The protozoan Trypanosoma cruzi has a family of genes resembling the mucin genes of mammalian cells

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

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

An Evolutionary View of Trypanosoma Cruzi Telomeres

Like in most eukaryotes, the linear chromosomes of Trypanosoma cruzi end in a nucleoprotein structure called the telomere, which is preceded by regions of variable length called subtelomeres. Together telomeres and subtelomeres are dynamic sites where DNA sequence rearrangements can occur without compromising essential interstitial genes or chromosomal synteny. Good examples of subtelomeres involvement are the expansion of human olfactory receptors genes, variant surface antigens in Trypanosoma brucei, and Saccharomyces cerevisiae mating types. T. cruzi telomeres are made of long stretches of the hexameric repeat 5′-TTAGGG-OH-3′, and its subtelomeres are enriched in genes and pseudogenes from the large gene families RHS, TS and DGF1, DEAD/H-RNA helicase and N-acetyltransferase, intermingled with sequences of retrotransposons elements. In particular, members of the Trans-sialidase type II family appear to have played a role in shaping the current T. cruzi telomere structure. Although the structure and function of T. cruzi telomeric and subtelomeric regions have been documented, recent experiments are providing new insights into T. cruzi's telomere-subtelomere dynamics. In this review, I discuss the co-evolution of telomere, subtelomeres and the TS gene family, and the role that these regions may have played in shaping T. cruzi's genome.

Theft and Reception of Host Cell's Sialic Acid: Dynamics of Trypanosoma Cruzi Trans-sialidases and Mucin-Like Molecules on Chagas' Disease Immunomodulation

The last decades have produced a plethora of evidence on the role of glycans, from cell adhesion to signaling pathways. Much of that information pertains to their role on the immune system and their importance on the surface of many human pathogens. A clear example of this is the flagellated protozoan Trypanosoma cruzi, which displays on its surface a great variety of glycoconjugates, including O-glycosylated mucin-like glycoproteins, as well as multiple glycan-binding proteins belonging to the trans-sialidase (TS) family. Among the latter, different and concurrently expressed molecules may present or not TS activity, and are accordingly known as active (aTS) and inactive (iTS) members. Over the last thirty years, it has been well described that T. cruzi is unable to synthesize sialic acid (SIA) on its own, making use of aTS to steal the host's SIA. Although iTS did not show enzymatic activity, it retains a substrate specificity similar to aTS (α-2,3 SIA-containing glycotopes), displaying lectinic properties. It is accepted that aTS members act as virulence factors in mammals coursing the acute phase of the T. cruzi infection. However, recent findings have demonstrated that iTS may also play a pathogenic role during T. cruzi infection, since it modulates events related to adhesion and invasion of the parasite into the host cells. Since both aTS and iTS proteins share structural substrate specificity, it might be plausible to speculate that iTS proteins are able to assuage and/or attenuate biological phenomena depending on the catalytic activity displayed by aTS members. Since SIA-containing glycotopes modulate the host immune system, it should not come as any surprise that changes in the sialylation of parasite's mucin-like molecules, as well as host cell glycoconjugates might disrupt critical physiological events, such as the building of effective immune responses. This review aims to discuss the importance of mucin-like glycoproteins and both aTS and iTS for T. cruzi biology, as well as to present a snapshot of how disturbances in both parasite and host cell sialoglycophenotypes may facilitate the persistence of T. cruzi in the infected mammalian host.

Structure, Function and Gene Expression of Epithelial Mucins

In this review the main characteristics, i.e., structure, function and gene expression, of the different mucins are discussed. Mucin-type molecules consist of a core protein moiety (apomucin) where a number of carbohydrate chains are attached to serines and threonines by glycosidic bonds. O-linked carbohydrates form up to 80% of the molecule and the length of the glucidic side chains varies from one to more than 20 residues. At least eight mucin-like genes have been isolated so far, and the main characteristic is the presence of a central domain composed of a variable number of “tandem repeats”. The sequence homology of the central domain among the different members of the mucin-type family is limited, indicating that this internal domain is unique for each mucin. Thanks to the integrated results of genetic, immunological and biochemical studies, it is now possible to identify eight apomucin genes, namely MUC1, MUC2, MUC3, MUC4, MUC5AC, MUC5B, MUC6 and MUC7. MUC1 is the best characterized mucin and it is expressed on the apical surface of most polarized epithelial cells. The MUC1 gene has been cloned and sequenced. The MUC2 gene encodes a typical secretory gel-forming mucin which represents the predominant form in human intestinal and colon tissues. Another intestinal mucin is MUC3. The MUC4, MUC5AC and MUC5B genes have been isolated from a bronchial tissue cDNA library. The MUC4 and MUC5AC genes are mainly expressed in the respiratory tract, in gastric and reproductive mucosa, while MUC5B is highly detectable only in the bronchial glands. The MUC6 gene is expressed by gastric tissue and, recently, MUC7 has been cloned and sequenced using a salivary cDNA library.

A Brief View of the Surface Membrane Proteins from Trypanosoma cruzi

Trypanosoma cruzi is the causal agent of Chagas' disease which affects millions of people around the world mostly in Central and South America. T. cruzi expresses a wide variety of proteins on its surface membrane which has an important role in the biology of these parasites. Surface molecules of the parasites are the result of the environment to which the parasites are exposed during their life cycle. Hence, T. cruzi displays several modifications when they move from one host to another. Due to the complexity of this parasite's cell surface, this review presents some membrane proteins organized as large families, as they are the most abundant and/or relevant throughout the T. cruzi membrane.

Evasion and Immuno-Endocrine Regulation in Parasite Infection: Two Sides of the Same Coin in Chagas Disease?

Chagas disease is a serious illness caused by the protozoan parasite Trypanosoma cruzi. Nearly 30% of chronically infected people develop cardiac, digestive, or mixed alterations, suggesting a broad range of host-parasite interactions that finally impact upon chronic disease outcome. The ability of T. cruzi to persist and cause pathology seems to depend on diverse factors like T. cruzi strains, the infective load and the route of infection, presence of virulence factors, the parasite capacity to avoid protective immune response, the strength and type of host defense mechanisms and the genetic background of the host. The host-parasite interaction is subject to a constant neuro-endocrine regulation that is thought to influence the adaptive immune system, and as the infection proceeds it can lead to a broad range of outcomes, ranging from pathogen elimination to its continued persistence in the host. In this context, T. cruzi evasion strategies and host defense mechanisms can be envisioned as two sides of the same coin, influencing parasite persistence and different outcomes observed in Chagas disease. Understanding how T. cruzi evade host's innate and adaptive immune response will provide important clues to better dissect mechanisms underlying the pathophysiology of Chagas disease.

