Actomyosin motor in the merozoite of the malaria parasite, Plasmodium falciparum: implications for red cell invasion

Cellular and Molecular Mechanics of Gliding Locomotion in Eukaryotes

Gliding is a form of substrate-dependent cell locomotion exploited by a variety of disparate cell types. Cells may glide at rates well in excess of 1 microm/sec and do so without the gross distortion of cellular form typical of amoeboid crawling. In the absence of a discrete locomotory organelle, gliding depends upon an assemblage of molecules that links cytoplasmic motor proteins to the cell membrane and thence to the appropriate substrate. Gliding has been most thoroughly studied in the apicomplexan parasites, including Plasmodium and Toxoplasma, which employ a unique assortment of proteins dubbed the glideosome, at the heart of which is a class XIV myosin motor. Actin and myosin also drive the gliding locomotion of raphid diatoms (Bacillariophyceae) as well as the intriguing form of gliding displayed by the spindle-shaped cells of the primitive colonial protist Labyrinthula. Chlamydomonas and other flagellated protists are also able to abandon their more familiar swimming locomotion for gliding, during which time they recruit a motility apparatus independent of that driving flagellar beating.

Parasite genomes

The availability of genome sequences and the associated transcriptome and proteome mapping projects has revolutionised research in the field of parasitology. As more parasite species are sequenced, comparative and phylogenetic comparisons are improving the quality of gene prediction and annotation. Genome sequences of parasites are also providing important data sets for understanding parasite biology and identifying new vaccine candidates and drug targets. We review some of the preliminary conclusions from examination of parasite genome sequences and discuss some of the bioinformatics approaches taken in this analysis.

Cellular adhesive phenomena in apicomplexan parasites of red blood cells

The apicomplexan parasites Babesia and Plasmodium are related, yet phylogenetically distinct haemoprotozoa that infect red blood cells and cause severe diseases of major human and veterinary importance. A variety of cellular and molecular interactions are pivotal in many aspects of the pathogenicity of these two parasites. Comparison of the cellular and molecular mechanisms that culminate in accumulation of parasitised red blood cells in the microvasculature of cattle infected with Babesia bovis (babesiosis) and humans infected with Plasmodium falciparum (falciparum malaria) is particularly instructive given the striking similarities in the pathophysiology of these two important medical and veterinary diseases. While such adhesive phenomena have been studied extensively in malaria, they have received relatively little attention in babesiosis. In this review, we summarise the findings of more than 25 years of research into cellular adhesive phenomena in malaria and speculate on how this body of work can now be applied to Babesia parasites. Such information is fundamental if we are to learn more about the biology of Babesia parasites, the cellular and molecular mechanisms by which they cause infection and disease and how to develop novel therapeutic strategies or vaccines for both Babesia and malaria infections.

Plasmodium falciparum myosins: transcription and translation during asexual parasite development

Six myosins genes are now annotated in the Plasmodium falciparum Genome Project. Malaria myosins have been named alphabetically; accordingly, we refer to the two latest additions as Pfmyo-E and Pfmyo-F. Both new myosins contain regions characteristic of the functional motor domain of "true" myosins and, unusually for P. falciparum myosins, Pfmyo-F encodes two consensus IQ light chain-binding motifs. Phylogenetic analysis of the 17 currently known apicomplexan myosins together with one representative of each myosin class clusters all but one of the apicomplexan sequences together in Class XIV. This refines the earlier definition of the Class XIV Subclasses XIVa and XIVb. RT-PCR on blood stage parasite mRNA amplifies a specific product for all six myosins and each shows developmentally regulated transcription. Thus: Pfmyo-A and Pfmyo-B genes are transcribed throughout development; Pfmyo-C is predominant in trophozoites; Pfmyo-D occurs in trophozoites and schizonts; Pfmyo-E though barely present in earlier stages is abundant in schizonts; Pfmyo-F increases steadily throughout development and maturation. It is known that Pfmyo-A and Pfmyo-B are synthesised during late schizogony and we now show that Pfmyo-D expression is also temporally regulated to late trophozoites and schizonts where it distributes close to segregating nuclei. Thus, in asexual stages myosin synthesis does not always parallel transcript accumulation, showing that translation is also regulated. The implication is that the mRNAs are either subjected to turnover, synthesised and degraded, or that they are sequestered in an inactivate form until required for protein synthesis.

The hydration state of human red blood cells and their susceptibility to invasion by Plasmodium falciparum

In most inherited red blood cell (RBC) disorders with high gene frequencies in malaria-endemic regions, the distribution of RBC hydration states is much wider than normal. The relationship between the hydration state of circulating RBCs and protection against severe falciparum malaria remains unexplored. The present investigation was prompted by a casual observation suggesting that falciparum merozoites were unable to invade isotonically dehydrated normal RBCs. We designed an experimental model to induce uniform and stable isotonic volume changes in RBC populations from healthy donors by increasing or decreasing their KCl contents through a reversible K(+) permeabilization pulse. Swollen and mildly dehydrated RBCs were able to sustain Plasmodium falciparum cultures with similar efficiency to untreated RBCs. However, parasite invasion and growth were progressively reduced in dehydrated RBCs. In a parallel study, P falciparum invasion was investigated in density-fractionated RBCs from healthy subjects and from individuals with inherited RBC abnormalities affecting primarily hemoglobin (Hb) or the RBC membrane (thalassemias, hereditary ovalocytosis, xerocytosis, Hb CC, and Hb CS). Invasion was invariably reduced in the dense cell fractions in all conditions. These results suggest that the presence of dense RBCs is a protective factor, additional to any other protection mechanism prevailing in each of the different pathologies.

