Biosynthesis of glycosylphosphatidylinositols in mammals and unicellular microbes

Recent Advances in Chemical Synthesis of Amino Sugars

Amino sugars are a kind of carbohydrates with one or more hydroxyl groups replaced by an amino group. They play crucial roles in a broad range of biological activities. Over the past few decades, there have been continuing efforts on the stereoselective glycosylation of amino sugars. However, the introduction of glycoside bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, diastereomeric mixtures of O-glycoside are often produced if aminoglycoside lack a C2 substituent. This review focuses on the updated overview of the way to stereoselective synthesis of 1,2-cis-aminoglycoside. The scope, mechanism, and the applications in the synthesis of complex glycoconjugates for the representative methodologies were also included.

Inactivating the mannose-ethanolamine phosphotransferase Gpi7 confers caspofungin resistance in the human fungal pathogen Candida albicans

Understanding the molecular mechanisms governing antifungal resistance is crucial for identifying new cellular targets for developing new antifungal therapeutics. In this study, we performed a transposon-mediated genome-wide genetic screen in haploid Candida albicans to identify mutants resistant to caspofungin, the first member of the echinocandin class of antifungal drugs. A mutant exhibiting the highest resistance possessed a transposon insertion that inactivates GPI7, a gene encoding the mannose-ethanolamine phosphotransferase. Deleting GPI7 in diploid C. albicans caused similar caspofungin resistance. gpi7Δ/Δ cells showed significantly elevated cell wall chitin content and enhanced phosphorylation of Mkc1, a core component of the PKC-MAPK cell-wall integrity pathway. Deleting MKC1 suppressed the chitin elevation and caspofungin resistance of gpi7Δ/Δ cells, but overexpressing the dominant inactive form of RHO1, an upstream activator of PKC-MAPK signaling, did not. Transcriptome analysis uncovered 406 differentially expressed genes in gpi7Δ/Δ cells, many related to cell wall construction. Our results suggest that GPI7 deletion impairs cell wall integrity, which triggers the cell-wall salvage mechanism via the PKC-MAPK pathway independently of Rho1, resulting in the compensatory chitin synthesis to confer caspofungin resistance.

Sulfation of a FLAG tag mediated by SLC35B2 and TPST2 affects antibody recognition

A FLAG tag consisting of DYKDDDDK is an epitope tag that is frequently and widely used to detect recombinant proteins of interest. In this study, we performed a CRISPR-based genetic screening to identify factors involved in the detection of a FLAG-tagged misfolded model protein at the cell surface. In the screening, SLC35B2, which encodes 3’-phosphoadenosine-5’-phosphosulfate transporter 1, was identified as the candidate gene. The detection of FLAG-tagged misfolded proteins at the cell surface was significantly increased in SLC35B2-knockout cells. Furthermore, protein tyrosine sulfation mediated by tyrosyl-protein sulfotransferase 2 (TPST2) suppressed FLAG-tagged protein detection. Localization analysis of the FLAG-tagged misfolded proteins confirmed that defects in tyrosine sulfation are only responsible for enhancing anti-FLAG staining on the plasma membrane but not inducing the localization change of misfolded proteins on the plasma membrane. These results suggest that a FLAG tag on the misfolded protein would be sulfated, causing a reduced detection by the M2 anti-FLAG antibody. Attention should be required when quantifying the FLAG-tagged proteins in the secretory pathway.

The consensus Nglyco -X-S/T motif and a previously unknown Nglyco -N-linked glycosylation are necessary for growth and pathogenicity of Phytophthora

Asparagine (Asn, N)-linked glycosylation within N_glyco -X-S/T; X ≠ P motif is a ubiquitously distributed post-translational modification that participates in diverse cellular processes. In this work, N-glycosylation inhibitor was shown to prevent Phytophthora sojae growth, suggesting that N-glycosylation is necessary for oomycete development. We conducted a glycoproteomic analysis of P. sojae to identify and map N-glycosylated proteins and to quantify differentially expressed glycoproteins associated with mycelia, asexual cyst, and sexual oospore developmental stages. A total of 355 N-glycosylated proteins was found, containing 496 glycosites, potentially involved in glycan degradation, carbon metabolism, glycolysis, or other metabolic pathways. Through PNGase F deglycosylation assays and site-directed mutagenesis of a GPI transamidase protein (GPI16) upregulated in cysts and a heat shock protein 70 (HSP70) upregulated in oospores, we demonstrated that both proteins were N-glycosylated and that the N_glyco -N motif is a target site for asparagine - oligosaccharide linkage. Glycosite mutations of Asn 94 N_glyco -X-S/T in the GPI16 led to impaired cyst germination and pathogenicity, while mutation of the previously unknown Asn 270 N_glyco -N motif in HSP70 led to decreased oospore production. In addition to providing a map of the oomycete N-glycoproteome, this work confirms that P. sojae has evolved multiple N-glycosylation motifs essential for growth.

Calnexin mediates the maturation of GPI-anchors through ER retention

The protein folding and lipid moiety status of glycosylphosphatidylinositol-anchored proteins (GPI-APs) are monitored in the endoplasmic reticulum (ER), with calnexin playing dual roles in the maturation of GPI-APs. In the present study, we investigated the functions of calnexin in the quality control and lipid remodeling of GPI-APs in the ER. By directly binding the N-glycan on proteins, calnexin was observed to efficiently retain GPI-APs in the ER until they were correctly folded. In addition, sufficient ER retention time was crucial for GPI-inositol deacylation, which is mediated by post-GPI attachment protein 1 (PGAP1). Once the calnexin/calreticulin cycle was disrupted, misfolded and inositol-acylated GPI-APs could not be retained in the ER and were exposed on the plasma membrane. In calnexin/calreticulin-deficient cells, endogenous GPI-anchored alkaline phosphatase was expressed on the cell surface, but its activity was significantly decreased. ER stress induced surface expression of misfolded GPI-APs, but proper GPI-inositol deacylation occurred due to the extended time that they were retained in the ER. Our results indicate that calnexin-mediated ER quality control systems for GPI-APs are necessary for both protein folding and GPI-inositol deacylation.

