Plasmodium falciparum signal sequences: simply sequences or special signals?

A revised mechanism for how Plasmodium falciparum recruits and exports proteins into its erythrocytic host cell

Plasmodium falciparum exports ~10% of its proteome into its host erythrocyte to modify the host cell's physiology. The Plasmodium export element (PEXEL) motif contained within the N-terminus of most exported proteins directs the trafficking of those proteins into the erythrocyte. To reach the host cell, the PEXEL motif of exported proteins is processed by the endoplasmic reticulum (ER) resident aspartyl protease plasmepsin V. Then, following secretion into the parasite-encasing parasitophorous vacuole, the mature exported protein must be unfolded and translocated across the parasitophorous vacuole membrane by the Plasmodium translocon of exported proteins (PTEX). PTEX is a protein-conducting channel consisting of the pore-forming protein EXP2, the protein unfoldase HSP101, and structural component PTEX150. The mechanism of how exported proteins are specifically trafficked from the parasite's ER following PEXEL cleavage to PTEX complexes on the parasitophorous vacuole membrane is currently not understood. Here, we present evidence that EXP2 and PTEX150 form a stable subcomplex that facilitates HSP101 docking. We also demonstrate that HSP101 localises both within the parasitophorous vacuole and within the parasite's ER throughout the ring and trophozoite stage of the parasite, coinciding with the timeframe of protein export. Interestingly, we found that HSP101 can form specific interactions with model PEXEL proteins in the parasite's ER, irrespective of their PEXEL processing status. Collectively, our data suggest that HSP101 recognises and chaperones PEXEL proteins from the ER to the parasitophorous vacuole and given HSP101's specificity for the EXP2-PTEX150 subcomplex, this provides a mechanism for how exported proteins are specifically targeted to PTEX for translocation into the erythrocyte.

Epitopes of anti-RIFIN antibodies and characterization of rif-expressing Plasmodium falciparum parasites by RNA sequencing

Variable surface antigens of Plasmodium falciparum have been a major research focus since they facilitate parasite sequestration and give rise to deadly malaria complications. Coupled with its potential use as a vaccine candidate, the recent suggestion that the repetitive interspersed families of polypeptides (RIFINs) mediate blood group A rosetting and influence blood group distribution has raised the research profile of these adhesins. Nevertheless, detailed investigations into the functions of this highly diverse multigene family remain hampered by the limited number of validated reagents. In this study, we assess the specificities of three promising polyclonal anti-RIFIN antibodies that were IgG-purified from sera of immunized animals. Their epitope regions were mapped using a 175,000-peptide microarray holding overlapping peptides of the P. falciparum variable surface antigens. Through immunoblotting and immunofluorescence imaging, we show that different antibodies give varying results in different applications/assays. Finally, we authenticate the antibody-based detection of RIFINs in two previously uncharacterized non-rosetting parasite lines by identifying the dominant rif transcripts using RNA sequencing.

Plasmodium falciparum signal recognition particle components and anti-parasitic effect of ivermectin in blocking nucleo-cytoplasmic shuttling of SRP

Signal recognition particle (SRP) is a ubiquitous ribonucleoprotein complex that targets proteins to endoplasmic reticulum (ER) in eukaryotes. Here we report that Plasmodium falciparum SRP is composed of six polypeptides; SRP9, SRP14, SRP19, SRP54, SRP68 and SRP72 and a 303nt long SRP RNA. We generated four transgenic parasite lines expressing SRP-GFP chimeric proteins and co-localization studies showed the nucleo-cytoplasmic localization for these proteins. The evaluation of the effect of known SRP and nuclear import/export inhibitors on P. falciparum revealed that ivermectin, an inhibitor of importin α/β mediated nuclear import inhibited the nuclear import of PfSRP polypeptides at submicromolar concentration, thereby killing the parasites. These findings provide insights into dynamic structure of P. falciparum SRP and also raise the possibility that ivermectin could be used in combination with other antimalarial agents to control the disease.

