Cell-free synthesis of glycosyl-phosphatidylinositol precursors for the glycolipid membrane anchor of Trypanosoma brucei variant surface glycoproteins. Structural characterization of putative biosynthetic intermediates

Identification of the glycosylphosphatidylinositol-specific phospholipase A2 (GPI-PLA2) that mediates GPI fatty acid remodeling in Trypanosoma brucei

The biosynthesis of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) in the parasitic protozoan Trypanosoma brucei involves fatty acid remodeling of the GPI precursor molecules before they are transferred to protein in the endoplasmic reticulum. The genes encoding the requisite phospholipase A2 and A1 activities for this remodeling have thus far been elusive. Here, we identify a gene, Tb927.7.6110, that encodes a protein that is both necessary and sufficient for GPI-phospholipase A2 (GPI-PLA2) activity in the procyclic form of the parasite. The predicted protein product belongs to the alkaline ceramidase, PAQR receptor, Per1, SID-1, and TMEM8 (CREST) superfamily of transmembrane hydrolase proteins and shows sequence similarity to Post-GPI-Attachment to Protein 6 (PGAP6), a GPI-PLA2 that acts after transfer of GPI precursors to protein in mammalian cells. We show the trypanosome Tb927.7.6110 GPI-PLA2 gene resides in a locus with two closely related genes Tb927.7.6150 and Tb927.7.6170, one of which (Tb927.7.6150) most likely encodes a catalytically inactive protein. The absence of GPI-PLA2 in the null mutant procyclic cells not only affected fatty acid remodeling but also reduced GPI anchor sidechain size on mature GPI-anchored procyclin glycoproteins. This reduction in GPI anchor sidechain size was reversed upon the re-addition of Tb927.7.6110 and of Tb927.7.6170, despite the latter not encoding GPI precursor GPI-PLA2 activity. Taken together, we conclude that Tb927.7.6110 encodes the GPI-PLA2 of GPI precursor fatty acid remodeling and that more work is required to assess the roles and essentiality of Tb927.7.6170 and the presumably enzymatically inactive Tb927.7.6150.

Identification of TbPBN1 in Trypanosoma brucei reveals a conserved heterodimeric architecture for glycosylphosphatidylinositol-mannosyltransferase-I

Glycosylphosphatidylinositol (GPI)‐anchored proteins are found in all eukaryotes and are especially abundant on the surface of protozoan parasites such as Trypanosoma brucei. GPI‐mannosyltransferase‐I (GPI‐MT‐I) catalyzes the addition of the first of three mannoses that make up the glycan core of GPI. Mammalian and yeast GPI‐MT‐I consist of two essential subunits, the catalytic subunit PIG‐M/Gpi14 and the accessory subunit PIG‐X/Pbn1(mammals/yeast). T. brucei GPI‐MT‐I has been highlighted as a potential antitrypanosome drug target but has not been fully characterized. Here, we show that T. brucei GPI‐MT‐I also has two subunits, TbGPI14 and TbPBN1. Using TbGPI14 deletion, and TbPBN1 RNAi‐mediated depletion, we show that both proteins are essential for the mannosyltransferase activity needed for GPI synthesis and surface expression of GPI‐anchored proteins. In addition, using native PAGE and co‐immunoprecipitation analyses, we demonstrate that TbGPI14 and TbPBN1 interact to form a higher‐order complex. Finally, we show that yeast Gpi14 does not restore GPI‐MT‐I function in TbGPI14 knockout trypanosomes, consistent with previously demonstrated species specificity within GPI‐MT‐I subunit associations. The identification of an essential trypanosome GPI‐MT‐I subcomponent indicates wide conservation of the heterodimeric architecture unusual for a glycosyltransferase, leaving open the question of the role of the noncatalytic TbPBN1 subunit in GPI‐MT‐I function.

The Glycosylphosphatidylinositol Anchor: A Linchpin for Cell Surface Versatility of Trypanosomatids

The use of glycosylphosphatidylinositol (GPI) to anchor proteins to the cell surface is widespread among eukaryotes. The GPI-anchor is covalently attached to the C-terminus of a protein and mediates the protein’s attachment to the outer leaflet of the lipid bilayer. GPI-anchored proteins have a wide range of functions, including acting as receptors, transporters, and adhesion molecules. In unicellular eukaryotic parasites, abundantly expressed GPI-anchored proteins are major virulence factors, which support infection and survival within distinct host environments. While, for example, the variant surface glycoprotein (VSG) is the major component of the cell surface of the bloodstream form of African trypanosomes, procyclin is the most abundant protein of the procyclic form which is found in the invertebrate host, the tsetse fly vector. Trypanosoma cruzi, on the other hand, expresses a variety of GPI-anchored molecules on their cell surface, such as mucins, that interact with their hosts. The latter is also true for Leishmania, which use GPI anchors to display, amongst others, lipophosphoglycans on their surface. Clearly, GPI-anchoring is a common feature in trypanosomatids and the fact that it has been maintained throughout eukaryote evolution indicates its adaptive value. Here, we explore and discuss GPI anchors as universal evolutionary building blocks that support the great variety of surface molecules of trypanosomatids.

Elimination of GPI2 suppresses glycosylphosphatidylinositol GlcNAc transferase activity and alters GPI glycan modification in Trypanosoma brucei

Many eukaryotic cell-surface proteins are post-translationally modified by a glycosylphosphatidylinositol (GPI) moiety that anchors them to the cell membrane. The biosynthesis of GPI anchors is initiated in the endoplasmic reticulum by transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol. This reaction is catalyzed by GPI GlcNAc transferase, a multisubunit complex comprising the catalytic subunit Gpi3/PIG-A as well as at least five other subunits, including the hydrophobic protein Gpi2, which is essential for the activity of the complex in yeast and mammals, but the function of which is not known. To investigate the role of Gpi2, we exploited Trypanosoma brucei (Tb), an early diverging eukaryote and important model organism that initially provided the first insights into GPI structure and biosynthesis. We generated insect-stage (procyclic) trypanosomes that lack TbGPI2 and found that in TbGPI2-null parasites, (i) GPI GlcNAc transferase activity is reduced, but not lost, in contrast with yeast and human cells, (ii) the GPI GlcNAc transferase complex persists, but its architecture is affected, with loss of at least the TbGPI1 subunit, and (iii) the GPI anchors of procyclins, the major surface proteins, are underglycosylated when compared with their WT counterparts, indicating the importance of TbGPI2 for reactions that occur in the Golgi apparatus. Immunofluorescence microscopy localized TbGPI2 not only to the endoplasmic reticulum but also to the Golgi apparatus, suggesting that in addition to its expected function as a subunit of the GPI GlcNAc transferase complex, TbGPI2 may have an enigmatic noncanonical role in Golgi-localized GPI anchor modification in trypanosomes.

Proteomic identification of the UDP-GlcNAc: PI α1-6 GlcNAc-transferase subunits of the glycosylphosphatidylinositol biosynthetic pathway of Trypanosoma brucei

The first step of glycosylphosphatidylinositol (GPI) anchor biosynthesis in all eukaryotes is the addition of N-acetylglucosamine (GlcNAc) to phosphatidylinositol (PI) which is catalysed by a UDP-GlcNAc: PI α1-6 GlcNAc-transferase, also known as GPI GnT. This enzyme has been shown to be a complex of seven subunits in mammalian cells and a similar complex of six homologous subunits has been postulated in yeast. Homologs of these mammalian and yeast subunits were identified in the Trypanosoma brucei predicted protein database. The putative catalytic subunit of the T. brucei complex, TbGPI3, was epitope tagged with three consecutive c-Myc sequences at its C-terminus. Immunoprecipitation of TbGPI3-3Myc followed by native polyacrylamide gel electrophoresis and anti-Myc Western blot showed that it is present in a ~240 kDa complex. Label-free quantitative proteomics were performed to compare anti-Myc pull-downs from lysates of TbGPI-3Myc expressing and wild type cell lines. TbGPI3-3Myc was the most highly enriched protein in the TbGPI3-3Myc lysate pull-down and the expected partner proteins TbGPI15, TbGPI19, TbGPI2, TbGPI1 and TbERI1 were also identified with significant enrichment. Our proteomics data also suggest that an Arv1-like protein (TbArv1) is a subunit of the T. brucei complex. Yeast and mammalian Arv1 have been previously implicated in GPI biosynthesis, but here we present the first experimental evidence for physical association of Arv1 with GPI biosynthetic machinery. A putative E2-ligase has also been tentatively identified as part of the T. brucei UDP-GlcNAc: PI α1-6 GlcNAc-transferase complex.

