Selenoprotein Synthesis Research Articles

Selenoproteinless animals: Selenophosphate synthetase SPS1 functions in a pathway unrelated to selenocysteine biosynthesis

Proteins containing the 21st amino acid, selenocysteine (Sec), have been described in all three domains of life, but the composition of selenoproteomes in organisms varies significantly. Here, we report that aquatic arthropods possess many selenoproteins also detected in other animals and unicellular eukaryotes, and that most of these proteins were either lost or replaced with cysteine-containing homologs in insects. As a result of this selective selenoproteome reduction, fruit flies and mosquitoes have three known selenoproteins, and the honeybee, Apis mellifera, a single detected candidate selenoprotein. Moreover, we identified the red flour beetle, Tribolium castaneum, and the silkworm, Bombyx mori, as the first animals that lack any Sec-containing proteins. These insects also lost the Sec biosynthesis and insertion machinery, but selenophosphate synthetase 1 (SPS1), an enzyme previously implicated in Sec biosynthesis, is present in all insects, including T. castaneum and B. mori. These data indicate that SPS1 functions in a pathway unrelated to selenoprotein synthesis. Since SPS1 evolved from a protein that utilizes selenium for Sec biosynthesis, an attractive possibility is that SPS1 may define a new pathway of selenium utilization in animals.

Factors and Selenocysteine Insertion Sequence Requirements for the Synthesis of Selenoproteins from a Gram-Positive Anaerobe in Escherichia coli

Selenoprotein synthesis in Escherichia coli strictly depends on the presence of a specific selenocysteine insertion sequence (SECIS) following the selenocysteine-encoding UGA codon of the respective mRNA. It is recognized by the selenocysteine-specific elongation factor SelB, leading to cotranslational insertion of selenocysteine into the nascent polypeptide chain. The synthesis of three different selenoproteins from the gram-positive anaerobe Eubacterium acidaminophilum in E. coli was studied. Incorporation of (75)Se into glycine reductase protein B (GrdB1), the peroxiredoxin PrxU, and selenophosphate synthetase (SelD1) was negligible in an E. coli wild-type strain and was fully absent in an E. coli SelB mutant. Selenoprotein synthesis, however, was strongly increased if selB and selC (tRNA(Sec)) from E. acidaminophilum were coexpressed. Putative secondary structures downstream of the UGA codons did not show any sequence similarity to each other or to the E. coli SECIS element. However, mutations in these structures strongly reduced the amount of (75)Se-labeled protein, indicating that they indeed act as SECIS elements. UGA readthrough mediated by the three different SECIS elements was further analyzed using gst-lacZ translational fusions. In the presence of selB and selC from E. acidaminophilum, UGA readthrough was 36 to 64% compared to the respective cysteine-encoding UGC variant. UGA readthrough of SECIS elements present in Desulfomicrobium baculatum (hydV), Treponema denticola (selD), and Campylobacter jejuni (selW-like gene) was also considerably enhanced in the presence of E. acidaminophilum selB and selC. This indicates recognition of these SECIS elements and might open new perspectives for heterologous selenoprotein synthesis in E. coli.

Ex vivo correction of selenoprotein N deficiency in rigid spine muscular dystrophy caused by a mutation in the selenocysteine codon

Premature termination of translation due to nonsense mutations is a frequent cause of inherited diseases. Therefore, many efforts were invested in the development of strategies or compounds to selectively suppress this default. Selenoproteins are interesting candidates considering the idiosyncrasy of the amino acid selenocysteine (Sec) insertion mechanism. Here, we focused our studies on SEPN1, a selenoprotein gene whose mutations entail genetic disorders resulting in different forms of muscular diseases. Selective correction of a nonsense mutation at the Sec codon (UGA to UAA) was undertaken with a corrector tRNASec that was engineered to harbor a compensatory mutation in the anticodon. We demonstrated that its expression restored synthesis of a full-length selenoprotein N both in HeLa cells and in skin fibroblasts from a patient carrying the mutated Sec codon. Readthrough of the UAA codon was effectively dependent on the Sec insertion machinery, therefore being highly selective for this gene and unlikely to generate off-target effects. In addition, we observed that expression of the corrector tRNASec stabilized the mutated SEPN1 transcript that was otherwise more subject to degradation. In conclusion, our data provide interesting evidence that premature termination of translation due to nonsense mutations is amenable to correction, in the context of the specialized selenoprotein synthesis mechanism.