Immune Evasion Strategies of Trypanosoma cruzi

Microbes have evolved a diverse range of strategies to subvert the host immune system. The protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease, provides a good example of such adaptations. This parasite targets a broad spectrum of host tissues including both peripheral and central lymphoid tissues. Rapid colonization of the host gives rise to a systemic acute response which the parasite must overcome. The parasite in fact undermines both innate and adaptive immunity. It interferes with the antigen presenting function of dendritic cells via an action on host sialic acid-binding Ig-like lectin receptors. These receptors also induce suppression of CD4(+) T cells responses, and we presented evidence that the sialylation of parasite-derived mucins is required for the inhibitory effects on CD4 T cells. In this review we highlight the major mechanisms used by Trypanosoma cruzi to overcome host immunity and discuss the role of parasite colonization of the central thymic lymphoid tissue in chronic disease.

The Role of Sialic Acid-Binding Receptors (Siglecs) in the Immunomodulatory Effects of Trypanosoma cruzi Sialoglycoproteins on the Protective Immunity of the Host

Chagas disease is caused by the protozoan parasite Trypanosoma cruzi and is an important endemic infection in Latin America. Lately, it has also become a health concern in the United States and Europe. Most of the immunomodulatory mechanisms associated with this parasitic infection have been attributed to mucin-like molecules on the T. cruzi surface. Mucins are high molecular weight glycoproteins that are involved in regulating diverse cellular activities in both normal and pathological conditions. In Trypanosoma cruzi infection, the parasite-derived mucins are the main acceptors of sialic acid and it has been suggested that they play a role in various host-parasite interactions during the course of Chagas disease. Recently, we have presented evidence that sialylation of the mucins is required for the inhibitory effects on CD4(+) T cells. In what follows we propose that signaling via sialic acid-binding Ig-like lectin receptors for these highly sialylated structures on host cells contributes to the arrest of cell cycle progression in the G1 phase and may allow the parasite to modulate the immune system of the host.

Inhibitory effects of Trypanosoma cruzi sialoglycoproteins on CD4+ T cells are associated with increased susceptibility to infection

Background

The Trypanosoma cruzi infection is associated with severe T cell unresponsiveness to antigens and mitogens characterized by decreased IL-2 synthesis. Trypanosoma cruzi mucin (Tc Muc) has been implicated in this phenomenom. These molecules contain a unique type of glycosylation consisting of several sialylated O-glycans linked to the protein backbone via N-acetylglucosamine residues.

Methodology/Principal Findings

In this study, we evaluated the ability of Tc Muc to modulate the activation of CD4⁺ T cells. Our data show that cross-linking of CD3 on naïve CD4⁺ T cells in the presence of Tc Muc resulted in the inhibition of both cytokine secretion and proliferation. We further show that the sialylated O-Linked Glycan residues from tc mucin potentiate the suppression of T cell response by inducing G1-phase cell cycle arrest associated with upregulation of mitogen inhibitor p27^kip1. These inhibitory effects cannot be reversed by the addition of exogenous IL-2, rendering CD4⁺ T cells anergic when activated by TCR triggering. Additionally, in vivo administration of Tc Muc during T. cruzi infection enhanced parasitemia and aggravated heart damage. Analysis of recall responses during infection showed lower frequencies of IFN-γ producing CD4⁺ T cells in the spleen of Tc Muc treated mice, compared to untreated controls.

Conclusions/Significance

Our results indicate that Tc Muc mediates inhibitory efects on CD4⁺ T expansion and cytokine production, by blocking cell cycle progression in the G1 phase. We propose that the sialyl motif of Tc Muc is able to interact with sialic acid-binding Ig-like lectins (Siglecs) on CD4⁺ T cells, which may allow the parasite to modulate the immune system.

Trypanosoma cruzi: Entry into Mammalian Host Cells and Parasitophorous Vacuole Formation

Trypanosoma cruzi, the causative agent of Chagas disease, is transmitted to vertebrate hosts by blood-sucking insects. This protozoan is an obligate intracellular parasite. The infective forms of the parasite are the metacyclic trypomastigotes, amastigotes, and bloodstream trypomastigotes. The recognition between the parasite and mammalian host cell, involves numerous molecules present in both cell types, and similar to several intracellular pathogens, T. cruzi is internalized by host cells via multiple endocytic pathways. Morphological studies demonstrated that after the interaction of the infective forms of T. cruzi with phagocytic or non-phagocytic cell types, plasma membrane (PM) protrusions can form, showing similarity with those observed during canonical phagocytosis or macropinocytic events. Additionally, several molecules known to be molecular markers of membrane rafts, macropinocytosis, and phagocytosis have been demonstrated to be present at the invasion site. These events may or may not depend on the host cell lysosomes and cytoskeleton. In addition, after penetration, components of the host endosomal-lysosomal system, such as early endosomes, late endosomes, and lysosomes, participate in the formation of the nascent parasitophorous vacuole (PV). Dynamin, a molecule involved in vesicle formation, has been shown to be involved in the PV release from the host cell PM. This review focuses on the multiple pathways that T. cruzi can use to enter the host cells until complete PV formation. We will describe different endocytic processes, such as phagocytosis, macropinocytosis, and endocytosis using membrane microdomains and clathrin-dependent endocytosis and show results that are consistent with their use by this smart parasite. We will also discuss others mechanisms that have been described, such as active penetration and the process that takes advantage of cell membrane wound repair.

Structural features affecting trafficking, processing, and secretion of Trypanosoma cruzi mucins

Trypanosoma cruzi is wrapped by a dense coat of mucin-type molecules encoded by complex gene families termed TcSMUG and TcMUC, which are expressed in the insect- and mammal-dwelling forms of the parasite, respectively. Here, we dissect the contribution of distinct post-translational modifications on the trafficking of these glycoconjugates. In vivo tracing and characterization of tagged-variants expressed by transfected epimastigotes indicate that although the N-terminal signal peptide is responsible for targeting TcSMUG products to the endoplasmic reticulum (ER), the glycosyl phosphatidylinositol (GPI)-anchor likely functions as a forward transport signal for their timely progression along the secretory pathway. GPI-minus variants accumulate in the ER, with only a minor fraction being ultimately released to the medium as anchorless products. Secreted products, but not ER-accumulated ones, display several diagnostic features of mature mucin-type molecules including extensive O-type glycosylation, Galf-based epitopes recognized by monoclonal antibodies, and terminal Galp residues that become readily sialylated upon addition of parasite trans-sialidases. Processing of N-glycosylation site(s) is dispensable for the overall TcSMUG mucin-type maturation and secretion. Despite undergoing different O-glycosylation elaboration, TcMUC reporters yielded quite similar results, thus indicating that (i) molecular trafficking signals are structurally and functionally conserved between mucin families, and (ii) TcMUC and TcSMUG products are recognized and processed by a distinct repertoire of stage-specific glycosyltransferases. Thus, using the fidelity of a homologous expression system, we have defined some biosynthetic aspects of T. cruzi mucins, key molecules involved in parasite protection and virulence.