Protozoan Parasites Glycosylphosphatidylinositol Anchors: Structures, Functions and Trends for Drug Discovery

BACKGROUND

Glycosylphosphatidylinositol (GPI) anchors are molecules located on cell membranes of all eukaryotic organisms. Proteins, enzymes, and other macromolecules which are anchored by GPIs are essential elements for interaction between cells, and are widely used by protozoan parasites when compared to higher eukaryotes.

METHODS

More than one hundred references were collected to obtain broad information about mammalian and protozoan parasites' GPI structures, biosynthetic pathways, functions and attempts to use these molecules as drug targets against parasitic diseases. Differences between GPI among species were compared and highlighted. Strategies for drug discovery and development against protozoan GPI anchors were discussed based on what has been reported on literature.

RESULTS

There are many evidences that GPI anchors are crucial for parasite's survival and interaction with hosts' cells. Despite all GPI anchors contain a conserved glycan core, they present variations regarding structural features and biosynthetic pathways between organisms, which could offer adequate selectivity to validate GPI anchors as drug targets. Discussion was developed with focus on the following parasites: Trypanosoma brucei, Trypanosoma cruzi, Leishmania, Plasmodium falciparum and Toxoplasma gondii, causative agents of tropical neglected diseases.

CONCLUSION

This review debates the main variances between parasitic and mammalian GPI anchor biosynthesis and structures, as well as clues for strategic development for new anti-parasitic therapies based on GPI anchors.

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.

Lipid topogenesis--35years on

Glycerophospholipids are the principal fabric of cellular membranes. The pathways by which these lipids are synthesized were elucidated mainly through the work of Kennedy and colleagues in the late 1950s and early 1960s. Subsequently, attention turned to cell biological aspects of lipids: Where in the cell are lipids synthesized? How are lipids integrated into membranes to form a bilayer? How are they sorted and transported from their site of synthesis to other cellular destinations? These topics, collectively termed 'lipid topogenesis', were the subject of a review article in 1981 by Bell, Ballas and Coleman. We now assess what has been learned about early events of lipid topogenesis, i.e. "lipid synthesis, the integration of lipids into membranes, and lipid translocation across membranes", in the 35 years since the publication of this important review. We highlight the recent elucidation of the X-ray structures of key membrane enzymes of glycerophospholipid synthesis, progress on identifying lipid scramblase proteins needed to equilibrate lipids across membranes, and new complexities in the subcellular location and membrane topology of phosphatidylinositol synthesis revealed through a comparison of two unicellular model eukaryotes. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Virulent and avirulent strains of Toxoplasma gondii which differ in their glycosylphosphatidylinositol content induce similar biological functions in macrophages

Glycosylphosphatidylinositols (GPIs) from several protozoan parasites are thought to elicit a detrimental stimulation of the host innate immune system aside their main function to anchor surface proteins. Here we analyzed the GPI biosynthesis of an avirulent Toxoplasma gondii type 2 strain (PTG) by metabolic radioactive labeling. We determined the biological function of individual GPI species in the PTG strain in comparison with previously characterized GPI-anchors of a virulent strain (RH). The GPI intermediates of both strains were structurally similar, however the abundance of two of six GPI intermediates was significantly reduced in the PTG strain. The side-by-side comparison of GPI-anchor content revealed that the PTG strain had only ∼34% of the protein-free GPIs as well as ∼70% of the GPI-anchored proteins with significantly lower rates of protein N-glycosylation compared to the RH strain. All mature GPIs from both strains induced comparable secretion levels of TNF-α and IL-12p40, and initiated TLR4/MyD88-dependent NF-κBp65 activation in macrophages. Taken together, these results demonstrate that PTG and RH strains differ in their GPI biosynthesis and possess significantly different GPI-anchor content, while individual GPI species of both strains induce similar biological functions in macrophages.

Lipid remodelling of glycosylphosphatidylinositol (GPI) glycoconjugates in procyclic-form trypanosomes: biosynthesis and processing of GPIs revisited

The African trypanosome, Trypanosoma brucei, has been used as a model to study the biosynthesis of GPI (glycosylphosphatidylinositol) anchors. In mammalian (bloodstream)-form parasites, diacyl-type GPI precursors are remodelled in their lipid moieties before attachment to variant surface glycoproteins. In contrast, the GPI precursors of insect (procyclic)-form parasites, consisting of lyso-(acyl)PI (inositol-acylated acyl-lyso-phosphatidylinositol) species, remain unaltered before protein attachment. By using a combination of metabolic labelling, cell-free assays and complementary MS analyses, we show in the present study that GPI-anchored glycoconjugates in T. congolense procyclic forms initially receive tri-acylated GPI precursors, which are subsequently de-acylated either at the glycerol backbone or on the inositol ring. Chemical and enzymatic treatments of [3H]myristate-labelled lipids in combination with ESI-MS/MS (electrospray ionization-tandem MS) and MALDI-QIT-TOF-MS3 (matrix-assisted laser-desorption ionization-quadrupole ion trap-time-of-flight MS) analyses indicate that the structure of the lipid moieties of steady-state GPI lipids from T. congolense procyclic forms consist of a mixture of lyso-(acyl)PI, diacyl-PI and diacyl-(acyl)PI species. Interestingly, some of these species are myristoylated at the sn-2 position. To our knowledge, this is the first demonstration of lipid remodelling at the level of protein- or polysaccharide-linked GPI anchors in procyclic-form trypanosomes.

Lipid metabolism in Trypanosoma brucei

Trypanosoma brucei membranes consist of all major eukaryotic glycerophospholipid and sphingolipid classes. These are de novo synthesized from precursors obtained either from the host or from catabolised endocytosed lipids. In recent years, substantial progress has been made in the molecular and biochemical characterisation of several of these lipid biosynthetic pathways, using gene knockout or RNA interference strategies or by enzymatic characterization of individual reactions. Together with the completed genome, these studies have highlighted several possible differences between mammalian and trypanosome lipid biosynthesis that could be exploited for the development of drugs against the diseases caused by these parasites.

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.

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.

Activation of TLR2 and TLR4 by glycosylphosphatidylinositols derived from Toxoplasma gondii

GPIs isolated from Toxoplasma gondii, as well as a chemically synthesized GPI lacking the lipid moiety, activated a reporter gene in Chinese hamster ovary cells expressing TLR4, while the core glycan and lipid moieties cleaved from the GPIs activated both TLR4- and TLR2-expressing cells. MyD88, but not TLR2, TLR4, or CD14, is absolutely needed to trigger TNF-alpha production by macrophages exposed to T. gondii GPIs. Importantly, TNF-alpha response to GPIs was completely abrogated in macrophages from TLR2/4-double-deficient mice. MyD88(-/-) mice were more susceptible to death than wild-type (WT), TLR2(-/-), TLR4(-/-), TLR2/4(-/-), and CD14(-/-) mice infected with the ME-49 strain of T. gondii. The cyst number was higher in the brain of TLR2/4(-/-), but not TLR2(-/-), TLR4(-/-), and CD14(-/-), mice, as compared with WT mice. Upon infection with the ME-49 strain of T. gondii, we observed no decrease of IL-12 and IFN-gamma production in TLR2-, TLR4-, or CD14-deficient mice. Indeed, splenocytes from T. gondii-infected TLR2(-/-) and TLR2/4(-/-) mice produced more IFN-gamma than cells from WT mice in response to in vitro stimulation with parasite extracts enriched in GPI-linked surface proteins. Together, our results suggest that both TLR2 and TLR4 receptors may participate in the host defense against T. gondii infection through their activation by the GPIs and could work together with other MyD88-dependent receptors, like other TLRs or even IL-18R or IL-1R, to obtain an effective host response against T. gondii infection.

