Evidence for Direct Roles of Two Additional Factors, SECp43 and Soluble Liver Antigen, in the Selenoprotein Synthesis Machinery

Abstract

Several factors required for the specific incorporation of selenocysteine (Sec) 2 as the 21st amino acid in the genetic code have been identified, and their roles have been assessed in mammalian cells and tissues (see Refs. 1 and 2 for reviews). For example, the tRNA that specifically incorporates Sec into protein in response to Sec UGA codons has been characterized wherein the amino acid is biosynthesized on the tRNA following its aminoacylation with serine by seryl-tRNA synthetase (2). Thus, Sec tRNA has been designated Sec tRNA [Ser]Sec . There are two species of this tRNA in mammalian cells that differ by a single methyl group on the ribosyl moiety at position 34 (designated Um34; Ref.3). The form that does not contain Um34 is 5-methoxycarbonylmethyluridine (mcm 5 U) and that containing Um34 is 5-methoxycarbonylmethyl-2Ј-O-methyluridine (mcm 5 Um). The two species appear to have different roles in protein synthesis, wherein mcm 5 Um is associated with the synthesis of stress-related selenoproteins (e.g. glutathione peroxidase 1 [GPx1]), and mcm 5 U is associated with the synthesis of housekeeping selenoproteins (e.g. thioredoxin reductases) (4, 5).A specialized stem-loop structure in the 3Ј-untranslated region of mammalian selenoprotein genes, designated the SEC insertion sequence (SECIS) element, was identified that accounted for UGA codon specificity in dictating Sec incorporation into protein (6). The SECIS elements that have been identified in the 24 known selenoproteins in rodents and 25 known selenoproteins in humans (7) fall into two classes that differ in the occurrence of a ministem that is adjacent to the apical loop of the structure (8). A SECIS-binding protein, designated SBP2, was purified from various mammalian sources including liver, testes, and hepatoma cells (9) and shown to be required along with the SECIS element in selenoprotein mRNA for Sec insertion into protein (10). Recently, a ribosomal protein, L30, was found to bind SECIS elements and to have a role in mediating Sec incorporation at the ribosomal level (11). A specific elongation factor for Sec-tRNA [Ser]Sec has also been characterized (12, 13), and it recognizes only the selenocysteinyl-tRNA [Ser]Sec form of the tRNA and not, for example, the seryl-tRNA[Ser]Sec form (13). Additional factors that have a role in the biosynthesis of Sec and its insertion into protein in mammals are selenophosphate synthetases 1 (SPS1; Ref. 14) and 2 (SPS2) (15). These enzymes apparently are responsible for generating an active form of selenium for its utilization in Sec synthesis and/or possibly other selenium-requiring reactions. Interestingly, SPS2 is a selenoprotein and thus has been implicated in the autoregulation of selenoprotein synthesis (15).There are two other factors that also have been shown to be associated with Sec tRNA [Ser]Sec , SECp43 (16) and the soluble liver antigen, SLA (17). Protein overlay analysis provided evidence for the interaction of SECp43 with a 48-kDa protein in HeLa cytoplasmic extracts (16), whereas SLA was detected ...

New Developments in Selenium Biochemistry: Selenocysteine Biosynthesis in Eukaryotes and Archaea

“…This additional protein had previously been detected in patients with autoimmune chronic hepatitis as an autoimmune factor that co-precipitated tRNA [Ser]Sec in cell extracts of these individuals and was known as the SLA [19]. SLA, which is a PLPdependent transferase [20], also bound other protein components involved with Sec metabolism [22,23]. We found that SLA occurred in all eukaryotes and archaea-encoding selenoproteins, but not in those organisms examined that lacked selenoproteins.…”

“…The soluble liver antigen (SLA), which was initially identified as a 48-kDa protein bound to Sec tRNA [Ser]Sec and was targeted by antibodies in patients with an autoimmune chronic hepatitis [19], was reported to occur as a separate family within a larger superfamily of diverse PLP-dependent transferases [20] and proposed as a possible SecS in mammals [3,[20][21][22]. SLA was also found to exist in a protein complex with other factors involved in Sec biosynthesis and/or its insertion into protein providing further evidence that this protein is involved in selenium metabolism [22,23]. As described herein, SLA is indeed SecS in mammals [13].…”

Untitled

“…When coexpressed in cells, it appeared that SECp43 is able to sequester SLA from the cytoplasm to the nuclear compartment, where SECp43 normally resides. Combined depletion of both proteins causes a global decrease in selenoprotein synthesis, whereas SLA alone has no effect, and SECp43 appears to affect the expression of GPx1 only in a selenium-dependent manner (340). SECp43 also appears to regulate the levels of methylated Sec tRNA [Ser]Sec , which in turn is known to control the synthesis of stress-related selenoproteins.…”

“…(340). When coexpressed in cells, it appeared that SECp43 is able to sequester SLA from the cytoplasm to the nuclear compartment, where SECp43 normally resides.…”

Cloning and expression of selenoprotein W from pearl mussels Cristaria plicata

“…Firstly, a specialized mRNA secondary structure (selenocysteine insertion sequence, SECIS element) is required, and the Sec insertion sequence (SECIS) element locates in the 3′-untranslated region (3′UTR) of selenoprotein mRNAs at considerable distances from the UGA codon . Moreover, multiple cellular factors, include a Sec-tRNA specific elongation factor (eEFsec), SECIS-binding protein (SBP2), ribosomal protein L30, soluble liver antigen/liver protein and SECp43 are involved in the selenoprotein synthesis Chavatte et al, 2005;Xu et al, 2005;Small-Howard et al, 2006).…”