Loss of housekeeping selenoprotein expression in mouse liver modulates lipoprotein metabolism

Selenium is incorporated into proteins as selenocysteine (Sec), which is dependent on its specific tRNA, designated tRNA [Ser]Sec. Targeted removal of the tRNA [Ser]Sec gene ( Trsp) in mouse hepatocytes previously demonstrated the importance of selenoproteins in liver function. Herein, analysis of plasma proteins in this Trsp knockout mouse revealed increases in apolipoprotein E (ApoE) that was accompanied by elevated plasma cholesterol levels. The expression of genes involved in cholesterol biosynthesis, metabolism and transport were also altered in knockout mice. Additionally, in two transgenic Trsp mutant mouse lines (wherein only housekeeping selenoprotein synthesis was restored), the expression of ApoE, as well as genes involved in cholesterol biosynthesis, metabolism and transport were similar to those observed in wild type mice. These data correlate with reports that selenium deficiency results in increased levels of ApoE, indicating for the first time that housekeeping selenoproteins have a role in regulating lipoprotein biosynthesis and metabolism.

SBP2 Binding Affinity Is a Major Determinant in Differential Selenoprotein mRNA Translation and Sensitivity to Nonsense-Mediated Decay

Selenoprotein mRNAs are potential targets for degradation via nonsense-mediated decay due to the presence of in-frame UGA codons that can be decoded as either selenocysteine or termination codons. When UGA decoding is inefficient, as occurs when selenium is limiting, termination occurs at these positions. Based on the predicted exon-intron structure, 14 of the 25 human selenoprotein mRNAs are predicted to be sensitive to nonsense-mediated decay. Among these, sensitivity varies widely, resulting in a hierarchy of preservation or degradation of selenoprotein mRNAs and, thus, of selenoprotein synthesis. Potential factors in dictating the hierarchy of selenoprotein synthesis are the Sec insertion sequence RNA-binding proteins, SBP2 and nucleolin. To investigate the mechanistic basis for this hierarchy and the role of these two proteins, we carried out knockdowns of SBP2 expression and assessed the effects on selenoprotein mRNA levels. We also investigated in vivo binding of selenoprotein mRNAs by SBP2 and nucleolin via immunoprecipitation of the proteins and quantitation of bound mRNAs. We report that SBP2 exhibits strong preferential binding to some selenoprotein mRNAs over others, whereas nucleolin exhibits minimal differences in binding. Thus, SBP2 is a major determinant in dictating the hierarchy of selenoprotein synthesis via differential selenoprotein mRNA translation and sensitivity to nonsense-mediated decay.

Selenium–Mercury Interactions in Man and Animals

Selenium-mercury interactions were most extensively studied in relation to alleviation of Hg toxicity by added selenium. This presentation considers the influence of mercury on endogenous selenium, on its tissue and cellular "status" after lifelong or acute exposure to mercury vapor (Hg o). Discussed are data obtained from (1) humans living near or working in a mercury mine, and (2) rats experimentally exposed in the mine. Mercury vapor is unique--or similar to methylmercury--because of its ability to penetrate cell membranes and so invade all cells, where it is oxidized in the biologically active form (Hg++) by catalase. Such in situ-generated ions can react with endogenously generated highly reactive Se metabolites, like HSe-, and render a part of the selenium unavailable for selenoprotein synthesis. Data on human populations indicate that in moderate Hg exposure combined with an adequate selenium supply through diet, Se bioavailability can be preserved. On the other hand, the results of an acute exposure study emphasize the dual role of selenium in mercury detoxification. Besides the well-known Se coaccumulation through formation of nontoxic Hg-Se complexes, we observed noticeable Se (co)excretion, at least at the beginning of exposure. The higher Hg accumulation rate in the group of animals with lower basal selenium levels can also point to selenium involvement in mercury excretion. In such conditions there is a higher probability for decreased selenoprotein levels (synthesis) in some tissues or organs, depending on the synthesis hierarchy.