The role of inositol acylation and inositol deacylation in the Toxoplasma gondii glycosylphosphatidylinositol biosynthetic pathway

Toxoplasma gondii is a ubiquitous parasitic protozoan that invades nucleated cells in a process thought to be in part due to several surface glycosylphosphatidylinositol (GPI)-anchored proteins, like the major surface antigen SAG1 (P30), which dominates the plasma membrane. The serine protease inhibitors phenylmethylsulfonyl fluoride and diisopropyl fluoride were found to have a profound effect on the T. gondii GPI biosynthetic pathway, leading to the observation and characterization of novel inositol-acylated mannosylated GPI intermediates. This inositol acylation is acyl-CoA-dependent and takes place before mannosylation, but uniquely for this class of inositol-acyltransferase, it is inhibited by phenylmethylsulfonyl fluoride. The subsequent inositol deacylation of fully mannosylated GPI intermediates is inhibited by both phenylmethylsulfonyl fluoride and diisopropyl fluoride. The use of these serine protease inhibitors allows observations as to the timing of inositol acylation and subsequent inositol deacylation of the GPI intermediates. Inositol acylation of the non-mannosylated GPI intermediate D-GlcNalpha1-6-D-myo-inositol-1-HPO4-sn-lipid precedes mannosylation. Inositol deacylation of the fully mannosylated GPI intermediate allows further processing, i.e. addition of GalNAc side chain to the first mannose. Characterization of the phosphatidylinositol moieties present on both free GPIs and GPI-anchored proteins shows the presence of a diacylglycerol lipid, whose sn-2 position contains almost exclusively an C18:1 acyl chain. The data presented here identify key novel inositol-acylated mannosylated intermediates, allowing the formulation of an updated T. gondii GPI biosynthetic pathway along with identification of the putative genes involved.

Fatty acids isolated from Toxoplasma gondii reduce glycosylphosphatidylinositol-induced tumor necrosis factor alpha production through inhibition of the NF-kappaB signaling pathway

Glycosylphosphatidylinositols (GPIs) are involved in the pathogenicity of protozoan parasites and are known to induce inflammatory cytokines. However, we have previously shown that the family of six GPIs of Toxoplasma gondii extracted together from tachyzoites could not induce tumor necrosis factor alpha (TNF-alpha) secretion by macrophages, whereas GPIs individually separated from this extract by thin-layer chromatography (TLC) were able to stimulate the cells. In the present study we show that the TLC step makes it possible to eliminate inhibitors extracted together with the T. gondii GPIs. Among the non-GPI molecules we have isolated fatty acids able to inhibit the secretion of TNF-alpha induced by the T. gondii GPIs. Myristic and palmitic acids reduce the production of TNF-alpha through the inhibition of tyrosine phosphorylation of cytoplasmic proteins and the inhibition of NF-kappaB activation in a peroxisome proliferator-activated receptor-independent pathway and after a rapid entry into the cytoplasm of macrophages. GPIs are considered toxins inducing irreversible damage in the host, and fatty acids produced in parallel by the parasite could reduce the immune response, thus favoring the persistence of parasite infection.

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.

Toxoplasma gondii glycosylphosphatidylinositols are not involved in T. gondii-induced host cell survival

Toxoplasma gondii is an intracellular parasite able to both promote and inhibit apoptosis. T. gondii renders infected cells resistant to programmed cell death induced by multiple apoptotic triggers. On the other hand, increased apoptosis of immune cells after in vivo infection with T. gondii may suppress the immune response to the parasite. Glycosylphosphatidylinositol (GPI)-anchored proteins dominate the surface of T. gondii tachyzoites and GPIs are involved in the pathogenicity of protozoan parasites. In this report, we determine if GPIs are responsible for inhibition or induction of host cell apoptosis. We show here that T. gondii GPIs fail to block apoptosis that was triggered in human-derived cells via extrinsic or intrinsic apoptotic pathways. Furthermore, characteristics of apoptosis, e.g. caspase-3/7 activity, phosphatidylserine exposition at the cell surface or DNA strand breaks, were not observed in the presence of T. gondii GPIs. These results indicate that T. gondii GPIs are not involved in survival or in apoptosis of host cells. This absence of effect on apoptosis could be a feature common to GPIs of other parasites.

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 glycosylphosphatidylinositol (GPI) biosynthetic pathway of bloodstream-form Trypanosoma brucei is dependent on the de novo synthesis of inositol

In bloodstream-form Trypanosoma brucei (the causative agent of African sleeping sickness) the glycosylphosphatidylinositol (GPI) anchor biosynthetic pathway has been validated genetically and chemically as a drug target. The conundrum that GPI anchors could not be in vivo labelled with [3H]-inositol led us to hypothesize that de novo synthesis was responsible for supplying myo-inositol for phosphatidylinositol (PI) destined for GPI synthesis. The rate-limiting step of the de novo synthesis is the isomerization of glucose 6-phosphate to 1-D-myo-inositol-3-phosphate, catalysed by a 1-D-myo-inositol-3-phosphate synthase (INO1). When grown under non-permissive conditions, a conditional double knockout demonstrated that INO1 is an essential gene in bloodstream-form T. brucei. It also showed that the de novo synthesized myo-inositol is utilized to form PI, which is preferentially used in GPI biosynthesis. We also show for the first time that extracellular myo-inositol can in fact be used in GPI formation although to a limited extent. Despite this, extracellular inositol cannot compensate for the deletion of INO1. Supporting these results, there was no change in PI levels in the conditional double knockout cells grown under non-permissive conditions, showing that perturbation of growth is due to a specific lack of de novo synthesized myo-inositol and not a general inositol-less death. These results suggest that there is a distinction between de novo synthesized myo-inositol and that from the extracellular environment.

Regulation of surface coat exchange by differentiating African trypanosomes

African trypanosomes (Trypanosoma brucei) have a digenetic lifecycle that alternates between the mammalian bloodstream and the tsetse fly vector. In the bloodstream, replicating long slender parasites transform into non-dividing short stumpy forms. Upon transmission into the fly midgut, short stumpy cells differentiate into actively dividing procyclics. A hallmark of this process is the replacement of the bloodstream-stage surface coat composed of variant surface glycoprotein (VSG) with a new coat composed of procyclin. Pre-existing VSG is shed by a zinc metalloprotease activity (MSP-B) and glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC). We now provide a detailed analysis of the coordinate and inverse regulation of these activities during synchronous differentiation. MSP-B mRNA and protein levels are upregulated during differentiation at the same time as proteolysis whereas GPI-PLC levels decrease. When transcription or translation is inhibited, VSG release is incomplete and a substantial amount of protein stays cell-associated. Both modes of release are still evident under these conditions, but GPI hydrolysis plays a quantitatively minor role during normal differentiation. Nevertheless, GPI biosynthesis shifts early in differentiation from a GPI-PLC sensitive structure to a resistant procyclic-type anchor. Translation inhibition also results in a marked increase in the mRNA levels of both MSP-B and GPI-PLC, consistent with negative regulation by labile protein factors. The relegation of short stumpy surface GPI-PLC to a secondary role in differentiation suggests that it may play a more important role as a virulence factor within the mammalian host.

Removal or Maintenance of Inositol-linked Acyl Chain in Glycosylphosphatidylinositol Is Critical in Trypanosome Life Cycle

The protozoan parasite Trypanosoma brucei is coated by glycosylphosphatidylinositol (GPI)-anchored proteins. During GPI biosynthesis, inositol in phosphatidylinositol becomes acylated. Inositol is deacylated prior to attachment to variant surface glycoproteins in the bloodstream form, whereas it remains acylated in procyclins in the procyclic form. We have cloned a T. brucei GPI inositol deacylase (GPIdeAc2). In accordance with the acylation/deacylation profile, the level of GPIdeAc2 mRNA was 6-fold higher in the bloodstream form than in the procyclic form. Knockdown of GPIdeAc2 in the bloodstream form caused accumulation of an inositol-acylated GPI, a decreased VSG expression on the cell surface and slower growth, indicating that inositol-deacylation is essential for the growth of the bloodstream form. Overexpression of GPIdeAc2 in the procyclic form caused an accumulation of GPI biosynthetic intermediates lacking inositol-linked acyl chain and decreased cell surface procyclins because of release into the culture medium, indicating that overexpression of GPIdeAc2 is deleterious to the surface coat of the procyclic form. Therefore, the GPI inositol deacylase activity must be tightly regulated in trypanosome life cycle.

