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

Fungal cell wall organization and biosynthesis

The composition and organization of the cell walls from Saccharomyces cerevisiae, Candida albicans, Aspergillus fumigatus, Schizosaccharomyces pombe, Neurospora crassa, and Cryptococcus neoformans are compared and contrasted. These cell walls contain chitin, chitosan, β-1,3-glucan, β-1,6-glucan, mixed β-1,3-/β-1,4-glucan, α-1,3-glucan, melanin, and glycoproteins as major constituents. A comparison of these cell walls shows that there is a great deal of variability in fungal cell wall composition and organization. However, in all cases, the cell wall components are cross-linked together to generate a cell wall matrix. The biosynthesis and properties of each of the major cell wall components are discussed. The chitin and glucans are synthesized and extruded into the cell wall space by plasma membrane-associated chitin synthases and glucan synthases. The glycoproteins are synthesized by ER-associated ribosomes and pass through the canonical secretory pathway. Over half of the major cell wall proteins are modified by the addition of a glycosylphosphatidylinositol anchor. The cell wall glycoproteins are also modified by the addition of O-linked oligosaccharides, and their N-linked oligosaccharides are extensively modified during their passage through the secretory pathway. These cell wall glycoprotein posttranslational modifications are essential for cross-linking the proteins into the cell wall matrix. Cross-linking the cell wall components together is essential for cell wall integrity. The activities of four groups of cross-linking enzymes are discussed. Cell wall proteins function as cross-linking enzymes, structural elements, adhesins, and environmental stress sensors and protect the cell from environmental changes.

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.

Defining the boundaries of species specificity for the Saccharomyces cerevisiae glycosylphosphatidylinositol transamidase using a quantitative in vivo assay

In eukaryotes, GPI (glycosylphosphatidylinositol) lipid anchoring of proteins is an abundant post-translational modification. The attachment of the GPI anchor is mediated by GPI-T (GPI transamidase), a multimeric, membrane-bound enzyme located in the ER (endoplasmic reticulum). Upon modification, GPI-anchored proteins enter the secretory pathway and ultimately become tethered to the cell surface by association with the plasma membrane and, in yeast, by covalent attachment to the outer glucan layer. This work demonstrates a novel in vivo assay for GPI-T. Saccharomyces cerevisiae INV (invertase), a soluble secreted protein, was converted into a substrate for GPI-T by appending the C-terminal 21 amino acid GPI-T signal sequence from the S. cerevisiae Yapsin 2 [Mkc7p (Y21)] on to the C-terminus of INV. Using a colorimetric assay and biochemical partitioning, extracellular presentation of GPI-anchored INV was shown. Two human GPI-T signal sequences were also tested and each showed diminished extracellular INV activity, consistent with lower levels of GPI anchoring and species specificity. Human/fungal chimaeric signal sequences identified a small region of five amino acids that was predominantly responsible for this species specificity.

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.

Microvesicles/exosomes as potential novel biomarkers of metabolic diseases

Biomarkers are of tremendous importance for the prediction, diagnosis, and observation of the therapeutic success of common complex multifactorial metabolic diseases, such as type II diabetes and obesity. However, the predictive power of the traditional biomarkers used (eg, plasma metabolites and cytokines, body parameters) is apparently not sufficient for reliable monitoring of stage-dependent pathogenesis starting with the healthy state via its initiation and development to the established disease and further progression to late clinical outcomes. Moreover, the elucidation of putative considerable differences in the underlying pathogenetic pathways (eg, related to cellular/tissue origin, epigenetic and environmental effects) within the patient population and, consequently, the differentiation between individual options for disease prevention and therapy - hallmarks of personalized medicine - plays only a minor role in the traditional biomarker concept of metabolic diseases. In contrast, multidimensional and interdependent patterns of genetic, epigenetic, and phenotypic markers presumably will add a novel quality to predictive values, provided they can be followed routinely along the complete individual disease pathway with sufficient precision. These requirements may be fulfilled by small membrane vesicles, which are so-called exosomes and microvesicles (EMVs) that are released via two distinct molecular mechanisms from a wide variety of tissue and blood cells into the circulation in response to normal and stress/pathogenic conditions and are equipped with a multitude of transmembrane, soluble and glycosylphosphatidylinositol-anchored proteins, mRNAs, and microRNAs. Based on the currently available data, EMVs seem to reflect the diverse functional and dysfunctional states of the releasing cells and tissues along the complete individual pathogenetic pathways underlying metabolic diseases. A critical step in further validation of EMVs as biomarkers will rely on the identification of unequivocal correlations between critical disease states and specific EMV signatures, which in future may be determined in rapid and convenient fashion using nanoparticle-driven biosensors.