Torins are potent antimalarials that block replenishment of Plasmodium liver stage parasitophorous vacuole membrane proteins

Residence within a customized vacuole is a highly successful strategy used by diverse intracellular microorganisms. The parasitophorous vacuole membrane (PVM) is the critical interface between Plasmodium parasites and their possibly hostile, yet ultimately sustaining, host cell environment. We show that torins, developed as ATP-competitive mammalian target of rapamycin (mTOR) kinase inhibitors, are fast-acting antiplasmodial compounds that unexpectedly target the parasite directly, blocking the dynamic trafficking of the Plasmodium proteins exported protein 1 (EXP1) and upregulated in sporozoites 4 (UIS4) to the liver stage PVM and leading to efficient parasite elimination by the hepatocyte. Torin2 has single-digit, or lower, nanomolar potency in both liver and blood stages of infection in vitro and is likewise effective against both stages in vivo, with a single oral dose sufficient to clear liver stage infection. Parasite elimination and perturbed trafficking of liver stage PVM-resident proteins are both specific aspects of torin-mediated Plasmodium liver stage inhibition, indicating that torins have a distinct mode of action compared with currently used antimalarials.

Malaria parasite signal peptide peptidase is an ER-resident protease required for growth but not for invasion

The establishment of parasite infection within the human erythrocyte is an essential stage in the development of malaria disease. As such, significant interest has focused on the mechanics that underpin invasion and on characterization of parasite molecules involved. Previous evidence has implicated a presenilin-like signal peptide peptidase (SPP) from the most virulent human malaria parasite, Plasmodium falciparum, in the process of invasion where it has been proposed to function in the cleavage of the erythrocyte cytoskeletal protein Band 3. The role of a traditionally endoplasmic reticulum (ER) protease in the process of red blood cell invasion is unexpected. Here, using a combination of molecular, cellular and chemical approaches we provide evidence that PfSPP is, instead, a bona fide ER-resident peptidase that remains intracellular throughout the invasion process. Furthermore, SPP-specific drug inhibition has no effect on erythrocyte invasion whilst having low micromolar potency against intra-erythrocytic development. Contrary to previous reports, these results show that PfSPP plays no role in erythrocyte invasion. Nonetheless, PfSPP clearly represents a potential chemotherapeutic target to block parasite growth, supporting ongoing efforts to develop antimalarial-targeting protein maturation and trafficking during intra-erythrocytic development.

Isolation and characterization of type I signal peptidase of different malaria parasites

Type I signal peptidases are important membrane-bound serine proteases responsible for the cleavage of the signal peptide of the proteins. These enzymes are unique serine proteases that carry out catalysis using a serine/lysine catalytic dyad. In the present study, we report the isolation of type I signal peptidase from the malaria parasites Plasmodium falciparum, Plasmodium knowlesi, and Plasmodium yoelii and some characterization of type I signal peptidase of Plasmodium falciparum. We show that these enzymes are homologous to signal peptidases from various sources and also contain the conserved boxes present in other type I signal peptidases. The type I signal peptidase from P falciparum is an intron-less and a single-copy gene. The results also show that the enzyme from Plasmodium falciparum is subject to self-cleavage and it has been demonstrated to possess type I signal peptidase activity in E coli preprotein processing in vivo by complementation assay. This study will be helpful in understanding one of the important metabolic pathways “the secretory pathway” in the parasite and should make an important contribution in understanding the complex process of protein targeting in the parasite.

Identification of rhoptry trafficking determinants and evidence for a novel sorting mechanism in the malaria parasite Plasmodium falciparum

The rhoptry of the malaria parasite Plasmodium falciparum is an unusual secretory organelle that is thought to be related to secretory lysosomes in higher eukaryotes. Rhoptries contain an extensive collection of proteins that participate in host cell invasion and in the formation of the parasitophorous vacuole, but little is known about sorting signals required for rhoptry protein targeting. Using green fluorescent protein chimeras and in vitro pull-down assays, we performed an analysis of the signals required for trafficking of the rhoptry protein RAP1. We provide evidence that RAP1 is escorted to the rhoptry via an interaction with the glycosylphosphatidyl inositol-anchored rhoptry protein RAMA. Once within the rhoptry, RAP1 contains distinct signals for localisation within a sub-compartment of the organelle and subsequent transfer to the parasitophorous vacuole after invasion. This is the first detailed description of rhoptry trafficking signals in Plasmodium.