From Selenium to Selenoproteins: Synthesis, Identity, and Their Role in Human Health

The requirement of the trace element selenium for life and its beneficial role in human health has been known for several decades. This is attributed to low molecular weight selenium compounds, as well as to its presence within at least 25 proteins, named selenoproteins, in the form of the amino acid selenocysteine (Sec). Incorporation of Sec into selenoproteins employs a unique mechanism that involves decoding of the UGA codon. This process requires multiple features such as the selenocysteine insertion sequence (SECIS) element and several protein factors including a specific elongation factor EFSec and the SECIS binding protein 2, SBP2. The function of most selenoproteins is currently unknown; however, thioredoxin reductases (TrxR), glutathione peroxidases (GPx) and thyroid hormone deiodinases (DIO) are well characterised selenoproteins involved in redox regulation of intracellular signalling, redox homeostasis and thyroid hormone metabolism. Recent evidence points to a role for selenium compounds as well as selenoproteins in the prevention of some forms of cancer. A number of clinical trials are either underway or being planned to examine the effects of selenium on cancer incidence. In this review we describe some of the recent progress in our understanding of the mechanism of selenoprotein synthesis, the role of selenoproteins in human health and disease and the therapeutic potential of some of these proteins.

Syndromes of reduced sensitivity to thyroid hormone: genetic defects in hormone receptors, cell transporters and deiodination

At least six major steps are required for secreted thyroid hormone (TH) to exert its action on target tissues. Mutations interfering with three of these steps have been so far identified. The first recognized defect, which causes resistance to TH, involves the TH receptor beta gene and has been given the acronym RTH. Occurring in approximately 1 per 40,000 newborns, more than 1000 affected subjects, from 339 families, have been identified. The gene defect remains unknown in 15% of subjects with RTH. Two novel syndromes causing reduced sensitivity to TH were recently identified. One, producing severe psychomotor defects in > 100 males from 26 families, is caused by mutations in the cell-membrane transporter of TH, MCT8; the second, affecting the intracellular metabolism of TH in four individuals from two families, is caused by mutations in the SECISBP2 gene, which is required for the synthesis of selenoproteins, including TH deiodinases.

Selenophosphate synthetase 2 is essential for selenoprotein biosynthesis

Selenophosphate synthetase (SelD) generates the selenium donor for selenocysteine biosynthesis in eubacteria. One homologue of SelD in eukaryotes is SPS1 (selenophosphate synthetase 1) and a second one, SPS2, was identified as a selenoprotein in mammals. Earlier in vitro studies showed SPS2, but not SPS1, synthesized selenophosphate from selenide, whereas SPS1 may utilize a different substrate. The roles of these enzymes in selenoprotein synthesis in vivo remain unknown. To address their function in vivo, we knocked down SPS2 in NIH3T3 cells using small interfering RNA and found that selenoprotein biosynthesis was severely impaired, whereas knockdown of SPS1 had no effect. Transfection of SPS2 into SPS2 knockdown cells restored selenoprotein biosynthesis, but SPS1 did not, indicating that SPS1 cannot complement SPS2 function. These in vivo studies indicate that SPS2 is essential for generating the selenium donor for selenocysteine biosynthesis in mammals, whereas SPS1 probably has a more specialized, non-essential role in selenoprotein metabolism.

Apoptotic cellular events for selenium compounds involved in cancer prevention

Converging data from epidemiological, ecological, and clinical studies have shown that selenium (Se) can decrease the risk for some types of human cancers. Induction of apoptosis is considered an important cellular event that can account for the cancer preventive effects of Se. Prior to occurrence of apoptosis, Se compounds alter the expression and/or activities of signaling molecules, mitochondria-associated factors, transcriptional factors, tumor suppressor genes, and cellular reduced glutathione. Mechanistic studies have demonstrated that the methylselenol metabolite pool has many desirable attributes of chemoprevention, whereas the hydrogen selenide pool with excess of selenoprotein synthesis can lead to DNA single-strand breaks. To elucidate the effects of Se on cytotoxic events, it should be remembered that the chemical forms and the dose of Se, and the experimental system used, are determinants of its biological activities. This mini-review focuses on elucidation of the molecular mechanisms of cancer prevention by Se with the apoptotic approach.