The use of glycoinformatics in glycochemistry

Glycoinformatics is a small but growing branch of bioinformatics and chemoinformatics. Various resources are now available that can be of use to glycobiologists, but also to chemists who work on the synthesis or analysis of carbohydrates. This article gives an overview of existing glyco-specific databases and tools, with a focus on their application to glycochemistry: Databases can provide information on candidate glycan structures for synthesis, or on glyco-enzymes that can be used to synthesize carbohydrates. Statistical analyses of glycan databases help to plan glycan synthesis experiments. 3D-Structural data of protein-carbohydrate complexes are used in targeted drug design, and tools to support glycan structure analysis aid with quality control. Specific problems of glycoinformatics compared to bioinformatics for genomics or proteomics, especially concerning integration and long-term maintenance of the existing glycan databases, are also discussed.

Inhibitors incorporating zinc-binding groups target the GlcNAc-PI de-N-acetylase in Trypanosoma brucei, the causative agent of African sleeping sickness

Disruption of glycosylphosphatidylinositol biosynthesis is genetically and chemically validated as a drug target against the protozoan parasite Trypanosoma brucei, the causative agent of African sleeping sickness. The N-acetylglucosamine-phosphatidylinositol de-N-acetylase (deNAc) is a zinc metalloenzyme responsible for the second step of glycosylphosphatidylinositol biosynthesis. We recently reported the synthesis of eight deoxy-2-C-branched monosaccharides containing carboxylic acid, hydroxamic acid, or N-hydroxyurea substituents at the C2 position that may act as zinc-binding groups. Here, we describe the synthesis of a glucocyclitol-phospholipid incorporating a hydroxamic acid moiety and report the biochemical evaluation of the monosaccharides and the glucocyclitol-phospholipid as inhibitors of the trypanosome deNAc in the cell-free system and against recombinant enzyme. Monosaccharides with carboxylic acid or hydroxamic acid substituents were found to be the inhibitors of the trypanosome deNAc with IC₅₀ values 0.1–1.5 mm, and the glucocyclitol-phospholipid was found to be a dual inhibitor of the deNAc and the α1-4-mannose transferase with an apparent IC₅₀ = 19 ± 0.5 μm.

Mutual co-regulation between GPI-N-acetylglucosaminyltransferase and ergosterol biosynthesis in Candida albicans

A novel co-regulation exists between the first step of GPI (glycosylphosphatidylinositol) anchor biosynthesis and the rate-determining step of ergosterol biosynthesis in Candida albicans. Depleting CaGpi19p, an accessory subunit of the enzyme complex that initiates GPI biosynthesis, down-regulates ERG11, altering ergosterol levels and drug response. This effect is specific to CaGpi19p depletion and is not due to cell wall defects or GPI deficiency. Additionally, down-regulation of ERG11 down-regulates CaGPI19 and GPI biosynthesis.

The GPI anchor signal sequence dictates the folding and functionality of the Als5 adhesin from Candida albicans

Background

Proteins destined to be Glycosylphosphatidylinositol (GPI) anchored are translocated into the ER lumen completely before the C-terminal GPI anchor attachment signal sequence (SS) is removed by the GPI-transamidase and replaced by a pre-formed GPI anchor precursor. Does the SS have a role in dictating the conformation and function of the protein as well?

Methodology/Principal Findings

We generated two variants of the Als5 protein without and with the SS in order to address the above question. Using a combination of biochemical and biophysical techniques, we show that in the case of Als5, an adhesin of C. albicans, the C-terminal deletion of 20 amino acids (SS) results in a significant alteration in conformation and function of the mature protein.

Conclusions/Significance

We propose that the locking of the conformation of the precursor protein in an alternate conformation from that of the mature protein is one probable strategy employed by the cell to control the behaviour and function of proteins intended to be GPI anchored during their transit through the ER.

Trichomonas vaginalis pathobiology new insights from the genome sequence

The draft genome of the common sexually transmitted pathogen Trichomonas vaginalis encodes one of the largest known proteome with 60,000 candidate proteins. This provides parasitologists and molecular cell biologists alike with exciting, yet challenging, opportunities to unravel the molecular features of the parasite's cellular systems and potentially the molecular basis of its pathobiology. Here, recent investigations addressing selected aspects of the parasite's molecular cell biology are discussed, including surface and secreted virulent factors, membrane trafficking, cell signalling, the degradome, and the potential role of RNA interference in the regulation of gene expression.

A sweet spot in the FGFR signal transduction pathway

The hexosamine biosynthetic pathway, whose end product is UDP-N acetylglucosamine (UDP-GlcNAc), lies at the base of cellular glycosylation pathways, including glycosylation of lipids, formation of heparin sulfated proteoglycans, and N- and O-linked glycosylation of proteins. Forward genetic studies in Drosophila have revealed that mutations in genes encoding different enzymes of the hexosamine biosynthetic pathway result in reduction of UDP-GlcNAc to different extents, with a consequent disruption of distinct glycosylation pathways and developmental processes. A maternal and zygotic loss-of-function screen has identified mutations in nesthocker (nst), which encodes an enzyme in the hexosamine biosynthetic pathway. Embryos lacking maternal and zygotic nst gene products show defective O-GlcNAcylation of a fibroblast growth factor receptor (FGFR)-specific adaptor protein, which impairs FGFR-dependent migration of mesodermal and tracheal cells.

