Glycosylphosphatidylinositol biosynthesis defects in Gpi11p- and Gpi13p-deficient yeast suggest a branched pathway and implicate gpi13p in phosphoethanolamine transfer to the third mannose

A novel family of sugar-specific phosphodiesterases that remove zwitterionic modifications of GlcNAc

The zwitterions phosphorylcholine (PC) and phosphoethanolamine (PE) are often found esterified to certain sugars in polysaccharides and glycoconjugates in a wide range of biological species. One such modification involves PC attachment to the 6-carbon of N-acetylglucosamine (GlcNAc-6-PC) in N-glycans and glycosphingolipids (GSLs) of parasitic nematodes, a modification that helps the parasite evade host immunity. Knowledge of enzymes involved in the synthesis and degradation of PC and PE modifications is limited. More detailed studies on such enzymes would contribute to a better understanding of the function of PC modifications and have potential application in the structural analysis of zwitterion-modified glycans. In this study, we used functional metagenomic screening to identify phosphodiesterases encoded in a human fecal DNA fosmid library that remove PC from GlcNAc-6-PC. A novel bacterial phosphodiesterase was identified and biochemically characterized. This enzyme (termed GlcNAc-PDase) shows remarkable substrate preference for GlcNAc-6-PC and GlcNAc-6-PE, with little or no activity on other zwitterion-modified hexoses. The identified GlcNAc-PDase protein sequence is a member of the large endonuclease/exonuclease/phosphatase superfamily where it defines a distinct subfamily of related sequences of previously unknown function, mostly from Clostridium bacteria species. Finally, we demonstrate use of GlcNAc-PDase to confirm the presence of GlcNAc-6-PC in N-glycans and GSLs of the parasitic nematode Brugia malayi in a glycoanalytical workflow.

Effector secretion and stability in the maize anthracnose pathogen Colletotrichum graminicola requires N-linked protein glycosylation and the ER chaperone pathway

N-linked protein glycosylation is a conserved and essential modification mediating protein processing and quality control in the endoplasmic reticulum (ER), but how this contributes to the infection cycle of phytopathogenic fungi is largely unknown. In this study, we discovered that inhibition of protein N-glycosylation severely affected vegetative growth, hyphal tip development, conidial germination, appressorium formation, and, ultimately, the ability of the maize (Zea mays) anthracnose pathogen Colletotrichum graminicola to infect its host. Quantitative proteomics analysis showed that N-glycosylation can coordinate protein O-glycosylation, glycosylphosphatidylinositol anchor modification, and endoplasmic reticulum quality control (ERQC) by directly targeting the proteins from the corresponding pathway in the ER. We performed a functional study of the N-glycosylation pathway-related protein CgALG3 and of the ERQC pathway-related protein CgCNX1, which demonstrated that N-glycosylation of ER chaperone proteins is essential for effector stability, secretion, and pathogenicity of C. graminicola. Our study provides concrete evidence for the regulation of effector protein stability and secretion by N-glycosylation.

Glycosylphosphatidylinositol Anchor Biosynthesis Pathway-Related Protein GPI7 Is Required for the Vegetative Growth and Pathogenicity of Colletotrichum graminicola

Glycosylphosphatidylinositol (GPI) anchoring is a common post-translational modification in eukaryotic cells and has been demonstrated to have a wide range of biological functions, such as signal transduction, cellular adhesion, protein transport, immune response, and maintaining cell wall integrity. More than 25 proteins have been proven to participate in the GPI anchor synthesis pathway which occurs in the cytoplasmic and the luminal face of the ER membrane. However, the essential proteins of the GPI anchor synthesis pathway are still less characterized in maize pathogen Colletotrichum graminicola. In the present study, we analyzed the biological function of the GPI anchor synthesis pathway-related gene, CgGPI7, that encodes an ethanolamine phosphate transferase, which is localized in ER. The vegetative growth and conidia development of the ΔCgGPI7 mutant was significantly impaired in C. graminicola. and qRT-PCR results showed that the transcriptional level of CgGPI7 was specifically induced in the initial infection stage and that the pathogenicity of ΔCgGPI7 mutant was also significantly decreased compared with the wild type. Furthermore, the ΔCgGPI7 mutant displayed more sensitivity to cell wall stresses, suggesting that CgGPI7 may play a role in the cell wall integrity of C. graminicola. Cell wall synthesis-associated genes were also quantified in the ΔCgGPI7 mutant, and the results showed that chitin and β-1,3-glucans synthesis genes were significantly up-regulated in ΔCgGPI7 mutants. Our results suggested that CgGPI7 is required for vegetative growth and pathogenicity and might depend on the cell wall integrity of C. graminicola.

Endoplasmic Reticulum Export of GPI-Anchored Proteins

Protein export from the endoplasmic reticulum (ER) is an essential process in all eukaryotes driven by the cytosolic coat complex COPII, which forms vesicles at ER exit sites for transport of correctly assembled secretory cargo to the Golgi apparatus. The COPII machinery must adapt to the existing wide variety of different types of cargo proteins and to different cellular needs for cargo secretion. The study of the ER export of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs), a special glycolipid-linked class of cell surface proteins, is contributing to address these key issues. Due to their special biophysical properties, GPI-APs use a specialized COPII machinery to be exported from the ER and their processing and maturation has been recently shown to actively regulate COPII function. In this review, we discuss the regulatory mechanisms by which GPI-APs are assembled and selectively exported from the ER.

