2006 Articles

Kim HY, Fomenko DE, Yoon YE, Gladyshev VN. (2006) Catalytic Advantages Provided by Selenocysteine in Methionine-S-Sulfoxide Reductases. Biochemistry 45, 13697-13704.

AbstractMethionine sulfoxide reductases are key enzymes that repair oxidatively damaged proteins. Two distinct stereospecific enzyme families are responsible for this function: MsrA (methionine-S-sulfoxide reductase) and MsrB (methionine-R-sulfoxide reductase). In the present study, we identified multiple selenoprotein MsrA sequences in organisms from bacteria to animals. We characterized the selenocysteine (Sec)-containing Chlamydomonas MsrA and found that this protein exhibited 10-50-fold higher activity than either its cysteine (Cys) mutant form or the natural mouse Cys-containing MsrA, making this selenoenzyme the most efficient MsrA known. We also generated a selenoprotein form of mouse MsrA and found that the presence of Sec increased the activity of this enzyme when a resolving Cys was mutated in the protein. These data suggest that the presence of Sec improves the reduction of methionine sulfoxide by MsrAs. However, the oxidized selenoprotein could not always be efficiently reduced to regenerate the active enzyme. Overall, this study demonstrates that sporadically evolved Sec-containing forms of methionine sulfoxide reductases reflect catalytic advantages provided by Sec in these and likely other thiol-dependent oxidoreductases. More Information

Fernando MR, Lechner JM, Lofgren S, Gladyshev VN, Lou MF. (2006) Mitochondrial thioltransferase (glutaredoxin 2) has GSH-dependent and thioredoxin reductase-dependent peroxidase activities in vitro and in lens epithelial cells. FASEB J. 20, 2645-2647.

AbstractThioltransferase (or Grx) belongs to the oxidoreductase family and is known to regulate redox homeostasis in cells. Mitochondrial Grx2 is a recent discovery, but its function is largely unknown. In this study we investigate Grx2 function by examining its potential peroxidase activity using lens epithelial cells (LEC). cDNA for human and mouse Grx2 was cloned into pET21d(+) vector and used to produce respective recombinant Grx2 for kinetic studies. cDNA for human Grx2 was transfected into human LEC and used for in vivo studies. Both human and mouse Grx2 showed glutathione (GSH)-dependent and thioredoxin reductase (TR)-dependent peroxidase activity. The catalytic efficiency of human and mouse Grx2 was lower than that of glutathione peroxidases (2.5 and 0.8×10(4) s(-1) M(-1), respectively), but comparable with TR-dependent peroxiredoxins (16.5 and 2.7×10(4) s(-1) M(-1), respectively). TR-dependent peroxidase activity increased 2-fold in the transfected cells and was completely abolished by addition of anti-Grx2 antibody (Ab). Flow cytometry (FACS) analysis and confocal microscopy revealed that cells preloaded with pure Grx2 detoxified peroxides more efficiently. Grx2 over-expression protected cells against H2O2-mediated disruption of mitochondrial transmembrane potential. These results suggest that Grx2 has a novel function as a peroxidase, accepting electrons both from GSH and TR. This unique property may play a role in protecting the mitochondria from oxidative damage. More Information

Carlson, B.A., Xu, X.M., Shrimali, R., Sengupta, A., Yoo, M.H., Irons, R., Zhong, N., Hatfield, D.L., Lee, B.J., Lobanov, A.V., and Gladyshev, V.N. (2006) Mammalian and other eukaryotic selenocysteine tRNAs. In Selenium: Its molecular biology and role in human health (ed., Hatfield, D.L., Berry, M.J., Gladyshev, V.N.), Springer, pp. 31-40.

