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Yuan J, Palioura S, Salazar JC, Su D, O'Donoghue P, Hohn MJ, et al. RNA-dependent conversion of phosphoserine forms selenocysteine in eukaryotes and archaea. Proc Natl Acad Sci U S A 2006;103:18923-18927. (Reprinted with permission.)

Abstract

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The trace element selenium is found in proteins as selenocysteine (Sec), the 21st amino acid to participate in ribosome-mediated translation. The substrate for ribosomal protein synthesis is selenocysteinyl-tRNASec. Its biosynthesis from seryl-tRNASec has been established for bacteria, but the mechanism of conversion from Ser-tRNASec remained unresolved for archaea and eukarya. Here, we provide evidence for a different route present in these domains of life that requires the tRNASec-dependent conversion of O-phosphoserine (Sep) to Sec. In this two-step pathway, Ophosphoseryl-tRNASec kinase (PSTK) converts Ser-tRNASec to SeptRNASec. This misacylated tRNA is the obligatory precursor for a Sep-tRNA:Sec-tRNA synthase (SepSecS); this protein was previously annotated as SLA/LP. The human and archaeal Sep-SecS genes complement in vivo an Escherichia coli Sec synthase (SelA) deletion strain. Furthermore, purified recombinant SepSecS converts SeptRNASec into Sec-tRNASec in vitro in the presence of sodiumselenite and purified recombinant E. coli selenophosphate synthetase (SelD). Phylogenetic arguments suggest that Sec decoding was present in the last universal common ancestor. SepSecS and PSTK coevolved with the archaeal and eukaryotic lineages, but the history of PSTK is marked by several horizontal gene transfer events, including transfer to non-Sec-decoding Cyanobacteria and fungi.

Xu XM, Carlson BA, Mix H, Zhang Y, Saira K, Glass RS, et al. Biosynthesis of selenocysteine on its tRNA in eukaryotes. PLoS Biol 2006;5:e4. (Reprinted with permission.)

Abstract

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  2. Abstract
  3. Abstract
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Selenocysteine (Sec) is cotranslationally inserted into protein in response to UGA codons and is the 21st amino acid in the genetic code. However, the means by which Sec is synthesized in eukaryotes is not known. Herein, comparative genomics and experimental analyses revealed that the mammalian Sec synthase (SecS) is the previously identified pyridoxal phosphate-containing protein known as the soluble liver antigen. SecS required selenophosphate andO-phosphoseryl-tRNA[Ser]Secas substrates to generate selenocysteyl-tRNA[Ser]Sec. Moreover, it was found that Sec was synthesized on the tRNA scaffold from selenide, ATP, and serine using tRNA[Ser]Sec, seryl-tRNA synthetase,O-phosphoseryl-tRNA[Ser]Seckinase, selenophosphate synthetase, and SecS. By identifying the pathway of Sec biosynthesis in mammals, this study not only functionally characterized SecS but also assigned the function of theO-phosphoseryl-tRNA[Ser]Seckinase. In addition, we found that selenophosphate synthetase 2 could synthesize monoselenophosphate in vitro but selenophosphate synthetase 1 could not. Conservation of the overall pathway of Sec biosynthesis suggests that this pathway is also active in other eukaryotes and archaea that synthesize selenoproteins.

Comment

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The exact pathogenesis of autoimmune hepatitis (AIH), just like other autoimmune diseases, is not known. However, a loss of tolerance against specific self-antigens is the most likely final mechanism, and this loss of tolerance may be triggered by external stimuli in a genetically susceptible host. The identification of specific target antigens and their functional characterization may provide the critical clue to initiating events. In the case of soluble liver antigen/liver-pancreas (SLA/LP)–positive AIH, an important step in this process is marked by the 2 publications discussed here.

Antibodies to SLA/LP can be identified in about a quarter of all patients with AIH.1–3 Many of these patients do not have any other autoantibodies, whereas others display additional autoreactivity to nuclear antigens and/or smooth muscle antigens.4 Reactivity to SLA/LP is highly specific for AIH as these antibodies, in contrast to all other autoantibodies detectable in immune-mediated liver diseases, occur exclusively in patients with autoimmune liver disease.3 The immune response to SLA/LP is highly specific both with respect to its strict association with autoimmune liver disease and with respect to its target epitopes. Indeed, patient autoantibodies react specifically with an immunodominant region of the SLA/LP molecule located between amino acids 390 and 428.2 Even within this region, immune reactivity is mostly restricted to a linear epitope sequence of 20 amino acids.5 This high degree of specificity both in disease association and in epitope recognition suggests that anti-SLA/LP reactivity is antigen-driven and, furthermore, that this antigen-driven immune response is likely to be related to the pathogenesis of AIH, at least in those patients who display anti-SLA/LP reactivity. What then may be the nature of the driving antigen, and how could it overcome normal self-tolerance?

