Cell-free biosynthesis of lipophosphoglycan from Leishmania donovani. Characterization of microsomal galactosyltransferase and mannosyltransferase activities

Characterization of a ricin-resistant mutant of Leishmania donovani that expresses lipophosphoglycan

The abundant cell-surface lipophosphoglycan (LPG) of Leishmania parasites plays a central role throughout the eukaryote's life cycle. A number of LPG-defective mutants and their complementing genes have been isolated and have proven invaluable in assessing the importance of LPG and related glycoconjugates in parasite virulence. While ricin agglutination selection protocols frequently result in lpg- mutants, one  Leishmania donovani variant we isolated, named JABBA, was found to be lpg+. Procyclic (logarithmic) JABBA expresses significant amounts of a large-sized LPG, larger than observed from procyclic wild type but similar in size to LPG from wild type from metacyclic (stationary) phase. Structural analysis of the LPG from logarithmically grown JABBA by capillary electrophoresis protocols revealed that it averaged 30 repeat units composed of the unsubstituted Gal(β1,4)Man(α1)-PO4 typical of wild-type L. donovani. Analysis of JABBA LPG caps indicated that 20% is branched trisaccharide Gal(β1,4)[Glc(β1,2)]Man and tetrasaccharide Gal(β1,4)[Glc(β1,2)Man(α1,2)]Man instead of the usual Gal(β1,4)Man and Man(α1,2)Man terminating caps. Consistent with these structural observations, analyses of the relevant glycosyltransferases in JABBA microsomes involved in LPG biosynthesis showed a 2-fold increase in elongating mannosylphosphoryltransferase activity and up-regulation of a β-glucosyltransferase activity. Furthermore, the caps of JABBA LPG are cryptic in presentation as shown by the loss of binding by the lectins, ricin, peanut agglutinin and concanavalin A and reduced accessibility of the terminal galactose residues to oxidation by galactose oxidase. These results indicate that LPG from JABBA is intriguingly similar to the larger LPG in wild-type parasites that arises following the differentiation of the non-infectious procyclic promastigotes to infectious, metacyclic forms.

Probing elongating and branching β-D-galactosyltransferase activities in Leishmania parasites by making use of synthetic phosphoglycans

Protozoan parasites of the genus Leishmania synthesize lipophosphoglycans (LPGs), phosphoglycans and proteophosphoglycans that contain phosphosaccharide repeat units of [-6)Gal(β1-4)Man(α1-OPO(3)H-]. The repeat structures are assembled by sequential addition of Manα1-OPO(3)H and β-Gal. In this study, an UDP-Gal-dependent activity was detected in L. donovani and L. major membranes using synthetic phospho-oligosaccharide fragments of lipophosphoglycan as acceptor substrates. Incubation of a microsomal preparation from L. donovani or L. major parasites with synthetic substrates and UDP-[6-(3)H]Gal resulted in incorporation of radiolabel into these exogenous acceptors. The [(3)H]galactose-labeled products were characterized by degradation into radioactive, low molecular mass fragments upon hydrolysis with mild acid and treatment with β-galactosidases. We showed that the activity detected with L. donovani membranes is the elongating β-d-galactosyltransferase associated with LPG phosphosaccharide backbone biosynthesis (eGalT). The eGalT activity showed a requirement for the presence of at least one phosphodiester group in the substrate and it was enhanced dramatically when two or three phosphodiester groups were present. Using the same substrates we detected two types of galactosyltransferase activity in L. major membranes: the elongating β-d-galactosyltransferase and a branching β-d-galactosyltransferase (bGalT). Both L. major enzymes required a minimum of one phosphodiester group present in the substrate, but acceptors with two or three phosphodiester groups were found to be superior.

Biosynthesis of galactofuranose in kinetoplastids: novel therapeutic targets for treating leishmaniasis and chagas' disease

Cell surface proteins of parasites play a role in pathogenesis by modulating mammalian cell recognition and cell adhesion during infection. β-Galactofuranose (Galf) is an important component of glycoproteins and glycolipids found on the cell surface of Leishmania spp. and Trypanosoma cruzi. β-Galf-containing glycans have been shown to be important in parasite-cell interaction and protection against oxidative stress. Here, we discuss the role of β-Galf in pathogenesis and recent studies on the Galf-biosynthetic enzymes: UDP-galactose 4′ epimerase (GalE), UDP-galactopyranose mutase (UGM), and UDP-galactofuranosyl transferase (GalfT). The central role in Galf formation, its unique chemical mechanism, and the absence of a homologous enzyme in humans identify UGM as the most attractive drug target in the β-Galf-biosynthetic pathway in protozoan parasites.