Synthesis and evaluation of Nα,Nε-diacetyl-l-lysine-inositol conjugates as cancer-selective probes for metabolic engineering of GPIs and GPI-anchored proteins

Two myo-inositol derivatives having an Nα,Nε-diacetyl-l-lysine (Ac2Lys) moiety linked to the inositol 1-O-position through a self-cleavable linker and a metabolically stable 2-azidoethyl group linked to the inositol 3-O- and 4-O-positions, respectively, were designed and synthesized. The Ac2Lys moiety blocking the inositol 1-O-position required for GPI biosynthesis was expected to be removable by a combination of two enzymes, histone deacetylase (HDAC) and cathepsin L (CTSL), abundantly expressed in cancer cells, but not in normal cells, to transform these inositol derivatives into biosynthetically useful products with a free 1-O-position. As a result, it was found that these inositol derivatives could be incorporated into the glycosylphosphatidylinositol (GPI) biosynthetic pathway by cancer cells, but not by normal cells, to express azide-labeled GPIs and GPI-anchored proteins on cell surfaces. Consequently, this study has established a novel strategy and new molecular tools for selective metabolic labeling of cancer cells, which should be useful for various biological studies and applications.

Metabolic Labeling and Structural Analysis of Glycosylphosphatidylinositols from Parasitic Protozoa

Glycosylphosphatidylinositol (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 and represent the major carbohydrate modification of many cell-surface parasite proteins. A minimal GPI-anchor precursor consists of core glycan (ethanolamine-PO₄-Manα1-2Manα1-6Manα1-4GlcNH₂) linked to the 6-position of the D-myo-inositol ring of phosphatidylinositol. Although the GPI core glycan is conserved in all organisms, many differences in additional modifications to GPI structures and biosynthetic pathways have been reported. The preassembled GPI-anchor precursor is post-translationally transferred to a variety of membrane proteins in the lumen of the endoplasmic reticulum in a transamidase-like reaction during which a C-terminal GPI attachment signal is released. Increasing evidence shows that a significant proportion of the synthesized GPIs are not used for protein anchoring, particularly in protozoa in which a large amount of free GPIs are being displayed at the cell surface. The characteristics of GPI biosynthesis are currently being explored for the development of parasite-specific inhibitors. Especially this pathway, at least for Trypanosoma brucei, has been validated as a drug target. Furthermore, thanks to an increase of new innovative strategies to produce pure synthetic carbohydrates, a novel era in the use of GPIs in diagnostic, anti-GPI antibody production, as well as parasitic protozoa GPI-based vaccine approach is developing fast.

Fungal Biofilms: Inside Out

We focus this article on turning a biofilm inside out. The "inside" of the biofilm comprises the individual biofilm-related phenotypes, their environmental drivers and genetic determinants, and the coordination of gene functions through transcriptional regulators. Investigators have viewed the inside of the biofilm through diverse approaches, and this article will attempt to capture the essence of many. The ultimate goal is to connect the inside to the "outside," which we view as biofilm structure, development, pharmacological attributes, and medical impact.

Fungal Biofilms: Inside Out

We focus this article on turning a biofilm inside out. The "inside" of the biofilm comprises the individual biofilm-related phenotypes, their environmental drivers and genetic determinants, and the coordination of gene functions through transcriptional regulators. Investigators have viewed the inside of the biofilm through diverse approaches, and this article will attempt to capture the essence of many. The ultimate goal is to connect the inside to the "outside," which we view as biofilm structure, development, pharmacological attributes, and medical impact.

In Vivo Rat T-Lymphocyte Pig-a Assay: Detection and Expansion of Cells Deficient in the GPI-Anchored CD48 Surface Marker for Analysis of Mutation in the Endogenous Pig-a Gene

The Pig-a assay is being developed as an in vivo gene mutation assay for regulatory safety assessments. The assay is based on detecting mutation in the endogenous Pig-a gene of treated rats by using flow cytometry to measure changes in cell surface markers of peripheral blood cells. Here we present a methodology for demonstrating that phenotypically mutant rat T-cells identified by flow cytometry contain mutations in the Pig-a gene, an important step for validating the assay. In our approach, the mutant phenotype T-cells are sorted into individual wells of 96-well plates and expanded into clones. Subsequent sequencing of genomic DNA from the expanded clones confirms that the Pig-a assay detects exactly what it claims to detect-cells with mutations in the endogenous Pig-a gene. In addition, determining the spectra of Pig-a mutations provides information for better understanding the mutational mechanism of compounds of interest. Our methodology of combining phenotypic antibody labeling, magnetic enrichment, sorting, and single-cell clonal expansion can be used in genotoxicity/mutagenicity studies and in other general immunotoxicology research requiring identification, isolation, and expansion of extremely rare subpopulations of T-cells.

Antifungal Drugs: The Current Armamentarium and Development of New Agents

Invasive fungal infections are becoming an increasingly important cause of human mortality and morbidity, particularly for immunocompromised populations. The fungal pathogens Candida albicans , Cryptococcus neoformans , and Aspergillus fumigatus collectively contribute to over 1 million human deaths annually. Hence, the importance of safe and effective antifungal therapeutics for the practice of modern medicine has never been greater. Given that fungi are eukaryotes like their human host, the number of unique molecular targets that can be exploited for drug development remains limited. Only three classes of molecules are currently approved for the treatment of invasive mycoses. The efficacy of these agents is compromised by host toxicity, fungistatic activity, or the emergence of drug resistance in pathogen populations. Here we describe our current arsenal of antifungals and highlight current strategies that are being employed to improve the therapeutic safety and efficacy of these drugs. We discuss state-of-the-art approaches to discover novel chemical matter with antifungal activity and highlight some of the most promising new targets for antifungal drug development. We feature the benefits of combination therapy as a strategy to expand our current repertoire of antifungals and discuss the antifungal combinations that have shown the greatest potential for clinical development. Despite the paucity of new classes of antifungals that have come to market in recent years, it is clear that by leveraging innovative approaches to drug discovery and cultivating collaborations between academia and industry, there is great potential to bolster the antifungal armamentarium.