Differential regulation of expression of cytosolic and mitochondrial thioredoxin reductase in rat liver and kidney

Adequate supply of selenium (Se) is critical for synthesis of selenoproteins through selenocysteine insertion mechanism. To explore this process we investigated the expression of the cytosolic and mitochondrial isoenzymes of thioredoxin reductase (TrxR1 and TrxR2) in response to altered Se supply. Rats were fed diets containing different quantities of selenium and the levels of TrxR1 and TrxR2 protein and their corresponding mRNAs were determined in liver and kidney. Expression of the two isoenzymes was differentially affected, with TrxR1 being more sensitive to Se depletion than TrxR2 and greater changes in liver than kidney. In order to determine if the selenocysteine incorporation sequence (SECIS) element was critical in this response liver and kidney cell lines (H4 and NRK-52E) were transfected with reporter constructs in which expression of luciferase required read-through at a UGA codon and which contained either the TrxR1 or TrxR2 3′UTR, or a combination of the TrxR1 5′ and 3′UTRs. Cell lines expressing constructs with the TrxR1 3′UTR demonstrated no response to restricted Se supply. In comparison the Se-deficient cells expressing constructs with the TrxR2 3′UTR showed considerably less luciferase activity than the Se-adequate cells. No disparity of response to Se supply was observed in the constructs containing the different TrxR1 5′UTR variants. The data show that there is a prioritisation of TrxR2 over TrxR1 during Se deficiency such that TrxR1 expression is more sensitive to Se supply than TrxR2 but this sensitivity of TrxR1 was not fully accounted for by TrxR1 5′ or 3′UTR sequences when assessed using luciferase reporter constructs.

Impaired Selenoprotein Synthesis Caused by SBP2 Gene Mutations Manifests as an Inherited Defect in Intracellular Thyroid Hormone Metabolism

Impaired Selenoprotein Synthesis Caused by SBP2 Gene Mutations Manifests as an Inherited Defect in Intracellular Thyroid Hormone Metabolism

Soluble Liver Antigen/Liver and Pancreas, an Antigen in Autoimmune Hepatitis Patients: Influence on Selenocysteine Synthesis and its Complex with HSP70

Autoimmune hepatitis (AIH) serum contains autoantibodies against solubl el iver antigen/liver and pancreas (SLA/LP), a protein that interacts with the selenocysteine (Sec)-tRNA Sec .W e investigated the infl uence of AIH serum on Sec production. Two (#3 and #4) of four AIH sera inhibited its synthesis, with serum #4 showing the strongest inhibition. This did not parallel the level of antibodies to SLA/LP (anti-SLA/LP) in sera, because the titer in serum #3 was higher than that in serum #4. Thus, w ef ound that AIH inhibited Sec synthesis but we could not conclude that this inhibition depended on an autoantibody against SLA/LP. To clarify the function of SLA/LP, we prepared an antibody against an SLA/LP peptide. Inhibition of Sec synthesis by this anti-peptide antibody was incomplete and sometimes did not occur. Absorption of autoantibodies in the AIH serum with the SLA/LP protein did not have any effects on Sec synthesis by Sec synthase (SecS) which converts Seryl (Ser)-tRNA Sec to Sec-tRNA Sec .W e then used SLA/L Pi nstead of active bovine SecS but we could not detect SecS activity in SLA/LP itself. Next, we studied the distribution of SLA/LP in HEK293 cells transfected with an SLA/LP expression vector. Immunofluorescence microscopy detected SLA/LP expression in the cytoplasm, but not the nucleus of HEK293 cells. Higher levels of SLA/LP in the cytosol did not correlate with higher Sec production, which remained identical to mock-transfected cells. Interestingly, we found that S LA/LP formed a complex in the cytosol of HEK293 cells. This complex was purified and the components analy zed using Nano-LC and MS/MS, which we identified as HSP7 0a nd tubulin. Altogether, our data suggest that SLA/LP may play an indirect role to produce Sec in the selenoprotein synthesis machinery.