Characterization of Plasmodium falciparum integral membrane protein Pf25-IMP and identification of its red blood cell binding sequences inhibiting merozoite invasion in vitro

The identification of proteins present on the surface of Plasmodium falciparum-infected red blood cells as well as of free merozoites has been widely considered as one of the main areas of research in the development of an antimalarial vaccine due to their involvement in the parasite's pathogenesis and invasion mechanisms. Major advances had been accomplished in this area thanks to the analysis of the reported genomic sequence of P. falciparum, allowing for the identification of genes encoding for putative integral membrane proteins. This study reports for the first time the transcription of the MAL8P1.3 gene, which codifies for a 25-kDa integral membrane protein of P. falciparum (FCB-2 strain), namely, Pf25-IMP. Western blot and immunofluorescence assays using goat polyclonal sera indicate that this protein is expressed in erythrocytic asexual blood stages. A highly robust, sensible, and specific receptor-ligand interaction assay allowed identification of two high activity binding peptides (HABPs) derived from Pf25-IMP: 30577 ((41)YKTANENVKLASSLSDRLSR(60)) and 30583 ((161)LNKKTVVRKIAEGLGYTIVF(180)). Both HABPs bound with high affinity to human red blood cells (RBCs), and such binding was susceptible to enzyme treatment with trypsin. A common RBC surface receptor of apparently 48 kDa was found for both HABPs, plus an additional 31-kDa receptor for HABP 30577. HABP 30577 inhibited merozoite invasion in vitro by 73%, while HABP 30583 showed a 59% inhibition at 200 microM concentration. The data suggest a possible role of Pf25-IMP in merozoite invasion to RBCs and support its inclusion in further immunological studies for evaluating its potential as vaccine candidates.

Plasmodium falciparum signal peptidase is regulated by phosphorylation and required for intra-erythrocytic growth

The human malaria parasite Plasmodium falciparum exports a variety of its proteins through its endoplasmic reticulum (ER) based secretory pathway in order to survive in the host erythrocyte. Signal peptidases are membrane-bound endopeptidases and have an important role in the transport and maturation of these parasite proteins. Prokaryotic signal peptidases are indispensable enzymes required for the removal of N-terminal signal peptide from the secretory proteins. Eukaryotic signal peptidases exist as multimeric protein complex in the ER and the catalytic subunit of this complex catalyzes removal of the N-terminal signal peptide from preproteins. All the signal peptidases contain five regions of high-sequence similarity referred to as boxes A-E. Here we report characterization of the catalytic subunit of signal peptidase complex (SPC) from P. falciparum. This protein designated as PfSP21 shows homology with the similar subunit from other sources and contains all the conserved boxes A-E. PfSP21 is able to cleave the peptide substrate containing the signal peptidase cleavage site. PfSP21 is phosphorylated by protein kinase C and its enzyme activity was upregulated after this phosphorylation. Immunofluorescence assay studies revealed that PfSP21 is localized in the ER of P. falciparum. PfSP21 dsRNA specifically inhibits the growth of P. falciparum in culture and this inhibition is most likely due to the decrease in the amount of endogenous PfSP21 protein. These studies demonstrate the characterization of a functional subunit of SPC from P. falciparum and should make an important contribution in our better understanding of the complex process of protein translocation in the parasite.

Traffic to the Malaria Parasite Food Vacuole

Phosphatidylinositol 3-phosphate (PI3P) is a key ligand for recruitment of endosomal regulatory proteins in higher eukaryotes. Subsets of these endosomal proteins possess a highly selective PI3P binding zinc finger motif belonging to the FYVE domain family. We have identified a single FYVE domain-containing protein in Plasmodium falciparum which we term FCP. Expression and mutagenesis studies demonstrate that key residues are involved in specific binding to PI3P. In contrast to FYVE proteins in other organisms, endogenous FCP localizes to a lysosomal compartment, the malaria parasite food vacuole (FV), rather than to cytoplasmic endocytic organelles. Transfections of deletion mutants further indicate that FCP is essential for trophozoite and FV maturation and that it traffics to the FV via a novel constitutive cytoplasmic to vacuole targeting pathway. This newly discovered pathway excludes the secretory pathway and is directed by a C-terminal 44-amino acid peptide domain. We conclude that an FYVE protein that might be expected to participate in vesicle targeting in the parasite cytosol instead has a vital and functional role in the malaria parasite FV.