Malaria Parasite Actin Filaments are Very Short

A novel form of actomyosin regulation has recently been proposed in which the polymerisation of new actin filaments regulates apicomplexan parasite motility. Here, we identified actin I in the merozoites of Plasmodium falciparum by mass spectrometry. The only post-translational modification is acetylation of the N terminus (acetyl-Gly-Glu-actin), while methylation of histidine 73, a common modification for actin, is absent. Results obtained with anti-actin antibodies suggest that, in contrast to a previous report, there is no actin-ubiquitin conjugate in merozoites. About half of the extracted monomeric actin polymerised and actin filaments could be sedimented at 500,000g. In contrast, centrifugation at 100,000g, conditions commonly used to sediment filamentous actin, yielded very little F-actin. In a functional characterisation using an in vitro motility assay, actin filaments moved over myosin at a velocity indistinguishable from that of rabbit skeletal actin. Filament length, however, was too short to be resolved by conventional fluorescence microscopy. On electron micrographs an average filament length of approximately 100nm was determined. We also identified by mass spectrometry proteins co-purifying with filamentous actin, which are potential actin-binding proteins. Our results demonstrate differences in actin filament dynamics for an apicomplexan parasite, which could be due to specific properties of the actin and/or actin-regulatory proteins.

Malaria and the red blood cell membrane

Malaria is the most serious and widespread parasitic disease of humans and is arguably the commonest disease of red blood cells (RBCs). Malaria has exerted a powerful effect on human evolution and selection for resistance has led to the appearance and persistence of a number of inherited diseases. After parasite invasion, RBCs are progressively and dramatically modified. New structures appear inside the RBC and novel parasite proteins are exported to the erythrocyte cytoplasm and membrane skeleton. Radical biochemical, morphological, and rheological alterations manifest as increased membrane rigidity, reduced cell deformability, and greater adhesiveness for the vascular endothelium and other blood cells. Numerous protein-protein interactions between the malaria-parasite and the host RBC are important for many aspects of parasite biology and the pathogenesis of malaria. In addition, there are many other parasite proteins located within the infected red cell and at the membrane skeleton, for which no precise functional roles have yet been elucidated. Sequencing and annotation of the complete genome of Plasmodium falciparum, the production of proteomic and transcriptomic profiles of parasites, and the development of a transfection system for the asexual stage of the parasite are all recent achievements that should advance understanding of the molecular mechanisms that underlie the parasite-induced functional alterations in red cells.

Anillin binds nonmuscle myosin II and regulates the contractile ring

We demonstrate that the contractile ring protein anillin interacts directly with nonmuscle myosin II and that this interaction is regulated by myosin light chain phosphorylation. We show that despite their interaction, anillin and myosin II are independently targeted to the contractile ring. Depletion of anillin in Drosophila or human cultured cells results in cytokinesis failure. Human cells depleted for anillin fail to properly regulate contraction by myosin II late in cytokinesis and fail in abscission. We propose a role for anillin in spatially regulating the contractile activity of myosin II during cytokinesis.

The glideosome: a molecular machine powering motility and host-cell invasion by Apicomplexa

The apicomplexans are obligate intracellular protozoan parasites that rely on gliding motility for their migration across biological barriers and for host-cell invasion and egress. This unusual form of substrate-dependent motility is powered by the "glideosome", a macromolecular complex consisting of adhesive proteins that are released apically and translocated to the posterior pole of the parasite by the action of an actomyosin system anchored in the inner membrane complex of the parasite. Recent studies have revealed new insights into the composition and biogenesis of Toxoplasma gondii myosin-A motor complex and have identified an exciting set of small molecules that can interfere with different aspects of glideosome function.

Correlation of structural development and differential expression of invasion-related molecules in schizonts ofPlasmodium falciparum

During asexual developmentPlasmodiumschizonts undergo a series of complex biochemical and structural changes. Using tightly synchronized cultures of 2P. falciparumlines (clone C10 and strain ITO4) for light microscopy and fluorescence imaging we monitored the timing and sequence of expression of proteins associated with invasion-related organelles. Antibodies to rhoptry, micronemal and dense granule proteins (Rhoptry Associated Protein 1, Apical Membrane Antigen 1, Erythrocyte Binding Antigen 175, Ring-infected Erythrocyte Surface Antigen) and to pellicle-associated proteins (Merozoite Surface Protein 1, PfMyosin-A) were used. Clone C10 developed faster than ITO4; this difference was also found in the timing of protein expression seen by immunofluorescence. Light microscopic data were combined with transmission electron microscopic analysis using serial sectioning of ITO4 schizonts to determine nuclear number and organellar development. Thus a timetable of schizont structural maturation was established. Generally, the timing of organelle-specific antigen expression correlates well with the ultrastructural data. Rhoptries are formed mainly between second and fourth nuclear divisions, micronemes between the end of the fourth nuclear division and merozoite separation from the residual body, while dense granules are generated mainly after the micronemes. PfAMA-1 appears in micronemes before EBA-175, suggesting micronemal heterogeneity.

Differential transcriptome profiling identifies Plasmodium genes encoding pre-erythrocytic stage-specific proteins

Invasive sporozoite and merozoite stages of malaria parasites that infect mammals enter and subsequently reside in hepatocytes and red blood cells respectively. Each invasive stage may exhibit unique adaptations that allow it to interact with and survive in its distinct host cell environment, and these adaptations are likely to be controlled by differential gene expression. We used suppression subtractive hybridization (SSH) of Plasmodium yoelii salivary gland sporozoites versus merozoites to identify stage-specific pre-erythrocytic transcripts. Sequencing of the SSH library and matching the cDNA sequences to the P. yoelii genome yielded 25 redundantly tagged genes including the only two previously characterized sporozoite-specific genes encoding the circumsporozoite protein (CSP) and thrombospondin-related anonymous protein (TRAP). Twelve novel genes encode predicted proteins with signal peptides, indicating that they enter the secretory pathway of the sporozoite. We show that one novel protein bearing a thrombospondin type 1 repeat (TSR) exhibits an expression pattern that suggests localization in the sporozoite secretory rhoptry organelles. In addition, we identified a group of four genes encoding putative low-molecular-mass proteins. Two proteins in this group exhibit an expression pattern similar to TRAP, and thus possibly localize in the sporozoite secretory micronemes. Proteins encoded by the differentially expressed genes identified here probably mediate specific interactions of the sporozoite with the mosquito vector salivary glands or the mammalian host hepatocyte and are not used during merozoite-red blood cell interactions.

