The first step of glycosylphosphatidylinositol biosynthesis is mediated by a complex of PIG-A, PIG-H, PIG-C and GPI1

PathLocdb: a comprehensive database for the subcellular localization of metabolic pathways and its application to multiple localization analysis

Background

In eukaryotes, the cell is divided into several compartments enclosed by unitary membranes. Such compartmentalization is critical for cells to restrict different pathways to be carried out in different subcellular regions. The summary and classification of subcellular localizations of metabolic pathways are the first steps towards understanding their roles in spatial differentiation and the specialization of metabolic pathways in different organisms.

Results

Integrating the subcellular localization of enzymes and their pathways from UniProt Knowledgebase and KEGG pathway databases, we present the first database for subcellular localization of 43014 pathways from 80676 UniProt entries and their pathway annotations from UniProt and KEGG pathway databases. To extract pathway localization across organisms, we defined 889 superpathways as clusters of basic pathways with the same pathway annotations from different organisms. Over eighty-eight percent of superpathways in the Swiss-Prot dataset occur in cytoplasm and mitochondria. And over seventy percent of UniProt superpathways have multiple localization annotations. We summarized four common reasons for the multiple localization of superpathways. Based on this database, we also discovered 88 potential transport systems between different steps of multiply localized pathways and 45 duplicated genes from 17 pathways, occurring in parallel in several locations in humans.

Conclusions

PathLocdb is a free web-accessible database that enables biochemical researchers to quickly access summarized subcellular localization of pathways from UniProt and KEGG pathway databases. As the first effort to systematically integrate pathway localization, this database is very useful in discovering the variation of localization of pathways between organisms and also cross-talk between different organelles within a pathway. The Pathlocdb database is available at http://pathloc.cbi.pku.edu.cn.

The Candida albicans homologue of PIG-P, CaGpi19p: gene dosage and role in growth and filamentation

Glycosylphosphatidyl inositol (GPI)-anchored proteins in Candida albicans are responsible for a vast range of functions, and deletions in certain GPI-anchored proteins severely reduce adhesion and virulence of this organism. In addition, completely modified GPIs are necessary for virulence. GPI anchor biosynthesis is essential for viability and starts with the transfer of N-acetylglucosamine to phosphatidylinositol. This step is catalysed by a multi-subunit complex, GPI–N-acetylglucosaminyltransferase (GPI–GnT). In this, the first report to our knowledge on a subunit of the Candida GPI–GnT complex, we show that CaGpi19p is the functional equivalent of the Saccharomyces cerevisiae Gpi19p. An N-terminal truncation mutant of CaGpi19p functionally complements a conditionally lethal S. cerevisiae gpi19 mutant. Further, we constructed a conditional null mutant of CaGPI19 by disrupting one allele and placing the remaining copy under the control of the MET3 promoter. Repression leads to growth defects, cell wall biogenesis aberrations, azole sensitivity and hyperfilamention. In addition, there is a noticeable gene dosage effect, with the heterozygote also displaying intermediate degrees of most phenotypes. The mutants also displayed a reduced susceptibility to the antifungal agent amphotericin B. Collectively, the results suggest that CaGPI19 is required for normal morphology and cell wall architecture.

Convergent extension movements in growth plate chondrocytes require gpi-anchored cell surface proteins

Proteins that are localized to the cell surface via glycosylphosphatidylinositol (gpi) anchors have been proposed to regulate cell signaling and cell adhesion events involved in tissue patterning. Conditional deletion of Piga, which encodes the catalytic subunit of an essential enzyme in the gpi-biosynthetic pathway, in the lateral plate mesoderm results in normally patterned limbs that display chondrodysplasia. Analysis of mutant and mosaic Piga cartilage revealed two independent cell autonomous defects. First, loss of Piga function interferes with signal reception by chondrocytes as evidenced by delayed maturation. Second, the proliferative chondrocytes, although present, fail to flatten and arrange into columns. We present evidence that the abnormal organization of mutant proliferative chondrocytes results from errors in cell intercalation. Collectively, our data suggest that the distinct morphological features of the proliferative chondrocytes result from a convergent extension-like process that is regulated independently of chondrocyte maturation.

Trypanosome glycosylphosphatidylinositol biosynthesis

Trypanosoma brucei, a protozoan parasite, causes sleeping sickness in humans and Nagana disease in domestic animals in central Africa. The trypanosome surface is extensively covered by glycosylphosphatidylinositol (GPI)-anchored proteins known as variant surface glycoproteins and procyclins. GPI anchoring is suggested to be important for trypanosome survival and establishment of infection. Trypanosomes are not only pathogenically important, but also constitute a useful model for elucidating the GPI biosynthesis pathway. This review focuses on the trypanosome GPI biosynthesis pathway. Studies on GPI that will be described indicate the potential for the design of drugs that specifically inhibit trypanosome GPI biosynthesis.

Silencing of genes required for glycosylphosphatidylinositol anchor biosynthesis in Burkitt lymphoma

OBJECTIVE

To investigate the mechanism of glycosylphosphatidylinositol (GPI) anchor deficiency in Burkitt lymphoma cell lines.

METHODS

We identified a large GPI anchor protein deficient population in three different Burkitt lymphoma cell lines through proaerolysin treatment of the cells and flow cytometry analysis using a proaerolysin variant (FLAER). The mechanism of GPI anchor protein deficiency was studied by DNA gene sequencing, a cell-free assay to investigate the GPI anchor biosynthetic pathway, microarray analysis, and quantitative real-time polymerase chain reaction.

RESULTS

Burkitt lymphoma cell lines harbor large populations of FLAER(neg) cells, which are resistant to proaerolysin. In all three cell lines, silencing of a gene involved in an early step in GPI-anchor biosynthesis was responsible for the lack of GPI-anchored proteins on the cell surface. Quantitative polymerase chain reaction and microarray analysis demonstrate that the level of mRNA for PIGL and PIGY is lower in the FLAER(neg) Ramos cells and that mRNA levels of PIGY are reduced in the Akata and Daudi cells. Hypermethylation of these genes was associated with the low levels of mRNA and treatment of the cells with 5-aza-2' deoxycytidine restored cell surface GPI-anchored proteins to the FLAER(neg) cells.

CONCLUSION

GPI-anchored protein deficiency in Burkitt lymphoma cells is not due to a genetic mutation (e.g., PIGA); rather, the lack of GPI-anchored proteins results from transcriptional silencing of PIGL and PIGY.