Trypanosomatid comparative genomics: Contributions to the study of parasite biology and different parasitic diseases

In 2005, draft sequences of the genomes of Trypanosoma brucei, Trypanosoma cruzi and Leishmania major, also known as the Tri-Tryp genomes, were published. These protozoan parasites are the causative agents of three distinct insect-borne diseases, namely sleeping sickness, Chagas disease and leishmaniasis, all with a worldwide distribution. Despite the large estimated evolutionary distance among them, a conserved core of ~6,200 trypanosomatid genes was found among the Tri-Tryp genomes. Extensive analysis of these genomic sequences has greatly increased our understanding of the biology of these parasites and their host-parasite interactions. In this article, we review the recent advances in the comparative genomics of these three species. This analysis also includes data on additional sequences derived from other trypanosmatid species, as well as recent data on gene expression and functional genomics. In addition to facilitating the identification of key parasite molecules that may provide a better understanding of these complex diseases, genome studies offer a rich source of new information that can be used to define potential new drug targets and vaccine candidates for controlling these parasitic infections.

The genome and its implications

Trypanosoma cruzi has a heterogeneous population composed of a pool of strains that circulate in the domestic and sylvatic cycles. Genome sequencing of the clone CL Brener revealed a highly repetitive genome of about 110Mb containing an estimated 22,570 genes. Because of its hybrid nature, sequences representing the two haplotypes have been generated. In addition, a repeat content close to 50% made the assembly of the estimated 41 pairs of chromosomes quite challenging. Similar to other trypanosomatids, the organization of T. cruzi chromosomes was found to be very peculiar, with protein-coding genes organized in long polycistronic transcription units encoding 20 or more proteins in one strand separated by strand switch regions. Another remarkable feature of the T. cruzi genome is the massive expansion of surface protein gene families. Because of the high genetic diversity of the T. cruzi population, sequencing of additional strains and comparative genomic and transcriptome analyses are in progress. Five years after its publication, the genome data have proven to be an essential tool for the study of T. cruzi and increasing efforts to translate this knowledge into the development of new modes of intervention to control Chagas disease are underway.

Review on Trypanosoma cruzi: Host Cell Interaction

Trypanosoma cruzi, the causative agent of Chagas' disease, which affects a large number of individuals in Central and South America, is transmitted to vertebrate hosts by blood-sucking insects. This protozoan is an obligate intracellular parasite. The infective forms of the parasite are metacyclic and bloodstream trypomastigote and amastigote. Metacyclic trypomastigotes are released with the feces of the insect while amastigotes and bloodstream trypomastigotes are released from the infected host cells of the vertebrate host after a complex intracellular life cycle. The recognition between parasite and mammalian host cell involves numerous molecules present in both cell types. Here, we present a brief review of the interaction between Trypanosoma cruzi and its host cells, mainly emphasizing the mechanisms and molecules that participate in the T. cruzi invasion process of the mammalian cells.

Genomic organization and expression profile of the mucin-associated surface protein (masp) family of the human pathogen Trypanosoma cruzi

A novel large multigene family was recently identified in the human pathogen Trypanosoma cruzi, causative agent of Chagas disease, and corresponds to ∼6% of the parasite diploid genome. The predicted gene products, mucin-associated surface proteins (MASPs), are characterized by highly conserved N- and C-terminal domains and a strikingly variable and repetitive central region. We report here an analysis of the genomic organization and expression profile of masp genes. Masps are not randomly distributed throughout the genome but instead are clustered with genes encoding mucin and other surface protein families. Masp transcripts vary in size, are preferentially expressed during the trypomastigote stage and contain highly conserved 5′ and 3′ untranslated regions. A sequence analysis of a trypomastigote cDNA library reveals the expression of multiple masp variants with a bias towards a particular masp subgroup. Immunofluorescence assays using antibodies generated against a MASP peptide reveals that the expression of particular MASPs at the cell membrane is limited to subsets of the parasite population. Western blots of phosphatidylinositol-specific phospholipase C (PI-PLC)-treated parasites suggest that MASP may be GPI-anchored and shed into the medium culture, thus contributing to the large repertoire of parasite polypeptides that are exposed to the host immune system.

Perspectives on the Trypanosoma cruzi-host cell receptor interactions

Chagas disease is caused by the parasite Trypanosoma cruzi. The critical initial event is the interaction of the trypomastigote form of the parasite with host receptors. This review highlights recent observations concerning these interactions. Some of the key receptors considered are those for thromboxane, bradykinin, and for the nerve growth factor TrKA. Other important receptors such as galectin-3, thrombospondin, and laminin are also discussed. Investigation into the molecular biology and cell biology of host receptors for T. cruzi may provide novel therapeutic targets.

Trypanosoma cruzi surface mucins: host-dependent coat diversity

The surface of the protozoan parasite Trypanosoma cruzi is covered in mucins, which contribute to parasite protection and to the establishment of a persistent infection. Their importance is highlighted by the fact that the approximately 850 mucin-encoding genes comprise approximately 1% of the parasite genome and approximately 6% of all predicted T. cruzi genes. The coordinate expression of a large repertoire of mucins containing variable regions in the mammal-dwelling stages of the T. cruzi life cycle suggests a possible strategy to thwart the host immune response. Here, we discuss the expression profiling of T. cruzi mucins, the mechanisms leading to the acquisition of mucin diversity and the possible consequences of a mosaic surface coat in the interplay between parasite and host.