Toxoplasma gondii grown in human cells uses GalNAc-containing glycosylphosphatidylinositol precursors to anchor surface antigens while the immunogenic Glc–GalNAc-containing precursors remain free at the parasite cell surface

Toxoplasma gondii is a ubiquitous parasite that infects nearly all warm-blooded animals. Developmental switching in T. gondii, from the virulent tachyzoite to the relatively quiescent bradyzoite stage, is responsible for the disease propagation after alteration of the immune status of the carrier. The redifferentiation event is characterized by an over expression of a tachyzoite specific set of glycosylphosphatidylinositol anchored surface antigens and free GPIs. T. gondii grown in animal cells uses two glycosylphosphatidylinositol precursors to anchor the parasite surface proteins. The first form has an N-acetylgalactosamine residue bound to a conserved three-mannosyl core glycan, while the second structure contains an additional terminal glucose linked to the N-acetylgalactosamine side branch. Sera from persons infected with T. gondii reacted only with the glucose-N-acetylgalactosamine-containing structure. Here we report that T. gondii cultured in human cells uses predominantly the N-acetylgalactosamine-containing structure to anchor the parasite surface antigens. On the other hand, glycosylphosphatidylinositol structures having an additional terminal glucose are found exclusively on the parasite cell surface as free glycolipids participating in the production of cytokines that are implicated in the pathogenesis of T. gondii. We also provide evidence that such free glycosylphosphatidylinositols are restricted mainly to the lipid microdomains in the parasite cell surface membrane and mostly associated with proteins involved in the parasite motility as well as invasion of the host cell.

Deletion of the Glucosidase II Gene in Trypanosoma brucei Reveals Novel N-Glycosylation Mechanisms in the Biosynthesis of Variant Surface Glycoprotein

The trypanosomatids are generally aberrant in their protein N-glycosylation pathways. However, protein N-glycosylation in the African trypanosome Trypanosoma brucei, etiological agent of human African sleeping sickness, is not well understood. Here, we describe the creation of a bloodstream-form T. brucei mutant that is deficient in the endoplasmic reticulum enzyme glucosidase II. Characterization of the variant surface glycoprotein, the main glycoprotein synthesized by the parasite with two N-glycosylation sites, revealed unexpected changes in the N-glycosylation of this molecule. Structural characterization by mass spectrometry, nuclear magnetic resonance spectroscopy, and chemical and enzymatic treatments revealed that one of the two glycosylation sites was occupied by conventional oligomannose structures, whereas the other accumulated unusual structures in the form of Glcalpha1-3Manalpha1-2Manalpha1-2Manalpha1-3(Manalpha1-6)Manbeta1-4GlcNAcbeta1-4GlcNAc, Glcalpha1-3Manalpha1-2Manalpha1-2Manalpha1-3(GlcNAcbeta1-2Manalpha1-6)Manbeta1-4GlcNAcbeta1-4GlcNAc, and Glcalpha1-3Manalpha1-2Manalpha1-2Manalpha1-3(Galbeta1-4GlcNAcbeta1-2Manalpha1-6)Manbeta1-4GlcNAcbeta1-4GlcNAc. The possibility that these structures might arise from Glc1Man9GlcNAc2 by unusually rapid alpha-mannosidase processing was ruled out using a mixture of alpha-mannosidase inhibitors. The results suggest that bloodstream-form T. brucei can transfer both Man9GlcNAc2 and Man5GlcNAc2 to the variant surface glycoprotein in a site-specific manner and that, unlike organisms that transfer exclusively Glc3Man9GlcNAc2, the T. brucei UDP-Glc: glycoprotein glucosyltransferase and glucosidase II enzymes can use Man5GlcNAc2 and Glc1Man5GlcNAc2, respectively, as their substrates. The ability to transfer Man5GlcNAc2 structures to N-glycosylation sites destined to become Man(4-3)GlcNAc2 or complex structures may have evolved as a mechanism to conserve dolichol-phosphate-mannose donors for glycosylphosphatidylinositol anchor biosynthesis and points to fundamental differences in the specificities of host and parasite glycosyltransferases that initiate the synthesis of complex N-glycans.

In vitro biosynthesis of glycosylphosphatidylinositol in Aspergillus fumigatus

Glycosylphosphatidylinositol (GPI) represents a mechanism for the attachment of proteins to the plasma membrane found in all eukaryotic cells. GPI biosynthesis has been mainly studied in parasites, yeast, and mammalian cells. Aspergillus fumigatus, a filamentous fungus, produces GPI-anchored molecules, some of them being essential in the construction of the cell wall. An in vitro assay was used to study the GPI biosynthesis in the mycelium form of this organism. In the presence of UDP-GlcNAc and coenzyme A, the cell-free system produces the initial intermediates of the GPI biosynthesis: GlcNAc-PI, GlcN-PI, and GlcN-(acyl)PI. Using GDP-Man, two types of mannosylation are observed. First, one or two mannose residues are added to GlcN-PI. This mannosylation, never described in fungi, does not require dolichol phosphomannoside (Dol-P-Man) as the monosaccharide donor. Second, one to five mannose residues are added to GlcN-(acyl)PI using Dol-P-Man as the mannose donor. The addition of ethanolamine phosphate groups to the first, second, and third mannose residue is also observed. This latter series of GPI intermediates identified in the A. fumigatus cell-free system indicates that GPI biosynthesis in this filamentous fungus is similar to the mammalian or yeast systems. Thus, these biochemical data are in agreement with a comparative genome analysis that shows that all but 3 of the 21 genes described in the Saccharomyces cerevisiae GPI pathways are found in A. fumigatus.

Deletion of the GPIdeAc Gene Alters the Location and Fate of Glycosylphosphatidylinositol Precursors in Trypanosoma brucei

Glycosylphosphatidylinositol (GPI) membrane anchors are ubiquitous among the eukaryotes. In most organisms, the pathway of GPI biosynthesis involves inositol acylation and inositol deacylation as discrete steps at the beginning and end of the pathway, respectively. The bloodstream form of the protozoan parasite Trypanosoma brucei is unusual in that these reactions occur on multiple GPI intermediates and that it can express side chains of up to six galactose residues on its mature GPI anchors. An inositol deacylase gene, T. brucei GPIdeAc, has been identified. A null mutant was created and shown to be capable of expressing normal mature GPI anchors on its variant surface glycoprotein. Here, we show that the null mutant synthesizes galactosylated forms of the mature GPI precursor, glycolipid A, at an accelerated rate (2.8-fold compared to wild type). These free GPIs accumulate at the cell surface as metabolic end products. Using continuous and pulse-chase labeling experiments, we show that there are two pools of glycolipid A. Only one pool is competent for transfer to nascent variant surface glycoprotein and represents 38% of glycolipid A in wild-type cells. This pool rises to 75% of glycolipid A in the GPIdeAc null mutant. We present a model for the pathway of GPI biosynthesis in T. brucei that helps to explain the complex phenotype of the GPIdeAc null mutant.

Roles of glycosylphosphatidylinositols of Toxoplasma gondii. Induction of tumor necrosis factor-alpha production in macrophages

Toxoplasma gondii is a ubiquitous parasitic protozoan, which causes congenital infectious diseases as well as severe encephalitis, a major cause of death among immune-deficient persons, such as AIDS patients. T. gondii is normally controlled by the immune system of healthy individuals, leading to an asymptomatic infection. T. gondii triggers early cytokine production, which, to a certain extent, protects the host against replication of tachyzoites, the infective form of the parasite. Glycosylphosphatidylinositols (GPIs) constitute a class of glycolipids that have various functions, the most fundamental being to link proteins to eucaryotic cell membranes. GPIs are involved in the pathogenicity of other protozoan parasites and are known to induce tumor necrosis factor-alpha (TNF alpha) production. We show that GPIs highly purified from T. gondii tachyzoites, as well as their core glycans, induce TNF alpha production in macrophages. A chemically synthesized GPI of T. gondii lacking its lipid moiety, GPIa, has the same effect as the natural GPIs, whereas a chemically synthesized molecule with dialkylglycerol instead of diacylglycerol as lipid moiety, GPIb, does not induce TNF alpha production. Moreover, GPIb inhibits the TNF alpha production induced by T. gondii GPI or by GPIa. The core glycan prepared from the two chemically synthesized molecules activates macrophages, showing that the lipid moiety may regulate signaling. Stimulation of macrophages with GPIs of T. gondii results in activation of the transcription factor NF-kappa B, which is inhibited by the chemically synthesized GPIb, suggesting the involvement of NF-kappa B in TNF alpha gene expression. Our results support the idea that T. gondii GPIs are bioactive factors that participate in the production of TNF alpha during toxoplasmal pathogenesis.

