The secretary pathway of plasmodium falciparum regulates transport of p82/RAP1 to the rhoptries

The Ins and Outs of Plasmodium Rhoptries, Focusing on the Cytosolic Side

Parasites of the genus Plasmodium cause human and animal malaria, leading to significant health and economic impacts. A key aspect of the complex life cycle of Plasmodium parasites is the invasion of the parasite into its host cell, which is mediated by secretory organelles. The largest of these organelles, the rhoptry, undergoes rapid and profound physiological changes when it secretes its contents during merozoite and sporozoite invasion of the host erythrocyte and hepatocyte, respectively. Here we discuss recent advancements in our understanding of the dynamic rhoptry biology during the parasite's invasive stages, with a focus on the roles of cytosolically exposed rhoptry-interacting proteins (C-RIPs). We explore potential similarities between the molecular mechanisms driving merozoite and sporozoite rhoptry function.

An Endoplasmic Reticulum CREC Family Protein Regulates the Egress Proteolytic Cascade in Malaria Parasites

The endoplasmic reticulum (ER) is thought to play an essential role during egress of malaria parasites because the ER is assumed to be required for biogenesis and secretion of egress-related organelles. However, no proteins localized to the parasite ER have been shown to play a role in egress of malaria parasites. In this study, we generated conditional mutants of the Plasmodium falciparum endoplasmic reticulum-resident calcium-binding protein (PfERC), a member of the CREC family. Knockdown of the PfERC gene showed that this gene is essential for asexual growth of P. falciparum Analysis of the intraerythrocytic life cycle revealed that PfERC is essential for parasite egress but is not required for protein trafficking or calcium storage. We found that PfERC knockdown prevents the rupture of the parasitophorous vacuole membrane. This is because PfERC knockdown inhibited the proteolytic maturation of the subtilisin-like serine protease SUB1. Using double mutant parasites, we showed that PfERC is required for the proteolytic maturation of the essential aspartic protease plasmepsin X, which is required for SUB1 cleavage. Further, we showed that processing of substrates downstream of the proteolytic cascade is inhibited by PfERC knockdown. Thus, these data establish that the ER-resident CREC family protein PfERC is a key early regulator of the egress proteolytic cascade of malaria parasites.IMPORTANCE The divergent eukaryotic parasites that cause malaria grow and divide within a vacuole inside a host cell, which they have to break open once they finish cell division. The egress of daughter parasites requires the activation of a proteolytic cascade, and a subtilisin-like protease initiates a proteolytic cascade to break down the membranes blocking egress. It is assumed that the parasite endoplasmic reticulum plays a role in this process, but the proteins in this organelle required for egress remain unknown. We have identified an early ER-resident regulator essential for the maturation of the recently discovered aspartic protease in the egress proteolytic cascade, plasmepsin X, which is required for maturation of the subtilisin-like protease. Conditional loss of PfERC results in the formation of immature and inactive egress proteases that are unable to breakdown the vacuolar membrane barring release of daughter parasites.

Discovery of functional motifs in h-regions of trypanosome signal sequences

N-terminal signal peptides direct secretory proteins into the ER (endoplasmic reticulum) of eukaryotes or the periplasmic space of prokaryotes. A hydrophobic core (h-region) is important for signal sequence function; however, the mechanism of h-region action is not resolved. To gain new insight into signal sequences, bioinformatic analysis of h-regions from humans, Saccharomyces cerevisiae, Trypanosoma brucei and Escherichia coli was performed. Each species contains a unique set of peptide motifs (h-motifs) characterized by identity components (i.e. sequence of conserved amino acids) joined by spacers. Human h-motifs have four identity components, whereas those from the other species utilize three identity components. Example of h-motifs are human Hs3 {L-x(2)-[AGILPV]-L-x(0,2)-L}, S. cerevisiae Sc1 [L-x(0,2)-S-x(0,3)-A], T. brucei Tb2 {L-x(1,2)-L-[AILV]} and E. coli Ec1 [A-x(0,2)-L-x(0,3)-A]. The physiological relevance of h-motifs was tested with a T. brucei microsomal system for translocation of a VSG (variant surface glycoprotein)-117 signal peptide. Disruption of h-motifs by scrambling of sequences in h-regions produced defective signal peptides, although the hydrophobicity of the peptide was not altered. We conclude that: (i) h-regions harbour h-motifs, and are not random hydrophobic amino acids; (ii) h-regions from different species contain unique sets of h-motifs; and (iii) h-motifs contribute to the biological activity of ER signal peptides. h-Regions are ‘scaffolds’ in which functional h-motifs are embedded. A hypothetical model for h-motif interactions with a Sec61p protein translocon is presented.