Immunocharacterization of the mucin-type proteins from the intracellular stage of Trypanosoma cruzi

The surface of Trypanosoma cruzi is covered by different groups of mucins that are differentially expressed during the parasite life cycle. We have previously identified the major mucins from the bloodstream trypomastigote stage. Here, we present additional evidence that together with our previous observations allows for the identification of a second mucin group also expressed in the mammal-dwelling stages, but predominant in the intracellular amastigote. These mucins are encoded by many genes, are mostly composed of tandem repeats and are highly conserved except for an exposed hypervariable (HV) N-terminal peptide. Antibodies against HV-peptides are restricted to approximately 50% of the chronically infected human population, are monospecific (i.e. directed towards a single HV), and display low-avidity. In contrast, immunization with a single HV-peptide triggers high-avidity, cross-reacting humoral responses against multiple HV sequences, but not against other T. cruzi surface antigens. The diversity present in the HV regions and the characteristics of the antibody response against them suggest a role of these molecules in eluding and/or modulating the mammalian host immune system.

A mucin like gene different from the previously reported members of the mucin like gene families is transcribed in Trypanosoma cruzi but not in Trypanosoma rangeli

Trypanosoma cruzi expresses mucin like glycoproteins encoded by a complex multigene family. In this work, we report the transcription in T. cruzi but not in T. rangeli of a mucin type gene automatically annotated by the T. cruzi genome project. The gene showed no nucleotide similarities with the previously reported T. cruzi mucin like genes, although the computational analysis of the deduced protein showed that it has the characteristic features of mucins: a signal peptide sequence, O-glycosylation sites, and glycosylphosphatidylinositol (GPI) anchor sequence. The presence in this gene of N-terminal and C-terminal coding sequences common to other annotated mucin like genes suggests the existence of a new mucin like gene family.

Utility of the Trypanosoma cruzi sequence database for identification of potential vaccine candidates by in silico and in vitro screening

Glycosylphosphatidylinositol (GPI)-anchored proteins are abundantly expressed in the infective and intracellular stages of Trypanosoma cruzi and are recognized as antigenic targets by both the humoral and cellular arms of the immune system. Previously, we demonstrated the efficacy of genes encoding GPI-anchored proteins in eliciting partially protective immunity to T. cruzi infection and disease, suggesting their utility as vaccine candidates. For the identification of additional vaccine targets, in this study we screened the T. cruzi expressed sequence tag (EST) and genomic sequence survey (GSS) databases. By applying a variety of web-based genome-mining tools to the analysis of approximately 2,500 sequences, we identified 348 (37.6%) EST and 260 (17.4%) GSS sequences encoding novel parasite-specific proteins. Of these, 19 sequences exhibited the characteristics of secreted and/or membrane-associated GPI proteins. Eight of the selected sequences were amplified to obtain genes TcG1, TcG2, TcG3, TcG4, TcG5, TcG6, TcG7, and TcG8 (TcG1-TcG8) which are expressed in different developmental stages of the parasite and conserved in the genome of a variety of T. cruzi strains. Flow cytometry confirmed the expression of the antigens encoded by the cloned genes as surface proteins in trypomastigote and/or amastigote stages of T. cruzi. When delivered as a DNA vaccine, genes TcG1-TcG6 elicited a parasite-specific antibody response in mice. Except for TcG5, antisera to genes TcG1-TcG6 exhibited trypanolytic activity against the trypomastigote forms of T. cruzi, a property known to correlate with the immune control of T. cruzi. Taken together, our results validate the applicability of bioinformatics in genome mining, resulting in the identification of T. cruzi membrane-associated proteins that are potential vaccine candidates.

The Surface Coat of the Mammal-dwelling Infective Trypomastigote Stage of Trypanosoma cruzi Is Formed by Highly Diverse Immunogenic Mucins

A thick coat of mucin-like glycoproteins covers the surface of Trypanosoma cruzi and plays a crucial role in parasite protection and infectivity and host immunomodulation. The appealing candidate genes coding for the mucins of the mammal-dwelling stages define a heterogeneous family termed TcMUC, which comprises up to 700 members, thus precluding a genetic approach to address the protein core identity. Here, we demonstrate by multiple approaches that the TcMUC II genes code for the majority of trypomastigote mucins. These molecules display a variable, non-repetitive, highly O-glycosylated central domain, followed by a short conserved C terminus and a glycosylphosphatidylinositol anchor. A simultaneous expression of multiple TcMUC II gene products was observed. Moreover, the C terminus of TcMUC II mucins, but not their central domain, elicited strong antibody responses in patients with Chagas' disease and T. crusi infected animals. This highly diverse coat of mucins may represent a refined parasite strategy to elude the mammalian host immune system.

Structural characterization of chromosome I size variants from a natural yeast strain

Many yeast strains isolated from the wild show karyotype instability during vegetative growth, with rearrangement rates of up to 10(-2) chromosomal changes per generation. Physical isolation and analysis of several chromosome I size variants of one of these strains revealed that they differed only in their subtelomeric regions, leaving the central 150 Kb unaltered. Fine mapping of these subtelomeric variable regions revealed gross alterations of two very similar loci, FLO1 and FLO9. These loci are located on the right and left arms, respectively, of chromosome I and encompass internal repetitive DNA sequences. Furthermore, some chromosome I variants lacking the FLO1 locus showed evidence of recombination at a DNA region on their right arm that is enriched in repeated sequences, including Ty LTRs. We propose that repetitive sequences in many subtelomeric regions in S. cerevisiae play a key role in karyotype hypervariability. As these regions encode several membrane-associated proteins, subtelomeric plasticity may allow rapid adaptive changes of the yeast strain to specific substrates. This pattern of semi-conservative chromosomal rearrangement may have profound implications, both in terms of evolution of wild strains and for biotechnological processes.

Cloning and expression analysis of two mucin-like genes encoding microfilarial sheath surface proteins of the parasitic nematodes Brugia and Litomosoides

In several filarial genera the first stage larvae (microfilariae) are enclosed by an eggshell-derived sheath that provides a major interface between the parasite and the host immune system. Analysis of the polypeptide constituents of the microfilarial sheath from the cotton rat filaria Litomosoides sigmodontis identified two abundant surface glycoproteins: Shp3a and Shp3. The corresponding genes and the orthologues of the human parasite Brugia malayi and the rodent filaria Brugia pahangi were cloned and sequenced. They encode secreted, mucin-like proteins with N-terminal Ser/Thr-rich repeats and a C-terminal anchor domain rich in aromatic amino acids. About 75% of the protein molecular masses result from post-translational modifications. The Ser/Thr-rich motifs are supposed to serve as targets for dimethylaminoethanol-phosphate substitutions. These modifications were detected only on the sheaths of the late developmental stage of stretched microfilariae, corresponding with the expression of the proteins in the epithelium of the distal part of the uterus and the specific transcription of shp3 and shp3a in the anterior female worm segment. Genomic analysis of all three species demonstrated a conserved linkage of the two genes. Their transcripts undergo cis- and trans-splicing. The transcription start sites of the primary transcripts were determined for the L. sigmodontis genes. The core promoter regions are remarkably conserved between the paralogue genes Ls-shp3a and Ls-shp3 and their orthologues in Brugia, implicating conserved regulatory elements.

