Biochemical and biophysical characterization of the selenium-binding and reducing site in Arabidopsis thaliana homologue to mammals selenium-binding protein 1

Methanethiol: A Scent Mark of Dysregulated Sulfur Metabolism in Cancer

In order to cope with increased demands for energy and metabolites as well as to enhance stress resilience, tumor cells develop various metabolic adaptations, representing a hallmark of cancer. In this regard, the dysregulation of sulfur metabolism that may result in elevated levels of volatile sulfur compounds (VSCs) in body fluids, breath, and/or excretions of cancer patients has recently gained attention. Besides hydrogen sulfide (H2S), methanethiol is the predominant cancer-associated VSC and has been proposed as a promising biomarker for non-invasive cancer diagnosis. Gut bacteria are the major exogenous source of exposure to this foul-smelling toxic gas, with methanethiol-producing strains such as Fusobacterium nucleatum highly abundant in the gut microbiome of colorectal carcinoma (CRC) patients. Physiologically, methanethiol becomes rapidly degraded through the methanethiol oxidase (MTO) activity of selenium-binding protein 1 (SELENBP1). However, SELENBP1, which is considered a tumor suppressor, is often downregulated in tumor tissues, and this has been epidemiologically linked to poor clinical outcomes. In addition to impaired removal, an increase in methanethiol levels may derive from non-enzymatic reactions, such as a Maillard reaction between glucose and methionine, two metabolites enriched in cancer cells. High methionine concentrations in cancer cells may also result in enzymatic methanethiol production in mitochondria. Moreover, enzymatic endogenous methanethiol production may occur through methyltransferase-like protein 7B (METTL7B), which is present at elevated levels in some cancers, including CRC and hepatocellular carcinoma (HCC). In conclusion, methanethiol contributes to the scent of cancer as part of the cancer-associated signature combination of volatile organic compounds (VOCs) that are increasingly being exploited for non-invasive early cancer diagnosis.

Selenium-binding protein 1 (SELENBP1) is a copper-dependent thiol oxidase

Selenium-binding protein 1 (SELENBP1) was reported to act as a methanethiol oxidase (MTO) in humans, catalyzing the conversion of methanethiol to hydrogen peroxide, hydrogen sulfide and formaldehyde. Here, we identify copper ions as essential to this novel MTO activity. Site-directed mutagenesis of putative copper-binding sites in human SELENBP1 produced as recombinant protein in E. coli resulted in loss of its enzymatic function. On the other hand, the eponymous binding of selenium (as selenite) was no requirement for MTO activity and only moderately increased SELENBP1-catalyzed oxidation of methanethiol. Furthermore, SEMO-1, the SELENBP1 ortholog recently identified in the nematode C. elegans, also requires copper ions, and MTO activity was enhanced or abrogated, respectively, if worms were grown in the presence of cupric chloride or of a Cu chelator. In addition to methanethiol, we identified novel substrates of SELENBP1 from the group of volatile sulfur compounds, ranging from ethanethiol to 1-pentanethiol as well as 2-propene-1-thiol. Gut microbiome-derived methanethiol as well as food-derived volatile sulfur compounds (VSCs) account for malodors that may contribute to extraoral halitosis in humans, if not metabolized properly. As SELENBP1 is particularly abundant in tissues exposed to VSCs, such as colon, liver, and lung, it appears to contribute to copper-dependent VSC degradation.

Incorporation of Selenium Derived from Nanoparticles into Plant Proteins in Vivo

Diets comprising selenium-deficient crops have been linked to immune disorders and cardiomyopathy. Selenium nanoparticles (SeNPs) have emerged as a promising nanoplatform for selenium-biofortified agriculture. However, SeNPs fail to reach field-scale applications due to a poor understanding of the fundamental principles of its behavior. Here, we describe the transport, transformation, and bioavailability of SeNPs through a combination of in vivo and in vitro experiments. We show synthesized amorphous SeNPs, when sprayed onto the leaves of Arabidopsis thaliana, are rapidly biotransformed into selenium(IV), nonspecifically incorporated as selenomethionine (SeMet), and specifically incorporated into two selenium-binding proteins (SBPs). The SBPs identified were linked to stress and reactive oxygen species (mainly H2O2 and O2-) reduction, processes that enhance plant growth and primary root elongation. Selenium is transported both upwards and downwards in the plant when SeNPs are sprayed onto the leaves. With the application of Silwet L-77 (a common agrochemical surfactant), selenium distributed throughout the whole plant including the roots, where pristine SeNPs cannot reach. Our results demonstrate that foliar application of SeNPs promotes plant growth without causing nanomaterial accumulation, offering an efficient way to obtain selenium-fortified agriculture.