Protein Glycosylation in Aspergillus fumigatus Is Essential for Cell Wall Synthesis and Serves as a Promising Model of Multicellular Eukaryotic Development

Glycosylation is a conserved posttranslational modification that is found in all eukaryotes, which helps generate proteins with multiple functions. Our knowledge of glycosylation mainly comes from the investigation of the yeast Saccharomyces cerevisiae and mammalian cells. However, during the last decade, glycosylation in the human pathogenic mold Aspergillus fumigatus has drawn significant attention. It has been revealed that glycosylation in A. fumigatus is crucial for its growth, cell wall synthesis, and development and that the process is more complicated than that found in the budding yeast S. cerevisiae. The present paper implies that the investigation of glycosylation in A. fumigatus is not only vital for elucidating the mechanism of fungal cell wall synthesis, which will benefit the design of new antifungal therapies, but also helps to understand the role of protein glycosylation in the development of multicellular eukaryotes. This paper describes the advances in functional analysis of protein glycosylation in A. fumigatus.

Versatile co-expression of graft-protective proteins using 2A-linked cassettes

BACKGROUND

Expression of multiple graft-protective proteins targeted to different locations (i.e., intracellular, cell surface, and secreted) has become an increasingly important goal in xenotransplantation. The 2A "ribosome skip" signal is used as a linker to enable transgene co-expression, but some studies have shown that post-translational modification and trafficking of 2A-linked proteins may be adversely affected depending on their position relative to 2A. We tested whether several relevant proteins, subject to a range of processing and localization mechanisms, could be efficiently co-expressed using the 2A system.

METHODS

Six expression cassettes were constructed, each containing up to four 2A-linked open reading frames, encoding combinations of human CD55, thrombomodulin (TBM), CD39, CTLA4-Ig and hygromycin resistance. Each linker incorporated a furin cleavage site to remove the carboxy-terminal extension that remains on upstream proteins after 2A processing. The cassettes were used to produce vectors for transfection, adenoviral transduction and transgenesis. Expression was detected by flow cytometry and/or Western blotting.

RESULTS

All proteins were expressed in the appropriate location following transient transfection of COS-7 cells, irrespective of the number of linked genes. The percentage of stable transfectants expressing a linked gene was increased 10-fold (from 4-5% to 58-67%) by incorporating the hygromycin resistance gene into the cassette. Stable transfection of transgenic GalT KO pig fibroblasts with a hygromycin- TBM-CD39 construct resulted in surface expression of both TBM and CD39 by the majority of hygromycin-resistant cells. Expression was maintained after flow cytometric sorting and expansion. Adenoviral transduction of NIT-1 mouse insulinoma cells with a TBM-CD39 construct resulted in strong expression of both genes on the cell surface. Mice transgenic for 3-gene (CD55- TBM-CD39) or 4-gene (CD55- TBM-CTLA4Ig-CD39) constructs expressed all genes except CD55.

CONCLUSIONS

These results confirm the versatility of the 2A system, and demonstrate that careful construct design can minimize potential problems with post-translational modification and trafficking. In addition, incorporation of a selection marker into the 2A-linked chain can dramatically increase the proportion of stable transfectants expressing proteins of interest. This provides a powerful method for the rapid modification of existing genetically modified pigs.

Purification and crystallization of yeast glycosylphosphatidylinositol transamidase subunit PIG-S (PIG-S(71-467))

The transfer of glycosylphosphatidylinositol (GPI) anchors onto eukaryotic proteins is catalyzed by the transamidase complex, which is composed of at least five subunits (PIG-K, PIG-S, PIG-T, PIG-U and GPAA1). Here, the recombinant protein PIG-S(71-467) from Saccharomyces cerevisiae, including residues 71-467 of the entire 534-residue protein, was cloned, expressed and purified to homogeneity. The monodisperse protein was crystallized by the vapour-diffusion method. A diffraction data set was collected to 3.2 Å resolution with 91.6% completeness. The crystals belonged to space group C2, with unit-cell parameters a = 106.72, b = 59.33, c = 124.3 Å, β = 114.19°, and contained two molecules in the asymmetric unit.

Functional convergence of signalling by GPI-anchored and anchorless forms of a salamander protein implicated in limb regeneration

The GPI-anchor is an established determinant of molecular localisation and various functional roles have been attributed to it. The newt GPI-anchored three-finger protein (TFP) Prod1 is an important regulator of cell behaviour during limb regeneration, but it is unclear how it signals to the interior of the cell. Prod1 was expressed by transfection in cultured newt limb cells and activated transcription and expression of matrix metalloproteinase 9 (MMP9) by a pathway involving ligand-independent activation of epidermal growth factor receptor (EGFR) signalling and phosphorylation of extracellular regulated kinase 1 and 2 (ERK1/2). This was dependent on the presence of the GPI-anchor and critical residues in the α-helical region of the protein. Interestingly, Prod1 in the axolotl, a salamander species that also regenerates its limbs, was shown to activate ERK1/2 signalling and MMP9 transcription despite being anchorless, and both newt and axolotl Prod1 co-immunoprecipitated with the newt EGFR after transfection. The substitution of the axolotl helical region activated a secreted, anchorless version of the newt molecule. The activity of the newt molecule cannot therefore depend on a unique property conferred by the anchor. Prod1 is a salamander-specific TFP and its interaction with the phylogenetically conserved EGFR has implications for our view of regeneration as an evolutionary variable.