AbstractSelenocysteine (Sec) tRNA occupies a prominent position in the expression of selenoproteins as it is essential for their synthesis and it provides the means by which selenium is co-translationally inserted into protein as the amino acid, Sec. Thus, Sec tRNA is regarded as the principle constituent in selenoprotein synthesis. Many features unique to this tRNA have been characterized over the years in mammals and other eukaryotes. In the last five years, the major advances have been in an elucidation of the different roles that the two major Sec tRNA isoforms play in selenoprotein biosynthesis and in Sec biosynthesis. One isoform appears to be responsible for the synthesis of selenoproteins that have roles in housekeeping functions and are less dependent on selenium status for their expression. The second isoforrn, that differs by only a single methyl group at the 2′-0-hydroxylribosyl moiety at position 34 (designated Um34), appears to be responsible for the expression of selenoproteins that have roles in stress-related phenomena and are highly dependent on selenium for their expression. Several new observations regarding Sec biosynthesis, which occurs on its tRNA, have also been recently made. Other recent advances involving Sec tRNA have used this molecule as a tool for determining whether eukaryotes outside the animal kingdom contain the machinery dedicated for the insertion of Sec into protein. These recent findings are discussed in this chapter. More Information

Salinas, G., Romero, H., Xu, X.M., Carlson, B.A., Hatfield, D.L. and Gladyshev, V.N. (2006) Evolution of Sec decoding and the key role of selenophosphate synthetase in the pathway of selenium utilization. In Selenium: Its molecular biology and role in human health (ed., Hatfield, D.L., Berry, M.J., Gladyshev, V.N.), Springer, pp. 41-52.

AbstractThe complete sequencing of genomes and the development of in silico methods for identification of genes encoding selenocysteine (Sec)-containing proteins have greatly contributed to shape our view on the evolution of selenium utilization in nature. Current evidence is consistent with the idea that Sec decoding is a late addition to the genetic code and it evolved once, before the separation of archaeal, bacterial and eukaryal domains. Many organisms have lost the Sec decoding trait, but recent evidence has shown that the loss is not irreversible. The distribution of organisms that use UGA as a Sec codon suggests that Sec decoding evolved as a result of speciation, differential gene loss and horizontal gene transfer. Selenium is also used in the synthesis 2-selenouridine, a modified base of unknown function located in the wobble position of certain tRNAs. It has been recently demonstrated that selenouridine and Sec-decoding traits can evolve independently of each other, but both require selenophosphate synthetase. This ATP-dependent enzyme emerged as a key feature of selenium utilization that allows separation of selenium from the pathways of sulfur utilization and non-specific use of selenium. Some animals, including mammals, evolved two selenophosphate synthetases, highlighting an unknown complexity of selenium utilization in nature. More Information

Gladyshev, V.N. (2006) Selenoproteins and selenoproteomes. In Selenium: Its molecular biology and role in human health (ed., Hatfield, D.L., Berry, M.J., Gladyshev, V.N.), Springer, pp. 101-112.

AbstractIn the past several years, progress in genome sequencing and development of specialized bioinformatics tools allowed efficient identification of selenocysteine-containing proteins encoded in completely sequenced genomes. Information is currently available on selenoproteomes from a variety of organisms, including humans, which contain 25 known selenoprotein genes. This review provides basic information about mammalian selenoproteins and other known selenoprotein families. Analysis of full sets of selenoproteins in organisms provides exciting avenues for examining selenoprotein evolution and dependence of organisms on the trace element selenium and allows linking selenoproteins with specific biological and biomedical effects of dietary selenium. More Information

Kim, H.Y., and Gladyshev, V.N. (2006) Selenium and methionine sulfoxide reduction. In Selenium: Its molecular biology and role in human health (ed., Hatfield, D.L., Berry, M.J., Gladyshev, V.N.), Springer, pp. 125-136.

