GPI-anchor synthesis

Complex formation of 70-kDa heat shock protein with acidic glycolipids and phospholipids

A new property of a heat-inducible heat shock protein (Hsp) 70.1 that it forms a complex with acidic lipids was first demonstrated. Based on the behaviors of the complexes on the native PAGE, the acidic lipid/Hsp70.1 complexes are categorized into two groups. The first group is the sulfatide-induced large-sized complex, which stays on the gel top on the native PAGE. Only the N-terminal ATPase domain is responsible for the complex formation. The second group is the ganglioside-induced complex, which is diffused in the resolution gel on the native PAGE. Both the N-terminal ATPase and the C-terminal peptide-binding domains are involved in the complex formation. No complex is formed by neutral glyco- and phospholipids. The complex formation with the acidic glyco- and phospholipids implicates the various functions of Hsp70 on the membrane surfaces.

Identification and characterization of D8C, a novel domain present in liver-specific LZP, uromodulin and glycoprotein 2, mutated in familial juvenile hyperuricaemic nephropathy

Present work reported a novel domain--D8C (domain with conserved eight cysteines in liver-specific ZP domain-containing protein, glycoprotein 2 (GP-2) and uromodulin (UMOD)), present in liver-specific LZP, UMOD, GP-2 and some uncharacterized proteins, most of which are membrane proteins, extracellular proteins or nuclear membrane proteins. D8C contains eight well-conserved cysteine residues, which were predicted to form four pairs of disulfide bridges. D8C is composed mainly of beta-strands. Mutation in the D8C at Cys217 in human UMOD is associated with familial juvenile hyperuricaemic nephropathy, which might be due to the disruption of the disulfide bridge. Identification of D8C would further the understandings of related proteins.

Critical roles of glycosylphosphatidylinositol for Trypanosoma brucei

Trypanosoma brucei, the protozoan parasite responsible for sleeping sickness, evades the immune response of mammalian hosts and digestion in the gut of the insect vector by means of its coat proteins tethered to the cell surface via glycosylphosphatidylinositol (GPI) anchors. To evaluate the importance of GPI for parasite survival, we cloned and disrupted a trypanosomal gene, TbGPI10, involved in biosynthesis of GPI. TbGPI10 encodes a protein of 558 amino acids having 25% and 23% sequence identity to human PIG-B and Saccharomyces cerevisiae Gpi10p, respectively. TbGPI10 restored biosynthesis of GPI in a mouse mutant cell line defective in mouse Pig-b gene. TbGPI10 also rescued the inviability of GPI10-disrupted S. cerevisiae, indicating that TbGPI10 is the orthologue of PIG-B/GPI10 that is involved in the transfer of the third mannose to GPI. The bloodstream form of T. brucei could not lose TbGPI10; therefore, GPI synthesis is essential for growth of mammalian stage parasites. Procyclic form cells (insect stage parasites) lacking the surface coat proteins because of disruption of TbGPI10 are viable and grow slower than normal, provided that they are cultured in nonadherent flasks. In regular flasks, they adhered to the plastic surface and died. Infectivity to tsetse flies is partially impaired, particularly in the early stage. Therefore, parasitespecific inhibition of GPI biosynthesis should be an effective chemotherapy target against African trypanosomiasis.

Glycosyl-phosphatidylinositol-anchor addition signals are processed in Nicotiana tabacum