Prediction of posttranslational modification of proteins from their amino acid sequence

If posttranslational modifications (PTMs) are chemical alterations of the protein primary structure during the protein's life cycle as a result of an enzymatic reaction, then the motif in the substrate protein sequence that is recognized by the enzyme can serve as basis for predictor construction that recognizes PTM sites in database sequences. The recognition motif consists generally of two regions: first, a small, central segment that enters the catalytic cleft of the enzyme and that is specific for this type of PTM and, second, a sequence environment of about 10 or more residues with linker characteristics (a trend for small and polar residues with flexible backbone) on either side of the central part that are needed to provide accessibility of the central segment to the enzyme's catalytic site. In this review, we consider predictors for cleavage of targeting signals, lipid PTMs, phosphorylation, and glycosylation.

Characterization of GPIT-1 and GPIT-2, two auxiliary components of the Neurospora crassa GPI transamidase complex

The glycosylphosphatidylinositol (GPI) transamidase contains five known subunits and functions in the lumen of the ER to produce GPI-anchored proteins. The transamidase cleaves proteins containing a GPI anchor attachment signal at their C terminus and generates an amide bond between the newly generated carboxyl terminus of the protein and a GPI anchor. We have identified and characterized GPIT-1 and GPIT-2, two of the transamidase subunits from Neurospora crassa. GPIT-1 and GPIT-2 are homologs of the human PIG-T and PIG-U transamidase subunits respectively. We demonstrated that GPIT-2 is required for the addition of GPI anchors onto GPI-anchored proteins. We employed the Neurospora RIP (repeat-induced point mutation) phenomenon to generate 106 "noncritical" amino acid changes in GPIT-1 and 84 "noncritical" amino acid changes in GPIT-2. We used the data to evaluate three-dimensional models for the structures of GPIT-1 and GPIT-2. The mutational data for GPIT-1 is consistent with a multiple-blade propeller structure containing a central channel. The mutational analysis for GPIT-2 supports a structural model based on the karyopherin alpha subunit.

Wnt signaling is regulated by endoplasmic reticulum retention

Precise regulation of Wnt signaling is important in many contexts, as in development of the vertebrate forebrain, where excessive or ectopic Wnt signaling leads to severe brain defects. Mutation of the widely expressed oto gene causes loss of the anterior forebrain during mouse embryogenesis. Here we report that oto is the mouse ortholog of the gpi deacylase gene pgap1, and that the endoplasmic reticulum (ER)-resident Oto protein has a novel and deacylase-independent function during Wnt maturation. Oto increases the hydrophobicities of Wnt3a and Wnt1 by promoting the addition of glycophosphatidylinositol (gpi)-like anchors to these Wnts, which results in their retention in the ER. We also report that oto-deficient embryos exhibit prematurely robust Wnt activity in the Wnt1 domain of the early neural plate. We examine the effect of low oto expression on Wnt1 in vitro by knocking down endogenous oto expression in 293 and M14 melanoma cells using shRNA. Knockdown of oto results in increased Wnt1 secretion which is correlated with greatly enhanced canonical Wnt activity. These data indicate that oto deficiency increases Wnt signaling in vivo and in vitro. Finally, we address the mechanism of Oto-mediated Wnt retention under oto-abundant conditions, by cotransfecting Wnt1 with gpi-specific phospholipase D (GPI-PLD). The presence of GPI-PLD in the secretory pathway results in increased secretion of soluble Wnt1, suggesting that the gpi-like anchor lipids on Wnt1 mediate its retention in the ER. These data now provide a mechanistic framework for understanding the forebrain defects in oto mice, and support a role for Oto-mediated Wnt regulation during early brain development. Our work highlights a critical role for ER retention in regulating Wnt signaling in the mouse embryo, and gives insight into the notoriously inefficient secretion of Wnts.

The Aspergillus nidulans pigP gene encodes a subunit of GPI-N-acetylglucosaminyltransferase which influences filamentation and protein secretion

Glycosylphosphatidylinositol (GPI) anchoring is the main mechanism allowing proper localization of secretory proteins in cell membranes. We have isolated an Aspergillus nidulans homolog of the human PIG-P gene, which encodes a subunit of acetylglucosaminyltransferase (GPI-GnT)-an enzyme involved in the synthesis of GPI anchors. A. nidulans pigP mutants have significantly decreased GPI synthesis. On solid media they show strong growth retardation (the "button" phenotype) while in liquid minimal media they show overall good growth but with hyperbranched and bulbous hyphae with impaired septation. Furthermore, the pigP strains, in contrast to the wild-type, abundantly secrete a 33-kDa alkaline serine protease (ALP) into the liquid medium.

