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Reviewing Glycosyl-Inositols: Natural Occurrence, Biological Roles, and Synthetic Techniques

Glycosyl-inositols are molecules consisting of one or more α- or β-D-glycosyl residues bonded primarily to inositol or methyl-inositol. These derivatives are found in plants, yeast, bacteria, and parasites, and exhibit diverse biological properties. The limited availability of glycosyl inositols from natural sources has led to significant interest in chemical and enzymatic synthesis techniques due to their potential applications in various fields. This review provides a comprehensive overview of inositols, methyl-inositols, and primarily glycosyl inositols, focusing on their classification, natural occurrence, biological roles, and potential applications across different disciplines. Inositols, particularly myo-inositol and its derivatives are widely distributed in plants and play essential roles in biochemical processes and metabolic functions in different organs and tissues. Glycosyl inositols, including glycosylphosphatidylinositols, glycosyl inositol phosphorylceramides, phosphatidylinositol mannosides, monoglycosyl and diglycosyl derivatives, are discussed, emphasizing their structural diversity and biological functions. Methods for their chemical and enzymatic synthesis are also reviewed, highlighting recent advances and challenges in the field. Overall, this comprehensive review underscores the significance of glycosyl inositols as versatile molecules with diverse biological functions and promising applications in scientific research and industry.

Computational prediction of the effects of non-synonymous single nucleotide polymorphisms on the GPI-anchor transamidase subunit GPI8p of Plasmodium falciparum

Drug resistance is increasingly evolving in malaria parasites; hence, it is important to discover and establish alternative drug targets. In this context, GPI-anchor transamidase (GPI-T) is a potential drug target primarily of its crucial role in the development and survival of the parasite in the GPI anchor biosynthesis pathway. The present investigation was undertaken to explore the plausible effects of nsSNP on the structure and functions of GPI-T subunit GPI8p of Plasmodium falciparum. The GPI8p (PF3D7_1128700) was analyzed using various sequence-based and structure-based computational tools such as SIFT, PROVEAN, PredictSNP, SNAP2, I-Mutant, MuPro, ConSurf, NetSurfP, MUSTER, COACH server and STRING server. Of the 34 nsSNPs submitted for functional analysis, 18 nsSNPs (R124 L, N143 K, Y145 F, V157I, T195S, K379E, I392 K, I437 T, Y438H, N439D, Y441H, N442D, N448D, N451D, D457A, D457Y, I458 L and N460 K) were predicted to have deleterious effects on the protein GPI8p. Additionally, I-Mutant 2.0 and MuPro both showed a decrease in stability after mutation as a result of these nsSNPs, suggesting the destabilization of protein. ConSurf findings suggest that most of the regions were highly conserved. In addition, COACH server was used to predict the ligand binding sites. It was found that no mutation was present at the predicted ligand binding site. The results of the STRING database showed that the protein GPI8p interacts with those proteins which either involve the biosynthetic process of attaching GPI anchor to protein or GPI anchor. The present study suggested that the GPI8p could be a novel target for anti-malarial drugs, which provides significant details for further experimentation.

GPI-Anchored Proteins and Glycosphingolipid-Rich Rafts: Platforms for Adhesion and Signaling

Glycosylphosphatidylinositol (GPI)-anchored proteins in mammalian cells play a role in adhesion and signaling. They are sorted in the trans-Golgi network into glycosphingolipid- and cholesterol-rich microdomains termed rafts. Such rafts can be isolated from many cell types including epithelial cells, neural cells, and lymphocytes. In polarized cells, the rafts segregate in distinct regions of the cell. The rafts constitute platforms for signal transduction via raft-associated srcfamily tyrosine kinases. This review compares the sorting, distribution, and signaling of GPI-anchored proteins and rafts in epithelial cells, lymphocytes, and neural cells. A possible involvement of rafts in distinct diseases is also addressed.

Labeling Cell Surface GPIs and GPI-Anchored Proteins through Metabolic Engineering with Artificial Inositol Derivatives

Glycosylphosphatidylinositol (GPI) anchoring of proteins to the cell surface is important for various biological processes, but GPI-anchored proteins are difficult to study. An effective strategy was developed for the metabolic engineering of cell-surface GPIs and GPI-anchored proteins by using inositol derivatives carrying an azido group. The azide-labeled GPIs and GPI-anchored proteins were then tagged with biotin on live cells through a click reaction, which allows further elaboration with streptavidin-conjugated dyes or other molecules. The strategy can be used to label GPI-anchored proteins with various tags for biological studies.

The role of surface glycoconjugates in Leishmania midgut attachment examined by competitive binding assays and experimental development in sand flies

Binding of promastigotes to the sand fly midgut epithelium is regarded as an essential part of the Leishmania life cycle in the vector. Among Leishmania surface molecules putatively involved in attachment to the sand fly midgut, two GPI-anchored molecules are the most prominent: lipophosphoglycan (LPG) and promastigote surface protease gp63. In this work, we examined midgut attachment of Leishmania lines mutated in GPI-anchored molecules and compared results from 2 different techniques: in vivo development in sand flies and in vitro competitive binding assays using fluorescently labelled parasites. In combination with previous studies, our data provide additional support for (1) an LPG-independent parasite-binding mechanism of Leishmania major within the midgut of the permissive vector Phlebotomus perniciosus, and provide strong support for (2) the crucial role of L. major LPG in specific vector Phlebotomus papatasi, and (3) a role for Leishmania amazonensis gp63 in Lutzomyia longipalpis midgut binding. Moreover, our results suggest a critical role for GPI-anchored proteins and gp63 in Leishmania mexicana attachment to L. longipalpis midguts, as the wild type (WT) line accounted for over 99% of bound parasites.

Plasmodium vivax: paroxysm-associated lipids mediate leukocyte aggregation

BACKGROUND

Paroxysms are recurrent febrile episodes, characteristic of Plasmodium vivax infections, which coincide with the rupture of schizont-infected erythrocytes in the patients' circulation. The present study describes the formation of prominent aggregates of leukocytes in vitro in the presence of parasite and host factors released during paroxysms.