Bioinformatics and functional analysis of an Entamoeba histolytica mannosyltransferase necessary for parasite complement resistance and hepatical infection

The glycosylphosphatidylinositol (GPI) moiety is one of the ways by which many cell surface proteins, such as Gal/GalNAc lectin and proteophosphoglycans (PPGs) attach to the surface of Entamoeba histolytica, the agent of human amoebiasis. It is believed that these GPI-anchored molecules are involved in parasite adhesion to cells, mucus and the extracellular matrix. We identified an E. histolytica homolog of PIG-M, which is a mannosyltransferase required for synthesis of GPI. The sequence and structural analysis led to the conclusion that EhPIG-M1 is composed of one signal peptide and 11 transmembrane domains with two large intra luminal loops, one of which contains the DXD motif, involved in the enzymatic catalysis and conserved in most glycosyltransferases. Expressing a fragment of the EhPIG-M1 encoding gene in antisense orientation generated parasite lines diminished in EhPIG-M1 levels; these lines displayed reduced GPI production, were highly sensitive to complement and were dramatically inhibited for amoebic abscess formation. The data suggest a role for GPI surface anchored molecules in the survival of E. histolytica during pathogenesis.

Parasite glycoconjugates. Part 16: Synthesis of a disaccharide and phosphorylated di- and tri-saccharides from Leishmania lipophosphoglycan

A neutral disaccharide beta-D-Galp-(1-->4)-alpha-D-Manp and phosphorylated di- and tri-saccharides beta-D-Galp-(1-->3)-[H(2)PO(3)-6]-beta-D-Galp-O[CH(2)](8)CHCH(2) and beta-D-Galp-(1-->3)-[H(2)PO(3)-6]-beta-D-Galp-(1-->4)-alpha-D-Manp, which are fragments of the phosphoglycan portion of the surface lipophosphoglycan from Leishmania donovani (the disaccharide) or Leishmania major (all three compounds), were prepared and used as TLC standards to help the identification and differentiation of the elongating and branching beta-D-galactosyl transferase activities in Leishmania. The phosphosaccharides were synthesised using the H-phosphonate method for phosphorylation.

Reduction of cell surface glycosylphosphatidylinositol conjugates in Entamoeba histolytica by antisense blocking of E. histolytica GlcNAc-phosphatidylinositol deacetylase expression: effect on cell proliferation, endocytosis, and adhesion to target cells

Glycosylphosphatidylinositol (GPI)-anchored molecules such as cell surface Gal/GalNAc lectin and proteophosphoglycans of the protozoan parasite Entamoeba histolytica are thought to be involved in pathogenesis. Here, we report the identification of genes that may be involved in the GPI biosynthetic pathway of E. histolytica by use of bioinformatic tools applied to the recently published genome sequence. Of the genes identified, one of the early genes, GlcNAc-phosphatidylinositol deacetylase (PIG-L), was partially characterized. Cell lines deficient in E. histolytica PIG-L (EhPL-AS) or overproducing it (EhPL-S) were generated by expressing the gene in the antisense or sense orientation, respectively, in a tetracycline-inducible system. The overexpressing cells showed higher EhPIG-L activity and increased production of GlcN-PI. Conversely, cells expressing the antisense RNA displayed reduced GlcN-PI production. The total number of GPI-containing molecules was also reduced in these cells, as demonstrated by Alexa 488 fluorescently labeled proaerolysin labeling. The distribution of GPI-linked PPG and Gal/GalNAc lectin was altered in the tetracycline-induced EhPL-AS cell lines. Further, the antisense-blocked cells showed 36% suppression of cell growth, 50 to 60% inhibition of fluid phase endocytosis, and about 50% inhibition of adhesion to target cells. Therefore, our data suggest the importance of GPI anchors in regulating some of the events in amoebic pathogenesis. They also demonstrated the use of antisense RNA-mediated inhibition of GPI biosynthetic enzymes as an approach to decrease the amount of GPI conjugates in E. histolytica.

Synthetic Fragments of Antigenic Lipophosphoglycans fromLeishmania major andLeishmania mexicana and Their Use for Characterisation of theLeishmania Elongating ?-D-Mannopyranosylphosphate Transferase

The phosphorylated branched heptasaccharides 7 and 8, the octasaccharide 9 and the phosphorylated trisaccharides 5 and 6, which are fragments of the phosphoglycan portion of the surface lipophosphoglycans from Leishmania mexicana (5) or L. major (6-9), were synthesised by using the glycosyl hydrogenphosphonate method for the preparation of phosphodiester bridges. The compounds were tested as acceptor substrates/putative inhibitors for the Leishmania elongating alpha-D-mannosylphosphate transferase.