Gene discovery in the freshwater fish parasite Trypanosoma carassii: identification of trans-sialidase-like and mucin-like genes

A total of 1,921 expressed sequence tags (ESTs) were obtained from bloodstream trypomastigotes of Trypanosoma carassii, a parasite of economic importance due to its high prevalence in fish farms. Analysis of the data set allowed us to identify a trans-sialidase (TS)-like gene and three ESTs coding for putative mucin-like genes. TS activity was detected in cell extracts of bloodstream trypomastigotes. We have also used the sequence information obtained to identify genes that have not been previously described in trypanosomatids. (Additional information on these ESTs can be found at http://genoma.unsam.edu.ar/projects/tca.)

A Trypanosoma cruzi small surface molecule provides the first immunological evidence that Chagas' disease is due to a single parasite lineage

Chagas' disease is a major health and economic problem caused by the protozoan Trypanosoma cruzi. Multiple independently evolving clones define a complex parasite population that can be arranged into two broad genetic lineages termed T. cruzi I and II. These lineages have different evolutionary origin and display distinct ecological and biological traits. Here we describe a novel molecule termed TSSA for trypomastigote small surface antigen that provides the first immunological marker allowing discrimination between lineages. TSSA is a surface, glycosylphosphatidyl inositol (GPI)-anchored mucin-like protein, highly antigenic during the infection. TSSA sequences from different parasite isolates reveal a population dimorphism that perfectly matches with the two T. cruzi lineages. Interestingly, this dimorphism is restricted to the central region of the molecule, which comprises the immunodominant B cell epitopes. This sequence variability has a major impact on TSSA antigenicity, leading to no immunological cross-reactivity between both isoforms for antibodies present either in immunization or infection sera. Furthermore, the absolute seroprevalence for TSSA in confirmed Chagasic patients is restricted to T. cruzi II isoform, strongly suggesting that human infections are due to this particular subgroup. Even though association of T. cruzi II with Chagas' disease has been proposed based on molecular markers, this is the first immunological evidence supporting this hypothesis. The implications of these results for the future research on Chagas' disease could be envisaged.

Evolution and classification of cystine knot-containing hormones and related extracellular signaling molecules

The cystine knot three-dimensional structure is found in many extracellular molecules and is conserved among divergent species. The identification of proteins with a cystine knot structure is difficult by commonly used pairwise alignments because the sequence homology among these proteins is low. Taking advantage of complete genome sequences in diverse organisms, we used a complementary approach of pattern searches and pairwise alignments to screen the predicted protein sequences of five model species (human, fly, worm, slime mold, and yeast) and retrieved proteins with low sequence homology but containing a typical cystine knot signature. Sequence comparison between proteins known to have a cystine knot three-dimensional structure (transforming growth factor-beta, glycoprotein hormone, and platelet-derived growth factor subfamily members) identified new crucial amino acid residues (two hydrophilic amino acid residues flanking cysteine 5 of the cystine knot). In addition to the well known members of the cystine knot superfamily, novel subfamilies of proteins (mucins, norrie disease protein, von Willebrand factor, bone morphogenetic protein antagonists, and slit-like proteins) were identified as putative cystine knot-containing proteins. Phylogenetic analysis revealed the ancient evolution of these proteins and the relationship between hormones [e.g. transforming growth factor-beta (TGFbeta)] and extracellular matrix proteins (e.g. mucins). They are absent in the unicellular yeast genome but present in nematode, fly, and higher species, indicating that the cystine knot structure evolved in extracellular signaling molecules of multicellular organisms. All data retrieved by this study can be viewed at http://hormone.stanford.edu/.

Trypanosoma cruzi surface mucins with exposed variant epitopes

The protozoan parasite Trypanosoma cruzi, the agent of Chagas disease, has a large number of mucin molecules on its surface, whose expression is regulated during the life cycle. These mucins are the main acceptors of sialic acid, a monosaccharide that is required by the parasite to infect and survive in the mammalian host. A large mucin-like gene family named TcMUC containing about 500 members has been identified previously in T. cruzi. TcMUC can be divided into two subfamilies according to the presence or absence of tandem repeats in the central region of the genes. In this work, T. cruzi parasites were transfected with one tagged member of each subfamily. Only the product from the gene with repeats was highly O-glycosylated in vivo. The O-linked oligosaccharides consisted mainly of beta-d-Galp(1-->4)GlcNAc and beta-d-Galp(1-->4)[beta-d-Galp(1-->6)]-d-GlcNAc. The same glycosyl moieties were found in endogenous mucins. The mature product was anchored by glycosylphosphatidylinositol to the plasma membrane and exposed to the medium. Sera from infected mice recognized the recombinant product of one repeats-containing gene thus showing that they are expressed during the infection. TcMUC genes encode a hypervariable region at the N terminus. We now show that the hypervariable region is indeed present in the exposed mature N termini of the mucins because sera from infected hosts recognized peptides having sequences from this region. The results are discussed in comparison with the mucins from the insect stages of the parasite (Di Noia, J. M., D'Orso, I., Sánchez, D. O., and Frasch, A. C. C. (2000) J. Biol. Chem. 275, 10218-10227) which do not have variable regions.

Mucins and molluscan calcification. Molecular characterization of mucoperlin, a novel mucin-like protein from the nacreous shell layer of the fan mussel Pinna nobilis (Bivalvia, pteriomorphia)

A cDNA expression library constructed from mantle tissue mRNA of the Mediterranean fan mussel Pinna nobilis was screened with antibodies raised against the acetic acid-soluble shell matrix of the same species. This resulted in the isolation of a 2138-base pair cDNA, containing 13 tandem repeats of 93 base pairs. The deduced protein has a molecular mass of 66.7 kDa and a isoelectric point of 4.8. This protein, which is enriched in serine and proline residues, was overexpressed, purified, and used for producing polyclonal antibodies. Immunological in situ and in vitro tests showed that the protein is localized in the nacreous aragonitic layer of P. nobilis, but not in the calcitic prisms. Because this protein of the nacre of P. nobilis exhibits some mucin-like characteristics, we propose the name mucoperlin. This is the first paper reporting the cloning of a molluscan mucin and the first molecular evidence for the involvement of a mucin in molluscan calcification. This finding corroborates our previous hypothesis that some of the proteinaceous constituents of the molluscan shell matrix would derive from mucins, common to many metazoan lineages of the late Precambrian (Marin, F., Smith, M., Isa, Y., Muyzer, G. and Westbroek, P. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 1554-1559). The adaptation of an ancestral mucin to a new function, the regulation of the mineralization process, may be one of the molecular events, among others, that would explain the simultaneous emergence of organized calcification in many metazoan lineages during the Cambrian explosion.

