Glycosylphosphatidylinositol (GPI) Anchors: Biochemistry and Cell Biology: Introduction to a Thematic Review Series

This article is available online at http://www.jlr.org Glycosylphosphatidylinositol (GPI) is a lipid anchor for many cell-surface proteins. The GPI anchor represents a posttranslational modifi cation of proteins with a glycolipid and is used ubiquitously in eukaryotes and most likely in some Archaea, but not in Eubacteria. GPI-anchored proteins are the major form of cell-surface proteins in protozoa. In fungi, many GPI-anchored proteins are ultimately incorporated into the cell wall. In humans, there are at least 150 GPI-anchored proteins, and they may function in a variety of roles serving as receptors, adhesion molecules, enzymes, transcytotic receptors and transporters, and protease inhibitors. The GPI anchor is assembled on a phosphatidylinositol lipid in the endoplasmic reticulum by a series of enzymatic reactions and then is covalently attached to the carboxyl terminus of proteins. The core of GPI consists of phosphatidylinositol, glycans comprising one glucosamine and three mannoses, and a terminal phosphoethanolamine, which is amide-bonded to the newly formed carboxyl terminus of the protein during the process of GPI attachment. Depending on the organism, cell type, and protein, the GPI backbone can be modifi ed with phosphoethanolamine and/or various glycan side-branches. The lipid moiety of the GPI anchor can be a 1-alkyl-2-acyl phosphatidylinositol, diacyl phosphatidylinositol, or inositol-phosphoceramide. In some cases, the inositol ring in the phosphatidylinositol moiety is palmitoylated or myristoylated. The carboxyl terminus of all GPI-anchored proteins contains a hydrophobic signal sequence that triggers the addition of the GPI anchor. During the process of adding the GPI anchor, the carboxyl-terminal stretch of hydrophobic amino acids is clipped off and replaced, via a transamidation reaction, with a preassembled GPI anchor. The GPI anchor is then further modifi ed in the endoplasmic reticulum and Golgi, a process that involves remodeling of both glycans and lipids, such as removal of mannose-linked phosphoethanolamine, removal of an inositol-linked acyl chain added during GPI assembly, or fatty acid remodeling. Once the various maturation steps are completed, GPI-anchored proteins are transported to the cell surface. GPI-anchored proteins have a number of hallmark features: they are typically associated with membrane microdomains (rafts) enriched in sphingolipids and cholesterol; they often exist on the cell surface as transient homodimers; they are endocytosed via a specifi c pathway; they transduce signals for proliferation or cell motility upon ligation and clustering; and they can be shed from the plasma membrane after cleavage of the GPI anchor. For each of these properties, the lipid and the glycan moieties of the GPI anchor are crucial. For example, the fatty acyl groups of GPI anchors in mammalian cells undergo remodeling in the Golgi apparatus, an enzymatic process that removes sn2-linked unsaturated fatty acids in the GPI and replaces them with a saturated fatty acid (stearic acid). The fatty acid remodeling of GPI is critical for the raft association of mammalian GPI-anchored proteins. Recent advances in human genetics, particularly exome sequencing, have revealed a number of genetic diseases associated with loss-of-function mutations in genes involved in the assembly, protein attachment, and remodeling of GPI anchors. These diseases, collectively termed “ inherited GPI defi ciency, ” have shown that normal amounts of GPI biosynthesis and proper maturation of GPI-anchors are crucial for human health. For instance, defects in the fatty acid remodeling of GPI anchors in the Golgi cause Mabry syndrome, which is characterized by hyperphosphatasia, intellectual disability and developmental delay, seizures, encephalopathy, facial dysmorphism, and other organ abnormalities. In this Thematic Review Series, four topics are reviewed by leading specialists in the fi eld. An article by Kinoshita and Fujita reviews the biosynthesis of GPI-anchored proteins in mammalian cells and yeast and discusses diseases caused by defective maturation of GPI anchors. The Glycosylphosphatidylinositol (GPI) Anchors: Biochemistry and Cell Biology: Introduction to a Thematic Review Series

Glycosylphosphatidylinositol (GPI) is a lipid anchor for many cell-surface proteins. The GPI anchor represents a posttranslational modifi cation of proteins with a glycolipid and is used ubiquitously in eukaryotes and most likely in some Archaea, but not in Eubacteria. GPI-anchored proteins are the major form of cell-surface proteins in protozoa. In fungi, many GPI-anchored proteins are ultimately incorporated into the cell wall. In humans, there are at least 150 GPI-anchored proteins, and they may function in a variety of roles serving as receptors, adhesion molecules, enzymes, transcytotic receptors and transporters, and protease inhibitors.
