The Selenium Transport Protein, Selenoprotein P, Requires Coding Sequence Determinants to Promote Efficient Selenocysteine Incorporation

Abstract

Selenoproteins are an essential and unique group of proteins in which selenocysteine (Sec) is incorporated in response to a stop codon (UGA). Reprograming of UGA for Sec insertion in eukaryotes requires a cis-acting stem loop structure in the 3’ untranslated region of selenoprotein mRNA and several trans-acting factors. Together these factors are sufficient for Sec incorporation in vitro, but the process is highly inefficient. An additional challenge is the synthesis of Selenoprotein P (SELENOP), which uniquely contains multiple UGA codons. Full SELENOP expression requires processive Sec incorporation, the mechanism for which is not understood. In this study, we identify core coding region sequence determinants within the SELENOP mRNA that govern SELENOP synthesis. Using 75Se labeling in cells, we determined that the N-terminal sequence (upstream of the 2nd UGA) and C-terminal sequence context are two independent determinants for efficient synthesis of full length SELENOP. In addition, the distance between the first UGA and the consensus signal peptide is also critical for efficiency.

Selenium at the Neural Barriers: AReview

“…The biosynthesis of SELENOP, involving the incorporation of multiple Sec moieties, is modulated by two SECIS elements in the 3 UTR region of SELENOP mRNA, reviewed by Shetty and Copeland (2018a). High high-energy demand from the cell for the incorporation of 7-17 or more, up to 35 (Shetty and Copeland, 2018b), Sec residues, depending on the biological species (Labunskyy et al, 2014), indicates the importance of the protein for the body. SELENOP contains two histidine-rich stretches in the N-terminal domain, which may bind to heparin (Hondal et al, 2001).…”

The selenoprotein P 3’ untranslated region is an RNA binding protein platform that fine tunes selenocysteine incorporation

“…In the case of oxidative stress, it is possible that the complex constituents are altered, allowing regulated SELENOP expression. The idea that an active SELENOP mRNP may form during pre-mRNA processing may explain why we have been unsuccessful in trying to express detectable protein from transfected cDNA [24]. One of the most likely mechanisms by which 3' UTR sequences would affect selenoprotein expression is by modulating SECIS function, either by blocking or possibly even enhancing SBP2 access or affinity.…”

Ribosome Fate during Decoding of UGA-Sec Codons

“…One version of the cDNA was wild-type, which contains multiple Sec codons, and the other had replaced those with Cys codons. Quantitative RT-PCR revealed that the steady state mRNA level for the Cys codon-containing construct was about twice that of the Sec codon counterpart [73]. Interpreting this result is confounded by the fact that SELENOP is an exception among selenoproteins in possessing multiple UGA codons.…”