Development of an in vivo gene mutation assay using the endogenous Pig-A gene: I. Flow cytometric detection of CD59-negative peripheral red blood cells and CD48-negative spleen T-cells from the rat

The product of the phosphatidylinositol glycan complementation group A gene (Pig-A) is involved in the synthesis of glycosylphosphatidylinositol (GPI) anchors that link various protein markers to the surface of several types of mammalian cells, including hematopoietic cells. Previous observations indicate that Pig-A mutation results in the lack of GPI synthesis and the absence of GPI-anchored proteins on the cell surface. As a first step in designing a rapid assay for measuring Pig-A mutation in the rat, we developed flow cytometry (FCM) strategies for detecting GPI-negative cells in rat peripheral blood and spleen. Anti-CD59 was used to detect GPI-anchored proteins on red blood cells (RBCs), and anti-CD48 was used to detect GPI-anchored proteins on spleen T-cells. The spontaneous frequency of CD59-negative RBCs in five male F344 rats ranged from 1 x 10(-6) to 27 x 10(-6). In contrast, treatment of five rats with three doses of 40 mg/kg N-ethyl-N-nitrosourea (ENU) increased the frequency of CD59-negative RBCs to 183 x 10(-6) to 249 x 10(-6) at 2 weeks and to 329 x 10(-6) to 413 x 10(-6) at 4 weeks after dosing. In the same 4-week posttreatment rats, the frequency of CD48-negative T-cells was 11 x 10(-6) to 16 x 10(-6) in control rats and 194 x 10(-6) to 473 x 10(-6) in ENU-treated rats. The frequencies of GPI-deficient cells were similar for RBCs and spleen T-cells. These results indicate that FCM detection of GPI-linked markers may form the basis for a rapid in vivo mutation assay. Although RBCs may be useful for a minimally invasive assay, T-cells are a promising tissue for both detecting GPI-deficient cells and confirming that Pig-A gene mutation is the cause of the phenotype.

Yeast ARV1 is required for efficient delivery of an early GPI intermediate to the first mannosyltransferase during GPI assembly and controls lipid flow from the endoplasmic reticulum

Glycosylphosphatidylinositol (GPI), covalently attached to many eukaryotic proteins, not only acts as a membrane anchor but is also thought to be a sorting signal for GPI-anchored proteins that are associated with sphingolipid and sterol-enriched domains. GPI anchors contain a core structure conserved among all species. The core structure is synthesized in two topologically distinct stages on the leaflets of the endoplasmic reticulum (ER). Early GPI intermediates are assembled on the cytoplasmic side of the ER and then are flipped into the ER lumen where a complete GPI precursor is synthesized and transferred to protein. The flipping process is predicted to be mediated by a protein referred as flippase; however, its existence has not been proven. Here we show that yeast Arv1p is an important protein required for the delivery of an early GPI intermediate, GlcN-acylPI, to the first mannosyltransferase of GPI synthesis in the ER lumen. We also provide evidence that ARV1 deletion and mutations in other proteins involved in GPI anchor synthesis affect inositol phosphorylceramide synthesis as well as the intracellular distribution and amounts of sterols, suggesting a role of GPI anchor synthesis in lipid flow from the ER.

Advances in the diagnosis and therapy of paroxysmal nocturnal hemoglobinuria

PNH is an uncommon acquired hemolytic anemia that often manifests with hemoglobinuria, abdominal pain, smooth muscle dystonias, fatigue, and thrombosis. The disease results from the expansion of hematopoietic stem cells harboring a mutation in a gene, PIG-A, that is required for the biosynthesis of a lipid moiety, glycosylphosphatidylinositol (GPI), that attaches dozens of different proteins to the cell surface. Thus, PNH cells are deficient in cell surface GPI anchored proteins; this deficiency on erythrocytes leads to intravascular hemolysis since certain GPI anchored proteins normally function as complement regulators. Free hemoglobin released from intravascular hemolysis leads to circulating nitric oxide depletion and is responsible for many of the clinical manifestations of PNH, including fatigue, erectile dysfunction, esophageal spasm, and thrombosis. Interestingly, rare PIG-A mutations can be found in virtually all healthy control subjects leading to speculation that PIG-A mutations in hematopoietic stem cells are common benign events. However, recent data reveals that most of these mutations in healthy controls are not derived from stem cells. The recently FDA approved complement inhibitor eculizumab has been shown to decrease hemolysis, decrease erythrocyte transfusion requirements, decrease the risk for thrombosis and improve quality of life for PNH patients.

Molecular recognition and interfacial catalysis by the essential phosphatidylinositol mannosyltransferase PimA from mycobacteria

Mycobacterial phosphatidylinositol mannosides (PIMs) and metabolically derived cell wall lipoglycans play important roles in host-pathogen interactions, but their biosynthetic pathways are poorly understood. Here we focus on Mycobacterium smegmatis PimA, an essential enzyme responsible for the initial mannosylation of phosphatidylinositol. The structure of PimA in complex with GDP-mannose shows the two-domain organization and the catalytic machinery typical of GT-B glycosyltransferases. PimA is an amphitrophic enzyme that binds mono-disperse phosphatidylinositol, but its transferase activity is stimulated by high concentrations of non-substrate anionic surfactants, indicating that the early stages of PIM biosynthesis involve lipid-water interfacial catalysis. Based on structural, calorimetric, and mutagenesis studies, we propose a model wherein PimA attaches to the membrane through its N-terminal domain, and this association leads to enzyme activation. Our results reveal a novel mode of phosphatidylinositol recognition and provide a template for the development of potential antimycobacterial compounds.

Thematic review series: lipid posttranslational modifications. GPI anchoring of protein in yeast and mammalian cells, or: how we learned to stop worrying and love glycophospholipids

Glycosylphosphatidylinositol (GPI) anchoring of cell surface proteins is the most complex and metabolically expensive of the lipid posttranslational modifications described to date. The GPI anchor is synthesized via a membrane-bound multistep pathway in the endoplasmic reticulum (ER) requiring >20 gene products. The pathway is initiated on the cytoplasmic side of the ER and completed in the ER lumen, necessitating flipping of a glycolipid intermediate across the membrane. The completed GPI anchor is attached to proteins that have been translocated across the ER membrane and that display a GPI signal anchor sequence at the C terminus. GPI proteins transit the secretory pathway to the cell surface; in yeast, many become covalently attached to the cell wall. Genes encoding proteins involved in all but one of the predicted steps in the assembly of the GPI precursor glycolipid and its transfer to protein in mammals and yeast have now been identified. Most of these genes encode polytopic membrane proteins, some of which are organized in complexes. The steps in GPI assembly, and the enzymes that carry them out, are highly conserved. GPI biosynthesis is essential for viability in yeast and for embryonic development in mammals. In this review, we describe the biosynthesis of mammalian and yeast GPIs, their transfer to protein, and their subsequent processing.

Biosynthesis and function of GPI proteins in the yeast Saccharomyces cerevisiae

Like most other eukaryotes, Saccharomyces cerevisiae harbors a GPI anchoring machinery and uses it to attach proteins to membranes. While a few GPI proteins reside permanently at the plasma membrane, a majority of them gets further processed and is integrated into the cell wall by a covalent attachment to cell wall glucans. The GPI biosynthetic pathway is necessary for growth and survival of yeast cells. The GPI lipids are synthesized in the ER and added onto proteins by a pathway comprising 12 steps, carried out by 23 gene products, 19 of which are essential. Some of the estimated 60 GPI proteins predicted from the genome sequence serve enzymatic functions required for the biosynthesis and the continuous shape adaptations of the cell wall, others seem to be structural elements of the cell wall and yet others mediate cell adhesion. Because of its genetic tractability S. cerevisiae is an attractive model organism not only for studying GPI biosynthesis in general, but equally for investigating the intracellular transport of GPI proteins and the peculiar role of GPI anchoring in the elaboration of fungal cell walls.

