Biochimica et Biophysica Acta (BBA) - General Subjects
ReviewLipid remodeling of GPI-anchored proteins and its function
Introduction
A number of proteins are modified with glycosylphosphatidylinositol (GPI). GPI anchoring is a post-translational modifications conserved among yeast, protozoans, plants, and animals [1], [2], [3]. The existence of GPI anchors and their modification of proteins were identified in the 1980s, and the complete structures of the GPI anchors of Trypanosoma brucei variant surface glycoprotein and rat brain Thy-1 were determined in 1988 [4], [5]. These structures provided the impetus for studying the biosynthesis and modification pathways for the GPI anchor, as well as for studying the function of individual GPI-anchored proteins. More than 150 plasma membrane proteins are presently known to be anchored by GPI in mammalian cells. GPI-anchored proteins are functionally diverse and are important for signal transduction, cell–cell interaction, cell adhesion, and host defense [2], [6]. In addition, some GPI-anchored proteins function as receptors for viruses and toxins [2]. In the genome of the yeast Saccharomyces cerevisiae, more than 60 genes are predicted to encode GPI-anchored proteins which play important roles in cell wall biogenesis and cell wall assembly [7]. Some GPI-anchored proteins are localized at the plasma membrane, whereas others are further processed at the plasma membrane and are covalently linked via the GPI moiety to β1,6-glucan components of the cell wall.
GPI anchors have variations in the side chain and lipid moieties, but the core structure is conserved among all species. The biosynthesis of GPI is essential for the growth of yeasts and bloodstream forms of protozoan, and for embryonic development in mammals [8], [9], [10]. The biosynthesis and attachment of GPI to proteins occur by a series of sequential reactions on the endoplasmic reticulum (ER) membrane. Phosphatidylinositol is modified by the stepwise additions of sugars and ethanolaminephosphates (EtN-P), forming a complete precursor lipid in the ER [11], [12]. Most genes involved in the GPI biosynthetic pathway have been characterized in mammalian cells and yeast. To date, more than 20 genes are known to be directly involved in this reaction [11], [12].
GPI-anchored proteins have unique properties that are different from those of soluble or membrane proteins. One of the most striking features of GPI-anchored proteins is their association with specialized lipid domain, called microdomains or lipid rafts [13], [14]. Microdomains or lipid rafts are assumed to be sphingolipid- and sterol-rich membranes and can be biochemically isolated as a detergent-resistant membrane (DRM) fraction [13], [15]. There is no satisfactory simple definition of microdomains, but certain lipids form specific membrane structures that function as a platform for intracellular signaling and are required for the selective transport of proteins [14], [15], [16]. The structure of GPI is important for the integration of GPI-anchored proteins into lipid microdomains. Although the GPI anchor is synthesized from phosphatidylinositol (PI) containing unsaturated fatty acyl chains at the sn-2 position, the lipid moieties of the GPI anchor are exchanged after GPI attachment to proteins in both yeast and mammalian cells [17], [18] (Fig. 1). This lipid remodeling process of GPI-anchored proteins is essential for their association with the lipid microdomains. In this review, we discuss the genes involved in the lipid remodeling pathways of GPI-anchored proteins and their functions in yeast and mammalian cells.
Section snippets
Evidence of lipid remodeling from structural analysis
Lipid remodeling of GPI-anchors has been well characterized in the African trypanosome T. brucei [19]. The fatty acids of the GPI intermediates are replaced by myristic acid (C14:0) through sequential deacylation and reacylation reactions at the sn-2 position followed by the sn-1 position before the GPI is attached to the protein in the bloodstream form of T. brucei [1], [19], [20]. Trypanosoma cruzi has GPI anchors containing ceramides, even though ceramides are not the first substrate in GPI
Yeast BST1 and mammalian PGAP1 involved in GPI inositol deacylation
Early in the GPI biosynthesis pathway, the inositol on the GPI intermediate GlcN-PI is acylated by yeast Gwt1p or mammalian PIG-W [40], [41]. This acylation step is clearly critical for efficient GPI biosynthesis and attachment to proteins, as the amounts of GPI-anchored proteins are greatly decreased in Gwt1p- or PIG-W-deficient mutant cells. Once the GPI anchor is attached to a protein, the inositol is usually deacylated in the ER. Except in human erythrocytes [42], [43], [44], almost all
Lipid remodeling and microdomain association
GPI anchoring to a protein confers a specific association with DRMs, which implies that association with a lipid-dependent structural unit has functional significance [13]. Several biochemical studies have proposed that GPI-anchored proteins are present in specialized membrane domains known as microdomains, which are sphingolipid- and sterol-rich domains in membrane [15]. However, until very recently, little was known about how GPI-anchored proteins associated with microdomains.
