfite by PAPS reductase. Sulfite is then reduced to free sulfide by sulfite reductase. Sulfide, either free or bound, is then incorporated into organic form (as cysteine) by the enzyme O-acetylserine (thiol) lyase. In order to better characterize the pathway we attempted to clone PAPS reductase cDNAs by functional complementation of an Escherichia coli cysH mutant to prototrophy. We found no evidence for PAPS reductase cDNAs but did identify cDNAs that encode a small family of novel, chloroplast-localized proteins with APS reductase activity that are new members of the thioredoxin superfamily. We show here that the thioredoxin domain of these proteins is functional. We speculate that rather than proceeding via either of the pathways proposed above, reductive sulfate assimilation proceeds via the reduction of APS to sulfite by APS reductase and the subsequent reduction of sulfite to sulfide by sulfite reductase. In this scheme the product of the APS kinase reaction, PAPS, is not a direct intermediate in the pathway but rather acts as a substrate for sulfotransferase action and perhaps as a store of activated sulfate that can be returned to the pathway as APS via phosphohydrolase action on PAPS. Interactions between enzyme isoforms within the chloroplast stroma may bring about substrate channeling of APS and contribute to the partitioning of APS between sulfotransferase reactions on the one hand and the synthesis of cysteine and related metabolites via the reductive sulfate assimilation pathway on the other.

Mutational analysis of domain II of flavonol 3-sulfotransferase

The flavonol 3- and 4'-sulfotransferases (ST) from Flaveria chloraefolia catalyze the transfer of the sulfonate group from 3'-phosphoadenosine 5'-phosphosulfate (PAdoPS) to position 3 of flavonol aglycones and position 4' of flavonol 3-sulfates. We identified previously a protein segment, designated domain II, that contains all the determinants responsible for the specificity of these enzymes. Within domain II, at least five amino acids specific to the 4'-ST that could bind the sulfate group of quercetin 3-sulfate were identified. In this study, these amino acid residues were introduced at equivalent positions in the flavonol 3-ST sequence by site-directed mutagenesis of the cloned cDNA. No reversal of the substrate specificity was observed after the individual mutations. However, mutation of Leu95 to Tyr had different effects on the kinetic constants depending on the substitution pattern of the flavonoid B ring, suggesting that the tyrosine side chain may be in direct contact with this part of the molecule. The function of conserved amino acids present in domain II was also investigated. Unconservative mutations at Lys134, Tyr137 and Tyr150 resulted in protein instability in solution, suggesting that these residues might be important for the structural stability of the enzyme. Replacement of Arg140 with Lys or Ser had no effect on protein stability, but resulted in a strong reduction in specific activity. The results of photoaffinity-labeling experiments with PAdoP[35S]S suggest that this residue is required to bind the cosubstrate. In addition, the reduced affinity of [Ser140]ST for 3'-phosphoadenosine 5'-phosphate (PAdoP)-agarose indicates that Arg140 is also involved in binding the coproduct. Replacement of His118 with Glu or Ala resulted in a strong reduction in catalytic activity. However, [Lys118]ST retained a significant amount of catalytic activity. The results of photoaffinity-labeling experiments with PAdoP[35S]S and affinity chromatography on PAdoP-agarose suggest that His118 might be involved in catalysis in the flavonol 3-ST.

Molecular and biochemical characterization of two nucleoside diphosphate kinase cDNA clones from Flaveria bidentis

Two nucleoside diphosphate kinase cDNA clones have been isolated from Flaveria bidentis by immunoscreening of an expression library with a polyclonal antibody raised against Flaveria chloraefolia flavonol 3-sulfotransferase (F3-ST). The clones represent members of a small multigene family in this species. The nucleotide sequences of the two cDNA clones show a high degree of sequence similarity to other reported nucleoside diphosphate kinases (NDPKs), including the putative human tumor suppressor gene NM23 and the Drosophila regulatory gene. When these cDNA clones were expressed in Escherichia coli, their gene products exhibited NDPK enzymatic activity. The immunocross reaction of the clones with the antibody raised against the F3-ST suggests a common immuno-epitope and a similarity of a nucleotide binding site for the two proteins.