Protein export from Plasmodium parasites

Many prokaryotic and eukaryotic intracellular pathogens survive by altering the host cell through the export of proteins. In contrast to the well-studied prokaryotic export systems, knowledge of protein export in eukaryotic pathogens is scant. The recent discovery that a short protein sequence targets a protein for export from the malaria parasite Plasmodium falciparum has shed light on the possible mechanism of proteins export and has allowed the preliminary identification of several hundred exported proteins. Among the exported proteins are the members of the paralogous protein families, previously identified exported proteins and many uncharacterized proteins. The interaction of the parasite with the host cell is thus much more complex, and involves more parasite proteins, than previously thought.

Biochemical properties and cellular localization of Plasmodium falciparum protein disulfide isomerase

We have previously reported the isolation of a 52,000 M(r) protein (Pf52) displaying consensus sequences for thiol:disulfide oxidoreductases. Pf52 therefore represents the plasmodial protein disulfide isomerase (PDI). It has been renamed PfPDI and correlates to MAL8P1.17 in the annotated genome of P. falciparum (3D7 strain). Antibodies were raised against recombinant (His)(6)-tagged forms of PfPDI devoid of its signal peptide sequence, demonstrating a major co-localization of PfPDI with endoplasmic reticulum-resident proteins, PfBIP and PfERC, but not with the Golgi marker PfERD2. Recombinant PfPDI displayed typical biochemical functions of PDIs: oxidase/isomerase and reductase activities, as well as a chaperone-like behavior on the denaturated protein rhodanese. These activities were comparable to those measured for the purified native bovine PDI and the human recombinant PDI. The antiplasmodial compound DS61 does inhibit the recombinant PfPDI oxidase/isomerase activity but not that of the human recombinant PDI, suggesting structural differences between both enzymes. However, a discrepancy between the inhibitory activity of DS61 on the recombinant PfPDI (IC(50) of 430 microM) and its in vitro antiplasmodial activity (IC(50) of 0.1 microM) was observed, suggesting that PfPDI is not the only target of DS61. Taking into account its biochemical properties and its intracellular localization, the involvement of PfPDI in the parasite protein folding is discussed, as well as its potential for the development of alternative antimalarial chemotherapy strategies.

Theileria parva candidate vaccine antigens recognized by immune bovine cytotoxic T lymphocytes

East Coast fever, caused by the tick-borne intracellular apicomplexan parasite Theileria parva, is a highly fatal lymphoproliferative disease of cattle. The pathogenic schizont-induced lymphocyte transformation is a unique cancer-like condition that is reversible with parasite removal. Schizont-infected cell-directed CD8(+) cytotoxic T lymphocytes (CTL) constitute the dominant protective bovine immune response after a single exposure to infection. However, the schizont antigens targeted by T. parva-specific CTL are undefined. Here we show the identification of five candidate vaccine antigens that are the targets of MHC class I-restricted CD8(+) CTL from immune cattle. CD8(+) T cell responses to these antigens were boosted in T. parva-immune cattle resolving a challenge infection and, when used to immunize naïve cattle, induced CTL responses that significantly correlated with survival from a lethal parasite challenge. These data provide a basis for developing a CTL-targeted anti-East Coast fever subunit vaccine. In addition, orthologs of these antigens may be vaccine targets for other apicomplexan parasites.