Adenylate kinase and GTP:AMP phosphotransferase of the malarial parasite Plasmodium falciparum

For coping with energetic and synthetic challenges, parasites require high activities of adenylate kinase (AK; ATP + AMP <==> 2 ADP) and GTP:AMP phosphotransferase (GAK; GTP + AMP <==> GDP + ADP). These enzymes were identified in erythrocytic stages of Plasmodium falciparum. The genes encoding PfAK and PfGAK are located on chromosomes 10 and 4, respectively. Molecular cloning and heterologous expression in E. coli yielded enzymatically active proteins of 28.9 (PfAK) and 28.0 kDa (PfGAK). Recombinant PfAK resembles authentic PfAK in its biochemical characteristics including the possible association with a stabilizing protein and the high specificity for AMP as the mononucleotide substrate. Specificity is less stringent for the triphosphate, with ATP as the best substrate (75 U/mg; kcat = 2160 min(-1) at 25 degrees C). PfAK contains the sequence of the amphiphatic helix that is known to mediate translocation of the cytosolic protein into the mitochondrial intermembrane space. PfGAK exhibits substrate preference for GTP and AMP (100 U/mg; kcat = 2800 min(-1) at 25 degrees C); notably, there is no detectable activity with ATP. In contrast to its human orthologue (AK3), PfGAK contains a zinc finger motif and binds ionic iron. The dinucleoside pentaphosphate compounds AP5A and GP5A inhibited PfAK and PfGAK, respectively, with Ki values of approximately 0.2 microM which is more than 250-fold lower than the KM values determined for the nucleotide substrates. The disubstrate inhibitors are useful for studying the enzymatic mechanism of PfAK and PfGAK as well as their function in adenine nucleotide homeostasis; in addition, the chimeric inhibitors represent interesting lead compounds for developing nucleosides to be used as antiparasitic agents.

Toxoplasma gondii: the model apicomplexan

Toxoplasma gondii is an obligate intracellular protozoan parasite which is a significant human and veterinary pathogen. Other members of the phylum Apicomplexa are also important pathogens including Plasmodium species (i.e. malaria), Eimeria species, Neospora, Babesia, Theileria and Cryptosporidium. Unlike most of these organisms, T. gondii is readily amenable to genetic manipulation in the laboratory. Cell biology studies are more readily performed in T. gondii due to the high efficiency of transient and stable transfection, the availability of many cell markers, and the relative ease with which the parasite can be studied using advanced microscopic techniques. Thus, for many experimental questions, T. gondii remains the best model system to study the biology of the Apicomplexa. Our understanding of the mechanisms of drug resistance, the biology of the apicoplast, and the process of host cell invasion has been advanced by studies in T. gondii. Heterologous expression of apicomplexan proteins in T. gondii has frequently facilitated further characterisation of proteins that could not be easily studied. Recent studies of Apicomplexa have been complemented by genome sequencing projects that have facilitated discovery of surprising differences in cell biology and metabolism between Apicomplexa. While results in T. gondii will not always be applicable to other Apicomplexa, T. gondii remains an important model system for understanding the biology of apicomplexan parasites.

Identification of the membrane receptor of a class XIV myosin in Toxoplasma gondii

Apicomplexan parasites exhibit a unique form of substrate-dependent motility, gliding motility, which is essential during their invasion of host cells and during their spread between host cells. This process is dependent on actin filaments and myosin that are both located between the plasma membrane and two underlying membranes of the inner membrane complex. We have identified a protein complex in the apicomplexan parasite Toxoplasma gondii that contains the class XIV myosin required for gliding motility, TgMyoA, its associated light chain, TgMLC1, and two novel proteins, TgGAP45 and TgGAP50. We have localized this complex to the inner membrane complex of Toxoplasma, where it is anchored in the membrane by TgGAP50, an integral membrane glycoprotein. Assembly of the protein complex is spatially controlled and occurs in two stages. These results provide the first molecular description of an integral membrane protein as a specific receptor for a myosin motor, and further our understanding of the motile apparatus underlying gliding motility in apicomplexan parasites.

Apical membrane antigen 1, a major malaria vaccine candidate, mediates the close attachment of invasive merozoites to host red blood cells

Apical membrane antigen 1 (AMA-1) of Plasmodium merozoites is established as a candidate molecule for inclusion in a human malaria vaccine and is strongly conserved in the genus. We have investigated its function in merozoite invasion by incubating Plasmodium knowlesi merozoites with red cells in the presence of a previously described rat monoclonal antibody (MAb R31C2) raised against an invasion-inhibitory epitope of P. knowlesi AMA-1 and then fixing the material for ultrastructural analysis. We have found that the random, initial, long-range (12 nm) contact between merozoites and red cells occurs normally in the presence of the antibody, showing that AMA-1 plays no part in this stage of attachment. Instead, inhibited merozoites fail to reorientate, so they do not bring their apices to bear on the red cell surface and do not make close junctional apical contact. We conclude that AMA-1 may be directly responsible for reorientation or that the molecule may initiate the junctional contact, which is then presumably dependent on Duffy binding proteins for its completion.

Toxoplasma as a novel system for motility

Motility is a characteristic of most living organisms and often requires specialized structures like cilia or flagella. An alternative is amoeboid movement, where the polymerization/depolymerization of actin leads to the formation of pseudopodia, filopodia and/or lamellipodia that enable the cell to crawl along a surface. Despite their lack of locomotive organelles and in absence of cell deformation, members of the apicomplexan parasites employ a unique form of locomotion called gliding motility to promote their migration across biological barriers and to power host-cell invasion and egress. Detailed studies in Toxoplasma gondii and Plasmodium species have revealed that this unique mode of movement is dependent on a myosin of class XIV and necessitates actin dynamics and the concerted discharge and processing of adhesive proteins. Gliding is essential for the survival and infectivity of these obligate intracellular parasites, which cause severe disease in humans and animals.