Evolutionary Aspects of Selenium Binding Protein (SBP)

Selenium-binding proteins represent a ubiquitous protein family and recently SBP1 was described as a new stress response regulator in plants. SBP1 has been characterized as a methanethiol oxidase, however its exact role remains unclear. Moreover, in mammals, it is involved in the regulation of anti-carcinogenic growth and progression as well as reduction/oxidation modulation and detoxification. In this work, we delineate the functional potential of certain motifs of SBP in the context of evolutionary relationships. The phylogenetic profiling approach revealed the absence of SBP in the fungi phylum as well as in most non eukaryotic organisms. The phylogenetic tree also indicates the differentiation and evolution of characteristic SBP motifs. Main evolutionary events concern the CSSC motif for which Acidobacteria, Fungi and Archaea carry modifications. Moreover, the CC motif is harbored by some bacteria and remains conserved in Plants, while modified to CxxC in Animals. Thus, the characteristic sequence motifs of SBPs mainly appeared in Archaea and Bacteria and retained in Animals and Plants. Our results demonstrate the emergence of SBP from bacteria and most likely as a methanethiol oxidase.

The SAH7 Homologue of the Allergen Ole e 1 Interacts with the Putative Stress Sensor SBP1 (Selenium-Binding Protein 1) in Arabidopsis thaliana

In this study, we focused on a member of the Ole e 1 domain-containing family, AtSAH7, in Arabidopsis thaliana. Our lab reports for the first time on this protein, AtSAH7, that was found to interact with Selenium-binding protein 1 (AtSBP1). We studied by GUS assisted promoter deletion analysis the expression pattern of AtSAH7 and determined that the sequence 1420 bp upstream of the transcription start can act as a minimal promoter inducing expression in vasculature tissues. Moreover, mRNA levels of AtSAH7 were acutely increased under selenite treatment in response to oxidative stress. We confirmed the aforementioned interaction in vivo, in silico and in planta. Following a bimolecular fluorescent complementation approach, we determined that the subcellular localization of the AtSAH7 and the AtSAH7/AtSBP1 interaction occur in the ER. Our results indicate the participation of AtSAH7 in a biochemical network regulated by selenite, possibly associated with responses to ROS production.

Illumina RNA and SMRT Sequencing Reveals the Mechanism of Uptake and Transformation of Selenium Nanoparticles in Soybean Seedlings

Selenium (Se) is an essential element for mammals, and its deficiency in the diet is a global problem. Agronomic biofortification through exogenous Se provides a valuable strategy to enhance human Se intake. Selenium nanoparticles (SeNPs) have been regarded to be higher bioavailability and less toxicity in comparison with selenite and selenate. Still, little has been known about the mechanism of their metabolism in plants. Soybean (Glycine max L.) can enrich Se, providing an ideal carrier for Se biofortification. In this study, soybean sprouts were treated with SeNPs, and a combination of next-generation sequencing (NGS) and single-molecule real-time (SMRT) sequencing was applied to clarify the underlying molecular mechanism of SeNPs metabolism. A total of 74,662 nonredundant transcripts were obtained, and 2109 transcription factors, 9687 alternative splice events, and 3309 long non-coding RNAs (lncRNAs) were predicted, respectively. KEGG enrichment analysis of the DEGs revealed that metabolic pathways, biosynthesis of secondary metabolites, and peroxisome were most enriched both in roots and leaves after exposure to SeNPs. A total of 117 transcripts were identified to be putatively involved in SeNPs transport and biotransformation in soybean. The top six hub genes and their closely coexpressed Se metabolism-related genes, such as adenylylsulfate reductase (APR3), methionine-tRNA ligase (SYM), and chloroplastic Nifs-like cysteine desulfurases (CNIF1), were screened by WGCNA and identified to play crucial roles in SeNPs accumulation and tolerance in soybean. Finally, a putative metabolism pathway of SeNPs in soybean was proposed. These findings have provided a theoretical foundation for future elucidation of the mechanism of SeNPs metabolism in plants.

