Identification and disruption of the gene encoding the third member of the low-molecular-mass rhoptry complex in Plasmodium falciparum

Molecular Characterization and Immunological Evaluation of Truncated Babesia microti Rhoptry Neck Protein 2 as a Vaccine Candidate

Babesia microti is a protozoan that infects red blood cells. Babesiosis is becoming a new global threat impacting human health. Rhoptry neck proteins (RONs) are proteins located at the neck of the rhoptry and studies indicate that these proteins play an important role in the process of red blood cell invasion. In the present study, we report on the bioinformatic analysis, cloning, and recombinant gene expression of two truncated rhoptry neck proteins 2 (BmRON2), as well as their potential for incorporation in a candidate vaccine for babesiosis. Western blot and immunofluorescence antibody (IFA) assays were performed to detect the presence of specific antibodies against BmRON2 in infected mice and the localization of N-BmRON2 in B. microti parasites. In vitro experiments were carried out to investigate the role of BmRON2 proteins during the B. microti invasion process and in vivo experiments to investigate immunoprotection. Homologous sequence alignment and molecular phylogenetic analysis indicated that BmRON2 showed similarities with RON2 proteins of other Babesia species. We expressed the truncated N-terminal (33-336 aa, designated rN-BmRON2) and C-terminal (915-1171 aa, designated rC-BmRON2) fragments of the BmRON2 protein, with molecular weights of 70 and 29 kDa, respectively. Western blot assays showed that the native BmRON2 protein is approximately 170 kDa, and that rN-BmRON2 was recognized by serum of mice experimentally infected with B. microti. Immunofluorescence analysis indicated that the BmRON2 protein was located at the apical end of merozoites, at the opposite end of the nucleus. In vitro red blood cell invasion inhibition studies with B. microti rBmRON2 proteins showed that relative invasion rate of rN-BmRON2 and rC-BmRON2 group is 45 and 56%, respectively. Analysis of the host immune response after immunization and B. microti infection showed that both rN-BmRON2 and rC-BmRON2 enhanced the immune response, but that rN-BmRON2 conferred better protection than rC-BmRON2. In conclusion, our results indicate that truncated rhoptry neck protein 2, especially its N-terminal fragment (rN-BmRON2), plays an important role in the invasion of host red blood cells, confers immune protection, and shows good potential as a candidate vaccine against babesiosis.

Identification of a Novel RAMA/RON3 Rhoptry Protein Complex in Plasmodium falciparum Merozoites

Malaria causes a half a million deaths annually. The parasite intraerythrocytic lifecycle in the human bloodstream is the major cause of morbidity and mortality. Apical organelles of merozoite stage parasites are involved in the invasion of erythrocytes. A limited number of apical organellar proteins have been identified and characterized for their roles during erythrocyte invasion or subsequent intraerythrocytic parasite development. To expand the repertoire of identified apical organellar proteins we generated a panel of monoclonal antibodies against Plasmodium falciparum schizont-rich parasites and screened the antibodies using immunofluorescence assays. Out of 164 hybridoma lines, 12 clones produced monoclonal antibodies yielding punctate immunofluorescence staining patterns in individual merozoites in late schizonts, suggesting recognition of merozoite apical organelles. Five of the monoclonal antibodies were used to immuno-affinity purify their target antigens and these antigens were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Two known apical organelle protein complexes were identified, the high-molecular mass rhoptry protein complex (PfRhopH1/Clags, PfRhopH2, and PfRhopH3) and the low-molecular mass rhoptry protein complex (rhoptry-associated proteins complex, PfRAP1, and PfRAP2). A novel complex was additionally identified by immunoprecipitation, composed of rhoptry-associated membrane antigen (PfRAMA) and rhoptry neck protein 3 (PfRON3) of P. falciparum. We further identified a region spanning amino acids Q₂₂₁-E₄₈₁ within the PfRAMA that may associate with PfRON3 in immature schizonts. Further investigation will be required as to whether PfRAMA and PfRON3 interact directly or indirectly.

Plasmodium falciparum parasites exit the infected erythrocyte after haemolysis with saponin and streptolysin O

Malaria is caused by unicellular parasites of the genus Plasmodium, which reside in erythrocytes during the clinically relevant stage of infection. To separate parasite from host cell material, haemolytic agents such as saponin are widely used. Previous electron microscopy studies on saponin-treated parasites reported both, parasites enclosed by the erythrocyte membrane and liberated from the host cell. These ambiguous reports prompted us to investigate haemolysis by live-cell time-lapse microscopy. Using either saponin or streptolysin O to lyse Plasmodium falciparum-infected erythrocytes, we found that ring-stage parasites efficiently exit the erythrocyte upon haemolysis. For late-stage parasites, we found that only approximately half were freed, supporting the previous electron microscopy studies. Immunofluorescence imaging indicated that freed parasites were surrounded by the parasitophorous vacuolar membrane. These results may be of interest for future work using haemolytic agents to enrich for parasite material.