AbstractMethionine residues in proteins can be readily oxidized to a diastereomeric mixture of methionine sulfoxides by reactive oxygen species. In most organisms, methionine sulfoxides are reversibly and stereospecifically reduced back to methionine by two distinct classes of repair enzymes, methionine-S-sulfoxide reductase (MsrA) and methionine-R-sulfoxide reductase (MsrB). Methionine sulfoxide reduction is thought to be an essential pathway that protects cells from oxidative stress and regulates protein function. This pathway is also implicated in delaying the aging process in organisms from yeast to mammals. The first selenoprotein identified using bioinformatics methods, SelR (also known as SelX or MsrB1), was recently found to be a selenocysteine-containing MsrB. In mammals, selenoprotein MsrB1 is a major MsrB, while MsrB2 and MsrB3 contain cysteine in place of selenocysteine. It has been found that selenocysteine- and cysteine-containing MsrBs employ different catalytic mechanisms. Interestingly, a selenocysteine-containing form of MsrA was also described, but so far was only detected in green algae. More Information

Labunskyy, V.M., Gladyshev, V.N., and Hatfield, D.L. (2006) The 15-kDa selenoprotein (Sep15): functional analysis and role in cancer. In Selenium: Its molecular biology and role in human health (ed., Hatfield, D.L., Berry, M.J., Gladyshev, V.N.), Springer, pp. 143-150.

AbstractThe 15-kDa selenoprotein (Sep15) was identified several years ago as a protein of unknown function. In recent years, several lines of evidence implicated Sepl5 in the effect of dietary selenium in cancer prevention. These lines of evidence include: 1) protein expression patterns in normal and malignant cells; 2) identification of polymorphic sites that regulate Sep15 levels and differentially respond to selenium supplementation; 3) location of the Sep15 gene in the human genome; and 4) correlation between Sep15 haplotype and susceptibility to cancer. Functional analyses revealed a specific interaction between Sep15 and a protein folding sensor in the endoplasmic reticulum of mammalian cells and identified Sep15 as a novel thioredoxin-like fold redox regulator. Sep15 defines a new protein family that occurs in several organisms from green algae to mammals and also contains selenoprotein M (SelM) and a recently identified fish-specific selenoprotein Fep15. More Information

Carlson, B.A., Xu, X.M., Shrimali, R., Sengupta, A., Yoo, M.H., Zhong, N., Hatfield, D.L., Irons, R., Davis, C., Lee , B.J., Novoselov, S.V., and Gladyshev, V.N. (2006) Mouse models for assessing the role of selenoproteins in health and development. In Selenium: Its molecular biology and role in human health (ed., Hatfield, D.L., Berry, M.J., Gladyshev, V.N.), Springer, pp.337-346.

AbstractMouse models have been generated to assess the roles of selenoproteins involved with housekeeping tasks and/or stress-related phenomena in development and health. Each mouse model has taken advantage of the fact that the synthesis of all selenoproteins is dependent on the expression of two selenocysteine (Sec) tRNA[Ser]Sec isoforms that differ fiom each other by a single methyl group on the ribosyl moiety at position 34. The endogenous (Sec) tRNA[Ser]Sec population was selectively altered by generating mouse models involving 1) transgenic animals carrying mutant or wild type (Sec) tRNA[Ser]Sec transgenes, 2) conditional knockout animals carrying a floxed (Sec) tRNA[Ser]Sec gene that was targeted for removal in specific tissues and organs using loxP-Cre technology and 3) transgenic/standard knockout animals carrying mutant or wild type transgenes and a knockout of the (Sec) tRNA[Ser]Sec gene wherein the animal’s survival is dependent on the transgene. These mouse models perturbed selenoprotein expression, often in a protein- and tissue-specific manner, permitting us to better assess their function in health and development. More Information

Salinas, G., Lobanov, A.V., and Gladyshev, V.N. (2006) Selenium in parasites. In Selenium: Its molecular biology and role in human health (ed., Hatfield, D.L., Berry, M.J., Gladyshev, V.N.), Springer, pp. 359-370.