Maurer's clefts: A novel multi-functional organelle in the cytoplasm of Plasmodium falciparum-infected erythrocytes

Discovered in 1902 by Georg Maurer as a peculiar dotted staining pattern observable by light microscopy in the cytoplasm of erythrocytes infected with the human malarial parasite Plasmodium falciparum, the function of Maurer's clefts have remained obscure for more than a century. The growing interest in protein sorting and trafficking processes in malarial parasites has recently aroused the Maurer's clefts from their deep slumber. Mounting evidence suggests that Maurer's clefts are a secretory organelle, which the parasite establishes within its host erythrocyte, but outside its own confines, to route parasite proteins across the host cell cytoplasm to the erythrocyte surface where they play a role in nutrient uptake and immune evasion processes. Moreover, Maurer's clefts seem to play a role in cell signaling, merozoite egress, phospholipid biosynthesis and, possibly, other biochemical pathways. Here, we review our current knowledge of the ultrastructure of Maurer's clefts, their proteinaceous composition and their function in protein trafficking.

Fractionation and identification of proteins by 2-DE and MS: towards a proteomic analysis ofPlasmodium falciparum

Since completion of genome sequencing of the malarial parasite Plasmodium falciparum, proteomic tools for the identification of parasite proteins have become particularly attractive as they allow a more thorough interpretation of these data. Recent advances in 2-D PAGE, MS, and bioinformatics have created great opportunities for mapping and characterization of protein populations. We employed these improvements in a proteomic approach for the analysis of proteins detected in two blood stages of P. falciparum, (i) in the schizont stage and (ii) in the merozoite stage. For the isolation of merozoites, we introduced a new protocol based on the preparation of clustered structures of merozoites upon treatment of cultures with the common cysteine proteinase inhibitor E64. Peptide mass fingerprints of excised and trypsinated protein spots, acquired by MALDI-TOF MS were generated to identify a variety of proteins. Moreover, prefractionation procedures were used to enrich and map low-abundance proteins in protein samples. The data demonstrate that classic proteomic analyses using 2-D PAGE are now feasible for P. falciparum and represent the first step in the direction of creating 2-D reference maps for this medically most relevant protozoon.

Pexel/VTS: a protein-export motif in erythrocytes infected with malaria parasites

The trafficking of proteins from the malaria parasite into the erythrocyte host has an important role in both the adaptation by the parasite of its immediate environment and the pathophysiology of disease. The molecular basis of these trafficking processes, particularly export from the parasitophorous vacuole (PV), is poorly understood and a matter of some controversy. In this article, we highlight two recent, independent reports that have provided new insights into protein translocation across the PV membrane, characterizing a novel signalling motif as a key signature in cataloguing the parasite secretome.

Biosynthesis, localization, and processing of falcipain cysteine proteases of Plasmodium falciparum

Falcipain-2 and -3 are cysteine proteases of erythrocytic Plasmodium falciparum parasites that appear to function principally as hemoglobinases. To better understand their biological roles, we analyzed the biosynthesis, localization, and processing of these enzymes in cultured parasites. Immunoprecipitation of metabolically labeled proteins indicated that falcipain-2 was synthesized through the trophozoite stage, falcipain-3 appeared in late trophozoites/early schizonts, and both proteases persisted for at least 6 h after synthesis. Falcipain-2 and -3 were localized to the food vacuole, the site of hemoglobin hydrolysis, by immunofluorescence and immunoelectron microscopy. Subcellular fractionation experiments indicated that both enzymes were synthesized as membrane bound proforms that were processed to soluble mature forms, but falcipain-2 was processed to the mature protease much more quickly than was falcipain-3. Cysteine protease inhibitors and brefeldin A, but not aspartic or serine protease inhibitors, blocked the processing of both enzymes, suggesting that falcipain-2 and -3 process by autohydrolysis after exiting the endoplasmic reticulum/Golgi network. However, although all tested cysteine protease inhibitors blocked hemoglobinase activity in the food vacuole, only lipophilic inhibitors (E-64d, Mu-Leu-Hph-VSPh, and ALLN), blocked intracellular processing of falcipain-2 and -3. More hydrophilic inhibitors (E-64 and leupeptin) did not block processing, suggesting that autocatalytic processing of falcipain-2 and -3 occurs in a specific cellular compartment before delivery to the food vacuole. Our results support overlapping but not fully redundant roles for falcipain-2 and -3, which are targeted to the food vacuole and activated sequentially to degrade hemoglobin in erythrocytic parasites.