Generating anchors only to lose them: The unusual story of glycosylphosphatidylinositol anchor biosynthesis and remodeling in yeast and fungi

Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are present ubiquitously at the cell surface in all eukaryotes. They play a crucial role in the interaction of the cell with its external environment, allowing the cell to receive signals, respond to challenges, and mediate adhesion. In yeast and fungi, they also participate in the structural integrity of the cell wall and are often essential for survival. Roughly four decades after the discovery of the first GPI-APs, this review provides an overview of the insights gained from studies of the GPI biosynthetic pathway and the future challenges in the field. In particular, we focus on the biosynthetic pathway in Saccharomyces cerevisiae, which has for long been studied as a model organism. Where available, we also provide information about the GPI biosynthetic steps in other yeast/ fungi. Although the core structure of the GPI anchor is conserved across organisms, several variations are built into the biosynthetic pathway. The present Review specifically highlights these variations and their implications. There is growing evidence to suggest that several phenotypes are common to GPI deficiency and should be expected in GPI biosynthetic mutants. However, it appears that several phenotypes are unique to a specific step in the pathway and may even be species-specific. These could suggest the points at which the GPI biosynthetic pathway intersects with other important cellular pathways and could be points of regulation. They could be of particular significance in the study of pathogenic fungi and in identification of new and specific antifungal drugs/ drug targets. © 2018 IUBMB Life, 70(5):355-383, 2018.

Aspergillus fumigatus phosphoethanolamine transferase gene gpi7 is required for proper transportation of the cell wall GPI-anchored proteins and polarized growth

In fungi many proteins, which play important roles in maintaining the function of the cell wall and participating in pathogenic processes, are anchored to the cell surface by a glycosylphosphatidylinositol (GPI) anchor. It has been known that modification and removal of phosphoethanolamine (EtN-P) on the second mannose residue in GPI anchors is important for maturation and sorting of GPI anchored proteins in yeast and mammalian cells, but is a step absent from some protist parasites. In Aspergillus fumigatus, an opportunistic fungal pathogen causing invasive aspergillosis in humans, GPI-anchored proteins are known to be involved in cell wall synthesis and virulence. In this report the gene encoding A. fumigatus EtN-P transferase GPI7 was investigated. By deletion of the gpi7 gene, we evaluated the effects of EtN-P modification on the morphogenesis of A. fumigatus and localization of GPI proteins. Our results showed that deletion of the gpi7 gene led to reduced cell membrane GPI anchored proteins, the mis-localization of the cell wall GPI anchored protein Mp1, abnormal polarity, and autophagy in A. fumigatus. Our results suggest that addition of EtN-P of the second mannose on the GPI anchor is essential for transportation and localization of the cell wall GPI-anchored proteins.

Plasmodium falciparum Choline Kinase Inhibition Leads to a Major Decrease in Phosphatidylethanolamine Causing Parasite Death

Malaria is a life-threatening disease caused by different species of the protozoan parasite Plasmodium, with P. falciparum being the deadliest. Increasing parasitic resistance to existing antimalarials makes the necessity of novel avenues to treat this disease an urgent priority. The enzymes responsible for the synthesis of phosphatidylcholine and phosphatidylethanolamine are attractive drug targets to treat malaria as their selective inhibition leads to an arrest of the parasite's growth and cures malaria in a mouse model. We present here a detailed study that reveals a mode of action for two P. falciparum choline kinase inhibitors both in vitro and in vivo. The compounds present distinct binding modes to the choline/ethanolamine-binding site of P. falciparum choline kinase, reflecting different types of inhibition. Strikingly, these compounds primarily inhibit the ethanolamine kinase activity of the P. falciparum choline kinase, leading to a severe decrease in the phosphatidylethanolamine levels within P. falciparum, which explains the resulting growth phenotype and the parasites death. These studies provide an understanding of the mode of action, and act as a springboard for continued antimalarial development efforts selectively targeting P. falciparum choline kinase.

An unrecognized function for COPII components in recruiting the viral replication protein BMV 1a to the perinuclear ER

Positive-strand RNA viruses invariably assemble their viral replication complexes (VRCs) by remodeling host intracellular membranes. How viral replication proteins are targeted to specific organelle membranes to initiate VRC assembly remains elusive. Brome mosaic virus (BMV), whose replication can be recapitulated in Saccharomyces cerevisiae, assembles its VRCs by invaginating the outer perinuclear endoplasmic reticulum (ER) membrane. Remarkably, BMV replication protein 1a (BMV 1a) is the only viral protein required for such membrane remodeling. We show that ER-vesicle protein of 14 kD (Erv14), a cargo receptor of coat protein complex II (COPII), interacts with BMV 1a. Moreover, the perinuclear ER localization of BMV 1a is disrupted in cells lacking ERV14 or expressing dysfunctional COPII coat components (Sec13, Sec24 or Sec31). The requirement of Erv14 for the localization of BMV 1a is bypassed by addition of a Sec24-recognizable sorting signal to BMV 1a or by overexpressing Sec24, suggesting a coordinated effort by both Erv14 and Sec24 for the proper localization of BMV 1a. The COPII pathway is well known for being involved in protein secretion; our data suggest that a subset of COPII coat proteins have an unrecognized role in targeting proteins to the perinuclear ER membrane.