Different GPI-attachment signals affect the oligomerisation of GPI-anchored proteins and their apical sorting

To understand the mechanism involved in the apical sorting of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) we fused to the C-terminus of GFP the GPI-anchor-attachment signal of the folate receptor (FR) or of the prion protein (PrP), two native GPI-anchored proteins that are sorted apically or basolaterally, respectively, in MDCK cells. We investigated the behaviour of the resulting fusion proteins GFP-FR and GFP-PrP by analysing three parameters: their association with DRMs, their oligomerisation and their apical sorting. Strikingly, we found that different GPI-attachment signals differently modulate the ability of the resulting GFP-fusion protein to oligomerise and to be apically sorted. This is probably owing to differences in the GPI anchor and/or in the surrounding lipid microenvironment. Accordingly, we show that addition of cholesterol to the cells is necessary and sufficient to drive the oligomerisation and consequent apical sorting of GFP-PrP, which under control conditions does not oligomerise and is basolaterally sorted.

Characterization of the glycosylphosphatidylinositol-anchor signal sequence of human Cryptic with a hydrophilic extension

Epidermal Growth Factor-Cripto-1/FRL-1/Cryptic (EGF-CFC) proteins, including human Cripto-1 (hCFC2/hCR-1) and human Cryptic (hCFC1), are membrane-associated Nodal co-receptors, which have critical roles in vertebrate development. Most of the EGF-CFC proteins have been experimentally proven or predicted to be glycosylphosphatidylinositol (GPI)-anchored proteins. However, unlike other EGF-CFC proteins, hCFC1 does not exhibit a typical GPI-signal sequence, containing a 32-amino acid hydrophilic extension in its COOH-terminal end. Here we experimentally demonstrate that the COOH-terminal sequence of hCFC1 functions as a GPI-anchoring signal. Moreover, addition of a hydrophilic epitope tag of 55-amino acids (V5-His) after the GPI signal of hCR-1 interfered with generation of a GPI-anchored form of hCR-1. In contrast, addition of the same epitope tag to the end of GPI signal of hCFC1 did not affect the GPI-attachment of hCFC1. The COOH-terminal signal of hCFC1 could produce two different forms of the protein; a GPI-anchored form and an unprocessed form which was more prone to be secreted into the conditioned medium. The hydrophilic extension of hCFC1 negatively regulates the activity of hCFC1 as a Nodal co-receptor. These results demonstrate the presence of endogenous GPI-signal sequence with a hydrophilic extension, which can generate both GPI-anchored and soluble forms of the protein.

Probing enzymes late in the trypanosomal glycosylphosphatidylinositol biosynthetic pathway with synthetic glycosylphosphatidylinositol analogues

Glycosylphosphatidylinositol (GPI)-anchored proteins are abundant in the protozoan parasite Trypanosoma brucei, the causative agent of African sleeping sickness in humans and the related disease Nagana in cattle, and disruption of GPI biosynthesis is genetically and chemically validated as a drug target. Here, we examine the ability of enzymes of the trypanosomal GPI biosynthetic pathway to recognize and process a series of synthetic dimannosyl-glucosaminylphosphatidylinositol analogues containing systematic modifications on the mannose residues. The data reveal which portions of the natural substrate are important for recognition, explain why mannosylation occurs prior to inositol acylation in the trypanosomal pathway, and identify the first inhibitor of the third alpha-mannosyltransferase of the GPI biosynthetic pathway.

Biosynthesis, remodelling and functions of mammalian GPI-anchored proteins: recent progress

More than 100 mammalian proteins are post-translationally modified by glycosylphosphatidylinositol (GPI) at their C-termini and are anchored to the cell surface membrane via the lipid portion. GPI-anchored proteins (GPI-APs) have various functions, such as hydrolytic enzymes, receptors, adhesion molecules, complement regulatory proteins and other immunologically important proteins. GPI-anchored proteins are mainly associated with membrane microdomains or membrane rafts enriched in sphingolipids and cholesterol. It is thought that association with membrane rafts is important for GPI-APs in signal transduction and other functions. Here, we review recent progress in studies on biosynthesis, remodelling and functions of mammalian GPI-APs.

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.

Unique motifs identify PIG-A proteins from glycosyltransferases of the GT4 family

Background

The first step of GPI anchor biosynthesis is catalyzed by PIG-A, an enzyme that transfers N-acetylglucosamine from UDP-N-acetylglucosamine to phosphatidylinositol. This protein is present in all eukaryotic organisms ranging from protozoa to higher mammals, as part of a larger complex of five to six 'accessory' proteins whose individual roles in the glycosyltransferase reaction are as yet unclear. The PIG-A gene has been shown to be an essential gene in various eukaryotes. In humans, mutations in the protein have been associated with paroxysomal noctural hemoglobuinuria. The corresponding PIG-A gene has also been recently identified in the genome of many archaeabacteria although genes of the accessory proteins have not been discovered in them. The present study explores the evolution of PIG-A and the phylogenetic relationship between this protein and other glycosyltransferases.