Trafficking of the major virulence factor to the surface of transfected P. falciparum-infected erythrocytes

After invading human red blood cells (RBCs) the malaria parasite Plasmodium falciparum remodels the host cell by trafficking proteins to the RBC compartment. The virulence protein P. falciparum erythrocyte membrane protein 1 (PfEMP1) is responsible for cytoadherence of infected cells to host endothelial receptors. This protein is exported across the parasite plasma membrane and parasitophorous vacuole membrane and inserted into the RBC membrane. We have used green fluorescent protein chimeras and fluorescence photobleaching experiments to follow PfEMP1 export through the infected RBC. Our data show that a knob-associated histidine-rich protein (KAHRP) N-terminal protein export element appended to the PfEMP1 transmembrane and C-terminal domains was sufficient for efficient trafficking of protein domains to the outside of the P. falciparum-infected RBC. The physical state of the exported proteins suggests trafficking as a complex rather than in vesicles and supports the hypothesis that endogenous PfEMP1 is trafficked in a similar manner. This study identifies the sequences required for expression of proteins to the outside of the P. falciparum-infected RBC membrane.

A potential novel mechanism for the insertion of a membrane protein revealed by a biochemical analysis of the Plasmodium falciparum cytoadherence molecule PfEMP-1

Plasmodium falciparum erythrocyte membrane protein-1 (PfEMP-1) is exposed on the surface of infected erythrocytes where it both acts as an important pathogenicity factor in malaria and undergoes antigenic variation as a means of immune evasion. Because the mammalian erythrocyte lacks a protein secretory machinery there has been much interest in elucidating the mechanism whereby this protein is transferred from its site of synthesis within the parasite to its final destination. Current opinion favours a mechanism whereby PfEMP-1 becomes cotranslationally inserted into the endoplasmic reticulum of the parasite and is subsequently transported as an integral part of an erythrocyte cytoplasmic membrane system derived from the parasite. Here we show that the solubility characteristics of this protein during several stages of its transport pathway are inconsistent with this view. Instead we propose that the protein is synthesized as a peripheral membrane protein which only when it arrives at its final destination assumes a transmembrane topology. Even in this state, the extractability of the protein with urea suggest that it is anchored in the membrane by protein-protein rather than by protein-lipid interaction.

Protein transport and trafficking inPlasmodium falciparum-infected erythrocytes

The human malarial parasitePlasmodium falciparumextensively modifies its host erythrocyte, and to this end, is faced with an interesting challenge. It must not only sort proteins to common organelles such as endoplasmic reticulum, Golgi and mitochondria, but also target proteins across the ‘extracellular’ cytosol of its host cell. Furthermore, as a member of the phylum Apicomplexa, the parasite has to sort proteins to novel organelles such as the apicoplast, micronemes and rhoptries. In order to overcome these difficulties, the parasite has created a novel secretory system, which has been characterized in ever-increasing detail in the past decade. Along with the ‘hardware’ for a secretory system, the parasite also needs to ‘program’ proteins to enable high fidelity sorting to their correct subcellular location. The nature of these sorting signals has remained until relatively recently, enigmatic. Experimental work has now begun to dissect the sorting signals responsible for correct subcellular targeting of parasite-encoded proteins. In this review we summarize the current understanding of such signals, and comment on their role in protein sorting in this organism, which may become a model for the study of novel protein trafficking mechanisms.

The apicoplast: a plastid in Plasmodium falciparum and other Apicomplexan parasites

Apicomplexan parasites cause severe diseases such as malaria, toxoplasmosis, and coccidiosis (caused by Plasmodium spp., Toxoplasma, and Eimeria, respectively). These parasites contain a relict plastid-termed "apicoplast"--that originated from the engulfment of an organism of the red algal lineage. The apicoplast is indispensable but its exact role in parasites is unknown. The apicoplast has its own genome and expresses a small number of genes, but the vast majority of the apicoplast proteome is encoded in the nuclear genome. The products of these nuclear genes are posttranslationally targeted to the organelle via the secretory pathway courtesy of a bipartite N-terminal leader sequence. Apicoplasts are nonphotosynthetic but retain other typical plastid functions such as fatty acid, isoprenoid and heme synthesis, and products of these pathways might be exported from the apicoplast for use by the parasite. Apicoplast pathways are essentially prokaryotic and therefore excellent drug targets. Some antibiotics inhibiting these molecular processes are already in chemotherapeutic use, whereas many new drugs will hopefully spring from our growing understanding of this intriguing organelle.

