More than 100 mammalian proteins are post-translationally modified by glycosylphosphatidylinositol (GPI) at their C-termini and are anchored to the cell surface membrane via the lipid portion. GPI-anchored proteins (GPI-APs) have various functions, such as hydrolytic enzymes, receptors, adhesion molecules, complement regulatory proteins and other immunologically important proteins. GPI-anchored proteins are mainly associated with membrane microdomains or membrane rafts enriched in sphingolipids and cholesterol. It is thought that association with membrane rafts is important for GPI-APs in signal transduction and other functions. Here, we review recent progress in studies on biosynthesis, remodelling and functions of mammalian GPI-APs.

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Whereas most of the cellular phosphatidylinositol (PI) contain unsaturated fatty chains and are excluded from rafts, GPI-anchored proteins (APs) unusually contain two saturated fatty chains in their PI moiety, and they are typically found within lipid rafts. However, the origin of the saturated chains and whether they are essential for raft association are unclear. Here, we report that GPI-APs, with two saturated fatty chains, are generated from those bearing an unsaturated chain by fatty acid remodeling that occurs most likely in the Golgi and requires post-GPI-attachment to proteins (PGAP)2 and PGAP3. The surface GPI-APs isolated from the PGAP2 and -3 double-mutant Chinese hamster ovary (CHO) cells had unsaturated chains, such as oleic, arachidonic, and docosatetraenoic acids in the sn-2 position, whereas those from wild-type CHO cells had exclusively stearic acid, a saturated chain, indicating that the sn-2 chain is exchanged to a saturated chain. We then assessed the association of GPI-APs with lipid rafts. Recovery of unremodeled GPI-APs from the double-mutant cells in the detergent-resistant membrane fraction was very low, indicating that GPI-APs become competent to be incorporated into lipid rafts by PGAP3-and PGAP2-mediated fatty acid remodeling. We also show that the remodeling requires the preceding PGAP1-mediated deacylation from inositol of GPI-APs in the endoplasmic reticulum.

Glycosylphoshatidylinositol (GPI) anchors are remodeled during their transport to the cell surface. Newly synthesized proteins are transferred to a GPI anchor, consisting of diacylglycerol with conventional C16 and C18 fatty acids, whereas the lipid moiety in mature GPI-anchored proteins is exchanged to either diacylglycerol containing a C26:0 fatty acid in the sn-2 position or ceramide in Saccharomyces cerevisiae. Here, we report on PER1, a gene encoding a protein that is required for the GPI remodeling pathway. We found that GPI-anchored proteins could not associate with the detergent-resistant membranes in per1Delta cells. In addition, the mutant cells had a defect in the lipid remodeling from normal phosphatidylinositol (PI) to a C26 fatty acid-containing PI in the GPI anchor. In vitro analysis showed that PER1 is required for the production of lyso-GPI, suggesting that Per1p possesses or regulates the GPI-phospholipase A2 activity. We also found that human PERLD1 is a functional homologue of PER1. Our results demonstrate for the first time that PER1 encodes an evolutionary conserved component of the GPI anchor remodeling pathway, highlighting the close connection between the lipid remodeling of GPI and raft association of GPI-anchored proteins.

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p24 complexes act as cargo receptors for sorting GPI-anchored proteins into COPII vesicles.

Many eukaryotic proteins are attached to the cell surface via glycosylphosphatidylinositol (GPI) anchors. How GPI-anchored proteins (GPI-APs) are trafficked from the endoplasmic reticulum (ER) to the cell surface is poorly understood, but the GPI moiety has been postulated to function as a signal for sorting and transport. Here, we established mutant cells that were selectively defective in transport of GPI-APs from the ER to the Golgi. We identified a responsible gene, designated PGAP5 (post-GPI-attachment to proteins 5). PGAP5 belongs to a dimetal-containing phosphoesterase family and catalyzed the remodeling of the glycan moiety on GPI-APs. PGAP5 catalytic activity is a prerequisite for the efficient exit of GPI-APs from the ER. Our data demonstrate that GPI glycan acts as an ER-exit signal and suggest that glycan remodeling mediated by PGAP5 regulates GPI-AP transport in the early secretory pathway.

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