Generation of an erythrocyte vesicle transport system by Plasmodium falciparum malaria parasites

The asexual maturation of Plasmodium falciparum is accompanied by the transport of parasite-encoded proteins to the erythrocyte plasma membrane. Activation of G proteins by treatment with aluminum fluoride produced an accumulation within the erythrocyte cytosol of vesicles coated with Plasmodium homologues of COPII and N-ethylmaleimide-sensitive factor, proteins involved in intracellular transport between the Golgi apparatus and the endoplasmic reticulum. These vesicles contain malarial proteins that appear on the erythrocyte plasma membrane, as well as actin and myosin. It is proposed that the parasite adapted a process well established for intracellular transport to mediate the extracellular movement of its proteins through the erythrocyte cytosol to the surface membrane.

Sites of interaction between aldolase and thrombospondin-related anonymous protein in plasmodium

Gliding motility and host cell invasion by apicomplexan parasites are empowered by an acto-myosin motor located underneath the parasite plasma membrane. The motor is connected to host cell receptors through trans-membrane invasins belonging to the thrombospondin-related anonymous protein (TRAP) family. A recent study indicates that aldolase bridges the cytoplasmic tail of MIC2, the homologous TRAP protein in Toxoplasma, and actin. Here, we confirm these unexpected findings in Plasmodium sporozoites and identify conserved features of the TRAP family cytoplasmic tail required to bind aldolase: a subterminal tryptophan residue and two noncontiguous stretches of negatively charged amino acids. The aldolase substrate and other compounds that bind to the active site inhibit its interaction with TRAP and with F-actin, suggesting that the function of the motor is metabolically regulated. Ultrastructural studies in salivary gland sporozoites localize aldolase to the periphery of the secretory micronemes containing TRAP. Thus, the interaction between aldolase and the TRAP tail takes place during or preceding the biogenesis of the micronemes. The release of their contents in the anterior pole of the parasite upon contact with the target cells should bring simultaneously aldolase, TRAP and perhaps F-actin to the proper subcellular location where the motor is engaged.

The cytoplasmic domain of the Plasmodium falciparum ligand EBA-175 is essential for invasion but not protein trafficking

The invasion of host cells by the malaria parasite Plasmodium falciparum requires specific protein-protein interactions between parasite and host receptors and an intracellular translocation machinery to power the process. The transmembrane erythrocyte binding protein-175 (EBA-175) and thrombospondin-related anonymous protein (TRAP) play central roles in this process. EBA-175 binds to glycophorin A on human erythrocytes during the invasion process, linking the parasite to the surface of the host cell. In this report, we show that the cytoplasmic domain of EBA-175 encodes crucial information for its role in merozoite invasion, and that trafficking of this protein is independent of this domain. Further, we show that the cytoplasmic domain of TRAP, a protein that is not expressed in merozoites but is essential for invasion of liver cells by the sporozoite stage, can substitute for the cytoplasmic domain of EBA-175. These results show that the parasite uses the same components of its cellular machinery for invasion regardless of the host cell type and invasive form.

Gene discovery in the apicomplexa as revealed by EST sequencing and assembly of a comparative gene database

Large-scale EST sequencing projects for several important parasites within the phylum Apicomplexa were undertaken for the purpose of gene discovery. Included were several parasites of medical importance (Plasmodium falciparum, Toxoplasma gondii) and others of veterinary importance (Eimeria tenella, Sarcocystis neurona, and Neospora caninum). A total of 55192 ESTs, deposited into dbEST/GenBank, were included in the analyses. The resulting sequences have been clustered into nonredundant gene assemblies and deposited into a relational database that supports a variety of sequence and text searches. This database has been used to compare the gene assemblies using BLAST similarity comparisons to the public protein databases to identify putative genes. Of these new entries, approximately 15%-20% represent putative homologs with a conservative cutoff of p < 10(-9), thus identifying many conserved genes that are likely to share common functions with other well-studied organisms. Gene assemblies were also used to identify strain polymorphisms, examine stage-specific expression, and identify gene families. An interesting class of genes that are confined to members of this phylum and not shared by plants, animals, or fungi, was identified. These genes likely mediate the novel biological features of members of the Apicomplexa and hence offer great potential for biological investigation and as possible therapeutic targets.

Actin filament polymerization regulates gliding motility by apicomplexan parasites

Host cell entry by Toxoplasma gondii depends critically on actin filaments in the parasite, yet paradoxically, its actin is almost exclusively monomeric. In contrast to the absence of stable filaments in conventional samples, rapid-freeze electron microscopy revealed that actin filaments were formed beneath the plasma membrane of gliding parasites. To investigate the role of actin filaments in motility, we treated parasites with the filament-stabilizing drug jasplakinolide (JAS) and monitored the distribution of actin in live and fixed cells using yellow fluorescent protein (YFP)-actin. JAS treatment caused YFP-actin to redistribute to the apical and posterior ends, where filaments formed a spiral pattern subtending the plasma membrane. Although previous studies have suggested that JAS induces rigor, videomicroscopy demonstrated that JAS treatment increased the rate of parasite gliding by approximately threefold, indicating that filaments are rate limiting for motility. However, JAS also frequently reversed the normal direction of motility, disrupting forward migration and cell entry. Consistent with this alteration, subcortical filaments in JAS-treated parasites occurred in tangled plaques as opposed to the straight, roughly parallel orientation observed in control cells. These studies reveal that precisely controlled polymerization of actin filaments imparts the correct timing, duration, and directionality of gliding motility in the Apicomplexa.

Gliding motility: the molecules behind the motion

In apicomplexan parasites, gliding motility and host cell invasion are driven by an actomyosin-based system. Recent studies have characterized several components of the gliding motility apparatus and have provided new insight into the molecular architecture of this locomotory system.