AbstractParasites, which cause an enormous burden in the population of the third world, are a diverse group of organisms, many of which are sensitive to oxidative stress imposed by their hosts. In recent years, several selenoprotein families, some with antioxidant properties, have been described and characterized in metazoan parasites. Glutathione peroxidase and thioredoxin glutathione reductase (TGR) appear to be essential selenoproteins in flatworms (phylum Platyhelminthes). TGR is the single enzyme that provides reducing equivalents to both thioredoxin and glutathione pathways, in contrast to hosts, which evolve parallel pathways. In roundworms (phylum Nematoda), selenoproteins have recently been described, revealing species differences in the Sec/Cys protein sets and the presence of an unusual SECIS element. Plasmodium sp, one of the most important protozoan parasites that affect humans, also decode Sec. The selenoprotein families encoded by Plasmodial genomes have neither Sec nor Cys homologs in their hosts, raising the possibility that targeting their selenoproteomes may provide new treatment strategies. More Information

Zhang Y, Romero H, Salinas G, Gladyshev VN. (2006) Dynamic evolution of selenocysteine utilization in bacteria: a balance between selenoprotein loss and evolution of selenocysteine from redox-active cysteine residues. Genome Biology 7, R94.

AbstractBACKGROUND: Selenocysteine (Sec) is co-translationally inserted into protein in response to UGA codons. It occurs in oxidoreductase active sites and often is catalytically superior to cysteine (Cys). However, Sec is used very selectively in proteins and organisms. The wide distribution of Sec and its restricted use have not been explained. RESULTS: We conducted comparative genomics and phylogenetic analyses to examine dynamics of Sec decoding in bacteria at both selenium utilization trait and selenoproteome levels. These searches revealed that 21.5% of sequenced bacteria utilize Sec, their selenoproteomes have 1 to 31 selenoproteins, and selenoprotein-rich organisms are mostly Deltaproteobacteria or Firmicutes/Clostridia. Evolutionary histories of selenoproteins suggest that Cys-to-Sec replacement is a general trend for most selenoproteins. In contrast, only a small number of Sec-to-Cys replacements were detected, and these were mostly restricted to formate dehydrogenase and selenophosphate synthetase families. In addition, specific selenoprotein gene losses were observed in many sister genomes. Thus, the Sec/Cys replacements were mostly unidirectional, and increased utilization of Sec by existing protein families was counterbalanced by loss of selenoprotein genes or entire selenoproteomes. Lateral transfers of the Sec trait were an additional factor, and we describe the first example of selenoprotein gene transfer between archaea and bacteria. Finally, oxygen requirement and optimal growth temperature were identified as environmental factors that correlate with changes in Sec utilization. CONCLUSION: Our data reveal a dynamic balance between selenoprotein origin and loss, and may account for the discrepancy between catalytic advantages provided by Sec and the observed low number of selenoprotein families and Sec-utilizing organisms. More Information

Lobanov AV, Gromer S, Salinas G, Gladyshev VN. (2006) Selenium metabolism in Trypanosoma: characterization of selenoproteomes and identification of a Kinetoplastida-specific selenoprotein. Nucleic Acids Res. 34, 4012-4024.

AbstractProteins containing the 21st amino acid selenocysteine (Sec) are present in the three domains of life. However, within lower eukaryotes, particularly parasitic protists, the dependence on the trace element selenium is variable as many organisms lost the ability to utilize Sec. Herein, we analyzed the genomes of Trypanosoma and Leishmania for the presence of genes coding for Sec-containing proteins. The selenoproteomes of these flagellated protozoa have three selenoproteins, including distant homologs of mammalian SelK and SelT, and a novel multidomain selenoprotein designated SelTryp. In SelK and SelTryp, Sec is near the C-terminus, and in all three selenoproteins, it is within predicted redox motifs. SelTryp has neither Sec- nor cysteine-containing homologs in the human host and appears to be a Kinetoplastida-specific protein. The use of selenium for protein synthesis was verified by metabolically labeling Trypanosoma cells with 75Se. In addition, genes coding for components of the Sec insertion machinery were identified in the Kinetoplastida genomes. Finally, we found that Trypanosoma brucei brucei cells were highly sensitive to auranofin, a compound that specifically targets selenoproteins. Overall, these data establish that Trypanosoma, Leishmania and likely other Kinetoplastida utilize and depend on the trace element selenium, and this dependence is due to occurrence of selenium in at least three selenoproteins. More Information

Hatfield DL, Carlson BA, Xu XM, Mix H, Gladyshev VN. (2006) Selenocysteine incorporation machinery and the role of selenoproteins in development and health. Prog. Nucleic Acid Res. Mol. Biol. 81, 97-142.