Characterization of proteins localized to a subcellular compartment associated with an alternate secretory pathway of the malaria parasite

Monoclonal antibodies recognizing proteins localized to a unique subcellular compartment within the malaria parasite are described in this report. These monoclonal antibodies recognize Plasmodium falciparum proteins of 68, 45 and 22 kDa proteins which are also conserved in rodent Plasmodium species. Co-localization studies indicate that these proteins are located in a brefeldin A-induced compartment which was previously proposed to be an early step in the export of proteins from the parasite into the infected erythrocyte. COPII coat proteins, Sar1p and Sec31p, and the endoplasmic reticulum-associated chaperone, BiP, all partially co-localize with the 68 and 22 kDa proteins, thus suggesting that this subcellular compartment has some similarities to the endoplasmic reticulum or that this compartment represents a domain of the endoplasmic reticulum. The 68 and 22 kDa proteins are highly soluble in non-ionic detergent and are likely to be located within the lumen of a membrane-bound compartment. These proteins found within this subcellular compartment are present throughout the blood stage from very early rings to segmenters. The results of this study further substantiate the existence of an alternate secretory pathway in the malaria parasite which plays a role in the export of proteins into the host erythrocyte.

The signal sequence of exported protein-1 directs the green fluorescent protein to the parasitophorous vacuole of transfected malaria parasites

The malaria parasite, Plasmodium falciparum, spends part of its life cycle inside the erythrocytes of its human host. In the mature stages of intraerythrocytic growth, the parasite undertakes extensive remodeling of its adopted cellular home by exporting proteins beyond the confines of its own plasma membrane. To examine the signals involved in export of parasite proteins, we have prepared transfected parasites expressing a chimeric protein comprising the N-terminal region of the Plasmodium falciparum exported protein-1 appended to green fluorescent protein. The majority of the population of the chimeric protein appears to be correctly processed and trafficked to the parasitophorous vacuole, indicating that this is the default destination for protein secretion. Some of the protein is redirected to the parasite food vacuole and further degraded. Photobleaching studies reveal that the parasitophorous vacuole contains subcompartments that are only partially interconnected. Dual labeling with the lipid probe, BODIPY-TR-ceramide, reveals the presence of membrane-bound extensions that can bleb from the parasitophorous vacuole to produce double membrane-bound compartments. We also observed regions and extensions of the parasitophorous vacuole, where there is segregation of the lumenal chimera from the lipid components. These regions may represent sites for the sorting of proteins destined for the trafficking to sites beyond the parasitophorous vacuole membrane.

Protein and lipid trafficking induced in erythrocytes infected by malaria parasites

The human malaria parasite Plasmodium falciparum develops in a parasitophorous vacuolar membrane (PVM) within the mature red cell and extensively modifies structural and antigenic properties of this host cell. Recent studies shed significant new, mechanistic perspective on the underlying processes. There is finally, definitive evidence that despite the absence of endocytosis, transmembrane proteins in the host red cell membrane are imported in to the PVM. These are not major erythrocyte proteins but components that reside in detergent resistant membrane (DRM) rafts in red cell membrane and are detected in rafts in the PVM. Disruption of either erythrocyte or vacuolar rafts is detrimental to infection suggesting that raft proteins and lipids are essential for the parasitization of the red cell. On secretory export of parasite proteins: an ER secretory signal (SS) sequence is required for protein secretion to the PV. Proteins carrying an additional plastid targeting sequence (PTS) are also detected in the PV but subsequently delivered to the plastid organelle within the parasite, suggesting that the PTS may have a second function as an endocytic sorting signal. A distinct but yet undefined peptidic motif underlies protein transport across the PVM to the red cell (although all of the published data does not yet fit this model). Further multiple exported proteins transit through secretory 'cleft' structures, suggesting that clefts may be sorting compartments assembled by the parasite in the red cell.