Biosynthesis of GPI-anchored proteins: special emphasis on GPI lipid remodeling

Glycosylphosphatidylinositols (GPIs) act as membrane anchors of many eukaryotic cell surface proteins. GPIs in various organisms have a common backbone consisting of ethanolamine phosphate (EtNP), three mannoses (Mans), one non-N-acetylated glucosamine, and inositol phospholipid, whose structure is EtNP-6Manα-2Manα-6Manα-4GlNα-6myoinositol-P-lipid. The lipid part is either phosphatidylinositol of diacyl or 1-alkyl-2-acyl form, or inositol phosphoceramide. GPIs are attached to proteins via an amide bond between the C-terminal carboxyl group and an amino group of EtNP. Fatty chains of inositol phospholipids are inserted into the outer leaflet of the plasma membrane. More than 150 different human proteins are GPI anchored, whose functions include enzymes, adhesion molecules, receptors, protease inhibitors, transcytotic transporters, and complement regulators. GPI modification imparts proteins with unique characteristics, such as association with membrane microdomains or rafts, transient homodimerization, release from the membrane by cleavage in the GPI moiety, and apical sorting in polarized cells. GPI anchoring is essential for mammalian embryogenesis, development, neurogenesis, fertilization, and immune system. Mutations in genes involved in remodeling of the GPI lipid moiety cause human diseases characterized by neurological abnormalities. Yeast Saccharomyces cerevisiae has >60 GPI-anchored proteins (GPI-APs). GPI is essential for growth of yeast. In this review, we discuss biosynthesis of GPI-APs in mammalian cells and yeast with emphasis on the lipid moiety.

New insights into the functions of PIGF, a protein involved in the ethanolamine phosphate transfer steps of glycosylphosphatidylinositol biosynthesis

PIGF is a protein involved in the ethanolamine phosphate (EtNP) transfer steps of glycosylphosphatidylinositol (GPI) biosynthesis. PIGF forms a heterodimer with either PIGG or PIGO, two enzymes that transfer an EtNP to the second or third mannoses of GPI respectively. Heterodimer formation is essential for stable and regulated expression of PIGO and PIGG, but the functional significance of PIGF remains obscure. In the present study, we show that PIGF binds to PIGO and PIGG through distinct molecular domains. Strikingly, C-terminal half of PIGF was sufficient for its binding to PIGO and PIGG and yet this truncation mutant could not complement the PIGF defective mutant cells, suggesting that heterodimer formation is not sufficient for PIGF function. Furthermore, we identified a highly conserved motif in PIGF and demonstrated that the motif is not involved in binding to PIGO or PIGG, but critical for its function. Finally, we identified a PIGF homologue from Trypanosoma brucei and showed that it binds specifically to the T. brucei PIGO homologue. These data together support the notion that PIGF plays a critical and evolutionary conserved role in the ethanolamine-phosphate transfer-step, which cannot be explained by its previously ascribed binding/stabilizing function. Potential roles of PIGF in GPI biosynthesis are discussed.

Comparative Analysis of Protein Glycosylation Pathways in Humans and the Fungal Pathogen Candida albicans

Protein glycosylation pathways are present in all kingdoms of life and are metabolic pathways found in all the life kingdoms. Despite sharing commonalities in their synthesis, glycans attached to glycoproteins have species-specific structures generated by the presence of different sets of enzymes and acceptor substrates in each organism. In this review, we present a comparative analysis of the main glycosylation pathways shared by humans and the fungal pathogen Candida albicans: N-linked glycosylation, O-linked mannosylation and glycosylphosphatidylinositol-anchorage. The knowledge of similarities and divergences between these metabolic pathways could help find new pharmacological targets for C. albicans infection.

Architecture and biosynthesis of the Saccharomyces cerevisiae cell wall

The wall gives a Saccharomyces cerevisiae cell its osmotic integrity; defines cell shape during budding growth, mating, sporulation, and pseudohypha formation; and presents adhesive glycoproteins to other yeast cells. The wall consists of β1,3- and β1,6-glucans, a small amount of chitin, and many different proteins that may bear N- and O-linked glycans and a glycolipid anchor. These components become cross-linked in various ways to form higher-order complexes. Wall composition and degree of cross-linking vary during growth and development and change in response to cell wall stress. This article reviews wall biogenesis in vegetative cells, covering the structure of wall components and how they are cross-linked; the biosynthesis of N- and O-linked glycans, glycosylphosphatidylinositol membrane anchors, β1,3- and β1,6-linked glucans, and chitin; the reactions that cross-link wall components; and the possible functions of enzymatic and nonenzymatic cell wall proteins.

Uptake of radiolabeled GlcNAc into Saccharomyces cerevisiae via native hexose transporters and its in vivo incorporation into GPI precursors in cells expressing heterologous GlcNAc kinase

Yeast glycan biosynthetic pathways are commonly studied through metabolic incorporation of an exogenous radiolabeled compound into a target glycan. In Saccharomyces cerevisiae glycosylphosphatidylinositol (GPI) biosynthesis, [³H]inositol has been widely used to identify intermediates that accumulate in conditional GPI synthesis mutants. However, this approach also labels non-GPI lipid species that overwhelm detection of early GPI intermediates during chromatography. In this study, we show that despite lacking the ability to metabolize N-acetylglucosamine (GlcNAc), S. cerevisiae is capable of importing low levels of extracellular GlcNAc via almost all members of the hexose transporter family. Furthermore, expression of a heterologous GlcNAc kinase gene permits efficient incorporation of exogenous [¹⁴C]GlcNAc into nascent GPI structures in vivo, dramatically lowering the background signal from non-GPI lipids. Utilizing this new method with several conditional GPI biosynthesis mutants, we observed and characterized novel accumulating lipids that were not previously visible using [³H]inositol labeling. Chemical and enzymatic treatments of these lipids indicated that each is a GPI intermediate likely having one to three mannoses and lacking ethanolamine phosphate (Etn-P) side-branches. Our data support a model of yeast GPI synthesis that bifurcates after the addition of the first mannose and that includes a novel branch that produces GPI species lacking Etn-P side-branches.