METHODS

Whole blood cells from uninfected malaria-naïve donors were incubated with plasma taken during a paroxysm or normal human plasma as a control and cell smears were observed under the microscope for the presence of leukocyte aggregates. Plasma factors involved in mediating the leukocyte aggregation were identified using immune depletion and reconstitution experiments. Furthermore, biochemical characterization was carried out to determine the chemical nature of the active moieties in plasma present during paroxysms.

RESULTS

Leukocyte aggregates were seen exclusively when cells were incubated in plasma collected during a paroxysm. Immune depletion and reconstitution experiments revealed that the host cytokines TNF-alpha, GM-CSF, IL-6 and IL-10 and two lipid fractions of paroxysm plasma comprise the necessary and sufficient mediators of this phenomenon. The two lipid components of the paroxysm plasmas speculated to be of putative parasite origin, were a phospholipid-containing fraction and another containing cholesterol and triglycerides. The phospholipid fraction was dependent upon the presence of cytokines for its activity unlike the cholesterol/triglyceride-containing fraction which in the absence of added cytokines was much more active than the phospholipids fraction. The biological activity of the paroxysm plasmas from non-immune patients who presented with acute P. vivax infections was neutralized by immune sera raised against schizont extracts of either P. vivax or Plasmodium falciparum. However, immune sera against P. vivax were more effective than that against P. falciparum indicating that the parasite activity involved may be antigenically at least partially parasite species-specific.

CONCLUSION

Leukocyte aggregation was identified as associated with paroxysms in P. vivax infections. This phenomenon is mediated by plasma factors including host-derived cytokines and lipids of putative parasite origin. The characteristics of the phospholipid fraction in paroxysm plasma are congruent with those of the parasite-derived, TNF-inducing GPI moieties described by others. The more active cholesterol/triglyceride(s), however, represent a novel malarial toxin, which is a new class of biologically active lipid associated with the paroxysm of P. vivax malaria.

Making sense of the schistosome surface

The syncytial cytoplasmic layer, termed the tegument, which covers the entire surface of adult schistosomes, is a major interface between the parasite and its host. Since schistosomes can survive for decades within the host bloodstream, they are clearly able to evade host immune responses, and their ability is dependent on the properties of the tegument surface. We review here the molecular organization and biochemical functions of the tegument, combining the extensive literature over the last three decades with recent proteomic studies. We have interpreted the organization of the tegument surface as bounded by a conventional plasma membrane overlain by a membrane-like secretion, the membranocalyx, with which host molecules can associate. The range of parasite proteins, glycans and lipids found in the surface complex is evaluated, together with the host molecules detected. We consider the way in which the tegument surface is formed after cercarial penetration into the skin, and changes that occur as parasites develop to maturity. Lastly, we review the evidence on surface dynamics and turnover.

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

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

Inositide evolution - towards turtle domination?

When viewing the changes in our understanding of inositides over the last 20 years, it is difficult to know whether to be more impressed by the proliferation in the number of inositides themselves (e.g. seven polyphosphoinositol lipids, more than 30 inositol phosphates), or by the number of functions for each. This review will focus on two specific aspects of this diversity: the evolution of the polyphosphoinositides, and the synthesis and functions of the higher inositol phosphates.

Malarial toxins and the regulation of parasite density

For over a century it has been recognized that many of the clinical symptoms of malaria are caused by toxins released by rupturing schizonts, but it is only in the past few years that the underlying mechanisms have begun to be understood. Dominic Kwiatkowski here focuses on the toxins that cause malaria fever by stimulating host cells to produce tumour necrosis factor a (TNF) and other pyrogenic cytokines. Both TNF and fever have antiparasite properties, and it is proposed that the release of these toxins plays an important role in the regulation of parasite density within the host. Cerebral malaria is related to excessive TNF production. Recent data indicate that this can be the consequence of genetic variation in the host's propensity to produce TNF.

Endosomes, glycosomes, and glycosylphosphatidylinositol catabolism in Leishmania major

Glycosylphosphatidylinositols (GPIs) serve as membrane anchors of polysaccharides and proteins in the protozoan parasite Leishmania major. Free GPIs that are not attached to macromolecules are present in L. major as intermediates of protein-GPI and polysaccharide-GPI synthesis or as terminal glycolipids. The importance of the intracellular location of GPIs in vivo for functions of the glycolipids is not appreciated. To examine the roles of intracellular free GPI pools for attachment to polypeptide, a GPI-specific phospholipase C (GPI-PLCp) from Trypanosoma brucei was used to probe trafficking of GPI pools inside L. major. The locations of GPIs were determined, and their catabolism by GPI-PLCp was analyzed with respect to the intracellular location of the enzyme. GPIs accumulated on the endo-lysosomal system, where GPI-PLCp was also detected. A peptide motif [CS][CS]-x(0,2)-G-x(1)-C-x(2,3)-S-x(3)-L formed part of an endosome targeting signal for GPI-PLCp. Mutations of the endosome targeting motif caused GPI-PLCp to associate with glycosomes (peroxisomes). Endosomal GPI-PLCp caused a deficiency of protein-GPI in L. major, whereas glycosomal GPI-PLCp failed to produce the GPI deficiency. We surmise that (i) endo-lysosomal GPIs are important for biogenesis of GPI-anchored proteins in L. major; (ii) sequestration of GPI-PLCp to glycosomes protects free protein-GPIs from cleavage by the phospholipase. In T. brucei, protein-GPIs are concentrated at the endoplasmic reticulum, separated from GPI-PLCp. These observations support a model in which glycosome sequestration of a catabolic GPI-PLCp preserves free protein-GPIs in vivo.