Cerebral visual impairment and intellectual disability caused by PGAP1 variants

Homozygous variants in PGAP1 (post-GPI attachment to proteins 1) have recently been identified in two families with developmental delay, seizures and/or spasticity. PGAP1 is a member of the glycosylphosphatidylinositol anchor biosynthesis and remodeling pathway and defects in this pathway are a subclass of congenital disorders of glycosylation. Here we performed whole-exome sequencing in an individual with cerebral visual impairment (CVI), intellectual disability (ID), and factor XII deficiency and revealed compound heterozygous variants in PGAP1, c.274_276del (p.(Pro92del)) and c.921_925del (p.(Lys308Asnfs*25)). Subsequently, PGAP1-deficient Chinese hamster ovary (CHO)-cell lines were transfected with either mutant or wild-type constructs and their sensitivity to phosphatidylinositol-specific phospholipase C (PI-PLC) treatment was measured. The mutant constructs could not rescue the PGAP1-deficient CHO cell lines resistance to PI-PLC treatment. In addition, lymphoblastoid cell lines (LCLs) of the affected individual showed no sensitivity to PI-PLC treatment, whereas the LCLs of the heterozygous carrier parents were partially resistant. In conclusion, we report novel PGAP1 variants in a boy with CVI and ID and a proven functional loss of PGAP1 and show, to our knowledge, for the first time this genetic association with CVI.

Null mutation in PGAP1 impairing Gpi-anchor maturation in patients with intellectual disability and encephalopathy

Many eukaryotic cell-surface proteins are anchored to the membrane via glycosylphosphatidylinositol (GPI). There are at least 26 genes involved in biosynthesis and remodeling of GPI anchors. Hypomorphic coding mutations in seven of these genes have been reported to cause decreased expression of GPI anchored proteins (GPI-APs) on the cell surface and to cause autosomal-recessive forms of intellectual disability (ARID). We performed homozygosity mapping and exome sequencing in a family with encephalopathy and non-specific ARID and identified a homozygous 3 bp deletion (p.Leu197del) in the GPI remodeling gene PGAP1. PGAP1 was not described in association with a human phenotype before. PGAP1 is a deacylase that removes an acyl-chain from the inositol of GPI anchors in the endoplasmic reticulum immediately after attachment of GPI to proteins. In silico prediction and molecular modeling strongly suggested a pathogenic effect of the identified deletion. The expression levels of GPI-APs on B lymphoblastoid cells derived from an affected person were normal. However, when those cells were incubated with phosphatidylinositol-specific phospholipase C (PI-PLC), GPI-APs were cleaved and released from B lymphoblastoid cells from healthy individuals whereas GPI-APs on the cells from the affected person were totally resistant. Transfection with wild type PGAP1 cDNA restored the PI-PLC sensitivity. These results indicate that GPI-APs were expressed with abnormal GPI structure due to a null mutation in the remodeling gene PGAP1. Our results add PGAP1 to the growing list of GPI abnormalities and indicate that not only the cell surface expression levels of GPI-APs but also the fine structure of GPI-anchors is important for the normal neurological development.

Glycosylphosphatidylinositols (GPI) are glycolipid anchors that anchor various proteins to the cell surface. At least 26 genes are involved in biosynthesis and modification of the GPI anchors. Recently, mutations in eight of those genes have been described. Although those mutations do not fully abolish the functions of encoded enzymes, they lead to a decreased expression of surface GPI-anchored proteins and to different forms of intellectual disability. Here we report a mutation in PGAP1 that encodes a protein that modifies the GPI anchor. We found that the mutation leads to a full loss of PGAP1 enzyme activity, but that the patient cells still express normal levels of surface GPI-anchored proteins. However, the GPI anchors have an abnormal lipid structure that is resistant to cleavage by phosphatidylinositol-specific phospholipase C. Our results add PGAP1 to the growing list of GPI abnormalities that cause intellectual disability and indicate that the fine structure of GPI-anchors is also important for a normal neurological development.

Recent progress in synthetic and biological studies of GPI anchors and GPI-anchored proteins

Covalent attachment of glycosylphosphatidylinositols (GPIs) to the protein C-terminus is one of the most common posttranslational modifications in eukaryotic cells. In addition to anchoring surface proteins to the cell membrane, GPIs also have many other important biological functions, determined by their unique structure and property. This account has reviewed the recent progress made in disclosing GPI and GPI-anchored protein biosynthesis, in the chemical and chemoenzymatic synthesis of GPIs and GPI-anchored proteins, and in understanding the conformation, organization, and distribution of GPIs in the lipid membrane.

Genetic and structural validation of Aspergillus fumigatus N-acetylphosphoglucosamine mutase as an antifungal target

Aspergillus fumigatus is the causative agent of IA (invasive aspergillosis) in immunocompromised patients. It possesses a cell wall composed of chitin, glucan and galactomannan, polymeric carbohydrates synthesized by processive glycosyltransferases from intracellular sugar nucleotide donors. Here we demonstrate that A. fumigatus possesses an active AfAGM1 (A. fumigatus N-acetylphosphoglucosamine mutase), a key enzyme in the biosynthesis of UDP (uridine diphosphate)–GlcNAc (N-acetylglucosamine), the nucleotide sugar donor for chitin synthesis. A conditional agm1 mutant revealed the gene to be essential. Reduced expression of agm1 resulted in retarded cell growth and altered cell wall ultrastructure and composition. The crystal structure of AfAGM1 revealed an amino acid change in the active site compared with the human enzyme, which could be exploitable in the design of selective inhibitors. AfAGM1 inhibitors were discovered by high-throughput screening, inhibiting the enzyme with IC₅₀s in the low μM range. Together, these data provide a platform for the future development of AfAGM1 inhibitors with antifungal activity.