Functional diversity in the trans-sialidase and mucin families in Trypanosoma cruzi

Trypanosomes are unable to synthesize the monosaccharide sialic acid, but some African trypanosomes and the American Trypanosoma cruzi can incorporate sialic acid derived from the host. To do so, T. cruzi expresses a trans-sialidase, an enzyme that catalyzes the transfer of sialic acid from host glycoconjugates to mucin-like molecules located on the parasite surface membrane. The importance of the process is indicated by the fact that T. cruzi has hundreds of genes encoding trans-sialidase, trans-sialidase-like proteins and mucin core proteins. Sequence divergence of members of these families has resulted in some molecules having functions unrelated to the acquisition of sialic acid. In this article, Alberto Frasch reviews the structure and possible function of the proteins making up these families.

AU-rich elements in the 3'-untranslated region of a new mucin-type gene family of Trypanosoma cruzi confers mRNA instability and modulates translation efficiency

Trypanosoma cruzi has a complex mucin gene family of 500 members with hypervariable regions expressed preferentially in vertebrate associated stages of the parasite. In this work, a novel mucin-type gene family is reported, composed of two groups of genes organized in independent tandems and having very short open reading frames. The structures of deduced proteins share the N and C termini but differ in central regions. One group has repeats with the consensus Lys-Asn-Thr(7)-Ser-Thr(3)-Ser(Ser/Lys)-Ala-Pro and the other a Thr-rich sequence of the type Asp-Gln-Thr(17-20)-Asn-Ala-Pro-Ala-Lys-Asp-Thr(5-7)-Asn-Ala-Pro-Ala-L ys. In both cases, expected mature core proteins are around 7 kDa. Both groups, named L and S, respectively, differ in the structure of genomic loci and mRNA, with differential blocks in the 3'-untranslated region. The highest mRNA level for S and L groups are in the epimastigote stage but they show distinct developmentally regulated patterns. Transcripts are short lived and their steady-state abundance is regulated post-transcriptionally with increased mRNA stability in insect stage epimastigote. AU-rich sequences, similar to ARE motives known to cause mRNA instability in higher eukaryotes, are present in the 3'-untranslated region of the transcripts. In transfection experiments this sequence is shown to be functional for the L group destabilizing its mRNA in a stage-specific manner. Furthermore, an effect of this AU-rich region on translation efficiency is shown. To our knowledge, this is the first time that a functional ARE sequence-dependent post-transcriptional regulation mechanism is reported in a lower eukaryote.

A cell surface mucin specifically expressed in the midgut of the malaria mosquito Anopheles gambiae

An invertebrate intestinal mucin gene, AgMuc1, was isolated from the malaria vector mosquito Anopheles gambiae. The predicted 122-residue protein consists of a central core of seven repeating TTTTVAP motifs flanked by hydrophobic N- and C-terminal domains. This structure is similar to that of mucins that coat the protozoan parasite Trypanosoma cruzi. Northern blot analysis indicated that the gene is expressed exclusively in the midgut of adult mosquitoes. A length polymorphism and in situ hybridization were used to genetically and cytogenetically map AgMuc1 to division 7A of the right arm of the second chromosome. The subcellular localization of the encoded protein in tissue culture cells was examined by using a baculovirus vector to express AgMuc1 protein tagged with the green fluorescent protein (GFP). The results indicated that this protein is found at the cell surface and that both hydrophobic domains are required for cell surface targeting. We propose that AgMuc1 is an abundant mucin-like protein that lines the surface of the midgut microvilli, potentially protecting the intestinal epithelium from the proteinase-rich environment of the gut lumen. An intriguing possibility is that, as an abundant surface protein, AgMuc1 may also interact with the malaria parasite during its invasion of the mosquito midgut.

Expression of trans-sialidase and 85-kDa glycoprotein genes in Trypanosoma cruzi is differentially regulated at the post-transcriptional level by labile protein factors

To adapt to different environments, Trypanosoma cruzi, the protozoan parasite that causes Chagas' disease, expresses a different set of proteins during development. To begin to understand the mechanism that controls this differential gene expression, we have analyzed the levels of amastin and trans-sialidase mRNAs and the mRNAs encoding members of the 85-kDa glycoprotein gene family, which are differentially expressed in the T. cruzi stages found in the mammalian host. Amastin mRNA is expressed predominantly in intracellular and proliferative amastigotes. trans-Sialidase mRNAs are found mostly in forms undergoing transformation from amastigotes to trypomastigotes inside infected cells, whereas mRNAs encoding the 85-kDa glycoproteins appear only in the infective trypomastigotes released from the cells. The genes coding for these mRNA species are constitutively transcribed in all stages of T. cruzi cells, suggesting that expression is controlled post-transcriptionally during differentiation. Inhibition of transcription by actinomycin D revealed that each mRNA species has a relatively long half-life in stages where it accumulates. In the case of the trans-sialidase and 85-kDa glycoprotein genes, mRNA accumulation was induced by treatment with the protein synthesis inhibitor cycloheximide at the stages that preceded the normal accumulation. Therefore, mRNA stabilization may account for mRNA accumulation. mRNA degradation could be promoted by proteins with high turnover, or stabilization could be promoted by forming a complex with the translational machinery at defined times in development. Identification of the factors that induce mRNA degradation or stabilization is essential to the understanding of control of gene expression in these organisms.

Gene discovery through expressed sequence Tag sequencing in Trypanosoma cruzi

Analysis of expressed sequence tags (ESTs) constitutes a useful approach for gene identification that, in the case of human pathogens, might result in the identification of new targets for chemotherapy and vaccine development. As part of the Trypanosoma cruzi genome project, we have partially sequenced the 5' ends of 1, 949 clones to generate ESTs. The clones were randomly selected from a normalized CL Brener epimastigote cDNA library. A total of 14.6% of the clones were homologous to previously identified T. cruzi genes, while 18.4% had significant matches to genes from other organisms in the database. A total of 67% of the ESTs had no matches in the database, and thus, some of them might be T. cruzi-specific genes. Functional groups of those sequences with matches in the database were constructed according to their putative biological functions. The two largest categories were protein synthesis (23.3%) and cell surface molecules (10.8%). The information reported in this paper should be useful for researchers in the field to analyze genes and proteins of their own interest.