Turanov AA, Su D, Gladyshev VN. (2006) Mouse mitochondrial thioredoxin reductase: Characterization of alternative cytosolic forms and cellular targets. J. Biol. Chem. 281, 22953-22963.

AbstractThioredoxin reductase (TR) and thioredoxin (Trx) define a major cellular redox system that maintains cysteine residues in numerous proteins in the reduced state. Both cytosolic (TR1 and Trx1) and mitochondrial (TR3 and Trx2) enzymes are essential in mammals, but the function of the mitochondrial system is less understood. In this study, we characterized subcellular localization of three TR3 forms that are generated by alternative first exon splicing and that differ in their N-terminal sequences. Only one of these forms resides in mitochondria, whereas the two other isoforms are cytosolic. Consistent with this finding, TR3 did not have catalytic preferences for mitochondrial Trx2 versus cytosolic Trx1, both of which could serve as TR3 substrates. Similarly, TR1 was equally active with Trx1, Trx2, or a bacterial Trx. We generated recombinant selenoprotein forms of TR1 and TR3 and found that these enzymes were inhibited by zinc, but not by calcium or cobalt ions. We further developed a proteomic method for identification of targets of TRs in mammalian cells utilizing affinity columns containing recombinant TR3 forms differing in C-terminal sequences. Using this procedure, we found that Trx1 was the major target of TR3 in both rat and mouse liver cytosol. The truncated form of TR3 lacking selenocysteine was particularly efficient in binding Trx1, consistent with the previously observed role of truncated TR1 in apoptosis. Overall, these data establish that the function of TR3 is not limited to its role in Trx2 reduction. More Information

Lobanov AV, Kryukov GV, Hatfield DL, Gladyshev VN. (2006) Is there a twenty third amino acid in the genetic code?. Trends Genet. 22, 357-360.

AbstractThe universal genetic code includes 20 common amino acids. In addition, selenocysteine (Sec) and pyrrolysine (Pyl), known as the twenty first and twenty second amino acids, are encoded by UGA and UAG, respectively, which are the codons that usually function as stop signals. The discovery of Sec and Pyl suggested that the genetic code could be further expanded by reprogramming stop codons. To search for the putative twenty third amino acid, we employed various tRNA identification programs that scanned 16 archaeal and 130 bacterial genomes for tRNAs with anticodons corresponding to the three stop signals. Our data suggest that the occurrence of additional amino acids that are widely distributed and genetically encoded is unlikely. More Information

Eckenroth B, Harris K, Turanov AA, Gladyshev VN, Raines RT, Hondal RJ. (2006) Semisynthesis and Characterization of Mammalian Thioredoxin Reductase. Biochemistry 45, 5158-5170.