Wheat Selenium-binding protein TaSBP-A enhances cadmium tolerance by decreasing free Cd2+ and alleviating the oxidative damage and photosynthesis impairment

Cadmium, one of the toxic heavy metals, robustly impact crop growth and development and food safety. In this study, the mechanisms of wheat (Triticum aestivum L.) selenium-binding protein-A (TaSBP-A) involved in response to Cd stress was fully investigated by overexpression in Arabidopsis and wheat. As a cytoplasm protein, TaSBP-A showed a high expression in plant roots and its expression levels were highly induced by Cd treatment. The overexpression of TaSBP-A enhanced Cd-toleration in yeast, Arabidopsis and wheat. Meanwhile, transgenic Arabidopsis under Cd stress showed a lower H2O2 and malondialdehyde content and a higher photochemical efficiency in the leaf and a reduction of free Cd2+ in the root. Transgenic wheat seedlings of TaSBP exhibited an increment of Cd content in the root, and a reduction Cd content in the leaf under Cd2+ stress. Cd2+ binding assay combined with a thermodynamics survey and secondary structure analysis indicated that the unique CXXC motif in TaSBP was a major Cd-binding site participating in the Cd detoxification. These results suggested that TaSBP-A can enhance the sequestration of free Cd2+ in root and inhibit the Cd transfer from root to leaf, ultimately conferring plant Cd-tolerance via alleviating the oxidative stress and photosynthesis impairment triggered by Cd stress.

Selenium-binding Protein 1 (SBD1): A stress response regulator in Chlamydomonas reinhardtii

Selenium-binding proteins (SBPs) represent a ubiquitous protein family implicated in various environmental stress responses, although the exact molecular and physiological role of the SBP family remains elusive. In this work, we report the identification and characterization of CrSBD1, an SBP homolog from the model microalgae Chlamydomonas reinhardtii. Growth analysis of the C. reinhardtii sbd1 mutant strain revealed that the absence of a functional CrSBD1 resulted in increased growth under mild oxidative stress conditions, although cell viability rapidly declined at higher hydrogen peroxide (H2O2) concentrations. Furthermore, a combined global transcriptomic and metabolomic analysis indicated that the sbd1 mutant exhibited a dramatic quenching of the molecular and biochemical responses upon H2O2-induced oxidative stress when compared to the wild-type. Our results indicate that CrSBD1 represents a cell regulator, which is involved in the modulation of C. reinhardtii early responses to oxidative stress. We assert that CrSBD1 acts as a member of an extensive and conserved protein-protein interaction network including Fructose-bisphosphate aldolase 3, Cysteine endopeptidase 2, and Glutaredoxin 6 proteins, as indicated by yeast two-hybrid assays.

Transcriptome, proteome, and metabolome reveal the mechanism of tolerance to selenate toxicity in Cardamine violifolia

Cardamine violifolia was found here to accumulate selenium (Se) to over 9000 mg kg^-1 dry weight. To investigate the mechanism of Se accumulation and tolerance in C. violifolia, metabolome, transcriptome, and proteome technologies were applied to C. violifolia seedlings treated with selenate. Several sulfate transporter (Sultr) genes (Sultr1;1, Sultr1;2, and Sultr2;1) and sulfur assimilatory enzyme genes showed high expression levels in response to selenate. Many calcium protein and cysteine-rich kinase genes of C. violifolia were downregulated, whereas selenium-binding protein 1 (SBP1) and protein sulfur deficiency-induced 2 (SDI2) of C. violifolia were upregulated by selenate. The expression of genes involved in the ribosome and posttranslational modifications and chaperones in C. violifolia were also detected in response to selenate. Based on the results of this study and previous findings, we suggest that the downregulated expression of calcium proteins and cysteine-rich kinases, and the upregulated expression of SBP1 and SDI2, were important contributors to the Se tolerance of C. violifolia. The downregulation of cysteine-rich kinases and calcium proteins would enhance Se tolerance of C. violifolia is a novel proposition that has not been reported on other Se hyperaccumulators. This study provides us novel insights to understand Se accumulation and tolerance in plants.