Glycosylphosphatidylinositol-anchored proteins: structure, function, and cleavage by phosphatidylinositol-specific phospholipase C

A wide variety of proteins are tethered by a glycosylphosphatidylinositol (GPI) anchor to the extracellular face of eukaryotic plasma membranes, where they are involved in a number of functions ranging from enzymatic catalysis to adhesion. The exact function of the GPI anchor has been the subject of much speculation. It appears to act as an intracellular signal targeting proteins to the apical surface in polarized cells. GPI-anchored proteins are sorted into sphingolipid- and cholesterol-rich microdomains, known as lipid rafts, before transport to the membrane surface. Their localization in raft microdomains may explain the involvement of this class of proteins in signal transduction processes. Substantial evidence suggests that GPI-anchored proteins may interact closely with the bilayer surface, so that their functions may be modulated by the biophysical properties of the membrane. The presence of the anchor appears to impose conformational restraints, and its removal may alter the catalytic properties and structure of a GPI-anchored protein. Release of GPI-anchored proteins from the cell surface by specific phospholipases may play a key role in regulation of their surface expression and functional properties. Reconstitution of GPI-anchored proteins into bilayers of defined phospholipids provides a powerful tool with which to explore the interactions of these proteins with the membrane and investigate how bilayer properties modulate their structure, function, and cleavage by phospholipases.

Cloning of Trypanosoma brucei and Leishmania major genes encoding the GlcNAc-phosphatidylinositol de-N-acetylase of glycosylphosphatidylinositol biosynthesis that is essential to the African sleeping sickness parasite

The second step of glycosylphosphatidylinositol anchor biosynthesis in all eukaryotes is the conversion of D-GlcNAcalpha1-6-d-myo-inositol-1-HPO(4)-sn-1,2-diacylglycerol (GlcNAc-PI) to d-GlcNalpha1-6-d-myo-inositol-1-HPO(4)-sn-1,2-diacylglycerol by GlcNAc-PI de-N-acetylase. The genes encoding this activity are PIG-L and GPI12 in mammals and yeast, respectively. Fragments of putative GlcNAc-PI de-N-acetylase genes from Trypanosoma brucei and Leishmania major were identified in the respective genome project data bases. The full-length genes TbGPI12 and LmGPI12 were subsequently cloned, sequenced, and shown to complement a PIG-L-deficient Chinese hamster ovary cell line and restore surface expression of GPI-anchored proteins. A tetracycline-inducible bloodstream form T. brucei TbGPI12 conditional null mutant cell line was created and analyzed under nonpermissive conditions. TbGPI12 mRNA levels were reduced to undetectable levels within 8 h of tetracycline removal, and the cells died after 3-4 days. This demonstrates that TbGPI12 is an essential gene for the tsetse-transmitted parasite that causes Nagana in cattle and African sleeping sickness in humans. It also validates GlcNAc-PI de-N-acetylase as a potential drug target against these diseases. Washed parasite membranes were prepared from the conditional null mutant parasites after 48 h without tetracycline. These membranes were shown to be greatly reduced in GlcNAc-PI de-N-acetylase activity, but they retained their ability to make GlcNAc-PI and to process d-GlcNalpha1-6-d-myo-inositol-1-HPO(4)-sn-1,2-diacylglycerol to later glycosylphosphatidylinositol intermediates. These results suggest that the stabilities of other glycosylphosphatidylinositol pathway enzymes are not dependent on GlcNAc-PI de-N-acetylase levels.

Substrate specificity of the Plasmodium falciparum glycosylphosphatidylinositol biosynthetic pathway and inhibition by species-specific suicide substrates

The substrate specificities of the early glycosylphosphatidylinositol biosynthetic enzymes of Plasmodium were determined using substrate analogues of D-GlcN(alpha)1-6-D-myo-inositol-1-HPO(4)-sn-1,2-dipalmitoylglycerol (GlcN-PI). Similarities between the Plasmodium and mammalian (HeLa) enzymes were observed. These are as follows: (i) The presence and orientation of the 2'-acetamido/amino and 3'-OH groups are essential for substrate recognition for the de-N-acetylase, inositol acyltransferase, and first mannosyltransferase enzymes. (ii) The 6'-OH group of the GlcN is dispensable for the de-N-acetylase, inositol acyltransferase, all four of the mannosyltransferases, and the ethanolamine phosphate transferase. (iii) The 4'-OH group of GlcNAc is not required for recognition, but substitution interferes with binding to the de-N-acetylase. The 4'-OH group of GlcN is essential for the inositol acyltransferase and first mannosyltransferase. (iv) The carbonyl group of the natural 2-O-hexadecanyl ester of GlcN-(acyl)PI is essential for substrate recognition by the first mannosyltransferase. However, several differences were also discovered: (i) Plasmodium-specific inhibition of the inositol acyltransferase was detected with GlcN-[L]-PI, while GlcN-(2-O-alkyl)PI weakly inhibited the first mannosyltransferase in a competitive manner. (ii) The Plasmodium de-N-acetylase can act on analogues containing N-benzoyl, GalNAc, or betaGlcNAc whereas the human enzyme cannot. Using the parasite specificity of the later two analogues with the known nonspecific de-N-acetylase suicide inhibitor [Smith, T. K., et al. (2001) EMBO J. 20, 3322-3332], GalNCONH(2)-PI and GlcNCONH(2)-beta-PI were designed and found to be potent (IC(50) approximately 0.2 microM), Plasmodium-specific suicide substrate inhibitors. These inhibitors could be potential lead compounds for the development of antimalaria drugs.

Specificities of enzymes of glycosylphosphatidylinositol biosynthesis in Trypanosoma brucei and HeLa cells

A series of synthetic analogues of d-GlcN alpha 1-6-d-myo-inositol-1-HPO(4)-sn-1,2-dipalmitoylglycerol, consisting of 22 variants of the d-GlcN or lipid components, were tested in trypanosomal and human (HeLa) cell-free systems. The assays measured the abilities of the analogues to act as substrates or inhibitors of the enzymes of glycosylphosphatidylinositol biosynthesis downstream of GlcNAc-phosphatidylinositol (GlcNAc-PI) de-N-acetylase. One compound, 4-deoxy-d-GlcN alpha 1-6-d-myo-inositol-1-HPO(4)-sn-1,2-dipalmitoylglycerol, proved to be an inhibitor of both the trypanosomal and HeLa pathways, whereas 4-O-methyl-d-GlcN alpha 1-6-d-myo-inositol-1-HPO(4)-sn-1,2-dipalmitoylglycerol and the 4'-epimer, d-GalN-alpha1-6-d-myo-inositol-1-HPO(4)-sn-1,2-dipalmitoylglycerol, were neither substrates nor inhibitors. The results with other analogues showed that the 6-OH of the alpha-d-GlcN residue is not required for substrate recognition in the trypanosomal and human pathways, whereas the 3-OH group is essential for both. Parasite-specific recognition of the beta-linked analogue d-GlcN beta 1-6-d-myo-inositol-1-HPO(4)-sn-1,2-dipalmitoylglycerol is striking. This suggests that, like the GlcNAc-PI de-N-acetylase, the trypanosomal glycosylphosphatidylinositol alpha-mannosyltransferases, inositol acyltransferse and ethanolamine phosphate transferase, do not recognize the 2-, 3-, 4-, and 5-OH groups of the d-myo-inositol residue, whereas the human inositol acyltransferase and/or first alpha-mannosyltransferase recognizes one or more of these groups. All of the various lipid analogues tested served as substrates in both the trypanosomal and HeLa cell-free systems, suggesting that a precise lipid structure and stereochemistry are not essential for substrate recognition. However, an analogue containing a single C18:0 alkyl chain in place of sn-1,2-dipalmitoylglycerol proved to be a better substrate in the trypanosomal than in the HeLa cell-free system. These findings should have a bearing on the design of future generations of specific inhibitors of the trypanosomal glycosylphosphatidylinositol biosynthetic pathway.