A multistage antimalarial targets the plasmepsins IX and X essential for invasion and egress

Regulated exocytosis by secretory organelles is important for malaria parasite invasion and egress. Many parasite effector proteins, including perforins, adhesins, and proteases, are extensively proteolytically processed both pre- and postexocytosis. Here we report the multistage antiplasmodial activity of the aspartic protease inhibitor hydroxyl-ethyl-amine-based scaffold compound 49c. This scaffold inhibits the preexocytosis processing of several secreted rhoptry and microneme proteins by targeting the corresponding maturases plasmepsins IX (PMIX) and X (PMX), respectively. Conditional excision of PMIX revealed its crucial role in invasion, and recombinantly active PMIX and PMX cleave egress and invasion factors in a 49c-sensitive manner.

Disrupting CD147-RAP2 interaction abrogates erythrocyte invasion by Plasmodium falciparum

Effective vaccines against malaria caused by Plasmodium falciparum are still lacking, and the molecular mechanism of the host-parasite interaction is not fully understood. Here we demonstrate that the interaction of RAP2, a parasite-secreted rhoptry protein that functions in the parasitophorous vacuole formation stage of the invasion, and CD147 on the host erythrocyte is essential for erythrocyte invasion by P falciparum and is independent from all previously identified interactions involved. Importantly, the blockade of the CD147-RAP2 interaction by HP6H8, a humanized CD147 antibody, completely abolished the parasite invasion with both cure and preventative functions in a humanized mouse model. Together with its long half-life on human red blood cells and its safety profile in cynomolgus monkeys, HP6H8 is the first antibody that offers an advantageous approach by targeting a more conserved late-stage parasite ligand for preventing as well as treating severe malaria.

Comparative genomics and proteomic analyses between lethal and nonlethal strains of Plasmodium berghei

Plasmodium berghei (Pb) XAT, a rodent malaria parasite, is an irradiation-attenuated variant derived from the lethal strain Pb NK65. Differences in genome sequence, protein structure and function between Pb XAT and Pb NK65 are currently unknown. In this study, to investigate genetic alterations in Pb XAT, we performed comparative genomics and proteomics analyses of nonlethal and lethal strains of Pb. We found mutations, such as a deletion mutation in rhoptry-associated protein (rap) 1, and deletion of rap2/3 and skeleton-binding protein 1 (sbp1), in Pb XAT. RAP1 is required for targeting of RAP2 to the rhoptries. However, the contribution of RAP2/3 to the lethality of Plasmodium is unclear. Therefore, we generated RAP1- and RAP2/3-deficient mutants of Pb ANKA, a reference strain of P. berghei. Furthermore, we investigated the effect of RAP1 and RAP2/3 deficiency on the outcome of infection. The parasitemia in mice infected with RAP1-deficient parasites was increased compared to that in control parasite-infected mice during the early phase of infection. However, mice infected with RAP1-deficient parasites survived longer than did control parasite-infected mice. Moreover, mice infected with RAP2/3-deficient parasites showed low levels of parasitemia and ultimately recovered from the infection The aim of this study was to investigate the effect of RAP2/3 expression on the outcome of infection with Pb XAT using a RAP2/3-expressing Pb XAT. Results showed that complementation of RAP2/3 expression in Pb XAT partially restored virulence. Our findings suggest that RAP1 and RAP2/3 contribute to virulence and a decrease in their expression explains the loss of virulence of the Pb XAT strain.

CD81 is required for rhoptry discharge during host cell invasion by Plasmodium yoelii sporozoites

Plasmodium sporozoites are transmitted by Anopheles mosquitoes and first infect the liver of their mammalian host, where they develop as liver stages before the onset of erythrocytic infection and malaria symptoms. Sporozoite entry into hepatocytes is an attractive target for anti-malarial prophylactic strategies but remains poorly understood at the molecular level. Apicomplexan parasites invade host cells by forming a parasitophorous vacuole that is essential for parasite development, a process that involves secretion of apical organelles called rhoptries. We previously reported that the host membrane protein CD81 is required for infection by Plasmodium falciparum and Plasmodium yoelii sporozoites. CD81 acts at an early stage of infection, possibly at the entry step, but the mechanisms involved are still unknown. To investigate the role of CD81 during sporozoite entry, we generated transgenic P. yoelii parasites expressing fluorescent versions of three known rhoptry proteins, RON2, RON4 and RAP2/3. We observed that RON2 and RON4 are lost following rhoptry discharge during merozoite and sporozoite entry. In contrast, our data indicate that RAP2/3 is secreted into the parasitophorous vacuole during infection. We further show that sporozoite rhoptry discharge occurs only in the presence of CD81, providing the first direct evidence for a role of CD81 during sporozoite productive invasion.