Membrane topology and transient acylation of Toxoplasma gondii glycosylphosphatidylinositols

Using hypotonically permeabilized Toxoplasma gondii tachyzoites, we investigated the topology of the free glycosylphosphatidylinositols (GPIs) within the endoplasmic reticulum (ER) membrane. The morphology and permeability of parasites were checked by electron microscopy and release of a cytosolic protein. The membrane integrity of organelles (ER and rhoptries) was checked by protease protection assays. In initial experiments, GPI biosynthetic intermediates were labeled with UDP-[6-(3)H]GlcNAc in permeabilized parasites, and the transmembrane distribution of the radiolabeled lipids was probed with phosphatidylinositol-specific phospholipase C (PI-PLC). A new early intermediate with an acyl modification on the inositol was identified, indicating that inositol acylation also occurs in T. gondii. A significant portion of the early GPI intermediates (GlcN-PI and GlcNAc-PI) could be hydrolyzed following PI-PLC treatment, indicating that these glycolipids are predominantly present in the cytoplasmic leaflet of the ER. Permeabilized T. gondii parasites labeled with either GDP-[2-(3)H]mannose or UDP-[6-(3)H]glucose showed that the more mannosylated and side chain (Glc-GalNAc)-modified GPI intermediates are also preferentially localized in the cytoplasmic leaflet of the ER.

PIG-A mutations in paroxysmal nocturnal hemoglobinuria and in normal hematopoiesis

PIG-A is an X-linked gene that is essential for the first step in the biosynthesis of glycosylphosphatidyl-inositol (GPI) anchors. A rare clonal hematopoietic stem cell disease, paroxysmal nocturnal hemoglobinuria (PNH), is caused by mutations in the PIG-A gene. PNH is an acquired disease that may arise de novo or emanate from aplastic anemia. PNH blood cells have an absence or marked deficiency of all GPI anchored proteins. Interestingly, rare GPI anchor deficient blood and marrow cells that harbor PIG-A mutations can also be found in most healthy controls. This review examines the clinical and biological relevance of PIG-A mutations in PNH, aplastic anemia and healthy controls.

Gpi19, the Saccharomyces cerevisiae homologue of mammalian PIG-P, is a subunit of the initial enzyme for glycosylphosphatidylinositol anchor biosynthesis

Glycosylphosphatidylinositols (GPIs) are attached to the C termini of some glycosylated secretory proteins, serving as membrane anchors for many of those on the cell surface. Biosynthesis of GPIs is initiated by the transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to phosphatidylinositol. This reaction is carried out at the endoplasmic reticulum (ER) by an enzyme complex called GPI-N-acetylglucosaminyltransferase (GPI-GlcNAc transferase). The human enzyme has six known subunits, at least four of which, GPI1, PIG-A, PIG-C, and PIG-H, have functional homologs in the budding yeast Saccharomyces cerevisiae. The uncharacterized yeast gene YDR437w encodes a protein with some sequence similarity to human PIG-P, a fifth subunit of the GPI-GlcNAc transferase. Here we show that Ydr437w is a small but essential subunit of the yeast GPI-GlcNAc transferase, and we designate its gene GPI19. Similar to other mutants in the yeast enzyme, temperature-sensitive gpi19 mutants display cell wall defects and hyperactive Ras phenotypes. The Gpi19 protein associates with the yeast GPI-GlcNAc transferase in vivo, as judged by coimmuneprecipitation with the Gpi2 subunit. Moreover, conditional gpi19 mutants are defective for GPI-GlcNAc transferase activity in vitro. Finally, we present evidence for the topology of Gpi19 within the ER membrane.

CHO Glycosylation Mutants: GPI Anchor

Glycosylphosphatidylinositol (GPI) is used for anchoring many cell surface proteins to the plasma membrane. Biosynthesis of GPI anchor, its attachment to proteins, and modification of GPI-anchored proteins (GPI-APs) en route to the plasma membrane are complex processes (Ferguson, 1999; Kinoshita and Inoue, 2000). GPI-AP-defective mutant cell lines derived from CHO and other cells have been very useful in elucidating GPI biosynthetic pathway and cloning genes involved in these processes. In this chapter, we overview GPI-AP biosynthesis, establishment and characterization of GPI-AP-defective mutant cell lines, expression cloning using those mutant cells, and characteristics of GPI-AP-defective mutant cell lines.

New insights into paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an uncommon intravascular hemolytic anemia that results from the clonal expansion of hematopoietic stem cells harboring somatic mutations in an X-linked gene, termed PIG-A. PIG-A mutations block glycosylphosphatidylinositol (GPI) anchor biosynthesis, resulting in a deficiency or absence of all GPI-anchored proteins on the cell surface. CD55 and CD59 are GPI-anchored complement regulatory proteins. Their absence on PNH red cells is responsible for the complement-mediated intravascular hemolysis. Intravascular hemolysis leads to release of free hemoglobin, which contributes to many of the clinical manifestations of PNH including fatigue, pain, esophageal spasm, erectile dysfunction and possibly thrombosis. Interestingly, rare PIG-A mutations can be found in virtually all healthy control subjects, leading to speculation that PIG-A mutations in hematopoietic stem cells are common benign events. However, negative selection of PIG-A mutant colony-forming cells with proaerolysin, a toxin that targets GPI-anchored proteins, reveals that most of these mutations are not derived from stem cells. Recently, a humanized monoclonal antibody directed against the terminal complement protein C5 has been shown to reduce hemolysis and greatly improve symptoms and quality of life for PNH patients.

The initial enzyme for glycosylphosphatidylinositol biosynthesis requires PIG-Y, a seventh component

Biosynthesis of glycosylphosphatidylinositol (GPI) is initiated by an unusually complex GPI-N-acetylglucosaminyltransferase (GPI-GnT) consisting of at least six proteins. Here, we report that human GPI-GnT requires another component, termed PIG-Y, a 71 amino acid protein with two transmembrane domains. The Burkitt lymphoma cell line Daudi, severely defective in the surface expression of GPI-anchored proteins, was a null mutant of PIG-Y. A complex of six components was formed without PIG-Y. PIG-Y appeared to be directly associated with PIG-A, implying that PIG-Y is the key molecule that regulates GPI-GnT activity by binding directly to the catalytic subunit PIG-A. PIG-Y is probably homologous to yeast Eri1p, a component of GPI-GnT. We did not obtain evidence for a functional linkage between GPI-GnT and ras GTPases in mammalian cells as has been reported for yeast cells. A single transcript encoded PIG-Y and, to its 5' side, another protein PreY that has homologues in a wide range of organisms and is characterized by a conserved domain termed DUF343. These two proteins are translated from one mRNA by leaky scanning of the PreY initiation site.

Mammalian PIG-X and yeast Pbn1p are the essential components of glycosylphosphatidylinositol-mannosyltransferase I

Within the endoplasmic reticulum (ER), mannoses and glucoses, donated from dolichol-phosphate-mannose and -glucose, are transferred to N-glycan and GPI-anchor precursors, and serine/threonine residues in many proteins. Glycosyltransferases that mediate these reactions are ER-resident multitransmembrane proteins with common characteristics, forming a superfamily of >10 enzymes. Here, we report an essential component of glycosylphosphatidylinositol-mannosyltransferase I (GPI-MT-I), which transfers the first of the four mannoses in the GPI-anchor precursors. We isolated a Chinese hamster ovary (CHO) cell mutant defective in GPI-MT-I but not its catalytic component PIG-M. The mutant gene, termed phosphatidylinositolglycan-class X (PIG-X), encoded a 252-amino acid ER-resident type I transmembrane protein with a large lumenal domain. PIG-X and PIG-M formed a complex, and PIG-M expression was <10% in the absence of PIG-X, indicating that PIG-X stabilizes PIG-M. We found that Saccharomyces cerevisiae Pbn1p/YCL052Cp, which was previously reported to be involved in autoprocessing of proproteinase B, is the functional homologue of PIG-X; Pbn1p is critical for Gpi14p/YJR013Wp function, the yeast homologue of PIG-M. This is the first report of an essential subcomponent of glycosyltransferases using dolichol-phosphate-monosaccharide.