ER to Golgi transport of GPI-anchored proteins
Both yeast Bst1p and mammalian PGAP1 are required for the efficient transport of GPI-anchored proteins from the ER to the Golgi [35], [47]. Inositol deacylation might trigger the concentration or sorting of GPI-anchored proteins into the ER-exit site. In yeast, the transport of GPI-anchored proteins from the ER to the Golgi is substantially delayed in gup1Δ and per1Δ cells [32], [36]. On the other hand, fatty acid remodeling of GPI-anchored proteins does not affect their transport from the ER
Quality control of GPI-anchored proteins and inositol deacylation
Before exiting from the ER, proteins are monitored by a quality control system that ensures their correct folding and oligomerization. A number of chaperones and enzymes in the ER are required for proper protein folding [74]. Misfolded proteins that fail to pass the quality control checkpoint are transported back to the cytosol and are degraded by an ER-associated degradation mechanism that involves the ubiquitin–proteasome pathway [75], [76]. Post-translational modification of proteins,
Targeting of GPI-anchored proteins in yeast
Yeasts exhibit several types of cell polarity depending on their growth stage. One example is polarized growth during mating. After an alpha factor pheromone binds to a receptor, signals are transmitted inside the cell by the mitogen activating kinase cascade [80], [81]. Then, polarized growth towards the mating partner is induced and the cell changes its shape to form shmoos [82]. A mating projection is formed during the polarized growth, and proteins required for cell fusion are concentrated
Transport of proteins associated with microdomains in yeast
GPI-anchored proteins also play important roles in the targeting of membrane proteins, which change their cellular localization in response to environmental and nutritional conditions, by associating with lipid microdomains [99]. A tryptophan permease, Tat2p, is associated with DRMs and is transported to the plasma membrane at low tryptophan concentrations, whereas at high tryptophan concentrations it is polyubiquitinated by the Rsp5p ubiquitin ligase complex in early endosomes and then
Apical transport in mammalian cells
Lipid remodeling of GPI-anchored proteins is interesting in the polarized transport of GPI-anchored proteins in mammalian cells. Many GPI-anchored proteins are transported to the apical side of the plasma membrane in several types of epithelial cells, and to the axonal region of neuronal cells [104], [105], [106], although in Fischer rat thyroid cells, the majority of GPI-anchored proteins are delivered to the basolateral membrane [107]. Treatment with Fumonisin B1, which is an inhibitor of
Endocytic pathway of GPI-anchored proteins
GPI-anchored proteins are endocytosed differently from other plasma membrane proteins, but there is still controversy about endocytic pathway of GPI-anchored proteins [14], [112]. Although most endocytic pathways require at least one of the numerous dynamin isoforms for the purpose of pinching-off at the cell surface, several GPI-anchored proteins are selectively internalized through dynamin-independent pathway [113], [114]. This internalization is regulated by the small GTPase, CDC42 [114].
Perspective
In this review, we described the pathway for the lipid remodeling of GPI-anchored proteins, the genes involved in this pathway, and the functions of these genes, proteins and pathways in yeast and mammalian cells. To understand the function and regulation of the GPI anchor, it is important to consider the problem at the level of lipid-dependent organization. Lipid remodeling of GPI-anchored proteins changes their physical properties and regulates their association with microdomains both in
Note added in proof
Two articles describing CWH43 have appeared recently: M. Umemura, M. Fujita, T. Yoko-o, A. Fukamizu, Y. Jigami, Saccharomyces cerevisiae CWH43 is involved in the remodeling of the lipid moiety of GPI anchors to ceramides, Mol. Biol. Cell (2007) in press; V. Ghugtyal, C. Vionnet, C. Roubaty, A. Conzelmann, CWH43 is required for the introduction of ceramides into GPI anchors in Saccharomyces cerevisiae, Mol. Microbiol. 65 (2007) 1493–1502.
Acknowledgements
We thank Mariko Umemura, Michiyo Okamoto, Hiroto Hirayama, Takuji Oka, and Takehiko Yoko-o for their helpful discussions. This research was partially supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS). M.F. was supported by a Research Fellowship for Young Scientists from the JSPS.
References (126)
- et al.