For some years now, there have been several clues that the SLA/LP molecule has an enzymatic function.2 This is not unusual. In fact, enzymes are frequently found as targets of autoantibodies. The best known examples in hepatology are the E2 subunits of the pyruvate dehydrogenase complex as targets of antimitochondrial antibodies and the cytochrome P450 isoenzyme 2D6 (CYP2D6) as the target of antibodies to the liver-kidney microsomal antigen (LKM).4 The occurrence of enzymes as autoantigens is likely to be related to their function and may also partly explain organ specificity. Not only are enzymes expressed differentially in different organs, but their function, ligands, and metabolites may differ in various cell types. Furthermore, the autoepitopes of these intracellular enzymes are highly conserved, and the autoantibodies usually are strong inhibitors of these enzymes. One such example is LKM-1 autoantibodies against CYP2D6.6 CYP2D6 is a genetically polymorphic enzyme; 10% of the Caucasian population are poor metabolizers and do not express this protein in their liver. However, all AIH type 2 patients test positive for LKM-1 antibodies, which inhibit CYP2D6 function in vivo, are extensive metabolizers, and express the CYP2D6 protein in their livers.6 Thus, the expression of the autoantigen seems to be another prerequisite for the induction of this autoimmune liver disease.6

Gelpi et al.7 detected an association of the SLA/LP molecule with selenocysteinyl-tRNASec, suggesting a link to selenoprotein synthesis. The cotranslational incorporation of selenocysteine into selenoproteins is facilitated by a specialized selenocysteinyl-tRNASec recognizing the codon UGA. The biosynthesis of selenocysteine, the 21st amino acid, had been elucidated in bacteria but remained unclear in eukaryotes and archaea. The 2 recent articles by Yuan et al.8 and Xu et al.9 now establish the eukaryotic and archaeal pathways of selenoprotein biosynthesis, in which SLA/LP has been demonstrated to function as selenocysteinyl-tRNA synthase, which generates selenocysteinyl-tRNASec from a phosphoseryl-tRNA precursor. The Nomenclature Commission of the Human Genome Organisation has therefore changed the name of the sla/lp gene to Sep (O-phosphoserine) tRNA:Sec (selenocysteine) tRNA synthase, with the symbol SEPSECS.

How then might the synthesis of selenocysteine be related to AIH? This crucial question remains open, but the aforementioned data will open new avenues of investigation. Selenoproteins are synthesized in various organs, but nutritional selenium is mainly metabolized in the liver, from which selenium is distributed to other organs in the form of selenoprotein P.10 Could nutritional selenium compounds or their metabolites modify SLA/LP in a way that gives rise to immunogenic neoantigens? Such initiation of autoimmunity was recently suggested to cause the generation of antimitochondrial antibodies in primary biliary cirrhosis11 and may be conceivable in SLA/LP-positive AIH. Alternatively, there may be situations in which the SLA/LP molecule changes its tertiary structure in a fashion that creates a novel antigenic structure. This, however, is less likely in view of the immune reactivity being primarily toward a linear epitope of the molecule rather than a structural epitope of the molecule. Could selenium deficiency be related to disease pathogenesis? Selenium deficiency could stimulate aberrant expression or subcellular distribution of SLA/LP molecules in the liver or the formation of immunogenic molecular complexes of SLA/LP with other proteins or nonprotein molecules. At least from our own unpublished data, we do not have any evidence of altered SLA/LP expression in the livers of AIH patients, but cellular localization or complex formation still could be relevant. Furthermore, it is interesting that SLA/LP is an RNA-processing molecule; note that many autoantigens, which are mainly observed in rheumatic diseases, are DNA-processing or RNA-processing molecules. However, autoantibodies to DNA or Ro/Sjögren's Syndrome-A can also be found in a considerable proportion of AIH patients.12, 13 Is there a common immunogenic mechanism that drives the generation of these autoantibodies?

The search is thus now open for the relation between the physiological function of the SLA/LP enzyme and its role as an autoimmune target. The identification of the unique function of SLA/LP in selenocysteine synthesis in combination with the high degree of disease and epitope specificity of anti-SLA/LP autoantibodies suggests that anti-SLA/LP–positive AIH, despite all its similarities to other forms of AIH, is a unique autoimmune disease. Independent of questions of nomenclature (type III AIH or SLA/LP-positive AIH), screening for SLA/LP autoantibodies remains an important step in the identification of these patients, many of whom are negative for other autoantibodies. It appears that these patients respond well to immunosuppression, as do other patients with AIH, but the need for lifelong immunosuppression in these patients appears to be universal.14 In addition, patients with primary biliary cirrhosis with reactivity for SLA/LP seem to display clinical and histological features of AIH and probably should receive immunosuppressive therapy, just as other patients with AIH do. Maybe the identification of the biochemical function of the SLA/LP enzyme will offer new perspectives for antigen-specific immunotherapy of this often serious chronic disease.

References

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  2. Abstract
  3. Abstract
  4. Comment
  5. References
  • 1
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    Yuan J, Palioura S, Salazar JC, Su D, O'Donoghue P, Hohn MJ, et al. RNA-dependent conversion of phosphoserine forms selenocysteine in eukaryotes and archaea. Proc Natl Acad Sci U S A 2006; 103: 1892318927.
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    Xu XM, Carlson BA, Mix H, Zhang Y, Saira K, Glass RS, et al. Biosynthesis of selenocysteine on its tRNA in eukaryotes. PLoS Biol 2006; 5: e4. DOI 10.1371/journal.pbio.0050004. Available at: http://biology.plosjournals.org.
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