Post-translational modifications in Plasmodium: more than you think!

Recent evidences indicate that transcription in Plasmodium may be hard-wired and rigid, deviating from the classical model of transcriptional gene regulation. Thus, it is important that other regulatory pathways be investigated as a comprehensive effort to curb the deadly malarial parasite. Research in post-translational modifications in Plasmodium is an emerging field that may provide new venues for drug discovery and potential new insights into how parasitic protozoans regulate their life cycle. Here, we discuss the recent findings of post-translational modifications in Plasmodium.

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.

Identification and characterization of a novel Plasmodium falciparum merozoite apical protein involved in erythrocyte binding and invasion

Proteins that coat Plasmodium falciparum merozoite surface and those secreted from its apical secretory organelles are considered promising candidates for the vaccine against malaria. In the present study, we have identified an asparagine rich parasite protein (PfAARP; Gene ID PFD1105w), that harbors a predicted signal sequence, a C-terminal transmembrane region and whose transcription and translation patterns are similar to some well characterized merozoite surface/apical proteins. PfAARP was localized to the apical end of the merozoites by GFP-targeting approach using an inducible, schizont-stage expression system, by immunofluorescence assays using anti-PfAARP antibodies. Immuno-electron microsopic studies showed that PfAARP is localized in the apical ends of the rhoptries in the merozoites. RBC binding assays with PfAARP expressed on COS cells surface showed that it binds to RBCs through its N-terminal region with a receptor on the RBC surface that is sensitive to trypsin and neuraminidase treatments. Sequencing of PfAARP from different P. falciparum strains as well as field isolates showed that the N-terminal region is highly conserved. Recombinant protein corresponding to the N-terminal region of PfAARP (PfAARP-N) was produced in its functional form in E. coli. PfAARP-N showed reactivity with immune sera from individuals residing in P. falciparum endemic area. The anti-PfAARP-N rabbit antibodies significantly inhibited parasite invasion in vitro. Our data on localization, functional assays and invasion inhibition, suggest a role of PfAARP in erythrocyte binding and invasion by the merozoite.

Protein targeting to destinations of the secretory pathway in the malaria parasite Plasmodium falciparum

The secretory pathway in the malaria parasite Plasmodium falciparum has many unique aspects in terms of protein destinations and trafficking mechanisms. Recently, several exciting insights into protein trafficking within this intracellular parasite have been unveiled: these include signals that are required for targeting of proteins to the red blood cell and the relict plastid (known as the apicoplast); and the elucidation of the pathways of the haemoglobin proteases targeted to the food vacuole. Protein-targeting to the apical organelles in P. falciparum, however, is still not very well understood, but available research offers a tantalising glimpse of the system.

Plasmodium rhoptries: how things went pear-shaped

Plasmodium parasites have three sets of specialised secretory organelles at the apical end of their invasive forms--rhoptries, micronemes and dense granules. The contents of these organelles are responsible for or contribute to host cell invasion and modification, and at least four apical proteins are leading vaccine candidates. Given the unusual nature of Plasmodium invasion, it is not surprising that unique proteins are involved in this process. Nowhere is this more evident than in rhoptries. We have collated data from several recent studies to compile a rhoptry proteome. Discussion is focussed here on rhoptry content and function.