A screen for deficiencies in GPI-anchorage of wall glycoproteins in yeast

Many of the genes and enzymes critical for assembly and biogenesis of yeast cell walls remain unidentified or poorly characterized. Therefore, we designed a high throughput genomic screen for defects in anchoring of GPI-cell wall proteins (GPI-CWPs), based on quantification of a secreted GFP-Sag1p fusion protein. Saccharomyces cerevisiae diploid deletion strains were transformed with a plasmid expressing the fusion protein under a GPD promoter, then GFP fluorescence was determined in culture supernatants after mid-exponential growth. Variability in the amount of fluorescent marker secreted into the medium was reduced by growth at 18 degrees C in buffered defined medium in the presence of sorbitol. Secondary screens included immunoblotting for GFP, fluorescence emission spectra, cell surface fluorescence, and cell integrity. Of 167 mutants deleted for genes affecting cell wall biogenesis or structure, eight showed consistent hyper-secretion of GFP relative to parental strain BY4743: tdh3 (glyceraldehyde-3-phosphate dehydrogenase), gda1 (guanosine diphosphatase), gpi13 and mcd4 (both ethanolamine phosphate-GPI-transferases), kre5 and kre1 (involved in synthesis of beta1,6 glucan), dcw1(implicated in GPI-CWP cross-linking to cell wall glucan), and cwp1 (a major cell wall protein). In addition, deletion of a number of genes caused decreased secretion of GFP. These results elucidate specific roles for specific genes in cell wall biogenesis, including differentiating among paralogous genes.

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.

Characterization of phosphatidylinositol-glycan biosynthesis protein class F gene in rice

The glycosylphosphatidylinositol (GPI) anchors are linked to glycosylphosphatidylinositol-anchored proteins (GAPs) which are essential for the growth of mammalian, yeast and protozoan cells. The GPI anchor is covalently linked to GAP by amide bond formation between the carboxyl terminus and phosphoethanolamine attached at the third mannose and mediated by a transamidase complex. Mediation of GPI synthesis is by the sequential additions of GPI-N-acetylglucosaminyltransferase (GPI-GnT) complex, the GlcN-PI de-N-acetylase, the GlcN-PI mannosyltransferases and the GPI lipid anchor phosphoethanolamine transferase complexes. We report a rice gene OsPIG-F that encodes a homolog to the human PIG-F protein, one of GPI lipid anchor phosphoethanolamine transferase complexes. The amino acid sequences of rice PIG-F consisted of six helix transmembrane domains, one glycosaminoglycan attachment site, one cGMP-dependent protein kinase phosphorylation site and a protein C phosphorylation site at the C-terminus. This unique structure of rice PIG-F indicates the typical membrane bound structure of a protein. Polyclonal antibody for rice PIG-F was found to be cross-reactive with a protein extracted from the leaves of rice. The levels of rice PIG-F transcripts were found to be abundant in leaves, moderately in the milky stage of seed development and less in the floral spikelet, indicating that the rice PIG-F gene was differentially regulated in specific tissues. Furthermore, the levels of rice PIG-F transcription were up-regulated by growth hormones including GA(3), NAA and kinetin. These results indicated that the rice PIG-F gene expression may medicated by these growth regulators.

Cwp2p, the plasma membrane receptor for Pichia membranifaciens killer toxin

PMKT is a channel-forming killer toxin secreted by Pichia membranifaciens. To identify novel genes that mediate cellular resistance to PMKT we screened a collection of 288 deletion mutants. We found 29 open reading frames (ORFs) that, when deleted, confer resistance to PMKT. In addition, the deletion of 15 ORFs was observed to increase protoplast resistance, in agreement with the initial assumption that a plasma membrane receptor for PMKT exists. Whole cells and protoplasts of a cwp2Delta mutant were found to be completely resistant to PMKT and were unable to bind PMKT, indicating that Cwp2p interacts with it. A protein with a molecular mass of 11.7 kDa was purified from PMKT-affinity columns. This protein was sequenced and identified as Cwp2p. Glycosylphosphatidylinositol (GPI) anchoring-defective mutants were much less sensitive to PMKT, as were wild-type protoplasts pretreated with phosphatidylinositol-specific phospholipase C to remove GPI-anchored proteins, indicating that the GPI-anchored precursor of Cwp2p is also necessary for PMKT activity. Carboxyfluorescein-entrapped liposomes containing a purified GFP-Cwp2p fusion protein in their membranes were much more sensitive to PMKT than protein-free liposomes. Cwp2p and its GPI-anchored precursor are proposed for the first time to be involved as PMKT secondary receptors.

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.