Recent studies have demonstrated the existence of glycosyl-phosphatidylinositol (GPI)-anchored proteins in higher plants. In this study we tested whether GPI-addition signals from diverse evolutionary sources would function to link a GPI-anchor to a reporter protein in plant cells. Tobacco protoplasts were transiently transfected with a truncated form of the Clostridium thermocellum endoglucanase E reporter gene (celE') fused with a tobacco secretion signal (PR-1a) at the N-terminus and either a yeast (GAS1), mammalian (Thy-1) or putative plant (LeAGP-1) GPI-anchor addition signal at the C-terminus. The yeast and plant C-terminal signals were found to be capable of directing the addition of a GPI-anchor to the endoglucanase protein (EGE') as shown by the sensitivity of the lipid component of GPI to phosphatidylinositol-specific phospholipase C (PI-PLC) digestion. In contrast, the mammalian signal was poorly processed for anchor addition. When EGE' was fused to a truncated form of the LeAGP-1 signal (missing three amino acids predicted to be critical to signal cleavage and anchor addition), a GPI-anchor was not linked to the EGE' protein indicating the necessity for the missing amino acids. Our results show the conservation of the properties of GPI-signals in plant cells and that there may be some similar preferences in GPI-addition signal sequences for yeast and plant cells.

Evidence that free GPI glycolipids are essential for growth of Leishmania mexicana

The cell surface of the parasitic protozoan Leishmania mexicana is coated by glycosylphosphatidylinositol (GPI)-anchored glycoproteins, a GPI-anchored lipophosphoglycan and a class of free GPI glycolipids. To investigate whether the anchor or free GPIs are required for parasite growth we cloned the L.mexicana gene for dolichol-phosphate-mannose synthase (DPMS) and attempted to create DPMS knockout mutants by targeted gene deletion. DPMS catalyzes the formation of dolichol-phosphate mannose, the sugar donor for all mannose additions in the biosynthesis of both the anchor and free GPIs, except for a alpha1-3-linked mannose residue that is added exclusively to the free GPIs and lipophosphoglycan anchor precursors. The requirement for dolichol-phosphate-mannose in other glycosylation pathways in L.mexicana is minimal. Deletion of both alleles of the DPMS gene (lmdpms) consistently resulted in amplification of the lmdpms chromosomal locus unless the promastigotes were first transfected with an episomal copy of lmdpms, indicating that lmdpms, and possibly GPI biosynthesis, is essential for parasite growth. As evidence presented in this and previous studies indicates that neither GPI-anchored glycoproteins nor lipophosphoglycan are required for growth of cultured parasites, it is possible that the abundant and functionally uncharacterized free GPIs are essential membrane components.

Saccharomyces cerevisiae GPI10, the functional homologue of human PIG-B, is required for glycosylphosphatidylinositol-anchor synthesis

An increasing number of plasma membrane proteins have been shown to be attached to the membrane via a glycosylphosphatidylinositol (GPI) moiety. All eukaryotes share a highly conserved GPI-core structure EthN-P-Man3-GlcN-PI, where EthN is ethanolamine. We have identified a protein encoded by the yeast open reading frame YGL142C that shares 33% identity with the human Pig-B protein. Deletion of this essential gene leads to a block in GPI anchor biosynthesis. We therefore named the gene GPI10. Gpi10p and Pig-B are functional homologues and the lethal deletion of GPI10 can be rescued by expression of the PIG-B cDNA. As found for PIG-B mutant cells, gpi10 deletant cells cannot attach the third mannose in an alpha-1,2 linkage to the GPI core-structure intermediate. Overexpression of GPI10 gives partial resistance to the GPI-synthesis inhibitor YW3548, suggesting that this gene product may affect the target of the inhibitor.

Identification of a species-specific inhibitor of glycosylphosphatidylinositol synthesis

Glycosylphosphatidylinositol (GPI)-anchoring represents a mechanism for attaching proteins to the cell surface that is used among all eukaryotes. A common core structure, EthN-P-Man3-GlcN-PI, is synthesized by sequential transfer of sugars and ethanolamine-P to PI and is highly conserved between organisms. We have screened for natural compounds that inhibit GPI-anchoring in yeast and have identified a terpenoid lactone, YW3548, that specifically blocks the addition of the third mannose to the intermediate structure Man2-GlcN-acyIPI. Consistent with the block in GPI synthesis, YW3548 prevents the incorporation of [3H]myo-inositol into proteins, transport of GPI-anchored proteins to the Golgi and is toxic. The compound inhibits the same step of GPI synthesis in mammalian cells, but has no significant activity in protozoa. These results suggest that despite the conserved core structure, the GPI biosynthetic machinery may be different enough between mammalian and protozoa to represent a target for anti-protozoan chemotherapy.