Epidermal-specific defect of GPI anchor in Pig-a null mice results in Harlequin ichthyosis-like features

We previously demonstrated that the epidermal-specific glycosylphosphatidylinositol (GPI)-anchor-deficient mice, generated by Pig-a gene disruption (Pig-a null mice), exhibited wrinkled and dry skin with hyperkeratosis and abnormal differentiation, and they died within a few days after birth. Here, we investigated the basis for the early demise of these animals, and the potential role of epidermal structural and biochemical abnormalities. The rapid demise of these animals was associated with both diminished epidermal permeability barrier function and decreased stratum corneum (SC) water content. The barrier abnormality could be attributed abnormal internal contents of lamellar bodies, with a downstream failure to generate normal extracellular lamellar bilayers in the SC. Moreover, processing profilaggrin to its monomeric form was impaired in Pig-a null mouse epidermis, while levels of the differentiation-specific proteins, involucrin, loricrin and profilaggrin were normal. Failure of filaggrin processing was accompanied by decreased activity of protein phosphatase 2A, an enzyme involved in profilaggrin to filaggrin processing. Thus, these studies demonstrate a critical role for GPI anchor and GPI-anchored proteins in divergent arms of epidermal terminal differentiation. While the permeability barrier abnormality can be attributed to defects in the lamellar body secretory system, the hydration abnormality is, in part, due to lack of availability of filaggrin-derived proteolytic products. Finally, since the dual abnormalities in the lamellar body secretory system and filaggrin processing resemble two key features of human Harlequin ichthyosis, Pig-a null mice could provide an appropriate analog for further studies of this disease.

Biosynthesis of glycosylphosphatidylinositol is essential to the survival of the protozoan parasite Toxoplasma gondii

The PIGA gene from Toxoplasma gondii has been cloned and characterized. Like mammalian PIGA, the transmembrane and C-terminal domains are sufficient to direct localization to the parasite endoplasmic reticulum. A functional copy of PIGA is required for tachyzoite viability, demonstrating that glycosylphosphatidylinositol biosynthesis is an essential process in T. gondii.

A new aspect of the molecular pathogenesis of paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal hematologic disorder which is manifest by complement-mediated hemolysis, venous thrombosis, and bone marrow failure. Complement-mediated hemolysis in PNH is explained by the deficiency of glycosylphosphatidylinositol (GPI)-anchored proteins, CD55 and CD59 on erythrocyte surfaces. All the PNH patients had phosphatidylinositol glycan-class A (PIG-A) gene abnormalities in various cell types, indicating that PIG-A gene mutations cause the defects in GPI-anchored proteins that are essential for the pathogenesis of PNH. In addition, a PIG-A gene abnormality results in a PNH clone. Bone marrow failure causes cytopenias associated with a proliferative decrease of its hematopoietic stem cells and appears to be related to a pre-leukemic state. Although it is unclear how a PNH clone expands in bone marrow, it is considered that the most important hypothesis implicates negative selection of a PNH clone, but it does not explain the changes in the clinical features at the terminal stage of PNH. Recently, it has been suggested that an immune mechanism, in an HLA-restricted manner, plays an important role in the occurrence or selection of a PNH clone and GPI may be a target for cytotoxic-T lymphocytes. Also, it has been indicated that the Wilms' tumor gene (WT1) product is related to a PNH clone, but the significance of WT1 expression is not clear because of the functional diversity of the gene. To elucidate this problem, it is important to know the pathophysiology of bone marrow failure in detail and how bone marrow failure affects hematopoietic stem cells and immune mechanisms in bone marrow failure syndromes.

Comparative importance in vivo of conserved glutamate residues in the EX7E motif retaining glycosyltransferase Gpi3p, the UDP-GlcNAc-binding subunit of the first enzyme in glycosylphosphatidylinositol assembly

Saccharomyces cerevisiae Gpi3p is the UDP-GlcNAc-binding and presumed catalytic subunit of the enzyme that forms GlcNAc-phosphatidylinositol in glycosylphosphatidylinositol biosynthesis. It is an essential protein with an EX7E motif that is conserved in four families of retaining glycosyltransferases. All Gpi3ps contain a cysteine residue four residues C-terminal to EX7E. To test their importance for Gpi3p function in vivo, Glu289 and 297 in the EX7E motif of S. cerevisiae Gpi3p, as well as Cys301, were altered by site-specific mutagenesis, and the mutant proteins tested for their ability to complement nonviable GPI3-deleted haploids. Gpi3p-C301A supported growth but membranes from C301A-expressing cells had low in vitro N-acetylglucosaminylphosphatidylinositol (GlcNAc-PI) synthetic activity. Haploids harboring Gpi3p-E289A proved viable, although slow growing but Gpi3-E297A did not support growth. The E289D and E297D mutants both supported growth at 25 degrees C, but, whereas the E289D strain grew at 37 degrees C, the E297D mutant did not. Membranes from E289D mutants had severely reduced in vitro GlcNAc-PI synthetic activity and E297D membranes had none. The mutation of the first Glu in the EX7E motif of Schizosaccharomyces pombe Gpi3p (Glu277) to Asp complemented the lethal null mutation in gpi3+ and supported growth at 37 degrees C, but the E285D mutant was nonviable. Our results suggest that the second Glu residue of the EX7E motif in Gpi3p is of greater importance than the first for function in vivo. Further, our findings do not support previous suggestions that the first Glu of an EX7E protein is the nucleophile and that Cys301 has an important role in UDP-GlcNAc binding by Gpi3ps.

Ether phospholipids and glycosylinositolphospholipids are not required for amastigote virulence or for inhibition of macrophage activation by Leishmania major

Ether phospholipids are major components of the membranes of humans and Leishmania. In protozoan parasites they occur separately or as part of the glycosylphosphatidylinositol (GPI) anchor of molecules implicated in virulence, such as lipophosphoglycan (LPG), smaller glycosylinositolphospholipids (GIPLs), and GPI-anchored proteins. We generated null mutants of the Leishmania major alkyldihydroxyacetonephosphate synthase (ADS), the first committed step of ether lipid synthesis. Enzymatic analysis and comprehensive mass spectrometric analysis showed that ads1- knock-outs lacked all ether phospholipids, including plasmalogens, LPG, and GIPLs. Leishmania ads1- thus represents the first ether lipid-synthesizing eukaryote for which a completely null mutant could be obtained. Remarkably ads1- grew well and maintained lipid rafts (detergent-resistant membranes). In virulence tests it closely resembled LPG-deficient L. major, including sensitivity to complement and an inability to survive the initial phase of macrophage infection. Likewise it retained the ability to inhibit host cell signaling and to form infectious amastigotes from the few parasites surviving the establishment defect. These findings counter current proposals that GIPLs are required for amastigote survival in the mammalian host or that parasite lyso-alkyl or alkylacyl-GPI anchors are solely responsible for inhibition of macrophage activation.

