Abstract
Glycosylphosphatidylinositol (GPI) is a post-translational modification that anchors cell surface proteins to the plasma membrane, and GPI modifications occur in all eukaryotes. Biosynthesis of GPI starts on the cytoplasmic face of the endoplasmic reticulum (ER) membrane, and GPI precursors flip from the cytoplasmic side to the luminal side of the ER, where biosynthesis of GPI precursors is completed. Gwt1p and PIG-W are inositol acyltransferases that transfer fatty acyl chains to the inositol moiety of GPI precursors in yeast and mammalian cells, respectively. To ascertain whether flipping across the ER membrane occurs before or after inositol acylation of GPI precursors, we identified essential residues of PIG-W and Gwt1p and determined the membrane topology of Gwt1p. Guided by algorithm-based predictions of membrane topology, we experimentally identified 13 transmembrane domains in Gwt1p. We found that Gwt1p, PIG-W, and their orthologs shared four conserved regions and that these four regions in Gwt1p faced the luminal side of the ER membrane. Moreover, essential residues of Gwt1p and PIG-W faced the ER lumen or were near the luminal edge of transmembrane domains. The membrane topology of Gwt1p suggested that inositol acylation occurred on the luminal side of the ER membrane. Rather than stimulate flipping of the GPI precursor across the ER membrane, inositol acylation of GPI precursors may anchor the precursors to the luminal side of the ER membrane, preventing flip-flops.