Two glycoforms are present in the GPI-membrane anchor of the surface antigen 1 (P30) of Toxoplasma gondii

SAG1 (P30) is the major surface protein of the Toxoplasma gondii tachyzoite, the life cycle stage associated with the acute phase of infection. The protein is inserted into the parasite's plasma membrane by a glycosyl-phosphatidylinositol anchor, a modification that is present on all T. gondii surface proteins characterized so far. Here we describe a detailed structural analysis of this anchor. GPI anchor peptides were isolated from [3H]glucosamine labeled purified P30 by protease digestion and phase partitioning. Neutral glycans were prepared from this material by dephosphorylation and deamination. Two glycoforms were characterized by gel filtration and high performance ion exchange chromatography in combination with exoglycosidase treatment. Both forms were shown to carry an N-acetylgalactosamine bound to the first mannose of the conserved three-mannosyl core. Glycan B carries an additional terminal hexose linked to GalNAc. To identify the nature of this hexose, bulk anchor peptide was prepared and glycans were purified by aminopropyl-HPLC. Highly purified glycans were subjected to MALDI-TOF-MS and, after derivatization, to FAB-MS and methylation linkage analysis. The structures of the two anchors found on SAG1 were determined to be: Man-alpha1,2-Man-alpha1,6-Man-[GalNAc-beta1,4-]-alpha1,4-GlcN-PI and Man-alpha1,2-Man-alpha1,6-Man [Glc-alpha1,4-GalNAc-beta1,4-]-alpha1,4-GlcN-PI. Comparison of these structures with free GPI glycolipid precursors characterized in T. gondii suggests that core modification of the anchor takes place prior to transfer to the protein.

Parasite-specific inhibition of the glycosylphosphatidylinositol biosynthetic pathway by stereoisomeric substrate analogues

The natural substrate for the first alpha-D-mannosyltransferase of glycosylphosphatidylinositol biosynthesis in the protozoan parasite Trypanosoma brucei is D-GlcNalpha1-6-D-myo-inositol-1-P-sn-1, 2-diacylglycerol. Here we show that a diastereoisomer, D-GlcNalpha1-6-L-myo-inositol-1-P-sn-1,2-diacylglycerol, is an inhibitor of this enzyme in a trypanosomal cell-free system. Tests with other L-myo-inositol-containing compounds revealed that L-myo-inositol-1-phosphate is the principal inhibitory component and that methylation of the 2-OH group of the L-myo-inositol residue abolishes any inhibition. Comparisons between the natural substrate and the inhibitors suggested that the inhibitors bind to the first alpha-D-mannosyltransferase by means of charge interactions with the 1-phosphate group and/or hydrogen bonds involving the 3-, 4-, and 5-OH groups of the L-myo-inositol residue, which are predicted to occupy orientations identical to those of the 1-phosphate and 5-, 4-, and 3-OH groups, respectively, of the D-myo-inositol residue of the natural substrate. However, additional experiments indicated that the 4-OH group of the D-myo-inositol residue is unlikely to be involved in substrate recognition. None of the L-myo-inositol-containing compounds that inhibited glycosylphosphatidylinositol (GPI) biosynthesis in a parasite cell-free system had any effect on GPI biosynthesis in a comparable human (HeLa) cell-free system, suggesting that other related parasite-specific inhibitors of this essential pathway might be developed.

Exploring the outcome of genetic modifications of glycosylation in cultured cell lines by concurrent isolation of the major classes of vertebrate glycans

In the preceding article (Manzi,A.E., Norgard-Sumnicht,K., Argade,S., Marth,J.D., van Halbeek,H. and Varki.A. [2000] GLYCOBIOLOGY:, 10, 669-688), we reported a comprehensive approach for the extraction, fractionation, and isolation of all of the major classes of sugar chains (glycans) from vertebrate tissues. Here we apply this "Glycan Isolation Protocol" to a variety of cultured mammalian cell lines, including two wild-type Chinese hamster ovary (CHO) cell lines and some of their genetically modified variants that were predicted or known to have defined abnormalities in the biosynthesis of one or more classes of glycans. We also use this approach to characterize clone 489, a new derivative of the GAG-deficient CHO clone pgsA-745, in which sulfation has been restored by transfection of a wild-type CHO cDNA library. By metabolically labeling the cell lines with [6-(3)H]glucosamine we were able to monitor the recovery of all major classes of glycans. The results allow us to reach several conclusions: first, the protocol described in the preceding paper is further validated by finding good recovery of total radioactivity and appropriate distribution of label in the correct glycan classes in the fractions from a variety of cell lines; second, the amount of radioactivity recovered in free glycosylphosphatidylinositol (GPI) lipids is remarkably high when compared to that found in GPI anchors, with the former being the dominant form in some cells; third, cells with known genetic mutations in specific glycosylation pathways are shown to have the expected changes in the distribution of recovered radioactivity in the appropriate fractions; fourth, the N- and O- glycans recovered via the protocol are of adequate quality to demonstrate marked differences in their structural profiles and/or content; fifth, the protocol can pick up unexpected differences of glycan classes not predicted to be affected by the primary defect; finally, the reappearance of sulfation in the novel clone 489 is not due to restoration of GAG sulfation, but rather due to the new expression of sulfation in the fraction enriched in N- and O-linked glycopeptides. These results demonstrate the power of this comprehensive approach for the concurrent exploration and profiling of the different major classes of glycans in cells.

Cloning, expression, and characterization of the acyl-CoA-binding protein in African trypanosomes

African trypanosomes are shielded from their hosts' defenses by a coat of variant surface glycoprotein molecules, each of which is attached to the plasma membrane by a glycosylphosphatidylinositol anchor. During the later stages of glycosylphosphatidylinositol biosynthesis, myristic acid is incorporated into the anchor from the donor myristoyl-CoA by a series of unique fatty acid remodeling and exchange reactions. We have cloned and expressed a recombinant trypanosome acyl-CoA-binding protein that has a preference for binding relatively short chain acyl-CoAs and that has a high affinity for binding myristoyl-CoA (K(d) = 3.5 x 10(-10) M). This protein enhances fatty acid remodeling of glycosylphosphatidylinositol precursors in the trypanosome cell-free system. We speculate that the trypanosome acyl-CoA-binding protein plays an active role in supplying myristoyl-CoA to the fatty acid remodeling machinery in the parasite.

Segregation of glycosylphosphatidylinositol biosynthetic reactions in a subcompartment of the endoplasmic reticulum

Glycosylphosphatidylinositols (GPIs) are synthesized in the endoplasmic reticulum (ER) via the sequential addition of monosaccharides, fatty acid, and phosphoethanolamine(s) to phosphatidylinositol (PI). While attempting to establish a mammalian cell-free system for GPI biosynthesis, we found that the assembly of mannosylated GPI species was impaired when purified ER preparations were substituted for unfractionated cell lysates as the enzyme source. To explore this problem we analyzed the distribution of the various GPI biosynthetic reactions in subcellular fractions prepared from homogenates of mammalian cells. The results indicate the following: (i) the initial reaction of GPI assembly, i.e. the transfer of GlcNAc to PI to form GlcNAc-PI, is uniformly distributed in the ER; (ii) the second step of the pathway, i.e. de-N-acetylation of GlcNAc-PI to yield GlcN-PI, is largely confined to a subcompartment of the ER that appears to be associated with mitochondria; (iii) the mitochondria-associated ER subcompartment is enriched in enzymatic activities involved in the conversion of GlcN-PI to H5 (a singly mannosylated GPI structure containing one phosphoethanolamine side chain; and (iv) the mitochondria-associated ER subcompartment, unlike bulk ER, is capable of the de novo synthesis of H5 from UDP-GlcNAc and PI. The confinement of these GPI biosynthetic reactions to a domain of the ER provides another example of the compositional and functional heterogeneity of the ER. The implications of these findings for GPI assembly are discussed.

Characterization of an immunogenic glycocalyx on the surfaces of Cryptosporidium parvum oocysts and sporozoites

Ruthenium red staining of Cryptosporidium parvum oocysts revealed the presence of a carbohydrate matrix on their outer bilayers that is characteristic of a glycocalyx. Surface labeling of intact oocysts identified material of high molecular weight (>10(6)) that reacted positively with sera from cryptosporidium-infected patients and with immunoglobulin A monoclonal antibodies.