The glutathione peroxidase family of Theobroma cacao: Involvement in the oxidative stress during witches' broom disease

The glutathione peroxidases (GPXs) are enzymes which are part of the cell antioxidant system inhibiting the ROS-induced damages of membranes and proteins. In cacao (Theobroma cacao L.) genome, five GPX genes were identified. Cysteine insertion codons (UGU) were found in TcPHGPX, TcGPX2, TcGPX4, TcGPX6 and tryptophan insertion codon (UGG) in TcGPX8. Multiple alignments revealed conserved domains between TcGPXs and other plants and human GPXs. Homology modeling was performed using the Populus trichocarpa GPX5 structure as template, and the molecular modeling showed that TcGPXs have affinity with selenometionine in their active site. In silico analysis of the TcGPXs promoter region revealed the presence of conserved cis-elements related to biotic stresses and hormone responsiveness. The expression analysis of TcGPXs in cacao plantlet meristems infected by M. perniciosa showed that TcGPXs are most expressed in susceptible variety than in resistant one, mainly in disease stages in which oxidative stress and programmed cell death occurred. This data, associated with phylogenetic and location analysis suggested that TcGPXs may play a role in protecting cells from oxidative stress as a try of disease progression reduction. To our knowledge, this is the first study of the overall GPX family from T. cacao.

Metabolic Response of the Yeast Candida utilis During Enrichment in Selenium

Selenium (Se) was found to inhibit the growth of the yeast Candida utilis ATCC 9950. Cells cultured in 30 mg selenite/L supplemented medium could bind 1368 µg Se/g of dry weight in their structures. Increased accumulation of trehalose and glycogen was observed, which indicated cell response to stress conditions. The activity of antioxidative enzymes (glutathione peroxidase, glutathione reductase, thioredoxin reductase, and glutathione S-transferase) was significantly higher than that of the control without Se addition. Most Se was bound to water-insoluble protein fraction; in addition, the yeast produced 20–30 nm Se nanoparticles (SeNPs). Part of Se was metabolized to selenomethionine (10%) and selenocysteine (20%). The HPLC-ESI-Orbitrap MS analysis showed the presence of five Se compounds combined with glutathione in the yeast. The obtained results form the basis for further research on the mechanisms of Se metabolism in yeast cells.

Cellular alteration and differential protein profile explain effects of GA3 and ABA and their inhibitor on Trichocline catharinensis (Asteraceae) seed germination

Seed physiology of wild species has not been studied as deeply as that of domesticated crop species. Trichocline catharinensis (Asteraceae) is an endemic wildflower species from the high-altitude fields of southern Brazil. This species is of interest as a source of genes to improve cultivated Asteraceae because of its ornamental features, disease resistance and ability to tolerate drought and poor soil conditions. We studied the effects of abscisic acid (ABA) and gibberellic acid (GA₃ ) and their inhibitors, fluridone (FLU) and paclobutrazol (PAC), on seed germination. We individually assessed ultrastructural changes and differential protein accumulation. The principal component analysis explained 69.66% of differential accumulation for 32 proteins at phase II of seed germination in response to hormone and inhibitor treatment. GA₃ -imbibed seed germination (98.75%) resulted in increased protein accumulation to meet energy demand, redox regulation, and reserve metabolism activation. FLU-imbibed seeds showed a higher germination speed index as a consequence of metabolism activation. ABA-imbibed seeds (58.75%) showed osmotolerance and flattened cells in the hypocotyl-radicular axis, suggesting that ABA inhibits cell expansion. PAC-imbibed seeds remained at phase II for 300 h, and germination was suppressed (7.5%) because of the increased signaling proteins and halted reserve mobilization. Therefore, our findings provide insight into the behavior of Asteraceae non-dormant seed germination, which broadens our knowledge of seed germination in a wild and endemic plant species from a threatened ecosystem.