Results

In this paper we show that out of the twelve conserved motifs identified by us eleven are exclusively present in PIG-A and, therefore, can be used as markers to identify PIG-A from newly sequenced genomes. Three of these motifs are absent in the primitive eukaryote, G. lamblia. Sequence analyses show that seven of these conserved motifs are present in prokaryote and archaeal counterparts in rudimentary forms and can be used to differentiate PIG-A proteins from glycosyltransferases. Using partial least square regression analysis and data involving presence or absence of motifs in a range of PIG-A and glycosyltransferases we show that (i) PIG-A may have evolved from prokaryotic glycosyltransferases and lipopolysaccharide synthases, members of the GT4 family of glycosyltransferases and (ii) it is possible to uniquely classify PIG-A proteins versus glycosyltransferases.

Conclusion

Besides identifying unique motifs and showing that PIG-A protein from G. lamblia and some putative PIG-A proteins from archaebacteria are evolutionarily closer to glycosyltransferases, these studies provide a new method for identification and classification of PIG-A proteins.

Phosphatidylethanolamine is the precursor of the ethanolamine phosphoglycerol moiety bound to eukaryotic elongation factor 1A

In addition to its conventional role during protein synthesis, eukaryotic elongation factor 1A is involved in other cellular processes. Several regions of interaction between eukaryotic elongation factor 1A and the translational apparatus or the cytoskeleton have been identified, yet the roles of the different post-translational modifications of eukaryotic elongation factor 1A are completely unknown. One amino acid modification, which so far has only been found in eukaryotic elongation factor 1A, consists of ethanolamine-phosphoglycerol attached to two glutamate residues that are conserved between mammals and plants. We now report that ethanolamine-phosphoglycerol is also present in eukaryotic elongation factor 1A of the protozoan parasite Trypanosoma brucei, indicating that this unique protein modification is of ancient origin. In addition, using RNA-mediated gene silencing against enzymes of the Kennedy pathway, we demonstrate that phosphatidylethanolamine is a direct precursor of the ethanolamine-phosphoglycerol moiety. Down-regulation of the expression of ethanolamine kinase and ethanolamine-phosphate cytidylyltransferase results in inhibition of phosphatidylethanolamine synthesis in T. brucei procyclic forms and, concomitantly, in a block in glycosylphosphatidylinositol attachment to procyclins and ethanolamine-phosphoglycerol modification of eukaryotic elongation factor 1A.

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.

Nutrient sensing by the early mouse embryo: hexosamine biosynthesis and glucose signaling during preimplantation development

Although mouse oocytes and cleavage-stage embryos are unable to utilize glucose as a metabolic fuel, they have a specific requirement for a short exposure to glucose prior to compaction. The reason for this requirement has been unclear. In this study we confirm that cleavage-stage exposure to glucose is required for blastocyst formation and show that the absence of glucose between 18-64 h after hCG causes an irreversible decrease in cellular proliferation and an increase in apoptosis. More importantly, this glucose signals to activate expression of Slc2a3 transcript and SLC2A3 protein, a facilitative glucose transporter (previously known as GLUT3) associated with developmental competence and increased glucose uptake used to fuel blastocyst formation. Glucosamine could substitute for glucose in these roles, suggesting that hexosamine biosynthesis may be a nutrient-sensing mechanism involved in metabolic differentiation. Inhibition of the rate-limiting enzyme in this pathway, glutamine-fructose-6-phosphate amidotransferase (GFPT), inhibited expression of the SLC2A3 transporter protein and blastocyst formation. Glucosamine, a substrate that enters this pathway downstream of GFPT, was able to overcome this inhibition and support SLC2A3 expression. These data suggest that early embryos rely on hexosamine biosynthesis as a glucose-sensing pathway to initiate metabolic differentiation.

Pga1 is an essential component of Glycosylphosphatidylinositol-mannosyltransferase II of Saccharomyces cerevisiae

The Saccharomyces cerevisiae essential gene YNL158w/PGA1 encodes an endoplasmic reticulum (ER)-localized membrane protein. We constructed temperature-sensitive alleles of PGA1 by error-prone polymerase chain reaction mutagenesis to explore its biological role. Pulse-chase experiments revealed that the pga1(ts) mutants accumulated the ER-form precursor of Gas1 protein at the restrictive temperature. Transport of invertase and carboxypeptidase Y were not affected. Triton X-114 phase separation and [(3)H]inositol labeling indicated that the glycosylphosphatidylinositol (GPI)-anchoring was defective in the pga1(ts) mutants, suggesting that Pga1 is involved in GPI synthesis or its transfer to target proteins. We found GPI18, which was recently reported to encode GPI-mannosyltransferase II (GPI-MT II), as a high-copy suppressor of the temperature sensitivity of pga1(ts). Both Gpi18 and Pga1 were detected in the ER by immunofluorescence, and they were coprecipitated from the Triton X-100-solubilized membrane. The gpi18(ts) and pga1(ts) mutants accumulated the same GPI synthetic intermediate at the restrictive temperature. From these results, we concluded that Pga1 is an additional essential component of the yeast GPI-MT II.

