NMR Structures of the Selenoproteins Sep15 and SelM Reveal Redox Activity of a New Thioredoxin-like Family

Abstract

Selenium 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 thioredoxinlike 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.The formation of disulfide bonds is an essential step in the structural maturation of secretory and membrane proteins. All living organisms have evolved mechanistically related pathways to catalyze the formation of disulfide bonds (1). In eukaryotes, disulfide bonds are formed primarily within the lumen of the endoplasmic reticulum. An intricate series of protein folding and quality control mechanisms localized to this organelle regulate the formation of disulfide bonds through thiol-disulfide exchange (2).Thiol-disulfide oxidoreductases catalyze thiol-disulfide exchange between proteins with free thiols and thiol-containing small molecules or proteins with disulfide bonds. The activities of these enzymes are dependent upon active-site cysteine residues that are arranged in a CXXC motif or CXXC-derived motifs, in which one of these cysteines may be replaced by selenocysteine, serine, or threonine (3). Enzymatic activity and the direction of exchange (oxidation or reduction) depend upon redox potential, interactions with other redox proteins, and the availability of terminal electron donors and acceptors (4).Selenium is an essential trace element that is incorporated into proteins as the 21st natural amino acid in the genetic code, selenocysteine. Selenoproteins are found throughout prokaryotic (5) and eukaryotic proteomes (6), and most have orthologs in which selenocysteine is replaced with cysteine. The identity of the active-site redox motif residues (selenocysteine or cysteine) regulates the redox potential and catalytic efficiency of these enzymes (7,8). Recent studies provide strong evidence that dietary selenium plays an important role in cancer prevention (9 -11), immune function (12), aging (13), and the male reproductive system (12,14). Although selenoproteins appear to mediate these effects, physiological roles for most of these proteins are unknown. Two recently identified eukaryotic selenoproteins, Sep15 (15) and SelM (16), have...

Polymorphisms in the selenoprotein S and 15-kDa selenoprotein genes are associated with altered susceptibility to colorectal cancer

“…Sep15, a member of a novel family of selenoproteins, is also involved in protein folding mechanisms within the endoplasmic reticulum [11,18]. Rs5845 and rs5859 are SNPs in a region of the SEP15 gene corresponding to the 3 0 UTR; they are closely linked, and their modulation of the ability of the 3 0 UTR to promote reporter gene activity [13] indicates that they have functional consequences.…”

“…Selenocysteine is incorporated into selenoproteins during their synthesis with a specific stem-loop structure in the 3 0 untranslated region (3 0 UTR) of the mRNAs required to recode a UGA codon from ''stop'' to selenocysteine [2]. These selenoproteins include the family of glutathione peroxidises [4], selenoprotein P (SePP) which has a Se transport role [5], selenoprotein S which is an endoplasmic reticulum protein involved in removing unfolded proteins and the 15-kDa selenoprotein which is also an endoplasmic reticulum protein involved in the unfolded protein response [11,18]. Because the selenoproteins have important roles in cell stress responses and redox control [2,4], it is possible that genetic variation due to single nucleotide polymorphisms (SNP) in the genes encoding the selenoproteins may influence susceptibility to cancer and Se requirements for optimal health.…”

Selenium biochemistry and its role for human health

“…86 In relation to its redox potential, this suggests for Sep15 a possible additional function of catalyzing the isomerization or reducing disulfide bonds. 88 Sep15 may also play a role in regulation of apoptosis, as shown in malignant mesothelioma and NIH3T3 cell lines, 89,90 but insufficient data exists to provide the evidence.…”

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“…Sep 15 is expressed in several tissues, with highest levels in brain, lung, testis, liver, thyroid, and kidney (191). Its expression is regulated in response to dietary selenium (103). The precise function of Sep 15 remains elusive.…”

“…However, based on its localization to the ER, and interaction with the UDP-glucose:glycoprotein glucosyltransferases (GTs), a potential involvement in the quality control of glycoprotein folding within the ER has been proposed (179,192). Recent structure-determination studies showed that Sep 15 is a member of the thioredoxin-like fold superfamily of proteins, highly homologous to the ER protein disulfide isomerase (PDI), and contains a surface-accessible, Sec-containing redox-active motif (-CXU-) (103). These data further strengthen the evidence that Sep15 may be a thioldisulfide isomerase, involved in disulfide bond formation in the ER.…”