Vaccination with enzymatically cleaved GPI-anchored proteins from Schistosoma mansoni induces protection against challenge infection

The flatworm Schistosoma mansoni is a blood fluke parasite that causes schistosomiasis, a debilitating disease that occurs throughout the developing world. Current schistosomiasis control strategies are mainly based on chemotherapy, but many researchers believe that the best long-term strategy to control schistosomiasis is through immunization with an antischistosomiasis vaccine combined with drug treatment. In the search for potential vaccine candidates, numerous tegument antigens have been assessed. As the major interface between parasite and mammalian host, the tegument plays crucial roles in the establishment and further course of schistosomiasis. Herein, we evaluated the potential of a GPI fraction, containing representative molecules located on the outer surface of adult worms, as vaccine candidate. Immunization of mice with GPI-anchored proteins induced a mixed Th1/Th2 type of immune response with production of IFN-γ and TNF-α, and low levels of IL-5 into the supernatant of splenocyte cultures. The protection engendered by this vaccination protocol was confirmed by 42% reduction in worm burden, 45% reduction in eggs per gram of hepatic tissue, 29% reduction in the number of granulomas per area, and 53% reduction in the granuloma fibrosis. Taken together, the data herein support the potential of surface-exposed GPI-anchored antigens from the S. mansoni tegument as vaccine candidate.

Drosophila GPI-mannosyltransferase 2 is required for GPI anchor attachment and surface expression of chaoptin

Glycosylphosphatidylinositol (GPI) anchors are critical for the membrane attachment of a wide variety of essential signaling and cell adhesion proteins. The GPI anchor is a complex glycolipid structure that utilizes glycosylphosphatidylinositol-mannosyltransferases (GPI-MTs) for the addition of three core mannose residues during its biosynthesis. Here, we demonstrate that Drosophila GPI-MT2 is required for the GPI-mediated membrane attachment of several GPI-anchored proteins, including the photoreceptor-specific cell adhesion molecule, chaoptin. Mutations in gpi-mt2 lead to defects in chaoptin trafficking to the plasma membrane in Drosophila photoreceptor cells. In gpi-mt2 mutants, loss of sufficient chaoptin in the membrane leads to microvillar instability, photoreceptor cell pathology, and retinal degeneration. Finally, using site-directed mutagenesis, we have identified key amino acids that are essential for GPI-MT2 function and cell viability in Drosophila. Our findings on GPI-MT2 provide a mechanistic link between GPI anchor biosynthesis and protein trafficking in Drosophila and shed light on a novel mechanism for inherited retinal degeneration.

Detection of PIGO-deficient cells using proaerolysin: a valuable tool to investigate mechanisms of mutagenesis in the DT40 cell system

While isogenic DT40 cell lines deficient in DNA repair pathways are a great tool to understand the DNA damage response to genotoxic agents by a comparison of cell toxicity in mutants and parental DT40 cells, no convenient mutation assay for mutagens currently exists for this reverse-genetic system. Here we establish a proaerolysin (PA) selection-based mutation assay in DT40 cells to identify glycosylphosphatidylinositol (GPI)-anchor deficient cells. Using PA, we detected an increase in the number of PA-resistant DT40 cells exposed to MMS for 24 hours followed by a 5-day period of phenotype expression. GPI anchor synthesis is catalyzed by a series of phosphatidylinositol glycan complementation groups (PIGs). The PIG-O gene is on the sex chromosome (Chromosome Z) in chicken cells and is critical for GPI anchor synthesis at the intermediate step. Among all the mutations detected in the sequence levels observed in DT40 cells exposed to MMS at 100 µM, we identified that ∼55% of the mutations are located at A:T sites with a high frequency of A to T transversion mutations. In contrast, we observed no transition mutations out of 18 mutations. This novel assay for DT40 cells provides a valuable tool to investigate the mode of action of mutations caused by reactive agents using a series of isogenic mutant DT40 cells.

Mannose receptor (MR) engagement by mesothelin GPI anchor polarizes tumor-associated macrophages and is blocked by anti-MR human recombinant antibody

Tumor-infiltrating macrophages respond to microenvironmental signals by developing a tumor-associated phenotype characterized by high expression of mannose receptor (MR, CD206). Antibody cross-linking of CD206 triggers anergy in dendritic cells and CD206 engagement by tumoral mucins activates an immune suppressive phenotype in tumor-associated macrophages (TAMs). Many tumor antigens are heavily glycosylated, such as tumoral mucins, and/or attached to tumor cells by mannose residue-containing glycolipids (GPI anchors), as for example mesothelin and the family of carcinoembryonic antigen (CEA). However, the binding to mannose receptor of soluble tumor antigen GPI anchors via mannose residues has not been systematically studied. To address this question, we analyzed the binding of tumor-released mesothelin to ascites-infiltrating macrophages from ovarian cancer patients. We also modeled functional interactions between macrophages and soluble mesothelin using an in vitro system of co-culture in transwells of healthy donor macrophages with human ovarian cancer cell lines. We found that soluble mesothelin bound to human macrophages and that the binding depended on the presence of GPI anchor and of mannose receptor. We next challenged the system with antibodies directed against the mannose receptor domain 4 (CDR4-MR). We isolated three novel anti-CDR4-MR human recombinant antibodies (scFv) using a yeast-display library of human scFv. Anti-CDR4-MR scFv #G11 could block mesothelin binding to macrophages and prevent tumor-induced phenotype polarization of CD206(low) macrophages towards TAMs. Our findings indicate that tumor-released mesothelin is linked to GPI anchor, engages macrophage mannose receptor, and contributes to macrophage polarization towards TAMs. We propose that compounds able to block tumor antigen GPI anchor/CD206 interactions, such as our novel anti-CRD4-MR scFv, could prevent tumor-induced TAM polarization and have therapeutic potential against ovarian cancer, through polarization control of tumor-infiltrating innate immune cells.