Deletion of the GPI pre-anchor sequence in human p97—a general approach for generating the soluble form of GPI-linked proteins

Melanotransferrin, also named p97, belongs to the transferrin-like group of iron-binding proteins. Unlike the other members of this family, p97 exists in two forms-one soluble form and one attached to the cell membrane by a glycosylphosphatidylinositol (GPI) anchor. The GPI-linked form plays a role in the uptake of iron, while the soluble form of p97 has the unique ability of traversing the blood-brain barrier and may be utilized to deliver drug conjugates into the brain. To investigate these possibilities, a recombinant soluble form of p97 from the GPI-linked p97 protein is required. The approach involved sequential deletions of the p97 GPI pre-anchor sequence (PAS) up to the putative site of cleavage/attachment, releasing p97 from attachment to the GPI-anchor and rendering it soluble. Transfection of the p97 deletion constructs into both the CHO and BHK TK(-) cells was performed with the aim of optimizing the production of p97 by utilizing the cell characteristics unique to each cell line. Altering the GPI PAS resulted in the generation of a recombinant soluble form that was secreted at significantly higher rates than from the full-length expressing cell lines. Increases were from 22 x 10(-9) to 241 x 10(-9)microg/cell/h for expression in the CHO cell system and from 220 x 10(-9) to 4970 x 10(-9)microg/cell/h for the BHK system. Furthermore, there appeared to be differences in the secretion rates between the various deletions suggesting the need for closer examination of the C-terminus in achieving maximum production of the altered proteins. The results of this study are likely applicable for expressing soluble forms of other GPI-linked proteins.

The glycoforms of a Trypanosoma brucei variant surface glycoprotein and molecular modeling of a glycosylated surface coat

The plasma membrane of the African sleeping sickness parasite Trypanosoma brucei is covered with a dense, protective surface coat. This surface coat is a monolayer of five million variant surface glycoprotein (VSG) dimers that form a macromolecular diffusion barrier. The surface coat protects the parasite from the innate immune system and, through antigenic variation, the specific host immune response. There are several hundred VSG genes per parasite, and they encode glycoproteins that vary in primary amino acid sequence, the number of N-glycosylation sites, and the types of N-linked oligosaccharides and glycosylphosphatidylinositol membrane anchors they contain. In this study, we show that VSG MITat.1.5 is glycosylated at all three potential N-glycosylation sites, and we assign the oligosaccharides present at each site. Using the most abundant oligosaccharides at each site, we construct a molecular model of the glycoprotein to assess the role of N-linked oligosaccharides in the architecture of the surface coat.

Structure and activity of glycosylphosphatidylinositol anchors of Plasmodium falciparum

The glycosylphosphatidylinositol (GPI) anchors of Plasmodium falciparum are thought to be etiologic agents of malaria based on their ability to induce proinflammatory cytokine production by macrophages and cause symptoms that resemble severe malaria illness in animals. This review summarizes the published information on the structures of P. falciparum GPIs, structure-activity relationship, and anti-GPI antibodies in the host.

Intracellular retention of glycosylphosphatidyl inositol-linked proteins in caveolin-deficient cells

The relationship between glycosylphosphatidyl inositol (GPI)-linked proteins and caveolins remains controversial. Here, we derived fibroblasts from Cav-1 null mouse embryos to study the behavior of GPI-linked proteins in the absence of caveolins. These cells lack morphological caveolae, do not express caveolin-1, and show a approximately 95% down-regulation in caveolin-2 expression; these cells also do not express caveolin-3, a muscle-specific caveolin family member. As such, these caveolin-deficient cells represent an ideal tool to study the role of caveolins in GPI-linked protein sorting. We show that in Cav-1 null cells GPI-linked proteins are preferentially retained in an intracellular compartment that we identify as the Golgi complex. This intracellular pool of GPI-linked proteins is not degraded and remains associated with intracellular lipid rafts as judged by its Triton insolubility. In contrast, GPI-linked proteins are transported to the plasma membrane in wild-type cells, as expected. Furthermore, recombinant expression of caveolin-1 or caveolin-3, but not caveolin-2, in Cav-1 null cells complements this phenotype and restores the cell surface expression of GPI-linked proteins. This is perhaps surprising, as GPI-linked proteins are confined to the exoplasmic leaflet of the membrane, while caveolins are cytoplasmically oriented membrane proteins. As caveolin-1 normally undergoes palmitoylation on three cysteine residues (133, 143, and 156), we speculated that palmitoylation might mechanistically couple caveolin-1 to GPI-linked proteins. In support of this hypothesis, we show that palmitoylation of caveolin-1 on residues 143 and 156, but not residue 133, is required to restore cell surface expression of GPI-linked proteins in this complementation assay. We also show that another lipid raft-associated protein, c-Src, is retained intracellularly in Cav-1 null cells. Thus, Golgi-associated caveolins and caveola-like vesicles could represent part of the transport machinery that is necessary for efficiently moving lipid rafts and their associated proteins from the trans-Golgi to the plasma membrane. In further support of these findings, GPI-linked proteins were also retained intracellularly in tissue samples derived from Cav-1 null mice (i.e., lung endothelial and renal epithelial cells) and Cav-3 null mice (skeletal muscle fibers).

The I-antigens of Ichthyophthirius multifiliis are GPI-anchored proteins

The parasitic ciliate Ichthyophthirius multifiliis has abundant surface membrane proteins (i-antigens) that when clustered, trigger rapid, premature exit from the host. Similar antigens are present in free-living ciliates and are GPI-anchored in both Paramecium and Tetrahymena. Although transmembrane signalling through GPI-anchored proteins has been well-documented in metazoan cells, comparable phenomena have yet to be described in protists. Since premature exit of Ichthyophthirius is likely to involve a transmembrane signalling event, we sought to determine whether i-antigens are GPI-anchored in these cells as well. Based on their solubility properties in Triton X-114, the i-antigens of Ichthyophthirius are amphiphilic in nature and partition with the detergent phase. Nevertheless, following treatment of detergent lysates with phospholipase C, the same proteins become hydrophilic. Concomitantly, they are recognized by antibodies against a cross-reacting determinant exposed on virtually all GPI-anchored proteins following cleavage with phospholipase C. Finally, when expressed in recombinant form in Tetrahymena thermophila, full-length i-antigens are restricted to the membrane, while those lacking hydrophobic C-termini are secreted from the cell. Taken together, these observations argue strongly that the i-antigens of Ichthyophthirius multifiliis are, in fact, GPI-anchored proteins.

