Mammalian Selenoprotein in Which Selenocysteine (Sec) Incorporation Is Supported by a New Form of Sec Insertion Sequence Element

Abstract

Selenocysteine (Sec), the 21st amino acid in protein, is encoded by UGA. The Sec insertion sequence (SECIS) element, which is the stem-loop structure present in 3 untranslated regions (UTRs) of eukaryotic selenoprotein-encoding genes, is essential for recognition of UGA as a codon for Sec rather than as a stop signal. We now report the identification of a new eukaryotic selenoprotein, designated selenoprotein M (SelM). The 3-kb human SelM-encoding gene has five exons and is located on chromosome 22 but has not been correctly identified by either Celera or the public Human Genome Project. We characterized human and mouse SelM cDNA sequences and expressed the selenoprotein in various mammalian cell lines. The 3 UTR of the human, mouse, and rat SelM-encoding genes lacks a canonical SECIS element. Instead, Sec is incorporated in response to a conserved mRNA structure, in which cytidines are present in place of the adenosines previously considered invariant. Substitution of adenosines for cytidines did not alter Sec incorporation; however, other mutant structures did not support selenoprotein synthesis, demonstrating that this new form of SECIS element is functional. SelM is expressed in a variety of tissues, with increased levels in the brain. It is localized to the perinuclear structures, and its N-terminal signal peptide is necessary for protein translocation.In addition to the 20 common amino acids found in proteins of all living organisms, an unusual cotranslationally incorporated amino acid, selenocysteine (Sec), is present in several proteins in representatives of the three major domains of life, bacteria, archaea, and eukaryotes. Sec is inserted into polypeptide chains in response to the codon UGA, which requires a Sec-specific tRNA and a specific elongation factor in ribosome-based protein synthesis (1).Despite having its own codon and specific factors for its biosynthesis and insertion into protein, Sec is a rare amino acid. Only 17 proteins in mammals have been reported to contain Sec. These proteins do not have a common amino acid sequence motif, biological function, tissue expression, or intracellular localization. Hence, sequence analysis is generally not sufficient to determine the presence of Sec in protein. Thus far, the only common feature in eukaryotic selenoprotein-encoding genes (besides Sec-encoding UGA) is the presence of a stemloop structure, called the Sec insertion sequence (SECIS) element, in the 3Ј untranslated region (UTR) (19). This structure is essential for insertion of Sec in response to UGA.The functional part of a eukaryotic SECIS element is composed of a helix containing non-Watson-Crick base pairs UGAN. . . . .NGAN (designated the quartet or core), an unpaired A preceding the quartet, and an unpaired AA motif in the apical loop or bulge that is separated from the quartet by 11 to 12 nucleotides (3, 24). While having low sequence conservation, the secondary structure and free energy of eukaryotic SECIS elements are strictly conserved and can help in the identification of these stem...

Selenium biochemistry and its role for human health

“…Differently from the latter, SelM lacks the UGGT-binding domain and presents an ER-retention signal, whereas its redox-active motif is in the form Cys-X-X-SeCys. 83 The binding partners and the specific role of SelM remain to be established.…”

Overexpression of human selenoprotein M differentially regulates the concentrations of antioxidants and H2O2, the activity of antioxidant enzymes, and the composition of white blood cells in a transgenic rat

“…This gene has a 145-amino acid open reading frame beginning with an ATG codon in favorable Kozak context and contains an in-frame TGA as the Sec codon. Furthermore, homologous proteins were found in the rat, zebra fish, and other vertebrates, and Sec was conserved in these homologs (7). All previously characterized eukaryotic selenoprotein genes contain a conserved version of the Sec insertion sequence (SECIS) element in the 3' UTR.…”

Bioinformatic prediction of selenoprotein genes in the dolphin genome

“…The SECIS element of dolphin Gpx1 is GUGA-AA-GA, which is similar to that of nematode thioredoxin reductase (TR) [15], and the SECIS element of dolphin selenoprotein O (Sel O) is AUGA-CC-GA, which is similar to that of human and mouse SelO [7].…”

“…Over the past decade, several bioinformatic methods for the prediction of selenoprotein genes have been developed, and have been used for the identification of selenoproteins from many species, including human, fish, protozoa, archaea and bacteria [5][6][7][8][9]. The number of human selenoproteome members increased from 14 to 25, via bioinformatic predictions and subsequent experimental verification [10].…”