The reduction of selenium(IV) by boreal Pseudomonas sp. strain T5-6-I - Effects on selenium(IV) uptake in Brassica oleracea

Selenium (Se) is an essential micronutrient but toxic when taken in excessive amounts. Therefore, understanding the metabolic processes related to selenium uptake and bacteria-plant interactions coupled with selenium metabolism are of high importance. We cultivated Brassica oleracea with the previously isolated heterotrophic aerobic Se(IV)-reducing Pseudomonas sp. T5-6-I strain to better understand the phenomena of bacteria-mediated Se(IV) reduction on selenium availability to the plants. B. oleracea grown on Murashige and Skoog medium (MS-salt agar) with and without of Pseudomonas sp. were amended with Se(IV)/⁷⁵Se(IV), and selenium transfer into plants was studied using autoradiography and gamma spectroscopy. XANES was in addition used to study the speciation of selenium in the B. oleracea plants. In addition, the effects of Se(IV) on the protein expression in B. oleracea was studied using HPLC-SEC. TEM and confocal microscopy were used to follow the bacterial/Se-aggregate accumulation in plants and the effects of bacterial inoculation on root-hair growth. In the tests using ⁷⁵Se(IV) on average 130% more selenium was translocated to the B. oleracea plants grown with Pseudomonas sp. compared to the plants grown with selenium, but without Pseudomonas sp.. In addition, these bacteria notably increased root hair density. Changes in the protein expression of B. oleracea were observed on the ∼30-58 kDa regions in the Se(IV) treated samples, probably connected e.g. to the oxidative stress induced by Se(IV) or expression of proteins connected to the Se(IV) metabolism. Based on the XANES measurements, selenium appears to accumulate in B. oleracea mainly in organic C-Se-H and C-Se-C bonds with and without bacteria inoculation. We conclude that the Pseudomonas sp. T5-6-I strain seems to contribute positively to the selenium accumulation in plants, establishing the high potential of Se⁰-producing bacteria in the use of phytoremediation and biofortification of selenium.

A Caenorhabditis elegans ortholog of human selenium-binding protein 1 is a pro-aging factor protecting against selenite toxicity

Human selenium-binding protein 1 (SELENBP1) was originally identified as a protein binding selenium, most likely as selenite. SELENBP1 is associated with cellular redox and thiol homeostasis in several respects, including its established role as a methanethiol oxidase that is involved in degradation of methanethiol, a methionine catabolite, generating hydrogen sulfide (H₂S) and hydrogen peroxide (H₂O₂). As both H₂S and reactive oxygen species (such as H₂O₂) are major regulators of Caenorhabditis elegans lifespan and stress resistance, we hypothesized that a SELENBP1 ortholog in C. elegans would likely be involved in regulating these aspects. Here we characterize Y37A1B.5, a putative selenium-binding protein 1 ortholog in C. elegans with 52% primary structure identity to human SELENBP1. While conferring resistance to toxic concentrations of selenite, Y37A1B.5 also attenuates resistance to oxidative stress and lowers C. elegans lifespan: knockdown of Y37A1B.5 using RNA interference resulted in an approx. 10% increase of C. elegans lifespan and an enhanced resistance against the redox cycler paraquat, as well as enhanced motility. Analyses of transgenic reporter strains suggest hypodermal expression and cytoplasmic localization of Y37A1B.5, whose expression decreases with worm age. We identify the transcriptional coregulator MDT-15 and transcription factor EGL-27 as regulators of Y37A1B.5 levels and show that the lifespan extending effect elicited by downregulation of Y37A1B.5 is independent of known MDT-15 interacting factors, such as DAF-16 and NHR-49. In summary, Y37A1B.5 is an ortholog of SELENBP1 that shortens C. elegans lifespan and lowers resistance against oxidative stress, while allowing for a better survival under toxic selenite concentrations.