GPI anchoring leads to sphingolipid-dependent retention of endocytosed proteins in the recycling endosomal compartment

Glycosylphosphatidylinositol (GPI) anchoring is important for the function of several proteins in the context of their membrane trafficking pathways. We have shown previously that endocytosed GPI-anchored proteins (GPI-APs) are recycled to the plasma membrane three times more slowly than other membrane components. Recently, we found that GPI-APs are delivered to endocytic organelles, devoid of markers of the clathrin-mediated pathway, prior to their delivery to a common recycling endosomal compartment (REC). Here we show that the rate-limiting step in the recycling of GPI-APs is their slow exit from the REC; replacement of the GPI anchor with a transmembrane protein sequence abolishes retention in this compartment. Depletion of endogenous sphingolipid levels using sphingolipid synthesis inhibitors or in a sphingolipid-synthesis mutant cell line specifically enhances the rate of endocytic recycling of GPI-APs to that of other membrane components. We have shown previously that endocytic retention of GPI-APs is also relieved by cholesterol depletion. These findings strongly suggest that functional retention of GPI-APs in the REC occurs via their association with sphingolipid and cholesterol-enriched sorting platforms or 'rafts'.

An accessory role for the diacylglycerol moiety of variable surface glycoprotein of African trypanosomes in the stimulation of bovine monocytes

The membrane-associated form of the variable surface glycoprotein (mfVSG) from African trypanosomes is a potent macrophage activator capable of inducing production of tumor necrosis factor alpha (TNFalpha) in both bovine and murine models. Using a bovine model, we have re-investigated the hypothesis that the diacylglycerol moiety of the glycosylphosphatodylinositol (GPI) anchor is involved in macrophage activation and might be the actual parasite toxin. The anchor of the variable surface glycoprotein (VSG) was labeled with (3)H-myristic acid and VSG purified in its membrane-associated form. The dimyristylglycerol moiety of the anchor was released by phospholipase C cleavage. Integrity of the anchor and efficiency of cleavage was verified by autoradiography and methanol:hexane extraction. For analysis of biological function, bovine monocytes were used which had been incubated with bovine interferon gamma (primed) or with culture medium (unprimed). The VSG purified in its membrane-associated form was found to stimulate both primed and unprimed cells to secrete TNFalpha. The same preparation from which the dimyristylglycerol moiety had been cleaved was no longer able to stimulate unprimed cells but could still stimulate primed cells. Our data indicate that the presence of the dimyristylglycerol is not an absolute requirement for induction of TNFalpha production but can substitute for the interferon gamma priming. Therefore, we favor the hypothesis that stimulation of macrophages to secrete TNFalpha by the mfVSG is mediated by an as yet unknown trigger moiety and is facilitated by the dimyristylglycerol anchor.

Developmental stage-specific biosynthesis of glycosylphosphatidylinositol anchors in intraerythrocytic Plasmodium falciparum and its inhibition in a novel manner by mannosamine

Glycosylphosphatidylinositols (GPIs) are the major glycoconjugates in intraerythrocytic stage Plasmodium falciparum. Several functional proteins including merozoite surface protein 1 are anchored to the cell surface by GPI modification, and GPIs are vital to the parasite. Here, we studied the developmental stage-specific biosynthesis of GPIs by intraerythrocytic P. falciparum. The parasite synthesizes GPIs exclusively during the maturation of early trophozoites to late trophozoites but not during the development of rings to early trophozoites or late trophozoites to schizonts and merozoites. Mannosamine, an inhibitor of GPI biosynthesis, inhibits the growth of the parasite specifically at the trophozoite stage, preventing further development to schizonts and causing death. Mannosamine has no effect on the development of either rings to early trophozoites or late trophozoites to schizonts and merozoites. The analysis of GPIs and proteins synthesized by the parasite in the presence of mannosamine demonstrates that the effect is because of the inhibition of GPI biosynthesis. The data also show that mannosamine inhibits GPI biosynthesis by interfering with the addition of mannose to an inositol-acylated GlcN-phosphatidylinositol (PI) intermediate, which is distinctively different from the pattern seen in other organisms. In other systems, mannosamine inhibits GPI biosynthesis by interfering with either the transfer of a mannose residue to the Manalpha1-6Manalpha1-4GlcN-PI intermediate or the formation of ManN-Man-GlcN-PI, an aberrant GPI intermediate, which cannot be a substrate for further addition of mannose. Thus, the parasite GPI biosynthetic pathway could be a specific target for antimalarial drug development.

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.

The gene for an abundant parasite coat protein predicts tandemly repetitive metal binding domains

Immobilization antigens are highly abundant surface membrane proteins that coat the surface of hymenostomatid ciliates. While their function is unknown, recent studies with the common fish parasite, Ichthyophthirius multifiliis, suggest their involvement in a novel mechanism of humoral immunity involving an effect of antibody on parasite behavior. To gain further insight into the nature of these proteins, we have cloned a gene encoding the 48kDa i-antigen of I. multifiliis. Analysis of the gene (designated IAG48[G1]) reveals a single, uninterrupted reading frame that predicts a protein of 442 amino acids. Based on its deduced amino acid sequence, the protein contains hydrophobic amino acid domains at its N- and C-terminus that are characteristic of signal peptide and GPI-anchor addition sites, respectively. The most striking feature of the predicted protein, however, is a series of tandem repeats that spans most of its length. The repeats themselves are characterized by periodic cysteine residues that fall into register when the homologous segments are aligned. Interestingly, the spacing of cysteines (C-X2,3-C) within a framework of larger (C-X2-C-X20-C-X3-C-X20-C-X2-C) motifs is entirely consistent with the structure of known zinc-binding proteins. Finally, comparison of the coding sequence of the 48kDa i-antigen gene with a partial cDNA previously thought to encode this protein reveals nearly complete identity except at their 3' ends, suggesting that alternative forms of the antigen exist.