Molecular cloning of a sulfotransferase in Arabidopsis thaliana and regulation during development and in response to infection with pathogenic bacteria

A cDNA clone (RaRO47) encoding a sulfotransferase (ST) has been isolated from Arabidopsis cell suspensions. The deduced polypeptide of 302 amino acids is highly related to plant flavonol sulfotransferases (FSTs), characterized for the first time in Flaveria, and also to STs from animal tissue. The expression of the Arabidopsis ST gene(s) corresponding to RaR047 was examined during different developmental stages. It was found that, at the level of steady-state mRNA, expression of gene(s) encoding this ST was rapidly induced in the aerial parts of young seedlings, and during growth of Arabidopsis cell cultures. No expression could be detected in roots. Treatment of Arabidopsis seedlings with hormonal or stress-related compounds, showed that RaR047 mRNA accumulation was more particularly induced in response to salicylic acid and methyl jasmonate. Furthermore, in the leaves of mature plants or in cell suspensions, accumulation of RaR047 mRNA was observed upon infection with bacterial pathogens. This expression was observed preferentially in response to avirulent pathogens causing an hypersensitive reaction, as compared to virulent pathogens, which lead to disease.

Purification, characterization, and molecular cloning of a novel rat liver Dopa/tyrosine sulfotransferase

A novel sulfotransferase was purified from the rat liver cytosol to electrophoretic homogeneity via five column chromatography steps (hydroxylapatite I, DEAE Bio-Gel, ATP-agarose I, hydroxylapatite II, and ATP-agarose II). The minimum molecular weight of the purified enzyme was determined by sodium dodecyl sulfatepolyacrylamide gel electrophoresis to be approximately 33,000. Gel filtration chromatography revealed a native molecular weight of approximately 34,000, indicating the enzyme being present in the monomeric form. The purified sulfotransferase displayed enzymatic activities, with a pH optimum of 9.25, toward various tyrosine and 3,4-dihydroxyphenylalanine (Dopa) isomers, except DL-ortho-tyrosine. Thyroid hormones, as well as dopamine and p-nitrophenol, could also be used as substrates. The apparent Km value of the enzyme (designated the Dopa/tyrosine sulfotransferase) for L-Dopa, determined at a constant 14 microM of 3'-phosphoadenosine 5'-phosphosulfate, was 0.76 mM. The intact enzyme was found to be N-blocked when subjected to N-terminal sequencing. Three internal partial amino acid sequences, obtained by analyzing its proteolytic fragments, were found to be distinct from the homologous sequences of other known rat liver sulfotransferases. The deduced amino acid sequence of a full-length cDNA isolated from a rat liver cDNA library confirmed the identity of the Dopa/tyrosine sulfotransferase as a new type of aryl sulfotransferase. Upon transfection of COS-7 cells with an expression vector (pMSG-CMV) harboring the full-length cDNA, a 33-kDa protein displaying enzymatic and immunological properties similar to those of the purified Dopa/tyrosine sulfotransferase was expressed.

Identification of amino acid residues critical for catalysis and cosubstrate binding in the flavonol 3-sulfotransferase

The comparison of the deduced amino acid sequences of plant and animal sulfotransferases (ST) has allowed the identification of four well conserved regions, and previous experimental evidence suggested that regions I and IV might be involved in the binding of the cosubstrate, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). Moreover, region IV is homologous to the glycine-rich phosphate binding loop (P-loop) motif known to be involved in nucleotide phosphate binding in several protein families. In this study, the function of amino acid residues within these two regions was investigated by site-directed mutagenesis of the plant flavonol 3-ST. In region I, our results identify Lys59 as critical for catalysis, since replacement of this residue with alanine resulted in a 300-fold decrease in specific activity, while a 15-fold reduction was observed after the conservative replacement with arginine. Photoaffinity labeling of K59R and K59A with [35S]PAPS revealed that Lys59 is not required for cosubstrate binding. However, the K59A mutant had a reduced affinity for 3'-phosphoadenosine 5'-phosphate (PAP)-agarose, suggesting that Lys59 may participate in the stabilization of an intermediate during the reaction. In region IV, all substitutions of Arg276 resulted in a marked decrease in specific activity. Conservative and unconservative replacements of Arg276 resulted in weak photoaffinity labeling with [35S]PAPS and the R276A/T73A and R276E enzymes displayed reduced affinities for PAP-agarose, suggesting that the Arg276 side chain is required to bind the cosubstrate. The analysis of the kinetic constants of mutant enzymes at residues Lys277, Gly281, and Lys284 allowed to confirm that region IV is involved in cosubstrate binding.