Biosynthesis of O-N-acetylglucosamine-linked glycans in Trypanosoma cruzi. Characterization of the novel uridine diphospho-N-acetylglucosamine:polypeptide N-acetylglucosaminyltransferase-catalyzing formation of N-acetylglucosamine alpha1-->O-threonine

In this study, we have characterized the activity of a uridine diphospho-N-acetylglucosamine:polypeptide-alpha-N-acetylglucosaminylt ransferase (O-alpha-GlcNAc-transferase) from Trypanosoma cruzi. The activity is present in microsomal membranes and is responsible for the addition of O-linked alpha-N-acetylglucosamine to cell surface proteins. This preparation adds N-acetylglucosamine to a synthetic peptide KPPTTTTTTTTKPP containing the consensus threonine-rich dodecapeptide encoded by T. cruzi MUC gene (Di Noia, J. M., Sánchez D. O., and Frasch, A. C. C. (1995) J. Biol. Chem. 270, 24146-24149). Incorporation of N-[3H]acetylglucosamine is linearly dependent on incubation time and concentration of enzyme and substrate. The transferase activity has an optimal pH of 7.5- 8.5, requires Mn2+, is unaffected by tunicamycin or amphomycin, and is strongly inhibited by UDP. The optimized synthetic peptide acceptor for the cytosolic O-GlcNAc-transferase (YSDSPSTST) (Haltiwanger, R. S., Holt, G. D., and Hart, G. W. (1990) J. Biol. Chem. 265, 2563-2568) is not a substrate for this enzyme. The glycosylated KPPTTTTTTTTKPP product is susceptible to base-catalyzed beta-elimination, and the presence of N-acetylglucosamine alpha-linked to threonine is supported by enzymatic digestion and nuclear magnetic resonance data. These results describe a unique biosynthetic pathway for T. cruzi surface mucin-like molecules, with potential chemotherapeutic implications.

Two distinct groups of mucin-like genes are differentially expressed in the developmental stages of Trypanosoma cruzi

Sialic acid acceptors of Trypanosoma cruzi are abundant mucin-like glycoproteins linked to the parasite membrane by a glycosylphosphatidyl inositol (GPI) anchor. They are heterogeneous and variable in different parasite stages. The protein portion of these mucins contains many threonine residues, and is thought to be encoded by a heterogeneous gene family. To investigate whether the high degree of heterogeneity in the mucin gene family is responsible for the diversity of mucins expressed on the parasite surface, we have studied the expression of mucin genes in several developmental stages of T. cruzi. We have found that mucins are expressed in all parasite stages. By using conserved sequences at 3' end of translated sequences of the gene family and the splice leader sequence, we have isolated 120 mucin-like cDNAs by RT-PCR from epimastigote and trypomastigote mRNAs. All transcribed genes contain conserved 5' and 3' regions, which code for the signal peptide, the sequence for GPI anchor addition, and a conserved domain rich in threonine residues. The internal portions of these genes are highly variable in size and sequence, and can be grouped in two major categories. One group contains KP(1-2)T(6-8) repeats, a motif found in mammalian mucins in the central region. This group is expressed preferentially in the trypomastigote forms ready to be released from the infected mammalian cell. The other has highly variable sequences in the central portion, and is expressed in all parasite stages. Because the number of synonymous substitutions is equivalent to the non-synonymous substitutions in the second group, they are probably evolving neutrally. On the other hand, the KP(1-2)T(6-8) containing genes have more synonymous substitutions and are most likely under a strong selective pressure. We propose that the group of KP(1-2)T(6-8) motif corresponds to the highly glycosylated mucins of the trypomastigote stages. In the other group proteolysis may remove the central domain yielding small mucins, such as the mucins found in insect derived stages of T. cruzi.

The Trypanosoma cruzi mucin family is transcribed from hundreds of genes having hypervariable regions

In previous works we have identified genes in the protozoan parasite Trypanosoma cruzi whose structure resemble those of mammalian mucin genes. Indirect evidence suggested that these genes might encode the core protein of parasite mucins, glycoproteins that were proposed to be involved in the interaction with, and invasion of, mammalian host cells. We now show that the mucin gene family from T. cruzi is much larger and diverse than expected. A minimal number of 484 mucin genes per haploid genome is calculated for a parasite clone. Most, if not all, genes are transcribed, as deduced from cDNA analysis. Comparison of the cDNA sequences showed evidences of a high mutation rate in localized regions of the genes. Sequence conservation among members of the family is much higher in the untranslated (UTR) regions than in the sequences encoding the mature mucin core protein. Transcription units can be classified into two main subfamilies according to the sequence homologies in the 5'-UTR, whereas the 3'-UTR is highly conserved in all clones analyzed. The common origin of members of this gene family as well as their relationships can be defined by sequence comparison of different domains in the transcription units. The regions encoding the N and C termini, supposed to correspond to the leader peptide and membrane-anchoring signal, respectively, (Di Noia, J. M., Sánchez, D. O., and Frasch, A. C. C. (1995) J. Biol. Chem. 270, 24146-24149) are highly conserved. Conversely, the central regions are highly variable. These regions encode the target sites for O-glycosylation and are made of a variable number of repetitive units rich in Thr and Pro residues or are nonrepetitive but still rich in Thr/Ser and Pro residues. The region putatively coding for the N-terminal domain of the mature core protein is hypervariable, being different in most of the transcripts sequenced. Nonrepetitive central domains are unique to each gene. Gene-specific probes show that the relative abundance of different mRNAs varies greatly within the same parasite clone.

The surface glycoconjugates of trypanosomatid parasites

Insect-transmitted protozoan parasites of the order Kinetoplastida, suborder Trypanosomatina, include Trypanosoma brucei (aetiological agent of African sleeping sickness), Trypanosoma cruzi (aetiological agent of Chagas' disease in South and Central America) and Leishmania spp. (aetiological agents of a variety of diseases throughout the tropics and sub-tropics). The structures of the most abundant cell-surface molecules of these organisms is reviewed and correlated with the different modes of parasitism of the three groups of parasites. The major surface molecules are all glycosylphosphatidylinositol (GPI)-anchored glycoproteins, such as the variant surface glycoproteins of T. brucei and the surface mucins of T. cruzi, or complex glycophospholipids, such as the lipophosphoglycans and glycoinositolphospholipids of the leishmanias. Significantly, all of the aforementioned structures share a motif of Man alpha 1-4GlcN alpha 1-6-myo-inositol-1-HPO4-lipid and can therefore be considered to be members of a GPI superfamily.