1-Oxabicyclic β-lactams as new inhibitors of elongating MPT–a key enzyme responsible for assembly of cell-surface phosphoglycans of Leishmania parasite

New iminosugars (1-oxabicyclic beta-lactam disaccharides) have been synthesized as inhibitors of elongating alpha-D-mannosyl phosphate transferase (eMPT), a key enzyme involved in the iterative biosynthesis of cell-surface phosphoglycans of the Leishmania parasite. The design is based on a transition-state model for this remarkable enzyme that transfers intact alpha-D-mannosyl-phosphate from GDP-Man. Since these phosphoglycans are unique to Leishmania and are essential for its infectivity and survival, their biosynthetic pathway has emerged as a novel target for anti-leishmanial drug and vaccine design.

Iterative synthesis of Leishmania phosphoglycans by solution, solid-phase, and polycondensation approaches without involving any glycosylation

A general strategy (solution, solid-phase, and polycondensation) for the synthesis of antigenic phosphoglycans (PG) of the protozoan parasite Leishmania is presented. Phosphoglycans constitute the variable structural and functional domain of major cell-surface lipophosphoglycan (LPG) and secreted proteophosphoglycan (PPG), the molecules involved in infectivity and survival of the Leishmania parasite inside human macrophages. We have shown that the chemically labile, anomerically phosphodiester-linked phosphoglycan repeats can be assembled in an iterative and efficient manner from a single key intermediate, without involving any glycosylation steps. Furthermore, the phosphoglycan chain can be extended toward either the nonreducing (6'-OH) or the reducing (1-OH) end. We also describe a new and efficient solid-phase methodology to construct phosphoglycans based on design and application of a novel cis-allylphosphoryl solid-phase linker that enabled the selective cleavage of the first anomeric-phosphodiester linkage without affecting any of the other internal anomeric-phosphodiester groups of the growing PG chain on the solid support. The strategy to construct larger phosphoglycans in a one-pot synthesis by polycondensation of a single key intermediate is also described, enabling CD spectrometric measurements to show the helical nature of phosphoglycans. Our versatile synthetic approach provides easy access to Leishmania phosphoglycans and the opportunity to address key immunological, biochemical, and biophysical questions pertaining to the phosphoglycan family (LPG and PPG) unique to the parasite.

Biosynthesis of Entamoeba histolytica proteophosphoglycan in vitro

A complex glycoconjugate proteophosphoglycan (PPG) is present on the surface of the pathogenic protozoan parasite Entamoeba histolytica but not in the non-pathogenic Entamoeba dispar. It is thought to be an important molecule involved in pathogenesis. In order to study its biosynthesis, an in vitro cell-free system was developed. The specificity of the system was demonstrated by various criteria including immunoprecipitation by a specific monoclonal antibody. The in vitro synthesized molecule was found to be susceptible to mild acid hydrolysis, digestion by phosphoinositol-specific phospholipase C and nitrous acid deamination, the salient features for a PPG-like molecule. The in vitro product was not synthesized when heat-treated cellular-extract was used in the assay or when the cell extract was prepared from Entamoeba invadens, a species that lacks these glycoconjugates. Analysis of the glycan side chains of the in vitro synthesized product by thin layer chromatography revealed side chains of variable sizes including a fraction greater than six glycan units. The crude membranes used in the cell-free system were further fractionated by sucrose density gradient centrifugation. The fraction containing the PPG synthesizing activity when used in the assay resulted in a 10-fold increase in specific activity. Development of this cell-free system will facilitate further studies on the nature of intracellular organelles and the pathways that are involved in PPG biosynthesis.

Secretory pathway of trypanosomatid parasites

The Trypanosomatidae comprise a large group of parasitic protozoa, some of which cause important diseases in humans. These include Trypanosoma brucei (the causative agent of African sleeping sickness and nagana in cattle), Trypanosoma cruzi (the causative agent of Chagas' disease in Central and South America), and Leishmania spp. (the causative agent of visceral and [muco]cutaneous leishmaniasis throughout the tropics and subtropics). The cell surfaces of these parasites are covered in complex protein- or carbohydrate-rich coats that are required for parasite survival and infectivity in their respective insect vectors and mammalian hosts. These molecules are assembled in the secretory pathway. Recent advances in the genetic manipulation of these parasites as well as progress with the parasite genome projects has greatly advanced our understanding of processes that underlie secretory transport in trypanosomatids. This article provides an overview of the organization of the trypanosomatid secretory pathway and connections that exist with endocytic organelles and multiple lytic and storage vacuoles. A number of the molecular components that are required for vesicular transport have been identified, as have some of the sorting signals that direct proteins to the cell surface or organelles in the endosome-vacuole system. Finally, the subcellular organization of the major glycosylation pathways in these parasites is reviewed. Studies on these highly divergent eukaryotes provide important insights into the molecular processes underlying secretory transport that arose very early in eukaryotic evolution. They also reveal unusual or novel aspects of secretory transport and protein glycosylation that may be exploited in developing new antiparasite drugs.