Microbial glycoconjugates

The surfaces of all microbes are 'sugar coated' with molecules such as lipopolysaccharides in Gram-negative bacteria, capsular polysaccharides in bacteria, lipoarabinomannans in mycobacteria and lipophosphoglycan in Leishmania. The basic structures of these glycoconjugates are known and, in the case of pathogens, they can function as virulence determinants. Recent publications have refined some of these structures and have elucidated interesting genes and proteins responsible for their biosynthesis.

Cholesterol-dependent retention of GPI-anchored proteins in endosomes

Several cell surface eukaryotic proteins have a glycosylphosphatidylinositol (GPI) modification at the Cterminal end that serves as their sole means of membrane anchoring. Using fluorescently labeled ligands and digital fluorescence microscopy, we show that contrary to the potocytosis model, GPI-anchored proteins are internalized into endosomes that contain markers for both receptor-mediated uptake (e.g. transferrin) and fluid phase endocytosis (e.g. dextrans). This was confirmed by immunogold electron microscopy and the observation that a fluorescent folate derivative bound to the GPI-anchored folate receptor is internalized into the same compartment as co-internalized horseradish peroxidase-transferrin; the folate fluorescence was quenched when cells subsequently were incubated with diaminobenzidine and H2O2. Most of the GPI-anchored proteins are recycled back to the plasma membrane but at a rate that is at least 3-fold slower than C6-NBD-sphingomyelin or recycling receptors. This endocytic retention is regulated by the level of cholesterol in cell membranes; GPI-anchored proteins are recycled back to the cell surface at the same rate as recycling transferrin receptors and C6-NBD-sphingomyelin in cholesterol-depleted cells. Cholesterol-dependent endocytic sorting of GPI-anchored proteins is consistent with the involvement of specialized lipid domains or 'rafts' in endocytic sorting. These results provide an alternative explanation for GPI-requiring functions of some GPI-anchored proteins.

Species-specificity in endoplasmic reticulum signal peptide utilization revealed by proteins from Trypanosoma brucei and Leishmania

N-Terminal signal peptides direct secretory and most membrane proteins into the exocytic pathway at the endoplasmic reticulum. Signal sequences can function across kingdoms. However, our attempts at translocating variant surface glycoprotein (VSG) 117, VSG MVAT7, VSG 221 and BiP from Trypanosoma brucei and gp63 from Leishmania chagasi into canine pancreas microsomes failed. On replacing the signal peptide of VSG 117 with that from yeast prepro-alpha-mating factor (ppalphaMF) the chimaeric protein was imported, indicating that the signal sequence of VSG 117 was incompatible with the protein-import machinery of mammalian microsomes. Replacement of the gp63-h-region with a hybrid composed of the N-terminal nine residues from the h-region of gp67 from Autographa californica nuclear polyhedrosis virus and the C-terminal 10 residues from the h-region of gp63 from L. major produced a functional signal peptide. Thus, the h-region of kinetoplastid signal peptides appears to be the subdomain that is non-functional at the mammalian translocon. The calculated biophysical properties and computed discriminant scores (predictive of importability of signal peptides into mammalian microsomes) of the kinetoplastid signal sequences nevertheless are similar to those of ppalphaMF and Escherichia coli beta-lactamase both of which were imported. These signal peptides are the first collection from one biological family that have been found to fail to function across a species barrier. They indicate that signal peptides are not as universally interchangeable as previously believed. Intriguingly, endoplasmic reticulum signal peptides from Leishmania and Crithidia fasciculata are reminiscent of signal peptides from Gram-positive bacteria.

Implications of Calreticulin Function in Parasite Biology

Calreticulin (CR) is a Ca(2+)-binding, multifunctional protein. The amazing array of CR-associated functions range from intracellular activities in secondary messenger release, protein folding and the modulation of gene expression to potential interactions with host receptors and signaling machinery and recognition by the host immune system. The multifunctional nature of CR may impact upon the ability of cells to recognize extracellular stimuli and coordinate appropriate responses. Identification of CR isolated from parasites and the conservation of its functions suggests that investigations into the contributions of CR to various aspects of parasite biology should be undertaken because it may reveal information regarding parasite interaction with the host and how the parasite may modulate its response to the host.

Cell activation mediated by glycosylphosphatidylinositol-anchored or transmembrane forms of CD14

CD14 is a glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein which functions as a receptor on myeloid cells for ligands derived from microbial pathogens such as lipopolysaccharide (LPS). We have studied the importance of the GPI tail of CD14 in signalling with the promonocytic cell line THP-1 expressing recombinant CD14 in a GPI-anchored form (THP1-wtCD14 cells) or in a transmembrane form (THP1-tmCD14). We found that, like other GPI-anchored molecules, GPI-anchored CD14 was recovered mainly from a Triton X-100-insoluble fraction, whereas transmembrane CD14 was fully soluble in Triton X-100. LPS induced cell activation of THP1-wtCD14 and of THP1-tmCD14 (protein tyrosine kinase phosphorylation, NF-kappaB activation, and cytokine production) in a very similar manner. However, anti-CD14 antibody-induced cross-linking caused a rapid calcium mobilization signal only in GPI-anchored CD14 cells. Studies with pharmacologic inhibitors of intracellular signalling events implicate phospholipase C and protein tyrosine kinases in the genesis of this antibody-induced calcium signal. Our results suggest that GPI anchoring and CD14 targeting to glycolipid-rich membrane microdomains are not required for LPS-mediated myeloid cell activation. GPI anchoring may however be important for other signalling functions, such as those events reflected by antibody cross-linking.