Myosin A tail domain interacting protein (MTIP) localizes to the inner membrane complex of Plasmodium sporozoites

Apicomplexan host cell invasion and gliding motility depend on the parasite's actomyosin system located beneath the plasma membrane of invasive stages. Myosin A (MyoA), a class XIV unconventional myosin, is the motor protein. A model has been proposed to explain how the actomyosin motor operates but little is known about the components, topology and connectivity of the motor complex. Using the MyoA neck and tail domain as bait in a yeast two-hybrid screen we identified MTIP, a novel 24 kDa protein that interacts with MyoA. Deletion analysis shows that the 15 amino-acid C-terminal tail domain of MyoA, rather than the neck domain, specifically interacts with MTIP. In Plasmodium sporozoites MTIP localizes to the inner membrane complex (IMC), where it is found clustered with MyoA. The data support a model for apicomplexan motility and invasion in which the MyoA motor protein is associated via its tail domain with MTIP, immobilizing it at the outer IMC membrane. The head domain of the immobilized MyoA moves actin filaments that, directly or via a bridging protein, connect to the cytoplasmic domain of a transmembrane protein of the TRAP family. The actin/TRAP complex is then redistributed by the stationary MyoA from the anterior to the posterior end of the zoite, leading to its forward movement on a substrate or to penetration of a host cell.

Infectivity-associated changes in the transcriptional repertoire of the malaria parasite sporozoite stage

Injection of Plasmodium salivary gland sporozoites into the vertebrate host by Anopheles mosquitoes initiates malaria infection. Sporozoites develop within oocysts in the mosquito midgut and then enter and mature in the salivary glands. Although morphologically similar, oocyst sporozoites and salivary gland sporozoites differ strikingly in their infectivity to the mammalian host, ability to elicit protective immune responses, and cell motility. Here, we show that differential gene expression coincides with these dramatic phenotypic differences. Using suppression subtractive cDNA hybridization we identified highly up-regulated mRNAs transcribed from 30 distinct genes in salivary gland sporozoites. Of those genes, 29 are not significantly expressed in the parasite's blood stages. The most frequently recovered transcript encodes a protein kinase. Developmental up-regulation of specific mRNAs in the infectious transmission stage of Plasmodium indicates that their translation products may have unique roles in hepatocyte infection and/or development of liver stages.

A proteomic view of the Plasmodium falciparum life cycle

The completion of the Plasmodium falciparum clone 3D7 genome provides a basis on which to conduct comparative proteomics studies of this human pathogen. Here, we applied a high-throughput proteomics approach to identify new potential drug and vaccine targets and to better understand the biology of this complex protozoan parasite. We characterized four stages of the parasite life cycle (sporozoites, merozoites, trophozoites and gametocytes) by multidimensional protein identification technology. Functional profiling of over 2,400 proteins agreed with the physiology of each stage. Unexpectedly, the antigenically variant proteins of var and rif genes, defined as molecules on the surface of infected erythrocytes, were also largely expressed in sporozoites. The detection of chromosomal clusters encoding co-expressed proteins suggested a potential mechanism for controlling gene expression.

Myosins of Babesia bovis: molecular characterisation, erythrocyte invasion, and phylogeny

Using degenerate primers, three putative myosin sequences were amplified from Australian isolates of Babesa bovis and confirmed as myosins (termed Bbmyo-A, Bbmyo-B, and Bbmyo-C) from in vitro cultures of the W strain of B. bovis. Comprehensive analysis of 15 apicomplexan myosins suggests that members of Class XIV be defined as those with greater than 35% myosin head sequence identity and that these be further subclassed into groups bearing above 50-60% identity. Bbmyo-A protein bears a strong similarity with other apicomplexan myosin-A type proteins (subclass XIVa), the Bbmyo-B myosin head protein sequence exhibits low identity (35-39%) with all members of Class XIV, and 5'-sequence of Bbmyo-C shows strong identity (60%) with P. falciparum myosin-C protein. Domain analysis revealed five divergent IQ domains within the neck of Pfmyo-C, and a myosin-N terminal domain as well as a classical IQ sequence unusually located within the head converter domain of Bbmyo-B. A cross-reacting antibody directed against P. falciparum myosin-A (Pfmyo-A) revealed a zone of approximately 85 kDa in immunoblots prepared with B. bovis total protein, and immunofluorescence inferred stage-specific myosin-A expression since only 25% of infected erythrocytes with mostly paired B. bovis were immuno-positive. Multiplication of B. bovis in in vitro culture was inhibited by myosin- and actin-binding drugs at concentrations lower than those that inhibit P. falciparum. This study identifies and classifies three myosin genes and an actin gene in B. bovis, and provides the first evidence for the participation of an actomyosin-based motor in erythrocyte invasion in this species of apicomplexan parasite.

'The glideosome': a dynamic complex powering gliding motion and host cell invasion by Toxoplasma gondii

Motion is an intrinsic property of all living organisms, and each cell displays a variety of shapes and modes of locomotion. How structural proteins support cellular movement and how cytoskeletal dynamics and motor proteins are harnessed to generate order and movement are among the fundamental and not fully resolved questions in biology today. Protozoan parasites belonging to the Apicomplexa are of enormous medical and veterinary significance, being responsible for a wide variety of diseases in human and animals, including malaria, toxoplasmosis, coccidiosis and cryptosporidiosis. These obligate intracellular parasites exhibit a unique form of actin-based gliding motility, which is essential for host cell invasion and spreading of parasites throughout the infected hosts. A motor complex composed of a small myosin of class XIV associated with a myosin light chain and a plasma membrane-docking protein is present beneath the parasite's plasma membrane. According to the capping model, this complex is connected directly or indirectly to transmembrane adhesin complexes, which are delivered to the parasite surface upon microneme secretion. Together with F-actin and as yet unknown bridging molecules and proteases, these complexes are among the structural and functional components of the 'glideosome'.