Roles of free GPIs in amastigotes of Leishmania

Glycosylated phosphatidylinositols (GPIs) are abundant cell surface molecules of the Leishmania. Amastigote-specific GPIs AmGPI-Y and AmGPI-Z, both ethanolamine (EtN)-containing glycolipids, were identified in Leishmania amazonensis. A paucity of GPI-anchored proteins in amastigotes of L. amazonensis made the kinetoplastid suitable for evaluating the importance of free (i.e. unconjugated to protein or polysaccharide) GPIs. A strain deficient in both AmGPI-Y and AmGPI-Z was produced by stable transfection of wild-type Leishmania with a GPI-phospholipase C gene. Phosphatidylinositol deficiency was not detected in the transfectants. GPI-deficient promastigotes infected murine macrophages in vitro and differentiated into amastigotes whose growth was arrested within the host cells. Cytostasis of amastigotes was also observed during axenic culture of GPI-deficient parasites. In a hamster model of leishmaniasis, GPI-deficient promastigotes produced smaller lesions with 20-fold fewer amastigotes than infections with control parasites. Together, these observations indicate that EtN-GPIs may be essential for amastigote viability, replication, and/or virulence. Implicit in these observations is the notion that drugs targeted against the GPI biosynthetic pathway might be of value in the management of human leishmaniasis.

Glucosylation of glycosylphosphatidylinositol membrane anchors: identification of uridine diphosphate-glucose as the direct donor for side chain modification in Toxoplasma gondii using carbohydrate analogues

Toxoplasma gondii is an obligate intracellular parasite of the phylum apicomplexa and a common and often life-threatening opportunistic infection associated with AIDS. A family of parasite-specific glycosylphosphatidylinositols containing a novel glucosylated side chain has been shown to be highly immunogenic in humans (Striepen et al. (1997) J. Mol. Biol. 266, 797-813). In contrast to trypanosomes in T. gondii side chain modification takes place before addition to protein in the endoplasmic reticulum. The biosynthesis of these modifications was studied in an in vitro system prepared from hypotonically lysed T. gondii parasites. Radiolabeled glucose-containing glycosylphosphatidylinositol precursors were synthesized by T. gondii membrane preparations upon incubation with uridine diphosphate-[3H]glucose. Synthesis of glucosylated glycolipids took place only in the presence of exogenous uridine diphosphate-glucose and was stimulated by unlabeled uridine diphosphate-glucose in a dose-dependent manner. In contrast to glycosylphosphatidylinositol mannosylation, glucosylation was shown to be insensitive to amphomycin treatment. In addition, the glucose analogue 2-deoxy-D-glucose was used to trace the glycosylphosphatidylinositol glucosylation pathway. Detailed analysis of glycolipids synthesized in vitro in the presence of UDP and GDP derivatives of D-glucose and 2-deoxy-D-glucose ruled out an involvement of dolichol phosphate-glucose and demonstrates direct transfer of glucose from uridine diphosphate-glucose.

A novel glycosylphosphatidylinositol in African trypanosomes. A possible catabolic intermediate

The major glycosylphosphatidylinositols (GPIs) in African trypanosomes are glycolipid A, the precursor of the variant surface glycoprotein membrane anchor, and glycolipid C, a species identical to glycolipid A except that it contains an acylated inositol. Both glycolipids A and C contain dimyristoyl glycerol and are efficiently labeled with [3H]myristate in a cell-free system. We now report a novel GPI known as lipid X. This GPI is radiolabeled strongly with [3H]palmitate (and very poorly with [3H]myristate or [3H]stearate) in digitonin-permeabilized cells. The structure of lipid X is Man1GlcNAc-(2O-palmitoyl)-D-myo-inositol-1-HPO4-3(lyso-pa lmitoylglyce rol). Metabolically, lipid X exists as an intermediate, and can be detected only under conditions in which its formation is stimulated (e.g. by EDTA) or its breakdown is inhibited (e.g. by Co2+). Lipid X has not been observed previously because these conditions do not support GPI biosynthesis. We speculate that lipid X is an intermediate in the catabolism of conventional trypanosome GPIs, possibly deriving from breakdown of glycolipid C.

GPI anchor biosynthesis in yeast: phosphoethanolamine is attached to the alpha1,4-linked mannose of the complete precursor glycophospholipid

Cells synthesize the GPI anchor carbohydrate core by successively adding N-acetylglucosamine, three mannoses, and phosphoethanolamine (EtN-P) onto phosphatidylinositol, thus forming the complete GPI precursor lipid which is then added to proteins. Previously, we isolated a GPI deficient yeast mutant accumulating a GPI intermediate containing only two mannoses, suggesting that it has difficulty in adding the third, alpha1,2-linked Man of GPI anchors. The mutant thus displays a similar phenotype as the mammalian mutant cell line S1A-b having a mutation in the PIG-B gene. The yeast mutant, herein named gpi10-1 , contains a mutation in YGL142C, a yeast homolog of the human PIG-B. YGL142C predicts a highly hydrophobic integral membrane protein which by sequence is related to ALG9, a yeast gene required for adding Man in alpha1,2 linkage to N-glycans. Whereas gpi10-1 cells grow at a normal rate and make normal amounts of GPI proteins, the microsomes of gpi10-1 are completely unable to add the third Man in an in vitro assay. Further analysis of the GPI intermediate accumulating in gpi10 shows it to have the structure Manalpha1-6(EtN-P-)Manalpha1-4GlcNalpha1-6(acyl) Inositol-P-lipid. The presence of EtN-P on the alpha1,4-linked Man of GPI anchors is typical of mammalian and a few other organisms but had not been observed in yeast GPI proteins. This additional EtN-P is not only found in the abnormal GPI intermediate of gpi10-1 but is equally present on the complete GPI precursor lipid of wild type cells. Thus, GPI biosynthesis in yeast and mammals proceeds similarly and differs from the pathway described for Trypanosoma brucei in several aspects.

Delineation of three pathways of glycosylphosphatidylinositol biosynthesis in Leishmania mexicana. Precursors from different pathways are assembled on distinct pools of phosphatidylinositol and undergo fatty acid remodeling

Glycosylphosphatidylinositol (GPI) glycolipids are major cell surface constituents in the Leishmania parasites. Distinct classes of GPI are present as membrane anchors for several surface glycoproteins and an abundant lipophosphoglycan as well as being the major glycolipids (GIPLs) in the plasma membrane. In this study we have identified putative precursors for the protein and lipophosphoglycan anchors and delineated the complete pathway for GIPL biosynthesis in Leishmania mexicana promastigotes. Based on the structural analyses of these GPI intermediates and their kinetics of labeling in vivo and in cell-free systems, we provide evidence that the GIPLs are the products of an independent biosynthetic pathway rather than being excess precursors of the anchor pathways. First, we show that the similar glycan head groups of the GIPL and protein/lipophosphoglycan anchor precursors are assembled on two distinct pools of PI corresponding to 1-O-(C18:0)alkyl-2-stearoyl-PI and 1-O-(C24:0/C26:0)-2-stearoyl-PI, respectively. These PI species account for 20 and 1% of the total PI pool, respectively, indicating a remarkable specificity in their selection. Second, analysis of the flux of intermediates through these pathways in vivo and in a cell-free system suggests that the GIPL and anchor pathways are independently regulated. We also show that GIPL biosynthesis requires fatty acid remodeling, in which the sn-2 stearoyl chains are replaced with myristoyl or lauroyl chains. Fatty acid remodeling is dependent on CoA and ATP and occurs on pre-existing but not on de novo synthesized GIPLs. We suggest that the compartmentalization of different GPI pathways may be important in regulating the species and stage-specific expression of different GPI structures in these parasites.

Proteins with glycosylphosphatidylinositol (GPI) signal sequences have divergent fates during a GPI deficiency. GPIs are essential for nuclear division in Trypanosoma cruzi

Glycosylphosphatidylinositols (GPIs) are membrane anchors for cell surface proteins of several major protozoan parasites of humans, including Trypanosoma cruzi, the causative agent of Chagas' disease. To investigate the general role of GPIs in T. cruzi, we generated GPI-deficient parasites by heterologous expression of T. brucei GPI-phospholipase C. Putative protein-GPI intermediates were depleted, causing the biochemical equivalent of a dominant-negative loss of function mutation in the GPI pathway. Cell surface expression of major GPI-anchored proteins was diminished in GPI-deficient T. cruzi. Four proteins that are normally GPI-anchored in T. cruzi exhibited different fates during the GPI shortage; Ssp-4 and p75 were secreted prematurely, while protease gp50/55 and p60 were degraded intracellularly. These observations demonstrate that secretion and intracellular degradation of GPI-anchored proteins may occur in the same genetic background during a GPI deficiency. We postulate that the interaction between a protein-GPI transamidase and the COOH-terminal GPI signal sequence plays a pivotal role in determining the fate of these proteins. At a nonpermissive GPI deficiency, T. cruzi amastigotes inside mammalian cells replicated their single kinetoplast but failed at mitosis. Hence, in these protozoans, GPIs appear to be essential for nuclear division, but not for mitochondrial duplication.