In vivo characterization of the GPI assembly defect in yeast mcd4-174 mutants and bypass of the Mcd4p-dependent step in mcd4Δ cells

Yeast mcd4-174 mutants are blocked in glycosylphosphatidylinositol (GPI) anchoring of protein, but the stage at which GPI biosynthesis is interrupted in vivo has not been identified, and Mcd4p has also been implicated in phosphatidylserine and ATP transport. We report that the major GPI that accumulates in mcd4-174 in vivo is Man(2)-GlcN-(acyl-Ins)PI, consistent with proposals that Mcd4p adds phosphoethanolamine to the first mannose of yeast GPI precursors. Mcd4p-dependent modification of GPIs can partially be bypassed in the mcd4-174/gpi11 double mutant and in mcd4Delta; mutants by high-level expression of PIG-B and GPI10, which respectively encode the human and yeast mannosyltransferases that add the third mannose of the GPI precursor. Rescue of mcd4Delta; by GPI10 indicates that Mcd4p-dependent addition of EthN-P to the first mannose of GPIs is not obligatory for transfer of the third mannose by Gpi10p.

Mutational analysis of the glycosylphosphatidylinositol (GPI) anchor pathway demonstrates that GPI-anchored proteins are required for cell wall biogenesis and normal hyphal growth in Neurospora crassa

Using mutational and proteomic approaches, we have demonstrated the importance of the glycosylphosphatidylinositol (GPI) anchor pathway for cell wall synthesis and integrity and for the overall morphology of the filamentous fungus Neurospora crassa. Mutants affected in the gpig-1, gpip-1, gpip-2, gpip-3, and gpit-1 genes, which encode components of the N. crassa GPI anchor biosynthetic pathway, have been characterized. GPI anchor mutants exhibit colonial morphologies, significantly reduced rates of growth, altered hyphal growth patterns, considerable cellular lysis, and an abnormal "cell-within-a-cell" phenotype. The mutants are deficient in the production of GPI-anchored proteins, verifying the requirement of each altered gene for the process of GPI-anchoring. The mutant cell walls are abnormally weak, contain reduced amounts of protein, and have an altered carbohydrate composition. The mutant cell walls lack a number of GPI-anchored proteins, putatively involved in cell wall biogenesis and remodeling. From these studies, we conclude that the GPI anchor pathway is critical for proper cell wall structure and function in N. crassa.

Cell wall construction inSaccharomyces cerevisiae

In this review, we discuss new insights in cell wall architecture and cell wall construction in the ascomycetous yeast Saccharomyces cerevisiae. Transcriptional profiling studies combined with biochemical work have provided ample evidence that the cell wall is a highly adaptable organelle. In particular, the protein population that is anchored to the stress-bearing polysaccharides of the cell wall, and forms the interface with the outside world, is highly diverse. This diversity is believed to play an important role in adaptation of the cell to environmental conditions, in growth mode and in survival. Cell wall construction is tightly controlled and strictly coordinated with progression of the cell cycle. This is reflected in the usage of specific cell wall proteins during consecutive phases of the cell cycle and in the recent discovery of a cell wall integrity checkpoint. When the cell is challenged with stress conditions that affect the cell wall, a specific transcriptional response is observed that includes the general stress response, the cell wall integrity pathway and the calcineurin pathway. This salvage mechanism includes increased expression of putative cell wall assemblases and some potential cross-linking cell wall proteins, and crucial changes in cell wall architecture. We discuss some more enzymes involved in cell wall construction and also potential inhibitors of these enzymes. Finally, we use both biochemical and genomic data to infer that the architectural principles used by S. cerevisiae to build its cell wall are also used by many other ascomycetous yeasts and also by some mycelial ascomycetous fungi.

Deletion of MCD 4 involved in glycosylphosphatidylinositol (GPI) anchor synthesis leads to an increase in beta-1,6-glucan level and a decrease in GPI-anchored protein and mannan levels in the cell wall of Saccharomyces cerevisiae

Most proteins involved in the synthesis of the GPI core structure of Saccharomyces cerevisiae are essential for growth. To explore the relationship between the GPI anchor structure and beta-1,6-glucan synthesis, we screened deletion mutants in genes involved in GPI synthesis for osmotic remedial growth. Heterozygous diploid strains were dissected on medium with osmotic support and slow growth of the mcd 4 deletion mutant was observed. The mcd 4 mutant showed abnormal morphology and cell aggregation, and was hypersensitive to SDS, hygromycin B and K1 killer toxin. Incorporation of GPI cell wall proteins was examined using a GPI-Flo 1 fusion protein. The result suggested that the mcd 4 deletion causes a decrease in GPI cell wall proteins levels. The mutation also caused a decrease in mannan levels and an increase in alkali-insoluble beta-1,6-glucan and chitin levels in the cell wall.

Saccharomyces cerevisiae Ybr004c and its human homologue are required for addition of the second mannose during glycosylphosphatidylinositol precursor assembly

Addition of the second mannose is the only obvious step in glycosylphosphatidylinositol (GPI) precursor assembly for which a responsible gene has not been discovered. A bioinformatics-based strategy identified the essential Saccharomyces cerevisiae Ybr004c protein as a candidate for the second GPI alpha-mannosyltransferase (GPI-MT-II). S. cerevisiae cells depleted of Ybr004cp have weakened cell walls and abnormal morphology, are unable to incorporate [3H]inositol into proteins, and accumulate a GPI intermediate having a single mannose that is likely modified with ethanolamine phosphate. These data indicate that Ybr004cp-depleted yeast cells are defective in second mannose addition to GPIs, and suggest that Ybr004cp is GPI-MT-II or an essential subunit of that enzyme. Ybr004cp homologues are encoded in all sequenced eukaryotic genomes, and are predicted to have 8 transmembrane domains, but show no obvious resemblance to members of established glycosyltransferase families. The human Ybr004cp homologue can substitute for its S. cerevisiae counterpart in vivo.