The role of phosphomannose isomerase in Leishmania mexicana glycoconjugate synthesis and virulence

Phosphomannose isomerase (PMI) catalyzes the reversible interconversion of fructose 6-phosphate and mannose 6-phosphate, which is the first step in the biosynthesis of activated mannose donors required for the biosynthesis of various glycoconjugates. Leishmania species synthesize copious amounts of mannose-containing glycolipids and glycoproteins, which are involved in virulence of these parasitic protozoa. To investigate the role of PMI for parasite glycoconjugate synthesis, we have cloned the PMI gene (lmexpmi) from Leishmania mexicana, generated gene deletion mutants (Delta lmexpmi), and analyzed their phenotype. Delta lmexpmi mutants lack completely the high PMI activity found in wild type parasites, but are, in contrast to fungi, able to grow in media deficient for free mannose. The mutants are unable to synthesize phosphoglycan repeats [-6-Gal beta 1-4Man alpha 1-PO(4)-] and mannose-containing glycoinositolphospholipids, and the surface expression of the glycosylphosphatidylinositol-anchored dominant surface glycoprotein leishmanolysin is strongly decreased, unless the parasite growth medium is supplemented with mannose. The Delta lmexpmi mutant is attenuated in infections of macrophages in vitro and of mice, suggesting that PMI may be a target for anti-Leishmania drug development. L. mexicana Delta lmexpmi provides the first conditional mannose-controlled system for parasite glycoconjugate assembly with potential applications for the investigation of their biosynthesis, intracellular sorting, and function.

The structure of a complex glycosylphosphatidyl inositol-anchored glucoxylan from the kinetoplastid protozoan Leptomonas samueli

The structure of a glycosylphosphatidyl inositol-anchored glucoxylan (GPI-glucoxylan) synthesized by the monogenetic trypanosomatid Leptomonas samueli has been determined. The glucoxylan is anchored to the membrane by phytoceramide and an oligosaccharide core, the structure of which is identical to glycoinositolphospholipids (GIPLs) expressed by this protozoan. The glucoxylan chain is linear, containing -->4Glcalpha1-->, -->4Xylbeta1--> and -->3Xylbeta1--> residues. A well defined sequence heterogeneity was analysed in terms of a series of overlapping trisaccharide substructures. A proportion of the chains are capped with a GlcAalpha1-->3Glcalpha1--> sequence. While an average GlcA-capped chain contained 10 Glc and 16 Xyl residues, uncapped chains have a higher molecular mass with an average of 30 Glc and 50 Xyl per chain. We propose a mode of biosynthesis based on the observed structural heterogeneity.

Characterization of the elongating alpha-D-mannosyl phosphate transferase from three species of Leishmania using synthetic acceptor substrate analogues

Leishmania express lipophosphoglycans and proteophosphoglycans that contain Galbeta1-4Manalpha1-P phosphosaccharide repeat structures assembled by the sequential addition of Manalpha1-P and betaGal. The synthetic acceptor substrate Galbeta1-4Manalpha1-P-decenyl and a series of analogues were used to probe Leishmania alpha-D-mannosyl phosphate transferase activity. We show that the activity detected with Galbeta1-4Manalpha1-P-decenyl is the elongating alpha-D-mannosyl phosphate transferase associated with lipophosphoglycan biosynthesis (eMPT(LPG)). Differences in the apparent K(m) values for the donor and acceptor substrates were found using L. major, L. mexicana, and L. donovani promastigote membranes, but total activity correlated with the number of lipophosphoglycan repeats. Further comparisons showed that lesion-derived L. mexicana amastigotes, that do not express lipophosphoglycan, lack eMPT(LPG) and that nondividing L. major metacyclic promastigotes contain 5-fold less eMPT(LPG) activity than dividing procyclic promastigotes. The fine specificity of promastigote eMPT(LPG) activity was determined using 24 synthetic analogues of Galbeta1-4Manalpha1-P-decenyl. The three species gave similar results: the negative charge of the phosphodiester and the C-6 hydroxyl of the alphaMan residue are essential for substrate recognition, the latter most likely acting as a hydrogen bond acceptor. The C-6' hydroxyl of the betaGal residue is required for substrate recognition as well as for catalysis. The rate of Manalpha1-P transfer declines with increasing acceptor substrate chain length. The presence of a monosaccharide substituent at the C-3 position of the terminal betaGal residue abrogates Man-P transfer, showing that chain elongation must precede side chain modification during lipophosphoglycan biosynthesis. In contrast, substitution of the penultimate phosphosaccharide repeat does not abrogate transfer but is slightly stimulatory in L. mexicana and inhibitory in L. major.

