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PubMed2006

A cleavable propeptide influences Toxoplasma infection by facilitating the trafficking and secretion of the TgMIC2-M2AP invasion complex.

Propeptides regulate protein function and trafficking in many eukaryotic systems and have emerged as important features of regulated secretory proteins in parasites of the phylum Apicomplexa. Regulated protein secretion from micronemes and host cell invasion are inextricably linked and essential processes for the apicomplexan parasite Toxoplasma gondii. TgM2AP is a propeptide-containing microneme protein found in a heterohexameric complex with the microneme protein TgMIC2, a protein that has a demonstrated fundamental role in gliding motility and invasion. TgM2AP function is also central to these processes, because disruption of TgM2AP (m2apKO) results in secretory retention of TgMIC2, leading to reduced TgMIC2 secretion from the micronemes and impaired invasion. Because the TgM2AP propeptide is predicted to be processed in an intracellular site near where TgMIC2 is retained in m2apKO parasites, we hypothesized that the propeptide and its proteolytic removal influence trafficking and secretion of the complex. We found that proTgM2AP traffics through endosomal compartments and that deletion of the propeptide leads to defective trafficking of the complex within or near this site, resulting in aberrant processing and decreased secretion of TgMIC2, impaired invasion, and reduced virulence in vivo, mirroring the phenotypes observed in m2apKO parasites. In contrast, mutation of several cleavage site residues resulted in normal localization, but it affected the stability and secretion of the complex from the micronemes. Therefore, the propeptide and its cleavage site influence distinct aspects of TgMIC2-M2AP function, with both impacting the outcome of infection.

Amino Acid Sequence↗
PubMed1999

Guanylyl cyclases with the topology of mammalian adenylyl cyclases and an N-terminal P-type ATPase-like domain in Paramecium, Tetrahymena and Plasmodium.

We cloned a guanylyl cyclase of 280 kDa from the ciliate Paramecium which has an N-terminus similar to that of a P-type ATPase and a C-terminus with a topology identical to mammalian adenylyl cyclases. Respective signature sequence motifs are conserved in both domains. The cytosolic catalytic C1a and C2a segments of the cyclase are inverted. Genes coding for topologically identical proteins with substantial sequence similarities have been cloned from Tetrahymena and were detected in sequences from Plasmodium deposited by the Malaria Genome Project. After 99 point mutations to convert the Paramecium TAA/TAG-Gln triplets to CAA/CAG, together with partial gene synthesis, the gene from Paramecium was heterologously expressed. In Sf9 cells, the holoenzyme is proteolytically processed into the two domains. Immunocytochemistry demonstrates expression of the protein in Paramecium and localizes it to cell surface membranes. The data provide a novel structural link between class III adenylyl and guanylyl cyclases and imply that the protozoan guanylyl cyclases evolved from an ancestral adenylyl cyclase independently of the mammalian guanylyl cyclase isoforms. Further, signal transmission in Ciliophora (Paramecium, Tetrahymena) and in the most important endoparasitic phylum Apicomplexa (Plasmodium) is, quite unexpectedly, closely related.

Amino Acid Sequence↗
PubMed2002

The Toxoplasma gondii protein MIC3 requires pro-peptide cleavage and dimerization to function as adhesin.

Attachment and invasion of host cells by apicomplexan parasites involve the exocytosis of the micronemal proteins (MICs). Most MICs are adhesins, which show homology with adhesive domains from higher eukaryote proteins and undergo proteolytic processing of unknown biological significance during their transport to micronemes. In Toxoplasma gondii, the micronemal homodimeric protein MIC3 is a potent adhesin that displays features shared by most Apicomplexa MICs. We have developed an original MIC3-binding assay by transfection of mammalian cells with complete or truncated MIC3 gene sequences and demonstrated that the receptor binding site of MIC3 is located in the N-terminal chitin-binding-like domain, which remains poorly accessible until the adjacent pro-peptide has been cleaved, and that binding requires dimerization. We have localized the dimerization domain in the C-terminal end of the protein and shown that it is able to convert MIC8, a monomeric micronemal protein sharing the MIC3 lectin-like domain, into a dimer able to interact with host cell receptors. These findings shed new light on molecular mechanisms that control functional maturation of MICs.