Enzymes and auxiliary factors for GPI lipid anchor biosynthesis and post-translational transfer to proteins

GPI lipid anchoring is an important post-translational modification of eukaryote proteins in the endoplasmic reticulum. In total, 19 genes have been directly implicated in the anchor synthesis and the substrate protein modification pathway. Here, the molecular functions of the respective proteins and their evolution are analyzed in the context of reported literature data and sequence analysis studies for the complete pathway (http://mendel.imp.univie.ac.at/SEQUENCES/gpi-biosynthesis/) and questions for future experimental investigation are discussed. Studies of two of these proteins have provided new mechanistic insights. The cytosolic part of PIG-A/GPI3 has a two-domain alpha/beta/alpha-layered structure; it is suggested that its C-terminal subsegment binds UDP-GlcNAc whereas the N-terminal domain interacts with the phosphatidylinositol moiety. The lumenal part of PIG-T/GPI16 apparently consists of a beta-propeller with a central hole that regulates the access of substrate protein C termini to the active site of the cysteine protease PIG-K/GPI8 (gating mechanism) as well as of a polypeptide hook that embraces PIG-K/GPI8. This structural proposal would explain the paradoxical properties of the GPI lipid anchor signal motif and of PIG-K/GPI8 orthologs without membrane insertion regions in some species.

Inhibitors of glycosyl-phosphatidylinositol anchor biosynthesis

Glycosyl-phosphatidylinositol (GPI) is a complex glycolipid structure that acts as a membrane anchor for many cell-surface proteins of eukaryotes. GPI-anchored proteins are particularly abundant in protozoa such as Trypanosoma brucei, Leishmania major, Plasmodium falciparum and Toxoplasma gondii, and represent the major carbohydrate modification of many cell-surface parasite proteins. Although the GPI core glycan is conserved in all organisms, many differences in additional modifications to GPI structures and biosynthetic pathways have been reported. Therefore, the characteristics of GPI biosynthesis are currently being explored for the development of parasite-specific inhibitors. In vitro and in vivo studies using sugars and substrate analogues as well as natural compounds have shown that it is possible to interfere with GPI biosynthesis at different steps in a species-specific manner. Here we review the recent and promising progress in the field of GPI inhibition.

A SIN lentiviral vector containing PIGA cDNA allows long-term phenotypic correction of CD34+-derived cells from patients with paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a hematopoietic stem cell (HSC) disorder in which an acquired somatic mutation of the X-linked PIGA gene results in a deficiency in GPI-anchored surface proteins. Clinically, PNH is dominated by a chronic hemolytic anemia, often associated with recurrent nocturnal exacerbations, neutropenia, thrombocytopenia, and thrombotic tendency. Allogenic bone marrow transplantation is the only potentially curative treatment for severe forms of PNH but is associated with a high treatment-related morbidity and mortality. HSC gene therapy could provide a new therapeutic option, especially when an HLA-matched donor is not available. To develop an efficient gene transfer approach, we have designed a new SIN lentiviral vector (TEPW) that contains the PIGA cDNA driven by the human elongation factor 1 alpha promoter, the central DNA flap of HIV-1, and the WPRE cassette. TEPW transduction led to a complete surface expression of the GPI anchor and CD59 in PIGA-deficient cell lines without any selection procedure. Moreover, efficient gene transfer was achieved in bone marrow and mobilized peripheral blood CD34(+) cells derived from two patients with severe PNH disease. This expression was stable during erythroid, myeloid, and megakaryocytic liquid culture differentiation. CD59 surface cell expression was fully restored during 5 weeks of long-term culture.

Genes for glycosylphosphatidylinositol toxin biosynthesis in Plasmodium falciparum

About 2.5 million people die of Plasmodium falciparum malaria every year. Fatalities are associated with systemic and organ-specific inflammation initiated by a parasite toxin. Recent studies show that glycosylphosphatidylinositol (GPI) functions as the dominant parasite toxin in the context of infection. GPIs also serve as membrane anchors for several of the most important surface antigens of parasite invasive stages. GPI anchoring is a complex posttranslational modification produced through the coordinated action of a multicomponent biosynthetic pathway. Here we present eight new genes of P. falciparum selected for encoding homologs of proteins essential for GPI synthesis: PIG-A, PIG-B, PIG-M, PIG-O, GPI1, GPI8, GAA-1, and DPM1. We describe the experimentally verified mRNA and predicted amino acid sequences and in situ localization of the gene products to the parasite endoplasmic reticulum. Moreover, we show preliminary evidence for the PIG-L and PIG-C genes. The biosynthetic pathway of the malaria parasite GPI offers potential targets for drug development and may be useful for studying parasite cell biology and the molecular basis for the pathophysiology of parasitic diseases.

Complete glycosylphosphatidylinositol anchors are required in Candida albicans for full morphogenesis, virulence and resistance to macrophages

Glycosylphosphatidylinositol (GPI)-anchored proteins are involved in cell wall integrity and cell-cell interactions. We disrupted the Candida albicans homologue of the Saccharomyces cerevisiae GPI7/LAS21 gene, which encodes a GPI anchor-modifying activity. In the mutant and on solid media, the yeast-to-hyphae transition was blocked, whereas chlamydospore formation was enhanced. However, the morphogenetic switch was normal in liquid medium. Abnormal budding patterns, cytokinesis and cell shape were observed in both liquid and solid media. The cell wall structure was also modified in the mutants, as shown by hypersensitivity to Calcofluor white. In vitro and in vivo assays revealed that the mutant interacted with its host in a modified way, resulting in reduced virulence in mice and reduced survival in the gastrointestinal environment of mice. The mitogen-activated protein (MAP) kinase pathway of macrophages was downregulated by the wild-type cells but not by the DeltaCagpi7 null strains. In agreement with this abnormal behaviour, mutant cells were more sensitive to the lytic action of macrophages. Our results indicate that a functional GPI anchor is required for full hyphal formation in C. albicans, and that perturbation of the GPI biosynthesis results in hypersensitivity to host defences.

Glycosylphosphatidylinositol (GPI)-anchored proteins

From the numerous studies developed at the last quarter of the 20th century, glycosylphosphatidylinositol (GPI) anchor has been established as a unique mode of protein binding to the plasma membrane. The core structure of this anchor consists of ethanolamine phosphate, trimannoside, glucosamine and inositol phospholipid in this order. The anchor is combined with the carboxyl-terminal of protein by the ethanolamine head. GPI-anchored proteins are ubiquitously distributed among Eucarya from vertebrates to protozoa, and also shown to be present in some of Archaeobacteria such as Sulfolobus. There is no evidence for the presence of GPI-anchored protein in Eubacteria. In the eucaryotic cells, both biosynthesis of GPI precursors and posttranslational protein modification with GPI proceed in the endoplasmic reticulum. On GPI modification, the carboxyl-terminal signal peptide is split off from the protein and the resulting new carboxyl-terminal is then combined with the amino group of ethanolamine residue in the GPI precursors. The whole process of cleavage and GPI attachment is catalyzed by GPI-transamidase complex. Many genes concerning GPI biosynthesis and protein modification have been cloned and sequenced. The carboxyl-terminal signal peptide containing hydrophobic tail is characterized by genetic analysis and shown to be essential for GPI modification of protein. Recent computational analysis further clarified the detailed requirement of the carboxyl-terminal regions for GPI-anchoring. GPI-anchored proteins are assumed to be transported from Golgi to the plasma membrane in the form of "lipid rafts", and expressed as the clusters in the cell surface.