Pathogenesis of chagas' disease: parasite persistence and autoimmunity

Acute Trypanosoma cruzi infections can be asymptomatic, but chronically infected individuals can die of Chagas' disease. The transfer of the parasite mitochondrial kinetoplast DNA (kDNA) minicircle to the genome of chagasic patients can explain the pathogenesis of the disease; in cases of Chagas' disease with evident cardiomyopathy, the kDNA minicircles integrate mainly into retrotransposons at several chromosomes, but the minicircles are also detected in coding regions of genes that regulate cell growth, differentiation, and immune responses. An accurate evaluation of the role played by the genotype alterations in the autoimmune rejection of self-tissues in Chagas' disease is achieved with the cross-kingdom chicken model system, which is refractory to T. cruzi infections. The inoculation of T. cruzi into embryonated eggs prior to incubation generates parasite-free chicks, which retain the kDNA minicircle sequence mainly in the macrochromosome coding genes. Crossbreeding transfers the kDNA mutations to the chicken progeny. The kDNA-mutated chickens develop severe cardiomyopathy in adult life and die of heart failure. The phenotyping of the lesions revealed that cytotoxic CD45, CD8(+) γδ, and CD8α(+) T lymphocytes carry out the rejection of the chicken heart. These results suggest that the inflammatory cardiomyopathy of Chagas' disease is a genetically driven autoimmune disease.

FungalRV: adhesin prediction and immunoinformatics portal for human fungal pathogens

Background

The availability of sequence data of human pathogenic fungi generates opportunities to develop Bioinformatics tools and resources for vaccine development towards benefitting at-risk patients.

Description

We have developed a fungal adhesin predictor and an immunoinformatics database with predicted adhesins. Based on literature search and domain analysis, we prepared a positive dataset comprising adhesin protein sequences from human fungal pathogens Candida albicans, Candida glabrata, Aspergillus fumigatus, Coccidioides immitis, Coccidioides posadasii, Histoplasma capsulatum, Blastomyces dermatitidis, Pneumocystis carinii, Pneumocystis jirovecii and Paracoccidioides brasiliensis. The negative dataset consisted of proteins with high probability to function intracellularly. We have used 3945 compositional properties including frequencies of mono, doublet, triplet, and multiplets of amino acids and hydrophobic properties as input features of protein sequences to Support Vector Machine. Best classifiers were identified through an exhaustive search of 588 parameters and meeting the criteria of best Mathews Correlation Coefficient and lowest coefficient of variation among the 3 fold cross validation datasets. The "FungalRV adhesin predictor" was built on three models whose average Mathews Correlation Coefficient was in the range 0.89-0.90 and its coefficient of variation across three fold cross validation datasets in the range 1.2% - 2.74% at threshold score of 0. We obtained an overall MCC value of 0.8702 considering all 8 pathogens, namely, C. albicans, C. glabrata, A. fumigatus, B. dermatitidis, C. immitis, C. posadasii, H. capsulatum and P. brasiliensis thus showing high sensitivity and specificity at a threshold of 0.511. In case of P. brasiliensis the algorithm achieved a sensitivity of 66.67%. A total of 307 fungal adhesins and adhesin like proteins were predicted from the entire proteomes of eight human pathogenic fungal species. The immunoinformatics analysis data on these proteins were organized for easy user interface analysis. A Web interface was developed for analysis by users. The predicted adhesin sequences were processed through 18 immunoinformatics algorithms and these data have been organized into MySQL backend. A user friendly interface has been developed for experimental researchers for retrieving information from the database.

Conclusion

FungalRV webserver facilitating the discovery process for novel human pathogenic fungal adhesin vaccine has been developed.

D-myo-inositol-3-phosphate affects phosphatidylinositol-mediated endomembrane function in Arabidopsis and is essential for auxin-regulated embryogenesis

In animal cells, myo-inositol is an important regulatory molecule in several physiological and biochemical processes, including signal transduction and membrane biogenesis. However, the fundamental biological functions of myo-inositol are still far from clear in plants. Here, we report the genetic characterization of three Arabidopsis thaliana genes encoding D-myo-inositol-3-phosphate synthase (MIPS), which catalyzes the rate-limiting step in de novo synthesis of myo-inositol. Each of the three MIPS genes rescued the yeast ino1 mutant, which is defective in yeast MIPS gene INO1, and they had different dynamic expression patterns during Arabidopsis embryo development. Although single mips mutants showed no obvious phenotypes, the mips1 mips2 double mutant and the mips1 mips2 mips3 triple mutant were embryo lethal, whereas the mips1 mips3 and mips1 mips2⁺/⁻ double mutants had abnormal embryos. The mips phenotypes resembled those of auxin mutants. Indeed, the double and triple mips mutants displayed abnormal expression patterns of DR5:green fluorescent protein, an auxin-responsive fusion protein, and they had altered PIN1 subcellular localization. Also, membrane trafficking was affected in mips1 mips3. Interestingly, overexpression of PHOSPHATIDYLINOSITOL SYNTHASE2, which converts myo-inositol to membrane phosphatidylinositol (PtdIns), largely rescued the cotyledon and endomembrane defects in mips1 mips3. We conclude that myo-inositol serves as the main substrate for synthesizing PtdIns and phosphatidylinositides, which are essential for endomembrane structure and trafficking and thus for auxin-regulated embryogenesis.

