Cell Biology

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Science  17 Mar 2006:
Vol. 311, Issue 5767, pp. 1523
DOI: 10.1126/science.311.5767.1523b

Insertion of the 21st amino acid, selenocysteine, into selenoproteins occurs at what is usually a translation stop codon, UGA. This creates something of a dilemma in eukaryotic cells, because mRNAs carrying a premature stop codon are normally subject to nonsense-mediated decay (NMD). NMD is a process that destroys the mRNA and prevents the cell from synthesizing potentially dangerous truncated proteins. Indeed, when selenoprotein synthesis is limiting, selenoprotein mRNAs can be degraded by NMD.

In eukaryotes, recoding of the UGA stop codon is achieved through a secondary structure, the SECIS element, in the 3′ untranslated region of the selenoprotein mRNA. This element binds a complex of the SECIS binding protein (SBP2) and the elongation factor EFsec. De Jesus et al. investigated the subcellular location of these two proteins. Both proteins possess functional nuclear localization and nuclear export signals, and SBP2 is capable of shuttling between the cytoplasm and the nucleus. SBP2 and EFsec co-localize, suggesting that SBP2 may contribute to nuclear retention of EFsec. Furthermore, the level of the SBP2 protein correlates with the level of selenoprotein mRNAs, suggesting that it might stabilize these mRNAs. Thus, the prompt nuclear deposition of the two proteins on the SECIS element may play a role in protecting the selenoprotein mRNA from the unwanted attentions of the NMD machinery. — GR

Mol. Cell. Biol. 26, 1795 (2006).

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