Impact of Trivalent Arsenicals on Selenoprotein Synthesis

BackgroundExposure to arsenic has been associated with development of skin, lung, bladder, liver, and kidney cancer. Recent evidence suggests that an increase in oxidative stress in cells treated with arsenicals represents the molecular mechanism behind arsenic-induced carcinogenesis. Selenium, in the form of selenocysteine, is necessary for the activity of several enzymes with a role in defense against reactive oxygen species. A mutual sparing effect between arsenic and selenium has been shown in animal studies when both metalloids are present in high concentrations.ObjectivesTo determine whether changes in selenoprotein synthesis may be an underlying mechanism behind arsenic-induced carcinogenesis, we analyzed the new synthesis of selenoproteins within cells after exposure to inorganic or methylated arsenicals using a human keratinocyte cell model.ResultsAddition of arsenite to culture medium blocked new synthesis of selenoproteins when selenium was present in the form of selenite, and appeared to stimulate the use of serum-derived selenium. Monomethylarsonous acid (MMAIII) treatment of cells, in contrast, did not block all new synthesis of selenoproteins but did result in an increase in cytosolic thioredoxin reductase (TrxR1) at both the mRNA and protein levels. MMAIII also reduced the new synthesis of cellular glutatione peroxidase (cGpx) and other smaller selenoproteins. Dimethylarsinous acid (DMAIII) stimulated selenoprotein synthesis by an as yet unknown mechanism.ConclusionsThese results suggest that arsenite and MMAIII are key metabolites that trigger higher levels of TrxR1, and both lead to a reduction in the expression of cGpx. Together these effects certainly could lead to carcinogenesis given the knowledge that many cancers have higher levels of TrxR, and reduced Gpx levels will reduce the cell’s ability to defend against reactive oxygen species. Based on these results, the impact of the trivalent arsenicals arsenite and MMAIII on selenoprotein synthesis may indeed represent a potential molecular mechanism for the higher rates of cancer observed in populations exposed to high levels of arsenic.

SECIS elements in the coding regions of selenoprotein transcripts are functional in higher eukaryotes

Expression of selenocysteine (Sec)-containing proteins requires the presence of a cis-acting mRNA structure, called selenocysteine insertion sequence (SECIS) element. In bacteria, this structure is located in the coding region immediately downstream of the Sec-encoding UGA codon, whereas in eukaryotes a completely different SECIS element has evolved in the 3′-untranslated region. Here, we report that SECIS elements in the coding regions of selenoprotein mRNAs support Sec insertion in higher eukaryotes. Comprehensive computational analysis of all available viral genomes revealed a SECIS element within the ORF of a naturally occurring selenoprotein homolog of glutathione peroxidase 4 in fowlpox virus. The fowlpox SECIS element supported Sec insertion when expressed in mammalian cells as part of the coding region of viral or mammalian selenoproteins. In addition, readthrough at UGA was observed when the viral SECIS element was located upstream of the Sec codon. We also demonstrate successful de novo design of a functional SECIS element in the coding region of a mammalian selenoprotein. Our data provide evidence that the location of the SECIS element in the untranslated region is not a functional necessity but rather is an evolutionary adaptation to enable a more efficient synthesis of selenoproteins.

Bioavailability of selenium from the selenotrisulphide derivative of lipoic acid

Selenium is a required micronutrient in mammals, needed for the activity of enzymes that contain selenocysteine at their active site. Several isoenzymes of glutathione peroxidase and thioredoxin reductase contain selenocysteine and thus the nutritional status of selenium in tissues can have significant impact on the steady state level of reactive oxygen species. The aims of this study were to evaluate the bioavailability of selenium derived from the selenotrisulfide derivative of lipoic acid (LASe) and determine the ability of this compound to be absorbed into skin. Bioavailability of selenium derived from LASe was determined using a keratinocyte cell model (HaCat). Efficiency of utilization of selenium was assessed by following the decrease in the incorporation of radiolabeled selenite (75Se) in the presence of increasing concentration of selenium compounds. Percutaneous absorption of LASe was measured by determining selenium levels in full thickness biopsy of skin using a Yorkshire pig model. LASe was efficiently absorbed topically into pig skin, a good model of human skin. In a keratinocyte cell line LASe was an efficient source of selenium for selenoprotein synthesis, demonstrating that LASe is a good candidate as a topical selenium micronutrient. Both L-selenomethionine and selenate were found to be poor sources of selenium for selenoprotein synthesis in the skin cell model and L-selenomethionine was poorly absorbed into pig skin. These results indicate that stable selenotrisulfides, such as LASe, are good candidates for testing as topical selenium supplements.

Selenoproteins and Their Impact on Human Health Through Diverse Physiological Pathways

In the last few decades, the importance of selenium in human health has been the subject of numerous studies. It is believed that the physiological effects of selenium occur mainly through the function of selenoproteins, which incorporate selenium in the form of one or more selenocysteine residues. Recent advances in understanding the complex regulation of selenoprotein synthesis and functional characterization of several members of the selenoprotein family have contributed to an improved comprehension of the role(s) of selenium in human health and the great diversity of physiological pathways influenced by this trace element.