Novel applications for glycosylphosphatidylinositol-anchored proteins in pharmaceutical and industrial biotechnology

Glycosylphosphatidylinositol (GPI)-anchored proteins have been regarded as typical cell surface proteins found in most eukaryotic cells from yeast to man. They are embedded in the outer plasma membrane leaflet via a carboxy-terminally linked complex glycolipid GPI structure. The amphiphilic nature of the GPI anchor, its compatibility with the function of the attached protein moiety and the capability of GPI-anchored proteins for spontaneous insertion into and transfer between artificial and cellular membranes initially suggested their potential for biotechnological applications. However, these expectations have been hardly fulfilled so far. Recent developments fuel novel hopes with regard to: (i) Automated online expression, extraction and purification of therapeutic proteins as GPI-anchored proteins based on their preferred accumulation in plasma membrane lipid rafts, (ii) multiplex custom-made protein chips based on GPI-anchored cell wall proteins in yeast, (iii) biomaterials and biosensors with films consisting of sets of distinct GPI-anchored binding-proteins or enzymes for sequential or combinatorial catalysis, and (iv) transport of therapeutic proteins across or into relevant tissue cells, e.g., enterocytes or adipocytes. Latter expectations are based on the demonstrated translocation of GPI-anchored proteins from plasma membrane lipid rafts to cytoplasmic lipid droplets and eventually further into microvesicles which upon release from donor cells transfer their GPI-anchored proteins to acceptor cells. The value of these technologies, which are all based on the interaction of GPI-anchored proteins with membranes and surfaces, for the engineering, production and targeted delivery of biomolecules for a huge variety of therapeutic and biotechnological purposes should become apparent in the near future.

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.

Molecular mechanisms of host cell invasion by Trypanosoma cruzi

The protozoan parasite Trypanosoma cruzi, the etiologic agent of Chagas disease, is an obligate intracellular protozoan pathogen. Overlapping mechanisms ensure successful infection, yet the relationship between these cellular events and clinical disease remains obscure. This review explores the process of cell invasion from the perspective of cell surface interactions, intracellular signaling, modulation of the host cytoskeleton and endosomal compartment, and the intracellular innate immune response to infection.

Inhibition of lipolysis by adiposomes containing glycosylphosphatidylinositol-anchored Gce1 protein in rat adipocytes

Small membrane vesicles released from large adipocytes and harbouring the glycosylphosphatidylinositol-anchored (GPI-) AMP-degrading protein CD73 have previously been demonstrated to stimulate the signal-induced esterification of free fatty acids into neutral lipids suggesting a role of these so-called adiposomes (ADIP) in the paracrine regulation of lipid metabolism in the adipose tissue. Here the involvement of another constituent GPI-protein of ADIP, the cAMP-degrading protein Gce1 in the signal-induced inhibition of lipolysis was investigated in primary rat adipocytes. Incubation of small, and to a lower degree, large adipocytes with ADIP inhibited lipolysis and increased its sensitivity toward inhibition by H(2)O(2), the anti-diabetic drug glimepiride and palmitate. This was accompanied by the transfer of Gce1 from the ADIP to detergent-insoluble glycolipid-enriched plasma membrane microdomains (DIGs) and its subsequent translocation to cytoplasmic lipid droplets (LD) of the acceptor adipocytes. The translocation from DIGs to LD rather than the transfer from ADIP to DIGs of Gce1 was stimulated by H(2)O(2) > glimepiride > palmitate. Both transfer and translocation led to salt- and carbonate-resistant association of Gce1 with DIGs and LD, respectively, and relied on the structural integrity of the DIGs and GPI anchor of Gce1. In conclusion, the trafficking of GPI-proteins from ADIP of donor adipocytes via DIGs to LD of acceptor adipocytes mediates paracrine regulation of lipolysis within adipose tissue.

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.

The GPI-modified proteins Pga59 and Pga62 of Candida albicans are required for cell wall integrity

The fungal cell wall is essential in maintaining cellular integrity and plays key roles in the interplay between fungal pathogens and their hosts. The PGA59 and PGA62 genes encode two short and related glycosylphosphatidylinositol-anchored cell wall proteins and their expression has been previously shown to be strongly upregulated when the human pathogen Candida albicans grows as biofilms. Using GFP fusion proteins, we have shown that Pga59 and Pga62 are cell-wall-located, N- and O-glycosylated proteins. The characterization of C. albicans pga59Δ/pga59Δ, pga62Δ/pga62Δ and pga59Δ/pga59Δ pga62Δ/pga62Δ mutants suggested a minor role of these two proteins in hyphal morphogenesis and that they are not critical to biofilm formation. Importantly, the sensitivity to different cell-wall-perturbing agents was altered in these mutants. In particular, simultaneous inactivation of PGA59 and PGA62 resulted in high sensitivity to Calcofluor white, Congo red and nikkomicin Z and in resistance to caspofungin. Furthermore, cell wall composition and observation by transmission electron microscopy indicated an altered cell wall structure in the mutant strains. Collectively, these data suggest that the cell wall proteins Pga59 and Pga62 contribute to cell wall stability and structure.

C-terminal signals regulate targeting of glycosylphosphatidylinositol-anchored proteins to the cell wall or plasma membrane in Candida albicans

Fungal glycosylphosphatidylinositol (GPI)-anchored proteins localize to the plasma membrane (PM), cell wall (CW), or both. To study signals that regulate PM versus CW targeting in Candida albicans, we (i) fused the N and/or C termini of the GPI CW protein Hwp1p and the GPI PM protein Ecm331p to green fluorescent protein (GFP) and (ii) expressed and localized the resulting fusions. Forty-seven amino acids from the C terminus of Hwp1p were sufficient to target GFP to the CW, and 66 amino acids from the C terminus of Ecm331p were sufficient to target GFP to the PM. Truncation and mutagenesis studies showed that G390 was the omega cleavage site in Ecm331p. Domain exchange and mutagenesis studies showed that (i) the 5 amino acids immediately N-terminal to the omega sites (the omega - 5 to omega - 1 amino acids) played key roles in targeting to the PM or CW; (ii) KK and FE residues at positions omega - 1 and omega - 2, respectively, targeted to the PM and CW; and (iii) a loss of I at position omega - 5 increased PM retention. Small fluorescent reporters can be used to study the peptide signals that regulate PM versus CW targeting of GPI proteins and may be useful for identifying proteins that interact with key targeting signals.