Ynl038wp (Gpi15p) is the Saccharomyces cerevisiae homologue of human Pig-Hp and participates in the first step in glycosylphosphatidylinositol assembly

Glycosylphosphatidylinositols (GPIs) are found in all eukaryotes and are synthesized in a pathway that starts with the transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to phosphatidylinositol (PI). This reaction is carried out by a protein complex, three of whose subunits in humans, hGpi1p, Pig-Cp and Pig-Ap, have sequence and functional homologues in the Saccharomyces cerevisiae Gpi1, Gpi2 and Gpi3 proteins, respectively. Human GlcNAc-PI synthase contains two further subunits, Pig-Hp and PigPp. We report that the essential YNL038w gene encodes the S. cerevisiae homologue of Pig-Hp. Haploid YNL038w-deletion strains were created, in which Ynl038wp could be depleted by repressing YNL038w expression using the GAL10 promoter. Depletion of Ynl038wp from membranes virtually abolished in vitro GlcNAc-PI synthetic activity, indicating that Ynl038wp is necessary for GlcNAc-PI synthesis in vitro. Further, depletion of Ynl038wp in an smp3 mutant background prevented the formation of the trimannosylated GPI intermediates that normally accumulate in this late-stage GPI assembly mutant. Ynl038wp is therefore required for GPI synthesis in vivo. Because YNL038w encodes a protein involved in GPI biosynthesis, we designate the gene GPI15. Potential Pig-Hp/Gpi15p counterparts are also encoded in the genomes of Schizosacchomyces pombe and Candida albicans.

Purification, cloning and characterization of a GPI inositol deacylase from Trypanosoma brucei

Inositol acylation is an obligatory step in glycosylphosphatidylinositol (GPI) biosynthesis whereas mature GPI anchors often lack this modification. The GPI anchors of Trypanosoma brucei variant surface glycoproteins (VSGs) undergo rounds of inositol acylation and deacylation during GPI biosynthesis and the deacylation reactions are inhibited by diisopropylfluorophosphate (DFP). Inositol deacylase was affinity labelled with [3H]DFP and purified. Peptide sequencing was used to clone GPIdeAc, which encodes a protein with significant sequence and hydropathy similarity to mammalian acyloxyacyl hydrolase, an enzyme that removes fatty acids from bacterial lipopolysaccharide. Both contain a signal sequence followed by a saposin domain and a GDSL-lipase domain. GPIdeAc(-/-) trypanosomes were viable in vitro and in animals. Affinity-purified HA-tagged GPIdeAc was shown to have inositol deacylase activity. However, total inositol deacylase activity was only reduced in GPIdeAc(-/-) trypanosomes and the VSG GPI anchor was indistinguishable from wild type. These results suggest that there is redundancy in T.brucei inositol deacylase activity and that there is another enzyme whose sequence is not recognizably related to GPIdeAc.

FES-Cre targets phosphatidylinositol glycan class A (PIGA) inactivation to hematopoietic stem cells in the bone marrow

A somatic mutation in the X-linked phosphatidylinositol glycan class A (PIGA) gene causes the loss of glycosyl phosphatidylinositol (GPI)-linked proteins on blood cells from patients with paroxysmal nocturnal hemoglobinuria. Because all blood cell lineages may be affected it is thought that the mutation occurs in a hematopoietic stem cell. In transgenic mice, germline transmission of an inactive Piga gene is embryonic lethal. To inactivate the murine Piga gene in early hematopoiesis we therefore chose conditional gene inactivation using the Cre/loxP system. We expressed Cre recombinase under the transcription regulatory sequences of the human c-fes gene. FES-Cre inactivated PIGA in hematopoietic cells of mice carrying a floxed Piga allele (LF mice). PIGA(-) cells were found in all hematopoietic lineages of definitive but not primitive hematopoiesis. Their proportions were low in newborn mice but subsequently increased continuously to produce for the first time mice that have almost exclusively PIGA(-) blood cells. The loss of GPI-linked proteins occurred mainly in c-kit(+)CD34(+)Lin(-) progenitor cells before the CFU-GEMM stage. Using bone marrow reconstitution experiments with purified PIGA(-) cells we demonstrate that LF mice have long-term bone marrow repopulating cells that lack GPI-linked proteins, indicating that recombination of the floxed Piga allele occurs in the hematopoietic stem cell.

The complex structures of arabinogalactan-proteins and the journey towards understanding function

Arabinogalactan-proteins (AGPs) are a family of complex proteoglycans found in all higher plants. Although the precise function(s) of any single AGP is unknown, they are implicated in diverse developmental roles such as differentiation, cell-cell recognition, embryogenesis and programmed cell death. DNA sequencing projects have made possible the identification of the genes encoding a large number of putative AGP protein backbones. In contrast, our understanding of how AGPs undergo extensive post-translational modification is poor and it is important to understand these processes since they are likely to be critical for AGP function. Genes believed to be responsible for post-translational modification of an AGP protein backbone, include prolyl hydroxylases, glycosyl transferases, proteases and glycosylphosphatidylinositol-anchor synthesising enzymes. Here we examine models for proteoglycan function in animals and yeast to highlight possible strategies for determining the function(s) of individual AGPs in plants.

PIG-S and PIG-T, essential for GPI anchor attachment to proteins, form a complex with GAA1 and GPI8

Many eukaryotic cell surface proteins are anchored to the plasma membrane via glycosylphosphatidylinositol (GPI). The GPI transamidase mediates GPI anchoring in the endoplasmic reticulum, by replacing a protein's C-terminal GPI attachment signal peptide with a pre-assembled GPI. During this transamidation reaction, the GPI transamidase forms a carbonyl intermediate with a substrate protein. It was known that the GPI transamidase is a complex containing GAA1 and GPI8. Here, we report two new components of this enzyme: PIG-S and PIG-T. To determine roles for PIG-S and PIG-T, we disrupted these genes in mouse F9 cells by homologous recombination. PIG-S and PIG-T knockout cells were defective in transfer of GPI to proteins, particularly in formation of the carbonyl intermediates. We also demonstrate that PIG-S and PIG-T form a protein complex with GAA1 and GPI8, and that PIG-T maintains the complex by stabilizing the expression of GAA1 and GPI8. Saccharomyces cerevisiae Gpi16p (YHR188C) and Gpi17p (YDR434W) are orthologues of PIG-T and PIG-S, respectively.