Adhesins, Bacterial↗
PubMed2002

Plasmodium sporozoite invasion into insect and mammalian cells is directed by the same dual binding system.

Plasmodium sporozoites, the transmission form of the malaria parasite, successively invade salivary glands in the mosquito vector and the liver in the mammalian host. Sporozoite capacity to invade host cells is mechanistically related to their ability to glide on solid substrates, both activities depending on the transmembrane protein TRAP. Here, we show that loss-of- function mutations in two adhesive modules of the TRAP ectodomain, an integrin-like A-domain and a thrombospondin type I repeat, specifically decrease sporozoite invasion of host cells but do not affect sporozoite gliding and adhesion to cells. Irrespective of the target cell, i.e. in mosquitoes, rodents and cultured human or hamster cells, sporozoites bearing mutations in one module are less invasive, while those bearing mutations in both modules are non-invasive. In Chinese hamster ovary cells, the TRAP modules interact with distinct cell receptors during sporozoite invasion, and thus act as independently active pass keys. As these modules are also present in other members of the TRAP family of proteins in Apicomplexa, they may account for the capacity of these parasites to enter many cell types of phylogenetically distant origins.

Amino Acid Sequence↗
PubMed2003

Phenotypic variation of Plasmodium falciparum merozoite proteins directs receptor targeting for invasion of human erythrocytes.

The members of the phylum Apicomplexa parasitize a wide range of eukaryotic host cells. Plasmodium falciparum, responsible for the most virulent form of malaria, invades human erythrocytes using several specific and high affinity ligand-receptor interactions that define invasion pathways. We find that members of the P. falciparum reticulocyte-binding homolog protein family, PfRh2a and PfRh2b, are expressed variantly in different lines. Targeted gene disruption shows that PfRh2b mediates a novel invasion pathway and that it functions independently of other related proteins. Phenotypic variation of the PfRh protein family allows P. falciparum to exploit different patterns of receptors on the erythrocyte surface and thereby respond to polymorphisms in erythrocyte receptors and to evade the host immune system.

Animals↗
PubMed2004

Phylogenetic mapping of intron positions: a case study of translation initiation factor eIF2gamma.

Eukaryotic translation initiation factor 2 (eIF2) is a G protein that delivers the methionyl initiator tRNA to the small ribosomal subunit and releases it upon GTP hydrolysis after the recognition of the initiation codon. eIF2 is composed of three subunits, alpha, beta, and gamma. Subunit gamma shows the strongest conservation, and it confers both tRNA and GTP/GDP binding. Using intron positioning and protein sequence alignment, here we show that eIF2gamma is a suitable phylogenetic marker for eukaryotes. We determined or completed the sequences of 13 arthropod eIF2gamma genes. Analyzing the phylogenetic distribution of 52 different intron positions in 55 distantly related eIF2gamma genes, we identified ancient ones and shared derived introns in our data set. Obviously, intron positioning in eIF2gamma is evolutionarily conserved. However, there were episodes of complete and partial intron losses followed by intron gains. We identified 17 clusters of intron positions based on their distribution. The evolution of these clusters appears to be connected with preferred exon length and can be used to estimate the relative timing of intron gain because nearby precursor introns had to be erased from the gene before the new introns could be inserted. Moreover, we identified a putative case of intron sliding that constitutes a synapomorphic character state supporting monophyly of Coleoptera, Lepidoptera, and Diptera excluding Hymenoptera. We also performed tree reconstructions using the eIF2gamma protein sequences and intron positioning as phylogenetic information. Our results support the monophyly of Viridoplantae, Ascomycota, Homobasidiomyceta, and Apicomplexa.

Amino Acid Sequence↗
PubMed2006

The evolutionary origin of peroxisomes: an ER-peroxisome connection.

The peroxisome is an essential eukaryotic organelle, crucial for lipid metabolism and free radical detoxification, development, differentiation, and morphogenesis from yeasts to humans. Loss of peroxisomes invariably leads to fatal peroxisome biogenesis disorders in man. The evolutionary origin of peroxisomes remains unsolved; proposals for either a symbiogenetic or cellular membrane invagination event are unconclusive. To address this question, we have probed with a peroxisomal proteome, an "ensemble" of 19 representative eukaryotic complete genomes. Molecular phylogenetic and sequence comparison tools allowed us to identify four proteins as peroxisomal markers for unequivocal in silico peroxisome detection. We have then detected the Apicomplexa phylum as the first group of organisms devoid of peroxisomes, in the presence of mitochondria. Finally, we deliver evidence against a prokaryotic ancestor of peroxisomes: (1) the peroxisomal membrane is composed of purely eukaryotic bricks and is thus useful to trace the eukaryotes in their evolutionary paths and (2) the peroxisomal matrix protein import system shares mechanistic similarities with the endoplasmic reticulum/proteasome degradation process, indicating a common evolutionary history.