Functional analysis of Candida albicans GPI-anchored proteins: roles in cell wall integrity and caspofungin sensitivity

The outer layer of the Candida albicans cell wall is enriched in highly glycosylated proteins. The major class, the GlycosylPhosphatidylInositol (GPI)-anchored proteins are tethered to the wall by GPI-anchor remnants and include adhesins, glycosyltransferases, yapsins and superoxide dismutases. In silico analysis suggested that C. albicans possesses 115 putative GPI anchored proteins (GpiPs), almost twice the number reported for Saccharomyces cerevisiae. A global approach to characterise in silico predicted GpiPs has been initiated by generating a library of 45 mutants. This library was subjected to a screen for cell wall modifications by testing the cell wall integrity (SDS and Calcofluor White sensitivity) and response to caspofungin. We showed that, when caspofungin sensitivity was modified, in more than half of the cases the susceptibility can be correlated to the level of chitin and cell wall thickness: sensitive strains have low level of chitin and a thin cell wall. We also identified, for the first time, genes that when deleted lead to decreased caspofungin sensitivity: DFG5, PHR1, PGA4 and PGA62. The role of two unknown GpiPs, Pga31 and Pga62 in the cell wall structure and composition was clearly demonstrated during this study.

The glycosylphosphatidylinositol anchor: a complex membrane-anchoring structure for proteins

Positioned at the C-terminus of many eukaryotic proteins, the glycosylphosphatidylinositol (GPI) anchor is a posttranslational modification that anchors the modified protein in the outer leaflet of the cell membrane. The GPI anchor is a complex structure comprising a phosphoethanolamine linker, glycan core, and phospholipid tail. GPI-anchored proteins are structurally and functionally diverse and play vital roles in numerous biological processes. While several GPI-anchored proteins have been characterized, the biological functions of the GPI anchor have yet to be elucidated at a molecular level. This review discusses the structural diversity of the GPI anchor and its putative cellular functions, including involvement in lipid raft partitioning, signal transduction, targeting to the apical membrane, and prion disease pathogenesis. We specifically highlight studies in which chemically synthesized GPI anchors and analogues have been employed to study the roles of this unique posttranslational modification.

Metabolic labeling and structural analysis of glycosylphosphatidylinositols from parasitic protozoa

Glycosylphosphatidylinositol (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 and represent the major carbohydrate modification of many cell-surface parasite proteins. A minimal GPI-anchor precursor consists of core glycan (ethanolamine-P-Manalpha1-2Manalpha1-6Manalpha1-4GlcNH2) linked to the 6-position of the D-myo-inositol ring of phos-phatidylinositol. Although the GPI core glycan is conserved in all organisms, many differences in additional modifications to GPI structures and biosynthetic pathways have been reported. The preassembled GPI-anchor precursor is post-translationally transferred to a variety of membrane proteins in the lumen of the endoplasmic reticulum in a transamidase-like reaction during which a C-terminal GPI attachment signal is released. Increasing evidence show that a significant proportion of the synthesized GPIs are not used for protein anchoring, particularly in protozoa in which a large amount of free GPIs are being displayed at the cell surface. The characteristics of GPI biosynthesis are currently being explored for the development of parasite-specific inhibitors. Especially as this pathway, at least for Trypanosoma brucei, has been validated as a drug target.

Enhanced proinflammatory response to the Candida albicans gpi7 null mutant by murine cells

The Candida albicans gpi7/gpi7 null mutant strain (Deltagpi7), which is affected in glycosylphosphatidylinositol (GPI) anchor biosynthesis, showed a reduced virulence following systemic infection of C57BL/6 mice. In vitro production of TNF-alpha, IL-6 and IL-1beta by macrophages in response to Deltagpi7 cells was significantly increased as compared to control (wild type GPI7/GPI7 and revertant gpi7/GPI7) cells; this probably contributes to the enhanced recruitment of neutrophils to the peritoneal cavity in response to Deltagpi7 cells. Survival of knockout mice for Toll-like receptor (TLR) 2 and TLR4 following intravenous injection of Deltagpi7 cells showed no significant differences as compared to C57BL/6 mice. In vitro production of TNF-alpha by macrophages and neutrophil recruitment were significantly inhibited in TLR2-/- mice in response to control yeast strains. Interestingly both TNF-alpha production and neutrophil recruitment in response to Deltagpi7 were significantly increased in all three types of mice, with no differences among them, and laminarin failed to inhibit this increased production of TNF-alpha. These results indicate that the enhanced proinflammatory response to Deltagpi7 does not involve recognition through TLR2, TLR4 nor dectin-1. Therefore, complete GPI anchors confer surface properties that are involved in modulation of cytokine production by macrophages in response to C. albicans.

A chemical approach to unraveling the biological function of the glycosylphosphatidylinositol anchor

The glycosylphosphatidylinositol (GPI) anchor is a C-terminal posttranslational modification found on many eukaryotic proteins that reside in the outer leaflet of the cell membrane. The complex and diverse structures of GPI anchors suggest a rich spectrum of biological functions, but few have been confirmed experimentally because of the lack of appropriate techniques that allow for structural perturbation in a cellular context. We previously synthesized a series of GPI anchor analogs with systematic deletions within the glycan core and coupled them to the GFP by a combination of expressed protein ligation and native chemical ligation [Paulick MG, Wise AR, Forstner MB, Groves JT, Bertozzi CR (2007) J Am Chem Soc 129:11543-11550]. Here we investigate the behavior of these GPI-protein analogs in living cells. These modified proteins integrated into the plasma membranes of a variety of mammalian cells and were internalized and directed to recycling endosomes similarly to GFP bearing a native GPI anchor. The GPI-protein analogs also diffused freely in cellular membranes. However, changes in the glycan structure significantly affected membrane mobility, with the loss of monosaccharide units correlating to decreased diffusion. Thus, this cellular system provides a platform for dissecting the contributions of various GPI anchor components to their biological function.