GPI7 is the second partner of PIG-F and involved in modification of glycosylphosphatidylinositol

Many eukaryotic cell surface proteins are anchored to the membrane via glycosylphosphatidylinositol (GPI). GPI is synthesized from phosphatidylinositol by stepwise reactions and attached en bloc to nascent proteins. In mammalian cells, the major GPI species transferred to proteins is termed H7. By attachment of an additional ethanolamine phosphate (EtNP) to the second mannose, H7 can be converted to H8, which acts as a minor type of protein-linked GPI and also exists as a free GPI on the cell surface. Yeast GPI7 is involved in the transfer of EtNP to the second mannose, but the corresponding mammalian enzyme has not yet been clarified. Here, we report that the human homolog of Gpi7p (hGPI7) forms a protein complex with PIG-F and is involved in the H7-to-H8 conversion. We knocked down hGPI7 by RNA interference and found that H7 accumulated with little production of H8. Immunoprecipitation experiments revealed that hGPI7 was associated with and stabilized by PIG-F, which is known to bind to and stabilize PIG-O, a protein homologous to hGPI7. PIG-O is a transferase that adds EtNP to the third mannose, rendering GPI capable of attaching to proteins. We further found that the overexpression of hGPI7 decreased the level of PIG-O and, therefore, decreased the level of EtNP transferred to the third mannose. Finally, we propose a mechanism for the regulation of GPI biosynthesis through competition between the two independent enzymes, PIG-O and hGPI7, for the common stabilizer, PIG-F.

GPI7 Involved in Glycosylphosphatidylinositol Biosynthesis Is Essential for Yeast Cell Separation

GPI7 is involved in adding ethanolaminephosphate to the second mannose in the biosynthesis of glycosylphosphatidylinositol (GPI) in Saccharomyces cerevisiae. We isolated gpi7 mutants, which have defects in cell separation and a daughter cell-specific growth defect at the non-permissive temperature. WSC1, RHO2, ROM2, GFA1, and CDC5 genes were isolated as multicopy suppressors of gpi7-2 mutant. Multicopy suppressors could suppress the growth defect of gpi7 mutants but not the cell separation defect. Loss of function mutations of genes involved in the Cbk1p-Ace2p pathway, which activates the expression of daughter-specific genes for cell separation after cytokinesis, bypassed the temperature-sensitive growth defect of gpi7 mutants. Furthermore, deletion of EGT2, one of the genes controlled by Ace2p and encoding a GPI-anchored protein required for cell separation, ameliorated the temperature sensitivity of the gpi7 mutant. In this mutant, Egt2p was displaced from the septal region to the cell cortex, indicating that GPI7 plays an important role in cell separation via the GPI-based modification of daughter-specific proteins in S. cerevisiae.

Human Smp3p Adds a Fourth Mannose to Yeast and Human Glycosylphosphatidylinositol Precursors in Vivo

Yeast and human glycosylphosphatidylinositol (GPI) precursors differ in the extent to which a fourth mannose is present as a side branch of the third core mannose. A fourth mannose addition to GPIs has scarcely been detected in studies of mammalian GPI synthesis but is an essential step in the Saccharomyces cerevisiae pathway. We report that human SMP3 encodes a functional homolog of the yeast Smp3 GPI fourth mannosyl-transferase. Expression of hSMP3 in yeast complements growth and biochemical defects of smp3 mutants and permits in vivo mannosylation of trimannosyl (Man(3))-GPIs. Immunolocalization shows that hSmp3p resides in the endoplasmic reticulum in human cells. Northern analysis of mRNA from human tissues and cell lines indicates that hSMP3 is expressed in most tissues, with the highest levels in brain and colon, but its mRNA is nearly absent from cultured human cell lines. Correspondingly, increasing expression of hSMP3 in cultured HeLa cells causes abundant formation of three putative tetramannosyl (Man(4))-GPIs. Our data indicate that hSmp3p functions as a mannosyltransferase that adds a fourth mannose to certain Man(3)-GPIs during biosynthesis of the human GPI precursor, and suggest it may do so in a tissue-specific manner.

Zinc and the Msc2 zinc transporter protein are required for endoplasmic reticulum function

In this report, we show that zinc is required for endoplasmic reticulum function in Saccharomyces cerevisiae. Zinc deficiency in this yeast induces the unfolded protein response (UPR), a system normally activated by unfolded ER proteins. Msc2, a member of the cation diffusion facilitator (CDF) family of metal ion transporters, was previously implicated in zinc homeostasis. Our results indicate that Msc2 is one route of zinc entry into the ER. Msc2 localizes to the ER when expressed at normal levels. UPR induction in low zinc is exacerbated in an msc2 mutant. Genetic and biochemical evidence indicates that this UPR induction is due to genuine ER dysfunction. Notably, we found that ER-associated protein degradation is defective in zinc-limited msc2 mutants. We also show that the vacuolar CDF proteins Zrc1 and Cot1 are other pathways of ER zinc acquisition. Finally, zinc deficiency up-regulates the mammalian ER stress response indicating a conserved requirement for zinc in ER function among eukaryotes.