Animals↗
PubMed1999

Cloning and analysis of a Toxoplasma gondii histone acetyltransferase: a novel chromatin remodelling factor in Apicomplexan parasites.

The yeast transcriptional adaptor GCN5 functions as a histone acetyltransferase, directly linking chromatin modification to transcriptional regulation. Homologues of yeast GCN5 have been found in Tetrahymena, Drosophila, Arabidopsis and human, suggesting that this pathway of chromatin remodelling is evolutionarily conserved. Consistent with this view, we have identified the Toxoplasma gondii homologue, referred to here as TgGCN5. The gene codes for a protein of 474 amino acids with an estimated molecular mass of 53 kDa. The protein reveals two regions of close similarity with the GCN5 family members, the HAT domain and the bromodomain. Tg GCN5 occurs in a single copy in the T.gondii genome. The introduction of a second copy of TgGCN5 in T.gondii tachyzoites is toxic unless the HAT activity is disrupted by a single point mutation. Full TgGCN5 does not complement the growth defect in a yeast gcn5 (-)mutant strain, but a chimera comprising the T.gondii HAT domain fused to the remainder of yGCN5 does. These data show that T.gondii GNC5 is a histone acetyltransferase attesting to the significance of chromatin remodelling in gene regulation of Apicomplexa.

Acetylation↗
PubMed2002

Intron gain and loss in the evolution of the conserved eukaryotic recombination machinery.

Intron conservation, intron gain or loss and putative intron sliding events were determined for a set of three genes (SPO11, MRE11 and DMC1) involved in basic aspects of recombination in eukaryotes. These are ancient genes and present in nearly all of the major kingdoms. MRE11 is of bacterial origin and can be found in all kingdoms. DMC1 is a specialized homolog of the bacterial RecA protein, whereas the SPO11 gene is of archaebacterial origin. Only unique homologs of SPO11 are found in animals and fungi whereas three distantly related SPO11 copies are present in plant genomes. A comparison of the respective intron positions and phases of all genes was performed, demonstrating that a quarter of the intron positions were perfectly conserved over more than 1 000 000 000 years. Regarding the remaining three quarters of the introns we found insertions to be about three times more frequent than deletions. Aligning the introns of the three different SPO11 homologs of Arabidopsis thaliana we propose a conclusive model of their evolution. We postulate that at least one duplication event occurred shortly after the divergence of plants from animals and fungi and that a respective homolog has been retained in a protist group, the apicomplexa.

Arabidopsis↗
PubMed2005

Composite genome map and recombination parameters derived from three archetypal lineages of Toxoplasma gondii.

Toxoplasma gondii is a highly successful protozoan parasite in the phylum Apicomplexa, which contains numerous animal and human pathogens. T.gondii is amenable to cellular, biochemical, molecular and genetic studies, making it a model for the biology of this important group of parasites. To facilitate forward genetic analysis, we have developed a high-resolution genetic linkage map for T.gondii. The genetic map was used to assemble the scaffolds from a 10X shotgun whole genome sequence, thus defining 14 chromosomes with markers spaced at approximately 300 kb intervals across the genome. Fourteen chromosomes were identified comprising a total genetic size of approximately 592 cM and an average map unit of approximately 104 kb/cM. Analysis of the genetic parameters in T.gondii revealed a high frequency of closely adjacent, apparent double crossover events that may represent gene conversions. In addition, we detected large regions of genetic homogeneity among the archetypal clonal lineages, reflecting the relatively few genetic outbreeding events that have occurred since their recent origin. Despite these unusual features, linkage analysis proved to be effective in mapping the loci determining several drug resistances. The resulting genome map provides a framework for analysis of complex traits such as virulence and transmission, and for comparative population genetic studies.