Dissection of brefeldin A-sensitive and -insensitive steps in apicoplast protein targeting

The apicoplast is a relict plastid found in many apicomplexans, including the pathogens Toxoplasma gondii and Plasmodium falciparum. Nucleus-encoded apicoplast proteins enter the ER, and after cleavage of the signal sequence, are routed to the apicoplast by virtue of a transit peptide, which is subsequently removed. To assess the mechanisms of localization we examined stable transfectants of T. gondii for the localization and processing of various GFP fusion proteins. GFP fusions bearing apicoplast targeting sequences targeted efficiently to the plastid, with no retention in the ER, even when an ER retention/retrieval sequence was added. Incubation with brefeldin A, which blocks ER-to-Golgi trafficking by inhibiting a GTP exchange factor required for retrograde trafficking, blocked the processing of the protein. Surprisingly, it did not affect the immunofluorescence pattern. To avoid the potentially misleading presence of pre-existing GFP fusion protein in the apicoplast, we used a ligand-regulated aggregation system to arrest the GFP fusion protein in the ER prior to trafficking. Upon addition of ligand to promote disaggregation, the fusion protein targeted to the plastid, even in the presence of brefeldin A. Ligand release at 15 degrees C, which blocks trafficking of Golgi-routed proteins, also allowed significant localization to the plastid. Our data indicate that apicoplast proteins can localize to the region of the plastid when Golgi trafficking is inhibited, but suggest that some steps in import or maturation of the proteins may require a brefeldin A-sensitive GTP exchange factor.

Identification of a piroplasm protein of Theileria orientalis that binds to bovine erythrocyte band 3

Theileria orientalis infects cattle and causes various disease symptoms, including anaemia and icterus. The erythrocytic stages are responsible for these symptoms but the molecular events involved in these stages have not yet been fully elucidated. In this study, we identified a T. orientalis cDNA that encodes a polypeptide related to identity to the microneme-rhoptry protein of Theileria parva. Analysis of its recombinant product (ToMRP) by indirect fluorescent-antibody test revealed that it is specifically expressed at the early erythrocytic stage after invasion. This expression disappears during the intermediate stages of intra-erythrocytic development. Its expression then reappears at the late stages after the parasite has divided by binary fission into diad or tetrad forms and before these forms are released from the host erythrocyte. In vitro erythrocyte binding assays showed that ToMRP associates with the Triton X-insoluble fraction of erythrocytes membrane but not with intact erythrocytes. Cosedimentation and Western blot analyses revealed that ToMRP binds to band 3, a membrane component of bovine erythrocytes. These observations suggest that ToMRP may be involved in the parasite's egress from and/or invasion into the host erythrocytes by interacting with a protein in the membrane skeleton of the erythrocyte and thereby modifying the structure and function of the cell.

Are rhoptries in Apicomplexan parasites secretory granules or secretory lysosomal granules?

The club-shaped rhoptries in Apicomplexan parasites are one of the most unusual secretory organelles among the eukaryotes, containing unusual lipid and protein cargo that is specialized for intracellular parasitism. Rhoptries have traditionally been viewed strictly as regulated secretory granules. We discuss in this article recent data on the cargo, function and biogenesis of rhoptries in two parasitic model systems, Toxoplasma and Plasmodium. Current findings suggest that rhoptries receive products from both biosynthetic and endocytic pathways and, therefore, they are most analogous to secretory lysosomal granules found in mammalian cells.

Specific erythrocyte binding capacity and biological activity of Plasmodium falciparum-derived rhoptry-associated protein 1 peptides

Rhoptry-associated protein 1 (RAP1) is a merozoite antigen within Plasmodium falciparum rhoptries as yet having no specific function described for it. Synthetic peptides spanning the RAP1 sequence were tested in erythrocyte binding assays to identify possible RAP1 functional regions. Five high activity binding peptides (HABPs) were identified; 26201, 26202, 26203 and 26204 spanned residues 461C-K540 within RAP1 Cys region, whilst 26188 (201T-Y220) was located in p67 amino terminal. The results showed that peptide binding was saturable, some HABPs inhibited in vitro merozoite invasion and specifically bound to a 72 kDa protein in red blood cell membrane. HABP possible function in merozoite invasion of erythrocytes is also discussed.