The first step of glycosylphosphatidylinositol biosynthesis is mediated by a complex of PIG-A, PIG-H, PIG-C and GPI1

Biosynthesis of glycosylphosphatidylinositol (GPI) is initiated by transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to phosphatidylinositol (PI). This chemically simple step is genetically complex because three genes are required in both mammals and yeast. Mammalian PIG-A and PIG-C are homologous to yeast GPI3 and GPI2, respectively; however, mammalian PIG-H is not homologous to yeast GPI1. Here, we report cloning of a human homolog of GPI1 (hGPI1) and demonstrate that four mammalian gene products form a protein complex in the endoplasmic reticulum membrane. PIG-L, which is involved in the second step in GPI synthesis, GlcNAc-PI de-N-acetylation, did not associate with the isolated complex. The protein complex had GPI-GlcNAc transferase (GPI-GnT) activity in vitro, but did not mediate the second reaction. Bovine PI was utilized approximately 100-fold more efficiently than soybean PI as a substrate, and lyso PI was a very inefficient substrate. These results suggest that GPI-GnT recognizes the fatty acyl chains of PI. The unusually complex organization of GPI-GnT may be relevant to selective usage of PI and/or regulation.

Glycosyl-phosphatidylinositol anchored membrane enzymes

Several mammalian enzymes are anchored to the outer surface of the plasma membrane by a covalently attached glycosyl-phosphatidylinositol (GPI) structure. These include acetylcholinesterase, alkaline phosphatase, membrane dipeptidase and 5'-nucleotidase. All GPI anchors determined to date have the conserved core structure ethanolamine-PO4-6Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcNH2 alpha 1-6myo-inositol-1-PO4- lipid. In most mammalian GPI anchors the lipid is 1-alkyl-2-acyl-glycerol, although in porcine membrane dipeptidase it is diacylglycerol. Attached to the conserved core are various side chain residues that appear to be either protein- or tissue-specific. In addition to membrane attachment, a GPI anchor may confer additional properties on the protein, such as the susceptibility to cleavage by phospholipases and the potential to cluster in detergent-insoluble domains. GPI anchors can also act as intracellular targeting signals, in transmembrane signalling, in the clathrin-independent endocytic process of potocytosis and as hormone mediators. Thus, a GPI anchor can confer additional properties on enzymes that may be important in their physiological and pathophysiological functioning.

Expression of intracellular and GPI-anchored forms of GPI-specific phospholipase D in COS-1 cells

Glycosylphosphatidylinositol (GPI)-specific phospholipase D (GPI-PLD) is a secretory protein present in high amounts in mammalian body fluids. Its cDNA has been isolated and encodes a signal peptide of 23 amino acids and the mature protein of 816 amino acids. We generated cDNAs encoding a signal peptide-deficient and a GPI-anchored form of GPI-PLD and transiently transfected these constructs into COS-1 cells. The signal peptide-deficient form of GPI-PLD was expressed as a 90-kDa protein that was catalytically active and was localized intracellularly. Cells transfected with cDNA encoding the GPI-anchored form of GPI-PLD expressed a catalytically active enzyme of 100 kDa that could be labelled with [3H]ethanolamine demonstrating its modification by a GPI structure. Expression of the GPI-anchored form of GPI-PLD resulted in the release of endogenous GPI-anchored alkaline phosphatase from COS-1 cells, whereas expression of the intracellular form of GPI-PLD had no effect on membrane attachment of endogenous alkaline phosphatase. Similarly, in cells cotransfected with GPI-anchored placental alkaline phosphatase (PLAP) and the GPI-anchored form of GPI-PLD, PLAP was released into the cell culture supernatant while expression of the signal peptide-deficient form of GPI-PLD did not affect the amount of cell-associated PLAP.

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.

Glycosylphosphatidylinositol anchors represent the major carbohydrate modification in proteins of intraerythrocytic stage Plasmodium falciparum

The nature and extent of carbohydrate modification in intraerythrocytic stage Plasmodium falciparum proteins have been controversial. This study describes the characterization of the carbohydrates in intraerythrocytic P. falciparum proteins and provides an overall picture of the nature of carbohydrate modification in the parasite proteins. P. falciparum strains were metabolically labeled with radioactive sugar precursors and ethanolamine at different developmental stages. The individual parasite proteins separated on SDS-polyacrylamide gels and whole parasite cell lysates were analyzed for the carbohydrate moieties. The results established the following: 1) glycosylphosphatidylinositol (GPI) anchors represent the major carbohydrate modification in the intraerythrocytic stage P. falciparum proteins; 2) in contrast to previous reports, O-linked carbohydrates are either absent or present only at very low levels in the parasite; and 3) P. falciparum contains low levels of N-glycosylation capability. The amount of N-linked carbohydrates in whole parasite proteins is approximately 6% compared with the GPI anchors attached to proteins based on radioactive GlcN incorporated into the proteins. The glycan cores of multiple parasite protein GPI anchors are all similar, consisting of protein-ethanolamine-phosphate-(Manalpha1-2)6Manalpha1-2M analpha1-6Ma nalpha1- 4GlcN. The fourth Man residues distal to GlcN of the GPI anchor glycan cores contain unidentified substituents that are susceptible to conditions of nitrous acid deamination. This unusual structural feature may contribute to the reported pathogenic properties of the P. falciparum GPI anchors.

Uptake and intracellular stability of glycosylphosphatidylinositol-specific phospholipase D in neuroblastoma cells

Glycosylphosphatidylinositol-specific phospholipase D from mammalian serum has been described to be relatively stable towards the action of proteases in vitro, and it has been speculated that the enzyme may only be active on glycosylphosphatidylinositol-anchored substrates after its proteolytic processing in an intracellular compartment following uptake from body fluids. To test this hypothesis, we studied the possible uptake and intracellular processing of purified glycosylphosphatidylinositol-specific phospholipase D into the mouse neuroblastoma cell line N2A. We found that after incubation of neuroblastoma cells with glycosylphosphatidylinositol-specific phospholipase D at 37 degrees C the amount of cell-associated glycosylphosphatidylinositol-specific phospholipase D activity increased in a concentration- and time-dependent way. A similar uptake was also observed with 125I-labeled intact and trypsin-treated form of glycosylphosphatidylinositol-specific phospholipase D. We found that the incorporated radiolabeled proteins were processed intracellularly to distinct low molecular mass products, and that this process was in part inhibited by the presence of chloroquine during incubation.