Animals↗
PubMed2000

Species concepts and malaria parasites: detecting a cryptic species of Plasmodium.

Species of malaria parasite (phylum Apicomplexa: genus Plasmodium) have traditionally been described using the similarity species concept (based primarily on differences in morphological or life-history characteristics). The biological species concept (reproductive isolation) and phylogenetic species concept (based on monophyly) have not been used before in defining species of Plasmodium. Plasmodium azurophilum, described from Anolis lizards in the eastern Caribbean, is actually a two-species cryptic complex. The parasites were studied from eight islands, from Puerto Rico in the north to Grenada in the south. Morphology of the two species is very similar (differences are indistinguishable to the eye), but one infects only erythrocytes and the other only white blood cells. Molecular data for the cytochrome b gene reveal that the two forms are reproductively isolated; distinct haplotypes are present on each island and are never shared between the erythrocyte-infecting and leucocyte-infecting species. Each forms a monophyletic lineage indicating that they diverged before becoming established in the anoles of the eastern Caribbean. This comparison of the similarity, biological and phylogenetic species concepts for malaria parasites reveals the limited value of using only similarity measures in defining protozoan species.

Animals↗
PubMed2004

Changes in Haemoproteus sex ratios: fertility insurance or differential sex lifespan?

There is little direct evidence of the fitness effects of changes in malaria gametocyte sex ratio. Gametocyte sex ratios in haemospororin parasites (phylum Apicomplexa) are usually female skewed. However, in some cases and especially in Haemoproteus parasites, less female-biased and even male-biased sex ratios are encountered. The 'fertility insurance hypothesis' tries to explain these biases as an evolutionary strategy to facilitate gamete encounter. Thus, the hypothesis predicts that, if there is a reduction in gametocyte density (intensity of infection) or other factors preventing gametes from meeting, a change to a higher proportion of male gametocytes may be favoured. By contrast, a change in sex ratio may be caused by other non-adaptive mechanisms, for example differential survival of the gametocytes of each sex. We study within-host changes in Haemoproteus majoris sex ratios following an experimental reduction in the density of the parasites in the blood in a breeding population of blue tits (Parus caeruleus). Medication with the antimalarial drug primaquine induced a significant reduction in Haemoproteus gametocyte infection intensity in two different breeding seasons and under two different doses of medication. Sex ratios became male skewed following the experimental treatment in agreement with the predictions of the 'fertility insurance' hypothesis. Also in support of the hypothesis, a significant change towards male-biased sex ratios emerged for non-medicated birds in one year, probably owing to the natural immune reduction of the density of the parasites in the blood. The alternative possibility that changes are caused by different lifespans of gametocytes is not supported by changes in sex ratios in control hosts, where new production and release of gametocytes occur.

Analysis of Variance↗
PubMed1998

Ultrastructural analysis of the sporozoite of Cryptosporidium parvum.

Cryopreparation of live sporozoites and oocysts of the apicomplexan parasite Cryptosporidium parvum, followed by transmission electron microscopy, was undertaken to show the 3D arrangement of organelles, their number and distribution. Profiles of parasites obtained from energy-filtering transmission electron microscopy of serial sections provided 3D reconstructions from which morphometric data and stereo images were derived. The results suggest that sporozoites have a single rhoptry containing an organized lamellar body, no mitochondria or conventional Golgi apparatus, and one or two crystalline bodies. Micronemes were shown to be spherical, numerous and apically located, and to account for 0.8% of the total cell volume. Dense granules were less numerous, larger, accounted for 5.8% of the cell volume, and were located more posteriorly than micronemes. A structure juxtaposed to the nucleus with similarities to the plastid-like organelle reported for other members of the Apicomplexa was observed. The detailed analysis illustrates the advantages of cryopreparation in retaining ultrastructural fidelity of labile or difficult to preserve structures such as the sporozoite of Cryptosporidium.

Animals↗
PubMed2003

Integrated mapping, chromosomal sequencing and sequence analysis of Cryptosporidium parvum.