Functional analysis of Plasmodium falciparum merozoite antigens: implications for erythrocyte invasion and vaccine development

Malaria is a major human health problem and is responsible for over 2 million deaths per year. It is caused by a number of species of the genus Plasmodium, and Plasmodium falciparum is the causative agent of the most lethal form. Consequently, the development of a vaccine against this parasite is a priority. There are a number of stages of the parasite life cycle that are being targeted for the development of vaccines. Important candidate antigens include proteins on the surface of the asexual merozoite stage, the form that invades the host erythrocyte. The development of methods to manipulate the genome of Plasmodium species has enabled the construction of gain-of-function and loss-of-function mutants and provided new strategies to analyse the role of parasite proteins. This has provided new information on the role of merozoite antigens in erythrocyte invasion and also allows new approaches to address their potential as vaccine candidates.

The malaria-infected red blood cell: structural and functional changes

The asexual stage of malaria parasites of the genus Plasmodium invade red blood cells of various species including humans. After parasite invasion, red blood cells progressively acquire a new set of properties and are converted into more typical, although still simpler, eukaryotic cells by the appearance of new structures in the red blood cell cytoplasm, and new proteins at the red blood cell membrane skeleton. The red blood cell undergoes striking morphological alterations and its rheological properties are considerably altered, manifesting as red blood cells with increased membrane rigidity, reduced deformability and increased adhesiveness for a number of other cells including the vascular endothelium. Elucidation of the structural changes in the red blood cell induced by parasite invasion and maturation and an understanding of the accompanying functional alterations have the ability to considerably extend our knowledge of structure-function relationships in the normal red blood cell. Furthermore, interference with these interactions may lead to previously unsuspected means of reducing parasite virulence and may lead to the development of novel antimalarial therapeutics.

Traffic jams: protein transport in Plasmodium falciparum

Protein targeting in malaria parasites is a complex process, involving several cellular compartments that distinguish these cells from more familiar systems, such as yeast or mammals. At least a dozen distinct protein destinations are known. The best studied of these is the vestigial chloroplast (the apicoplast), but new tools promise rapid progress in understanding how Plasmodium falciparum and related apicomplexan parasites traffic proteins to their invasion-related organelles, and how they modify the host by trafficking proteins into its cytoplasm and plasma membrane. Here, Giel van Dooren and colleagues discuss recent insights into protein targeting via the secretory pathway in this fascinating and important system. This topic emerged as a major theme at the Molecular Approaches to Malaria conference, Lorne, Australia, 2-5 February 2000.

Functional analysis of proteins involved in Plasmodium falciparum merozoite invasion of red blood cells

Plasmodium falciparum causes the most lethal form of malaria in humans and is responsible for over two million deaths per year. The development of a vaccine against this parasite is an urgent priority and potential protein targets include those on the surface of the asexual merozoite stage, the form that invades the host erythrocyte. The development of methods to transfect P. falciparum has enabled the construction of gain-of-function and loss-of-function mutants and provided new strategies to analyse the role of parasite proteins. In this review, we describe the use of this technology to examine the role of merozoite antigens in erythrocyte invasion and to address their potential as vaccine candidates.

RAP1 controls rhoptry targeting of RAP2 in the malaria parasite Plasmodium falciparum

Rhoptry associated protein 1 (RAP1) and 2 (RAP2), together with a poorly described third protein RAP3, form the low molecular weight complex within the rhoptries of Plasmodium falciparum. These proteins are thought to play a role in erythrocyte invasion by the extracellular merozoite and are important vaccine candidates. We used gene-targeting technology in P.falciparum blood-stage parasites to disrupt the RAP1 gene, producing parasites that express severely truncated forms of RAP1. Immunoprecipitation experiments suggest that truncated RAP1 species did not complex with RAP2 and RAP3. Consistent with this were the distinct subcellular localizations of RAP1 and 2 in disrupted RAP1 parasites, where RAP2 does not traffic to the rhoptries but is instead located in a compartment that appears related to the lumen of the endoplasmic reticulum. These results suggest that RAP1 is required to localize RAP2 to the rhoptries, supporting the hypothesis that rhoptry biogenesis is dependent in part on the secretory pathway in the parasite. The observation that apparently host-protective merozoite antigens are not essential for efficient erythrocyte invasion has important implications for vaccine design.

