Association between the 15-kDa Selenoprotein and UDP-glucose:Glycoprotein Glucosyltransferase in the Endoplasmic Reticulum of Mammalian Cells

Abstract

Mammalian selenocysteine-containing proteins characterized with respect to function are involved in redox processes and exhibit distinct expression patterns and cellular locations. A recently identified 15-kDa selenoprotein (Sep15) has no homology to previously characterized proteins, and its function is not known. Here we report the intracellular localization and identification of a binding partner for this selenoprotein which implicate Sep15 in the regulation of protein folding. The native Sep15 isolated from rat prostate and mouse liver occurred in a complex with a 150-kDa protein. The latter protein was identified as UDP-glucose:glycoprotein glucosyltransferase (UGTR), the endoplasmic reticulum (ER)-resident protein, which was previously shown to be involved in the quality control of protein folding. UGTR functions by glucosylating misfolded proteins, retaining them in the ER until they are correctly folded or transferring them to degradation pathways. To determine the intracellular localization of Sep15, we expressed a green fluorescent protein-Sep15 fusion protein in CV-1 cells, and this protein was localized to the ER and possibly other perinuclear compartments. We determined that Sep15 contained the N-terminal signal peptide that was essential for translocation and that it was cleaved in the mature protein. However, C-terminal sequences of Sep15 were not involved in trafficking and retention of Sep15. The data suggest that the association between Sep15 and UGTR is responsible for maintaining the selenoprotein in the ER. This report provides the first example of the ER-resident selenoprotein and suggests a possible role of the trace element selenium in the quality control of protein folding.

A Selenoprotein in the Plant Kingdom

“…The observed truncation of C. reinhardtii GPX at the N terminus is consistent with similar observations in other human and mammalian selenoproteins (28,47). It suggests that the protein is not located in the cytosol.…”

Protein Folding and Quality Control in the ER

“…The ER also contains a number of selenoproteins. One of these is Sep15, which binds to UGGT and presumably works as a reductase, as suggested by the reducing potential of selenocysteine (Korotkov et al 2001).…”

Selenium in cancer prevention: a review of the evidence and mechanism of action

“…(According to Combs (2001), the same may be true of other parts of the world, as there is little or no information on Se intake or status for most of Africa, South America and central and south Asia.) Furthermore, an updated study of Se requirements by Burk's group in collaboration with Chinese colleagues (Xia et al 2005) has shown that full expression of selenoprotein P requires a greater Se intake than that required Spallholz et al (1990), Diplock (1994), Sunde (1997), Allan et al (1999) 15 kDa selenoprotein Associated with the endoplasmic reticulum: may be involved in the regulation of protein folding Korotkov et al (2001) Gene located in a region often altered in human cancers Hu et al (2001) Allan et al (1999) More highly expressed in cancer cells than in normal cells and its expression is repressed by p53 Gladyshev et al (1998) for full expression of plasma GPx. Thus, it is even more likely that current intakes are inadequate for optimising the protective effects of the selenoproteins.…”