Phosphoglycosylation of a secreted acid phosphatase from Leishmania donovani

The secreted acid phosphatase (SAcP) of L.donovani is a heterogeneous glycoprotein that displays a wide array of N- and O-linked glycosylations. The O-linked sugars are of particular interest due to their similarity to the phosphoglycan structures of the major lipophosphoglycan surface antigen and released phosphoglycan (Turco et al., 1987; Greis et al., 1992). This study describes a structural analysis of the SAcP O-linked glycosylations using mass spectroscopy, amino acid sequencing, and enzymatic carbohydrate sequencing. Analysis of glycan chain lengths and peptide glycosylation site distribution was performed, revealing that the average O-linked structure was approximately 32 repeat units in length. Amino acid sequence analysis of glycosylated peptides showed that phosphoglycosylations did not occur randomly but were localized to specific serine residues within an array of degenerate serine/threonine-rich repeat sequences localized in the C-terminus. No evidence was obtained for modification of threonine residues. The observed pattern suggested that a consensus sequence may exist for localization of phosphoglycan structures.

Biosynthesis of glycosylphosphatidylinositols in mammals and unicellular microbes

Membrane anchoring of cell surface proteins via glycosylphosphatidylinositol (GPI) occurs in all eukaryotic organisms. In addition, GPI-related glycophospholipids are important constituents of the glycan coat of certain protozoa. Defects in GPI biosynthesis can retard, if not abolish growth of these organisms. In humans, a defect in GPI biosynthesis can cause paroxysmal nocturnal hemoglobinuria (PNH), a severe acquired bone marrow disorder. Here, we review advances in the characterization of GPI biosynthesis in parasitic protozoa, yeast and mammalian cells. The GPI core structure as well as the major steps in its biosynthesis are conserved throughout evolution. However, there are significant biosynthetic differences between mammals and microbes. First indications are that these differences could be exploited as targets in the design of novel pharmacotherapeutics that selectively inhibit GPI biosynthesis in unicellular microbes.

Characterization of a novel GDP-mannose:Serine-protein mannose-1-phosphotransferase from Leishmania mexicana

Protozoan parasites of the genus Leishmania secrete a number of glycoproteins and mucin-like proteoglycans that appear to be important parasite virulence factors. We have previously proposed that the polypeptide backbones of these molecules are extensively modified with a complex array of phosphoglycan chains that are linked to Ser/Thr-rich domains via a common Manalpha1-PO4-Ser linkage (Ilg, T., Overath, P., Ferguson, M. A. J., Rutherford, T., Campbell, D. G., and McConville, M. J. (1994) J. Biol. Chem. 269, 24073-24081). In this study, we show that Leishmania mexicana promastigotes contain a peptide-specific mannose-1-phosphotransferase (pep-MPT) activity that adds Manalpha1-P to serine residues in a range of defined peptides. The presence and location of the Manalpha1-PO4-Ser linkage in these peptides were determined by electrospray ionization mass spectrometry and chemical and enzymatic treatments. The pep-MPT activity was solubilized in non-ionic detergents, was dependent on Mn2+, utilized GDP-Man as the mannose donor, and was expressed in all developmental stages of the parasite. The pep-MPT activity was maximal against peptides containing Ser/Thr-rich domains of the endogenous acceptors and, based on competition assays with oligosaccharide acceptors, was distinct from other leishmanial MPTs involved in the initiation and elongation of lipid-linked phosphoglycan chains. In subcellular fractionation experiments, pep-MPT was resolved from the endoplasmic reticulum marker BiP, but had an overlapping distribution with the cis-Golgi marker Rab1. Although Man-PO4 residues in the mature secreted glycoproteins are extensively modified with mannose oligosaccharides and phosphoglycan chains, similar modifications were not added to peptide-linked Man-PO4 residues in the in vitro assays. Similarly, Man-PO4 residues on endogenous polypeptide acceptors were also poorly extended, although the elongating enzymes were still active, suggesting that the pep-MPT activity and elongating enzymes may be present in separate subcellular compartments.