Cell surface dynamics of GPI-anchored proteins

Several cell surface eukaryotic proteins have a glycosylphosphoinositol lipid (GPI) modification at the carboxy-terminal end that serves as their sole means of membrane anchoring. In this report we review recent observations regarding the surface dynamics of GPI-anchored proteins. We discuss the association of GPI-anchored proteins with caveolae at the cell surface and their role in signal transduction as determined by the ability of GPI-anchored proteins to form detergent-insoluble complexes enriched in several cytoplasmic proteins including non-receptor type tyrosine kinases and caveolin/VIP-21, a component of the striated coat of caveolae. We have shown by immunofluorescence and electron microscopy that GPI-anchored proteins are not constitutively concentrated in caveolae but may be enriched in these structures only after cross-linking. While caveolae occupy only a small fraction of the cell surface (< 4%) almost all of the GPI-anchored protein at the cell surface becomes incorporated into detergent-insoluble low-density complexes, suggesting that these proteins are intrinsically detergent-insoluble in the milieu of the plasma membrane, and their co-purification with caveolin is not reflective of their native distribution. The finding that GPI-anchored proteins are not normally clustered over caveolae raised questions about the involvement of caveolae in the internalization of GPI-anchored proteins. In recent studies we have found that GPI-anchored proteins are internalized into bona fide endosomes wherein they appear to be sorted from bulk membrane components. The implications of these observations on the biology of GPI-anchored proteins are discussed.

Distribution and biophysical properties of fluorescent lipids on the surface of adult Schistosoma mansoni

The properties of 4 fluorescent lipid compounds in the surface membrane of adult male Schistosoma mansoni worms were examined by fluorescent microscopy and fluorescent recovery after photobleaching (FRAP). The data suggest that the probes N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s- indacene-3-pentanoyl) sphingosine (BODIPY FL ceramide) and PKH2 pass through the outer membrane and enter structures in or below the membrane. In contrast 5-(N-octadecanoyl)aminofluorescein (AF18) and N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s- indacene-3-pentanoyl) sphingosylphosphocholine (BODIPY FL sphingomyelin) insert into the outer monolayer. The DL values of these latter 2 compounds, 8:83 +/- 2.35 x 10(-9)cm(2)s(-1) and 2.76 +/- 0.53 x 10(-9)cm(2)s(-1), respectively, suggest that they enter different domains. Furthermore, it was observed that both BODIPY FL ceramide and BODIPY FL sphingomyelin entered particular structures in or under the surface membrane. The possible nature of these particles is discussed.

Phosphatidylinositol hydrolysis by Trypanosoma brucei glycosylphosphatidylinositol phospholipase C

Detergent-solubilized glycosylphosphatidylinositol (GPI)-anchored structures can be cleaved by C-type phospholipases isolated from peanuts and bloodstream cells of the African trypanosome, Trypanosoma brucei. The two enzymes differ in their reported ability to hydrolyze phosphatidylinositol (PI); while the peanut enzyme readily hydrolyzes PI in vitro, the T. brucei enzyme was reported to be virtually inactive against PI and consequently named GPI-specific phospholipase C (GPI-PLC). In this paper, we describe experiments in which we reinvestigated the substrate specificity of T. brucei GPI-PLC by incubating the purified enzyme with Triton X-100/PI-mixed micelles and by studying PI hydrolysis. We found that PI hydrolysis occurred in a detergent-dependent fashion over the range of concentrations tested (5 microM to 1 mM PI). At 5 microM PI, hydrolysis was maximal at 0.005% Triton X-100, whereas at 1 mM PI, maximal hydrolysis required 0.05% Triton X-100. Hydrolysis of both PI and GPI was strongly affected by the presence of phospholipids. Endogenous PI was hydrolyzed during osmotic and detergent lysis of trypanosomes under conditions used to obtain quantitative hydrolysis of the GPI-anchored trypanosome variant surface glycoprotein. PI hydrolysis in the lysates was inhibited by sodium p-chloromercuriphenylsulfonate but unaffected by EGTA, consistent with the proposal that hydrolysis is due to GPI-PLC. These results suggest that the function of T. brucei GPI-PLC may be to regulate PI as well as (or instead of) GPI levels.

Cloning and functional expression of glycosyltransferases from parasitic protozoans by heterologous complementation in yeast: the dolichol phosphate mannose synthase from Trypanosoma brucei brucei

The gene for the enzyme dolichol phosphate mannose (Dol-P-Man) synthase from the parasitic protozoan Trypanosoma brucei brucei (T. brucei) was cloned by screening a T. brucei cDNA library and then sequenced. The library was constructed in a yeast expression vector and the positive clone was identified by complementation of a temperature-sensitive defect in the yeast strain DPM 1-6 [Orlean, Albright and Robbins (1988) J. Biol. Chem. 263, 17499-17507]. The insert of this clone displayed an open reading frame of 801 nucleotides coding for a putative protein of 267 amino acids. The deduced protein sequence showed an identity of 49% and a similarity of 69% with the published yeast sequence. Additional features of the T. brucei sequence are the presence of a putative signal sequence, a C-terminal transmembrane domain, a consensus sequence for phosphorylation by cAMP-dependent protein kinase and a stretch of five nucleotides immediately upstream from the putative initiation codon that could function as a prokaryotic ribosome binding site. A consensus sequence for dolichol binding (FI/VXF/YXXIPFXF/Y) found in the yeast protein could not be detected in the putative transmembrane domain of the T. brucei sequence. Biochemical characterization of the recombinant protein showed that it is functionally expressed in the yeast strain DPM 1-6 and Escherichia coli. In both constructs Dol-P-Man synthesis was shown in a cell-free system. Synthesis was stimulated by exogenous dolichol phosphate and inhibited by amphomycin. These results confirm that we have cloned the T. brucei Dol-P-Man synthase by heterologous complementation in yeast, an approach that might be applicable for other glycosyltransferases from various sources.

