Probable component of the GPI-GlcNAc transferase (GPI-GnT) complex in the endoplasmic reticulum, a complex that catalyzes transfer of GlcNAc from UDP-GlcNAc to an acceptor phosphatidylinositol, the first step in the production of GPI-anchors for cell surface proteins. Glycolipid biosynthesis; glycosylphosphatidylinositol-anchor biosynthesis. Interacts with GPI2, suggesting that it is a component of the GPI-GnT complex, probably composed of GPI1, GPI2, GPI3, GPI15, and ERI1.