A high-yield, enzymatic synthesis of GDP-D-[3H]arabinose and GDP-L-[3H]fucose

For assays involving glycosyltransferases or transporters, several GDP-sugars are either commercially unavailable or expensive. We describe an enzymatic synthesis of GDP-d-[3H]arabinosep and GDP-l-[3H]fucose that yields 66-95% nucleotide-sugar from the appropriate radiolabeled sugar in less than 30 min. The coupled reaction requires Mg2+, ATP, and GTP along with the appropriate radioactive monosaccharide, sugar-1-kinase, and pyrophosphorylase. The latter two activities are present in a cytosolic fraction of Crithidia fasciculata, which is easily grown at room temperature in simple culture medium without serum or added CO2. Addition of commercial yeast inorganic pyrophosphatase shifts the equilibrium of the pyrophosphorylase reaction toward nucleotide-sugar formation. To verify that these nucleotide-sugars are biologically active, we tested their ability to serve as substrates for glycosyltransferases. GDP-l-[3H]fucose functions as the donor substrate for recombinant human fucosyltransferase V, and GDP-d-[3H]arabinosep serves as the donor substrate for the arabinosyltransferase activities present in Leishmania major microsomes.

Leishmania donovani has distinct mannosylphosphoryltransferases for the initiation and elongation phases of lipophosphoglycan repeating unit biosynthesis

Lipophosphoglycan (LPG) is the predominant surface glycoconjugate of Leishmania promastigotes and plays several roles in the infectious cycle of this protozoan parasite. The salient feature of LPG is the presence of 15-30 copies of a disaccharide-phosphate repeating unit Gal(beta1,4)Man(alpha1-PO4), which is also found on many other secreted molecules (secretory acid phosphatase, phosphoglycan, proteophosphoglycan). This structural diversity suggests that a multiplicity of enzymes mediating repeating unit addition may exist, especially for the mannosylphosphoryltransferases (MPTs), which initiate repeating unit synthesis. This work has taken a combined biochemical-genetic approach to resolve this issue. An lpg- mutant of Leishmania donovani, JEDI, was obtained by antibody selection against cells expressing a repeating unit epitope of LPG. Metabolic and surface labeling experiments revealed that JEDI cells accumulated a truncated form of LPG bearing only a single repeating unit: [Gal(beta 1,4)Man(alpha1-PO4)][Gal(alpha1,6)Gal(alpha1,3)Gal(f)(beta1,3)[Glc(alpha 1-PO4)]Man(alpha1,3)Man(alpha1,4)GlcN(alpha1,6)]-PI. Enzymatic assays of microsomal preparations showed that JEDI lacked MPT activity when tested with a repeating unit acceptor but retained wild-type levels of the MPT activity with an LPG glycan core acceptor. These data indicate that at least two distinct MPT activities are required for LPG repeating unit synthesis: one involved in the 'initiation' of repeating unit synthesis on the LPG core (iMPT), and a second (lacking in JEDI) participating in the 'elongation' phase of repeating unit addition (eMPT), leading to the mature full-length LPG.

Stage-specific proteophosphoglycan from Leishmania mexicana amastigotes. Structural characterization of novel mono-, di-, and triphosphorylated phosphodiester-linked oligosaccharides

Intracellular amastigotes of the protozoan parasite Leishmania mexicana secrete a macromolecular proteophosphoglycan (aPPG) into the phagolysosome of their host cell, the mammalian macrophage. The structures of aPPG glycans were analyzed by a combination of high pH anion exchange high pressure liquid chromatography, gas chromatography-mass spectrometry, enzymatic digestions, electrospray-mass spectrometry as well as 1H and 31P NMR spectroscopy. Some glycans are identical to oligosaccharides known from Leishmania mexicana promastigote lipophosphoglycan and secreted acid phosphatase. However, the majority of the aPPG glycans represent amastigote stage-specific and novel structures. These include neutral glycans ([Glcbeta1-3]1-2Galbeta1-4Man, Galbeta1-3Galbeta1-4Man, Galbeta1-3Glcbeta1-3Galbeta1-4Man), several monophosphorylated glycans containing the conserved phosphodisaccharide backbone (R-3-[PO4-6-Gal]beta1-4Man) but carrying stage-specific modifications (R = Galbeta1-, [Glcbeta1-3]1-2Glcbeta1-), and monophosphorylated aPPG tri- and tetrasaccharides that are uniquely phosphorylated on the terminal hexose (PO4-6-Glcbeta1-3Galbeta1-4Man, PO4-6-Glcbeta1-3Glcbeta1-3Galbeta1-4Man, PO4-6-Galbeta1-3Glcbeta1-3Galbeta1-4Man). In addition aPPG contains highly unusual di- and triphosphorylated glycans whose major species are PO4-6-Glcbeta1-3Glcbeta1-3[PO4-6-Gal]beta1-4Man, PO4-6-Galbeta1-3Glcbeta1-3[PO4-6-Gal]beta1-4Man, PO4-6-Galbeta1-3Glcbeta1-3Glcbeta1-3[PO4-6-Gal]beta1-+ ++4Man, PO4-6-Glcbeta1-3[PO4-6-Glc]beta1-3[PO4-6-Gal]beta1-4Man, PO4-6-Galbeta1-3[PO4-6-Glc]beta1-3Glcbeta1-3[PO4-6-Gal]beta1 -4Man, and PO4-6-Glcbeta1-3[PO4-6-Glc]beta1-3Glcbeta1-3[PO4-6-Gal]beta1 -4Man. These glycans are linked together by the conserved phosphodiester R-Manalpha1-PO4-6-Gal-R or the novel phosphodiester R-Manalpha1-PO4-6-Glc-R and are connected to Ser(P) of the protein backbone most likely via the linkage R-Manalpha1-PO4-Ser. The variety of stage-specific glycan structures in Leishmania mexicana aPPG suggests the presence of developmentally regulated amastigote glycosyltransferases which may be potential anti-parasite drug targets.