Novel interactions of Selenium Binding Protein family with the PICOT containing proteins AtGRXS14 and AtGRXS16 in Arabidopsis thaliana

During abiotic stress the primary symptom of phytotoxicity can be ROS production which is strictly regulated by ROS scavenging pathways involving enzymatic and non-enzymatic antioxidants. Furthermore, ROS are well-described secondary messengers of cellular processes, while during the course of evolution, plants have accomplished high degree of control over ROS and used them as signalling molecules. Glutaredoxins (GRXs) are small and ubiquitous glutathione (GSH) -or thioredoxin reductase (TR)-dependent oxidoreductases belonging to the thioredoxin (TRX) superfamily which are conserved in most eukaryotes and prokaryotes. In Arabidopsis thaliana GRXs are subdivided into four classes playing a central role in oxidative stress responses and physiological functions. In this work, we describe a novel interaction of AtGRXS14 with the Selenium Binding Protein 1 (AtSBP1), a protein proposed to be integrated in a regulatory network that senses alterations in cellular redox state and acts towards its restoration. We further show that SBP protein family interacts with AtGRXS16 that also contains a PICOT domain, like AtGRXS14.

Selenium-binding protein 1 (SELENBP1) is a marker of mature adipocytes

Selenium-binding protein 1 (SELENBP1) has recently been reported to catalyse the oxidation of methanethiol, an organosulfur compound produced by gut microbiota. Two of the reaction products of methanethiol oxidation, hydrogen peroxide and hydrogen sulphide, serve as signalling molecules for cell differentiation. Indeed, colonocyte differentiation has been found to be associated with SELENBP1 induction. Here, we show that SELENBP1 is induced when 3T3-L1 preadipocytes undergo terminal differentiation and maturation to adipocytes. SELENBP1 induction succeeded the up-regulation of known marker proteins of white adipocytes and the intracellular accumulation of lipids. Immunofluorescence microscopy revealed predominant cytoplasmic localisation of SELENBP1 in 3T3-L1 adipocytes, as demonstrated by co-staining with the key lipogenic enzyme, acetyl-CoA-carboxylase (ACC), located in cytosol. In differentiating 3T3-L1 cells, the mTOR inhibitor rapamycin and the pro-inflammatory cytokine tumour necrosis factor alpha (TNF-α) likewise suppressed SELENBP1 induction, adipocyte differentiation and lipid accumulation. However, lipid accumulation per se is not linked to SELENBP1 induction, as hepatic SELENBP1 was down-regulated in high fructose-fed mice despite increased lipogenesis in the liver and development of non-alcoholic fatty liver disease (NAFLD). In conclusion, SELENBP1 is a marker of cell differentiation/maturation rather than being linked to lipogenesis/lipid accumulation.

Silencing of a Meloidogyne incognita selenium-binding protein alters the cuticular adhesion of Pasteuria penetrans endospores

Pasteuria penetrans is an endospore forming hyperparasitic bacterium of the plant-pathogenic root-knot nematode, Meloidogyne incognita. For successful parasitization, the first step is adherence of bacterial endospores onto the cuticle surface of nematode juveniles. The knowledge of molecular intricacies involved during this adherence is sparse. Here, we identified a M. incognita selenium-binding protein (Mi-SeBP-1) differentially expressed during the initial interaction of M. incognita and P. penetrans, and show that it is involved in modulating parasitic adhesion of bacterial endospores onto nematode cuticle. Selenium-binding proteins (SeBPs) are selenium associated proteins important for growth regulation, tumor prevention and modulation of oxidation/reduction in cells. Although reported to be present in several nematodes, the function of SeBPs is not known in Phylum Nematoda. In situ hybridization assay localized the Mi-SeBP-1 mRNA to the hypodermal cells. RNAi-mediated silencing of Mi-SeBP-1 significantly increased the adherence of P. penetrans endospores to the nematode juvenile cuticle. Silencing of Mi-SeBP-1 did not change the nematode's ability to parasitize plants and reproduction potential within the host. These results suggest that M. incognita Mi-SeBP-1 might be involved in altering the attachment of microbial pathogens on the nematode cuticle, but is not involved in nematode-host plant interaction. This is the first report for a function of SeBP in Phylum Nematoda.