Glycosylphosphatidylinositol (GPI) proteins of Saccharomyces cerevisiae contain ethanolamine phosphate groups on the alpha1,4-linked mannose of the GPI anchor

In humans and Saccharomyces cerevisiae the free glycosylphosphatidylinositol (GPI) lipid precursor contains several ethanolamine phosphate side chains, but these side chains had been found on the protein-bound GPI anchors only in humans, not yeast. Here we confirm that the ethanolamine phosphate side chain added by Mcd4p to the first mannose is a prerequisite for the addition of the third mannose to the GPI precursor lipid and demonstrate that, contrary to an earlier report, an ethanolamine phosphate can equally be found on the majority of yeast GPI protein anchors. Curiously, the stability of this substituent during preparation of anchors is much greater in gpi7Delta sec18 double mutants than in either single mutant or wild type cells, indicating that the lack of a substituent on the second mannose (caused by the deletion of GPI7) influences the stability of the one on the first mannose. The phosphodiester-linked substituent on the second mannose, probably a further ethanolamine phosphate, is added to GPI lipids by endoplasmic reticulum-derived microsomes in vitro but cannot be detected on GPI proteins of wild type cells and undergoes spontaneous hydrolysis in saline. Genetic manipulations to increase phosphatidylethanolamine levels in gpi7Delta cells by overexpression of PSD1 restore cell growth at 37 degrees C without restoring the addition of a substituent to Man2. The three putative ethanolamine-phosphate transferases Gpi13p, Gpi7p, and Mcd4p cannot replace each other even when overexpressed. Various models trying to explain how Gpi7p, a plasma membrane protein, directs the addition of ethanolamine phosphate to mannose 2 of the GPI core have been formulated and put to the test.

ATP uptake in the Golgi and extracellular release require Mcd4 protein and the vacuolar H+-ATPase

Extracellular nucleotides signal via a large group of purinergic receptors. Although much is known about these receptors, the mechanism of nucleotide transport out of the cytoplasm is unknown. We developed a functional screen for ATP release to the extracellular space and identified Mcd4p, a 919-amino acid membrane protein with 14 putative transmembrane domains, as a participant in glucose-dependent ATP release from Saccharomyces cerevisiae. This release occurred through the vesicular trafficking pathway initiated by ATP uptake into the Golgi compartment. Both the compartmental uptake and the extracellular release of ATP were regulated by the activity of the vacuolar H+-ATPase. It is likely that the Mcd4p pathway is generally involved in non-mitochondrial ATP movement across membranes, it is essential for Golgi and endoplasmic reticulum function, and its occurrence led to the appearance of P2 purinergic receptors.

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.

Genetic characterization of genes encoding enzymes catalyzing addition of phospho-ethanolamine to the glycosylphosphatidylinositol anchor in Saccharomyces cerevisiae

MPC1/GPI13/YLL031C, one of the genes involved in the addition of phospho-ethanolamine to the glycosylphosphatidylinositol (GPI) anchor core, is an essential gene. Three available temperature-sensitive mutant alleles, mpc1-3, mpc1-4, and mpc1-5, displayed different phenotypes to each other and, correspondingly, these mutants were found to have different mutations in the MPC1 ORF. Temperature-sensitivity of mpc1-5 mutants was suppressed by 5 mM ZnSO(4) and by 5 mM MnCl(2). Multicopy suppressors were isolated from mpc1-5 mutant. Suppressors commonly effective to mpc1-4 and mpc1-5 mutations are PSD1, encoding phosphatidylserine decarboxylase, and ECM33, which were found to suppress the temperature-sensitive phenotype shown by the fsr2-1 and las21delta mutants, those of which have defects in the GPI anchor synthesis. PSD2, encoding another phosphatidylserine decarboxylase that is localized in Golgi/vacuole, was found to be able to serve as a multicopy suppressor of mpc1 and fsr2-1 mutants but not of the las21 delta mutant. In contrast to psd1delta, psd2delta showed a synthetic growth defect with mpc1 mutants but not with fsr2-1 or las21delta. Furthermore, psd1delta psd2delta mpc1 triple mutants did not form colonies on nutrient medium unless ethanolamine was supplied to the medium, whereas psd1delta psd2 delta fsr2-1 or psd1delta psd2 delta las21delta triple mutants grew on nutrient medium without supplementation of ethanolamine. These observations suggest that Mpc1 preferentially utilizes phosphatidylethanolamine produced by Psd2 that is localized in Golgi/vacuole. fsr2-1 dpl1 Delta psd1delta strains showed slower growth than fsr2-1 dpl1delta psd2 delta, suggesting that Fsr2 enzyme depends more on Dpl1 and Psd1 for production of phosphatidylethanolamine. Las21 did not show preference for the metabolic pathway to produce phosphatidylethanolamine.

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.

Dynamics of cell wall structure in Saccharomyces cerevisiae

The cell wall of Saccharomyces cerevisiae is an elastic structure that provides osmotic and physical protection and determines the shape of the cell. The inner layer of the wall is largely responsible for the mechanical strength of the wall and also provides the attachment sites for the proteins that form the outer layer of the wall. Here we find among others the sexual agglutinins and the flocculins. The outer protein layer also limits the permeability of the cell wall, thus shielding the plasma membrane from attack by foreign enzymes and membrane-perturbing compounds. The main features of the molecular organization of the yeast cell wall are now known. Importantly, the molecular composition and organization of the cell wall may vary considerably. For example, the incorporation of many cell wall proteins is temporally and spatially controlled and depends strongly on environmental conditions. Similarly, the formation of specific cell wall protein-polysaccharide complexes is strongly affected by external conditions. This points to a tight regulation of cell wall construction. Indeed, all five mitogen-activated protein kinase pathways in bakers' yeast affect the cell wall, and additional cell wall-related signaling routes have been identified. Finally, some potential targets for new antifungal compounds related to cell wall construction are discussed.

