University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln
Vadim Gladyshev Publications Biochemistry, Department of
December 2006
SECIS elements in the coding regions of selenoprotein transcripts are functional in higher eukaryotes
Heiko Mix University of Nebraska-Lincoln
Alexey V. Lobanov University of Nebraska-Lincoln
Vadim Gladyshev University of Nebraska-Lincoln, [email protected]
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Mix, Heiko; Lobanov, Alexey V.; and Gladyshev, Vadim, "SECIS elements in the coding regions of selenoprotein transcripts are functional in higher eukaryotes" (2006). Vadim Gladyshev Publications. 17. https://digitalcommons.unl.edu/biochemgladyshev/17
This Article is brought to you for free and open access by the Biochemistry, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Vadim Gladyshev Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 414–423 Nucleic Acids Research, 2007, Vol. 35, No. 2 Published online 14 December 2006 doi:10.1093/nar/gkl1060 SECIS elements in the coding regions of selenoprotein transcripts are functional in higher eukaryotes Heiko Mix, Alexey V. Lobanov and Vadim N. Gladyshev*
Department of Biochemistry, University of Nebraska, Beadle Center Lincoln, NE 68588, USA
Received September 13, 2006; Revised November 6, 2006; Accepted November 7, 2006
ABSTRACT immediately downstream, which allows recoding of the codon from stop to Sec insertion (4). This secondary structure Expression of selenocysteine (Sec)-containing pro- is known as Sec insertion sequence (SECIS) element (5). In teins requires the presence of a cis-acting mRNA bacteria, the essential region for Sec insertion is located in structure, called selenocysteine insertion sequence the apical loop of the SECIS element, which includes the (SECIS) element. In bacteria, this structure is located binding site for a specialized elongation factor SelB. SelB in the coding region immediately downstream of the in turn recruits Sec tRNA and mediates elongation at the Sec-encoding UGA codon, whereas in eukaryotes in-frame UGA by competing with a release factor (3). As a a completely different SECIS element has evolved result, the process of Sec incorporation is relatively slow in the 30-untranslated region. Here, we report that and inefficient. SECIS elements in the coding regions of seleno- In archaea and eukaryotes, a different mechanism has protein mRNAs support Sec insertion in higher evolved, which has puzzled researchers for the past 15 years. Most strikingly, the essential cis-acting SECIS element is eukaryotes. Comprehensive computational analysis not located immediately downstream of the corresponding of all available viral genomes revealed a SECIS UGA codon but has evolved in the 30-untranslated regions element within the ORF of a naturally occurring (30-UTRs) of the selenoprotein-encoding mRNAs (6). The selenoprotein homolog of glutathione peroxidase location of the SECIS element immediately downstream of 4 in fowlpox virus. The fowlpox SECIS element UGA stalls the ribosome complex during elongation thereby supported Sec insertion when expressed in mamma- increasing efficiency of Sec insertion (3,7). The 30-UTR lian cells as part of the coding region of viral or location requires a different mechanism for the access of mammalian selenoproteins. In addition, readthrough Sec tRNA to the ribosomal A-site (8,9). A kink-turn structure at UGA was observed when the viral SECIS element located in the stem-region of the SECIS element is the bind- was located upstream of the Sec codon. We also ing site of SECIS-binding protein 2 (SBP2) (8). However, in demonstrate successful de novo design of a func- eukaryotic SECIS elements additional nucleotides have been identified in the stem region as being essential for the binding tional SECIS element in the coding region of a of SBP2, a protein belonging to the ribosomal protein L7Ae/ mammalian selenoprotein. Our data provide evi- L30e/S12e/Gadd45 family (10). Mutations in this region dence that the location of the SECIS element in have been shown to abrogate selenoprotein synthesis, indicat- the untranslated region is not a functional necessity ing that SBP2 is an essential factor for Sec insertion. In but rather is an evolutionary adaptation to enable eukaryotes, a Sec tRNA-specific elongation factor (EFSec) a more efficient synthesis of selenoproteins. has been identified which is a functional homolog of the bac- terial SelB gene product and has been found to be recruited by and to interact with SBP2 (2,11). In addition, archaeal and eukaryotic genomes code for several other factors implicated INTRODUCTION in selenoprotein synthesis which are described elsewhere (12). Selenocysteine (Sec)-containing proteins serve important oxi- Selenoproteins have been conserved throughout evolution, doreductase functions in a variety of organisms in all three most likely because of their unique physico-chemical proper- domains of life (1). Although the mechanism of Sec synthesis ties as compared with their sulfur-containing homologs. and incorporation into nascent peptide chains has been well However, identification of these proteins in sequence data- characterized in bacteria, a much different strategy for seleno- bases is challenging due to recognition of Sec-encoding UGA protein biosynthesis has evolved in archaea and eukaryotes codons as stop signals by sequence annotators. Recently, (2,3). To support Sec insertion into nascent peptide chains, bioinformatics tools have been developed that detect seleno- bacterial selenoprotein mRNA has to contain cis-acting ele- protein genes by searching for SECIS elements, cysteine- ments in the form of a UGA codon and a stem–loop structure containing homologs and coding nature of UGA codons
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