Cytoskeleton of apicomplexan parasites

The Apicomplexa are a phylum of diverse obligate intracellular parasites including Plasmodium spp., the cause of malaria; Toxoplasma gondii and Cryptosporidium parvum, opportunistic pathogens of immunocompromised individuals; and Eimeria spp. and Theileria spp., parasites of considerable agricultural importance. These protozoan parasites share distinctive morphological features, cytoskeletal organization, and modes of replication, motility, and invasion. This review summarizes our current understanding of the cytoskeletal elements, the properties of cytoskeletal proteins, and the role of the cytoskeleton in polarity, motility, invasion, and replication. We discuss the unusual properties of actin and myosin in the Apicomplexa, the highly stereotyped microtubule populations in apicomplexans, and a network of recently discovered novel intermediate filament-like elements in these parasites.

Myosins in protists

This review focuses on selected papers that illustrate an historical perspective and the current knowledge of myosin structure and function in protists. The review contains a general description of myosin structure, a phylogenetic tree of the myosin classes, and descriptions of myosin isoforms identified in protists. Each myosin is discussed within the context of the taxonomic group of the organism in which the myosin has been identified. Domain structure, cellular location, function, and regulation are described for each myosin.

Toxoplasma gondii myosins B/C: one gene, two tails, two localizations, and a role in parasite division

In apicomplexan parasites, actin-disrupting drugs and the inhibitor of myosin heavy chain ATPase, 2,3-butanedione monoxime, have been shown to interfere with host cell invasion by inhibiting parasite gliding motility. We report here that the actomyosin system of Toxoplasma gondii also contributes to the process of cell division by ensuring accurate budding of daughter cells. T. gondii myosins B and C are encoded by alternatively spliced mRNAs and differ only in their COOH-terminal tails. MyoB and MyoC showed distinct subcellular localizations and dissimilar solubilities, which were conferred by their tails. MyoC is the first marker selectively concentrated at the anterior and posterior polar rings of the inner membrane complex, structures that play a key role in cell shape integrity during daughter cell biogenesis. When transiently expressed, MyoB, MyoC, as well as the common motor domain lacking the tail did not distribute evenly between daughter cells, suggesting some impairment in proper segregation. Stable overexpression of MyoB caused a significant defect in parasite cell division, leading to the formation of extensive residual bodies, a substantial delay in replication, and loss of acute virulence in mice. Altogether, these observations suggest that MyoB/C products play a role in proper daughter cell budding and separation.

Microtubule associated motor proteins of Plasmodium falciparum merozoites

We have studied the occurrence, stage specificity and cellular location of key molecules associated with microtubules in Plasmodium falciparum merozoites. Antibodies to gamma tubulin, conventional kinesin and cytoplasmic dynein were used to determine the polarity of merozoite microtubules (mt), the stage specificity of the motor proteins and their location during merozoite development. We conclude that the minus ends of the mts are located at their apical pole. Kinesin was present throughout the lifecycle, appearing as a distinct crescent at the apex of developing merozoites. The vast majority of cytoplasmic dynein reactivity occurred in late merogony, also appearing at the merozoite apex. Destruction of mt with dinitroanilines did not affect the cellular location of kinesin or dynein. In invasion assays, dynein inhibitors reduced the number of ring stage parasites. Our results show that both conventional kinesin and cytoplasmic dynein are abundant, located at the negative pole of the merozoite mt and, intriguingly, appear there only in very late merogony, prior to merozoite release and invasion.

Exploring the transcriptome of the malaria sporozoite stage

Most studies of gene expression in Plasmodium have been concerned with asexual and/or sexual erythrocytic stages. Identification and cloning of genes expressed in the preerythrocytic stages lag far behind. We have constructed a high quality cDNA library of the Plasmodium sporozoite stage by using the rodent malaria parasite P. yoelii, an important model for malaria vaccine development. The technical obstacles associated with limited amounts of RNA material were overcome by PCR-amplifying the transcriptome before cloning. Contamination with mosquito RNA was negligible. Generation of 1,972 expressed sequence tags (EST) resulted in a total of 1,547 unique sequences, allowing insight into sporozoite gene expression. The circumsporozoite protein (CS) and the sporozoite surface protein 2 (SSP2) are well represented in the data set. A BLASTX search with all tags of the nonredundant protein database gave only 161 unique significant matches (P(N) < or = 10(-4)), whereas 1,386 of the unique sequences represented novel sporozoite-expressed genes. We identified ESTs for three proteins that may be involved in host cell invasion and documented their expression in sporozoites. These data should facilitate our understanding of the preerythrocytic Plasmodium life cycle stages and the development of preerythrocytic vaccines.

TgM2AP participates in Toxoplasma gondii invasion of host cells and is tightly associated with the adhesive protein TgMIC2

Like other members of the medically important phylum Apicomplexa, Toxoplasma gondii is an obligate intracellular parasite that secretes several classes of proteins involved in the active invasion of target host cells. Proteins in apical secretory organelles known as micronemes have been strongly implicated in parasite attachment to host cells. TgMIC2 is a microneme protein with multiple adhesive domains that bind target cells and is mobilized onto the parasite surface during parasite attachment. Here, we describe a novel parasite protein, TgM2AP, which is physically associated with TgMIC2. TgM2AP complexes with TgMIC2 within 15 min of synthesis and remains associated with TgMIC2 in the micronemes, on the parasite surface during invasion and in the culture medium after release from the parasite plasma membrane. TgM2AP is proteolytically processed initially when its propeptide is removed during transit through the golgi and later while it occupies the parasite surface after discharge from the micronemes. We show that TgM2AP is a member of a protein family expressed by coccidian parasites including Neospora caninum and Eimeria tenella. This phylogenic conservation and association with a key adhesive protein suggest that TgM2AP is a fundamental component of the T. gondii invasion machinery.