Truncation of the caspase-related subunit (Gpi8p) of Saccharomyces cerevisiae GPI transamidase: Dimerization revealed

Eukaryotic proteins can be post-translationally modified with a glycosylphosphatidylinositol (GPI) membrane anchor. This modification reaction is catalyzed by GPI transamidase (GPI-T), a multimeric, membrane-bound enzyme. Gpi8p, an essential component of GPI-T, shares low sequence similarity with caspases and contains all or part of the enzyme's active site [U. Meyer, M. Benghezal, I. Imhof, A. Conzelmann, Biochemistry 39 (2000) 3461-3471]. Structural predictions suggest that the soluble portion of Gpi8p is divided into two domains: a caspase-like domain that contains the active site machinery and a second, smaller domain of unknown function. Based on these predictions, we evaluated a soluble truncation of Gpi8p (Gpi8(23-306)). Dimerization was investigated due to the known proclivity of caspases to homodimerize; a Gpi8(23-306) homodimer was detected by native gel and confirmed by mass spectrometry and N-terminal sequencing. Mutations at the putative caspase-like dimerization interface disrupted dimer formation. When combined, these results demonstrate an organizational similarity between Gpi8p and caspases.

Glycosylphosphatidylinositol (GPI) anchor is required in Aspergillus fumigatus for morphogenesis and virulence

In yeast, glycosylphosphatidylinositol (GPI) is essential for viability and plays an important role in biosynthesis and organization of cell wall. Initiation of the GPI anchor biosynthesis is catalysed by the GPI-N-acetylglucosaminyltransferase complex (GPI-GnT). The GPI3 (SPT14) gene is thought to encode the catalytic subunit of GPI-GnT complex. In contrast to Saccharomyces cerevisiae, little is known about the GPI biosynthesis in filamentous fungi. In this study, the afpig-a gene was identified as the homologue of the GPI3/pig-A gene in Aspergillus fumigatus, an opportunistic fungal pathogen. By replacement of the afpig-a gene with a pyrG gene, we obtained the null mutants. Although the Deltaafpig-a mutant exhibited a significant increased cell lysis instead of temperature-sensitive or conditional lethal phenotype associated to the GPI3 mutant of yeast, they could survive at temperatures from 30 degrees C to 50 degrees C. The analysis of the mutants showed that a completely blocking of the GPI anchor synthesis in A. fumigatus led to cell wall defect, abnormal hyphal growth, rapid conidial germination and aberrant conidiation. In vivo assays revealed that the mutant exhibited a reduced virulence in immunocompromised mice. The GPI anchor was not essential for viability, but required for the cell wall integrity, morphogenesis and virulence in A. fumigatus.

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.

Computational prediction of subcellular localization

It is widely recognized that much of the information for determining the final subcellular localization of proteins is found in their amino acid sequences. Thus the prediction of protein localization sites is of both theoretical and practical interest. In most cases, the prediction has been attempted in two ways: one is based on the knowledge of experimentally characterized targeting signals, while the other utilizes the statistical differences of general sequence characteristics, such as amino acid composition, between localization sites. Both approaches have limitations, and it is recommended to check the results of various prediction methods based on different principles as well as training data. Recently, increased proteomic analyses of localization sites have provided new data to assess the current status of predictive methods. In this chapter we discuss these issues and close with an example illustrating the use of the WoLF PSORT web server for localization prediction.

Expression and immunogenicity of the Plasmodium falciparum circumsporozoite protein: the role of GPI signal sequence

Previous studies have shown that the immunogenicity of rodent malaria parasite-derived circumsporozoite protein (CS) can be improved by deleting the glycosyl-phosphatidyl-inositol (GPI) signal sequence. To study whether GPI signal sequence deletion would also improve immunogenicity of CS derived from the major plasmodium species causing mortality in humans (P. falciparum), we tested different variants of the P. falciparum CS protein in the context of a live vector-based vaccine carrier (rAd35). We demonstrate that deletion of the GPI signal sequence from CS did not result in altered expression or secretion. In contrast, cellular localization was clearly altered, which perhaps helps to explain the significant improvement of anti-CS antibody and T-cell responses observed in mice using deletion variants in the context of the rAd35 carrier. Our results show that rational design of antigens is warranted for further development of malaria vaccines.