Molecular structure of the "low molecular weight antigen" of Toxoplasma gondii: a glucose alpha 1-4 N-acetylgalactosamine makes free glycosyl-phosphatidylinositols highly immunogenic

Toxoplasma gondii is a ubiquitous parasitic protozoan causing congenital infection and severe encephalitis in the course of the acquired immunodeficiency syndrome. Glycosyl-phosphatidylinositols of T. gondii have been shown to be identical with the low molecular weight antigen which elicits an early immunoglobulin M immune response in humans. A detailed study of the structures of these glycolipid antigens was performed. Radiolabelled glycolipids were extensively analysed by chemical and exoglycosidase treatments in combination with high pH anion-exchange chromatography, gel-filtration and lectin affinity chromatography. In addition, carbohydrate fragments prepared and purified from bulk preparations of unlabelled glycolipids by high performance liquid chromatography were subjected to two-dimensional 1H nuclear magnetic resonance spectroscopy, fast-atom bombardment-mass spectrometry, and methylation linkage analysis in order to elucidate the structure of T. gondii GPIs. The following structures were identified: (ethanolamine-PO4)-Man alpha 1-2Man alpha 1-6(GalNAc beta 1-4)Man alpha 1-4GlcN alpha-inositol-PO4-lipid and the novel structure (ethanolamine-PO4)-Man alpha 1-2Man alpha 1-6(Glc alpha 1-4GalNAc beta 1-4)Man alpha 1-4 GlcN alpha-inositol-PO4-lipid both with and without terminal ethanolamine phosphate. Evidence is provided, that only T. gondii GPIs bearing the unique glucose-N-acetylgalactosamine side branch are immunogenic in humans and that this structure is widely distributed among T. gondii isolates. Monoclonal antibodies have been characterized to recognize structures with different degrees of side-chain modification. We suggest that these reagents in combination with recently devised techniques for insertional mutagenesis in T. gondii should greatly facilitate the cloning of genes essential for GPI side-chain modification.

Paroxysmal nocturnal hemoglobinuria as a molecular disease

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired, clonal disorder of hematopoietic cells caused by somatic mutation in the X-linked PIGA gene encoding a protein involved in the synthesis of the glycosylphosphatidylinositol (GPI) anchor by which many proteins are attached to the membrane of cells. About 15 proteins have been found to be lacking or markedly deficient on the abnormal blood cells. These defects result in a clinical syndrome that includes intravascular hemolysis mediated by complement, unusual venous thromboses, deficits of hematopoiesis, and other manifestations. Therapy is presently directed mainly at the consequences of the disorder rather than its basic causes and includes replacement of iron, folic acid, and whole blood; hormonal modulation (prednisone, androgens); anticoagulation; and bone marrow transplantation. PNH is a chronic disease with more than half of adult patients surviving 15 years or more; prognosis is less good in children.

Acylation of glucosaminyl phosphatidylinositol revisited. Palmitoyl-CoA dependent palmitoylation of the inositol residue of a synthetic dioctanoyl glucosaminyl phosphatidylinositol by hamster membranes permits efficient mannosylation of the glucosamine residue

Two critical steps in the assembly of yeast and mammalian glycosylphosphatidylinositol (GPI) anchor precursors are palmitoylation of the inositol residue and mannosylation of the glucosamine residue of the glucosaminyl phosphatidylinositol (GlcNalpha-PI) intermediate. Palmitoylation has been reported to be acyl-CoA dependent in yeast membranes (Costello, L. C., and Orlean, P. (1992) J. Biol. Chem. 267, 8599-8603) but strictly acyl-CoA independent in rodent membranes (Stevens, V. L., and Zhang, H. (1994) J. Biol. Chem. 269, 31397-31403), and thus poorly conserved. In addition, it was suggested that acylation must precede mannosylation in both yeast (Costello, L. C., and Orlean, P. (1992) J. Biol. Chem. 276, 8599-8603) and rodent (Urakaze, M., Kamitani, T., DeGasperi, R., Sugiyama, E., Chang, H.-M., Warren, C. D., and Yeh, E. T. H. (1992) J. Biol. Chem. 267, 6459-6462) cells because GlcNalpha-acyl-PI accumulates in vivo when mannosylation is blocked. However, GlcNalpha-acyl-PI accumulation would also be expected if mannosylation and acylation were independent of each other. These issues were addressed by the use of a synthetic dioctanoyl GlcNalpha-PI analogue (GlcNalpha-PI(C8)) as an in vitro substrate for GPI-synthesizing enzymes in Chinese hamster ovary cell membranes. GlcNalpha-PI(C8) was acylated in an manner requiring acyl-CoA. Thus, the process involving acyl-CoA reported for yeast has been conserved in mammals. Furthermore, both GlcNalpha-PI(C8) and GlcNalpha-acyl-PI(C8) could be mannosylated in vitro, but mannosylation of the latter was significantly more efficient. This provides direct support for the earlier suggestion that acylation precedes mannosylation in rodents cells. A similar result was also observed with the Saccharomyces cerevisiae mannosyltransferase. In contrast, it has been reported that mannosylation of endogenous GlcNalpha-PI by Trypansoma brucei membranes occurs without prior acylation. The same result was obtained with GlcNalpha-PI(C8), confirming that the mannosyltransferase of trypanosomes is divergent from those in yeasts and rodents.

Molecular species analysis and quantification of the glycosylphosphatidylinositol intermediate glycolipid C from Trypanosoma brucei

The precursor of the glycosylphosphatidylinositol membrane anchor of the variant surface glycoprotein of Trypanosoma brucei is known as glycolipid A and it has the structure: EtN-PO4-6Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcN alpha 1-6myo-inositol-PO4-(sn-1,2-dimyristoylglycerol). This precursor exists in equilibrium with its inositol-acylated form known as glycolipid C that contains a fatty acid attached to the inositol ring. In this study, we describe the purification to homogeneity of glycolipid C, its precise quantification and the analysis of the molecular species of glycolipid C by electrospray ionisation mass spectrometry. The results show that glycolipid C is present at 160000 copies per cell, that glycolipid C is acylated on the 2-position of the myo-inositol ring and that glycolipid C is heterogeneous with respect to the acyl chain attached to the inositol ring. The implications of these results with respect to the nature of the trypanosome inositol acyltransferase are discussed.

Substrate specificity of the dolichol phosphate mannose: glucosaminyl phosphatidylinositol alpha1-4-mannosyltransferase of the glycosylphosphatidylinositol biosynthetic pathway of African trypanosomes

The biosynthesis of glycosylphosphatidylinositol (GPI) precursors in Trypanosoma brucei involves the D-mannosylation of D-GlcN alpha 1-6-D-myo-inositol-1-PO4-sn-1,2-diacylglycerol (GlcN-PI). An assay for the first mannosyltransferase of the pathway, Dol-P-Man:GlcN-PI alpha 1-4-mannosyltransferase, is described. Analysis of the acceptor specificity revealed (a) that the enzyme requires the myo-inositol residue of the GlcN-PI substrate have the D configuration; (b) that the enzyme requires the presence of the NH2 group of the D-GlcN residue; (c) that GlcNAc-PI is more efficiently presented to the enzyme than GlcN-PI, suggesting a degree of substrate channelling via the preceding GlcNAc-PI de-N-acetylase enzyme; (d) that the fatty acid and phosphoglycerol components of the phosphatidyl moiety are important for enhancing substrate presentation and substrate recognition, respectively; and (e) that D-GlcN alpha 1-6-D-myo-inositol is the minimum structure that can support detectable acceptor activity. Analysis of the donor specificity revealed that short chain (C5 and C15) analogues of dolichol phosphate can act as substrates for the trypanosomal dolichol-phosphomannose synthetase, whereas the corresponding mannopyranosides cannot act as donors for the Dol-P-Man:GlcN-PI alpha 1-4-mannosyltransferase.