Specificity of GlcNAc-PI de-N-acetylase of GPI biosynthesis and synthesis of parasite-specific suicide substrate inhibitors

The substrate specificities of Trypanosoma brucei and human (HeLa) GlcNAc-PI de-N-acetylases were determined using 24 substrate analogues. The results show the following. (i) The de-N-acetylases show little specificity for the lipid moiety of GlcNAc-PI. (ii) The 3'-OH group of the GlcNAc residue is essential for substrate recognition whereas the 6'-OH group is dispensable and the 4'-OH, while not required for recognition, cannot be epimerized or substituted. (iii) The parasite enzyme can act on analogues containing betaGlcNAc or aromatic N-acyl groups, whereas the human enzyme cannot. (iv) Three GlcNR-PI analogues are de-N-acetylase inhibitors, one of which is a suicide inhibitor. (v) The suicide inhibitor most likely forms a carbamate or thiocarbamate ester to an active site hydroxy-amino acid or Cys or residue such that inhibition is reversed by certain nucleophiles. These and previous results were used to design two potent (IC50 = 8 nM) parasite-specific suicide substrate inhibitors. These are potential lead compounds for the development of anti-protozoan parasite drugs.

The Essential Smp3 Protein Is Required for Addition of the Side-branching Fourth Mannose during Assembly of Yeast Glycosylphosphatidylinositols

The major glycosylphosphatidylinositols (GPIs) transferred to protein in mammals and trypanosomes contain three mannoses. In Saccharomyces cerevisiae, however, the GPI transferred to protein bears a fourth, alpha1,2-linked Man on the alpha1,2-Man that receives the phosphoethanolamine (EthN-P) moiety through which GPIs become linked to protein. We report that temperature-sensitive smp3 mutants accumulate a GPI containing three mannoses and that smp3 is epistatic to the gpi11, gpi13, and gaa1 mutations, which normally result in the accumulation of Man(4)-GPIs, including the presumed substrate for the yeast GPI transamidase. The Smp3 protein, which is encoded by an essential gene, is therefore required for addition of the fourth Man to yeast GPI precursors. The finding that smp3 prevents the formation of the Man(4)-GPI that accumulates when addition of EthN-P to Man-3 is blocked in a gpi13 mutant suggests that the presence of the fourth Man is important for transfer of EthN-P to Man-3 of yeast GPIs. The Man(3)-GPI that accumulates in smp3 is a mixture of two dominant isoforms, one bearing a single EthN-P side branch on Man-1, the other with EthN-P on Man-2, and these isoforms can be placed in separate arms of a branched GPI assembly pathway. Smp3-related proteins are encoded in the genomes of Schizosaccharomyces pombe, Candida albicans, Drosophila melanogaster, and Homo sapiens and form a subgroup of a family of proteins, the other groups of which are defined by the Pig-B(Gpi10) protein, which adds the third GPI mannose, and by the Alg9 and Alg12 proteins, which act in the dolichol pathway for N-glycosylation. Because Man(4)-containing GPI precursors are normally formed in yeast and Plasmodium falciparum, whereas addition of a fourth Man during assembly of mammalian GPIs is rare and not required for GPI transfer to protein, Smp3p-dependent addition of a fourth Man represents a target for antifungal and antimalarial drugs.

The human GPI1 gene is required for efficient glycosylphosphatidylinositol biosynthesis

The first step in glycosylphosphatidylinositol (GPI) membrane anchor biosynthesis that is defective in paroxysmal nocturnal haemoglobinuria is mediated by an N-acetylglucosaminyl transferase expressed in the endoplasmic reticulum. Six human genes encode subunits of this enzyme, namely PIG-A, PIG-C, PIG-H, PIG-P, GPI1, and DPM2. Here, the human GPI1 gene is characterised. This gene is organised into eleven exons. The locus was mapped to chromosome 16p13.3 near the haemoglobin alpha chain locus. GPI1 is expressed ubiquitously in human cells and tissues. Expression levels are markedly elevated in haematopoietic tissues (bone marrow, foetal liver). To determine whether human GPI1 is essential for human GPI biosynthesis, antisense RNA was expressed in HEK293 cells. Transfectants exhibited a marked but incomplete decrease in the expression of a GPI-linked reporter protein, confirming that GPI1 is required for efficient GPI biosynthesis. In contrast, expression of GPI-linked proteins is normal in lymphatic cell lines from individuals with the alpha thalassaemia/mental retardation syndrome, which is characterised by large deletions from chromosome 16p removing one of the two GPI1 alleles along with the haemoglobin alpha locus. In conclusion, GPI1 plays an important role in the biosynthesis of GPI intermediates. Due to its autosomal localisation, the heterozygous deletion of GPI1 does not lead to an overt defect in the expression of GPI-linked proteins.

PIG-M transfers the first mannose to glycosylphosphatidylinositol on the lumenal side of the ER

Glycosylphosphatidylinositol (GPI) acts as a membrane anchor of many cell surface proteins. Its structure and biosynthetic pathway are generally conserved among eukaryotic organisms, with a number of differences. In particular, mammalian and protozoan mannosyltransferases needed for addition of the first mannose (GPI-MT-I) have different substrate specificities and are targets of species- specific inhibitors of GPI biosynthesis. GPI-MT-I, however, has not been molecularly characterized. Characterization of GPI-MT-I would also help to clarify the topology of GPI biosynthesis. Here, we report a human cell line defective in GPI-MT-I and the gene responsible, PIG-M. PIG-M encodes a new type of mannosyltransferase of 423 amino acids, bearing multiple transmembrane domains. PIG-M has a functionally important DXD motif, a characteristic of many glycosyltransferases, within a domain facing the lumen of the endoplasmic reticulum (ER), indicating that transfer of the first mannose to GPI occurs on the lumenal side of the ER membrane.

Dissecting and manipulating the pathway for glycosylphos-phatidylinositol-anchor biosynthesis

The pathway for glycosylphosphatidylinositol-anchor biosynthesis consists of at least 10 reaction steps. Many of the genes encoding the enzymes and regulators involved in this pathway have been recently cloned and their products characterised. These studies have revealed the common and also different characteristics of glycosylphosphatidyl-inositol biosynthesis enzymes in different organisms, leading to the development of species-specific inhibitors of the pathway.

Glycosylphosphatidylinositol (GPI)-deficient Jurkat T cells as a model to study functions of GPI-anchored proteins

Many cell surface proteins attached to the membrane by GPI are involved in cell signalling. However, the role of the GPI membrane anchor itself remains poorly understood. GPI-defective cells from patients with paroxysmal nocturnal haemoglobinuria (PNH) are relatively resistant to apoptosis induction. We developed a Jurkat T cell model for GPI deficiency by isolating a GPI-negative mutant, which is defective in the GPI biosynthetic gene PIG-A. Using retroviral PIG-A gene transfer along with the transfer of a vector control, we obtained two genetically identical cell lines, distinguished only by expression of the PIG-A gene and, thus, their ability to produce GPI. Cell proliferation and survival were not affected by this difference. Apoptotic stimuli such as serum starvation and camptothecin exposure elicited similar responses. In contrast, GPI-defective Jurkat cells were more susceptible to Fas-mediated apoptosis than GPI-positive cells. These results indicate that a deficiency in GPI-anchored proteins, as is found in PNH, does not confer resistance to apoptosis.