PGAP1 knock-out mice show otocephaly and male infertility

A palmitate linked to the inositol in glycosylphosphatidylinositol (GPI) is removed in the endoplasmic reticulum immediately after the conjugation of GPI with proteins in most cells. Previously, we identified PGAP1 (post GPI attachment to proteins 1) as a GPI inositoldeacylase that removes the palmitate from inositol. A defect in PGAP1 caused a delay in the transport of GPI-anchored proteins (GPI-APs) from the endoplasmic reticulum to the cell surface in Chinese hamster ovary cells, although the cell-surface expression of GPI-APs in the steady state was normal. Nevertheless, in most cells, GPI-APs undergo deacylation. To elucidate the biological significance of PGAP1 in vivo, we established PGAP1 knock-out mice. Most PGAP1 knock-out mice showed otocephaly, a developmental defect, and died right after birth. However, some survived with growth retardation. Male knock-out mice showed severely reduced fertility despite the capability of ejaculation. Their spermatozoa were normal in number, motility, and ability to ascend the uterus, but were unable to go into the oviduct. In vitro, PGAP1-deficient spermatozoa showed weak attachment to the zona pellucida and a severely diminished rate of fertilization. Therefore, an extra acyl chain in GPI anchors caused severe deleterious effects to development and sperm function.

Flocculation, adhesion and biofilm formation in yeasts

Yeast cells possess a remarkable capacity to adhere to abiotic surfaces, cells and tissues. These adhesion properties are of medical and industrial relevance. Pathogenic yeasts such as Candida albicans and Candida glabrata adhere to medical devices and form drug-resistant biofilms. In contrast, cell-cell adhesion (flocculation) is a desirable property of industrial Saccharomyces cerevisiae strains that allows the easy separation of cells from the fermentation product. Adhesion is conferred by a class of special cell wall proteins, called adhesins. Cells carry several different adhesins, each allowing adhesion to specific substrates. Several signalling cascades including the Ras/cAMP/PKA and MAP kinase (MAPK)-dependent filamentous growth pathways tightly control synthesis of the different adhesins. Together, these pathways trigger adhesion in response to stress, nutrient limitation or small molecules produced by the host, such as auxin in plants or NAD in mammals. In addition, adhesins are subject to subtelomeric epigenetic switching, resulting in stochastic expression patterns. Internal tandem repeats within adhesin genes trigger recombination events and the formation of novel adhesins, thereby offering fungi an endless reservoir of adhesion properties. These aspects of fungal adhesion exemplify the impressive phenotypic plasticity of yeasts, allowing them to adapt quickly to stressful environments and exploit new opportunities.

Enzymes of UDP-GlcNAc biosynthesis in yeast

D-Glucosamine is an important building block of major structural components of the fungal cell wall, namely chitin, chitosan and mannoproteins. Other amino sugars, such as D-mannosamine and D-galactosamine, relatively abundant in higher eukaryotes, rarely occur in fungal cells and are actually absent from yeast and yeast-like fungi. The glucosamine-containing sugar nucleotide UDP-GlcNAc is synthesized in yeast cells in a four-step cytoplasmic pathway. This article provides a comprehensive overview of the present knowledge on the enzymes catalysing the particular steps of the pathway in Candida albicans and Saccharomyces cerevisiae, with a special emphasis put on mechanisms of the catalysed reactions, regulation of activity and perspectives for exploitation of enzymes participating in UDP-GlcNAc biosynthesis as potential targets for antifungal chemotherapy.

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.

TbGPI16 is an essential component of GPI transamidase inTrypanosoma brucei

Glycosylphosphatidylinositol (GPI) is widely used by eukaryotic cell surface proteins for membrane attachment. De novo synthesized GPI precursors are attached to proteins post-translationally by the enzyme complex, GPI transamidase. TbGPI16, a component of the trypanosome transamidase, shares similarity with human PIG-T. Here, we show that TbGPI16 is the orthologue of PIG-T and an essential component of GPI transamidase by creating a TbGPI16 knockout. TbGPI16 forms a disulfide-linked complex with TbGPI8. A cysteine to serine mutant of TbGPI16 was unable to fully restore the surface expression of GPI-anchored proteins upon transfection into the knockout cells, indicating that its disulfide linkage with TbGPI8 is important for the full transamidase activity.

New mutant Chinese hamster ovary cell representing an unknown gene for attachment of glycosylphosphatidylinositol to proteins

Aerolysin, a secreted bacterial toxin from Aeromonas hydrophila, binds to glycosylphosphatidylinositol (GPI)-anchored protein and kills the cells by forming pores. Both GPI and N-glycan moieties of GPI-anchored proteins are involved in efficient binding of aerolysin. We isolated various Chinese hamster ovary (CHO) mutant cells resistant to aerolysin. Among them, CHOPA41.3 mutant cells showed several-fold decreased expression of GPI-anchored proteins. After transfection of N-acetylglucosamine transferase I (GnT1) cDNA, aerolysin was efficiently bound to the cells, indicating that the resistance against aerolysin in this cells was mainly ascribed to the defect of N-glycan maturation. CHOPA41.3 cells also accumulated GPI intermediates lacking ethanolamine phosphate modification on the first mannose. After stable transfection of PIG-N cDNA encoding GPI-ethanolamine phosphate transferase1, a profile of accumulated GPI intermediates became similar to that of GPI transamidase mutant cells. It indicated, therefore, that CHOPA41.3 cells are defective in GnT1, ethanolamine phosphate modification of the first mannose, and attachment of GPI to proteins. The GPI accumulation in CHOPA41.3 cells carrying PIG-N cDNA was not normalized after transfection with cDNAs of all known components in GPI transamidase complex. Microsomes from CHOPA41.3 cells had normal GPI transamidase activity. Taken together, there is an unknown gene required for efficient attachment of GPI to proteins.

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.

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.