G-rich is a Drosophila melanogaster selenoprotein, which is a homologue of human and
mouse SelK. Subcellular localization analysis using GFP-tagged G-rich showed that
G-rich was localized in the Golgi apparatus. The fusion protein was co-localized with
the Golgi marker proteins but not with an endoplasmic reticulum (ER) marker protein
in Drosophila SL2 cells. Bioinformatic analysis of G-rich suggests that this protein
is either type II or type III transmembrane protein. To determine the type of transmembrane
protein experimentally, GFP-G-rich in which GFP was tagged at the N-terminus of G-rich,
or G-rich-GFP in which GFP was tagged at the C-terminus of G-rich, were expressed
in SL2 cells. The tagged proteins were then digested with trypsin, and analyzed by
Western blot analysis. The results showed that the C-terminus of the G-rich protein
was exposed to the cytoplasm indicating it is a type III microsomal membrane protein.
G-rich is the first selenoprotein identified in the Golgi apparatus.