Photoaffinity labelling with P3-(4-azidoanilido)uridine 5'-triphosphate identifies gpi3p as the UDP-GlcNAc-binding subunit of the enzyme that catalyses formation of GlcNAc-phosphatidylinositol, the first glycolipid intermediate in glycosylphosphatidylinositol synthesis

Glycosylphosphatidylinositols (GPIs) are made by all eukaryotes. The first step in their synthesis is the transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol (PI). Four proteins in mammals and at least three in yeast make up a complex that carries out this reaction. Three of the proteins are highly conserved between yeast and mammals: the Gpi1 protein, the Pig-C/Gpi2 protein and the Pig-A/Gpi3 protein. The function of the individual subunits is not known, but of the three, the Pig-A/Gpi3 proteins resemble members of a large family of nucleotide-sugar-utilizing glycosyltransferases. To establish whether Gpi3p is the UDP-GlcNAc-binding subunit of the yeast GlcNAc-PI synthetic complex, we tested its ability to become cross-linked to the photoactivatable substrate analogue P(3)-(4-azidoanilido)-uridine 5'-triphosphate (AAUTP). We report that Gpi3p bearing the FLAG epitope at its C-terminus becomes cross-linked to AAUTP[alpha-(32)P], but that Gpi2p-FLAG does not. Furthermore, Gpi3p-FLAG expressed in Escherichia coli is also cross-linked. These results indicate that Gpi3p is the UDP-GlcNAc-binding and probable catalytic subunit of the GlcNAc-PI synthetic complex.

Location of the glucuronosyltransferase domain in the heparan sulfate copolymerase EXT1 by analysis of Chinese hamster ovary cell mutants

Heparan sulfate formation occurs by the copolymerization of glucuronic acid (GlcA) and N-acetylglucosamine (GlcNAc) residues. Recent studies have shown that these reactions are catalyzed by a copolymerase encoded by EXT1 and EXT2, members of the exostosin family of putative tumor suppressors linked to hereditary multiple exostoses. Previously, we identified a collection of Chinese hamster ovary cell mutants (pgsD) that failed to make heparan sulfate (Lidholt, K., Weinke, J. L., Kiser, C. S., Lugemwa, F. N., Bame, K. J., Cheifetz, S., Massagué, J., Lindahl, U., and Esko, J. D. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 2267-2271). Here, we show that pgsD mutants contain mutations that either alter GlcA transferase activity selectively or that affect both GlcNAc and GlcA transferase activities. Expression of EXT1 corrects the deficiencies in the mutants, whereas EXT2 and the related EXT-like cDNAs do not. Analysis of the EXT1 mutant alleles revealed clustered missense mutations in a domain that included a (D/E)X(D/E) motif thought to bind the nucleotide sugar from studies of other transferases. These findings provide insight into the location of the GlcA transferase subdomain of the enzyme and indicate that loss of the GlcA transferase domain may be sufficient to cause hereditary multiple exostoses.

Chimaeric mice with disruption of the gene coding for phosphatidylinositol glycan class A (Pig-a) were defective in embryogenesis and spermatogenesis

Mutations in the gene encoding PIG-A (phosphatidylinositol glycan class A) are found in patients with paroxysmal nocturnal haemoglobinuria (PNH), an acquired haematopoietic stem cell disorder. Individuals with hereditary PIG-A mutations have never been identified, which is also manifested by the difficulties in generating Pig-a knockout (KO) mice. This study investigated the effect of Pig-a mutations on the development of visceral and genital organs in addition to the haematopoietic system by the generation of Pig-a KO chimaeric mice. Of a total of 54 live births out of 1684 blastocysts injected, chimaerism for Pig-a knockout was detected in 29 mice, suggesting the importance of Pig-a in embryogenesis and in live birth. Quantification of the degree of chimaerism in different organs of the surviving chimaeric mice revealed extremely low levels of Pig-a KO cells in the liver and spleen. In contrast, high levels of KO signals were usually detected in the brain, heart, lung and kidney. Haematopoiesis proceeded normally in these chimaeric mice (as measured by 'complete blood cell counting') and the Pig-a KO cells were present at low levels in red blood cells and B lymphocytes but at high levels in T lymphocytes, although these KO cells did not gain any growth advantage. The effect of Pig-a knockout was also prominent in the reproductive system, another organ with high mitotic activity. Breeding the male chimaeras revealed a high rate of infertility and abnormality in the male genital organs, including abnormally shaped testes, epididymis and seminal vesicles. Even in the absence of gross abnormalities of the genital organs, low counts of motile sperm were also discernible. Pig-a KO sperm was detected in these organs; however, no transmission of the KO allele was observed. The results suggest a possible mechanism underlying the non-transmission of the Pig-a KO gene in germlines.

Initial enzyme for glycosylphosphatidylinositol biosynthesis requires PIG-P and is regulated by DPM2

Glycosylphosphatidylinositols (GPIs) are attached to the C-termini of many proteins, thereby acting as membrane anchors. Biosynthesis of GPI is initiated by GPI-N-acetylglucosaminyltransferase (GPI-GnT), which transfers N-acetylglucosamine from UDP- N-acetylglucosamine to phosphatidylinositol. GPI-GnT is a uniquely complex glycosyltransferase, consisting of at least four proteins, PIG-A, PIG-H, PIG-C and GPI1. Here, we report that GPI-GnT requires another component, termed PIG-P, and that DPM2, which regulates dolichol-phosphate-mannose synthase, also regulates GPI-GnT. PIG-P, a 134-amino acid protein having two hydrophobic domains, associates with PIG-A and GPI1. PIG-P is essential for GPI-GnT since a cell lacking PIG-P is GPI-anchor negative. DPM2, but not two other components of dolichol-phosphate-mannose synthase, associates with GPI-GnT through interactions with PIG-A, PIG-C and GPI1. Lec15 cell, a null mutant of DPM2, synthesizes early GPI intermediates, indicating that DPM2 is not essential for GPI-GnT; however, the enzyme activity is enhanced 3-fold in the presence of DPM2. These results reveal new essential and regulatory components of GPI-GnT and imply co-regulation of GPI-GnT and the dolichol-phosphate-mannose synthase that generates a mannosyl donor for GPI.

Requirement of PIG-F and PIG-O for transferring phosphoethanolamine to the third mannose in glycosylphosphatidylinositol

Many eukaryotic proteins are anchored by glycosylphosphatidylinositol (GPI) to the cell surface membrane. The GPI anchor is linked to proteins by an amide bond formed between the carboxyl terminus and phosphoethanolamine attached to the third mannose. Here, we report the roles of two mammalian genes involved in transfer of phosphoethanolamine to the third mannose in GPI. We cloned a mouse gene termed Pig-o that encodes a 1101-amino acid PIG-O protein bearing regions conserved in various phosphodiesterases. Pig-o knockout F9 embryonal carcinoma cells expressed very little GPI-anchored proteins and accumulated the same major GPI intermediate as the mouse class F mutant cell, which is defective in transferring phosphoethanolamine to the third mannose due to mutant Pig-f gene. PIG-O and PIG-F proteins associate with each other, and the stability of PIG-O was dependent upon PIG-F. However, the class F cell is completely deficient in the surface expression of GPI-anchored proteins. A minor GPI intermediate seen in Pig-o knockout but not class F cells had more than three mannoses with phosphoethanolamines on the first and third mannoses, suggesting that this GPI may account for the low expression of GPI-anchored proteins. Therefore, mammalian cells have redundant activities in transferring phosphoethanolamine to the third mannose, both of which require PIG-F.