Suppressor gene analysis reveals an essential role for sphingolipids in transport of glycosylphosphatidylinositol-anchored proteins in Saccharomyces cerevisiae

Sphingolipids are normally necessary for growth of Saccharomyces cerevisiae cells, but mutant strains that bypass the need for sphingolipids have been identified. Such bypass mutants fail to grow under stressful conditions, including low pH (pH 4.1), when they lack sphingolipids. To begin to understand why sphingolipids seem to be necessary for coping with low-pH stress, we screened a genomic library and selected a suppressor gene, CWP2 (cell wall protein 2), that when present in multiple copies partially compensates for the lack of sphingolipids and enhances survival at low pH. To explain these results, we present evidence that sphingolipids are required for a normal rate of transport of glycosylphosphatidylinositol (GPI)-anchored proteins, including Cwp2 and Gas1/Gpg1, from the endoplasmic reticulum (ER) to the Golgi apparatus. The effect of sphingolipids is specific for transport of GPI-anchored proteins because no effect on the rate of transport of carboxypeptidase Y, a non-GPI-anchored protein, was observed. Since the Gasl protein accumulated in the ER with a GPI anchor in cells lacking sphingolipids, we conclude that sphingolipids are not necessary for anchor attachment. Therefore, sphingolipids must be necessary for a step in formation of COPII vesicles or for their transport to the Golgi apparatus. Our data identify the Cwp2 protein as a vital component in protecting cells from the stress of low pH.

Synthetic phospho-oligosaccharide fragments of lipophosphoglycan as acceptors for Leishmania major alpha-D-mannosylphosphate transferase

Protozoan parasites of the genus Leishmania synthesise lipophosphoglycans, phosphoglycans and proteophosphoglycans that contain phosphosaccharide-repeat units of [-6Gal beta 1-4Man alpha 1-P-]. In this study, a GDP-Man-dependent alpha-mannosylphosphate-transferase activity was detected in washed Leishmania major membranes using synthetic phospho-oligosaccharide fragments of lipophosphoglycan as acceptor substrates. The divalent-cation-dependent alpha-mannosylphosphate-transferase activity had an apparent K(m) for GDP-Man of about 15-20 microM and a pH optimum of 7.0. The activity showed a requirement for a non-reducing terminal beta Gal residue and for one or more phosphodiester units preceding the acceptor site. Based on these results, the activity may be defined as a GDP-Man: Gal beta 1-4Man alpha 1-P-R alpha-mannosylphosphate-transferase. This acceptor specificity is consistent with a role for the alpha-mannosylphosphate transferase in the elongation of phosphosaccharide-repeat domains of Leishmania glycoconjugates rather than in the priming of these domains. An identical or similar activity must exist in the amastigote forms of the Leishmania that produce and secrete proteophosphoglycan material and the activity therefore represents a feasible target for the development of chemotherapeutics.