Spatial and temporal dynamics of the secretory pathway during differentiation of the Plasmodium yoelii schizont

A specialized complex of apical organelles facilitates Plasmodium merozoite invasion into the erythrocyte. Even though the apical organelles are crucial to the invasion process, relatively little is known about how they function or their biosynthesis during asexual replication. MAEBL is an erythrocyte binding protein located in the rhoptries and on the surface of mature merozoites and is expressed at the beginning of schizogony before the first nuclear division. Therefore, we have characterized MAEBL as a marker for the biosynthetic pathway of the rhoptry apical organelle during the final phase of intraerythrocytic development and as a marker for the nascent rhoptry vesicle in the immature schizont. An extensive proliferation of the endoplasmic reticulum occurred at the onset of schizogony and was seen as a complex but transient tubule array near the parasite surface. Both the rhoptry protein MAEBL and surface protein MSP-1 appeared to be present in this tubular reticular network together with endoplasmic reticulum markers. MAEBL then transits through Golgi bodies positioned near the parasite plasma membrane, directly adjacent to the network. Rhoptry organelle precursors are seen at the three to four nuclei stage of schizont development, remaining near the plasma membrane throughout schizogony. These studies constitute the first direct evidence that proteins of the rhoptry organelles transit through compartments of the 'classical' secretory pathway.

Plasmodium yoelii: identification of a gene encoding a putative ADP-ribosylation factor-1 GTPase-activating protein, PyAG1

The PyAG1 gene, identified by the screening of a Plasmodium yoelii genomic DNA library with a rhoptry-specific Mab, encodes a protein with a zinc finger structure immediately followed by the consensus sequence of the Arf GAP catalytic site. The serum of mice immunized with the recombinant protein recognized specifically the rhoptries of the late infected erythrocytic stages. Blast analysis using the Genbank database gave the highest scores with four proteins presenting an Arf1 GAP activity. If presenting also this activity, the PyAG1 protein could be involved in the regulation of the secreted protein vesicular transport and, consequently, in the rhoptry biogenesis.

Protein trafficking to the plastid of Plasmodium falciparum is via the secretory pathway

The plastid of Plasmodium falciparum (or 'apicoplast') is the evolutionary homolog of the plant chloroplast and represents a vestige of a photosynthetic past. Apicoplast indispensability indicates that it still provides essential functions to parasites. Similar to plant chloroplasts, the apicoplast is dependent on many nucleus-encoded genes to provide these functions. The apicoplast is surrounded by four membranes, two more than plant chloroplasts. Thus, protein targeting to the apicoplast must overcome additional membrane barriers. In P.falciparum we have analyzed apicoplast targeting using green fluorescent protein (GFP). We demonstrate that protein targeting is at least a two-step process mediated by bipartite N-terminal pre-sequences that consist of a signal peptide for entry into the secretory pathway and a plant-like transit peptide for subsequent import into the apicoplast. The P.falciparum transit peptide is exceptional compared with other known plastid transit peptides in not requiring serine or threonine residues. The pre-sequence components are removed stepwise during apicoplast targeting. Targeting GFP to the apicoplast has also provided the first opportunity to examine apicoplast morphology in live P. falciparum.

The transport of the histidine-rich protein I from Plasmodium falciparum is insensitive to brefeldin A

During its intraerythrocytic development, Plasmodium falciparum synthesizes several proteins that are exported beyond its membrane. Some of these secreted antigens are involved in the formation of protuberances or knobs, a major virulence factor, at the erythrocyte membrane. Various secreted malarial polypeptides, the transport of which is sensitive to brefeldin A, are translocated in vitro into dog pancreatic microsomes. We present evidence that the histidine-rich protein I (PfHRPI) is secreted by the parasite via a novel pathway, independent of the ER/Golgi apparatus. The secretion of PfHRPI was not blocked by incubation of parasite cultures at 15 degrees C and 20 degrees C or 37 degrees C in the presence of brefeldin A. PfHRPI was not translocated into microsomes in an in vitro translation-translocation cell-free system. Unlike other polypeptides from eukaryotic cells that bypass the ER/Golgi pathway and do not have a signal peptide, PfHRPI has an atypical signal sequence consisting of 21 amino acids, including eight positively charged residues followed by 11 hydrophobic residues. We also found that the unusually charged PfHRPI signal sequence diverts Exp-1, which is usually exported, away from the translocation machinery of microsomal membranes.

