Glycosylphosphatidylinositols synthesized by asexual erythrocytic stages of the malarial parasite, Plasmodium falciparum. Candidates for plasmodial glycosylphosphatidylinositol membrane anchor precursors and pathogenicity factors

Structural analysis of the glycosyl-phosphatidylinositol membrane anchor of the merozoite surface proteins-1 and -2 of Plasmodium falciparum

Plasmodium falciparum accumulates the two merozoite surface proteins-1 and -2 during schizogony. Both proteins are proposed to be anchored in membranes by glycosyl-phosphatidylinositol membrane anchors. In this report the identity of these GPI-anchors is confirmed by labelling with tritiated precursors and additionally by specific enzymatic and chemical treatments. Detailed structural analysis of the core-glycans showed that the GPI-anchors of both proteins possess an extra alpha 1-2 linked mannose at the conserved trimannosyl-core-glycan. MSP-1 and MSP-2 labelled with tritiated myristic acid possess primarily radioactive myristic acid at inositol rings in both GPI-anchors. Additionally the hydrophobic fragments released from [3H]myristic acid labelled GPI-anchors were identified as diacyl-glycerols, carrying preferentially [3H]palmitic acid in an ester-linkage.

Biosynthesis of the side chain of yeast glycosylphosphatidylinositol anchors is operated by novel mannosyltransferases located in the endoplasmic reticulum and the Golgi apparatus

Glycosylphosphatidylinositol (GPI) anchors of the yeast Saccharomyces cerevisiae have been reported to contain three different types of side chains attached to contain three different types of side chains attached to the alpha 1,2-linked mannose of the conserved protein-ethanolamine-PO4-Man alpha 1,2Man alpha 1,6Man alpha 1,4GlcNH2-inositol glycan core. The possible side chains are Man alpha 1,2- or Man alpha 1,2Man alpha 1,2- or Man alpha 1,3Man alpha 1,2- (Fankhauser, C., Homan, S. W., Thomas Oates, J. E., McConville, M. J., Desponds, C., Conzelmann, A., and Ferguson, M. A. (1993) J. Biol. Chem. 268, 26365-26374). To determine in what subcellular compartment these side chains are made, we metabolically labeled GPI-anchored membrane proteins with myo-[2-3H]inositol in secretion mutants blocked at various stages of the secretory pathway and analyzed the anchor structure of the labeled glycoproteins. When the exit of vesicles from the endoplasmic reticulum or entry into the cis-Golgi were blocked in sec12 or sec18 cells, all anchors contained a side chain consisting of a single alpha 1,2-linked mannose. GPI proteins trapped in the cis-Golgi of sec7 contained Man alpha 1,3Man alpha 1,2- but no Man alpha 1,2Man alpha 1,2-side chains. Mutants blocked at later stages of the secretory pathway made increased amounts of side chains containing two mannoses. Man alpha 1,2Man alpha 1,2- and Man alpha 1,3Man alpha 1,2- side chains were preferentially associated with ceramide- and diacylglycerol-containing GPI anchors, respectively. Mnn1, mnn2, mnn3, mnn5, and mnt1(=kre2), i.e. mutants which lack or down-regulate 1,2- and 1,3- mannosyltransferases used in the elongation of N- and O-glycans in the Golgi, add the fifth mannose to GPI anchors normally. The same conclusion was reached through the analysis of deletion mutants in KTR1, KTR2, KTR3, KTR4, and YUR1 which all are open reading frames with high homology to MNT1. Mutants deficient in the Golgi elongation of N-glycans such as anp1, van1, mnn9 are deficient in the maturation of the N-glycans of GPI-anchored glycoproteins, but process the GPI anchor side chain normally. Data are consistent with the idea that the fourth mannose is added to proteins as part of the anchor precursor glycolipid in the endoplasmic reticulum, whereas the fifth mannose is added by not yet identified alpha 1,3- and alpha 1,2-mannosyltransferases located in the Golgi apparatus.

Initiation of chemical studies on the immunoreactive glycolipids of adult Ascaris suum

There is a general lack of basic information concerning one class of glycoconjugate, the glycolipids, from parasitic nematodes. As the prototype, the neutral glycolipid fraction derived from adult males of Ascaris suum was investigated as to its chromatographic, differential chemical staining, antigenic and chemical properties. The thin-layer chromatography-resolved neutral fraction glycolipids could be classified into components of fast and slow migrating band groups. Immunoreactivity was restricted to the latter as detected by IgG and IgM anti-neutral fraction glycolipid antibody levels in serial infection sera of mice. Similarities of chromatography, antigenicity and serological cross-reactivity have been extended to the neutral glycolipid fractions of other parasitic nematodes: Litomosoides carinii and Nippostrongylus brasiliensis. Chemical, differential chemical staining and enzymatic analyses identified the Ascaris suum antigenic, slow migrating band group of components as amphoteric glycosphingolipids, and not the originally hypothesized glycoglycerolipids or glycosylphosphatidylinositols, that contained typical neutral monosaccharide constituents and a zwitterionic phosphodiester linkage, most probably phosphocholine. Glycosphingolipid-immunoreactivity is eliminated on cleavage of the zwitterionic phosphodiester linkage by hydrofluoric acid treatment.

Malaria: toxins, cytokines and disease

In this review the old concept of severe malaria as a toxic disease is re-examined in the light of recent discoveries in the field of cytokines. Animal studies suggest that the induction of TNF by parasite-derived molecules may be partly responsible for cerebral malaria and anemia, while hypoglycaemia may be due to direct effects of similar molecules on glucose metabolism. These molecules appear to be phospholipids and we suggest that when fully characterized they might form the basis of antitoxic therapy for malaria.

Malaria pathogenesis

Malaria is a disease caused by repeated cycles of growth of the parasite Plasmodium in the erythrocyte. Various cellular and molecular strategies allow the parasite to evade the human immune response for many cycles of parasite multiplication. Under certain circumstances Plasmodium infection causes severe anemia or cerebral malaria; the expression of disease is influenced by both parasite and host factors, as exemplified by the exacerbation of disease during pregnancy. This article provides an overview of malaria pathogenesis, synthesizing the recent field, laboratory, and epidemiological data that will lead to the development of strategies to reduce mortality and morbidity.

Proteins on the surface of the malaria parasite and cell invasion

The malaria parasite exists in an extracellular form at several stages in its life cycle. Within the vertebrate host, sporozoites and merozoites have to invade specific cell types. Proteins on the surface of the parasite or externalized from specialized organelles have been implicated as ligands for receptors on the host cell surface. Direct binding studies have identified parasite proteins that interact with the target cell surface. Examination of the deduced amino acid sequences has allowed the identification of primary structural motifs which may have roles in this process. On the sporozoite, the circumsporozoite protein and sporozoite surface protein-2, a protein initially located within micronemes, have been found to contain an amino acid sequence thought to be involved in mediating recognition of sulphated polysaccharides on the surface of a liver cell. On the merozoite, merozoite surface protein-1 may be involved in the initial recognition of red blood cells; this protein undergoes a complex series of modifications in the time between its synthesis as a precursor molecule and successful erythrocyte invasion. Other merozoite proteins located at the apical end of the parasite have been identified as erythrocyte or reticulocyte binding proteins.