Purification and structural characterization of a filamentous, mucin-like proteophosphoglycan secreted by Leishmania parasites

Parasitic protozoa of the genus Leishmania secrete a filamentous macromolecule that forms networks and appears to be associated with cell aggregation. We report here the purification of this parasite antigen from Leishmania major culture supernatant and its compositional (75.6% carbohydrate, 20% phosphate, 4.4% amino acids, w/w), structural, and ultrastructural characterization as a highly unusual proteophosphoglycan (PPG). Mild acid hydrolysis, which cleaves preferentially hexose 1-phosphate bonds, releases the PPG glycans. Their structures are Galbeta1-4Man, Manalpha1-2Man, Galbeta1-3Galbeta1-4Man, PO4-6(Galbeta1-3)0-2Galbeta1-4Man, and PO4-6(Arabeta1-2Galbeta1-3)Galbeta1-4Man. These glycans are also components of the parasite glycolipid lipophosphoglycan, but their relative abundance and structural organization in PPG are different. Some of them represent novel forms of protein glycosylation. 31P NMR on native PPG demonstrates that phosphate is exclusively in phosphodiester bonds and that the basic structure R-Manalpha1-PO4-6-Gal-R connects the glycans. A phosphodiester linkage to phosphoserine (most likely R-Manalpha1-PO4-Ser) anchors the PPG oligosaccharides to the polypeptide. PPG has a unique amino acid composition; glycosylated phosphoserine (>43 mol %), serine, alanine, and proline account for more than 87 mol % and appear to be clustered in large proteinase-resistant domains. Electron microscopy of purified PPG reveals cable-like, flexible, long (to 6 microm), and unbranched filaments. The overall structure of PPG shows many similarities to mammalian mucins. Potential functions of this novel mucin-like molecule for the parasites are discussed.

The surface glycoconjugates of parasitic protozoa: potential targets for new drugs

Protozoan parasites are the cause of many disease in humans and their domestic livestock. Glycoconjugates (i.e. glycoproteins, glycolipids) on the cell surface of these extremely diverse and very primative eukaryotes play a crucial role in determining the specificity of the host-parasite interaction and in protecting the parasites within their respective hosts. These molecules frequently share a common structural feature in that they are attached to the plasma membrane via a glycosylphosphatidylinositol (GPI) glycolipid. While GPI protein-membrane anchors are ubiquitous among the eukaryotes, they are used with exceptionally high frequency in the protozoa. Some kinetopastid parasites also synthesise very high levels of GPI-related glycolipids that are not linked to protein. Thus GPI-anchored molecules or free GPI glycolipids send to dominate the cell surface molecular architecture of these organisms. The highly elevated levels and specialised nature of GPI metabolism in the kinetoplastid and other parasites suggests that the GPI biosynthetic pathway might be a good target for the development of new chemotherapeutic agents. This article reviews the wide range of functions that GPI protein anchors and GPI-related glycolipids are thought to perform in these organisms and some aspects of their biosynthesis.

A specialized pathway affecting virulence glycoconjugates of Leishmania

For virulence and transmission, the protozoan parasite Leishmania must assemble a complex glycolipid on the cell surface, the lipophosphoglycan (LPG). Functional complementation identified the gene LPG2, which encodes an integral Golgi membrane protein implicated in intracellular compartmentalization of LPG biosynthesis. Ipg2- mutants lack only characteristic disaccharide-phosphate repeats, normally present on both LPG and other surface or secreted molecules considered critical for infectivity. In contrast, a related yeast gene, VAN2/VRG4, is essential and required for general Golgi function. These results suggest that LPG2 participates in a specialized virulence pathway, which may offer an attractive target for chemotherapy.

Biosynthesis of lipophosphoglycan from Leishmania donovani: characterization of mannosylphosphate transfer in vitro

Lipophosphoglycan (LPG) is the major surface glycoconjugate of Leishmania donovani promastigotes and is composed of a capped polymer of repeating PO4-6Gal(beta 1,4)Man alpha 1 disaccharide units linked via a phosphosaccharide core to a lyso-1-O-alkylphosphatidylinositol anchor. An exogenous acceptor composed of the glycolipid anchor portion of LPG was shown to stimulate the enzymatic synthesis of the repeating phosphorylated disaccharide units of LPG in a cell-free system. Using the exogenous acceptor, GDP-[3H]Man, [beta-32P]GDP-Man, and unlabeled UDP-Gal as substrates, membrane preparations from an LPG-defective mutant of L. donovani that lacks endogenous acceptors catalyzed the incorporation of the doubly labeled mannosylphosphate unit into a product that exhibited the chemical and chromatographic characteristics of LPG. Analysis of fragments generated by mild acid hydrolysis of the radiolabeled product indicated that [3H]mannose-1-[32P]PO4 had been transferred from the dual-labeled sugar nucleotide. These results are consistent with the proposal that the repeating units of the L. donovani LPG are synthesized by the alternating transfer of mannose 1-phosphate and galactose from their respective nucleotide donors.