An alternative secretory pathway in Plasmodium: more questions than answers

The malaria parasite extensively modifies the host erythrocyte. Many of these modifications are mediated by proteins exported from the parasite and targeted to specific locations within the infected erythrocyte. However, little is known about how the parasite targets proteins to specific locations beyond its own plasma membrane. Treatment of infected erythrocytes with brefeldin A results in the accumulation of many exported Plasmodium proteins into a compartment distinct from the ER. Proteins destined for the host erythrocyte membrane, the parasitophorous vacuole or inclusions within the erythrocyte cytoplasm accumulate in this novel compartment, and co-localization studies indicate that there is a single compartment per parasite. Exported proteins only accumulate in this novel compartment if brefeldin A treatment is concurrent with their synthesis. This novel compartment is probably a membrane-bound organelle located at the parasite periphery, and may be the first step in an alternative secretory pathway that specializes in the export of proteins into the host cell. Such an alternative secretory pathway raises questions about how exported proteins are differentially targeted to this novel organelle versus the ER and the fate of exported proteins after this novel organelle.

Theileria parva 104 kDa microneme--rhoptry protein is membrane-anchored by a non-cleaved amino-terminal signal sequence for entry into the endoplasmic reticulum

The 104 kDa microneme-rhoptry protein (p104) is the only known apical complex organelle-specific protein of Theileria parva. p104 exhibits striking structural similarities to circumsporozoite protein and sporozoite surface protein 2 of Plasmodium yoelii. Their primary sequences contain two hydrophobic segments, located at the amino-and the carboxy-terminus. The p104 amino-terminal hydrophobic region was suggested to be a signal peptide for entry into the endoplasmic reticulum and the extreme carboxy-terminal region to function as a membrane anchor. We have studied the biogenesis of p104 in a cell-free expression system and found that p104 is co-translationally transported into membranes derived from endoplasmic reticulum. The amino-terminal signal peptide is not cleaved off and anchors the protein in the membrane with the carboxy-terminal portion translocated into the lumen. We suggest that in vivo p104 is co-translationally integrated into the membrane of the endoplasmic reticulum, from where it is further transported to the microneme-rhoptry complex. Thus, p104 appears to be a suitable marker to study the development of micronemes and rhoptries in T. parva.

Processing of Toxoplasma ROP1 protein in nascent rhoptries

Secretion in the obligate intracellular parasite, Toxoplasma gondii, occurs through a number of regulated compartments. Among these are the apical organelles known as rhoptries which release their contents as part of the invasion process. We are interested in the processing, targeting and ultimate function of rhoptry proteins (and have focused our analyses on rhoptry protein 1 (ROP1). In this paper, we address the issue of processing: using a number of engineered forms of the ROP1 gene (introduced into a ROP1- background), we show that ROP1 is synthesized as a pre-pro-protein that is subject to proteolytic cleavages to remove the pre-sequence and the 'pro' region, at the N-terminus. Using brefeldin A (BFA) and reduced temperature we show that this processing occurs late in the secretory pathway of the parasite. Immunolocalization studies with epitope-tagged constructs indicate that processing is apparently occurring in the nascent rhoptries of dividing parasites. The results are discussed in the context of the targeting and possible function of the ROP1 protein.

A subtilisin-like protein in secretory organelles of Plasmodium falciparum merozoites

In the vertebrate host, the malaria parasite invades and replicates asexually within circulating erythrocytes. Parasite proteolytic enzymes play an essential but poorly understood role in erythrocyte invasion. We have identified a Plasmodium falciparum gene, denoted pfsub-1, encoding a member of the subtilisin-like serine protease family (subtilases). The pfsub-1 gene is expressed in asexual blood stages of P. falciparum, and the primary gene product (PfSUB-1) undergoes post-translational processing during secretory transport in a manner consistent with its being converted to a mature, enzymatically active form, as documented for other subtilases. In the invasive merozoite, the putative mature protease (p47) is concentrated in dense granules, which are secretory organelles located toward the apical end of the merozoite. At some point following merozoite release and completion of erythrocyte invasion, p47 is secreted from the parasite in a truncated, soluble form. The subcellular location and timing of secretion of p47 suggest that it is likely to play a role in erythrocyte invasion. PfSUB-1 is a new potential target for antimalarial drug development.