Delivery of selenium to selenophosphate synthetase for selenoprotein biosynthesis

BACKGROUND

Selenophosphate, the key selenium donor for the synthesis of selenoprotein and selenium-modified tRNA, is produced by selenophosphate synthetase (SPS) from ATP, selenide, and H₂O. Although free selenide can be used as the in vitro selenium substrate for selenophosphate synthesis, the precise physiological system that donates in vivo selenium substrate to SPS has not yet been characterized completely.

SCOPE OF REVIEW

In this review, we discuss selenium metabolism with respect to the delivery of selenium to SPS in selenoprotein biosynthesis.

MAJOR CONCLUSIONS

Glutathione, selenocysteine lyase, cysteine desulfurase, and selenium-binding proteins are the candidates of selenium delivery system to SPS. The thioredoxin system is also implicated in the selenium delivery to SPS in Escherichia coli.

GENERAL SIGNIFICANCE

Selenium delivered via a protein-bound selenopersulfide intermediate emerges as a central element not only in achieving specific selenoprotein biosynthesis but also in preventing the occurrence of toxic free selenide in the cell. This article is part of a Special Issue entitled "Selenium research in biochemistry and biophysics - 200 year anniversary".

Promoter analysis and functional implications of the selenium binding protein (SBP) gene family in Arabidopsis thaliana

Selenium Βinding Protein (SBP, originally termed SBP56) was identified in mouse liver as a cytosolic protein that could bind radioactive selenium. SBPs are highly conserved proteins present in a wide array of species across all kingdoms and are likely to be involved in selenium metabolism. In Arabidopsis, the selenium binding protein (SBP) gene family comprises three genes (AtSBP1, AtSBP2 and AtSBP3). AtSBP1 and AtSBP2 are clustered in a head-to-tail arrangement on chromosome IV, while AtSBP3 is located on chromosome III. In this work, we studied the promoter activity of the Arabidopsis SBP genes, determined their tissue specificity and showed that they are differentially regulated by sodium selenite and sodium selenate. All three SBP genes are upregulated in response to externally applied selenium compounds and the antioxidant NAC selectively downregulates SBP2. Although the effect on SBP2 levels was the most prominent, in all cases, the concurrent exposure of plants to selenite and the antioxidant supressed the expression of the SBP genes. We provide evidence that (at least) SBP1 expression is tightly linked to detoxification processes related to oxidative stress, since it is downregulated in the presence of NAC in selenium-treated plants. Furthermore, our results suggest that SBP genes may participate in the mechanisms that sense redox imbalance.

Determination of Proteinaceous Selenocysteine in Selenized Yeast

A method for the quantitation of proteinaceous selenocysteine (SeCys) in Se-rich yeast was developed. The method is based on the reduction of the Se-Se and S-Se bridges with dithiotretiol, derivatization with iodoacetamide (carbamidomethylation), followed by HPLC-ICP MS. The chromatographic conditions were optimized for the total recovery of the proteinaceous selenocysteine, the minimum number of peaks in the chromatogram (reduction of derivatization products of other Se-species present) and the baseline separation. A typical chromatogram of a proteolytic digest of selenized yeast protein consisted of up to five peaks (including SeMet, carbamidomethylated (CAM)-SeCys, and Se(CAM)₂) identified by retention time matching with available standards and electrospray MS. Inorganic selenium non-specifically attached to proteins and selenomethionine could be quantified (in the form of Se(CAM)₂) along with SeCys. Selenocysteine, selenomethionine, inorganic selenium, and the water soluble-metabolite fraction accounted for the totality of selenium species in Se-rich yeast.