The apicomplexan Cryptosporidium parvum is one of the most prevalent protozoan parasites of humans. We report the physical mapping of the genome of the Iowa isolate, sequencing and analysis of chromosome 6, and approximately 0.9 Mbp of sequence sampled from the remainder of the genome. To construct a robust physical map, we devised a novel and general strategy, enabling accurate placement of clones regardless of clone artefacts. Analysis reveals a compact genome, unusually rich in membrane proteins. As in Plasmodium falciparum, the mean size of the predicted proteins is larger than that in other sequenced eukaryotes. We find several predicted proteins of interest as potential therapeutic targets, including one exhibiting similarity to the chloroquine resistance protein of Plasmodium. Coding sequence analysis argues against the conventional phylogenetic position of Cryptosporidium and supports an earlier suggestion that this genus arose from an early branching within the Apicomplexa. In agreement with this, we find no significant synteny and surprisingly little protein similarity with Plasmodium. Finally, we find two unusual and abundant repeats throughout the genome. Among sequenced genomes, one motif is abundant only in C. parvum, whereas the other is shared with (but has previously gone unnoticed in) all known genomes of the Coccidia and Haemosporida. These motifs appear to be unique in their structure, distribution and sequences.

Animals↗
PubMed2007

A novel protein kinase family in Plasmodium falciparum is differentially transcribed and secreted to various cellular compartments of the host cell.

Processes at the surface of Plasmodium falciparum-infected erythrocytes such as antigenic variation and cytoadhesion may be modulated by active signalling between host and parasite. Potential candidates for this role include the putative kinases of the FIKK family. The novel Apicomplexa-specific FIKK gene has expanded in P. falciparum to 20 sequence-related members distributed between 11 chromosomes. Specific antibodies raised against different members indicated that most FIKK proteins locate to punctate foci in the erythrocyte cytoplasm that colocalized with Maurer's clefts proteins. One FIKK member dissociates at the trophozoite stage from the Maurer's clefts and relocates with the erythrocyte cytoskeleton. Another FIKK protein, despite having a PEXEL motif, remains located within the parasite. FIKK proteins possess the essential residues for phosphotransferase activity. We show that protein kinase activity was detected in immunoprecipitates obtained with two anti-FIKK antibodies. Quantitative PCR analysis revealed differential gene transcription of the FIKK paralogues in asexual blood stages parasites. We observed significant changes in the transcription pattern between parasites with different adhesion phenotypes. Our data suggest a role of FIKK proteins in the remodelling of the erythrocyte surface and reveal the existence of an adaptable parasite system able to sense intra- and possibly extracellular changes.

Adaptation, Physiological↗
PubMed2005

Feline cutaneous toxoplasmosis: a case report.

A 9-year-old female, domestic short hair cat was presented with sudden onset of polyuria/polydipsia, and hundreds of cutaneous nodules. Prior to referral, the cat had had four skin nodules that were treated with steroids. The four skin nodules then multiplied to form more than 100 ulcerated and nonulcerated nodules located all over the trunk. Clinical evaluation revealed hypothermia and respiratory distress. Cytology from both skin nodules and bronchoalveolar lavage showed macrophages and small organisms whose shape and size were indicative of Toxoplasma spp., or similar organisms. Feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV) serology results were negative. The cat was seropositive for Toxoplasma (IgG 1 : 640) and Neospora (1 : 80) infections. The cat died soon after referral. Necropsy revealed pyothorax, necrotic/purulent pneumonia, haemorrhagic spots on kidneys and mesentery. Histopathology from skin nodules showed diffuse, deep necrotic dermatitis/panniculitis, vasculitis and disseminated free and grouped protozoa. The parasites were found in lungs, spleen, kidneys and liver. Immunohistochemistry on skin tissue with anti-Toxoplasma gondii and Neospora caninum antibodies gave positive results with both. Electron microscopy showed single and grouped tachyzoites with morphological features of T. gondii, often within macrophages. Samples of cutaneous nodules and bronchoalveolar fluid were examined by a polymerase chain reaction (PCR) assay for detecting apicomplexa coccidia. PCR results were consistent only with T. gondii infection. Therefore, immunohistochemistry positivity for N. caninum was considered a cross-reaction and a diagnosis of cutaneous and visceral toxoplasmosis was made.

Animals↗
PubMed2004

Trans-genera reconstitution and complementation of an adhesion complex in Toxoplasma gondii.