Ethanolamine phosphate linked to the first mannose residue of glycosylphosphatidylinositol (GPI) lipids is a major feature of the GPI structure that is recognized by human GPI transamidase

Glycosylphosphatidylinositol (GPI) anchoring of proteins is catalyzed by GPI transamidase (GPIT), a multisubunit, endoplasmic reticulum (ER)-localized enzyme. GPIT recognizes ER-translocated proteins that have a GPI-directing C-terminal signal sequence and replaces this sequence with a preassembled GPI anchor. Although the GPI signal sequence has been extensively characterized, little is known about the structural features of the GPI lipid substrate that enable its recognition by GPIT. In a previous study we showed that mature GPIs could be co-immunoprecipitated with GPIT complexes containing functional subunits (Vainauskas, S., and Menon, A. K. (2004) J. Biol. Chem. 279, 6540-6545). We now use this approach, as well as a method that reconstitutes the interaction between GPIs and GPIT, to define the basis of the interaction between GPI and human GPIT. We report that (i) human GPIT can interact with GPI biosynthetic intermediates, not just mature GPIs competent for transfer to protein, (ii) the ethanolamine phosphate group on the third mannose residue of the GPI glycan is not critical for GPI recognition by GPIT, (iii) the ethanolamine phosphate residue linked to the first mannose of the GPI structure is a major feature of GPIs that is recognized by human GPIT, and (iv) the simplest GPI recognized by human GPIT is EtN-P-2Manalpha1-4GlcN-(acyl)-phosphatidyl-inositol. These studies define the molecular characteristics of GPI that are recognized by GPIT and open the way to identifying GPIT subunits that are involved in this process.

The structure and synthesis of the fungal cell wall

The fungal cell wall is a dynamic structure that protects the cell from changes in osmotic pressure and other environmental stresses, while allowing the fungal cell to interact with its environment. The structure and biosynthesis of a fungal cell wall is unique to the fungi, and is therefore an excellent target for the development of anti-fungal drugs. The structure of the fungal cell wall and the drugs that target its biosynthesis are reviewed. Based on studies in a number of fungi, the cell wall has been shown to be primarily composed of chitin, glucans, mannans and glycoproteins. The biosynthesis of the various components of the fungal cell wall and the importance of the components in the formation of a functional cell wall, as revealed through mutational analyses, are discussed. There is strong evidence that the chitin, glucans and glycoproteins are covalently cross-linked together and that the cross-linking is a dynamic process that occurs extracellularly.

PER1 is required for GPI-phospholipase A2 activity and involved in lipid remodeling of GPI-anchored proteins

Glycosylphoshatidylinositol (GPI) anchors are remodeled during their transport to the cell surface. Newly synthesized proteins are transferred to a GPI anchor, consisting of diacylglycerol with conventional C16 and C18 fatty acids, whereas the lipid moiety in mature GPI-anchored proteins is exchanged to either diacylglycerol containing a C26:0 fatty acid in the sn-2 position or ceramide in Saccharomyces cerevisiae. Here, we report on PER1, a gene encoding a protein that is required for the GPI remodeling pathway. We found that GPI-anchored proteins could not associate with the detergent-resistant membranes in per1Delta cells. In addition, the mutant cells had a defect in the lipid remodeling from normal phosphatidylinositol (PI) to a C26 fatty acid-containing PI in the GPI anchor. In vitro analysis showed that PER1 is required for the production of lyso-GPI, suggesting that Per1p possesses or regulates the GPI-phospholipase A2 activity. We also found that human PERLD1 is a functional homologue of PER1. Our results demonstrate for the first time that PER1 encodes an evolutionary conserved component of the GPI anchor remodeling pathway, highlighting the close connection between the lipid remodeling of GPI and raft association of GPI-anchored proteins.

Recent developments in the molecular, biochemical and functional characterization of GPI8 and the GPI-anchoring mechanism [review]

Glycoconjugates are utilized by eukaryotic organisms ranging from yeast to humans for the cell surface expression of a wide variety of proteins and lipids. These glycoconjugates are expressed as enzymes or receptors and serve a diversity of functions, including cell signaling and cell survival. In parasitic protozoans, glycoconjugates play roles in infectivity, survival, virulence and immune evasion. Among the alternate glycoconjugate structures that have been identified, glycosylphosphatidylinositols (GPIs) represent a universal structure for the anchorage of proteins, lipids, and phosphosaccharides to cellular membranes. Biosynthesis of the GPI is a multi-step process that culminates in the attachment of the assembled GPI to a precursor protein. This final step in the transfer of the GPI to a protein is catalyzed by GPI8 of the putative transamidase complex (TAM). GPI8 functions dually to perform the proteolytic cleavage of the C-terminal signal sequence of the precursor protein, followed by the formation of an amide bond between the protein and the ethanolamine phosphate of the GPI. This review summarizes the current aggregate of biochemical, gene-disruption and active site mutagenesis studies, which provide evidence that GPI8 is responsible for the protein-GPI anchoring reaction. We describe recently published studies that have identified other potential components of the TAM complex and that have elucidated their likely role in protein-GPI attachment. Further, we discuss the biochemical, molecular and functional differences between protozoan and mammalian GPI8 and the protein-GPI anchoring machinery. Finally, we will present the implications of these studies for the development of anti-parasite drug therapies.