Praziquantel: An urgent and exciting challenge

The anthelmintic drug praziquantel has proved useful in the treatment of schistosomiasis. The precise mechanism by which praziquantel kills the parasites has yet to be elucidated. Here, John Kusel and colleagues review the current theories on praziquantel action and suggest future avenues for research, which becomes urgent in the light of some reports of drug resistance.

The hydrophobic mannoside Man alpha 1-6Man alpha 1-S-(CH2)7-CH3 acts as an acceptor for the UDP-Gal:glycosylphosphatidylinositol anchor alpha 1,3-galactosyltransferase of Trypanosoma brucei

The variant surface glycoproteins (VSGs) of Trypanosoma brucei are attached to the plasma membrane via a glycosylphosphatidylinositol (GPI) membrane anchor. This anchor contains the core sequence ethanolamine-PO4-6Man alpha 1-2Man alpha 1-6Man alpha 1-4GlcN alpha 1-6myo-inositol, which is conserved in all GPI anchors, and a unique alpha Gal side chain attached to the 3-position of the alpha Man residue adjacent to the alpha GlcN residue. Here we report that trypanosome membranes can catalyse the transfer of Gal from UDP-Gal to the hydrophobic thioglycoside Man alpha 1-6Man alpha 1-S-(CH2)7-CH3. Characterization of the galactosylated products by electrospray mass spectrometry, exoglycosidase digestion and periodate-oxidation studies revealed that the major product was Man alpha 1-6(Gal alpha 1-3)Man alpha 1-S-(CH2)7-CH3. The similarity of this product to part of the mature VSG GPI anchor suggests that the thioglycoside is able to act as an acceptor for the trypanosome-specific UDP-Gal-GPI anchor alpha 1,3-galactosyltransferase.

Insolubility and redistribution of GPI-anchored proteins at the cell surface after detergent treatment

A diverse set of cell surface eukaryotic proteins including receptors, enzymes, and adhesion molecules have a glycosylphosphoinositol-lipid (GPI) modification at the carboxy-terminal end that serves as their sole means of membrane anchoring. These GPI-anchored proteins are poorly solubilized in nonionic detergent such as Triton X-100. In addition these detergent-insoluble complexes from plasma membranes are significantly enriched in several cytoplasmic proteins including nonreceptor-type tyrosine kinases and caveolin/VIP-21, a component of the striated coat of caveolae. These observations have suggested that the detergent-insoluble complexes represent purified caveolar membrane preparations. However, we have recently shown by immunofluorescence and electron microscopy that GPI-anchored proteins are diffusely distributed at the cell surface but may be enriched in caveolae only after cross-linking. Although caveolae occupy only a small fraction of the cell surface (< 4%), almost all of the GPI-anchored protein at the cell surface becomes incorporated into detergent-insoluble low-density complexes. In this paper we show that upon detergent treatment the GPI-anchored proteins are redistributed into a significantly more clustered distribution in the remaining membranous structures. These results show that GPI-anchored proteins are intrinsically detergent-insoluble in the milieu of the plasma membrane, and their co-purification with caveolin is not reflective of their native distribution. These results also indicate that the association of caveolae, GPI-anchored proteins, and signalling proteins must be critically re-examined.

The evolutionary expansion of the trypanosomatid flagellates

The trypanosomatids combine a relatively uniform morphology with ability to parasitise a very diverse range of hosts including animals, plants and other protists. Along with their sister family, the biflagellate bodonids, they are set apart from other eukaryotes by distinctive organisational features, such as the kinetoplast-mitochondrion and RNA editing, isolation of glycolysis enzymes in the glycosome, use of the flagellar pocket for molecular traffic into and out of the cell, a unique method of generating cortical microtubules, and bizarre nuclear organisation. These features testify to the antiquity and isolation of the kinetoplast-bearing flagellates (Kinetoplastida). Molecular sequencing techniques (especially small subunit ribosomal RNA gene sequencing) are now radically reshaping previous ideas on the phylogeny of these organisms. The idea that the monogenetic (MG) trypanosomatids gave rise to the digenetic (DG) genera is losing ground to a view that, after the bodonids, the African trypanosomes (DG) represent the most ancient lineage, followed by Trypanosoma cruzi (DG), then Blastocrithidia (MG), Herpetomonas (MG) and Phytomonas (DG), with Leptomonas (MG), Crithidia (MG), Leishmania (DG) and Endotrypanum (DG) forming the crown of the evolutionary tree. Vast genetic distances (12% divergence) separate T. brucei and T. cruzi, while the Leishmania species are separated by very short distances (less than 1% divergence). These phylogenetic conclusions are supported by studies on RNA editing and on the nature of the parasite surface. The trypanosomatids seem to be able to adapt with ease their energy metabolism to the availability of substrates and oxygen, and this may give them the ability to institute new life cycles if host behaviour patterns allow. Sexual processes, though present in at least some trypanosomatids, may have played only a minor part in generating diversity during trypanosomatid evolution. On the other hand, the development of altruistic behaviour on the part of some life cycle stages may be a hitherto unconsidered way of maximising fitness in this group. It is concluded that, owing to organisational constraints, the trypanosomatids can undergo substantial molecular variation while registering very little in the way of morphological change.

Sequestration of GPI-anchored proteins in caveolae triggered by cross-linking

Glycosyl-phosphatidylinositol (GPI)-anchored proteins have been reported to reside in clusters collected over small membrane invaginations called caveolae. The detection of different GPI-anchored proteins with fluorescently labeled monoclonal antibodies showed that these proteins are not constitutively concentrated in caveolae; they enter these structures independently after cross-linking with polyclonal secondary antibodies. Analysis of the cell surface distribution of the GPI-anchored folate receptor by electron microscopy confirms these observations. Thus, multimerization of GPI-anchored proteins regulates their sequestration in caveolae, but in the absence of agents that promote clustering they are diffusely distributed over the plasma membrane.