Analysis of the processing of Plasmodium falciparum rhoptry-associated protein 1 and localization of Pr86 to schizont rhoptries and p67 to free merozoites

The processing and localization of Plasmodium falciparum rhoptry-associated protein 1 (RAP-1) products were examined using polyclonal and monoclonal antibodies raised to a recombinant protein containing residues 1-294 of RAP-1. Immunoblot and epitope mapping results with antibodies that selectively bound epitopes in the RAP-1 products Pr86, p82, and p67 showed that p82 and p67 are formed from Pr86 by progressive removal of epitopes from the amino-terminus of the RAP-1 coding sequence. The capacity of Pr86 to form complexes was revealed after size fractionation of parasite proteins radiolabeled in the presence of brefeldin A to prevent processing of Pr86. Fractions containing complexed Pr86 also contained the RAP-2 product p39 and the RAP-3 product p37, suggesting that Pr86, p39 and p37 may form complexes similar to complexes previously reported for p82 and p67 with p39 or p37. Immunofluorescence localization and immunoblot studies revealed that Pr86 is present in the rhoptries, but only transiently, and that it is not detected in segmenting schizonts or extracellular merozoites. p67 and p82, on the other hand, were shown to be major RAP-1 components in purified merozoites. Neither p67 nor p82 were relocalized from the intracellular rhoptries to the merozoite surface under conditions that promoted relocalization of the rhoptry protein PF83/apical membrane antigen 1. These results suggest that processing of Pr86 begins after Pr86 complexes are transported to the forming rhoptries and that two site-selective processing reactions occur in the rhoptries, a rapid cleavage of Pr86 to p82 and a delayed cleavage of p82 to p67. Since p67 is missing from ring-stage parasites (Howard et al., Am J Trop Med Hyg, 1984;33:1055 59), the present results indicate there is a narrow time during which p67 may play a role in merozoite invasion of erythrocytes.

Analysis of inhibitory epitopes in the Plasmodium falciparum rhoptry protein RAP-1 including identification of a second inhibitory epitope

Immune responses to Plasmodium falciparum rhoptry-associated protein 1 (RAP-1), RAP-2, and RAP-3 appear to contribute to protection against infection by this human malarial parasite. This conclusion is suggested by results of monkey immunization trials and of cell culture studies showing antibody-dependent inhibition of erythrocyte invasion. In the present study, splenectomized owl monkeys were infected with P. falciparum in order to monitor anti-RAP-1 antibody production as antiparasite immunity developed. The monkeys responded to a primary infection with the production of antibodies to a fragment of RAP-1 containing amino acids 1 to 294 (RAP-1(1-294)). After drug cure and reinfection, the monkeys had a prolonged prepatent period, indicating they had already developed partial immunity to the parasite. Sera from these animals showed major increases in anti-RAP-1(1-294) antibodies. In contrast, only low levels of antibodies to inhibitory B-cell epitope 1 (iB-1), an inhibitory epitope in RAP-1(1-294) with the sequence N200TLTPLEELYPT211, was observed after the initial parasite infection, and the anti-iB-1 antibodies were not readily boosted upon reinfection. These results suggest that iB-1 is an immunogenic but not immunodominant epitope and that anti-iB-1 antibodies do not substantially contribute to early stages of naturally acquired immunity in the owl monkey model. To identify additional epitopes bound by inhibitory antibodies, mouse monoclonal antibodies were produced with a recombinant fusion protein containing RAP-1(1-294). Monoclonal antibody 1D6 inhibited parasite invasion of erythrocytes in vitro. 1D6 did not bind peptide iB-1 but rather bound a second inhibitory epitope called iB-2. iB-2, like iB-1, is found near the amino terminus of p67, a RAP-1 processing product thought to be involved in merozoite invasion of erythrocytes. Since anti-iB-1 antibodies were not readily produced during parasite infection, it may be desirable to direct antibody responses to particular epitopes in RAP-1, such as iB-1 and iB-2.