AbstractThioredoxin reductase and thioredoxin constitute the cellular thioredoxin system, which provides reducing equivalents to numerous intracellular target disulfides. Mammalian thioredoxin reductase contains the rare amino acid selenocysteine. Known as the 21st amino acid, selenocysteine is inserted into proteins by recoding UGA stop codons. Some model eukaryotic organisms lack the ability to insert selenocysteine, and prokaryotes have a recoding apparatus different from that of eukaryotes, thus making heterologous expression of mammalian selenoproteins difficult. Here, we present a semisynthetic method for preparing mammalian thioredoxin reductase. This method produces the first 487 amino acids of mouse thioredoxin reductase-3 as an intein fusion protein in Escherichia coli cells. The missing C-terminal tripeptide containing selenocysteine is then ligated to the thioester-tagged protein by expressed protein ligation. The semisynthetic version of thioredoxin reductase that we produce in this manner has k(cat) values ranging from 1500 to 2220 min(-)(1) toward thioredoxin and has strong peroxidase activity, indicating a functional form of the enzyme. We produced the semisynthetic thioredoxin reductase with a total yield of 24 mg from 6 L of E. coli culture (4 mg/L). This method allows production of a fully functional, semisynthetic selenoenzyme that is amenable to structure-function studies. A second semisynthetic system is also reported that makes use of peptide complementation to produce a partially active enzyme. The results of our peptide complementation studies reveal that a tetrapeptide that cannot ligate to the enzyme (Ac-Gly-Cys-Sec-Gly) can form a noncovalent complex with the truncated enzyme to form a weak complex. This noncovalent peptide-enzyme complex has 350-500-fold lower activity than the semisynthetic enzyme produced by peptide ligation. More Information

Yoo MH, Xu XM, Carlson BA, Gladyshev VN, Hatfield DL. (2006) Thioredoxin reductase 1 deficiency reverses tumor phenotype and tumorigenicity of lung carcinoma cells. J. Biol. Chem. 281, 13005-13008.

AbstractDietary selenium has potent cancer prevention activity. Both low molecular weight selenocompounds and selenoproteins are implicated in this effect. Thioredoxin reductase 1 (TR1) is one of the major antioxidant and redox regulators in mammals that supports p53 function and other tumor suppressor activities. However, this selenium-containing oxidoreductase is also overexpressed in many malignant cells and has been proposed as a target for cancer therapy. To further assess the role of TR1 in the malignancy process, we used RNA interference technology to decrease its expression in mouse lung carcinoma (LLC1) cells. Stable transfection of LLC1 cells with a small interfering RNA construct that specifically targets TR1 removal manifested a reversal in the morphology and anchorage-independent growth properties of these cancer cells that made them similar to those of normal cells. The expression of at least two cancer-related protein mRNAs, Hgf and Opn1, were reduced dramatically in the TR1 knockdown cells. Mice injected with the TR1 knockdown showed a dramatic reduction in tumor progression and metastasis compared with those mice injected with the corresponding control vector. In addition, tumors that arose from injected TR1 knockdown cells lost the targeting construct, suggesting that TR1 is essential for tumor growth in mice. These observations provide direct evidence that the reduction of TR1 levels in malignant cells is antitumorigenic and suggest that the enzyme is a prime target for cancer therapy. More Information

Kim HY, Gladyshev VN. (2006) Alternative first exon splicing regulates subcellular distribution of methionine sulfoxide reductases. BMC Mol. Biol. 7, 11.

AbstractBACKGROUND: Methionine sulfoxide reduction is an important protein repair pathway that protects against oxidative stress, controls protein function and has a role in regulation of aging. There are two enzymes that reduce stereospecifically oxidized methionine residues: MsrA (methionine-S-sulfoxide reductase) and MsrB (methionine-R-sulfoxide reductase). In many organisms, these enzymes are targeted to various cellular compartments. In mammals, a single MsrA gene is known, however, its product is present in cytosol, nucleus, and mitochondria. In contrast, three mammalian MsrB genes have been identified whose products are located in different cellular compartments. RESULTS: In the present study, we identified and characterized alternatively spliced forms of mammalian MsrA. In addition to the previously known variant containing an N-terminal mitochondrial signal peptide and distributed between mitochondria and cytosol, a second mouse and human form was detected in silico. This form, MsrA(S), was generated using an alternative first exon. MsrA(S) was enzymatically active and was present in cytosol and nucleus in transfected cells, but occurred below detection limits in tested mouse tissues. The third alternative form lacked the active site and could not be functional. In addition, we found that mitochondrial and cytosolic forms of both MsrA and MsrB in Drosophila could be generated by alternative first exon splicing. CONCLUSION: Our data suggest conservation of alternative splicing to regulate subcellular distribution of methionine sulfoxide reductases. More Information

Lobanov AV, Delgado C, Rahlfs S, Novoselov SV, Kryukov GV, Gromer S, Hatfield DL, Becker K, Gladyshev VN. (2006) The Plasmodium selenoproteome. Nucleic Acids Res. 34, 496-505.