Effect of mercury on selenium utilization and selenoperoxidase activity in LNCaP cells

Formation of stable complexes with protein thiols is the best-known mechanism of mercury toxicity. However, the solubility product of Hg 2+ with sulfides, although very low, is higher than that with selenides, suggesting that the fully reduced form of selenium might also be a relevant target for Hg 2+. In cells, selenide is the suggested intermediate for selenoprotein biosynthesis and selenoenzymes, in turn, contain reduced selenium as the catalytic moiety. Thus, inhibition of biological functions of selenium could be seen as a different mechanism of Hg 2+ toxicity. To address this issue, we investigated selenoperoxidase (SeGPx) activity in LNCaP cells exposed to HgCl 2. Cells growing in standard medium express a low GPx activity, which increases on addition of selenium donors such as selenite, selenomethionine, or methyl-Se-cysteine. HgCl 2 added to the medium has different effects depending on the type of Se donor. A progressive decrease of SeGPx activity is observed in cells grown in standard medium exposed to HgCl 2, while coadministration of suprastoichiometric amounts of HgCl 2 prevents the increase of SeGPx activity only when selenite, but not selenomethionine or methyl-Se-cysteine, is the selenium source. From this evidence we conclude that HgCl 2: (a) does not inhibit directly SeGPxs, as confirmed on isolated enzymes; (b) does not interfere with the intermediates of the metabolic pathway of selenoprotein synthesis; and (c) decreases the bioavailability of selenium only when ionic complexes can be formed.

Availability and metabolism of 77Se-methylseleninic acid compared simultaneously with those of three related selenocompounds

Nutritional selenocompounds are considered to be transformed into the common intermediate selenide for utilization as selenoenzymes and/or for excretion as selenosugar and trimethylselenonium (TMSe). Therefore, selenocompounds can only be traced with a labeled selenium atom. Methylseleninic (MSA(IV)) has been proposed to be a third nutritional selenium source, the other two being inorganic selenocompounds and organic selenoamino acids, and to be a proximate selenochemical for producing the assumed biologically active form methylselenol. Here we applied a new tracer method to compare the availability and metabolism of MSA(IV) with those of three related selenocompounds under exactly identical host and tracing conditions. (82)Se-Selenite, (78)Se-selenate, (77)Se-MSA(IV) and (76)Se-methylselenonic acids (MSA(VI)) were simultaneously administered orally, each at the dose of 25 microg Se/kg body weight, to rats that had been depleted of endogenous natural abundance selenium with a single stable isotope ((80)Se). Time-related changes in the concentrations and/or distributions of the four labeled isotopes in the serum, liver, kidney, pancreas, lung and urine were determined simultaneously by inductively coupled argon plasma mass spectrometry (ICP MS) and/or HPLC-ICP MS. The availability with different isotope ratios was in the decreasing order of selenate>selenite=MSA(IV)>MSA(VI). Although selenate and MSA(VI) were distributed in organs and urine partly in their intact forms, MSA(IV) and selenite were not detected in the intact forms at all. MSA(IV) and MSA(VI) but not selenite or selenate produced TMSe in organs other than the liver, suggesting the transformation of MSA(IV) into methylselenol, and then either into selenide for the synthesis of selenoproteins and selenosugar or directly into TMSe. Thus, selenosugar and TMSe were produced widely in the organs. However, TMSe was not detected in the liver. The organ- and selenium source-specific production of TMSe was discussed as to the differences in selenium sources, and demethylation and methylation activity.

Tagging recombinant proteins with a Sel-tag for purification, labeling with electrophilic compounds or radiolabeling with 11C

Selenocysteine (Sec; U in one-letter code) is the twenty-first naturally occurring amino acid, with a selenium atom that gives this cysteine (Cys) homolog unique biochemical properties, including a high nucleophilicity and significant reactivity with electrophilic agents. This can be used in biotechnological Sec-dependent applications. Here, we describe how Sec can be introduced into a carboxy-terminal tetrapeptide motif (-Gly-Cys-Sec-Gly-COOH, known as a Sel-tag) for recombinant proteins by tailoring the encoding gene to become compatible with the Escherichia coli selenoprotein synthesis machinery. We also describe how the Sel-tag can be used as a basis for efficient one-step protein purification, rapid Sec-targeting protein labeling with electrophilic compounds, or radiolabeling with the positron emitter 11C.