A novel alternate secretory pathway for the export of Plasmodium proteins into the host erythrocyte

The malarial parasite dramatically alters its host cell by exporting and targeting proteins to specific locations within the erythrocyte. Little is known about the mechanisms by which the parasite is able to carry out this extraparasite transport. The fungal metabolite brefeldin A (BFA) has been used to study the secretory pathway in eukaryotes. BFA treatment of infected erythrocytes inhibits protein export and results in the accumulation of exported Plasmodium proteins into a compartment that is at the parasite periphery. Parasite proteins that are normally localized to the erythrocyte membrane, to nonmembrane bound inclusions in the erythrocyte cytoplasm, or to the parasitophorous vacuolar membrane accumulate in this BFA-induced compartment. A single BFA-induced compartment is detected per parasite and the various exported proteins colocalize to this compartment regardless of their final destinations. Parasite membrane proteins do not accumulate in this novel compartment, but accumulate in the endoplasmic reticulum (ER), suggesting that the parasite has two secretory pathways. This alternate secretory pathway is established immediately after merozoite invasion and at least some dense granule proteins also use the alternate pathway. The BFA-induced compartment exhibits properties that are similar to the ER, but it is clearly distinct from the ER. We propose to call this new organelle the secondary ER of apicomplexa. This ER-like organelle is an early, if not the first, step in the export of Plasmodium proteins into the host erythrocyte.

Identification and localization of rab6, separation of rab6 from ERD2 and implications for an 'unstacked' Golgi, in Plasmodium falciparum

The rab6 gene product in mammalian cells and yeast is localized to and regulates protein transport in the medial and trans Golgi cisternae, as well as the trans Golgi network. We have identified a homologue in the malaria parasite Plasmodium falciparum which displays a rab-like sequence that is 62.4% identical to mammalian rab6. In addition the parasite gene (Pfrab6 gene) contains an N-terminal hydrophobic domain, unique to P. falciparum. Antibodies developed to Pfrab6 localize protein in 4-7 well-resolved sites in a ring-stage parasite, as detected by high resolution fluorescence microscopy. This suggests that there are multiple, distinct foci of medial/trans Golgi membranes in a ring. ERD2 is a cis Golgi marker in mammalian cells. The plasmodial homologue of ERD2 (PfERD2) is concentrated in a single perinuclear region in a ring-stage parasite. This site is distinct from the Pfrab6 membranes, indicating that early and late Golgi markers can be segregated in P. falciparum. Mammalian cells contain a single Golgi complex where cis medial and trans markers are tightly stacked in closely apposed cisternae. In P. falciparum-rings however, rab6-associated membranes are not invariably 'stacked' with an ERD2 structure. In immunoelectron microscopy studies, both the PfERD2- and Pfrab6-associated membranes appear tubulovesicular in nature, devoid of cisternal morphology. Hence the Golgi of ring stage parasites may comprise of multiple, 'unstacked' tubulovesicular clusters, suggesting a primitive organization of the organelle in Plasmodia.

Rhoptry organelles of the apicomplexa: Their role in host cell invasion and intracellular survival

Members of the phylum Apicomplexa are obligate intracellular parasites that invade erythrocytes, lymphocytes, macrophages or cells of the alimentary canal in various vertebrate species. Organelles within the apical complex of invasive stages facilitate host cell invasion. Parasites in this phylum cause some of the most debilitating diseases of medical and veterinary importance. These include malaria, toxoplasmosis, babesiosis, theileriosis (East Coast fever), and coccidiosis in poultry and livestock. In recent years, opportunistic infections caused by Cryptosporidium parvum, and recrudescent Toxoplasma gondii infections in AIDS patients have prompted intensified efforts in understanding the biology of these parasites. In this review, Tobili Sam-Yellowe examines the unifying and variant molecular features of rhoptry proteins, and addresses the role of multigene families in organelle function: the biogenesis of the rhoptries will also be examined, in an attempt to understand the sequence of events leading to successful packaging, modification and processing of proteins within the organelle.