Glycosylphosphatidylinositols are required for the development of Trypanosoma cruzi amastigotes

Induction of a glycosylphosphatidylinositol (GPI) deficiency in Trypanosoma cruzi by the heterologous expression of Trypanosoma brucei GPI-phospholipase C (GPI-PLC) results in decreased expression of major surface proteins (N. Garg, R. L. Tarleton, and K. Mensa-Wilmot, J. Biol. Chem. 272:12482-12491, 1997). To further explore the consequences of a GPI deficiency on replication and differentiation of T. cruzi, the in vitro and in vivo behaviors of GPI-PLC-expressing T. cruzi were studied. In comparison to wild-type controls, GPI-deficient T. cruzi epimastigotes exhibited a slight decrease in overall growth potential in culture. In the stationary phase of in vitro growth, GPI-deficient epimastigotes readily converted to metacyclic trypomastigotes and efficiently infected mammalian cells. However, upon conversion to amastigote forms within these host cells, the GPI-deficient parasites exhibited a limited capacity to replicate and subsequently failed to differentiate into trypomastigotes. Mice infected with GPI-deficient parasites showed a substantially lower rate of mortality, decreased tissue parasite burden, and a moderate tissue inflammatory response in comparison to those of mice infected with wild-type parasites. The decreased virulence exhibited by GPI-deficient parasites suggests that inhibition of GPI biosynthesis is a feasible strategy for chemotherapy of infections by T. cruzi and possibly other intracellular protozoan 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.

Role of phosphatidylinositol-linked proteins in paroxysmal nocturnal hemoglobinuria pathogenesis

Patients with paroxysmal nocturnal hemoglobinuria have one or more mutant hematopoietic stem cell clones deficient in glycosylphosphatidylinositol (GPI)-anchor synthesis owing to somatic mutations in the X-linked gene PIG-A. The progeny of mutant stem cells dominates the peripheral blood. The presence of a large number of GPI-anchor deficient, complement-sensitive erythrocytes leads to hemolytic anemia. The somatic mutations in PIG-A are small, various, and widely distributed in the coding regions and splice sites, indicating they occur randomly. Profiles of the mutations vary geographically, suggesting the presence of mutagen-induced mutations. The clonal dominance by the mutants does not seem to be solely due to the PIG-A mutation but may be caused either by autonomous expansion of the mutants due to a combination of the PIG-A mutation and some other genetic change(s) or by selection that preferentially suppresses normal stem cells.

The GPI anchor of cell-surface proteins is synthesized on the cytoplasmic face of the endoplasmic reticulum

Glycosylphosphatidylinositol (GPI) membrane protein anchors are synthesized from sugar nucleotides and phospholipids in the ER and transferred to newly synthesized proteins destined for the cell surface. The topology of GPI synthesis in the ER was investigated using sealed trypanosome microsomes and the membrane-impermeant probes phosphatidylinositol-specific phospholipase C, Con A, and proteinase K. All the GPI biosynthetic intermediates examined were found to be located on the external face of the microsomal vesicles suggesting that the principal steps of GPI assembly occur in the cytoplasmic leaflet of the ER. Protease protection experiments showed that newly GPI-modified trypanosome variant surface glycoprotein was primarily oriented towards the ER lumen, consistent with eventual expression at the cell surface. The unusual topographical arrangement of the GPI assembly pathway suggests that a biosynthetic intermediate, possibly the phosphoethanolamine-containing anchor precursor, must be translocated across the ER membrane bilayer in the process of constructing a GPI anchor.