A Role for Barley Calcium-Dependent Protein Kinase CPK2a in the Response to Drought

Increasing the drought tolerance of crops is one of the most challenging goals in plant breeding. To improve crop productivity during periods of water deficit, it is essential to understand the complex regulatory pathways that adapt plant metabolism to environmental conditions. Among various plant hormones and second messengers, calcium ions are known to be involved in drought stress perception and signaling. Plants have developed specific calcium-dependent protein kinases that convert calcium signals into phosphorylation events. In this study we attempted to elucidate the role of a calcium-dependent protein kinase in the drought stress response of barley (Hordeum vulgare L.), one of the most economically important crops worldwide. The ongoing barley genome project has provided useful information about genes potentially involved in the drought stress response, but information on the role of calcium-dependent kinases is still limited. We found that the gene encoding the calcium-dependent protein kinase HvCPK2a was significantly upregulated in response to drought. To better understand the role of HvCPK2a in drought stress signaling, we generated transgenic Arabidopsis plants that overexpressed the corresponding coding sequence. Overexpressing lines displayed drought sensitivity, reduced nitrogen balance index (NBI), an increase in total chlorophyll content and decreased relative water content. In addition, in vitro kinase assay experiments combined with mass spectrometry allowed HvCPK2a autophosphorylation sites to be identified. Our results suggest that HvCPK2a is a dual-specificity calcium-dependent protein kinase that functions as a negative regulator of the drought stress response in barley.

Identification of novel transcriptional regulators of Zat12 using comprehensive yeast one-hybrid screens

To appropriately acclimate to environmental stresses, plants have to rapidly activate a specific transcriptional program. Yet, the identity and function of many of the transcriptional regulators that mediate early responses to abiotic stress stimuli is still unknown. In this work we employed the promoter of the multi-stress-responsive zinc-finger protein Zat12 in yeast one-hybrid (Y1H) screens to identify early abiotic stress-responsive transcriptional regulators. Analysis of Zat12 promoter fragments fused to luciferase underlined an approximately 200 bp fragment responsive to NaCl and to reactive oxygen species (ROS). Using these segments and others as baits against Y1H control or stress Arabidopsis prey libraries, we identified 15 potential Zat12 transcriptional regulators. Among the prominent proteins identified were known transcription factors including bZIP29 and ANAC91 as well as unknown function proteins such as a homolog of the human USB1, a U6 small nuclear RNA (snRNA) processing protein, and dormancy/auxin-associated family protein 2 (DRM2). Altered expression of Zat12 during high light stress in the knockout mutants further indicated the involvement of these proteins in the regulation of Zat12. Using a state of the art microfluidic approach we showed that AtUSB1 and DRM2 can specifically bind dsDNA and were able to identify the preferred DNA-binding motif of all four proteins. Overall, the proteins identified in this work provide an important start point for charting the earliest signaling network of Zat12 and of other genes required for acclimation to abiotic stresses.

A Critical Role for Cysteine 57 in the Biological Functions of Selenium Binding Protein-1

The concentration of selenium-binding protein1 (SBP1) is often lower in tumors than in the corresponding tissue and lower levels have been associated with poor clinical outcomes. SBP1 binds tightly selenium although what role selenium plays in its biological functions remains unknown. Previous studies indicated that cysteine 57 is the most likely candidate amino acid for selenium binding. In order to investigate the role of cysteine 57 in SBP1, this amino acid was altered to a glycine and the mutated protein was expressed in human cancer cells. The SBP1 half-life, as well as the cellular response to selenite cytotoxicity, was altered by this change. The ectopic expression of SBP1^GLY also caused mitochondrial damage in HCT116 cells. Taken together, these results indicated that cysteine 57 is a critical determinant of SBP1 function and may play a significant role in mitochondrial function.

Regulation of Selenium Metabolism and Transport

Selenium is regulated in the body to maintain vital selenoproteins and to avoid toxicity. When selenium is limiting, cells utilize it to synthesize the selenoproteins most important to them, creating a selenoprotein hierarchy in the cell. The liver is the central organ for selenium regulation and produces excretory selenium forms to regulate whole-body selenium. It responds to selenium deficiency by curtailing excretion and secreting selenoprotein P (Sepp1) into the plasma at the expense of its intracellular selenoproteins. Plasma Sepp1 is distributed to tissues in relation to their expression of the Sepp1 receptor apolipoprotein E receptor-2, creating a tissue selenium hierarchy. N-terminal Sepp1 forms are taken up in the renal proximal tubule by another receptor, megalin. Thus, the regulated whole-body pool of selenium is shifted to needy cells and then to vital selenoproteins in them to supply selenium where it is needed, creating a whole-body selenoprotein hierarchy.