Defective glycosyl phosphatidylinositol anchor synthesis and paroxysmal nocturnal hemoglobinuria
Adv. Immunol.
(1995) - et al.
The contribution of cell wall proteins to the organization of the yeast cell wall
Biochim. Biophys. Acta
(1999) - et al.
A conditionally lethal yeast mutant blocked at the first step in glycosyl phosphatidylinositol anchor synthesis
J. Biol. Chem.
(1994) - et al.
Biosynthesis and function of GPI proteins in the yeast Saccharomyces cerevisiae
Biochim. Biophys. Acta
(2007) - et al.
Sorting of GPI-anchored proteins to glycolipid-enriched membrane subdomains during transport to the apical cell surface
Cell
(1992) - et al.
The GPI biosynthetic pathway as a therapeutic target for African sleeping sickness
Biochim. Biophys. Acta
(1999) - et al.
Inositolphosphoceramide is not a substrate for the first steps in the biosynthesis of glycoinositolphospholipids in Trypanosoma cruzi
Mol. Biochem. Parasitol.
(2004) - et al.
Structures of glycosylphosphatidylinositol membrane anchors from Saccharomyces cerevisiae
J. Biol. Chem.
(1993) - et al.
Acyl and alkyl chain length of GPI-anchors is critical for raft association in vitro
FEBS Lett.
(1999) - et al.
Structures of the glycosyl-phosphatidylinositol anchors of porcine and human renal membrane dipeptidase. Comprehensive structural studies on the porcine anchor and interspecies comparison of the glycan core structures
J. Biol. Chem.
(1995)
Identification of molecular species of glycerophospholipids and sphingomyelin using electrospray mass spectrometry
J. Lipid Res.
Changes in molecular species profiles of glycosylphosphatidylinositol-anchor precursors in early stages of biosynthesis
J. Lipid Res.
Inositol deacylation of glycosylphosphatidylinositol-anchored proteins is mediated by mammalian PGAP1 and yeast Bst1p
J. Biol. Chem.
Deletion of GPI7, a yeast gene required for addition of a side chain to the glycosylphosphatidylinositol (GPI) core structure, affects GPI protein transport, remodeling, and cell wall integrity
J. Biol. Chem.
Ethanolaminephosphate side chain added to glycosylphosphatidylinositol (GPI) anchor by mcd4p is required for ceramide remodeling and forward transport of GPI proteins from endoplasmic reticulum to Golgi
J. Biol. Chem.
GWT1 gene is required for inositol acylation of glycosylphosphatidylinositol anchors in yeast
J. Biol. Chem.
Glycan components in the glycoinositol phospholipid anchor of human erythrocyte acetylcholinesterase. Novel fragments produced by trifluoroacetic acid
J. Biol. Chem.
The glycosylation of the complement regulatory protein, human erythrocyte CD59
J. Biol. Chem.
Screening for mutants defective in secretory protein maturation and ER quality control
Methods
A superfamily of membrane-bound O-acyltransferases with implications for wnt signaling
Trends Biochem. Sci.
Isolation of novel heart-specific genes using the BodyMap database
Genomics
Identification of ISC1 (YER019w) as inositol phosphosphingolipid phospholipase C in Saccharomyces cerevisiae
J. Biol. Chem.
SAC1-like domains of yeast SAC1, INP52, and INP53 and of human synaptojanin encode polyphosphoinositide phosphatases
J. Biol. Chem.
INP51, a yeast inositol polyphosphate 5-phosphatase required for phosphatidylinositol 4,5-bisphosphate homeostasis and whose absence confers a cold-resistant phenotype
J. Biol. Chem.
The yeast inositol polyphosphate 5-phosphatase Inp54p localizes to the endoplasmic reticulum via a C-terminal hydrophobic anchoring tail: regulation of secretion from the endoplasmic reticulum
J. Biol. Chem.
Exploration of the function and organization of the yeast early secretory pathway through an epistatic miniarray profile
Cell
Acyl chain unsaturation in PEs modulates phase separation from lipid raft molecules
Biochem. Biophys. Res. Commun.
Protein sorting upon exit from the endoplasmic reticulum
Cell
The Rab GTPase Ypt1p and tethering factors couple protein sorting at the ER to vesicle targeting to the Golgi apparatus
Dev. Cell
Sphingolipids are required for the stable membrane association of glycosylphosphatidylinositol-anchored proteins in yeast
J. Biol. Chem.
A novel inhibitor of ceramide trafficking from the endoplasmic reticulum to the site of sphingomyelin synthesis
J. Biol. Chem.
Differential ER exit in yeast and mammalian cells
Curr. Opin. Cell Biol.