Molecular cloning and sequencing of a novel invertebrate intestinal mucin cDNA

The first invertebrate intestinal mucin, termed insect intestinal mucin (IIM), was recently identified from Trichoplusia ni larvae (Wang, P., and Granados, R. R. (1997) Proc. Natl. Acad. Sci. U. S. A. , in press). We report the cDNA cloning and sequencing of IIM, which is only the second completely sequenced intestinal mucin after human intestinal mucin, MUC2. To clone and sequence the cDNA for IIM, a T. ni larval midgut cDNA expression library was constructed and screened with an anti-IIM antiserum. Two full-length cDNA clones for IIM were identified and sequenced. The deduced proteins from the two cDNA clones contained 807 and 788 amino acid residues, respectively. The structural organization of IIM is similar to that of MUC2, containing a 25-amino acid signal leading sequence and two threonine/proline/alanine-rich tandem repeat domains flanked by cysteine-rich sequences. One tandem repeat domain contained two repeating units, TTTQAP and AATTP, and the other contained one repeating unit, TAAP. The cysteine-rich regions showed potential chitin binding features. By immunolocalization in tissue sections, it was determined that IIM is expressed in midgut tissues. The IIM mRNA is abundant in the midgut tissue, and Northern blot analysis indicated that IIM transcripts were not polydispersed as is found in mammalian mucin transcription.

An intestinal mucin is the target substrate for a baculovirus enhancin

An invertebrate intestinal mucin (IIM) was identified from a lepidopterous insect, Trichoplusia ni. The IIM is a major protein constituent of the peritrophic membrane that facilitates the digestive process, as well as protecting invertebrate digestive tracts from microbial infections. The IIM demonstrated biochemical characteristics similar to vertebrate mucins, but exhibited strong association with the chitin-containing peritrophic membrane matrix. We have demonstrated that a baculovirus enhancin, which is encoded and carried by specific baculoviruses, has mucin-degrading activity both in vitro and in vivo. The in vivo degradation of IIM by enhancin was correlated with the enhancement of baculovirus infections in insects. These findings have shown that viruses have evolved a novel strategy to overcome intestinal mucinous barriers against microorganisms by utilizing a mucin-degrading enzyme.

High diversity in mucin genes and mucin molecules in Trypanosoma cruzi

Mucins are highly O-glycosylated molecules which in mammalian cells accomplish essential functions, like cytoprotection and cell-cell interactions. In the protozoan parasite Trypanosoma cruzi, mucin-related glycoproteins have been shown to play a relevant role in the interaction with and invasion of host cells. We have previously reported a family of mucin-like genes in T. cruzi whose overall structure resembled that of mammalian mucin genes. We have now analyzed the relationship between these genes and mucin proteins. A monoclonal antibody specific for a mucin sugar epitope and a polyclonal serum directed to peptide epitopes in a MUC gene-encoded recombinant protein, detected identical bands in three out of seven strains of T. cruzi. Immunoprecipitation experiments confirmed these results. When expressed in eukaryotic cells, the MUC gene product is post-translationally modified, most likely, through extensive O-glycosylation. Gene sequencing showed that the central domains encoding the repeated sequences with the consensus T8KP2, varies in number from 1 to 10, and the number of Thr residues in each repeat could be 7, 8, or 10. A run of 16 to 18 Thr residues was present in some, but not all, MUC gene-derived sequences. Direct compositional analysis of mucin core proteins showed that Thr residues are much more frequent than Ser residues. The same fact occurs in MUC gene-derived protein sequences. Molecular mass determinations of the 35-kDa glycoproteins further extend the heterogeneity of the family to the natural mucin molecules. Difficulties in assigning each of the several MUC genes identified to a mucin product arise from the high diversity and partial sequence conservation of the members of this family.

Gene cloning and cellular localization of a membrane-bound acid phosphatase of Leishmania mexicana

In a previous publication, we described the purification of a membrane-bound acid phosphatase of Leishmania mexicana as a heterogeneously N-glycosylated protein of an apparent molecular mass of 70000-72000 expressed in both the promastigote and the amastigote stage of the parasite [19]. Screening of a genomic DNA library of L. mexicana with degenerate oligonucleotides designed according to the NH2-terminus of the protein led to the cloning of the lmmbap gene, which is present in one copy per haploid genome. The open reading frame predicts a protein of 516 amino acids composed of a signal sequence, a large hydrophilic region, a trans-membrane alpha-helix and a short cytoplasmic tail. The sequence of the hydrophilic region is homologous to acid phosphatases from other organisms. While in wild-type promastigotes, the acid phosphatase is located in the endosomal/lysosomal compartment between the flagellar pocket and the nucleus, overexpression leads to its abundant exposure on the cell surface. In cells transfected with a construct lacking the region corresponding to the trans-membrane and the cytoplasmic parts, the resulting altered acid phosphatase is efficiently secreted into the culture medium. The potential of this system for studies on membrane trafficking in kinetoplastid organisms is discussed.

Mucin-like glycoprotein genes are closely linked to members of the trans-sialidase super-family at multiple sites in the Trypanosoma cruzi genome

In Trypanosoma cruzi a cell surface enzyme with trans-sialidase (TS) activity has been implicated as an important factor in establishing infection. The enzyme is encoded by genes belonging to a large super-family which on the basis of sequence has been subdivided into 4 groups. TS mediates the transfer of sialic acid residues from host glycoconjugates to acceptor molecules on the parasite surface. To study the organisation of the TS genes we isolated several distinct cosmids from a library constructed with DNA from the T. cruzi X10.6 clone. In these cosmids, the TS genes (group I) were present either as single copies or as a direct tandem repeat. A common feature of the cosmids was the presence of a related group III gene located 10-12kb downstream of the TS gene(s) and arranged in the same orientation. In several of the cosmids we also identified a mucin-like glycoprotein gene located between the group I and group III genes. The mucin-like genes are part of a large polymorphic family and contour clamped homogeneous electric field electrophoresis (CHEFE) analysis showed that they were linked to members of the TS super-family at multiple sites in the X10.6 genome. Screening of a second cosmid library made with DNA from the CL-Brener clone confirmed this multiple linkage suggesting that it is a common feature of the species. This genetic organisation may have important functional significance since the mucin-like glycoproteins are the major cell surface acceptors of sialic acid.