Calmodulin distribution and the actomyosin cytoskeleton in Toxoplasma gondii

The gliding motility of the protozoan parasite Toxoplasma gondii and its invasion of cells are powered by an actin-myosin motor. We have studied the spatial distribution and relationship between these two cytoskeleton proteins and calmodulin (CaM), the Ca(2+)-dependent protein involved in invasion by T. gondii. A 3D reconstruction using labeling and tomographic studies showed that actin was present as a V-like structure in the conoidal part of the parasite. The myosin distribution overlapped that of actin, and CaM was concentrated at the center of the apical pole. We demonstrated that the actomyosin network, CaM, and myosin light-chain kinases are confined to the apical pole of the T. gondii tachyzoite. MLCK could act as an intermediate molecule between CaM and the cytoskeleton proteins. We have developed a model of the organization of the actomyosin-CaM complex and the steps of a signaling pathway for parasite motility.

Regulation and expression of metazoan unconventional myosins

Unconventional myosins are molecular motors that convert adenosine triphosphate (ATP) hydrolysis into movement along actin filaments. On the basis of primary structure analysis, these myosins are represented by at least 15 distinct classes (classes 1 and 3-16), each of which is presumed to play a specific cellular role. However, in contrast to the conventional myosins-2, which drive muscle contraction and cytokinesis and have been studied intensively for many years in both uni- and multicellular organisms, unconventional myosins have only been subject to analysis in metazoan systems for a short time. Here we critically review what is known about unconventional myosin regulation, function, and expression. Several points emerge from this analysis. First, in spite of the high relative conservation of motor domains among the myosin classes, significant differences are found in biochemical and enzymatic properties of these motor domains. Second, the idea that characteristic distributions of unconventional myosins are solely dependent on the myosin tail domain is almost certainly an oversimplification. Third, the notion that most unconventional myosins function as transport motors for membranous organelles is challenged by recent data. Finally, we present a scheme that clarifies relationships between various modes of myosin regulation.

Gliding motility and cell invasion by Apicomplexa: insights from the Plasmodium sporozoite

Apicomplexa constitute one of the largest phyla of protozoa. Most Apicomplexa, including those pathogenic to humans, are obligate intracellular parasites. Their extracellular forms, which are highly polarized and elongated cells, share two unique abilities: they glide on solid substrates without changing their shape and reach an intracellular compartment without active participation from the host cell. There is now ample ultrastructural evidence that these processes result from the backward movement of extracellular interactions along the anteroposterior axis of the parasite. Recent work in several Apicomplexa, including genetic studies in the Plasmodium sporozoite, has provided molecular support for this 'capping' model. It appears that the same machinery drives both gliding motility and host cell invasion. The cytoplasmic motor, a transmembrane bridge and surface ligands essential for cell invasion are conserved among the main apicomplexan pathogens.

Molecular characterization of TgMIC5, a proteolytically processed antigen secreted from the micronemes of Toxoplasma gondii

During invasion of host cells, Toxoplasma gondii discharges the contents of small, apically located secretory organelles called micronemes. Micronemal proteins are known to be necessary for both parasite motility and invasion of host cells. To further define the contents of Toxoplasma micronemes, we used cell fractionation and secretion-modulating drugs to identify six novel, putative micronemal proteins. In this paper we describe preliminary characterization of one of these novel proteins, TgMIC5. Molecular cloning and DNA sequence analysis of the TgMIC5 cDNA and gene revealed that it encodes a previously identified immunodominant antigen called H4. TgMIC5 also possesses a consensus sequence unique to members of the parvulin family of peptidyl-prolyl cis-trans isomerases (PPIases). TgMIC5 is expressed as a preproprotein, which is proteolytically processed to a proprotein by signal peptidase before being further processed to a mature protein of 22 kDa. Using a combination of protein secretion experiments, immunofluorescence and immunoelectron microscopy, we demonstrated that TgMIC2 is stored in the micronemes of T. gondii tachyzoites before it is secreted into the surrounding medium. Based on its homology with parvulin-like PPIases, TgMIC5 may assist in the folding of other micronemal proteins that function in invasion of host cells by T. gondii tachyzoites.

Host cell invasion by malaria parasites

The complex life cycle of the malaria parasite includes three specialized invasive stages, distinct both in terms of their cellular architecture and in their choice of target host cell. Despite the dissimilarities between these forms, there are clear parallels in the manner by which they enter their respective host cells. Advances in the area of erythrocyte invasion by the malaria merozoite, outlined here by Chetan Chitnis and Mike Blackman and discussed at the Molecular Approaches to Malaria conference, Lorne, Australia, 2-5 February 2000, will undoubtedly impact on our understanding of mechanisms of cell entry by the other invasive forms. Similarly, recent progress in dissecting the functional role of surface proteins expressed by sporozoite and ookinete stages has provided fascinating insights into general aspects of invasion by all invasive stages of apicomplexan parasites.

Motile systems in malaria merozoites: how is the red blood cell invaded?

The ability of the malaria parasite to invade erythrocytes is central to the disease process, but is not thoroughly understood. In particular, little attention has been paid to the motor systems driving invasion. Here, Jennifer Pinder, Ruth Fowler and colleagues review motility in the merozoite. The components of an actomyosin motor are present, including a novel unconventional class XIV myosin, now called Pfmyo-A, which, because of its time of synthesis and location, is likely to generate the force required for invasion. In addition, there is a subpellicular microtubule assemblage in falciparum merozoites, the f-MAST, the integrity of which is necessary for invasion.

Immunization with parasite-derived apical membrane antigen 1 or passive immunization with a specific monoclonal antibody protects BALB/c mice against lethal Plasmodium yoelii yoelii YM blood-stage infection

We have purified apical merozoite antigen 1 (AMA-1) from extracts of red blood cells infected with the rodent malaria parasite Plasmodium yoelii yoelii YM. When used to immunize mice, the protein induced a strong protective response against a challenge with the parasite. Monoclonal antibodies specific for P. yoelii yoelii AMA-1 were prepared, and one was very effective against the parasite on passive immunization. A second protein that appears to be located in the apical rhoptry organelles and associated with AMA-1 was identified.