GPI7 affects cell-wall protein anchorage in Saccharomyces cerevisiae and Candida albicans

Glycosylphosphatidylinositol (GPI)-anchoring represents a mechanism for attaching proteins to the cell surface of all eukaryotic cells. Two localizations of GPI proteins have been observed in the yeasts Saccharomyces cerevisiae and Candida albicans: plasma membrane and cell wall. The signals and the mechanisms involved in this differential targeting are presently not well understood. Here several cell-wall-related phenotypes of a gpi7/las21 deletion are described, where GPI7/LAS21 encodes a GPI-anchor-modifying activity. In both organisms, the structure and composition of the cell wall was modified, with a clear increase in chitin deposition. Cell-wall-targeted proteins accumulated in the growth medium, whereas the protein content of the cell wall decreased significantly, suggesting inefficiency of the covalent linkage. The level of plasma-membrane-targeted GPI proteins was not affected. Sequence analyses revealed that gene families involved in the addition of phosphoethanolamines to the core GPI anchor are highly conserved between eukaryotes, with the exception of the Gpi7 family which seems to be fungus-specific. These data are compatible with the notion that the phosphoethanolamine added by Gpi7 protein to the GPI anchor is a key factor in the covalent linkage of cell-wall proteins to fungal cell-wall components.

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.

Conserved core structure and active site residues in alkaline phosphatase superfamily enzymes

Cofactor-independent phosphoglycerate mutase (iPGM) has been previously identified as a member of the alkaline phosphatase (AlkP) superfamily of enzymes, based on the conservation of the predicted metal-binding residues. Structural alignment of iPGM with AlkP and cerebroside sulfatase confirmed that all these enzymes have a common core structure and revealed similarly located conserved Ser (in iPGM and AlkP) or Cys (in sulfatases) residues in their active sites. In AlkP, this Ser residue is phosphorylated during catalysis, whereas in sulfatases the active site Cys residues are modified to formylglycine and sulfatated. Similarly located Thr residue forms a phosphoenzyme intermediate in one more enzyme of the AlkP superfamily, alkaline phosphodiesterase/nucleotide pyrophosphatase PC-1 (autotaxin). Using structure-based sequence alignment, we identified homologous Ser, Thr, or Cys residues in other enzymes of the AlkP superfamily, such as phosphopentomutase, phosphoglycerol transferase, phosphonoacetate hydrolase, and GPI-anchoring enzymes (glycosylphosphatidylinositol phosphoethanolamine transferases) MCD4, GPI7, and GPI13. We predict that catalytical cycles of all the enzymes of AlkP superfamily include phosphoenzyme (or sulfoenzyme) intermediates.

Defects in glycosylphosphatidylinositol (GPI) anchor synthesis activate Hog1 kinase and confer copper-resistance in Saccharomyces cerevisisae

Las21/Gpi7 contains a heavy-metal-associated motif at its N-terminus. When this motif was disrupted by amino acid substitution, the cells acquired weak copper-resistance. We found that the previously isolated las21 mutants were strongly resistant to copper. Metallothionein is necessary for the expression of the copper-resistance of the las21 mutants. However, hyper-production of metallothionein is unlikely to be the cause of copper-resistance of the las21 mutants. Copper-sensitive mutants (collectively called Cus mutants) were isolated from the las21delta and characterized. One of the Cus genes was found to be PBS2, which encodes Hog1 MAP kinase kinase, indicating that the Hog1 MAP kinase pathway is needed for the expression of copper-resistance of the las21 mutants. As expected, the las21delta hog1delta strain was no longer copper-resistant. We found that Hog1 was constitutively activated in las21delta cells and in ssk1delta las21delta cells but not in sho1delta las21delta cells. Inactivation of either FSR2/MCD4 or MPC1/GPI13, both of which are involved in GPI anchor synthesis, like LAS21, caused a similar level of constitutive activation of Hog1 kinase and copper-resistance as found in the las21delta strain. The constitutive activation was canceled by introducing the sskl mutation, but not the sho1 mutation, in each GPI anchor mutant tested, suggesting that the defect in GPI anchor synthesis specifically affects the Slnl branch of the MAP kinase pathway. Since the wild-type cells grown in YPD containing 0.5 M NaCl do not show copper-resistance, mere activation of Hog1 is not sufficient for expression of copper-resistance. We propose that a defect in GPI anchor synthesis has multiple consequences, including activation of the Hog1 MAP kinase cascade and conferring copper-resistance.

Differential effect of 1,10-phenanthroline on mammalian, yeast, and parasite glycosylphosphatidylinositol anchor synthesis

Glycosylphosphatidylinositol (GPI) anchoring of proteins to the plasma membrane is a common mechanism utilized by all eukaryotes including mammals, yeast, and the Trypanosoma brucei parasite. We have previously shown that in mammals phenanthroline (PNT) blocks the attachment of phosphoethanolamine (P-EthN) groups to mannose residues in GPI anchor intermediates, thus preventing the synthesis of mammalian GPI anchors. Therefore, PNT is likely to inhibit GPI-phosphoethanolamine transferases (GPI-PETs). Here we report that in yeast, PNT also inhibits the synthesis of the GPI anchor as well as GPI-anchored proteins. Interestingly, the mechanism of PNT inhibition of GPI synthesis is different from that of YW3548, another putative GPI-PET inhibitor. In contrast to mammals and yeast, the synthesis of GPIs in T. brucei is not affected by PNT. Our results indicate that the T. brucei GPI-PET could be a potential target for antiparasitic drugs.

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.

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.

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.