AbstractThe use of selenocysteine (Sec) as the 21st amino acid in the genetic code has been described in all three major domains of life. However, within eukaryotes, selenoproteins are only known in animals and algae. In this study, we characterized selenoproteomes and Sec insertion systems in protozoan Apicomplexa parasites. We found that among these organisms, Plasmodium and Toxoplasma utilized Sec, whereas Cryptosporidium did not. However, Plasmodium had no homologs of known selenoproteins. By searching computationally for evolutionarily conserved selenocysteine insertion sequence (SECIS) elements, which are RNA structures involved in Sec insertion, we identified four unique Plasmodium falciparum selenoprotein genes. These selenoproteins were incorrectly annotated in PlasmoDB, were conserved in other Plasmodia and had no detectable homologs in other species. We provide evidence that two Plasmodium SECIS elements supported Sec insertion into parasite and endogenous selenoproteins when they were expressed in mammalian cells, demonstrating that the Plasmodium SECIS elements are functional and indicating conservation of Sec insertion between Apicomplexa and animals. Dependence of the plasmodial parasites on selenium suggests possible strategies for antimalarial drug development. More Information

Ferguson AD, Labunskyy VM, Fomenko DE, Arac D, Chelliah Y, Amezcua CA, Rizo J, Gladyshev VN, Deisenhofer J. (2006) NMR structures of the selenoproteins Sep15 and SelM reveal redox activity of new thioredoxin-like family. J. Biol. Chem. 281, 3536-3543.

AbstractSelenium has significant health benefits, including potent cancer prevention activity and roles in immune function and the male reproductive system. Selenium-containing proteins, which incorporate this essential micronutrient as selenocysteine, are proposed to mediate the positive effects of dietary selenium. Presented here are the solution NMR structures of the selenoprotein SelM and an ortholog of the selenoprotein Sep15. These data reveal that Sep15 and SelM are structural homologs that establish a new thioredoxin-like protein family. The location of the active-site redox motifs within the fold together with the observed localized conformational changes after thiol-disulfide exchange and measured redox potential indicate that they have redox activity. In mammals, Sep15 expression is regulated by dietary selenium, and either decreased or increased expression of this selenoprotein alters redox homeostasis. A physiological role for Sep15 and SelM as thiol-disulfide oxidoreductases and their contribution to the quality control pathways of the endoplasmic reticulum are discussed. More Information

Novoselov SV, Hua D, Lobanov AV, Gladyshev VN. (2006) Identification and characterization of Fep15, a new selenocysteine-containing member of the Sep15 protein family. Biochem. J. 394, 575-579.

AbstractSec (selenocysteine) is a rare amino acid in proteins. It is co-translationally inserted into proteins at UGA codons with the help of SECIS (Sec insertion sequence) elements. A full set of selenoproteins within a genome, known as the selenoproteome, is highly variable in different organisms. However, most of the known eukaryotic selenoproteins are represented in the mammalian selenoproteome. In addition, many of these selenoproteins have cysteine orthologues. Here, we describe a new selenoprotein, designated Fep15, which is distantly related to members of the 15 kDa selenoprotein (Sep15) family. Fep15 is absent in mammals, can be detected only in fish and is present in these organisms only in the selenoprotein form. In contrast with other members of the Sep15 family, which contain a putative active site composed of Sec and cysteine, Fep15 has only Sec. When transiently expressed in mammalian cells, Fep15 incorporated Sec in an SECIS- and SBP2 (SECIS-binding protein 2)-dependent manner and was targeted to the endoplasmic reticulum by its N-terminal signal peptide. Phylogenetic analyses of Sep15 family members suggest that Fep15 evolved by gene duplication. More Information