Eimeria tenella and Toxoplasma gondii are obligate intracellular parasites belonging to the phylum Apicomplexa. In T. gondii, the microneme protein TgMIC2 contains two well-defined adhesive motifs and is thought to be a key participant in the attachment and invasion of host cells. However, several attempts by different laboratories to generate a knockout (KO) of TgMIC2 have failed, implying that TgMIC2 is an essential gene. As Eimeria and Toxoplasma utilize the same mechanisms of invasion and have highly conserved adhesive proteins, we hypothesized that the orthologous molecule in Eimeria, EtMIC1, could functionally substitute in Toxoplasma to allow a knockout of TgMIC2. TgMIC2 is partnered with a protein called TgM2AP, which corresponds to EtMIC2 in Eimeria. Because the activity of TgMIC2 is most likely tightly linked to its association with TgM2AP, it was thought that the activity of EtMIC1 might similarly require its partner EtMIC2. EtMIC1 and EtMIC2 were introduced into T. gondii, and the presence of EtMIC1 allowed the first knockout clone of TgMIC2 to be obtained. The TgMIC2 KO resulted in significantly decreased numbers of invaded parasites compared to the parental clone. In the absence of TgMIC2, TgM2AP was incorrectly processed and mistargeted to the parasitophorous vacuole instead of the micronemes. These findings indicate that the EtMIC1 can compensate for the essential requirement of TgMIC2, but it cannot fully functionally substitute for TgMIC2 in the invasion process or for supporting the correct maturation and targeting of TgM2AP.

Amino Acid Sequence↗
PubMed1980

[Ultrastructure and cytochemistry of the pellicle and apical complexes of the kinete of Babesia bigemina and Babesia ovis in the hemolymph and oavry of the tick].

The term kinete is used in this paper for the cigar-shaped, motile development stages (VERMICULE") OF Babesia occurring intra- and extracellularly in hemolymph and overy (including oocytes) of vectors, hard ticks (Ixodoidea). The structure of, and cytochemical activities of hydrolases (acid phosphatase, nonspecific esterase) in the pellicle and the apical complex was studied at the fine-structural level in kinetes of Babesia bigemina Smith & Kilborne, in hemolympho of female Boophilus microplus Canestrini. The cytochemistry of acid hydrolases was studied also in kinetes of Babesia ovis (Babès) Starcovici, in hemolymph and ovary of Rhipicephalus bursa Canestrini & Fanzago. The pellicle of the B. bigemina kinetes is composted of 3 membranes (pellicular complex): an outer membrane, approximately 8 nm thick (the plasmalemma) and 2 innder ones, each approximately nm thick, lying closely together. The outer membrane appears to be covered by a structureless coat, 3 nm thick. The space between the inner double membrane and the plasmalemma is 7.5 nm. The whole pellicular complex is 30 nm in diameter. The 2 inner pellicular membranes appear to be derived from the endoplasmic reticulum (ER) for the following reasons: (a) a layer of hydrolase-active material is enclosed by these membranes; (b) in the spheroid parasite stages which transform from kinetes inside hemocytes, the inner double membrane is apparently replaced by an ER cisterna; (c) the thickness of each of the inner pellicular membranes is approximately the same as that of the ER membrane. There are circular openings in the pellicular double membrane with average diameters of 100 nm; despite some similarity to micropores, they have a specific structure. The term Intrapellikularfenster (IPF) (intrapellicular windows) or pseudomicropores is proposed for these pellicular differentiations. The margin of an IPF is formed by the 2 inner membranes folding into each other; cytoplasmic, electron-dense material is accumulated alongside this edge. Unlike that of micropores, the plasmalemma of the IPF is not invaginated. The IPF appears as a single, dark ring in tangential sections. At times, rhoptry-like bodies are associated with the openings. The function of the IPF is not known. An intrapellicular opening similar to the IPF, although wider, is present at the apex of the parasite. Its margin coincides with the inners edge of the apical ring. Typical subpellicular microtubuli were not observed in the Babesia kinetes. The apical complex of the B. bigemina kinetes consists of an Apikalschirm (apical umbrella), a crown of microtubuli beneath it, and rhoptries: micronemes are also present in large numbers. The Apikalschirm is located beneath the pellicle of the apical pole of the parasite. It is a wheel-like structure composed of spokes radiating from a wide, hub=like central ring (apical ring). It should be stressed that the apical ring is not identical with the polar ring described as an integral part of the pellicular complex in other Apicomplexa...

Acid Phosphatase↗