The GPI anchor is assembled on a phosphatidylinositol lipid in the endoplasmic reticulum by a series of enzymatic reactions and then is covalently attached to the carboxyl terminus of proteins. The core of GPI consists of phosphatidylinositol, glycans comprising one glucosamine and three mannoses, and a terminal phosphoethanolamine, which is amide-bonded to the newly formed carboxyl terminus of the protein during the process of GPI attachment. Depending on the organism, cell type, and protein, the GPI backbone can be modifi ed with phosphoethanolamine and/or various glycan side-branches. The lipid moiety of the GPI anchor can be a 1-alkyl-2-acyl phosphatidylinositol, diacyl phosphatidylinositol, or inositol-phosphoceramide. In some cases, the inositol ring in the phosphatidylinositol moiety is palmitoylated or myristoylated.
The carboxyl terminus of all GPI-anchored proteins contains a hydrophobic signal sequence that triggers the addition of the GPI anchor. During the process of adding the GPI anchor, the carboxyl-terminal stretch of hydrophobic amino acids is clipped off and replaced, via a transamidation reaction, with a preassembled GPI anchor. The GPI anchor is then further modifi ed in the endoplasmic reticulum and Golgi, a process that involves remodeling of both glycans and lipids, such as removal of mannose-linked phosphoethanolamine, removal of an inositol-linked acyl chain added during GPI assembly, or fatty acid remodeling. Once the various maturation steps are completed, GPI-anchored proteins are transported to the cell surface.
GPI-anchored proteins have a number of hallmark features: they are typically associated with membrane microdomains (rafts) enriched in sphingolipids and cholesterol; they often exist on the cell surface as transient homodimers; they are endocytosed via a specifi c pathway; they transduce signals for proliferation or cell motility upon ligation and clustering; and they can be shed from the plasma membrane after cleavage of the GPI anchor. For each of these properties, the lipid and the glycan moieties of the GPI anchor are crucial. For example, the fatty acyl groups of GPI anchors in mammalian cells undergo remodeling in the Golgi apparatus, an enzymatic process that removes sn2-linked unsaturated fatty acids in the GPI and replaces them with a saturated fatty acid (stearic acid). The fatty acid remodeling of GPI is critical for the raft association of mammalian GPI-anchored proteins.
Recent advances in human genetics, particularly exome sequencing, have revealed a number of genetic diseases associated with loss-of-function mutations in genes involved in the assembly, protein attachment, and remodeling of GPI anchors. These diseases, collectively termed " inherited GPI defi ciency, " have shown that normal amounts of GPI biosynthesis and proper maturation of GPI-anchors are crucial for human health. For instance, defects in the fatty acid remodeling of GPI anchors in the Golgi cause Mabry syndrome, which is characterized by hyperphosphatasia, intellectual disability and developmental delay, seizures, encephalopathy, facial dysmorphism, and other organ abnormalities.
In this Thematic Review Series, four topics are reviewed by leading specialists in the fi eld. An article by Kinoshita and Fujita reviews the biosynthesis of GPI-anchored proteins in mammalian cells and yeast and discusses diseases caused by defective maturation of GPI anchors. The To whom correspondence should be addressed. e-mail: tkinoshi@biken.osaka-u.ac.jp Thematic review series authors focus on molecular mechanisms for lipid remodeling of GPI anchors as well as the physiological and pathological signifi cance of these remodeling steps. A review by Satyajit Mayor and colleagues focuses on the organization and dynamics of GPI-anchored proteins on the cell surface as documented by advanced biophysical and imaging approaches. Based on the localization and behavior of GPI-anchored proteins, the authors propose revisions to current models of plasma membrane organization in eukaryotic cells. Muniz and Riezman discuss the intracellular traffi cking of GPI-anchored proteins in yeast and mammalian cells. They introduce new concepts derived from studies of the traffi cking of GPI-anchored proteins, in particular, unique mechanisms for protein sorting in the endoplasmic reticulum of yeast. Finally, Fujihara and Ikawa review biological functions of GPIanchored proteins and the shedding of GPI anchoredproteins from the cell surface following cleavage of GPI anchors, focusing on the role of this process in embryo fertilization.