ER quality control: the cytoplasmic connection
Cell
Roles of O-mannosylation of aberrant proteins in reduction of the load for endoplasmic reticulum chaperones in yeast
J. Biol. Chem.
S. cerevisiae alpha pheromone receptors activate a novel signal transduction pathway for mating partner discrimination
Cell
Courtship in S. cerevisiae: both cell types choose mating partners by responding to the strongest pheromone signal
Cell
Pheromone signalling and polarized morphogenesis in yeast
Curr. Opin. Genet. Dev.
Slow diffusion of proteins in the yeast plasma membrane allows polarity to be maintained by endocytic cycling
Curr. Biol.
The yeast cell wall and septum as paradigms of cell growth and morphogenesis
J. Biol. Chem.
Yeast Cbk1 and Mob2 activate daughter-specific genetic programs to induce asymmetric cell fates
Cell
GPI7 involved in glycosylphosphatidylinositol biosynthesis is essential for yeast cell separation
J. Biol. Chem.
Bud-site selection and cell polarity in budding yeast
Curr. Opin. Microbiol.
Glycosylphosphatidylinositol-anchored proteins are required for the transport of detergent-resistant microdomain-associated membrane proteins Tat2p and Fur4p
J. Biol. Chem.
The structure, biosynthesis and functions of glycosylphosphatidylinositol anchors, and the contributions of trypanosome research
J. Cell Sci.
Glycosylphosphatidylinositol (GPI)-anchored proteins
Biol. Pharm. Bull.
The structure, biosynthesis and function of glycosylated phosphatidylinositols in the parasitic protozoa and higher eukaryotes
Biochem. J.
Glycosyl-phosphatidylinositol moiety that anchors Trypanosoma brucei variant surface glycoprotein to the membrane
Science
Complete structure of the glycosyl phosphatidylinositol membrane anchor of rat brain Thy-1 glycoprotein
Nature
Critical roles of glycosylphosphatidylinositol for Trypanosoma brucei
Proc. Natl. Acad. Sci. U. S. A.
Developmental abnormalities of glycosylphosphatidylinositol-anchor-deficient embryos revealed by Cre/loxP system
Lab. Invest.
Cited by (104)
Quality-controlled ceramide-based GPI-anchored protein sorting into selective ER exit sites
2022, Cell ReportsCitation Excerpt :The canonical ER quality-control system monitors and facilitates protein folding with the outcome of the degradation of misfolded proteins (Ellgaard and Helenius, 2003; Araki and Nagata, 2011; Sun and Brodsky, 2019; Phillips et al., 2020). Terminally misfolded GPI-APs are degraded by ubiquitin- and proteasome-dependent endoplasmic-reticulum-associated protein degradation (ERAD) or in vacuole/lysosomes by a process called rapid ER stress-induced export (RESET) (Fujita and Jigami, 2008; Satpute-Krishnan et al., 2014; Sikorska et al., 2016; Lopez et al., 2019; Kinoshita, 2020; Nakatsukasa, 2021; Lemus et al., 2021). In mammalian cells, N-glycans and their recognition by calnexin participate in protein-folding quality control and the GPI-inositol deacylation of GPI-APs (Liu et al., 2018; Guo et al., 2020).
Glycobiology of Yeast: Applications to Glycoprotein Expression and Remodeling
2021, Comprehensive Glycoscience: Second EditionGlycosylphosphatidylinositol Anchors and Lipids
2021, Comprehensive Glycoscience: Second EditionLipids and their (un)known effects on ER-associated protein degradation (ERAD)
2020, Biochimica et Biophysica Acta - Molecular and Cell Biology of LipidsThe natural anticancer agent cantharidin alters GPI-anchored protein sorting by targeting Cdc1-mediated remodeling in endoplasmic reticulum
2019, Journal of Biological ChemistryCitation Excerpt :Yeast cell wall biosynthesis and maintenance mainly depend on the GPI-anchored proteins, sorted by the ER–Golgi traffic system (1, 7, 58). PE also plays a crucial role in the regulation of this traffic system (4, 7, 44, 58). Thus, we hypothesized that the CTD-induced cell wall damage might be due to the defect in GPI-anchored protein sorting.
Biosynthesis of GPI-anchored proteins: Special emphasis on GPI lipid remodeling
2016, Journal of Lipid Research