Isolation and identification of actin-binding proteins in Plasmodium falciparum by affinity chromatography

The invasion of the erythrocyte by Plasmodium falciparum depends on the ability of the merozoite to move through the membrane invagination. This ability is probably mediated by actin dependent motors. Using affinity columns with G-actin and F-actin we isolated actin binding proteins from the parasite. By immunoblotting and immunoprecipitation with specific antibodies we identified the presence of tropomyosin, myosin, a-actinin, and two different actins in the eluate corresponding to F-actin binding proteins. In addition to these, a 240-260 kDa doublet, different in size from the erythrocyte spectrin, reacted with an antibody against human spectrin. All the above mentioned proteins were metabolically radiolabeled when the parasite was cultured with 35S-methionine. The presence of these proteins in P. falciparum is indicative of a complex cytoskeleton and supports the proposed role for an actin-myosin motor during invasion.

A dibasic motif in the tail of a class XIV apicomplexan myosin is an essential determinant of plasma membrane localization

Obligate intracellular parasites of the phylum Apicomplexa exhibit gliding motility, a unique form of substrate-dependent locomotion essential for host cell invasion and shown to involve the parasite actin cytoskeleton and myosin motor(s). Toxoplasma gondii has been shown to express three class XIV myosins, TgM-A, -B, and -C. We identified an additional such myosin, TgM-D, and completed the sequences of a related Plasmodium falciparum myosin, PfM-A. Despite divergent structural features, TgM-A purified from parasites bound actin in an ATP-dependent manner. Isoform-specific antibodies revealed that TgM-A and recombinant mycTgM-A were localized right beneath the plasma membrane, and subcellular fractionation indicated a tight membrane association. Recombinant TgM-D also had a peripheral although not as sharply defined localization. Truncation of their respective tail domains abolished peripheral localization and tight membrane association. Conversely, fusion of the tails to green fluorescent protein (GFP) was sufficient to confer plasma membrane localization and sedimentability. The peripheral localization of TgM-A and of the GFP-tail fusion did not depend on an intact F-actin cytoskeleton, and the GFP chimera did not localize to the plasma membrane of HeLa cells. Finally, we showed that the specific localization determinants were in the very C terminus of the TgM-A tail, and site-directed mutagenesis revealed two essential arginine residues. We discuss the evidence for a proteinaceous plasma membrane receptor and the implications for the invasion process.

Characterization of myosin-A and myosin-C: two class XIV unconventional myosins from Toxoplasma gondii

Two class XIV unconventional myosins from Toxoplasma gondii, Myosin-A (TgM-A) and Myosin-C (TgM-C), were characterized in terms of their biochemical properties and their expression in quiescent and motile stages of the parasite life cycle. In cell fractionation studies, both myosins partitioned with the major organelle/cell membrane fraction, and extraction studies indicated that both were tightly associated with membrane domains as detergent was necessary for their solubilization. In addition, both TgM-A and TgM-C demonstrated a hallmark feature of myosins in their ability to bind actin in the absence but not the presence of ATP. In parasites residing within the host cell parasitophorous vacuole, TgM-A was detected by immunofluorescence microscopy as a bright spot near the apical pole of the parasite. This pattern underwent a subtle change as the parasites became motile, with TgM-A then localizing more intimately with the parasite cell membrane domain in apically disposed spots or patches, consistent with the role of this myosin in gliding motility. TgM-C showed a distinct localization to the juxtanuclear region towards the apical pole of the parasite, consistent with an association with the Golgi apparatus.

Diatom gliding is the result of an actin-myosin motility system

Diatoms are a group of unicellular microalgae that are encased in a highly ornamented siliceous cell wall, or frustule. Pennate diatoms have bilateral symmetry and many genera possess an elongated slit in the frustule called the raphe, a feature synonymous with their ability to adhere and glide over a substratum, a process little understood. We have used cytoskeleton-disrupting drugs to investigate the roles of actin, myosin, and microtubules in diatom gliding or motility. No effect on diatom gliding was observed using the cytochalasins, known actin inhibitors, or the microtubule-inhibitors oryzalin and nocodazole. The latrunculins are a new group of anti-actin drugs, and we show here that they are potent inhibitors of diatom gliding, resulting in the complete disassociation of the raphe-associated actin cables. The recovery of actin staining and motility following latrunculin treatment was extremely fast. Cells exposed to latrunculin for 12 h recovered full function and actin staining within 5 sec of the drug being removed, demonstrating that the molecular components required for this motility system are immediately available. Butanedione monoxime (BDM), a known myosin inhibitor, also reversibly inhibited diatom gliding in a manner similar to the latrunculins. This work provides evidence that diatom gliding is based on an actin/myosin motility system.

Induction of an acrosomal process in Toxoplasma gondii: visualization of actin filaments in a protozoan parasite

The invasive stages of Toxoplasma gondii, an Apicomplexan parasite, actively invade their host cells in an actin-dependent way. However, despite containing biochemically significant amounts of actin, actin filaments have never been observed in these parasites. Jasplakinolide, a membrane-permeable actin-polymerizing and filament-stabilizing drug, induced the polymerization of actin filaments at the anterior end of each tachyzoite in association with the conoid, where they formed, in many cases, a prominent membrane-enclosed apical projection reminiscent of acrosomal processes of invertebrate sperm. These jasplakinolide-induced filaments decorated with myosin subfragment 1, demonstrating unequivocally that they were indeed actin. Jasplakinolide-treated tachyzoites were unable to invade host cells, but once the drug was removed the parasites were able to enter host cells. Actin polymerization at the apical end of the parasite is consistent with the role of the apical end in host-cell invasion powered by a jackhammer-like extension and retraction of the conoid complex coupled to the secretion and rearward capping of surface proteins.