Plasmodium rhoptry proteins: why order is important

Apicomplexan parasites, including the Plasmodium species that cause malaria, contain three unusual apical secretory organelles (micronemes, rhoptries, and dense granules) that are required for the infection of new host cells. Because of their specialized nature, the majority of proteins secreted from these organelles are unique to Apicomplexans and are consequently poorly characterized. Although rhoptry proteins of Plasmodium have been implicated in events central to invasion, there is growing evidence to suggest that proteins originating from this organelle play key roles downstream of parasite entry into the host cell. Here we discuss recent work that has advanced our knowledge of rhoptry protein trafficking and function, and highlight areas of research that require further investigation.

Functional analysis of the exported type IV HSP40 protein PfGECO in Plasmodium falciparum gametocytes

During Plasmodium falciparum infection, host red blood cell (RBC) remodeling is required for the parasite's survival. Such modifications are mediated by the export of parasite proteins into the RBC that alter the architecture of the RBC membrane and enable cytoadherence. It is probable that some exported proteins also play a protective role against the host defense response. This may be of particular importance for the gametocyte stage of the life cycle that is responsible for malaria transmission, since the gametocyte remains in contact with blood as it proceeds through five morphological stages (I to V) during its 12-day maturation. Using microarray analysis, we identified several genes with encoded secretory or export sequences that were differentially expressed during early gametocytogenesis. One of these, PfGECO, encodes a predicted type IV heat shock protein 40 (HSP40) that we show is expressed in gametocyte stages I to IV and is exported to the RBC cytoplasm. HSPs are traditionally induced under stressful conditions to maintain homeostasis, but PfGECO expression was not increased upon heat shock, suggesting an alternate function. Targeted disruption of PfGECO indicated that the gene is not essential for gametocytogenesis in vitro, and quantitative reverse transcriptase PCR (RT-PCR) showed that there was no compensatory expression of the other type IV HSP40 genes. Although P. falciparum HSP40 members are implicated in the trafficking of proteins to the RBC surface, removal of PfGECO did not affect the targeting of other exported gametocyte proteins. This work has expanded the repertoire of known gametocyte-exported proteins to include a type IV HSP40, PfGECO.

Disruption of plasmepsin-4 and merozoites surface protein-7 genes in Plasmodium berghei induces combined virulence-attenuated phenotype

Blood stage malaria parasites causing a mild and self limited infection in mice have been obtained with either radiation or chemical mutagenesis showing the possibility of developing an attenuated malaria vaccine. Targeted disruption of plasmepsin-4 (pm4) or the merozoite surface protein-7 (msp7) genes also induces a virulence-attenuated phenotype in terms of absence of experimental cerebral malaria (ECM), delayed increase of parasitemia and reduced mortality rate. The decrease in virulence in parasites lacking either pm4 or msp7 is however incomplete and dependent on the parasite and mouse strain combination. The sequential disruption of both genes induced remarkable virulence-attenuated blood-stage parasites characterized by a self-resolving infection with low levels of parasitemia and no ECM. Furthermore, convalescent mice were protected against the challenge with P. berghei or P. yoelii parasites for several months. These observations provide a proof-of-concept step for the development of human malaria vaccines based on genetically attenuated blood-stage parasites.

Antibodies to the Plasmodium falciparum rhoptry protein RAP-2/RSP-2 in relation to anaemia in Cameroonian children

Previous studies have implicated reactive antibodies to the low molecular weight rhoptry-associated proteins (RAP-1, RAP-2/RSP-2 and RAP-3) in erythroid cell destruction during Plasmodium falciparum infection. In this pilot study, the frequency, specificity and functional capacity of naturally acquired anti-RAP-2/RSP-2 antibodies were investigated in the sera of anaemic and nonanaemic malaria-infected Cameroonian children. All sera recognized RAP-2/RSP-2 by FACS, irrespective of the clinical status of the subjects. However, the anaemic children showed higher levels of IgG antibodies than the nonanaemic group, while both groups showed similar levels of IgM antibodies. Only few individuals had detectable levels of RAP-2/RSP-2-specific IgG1 and IgG3 subclass antibodies, while no IgG2 and IgG4 subclass antibodies were detected in these subjects. By ELISA, the anaemic group tended to show higher levels of antibodies to RAP-2/RSP-2 regarding all antibody classes tested, except for IgG4 and IgE. Unexpectedly, sera from the nonanaemic group activated complement to a greater extent than those from the anaemic group. These results need to be confirmed in extended studies but indicate that the effector functions of the RAP-2/RSP-2-reactive antibodies may be more important than their amounts. Such antibodies could play a role in both immunity and pathogenesis during P. falciparum infection.

Conserved regions of the Plasmodium falciparum rhoptry-associated protein 3 mediate specific host-pathogen interactions during invasion of red blood cells

Invasion of red blood cells (RBCs) by the Plasmodium falciparum malaria merozoite is mediated by parasite surface molecules and proteins contained within apical organelles that are capable of recognizing receptors on the membrane of RBCs. The identification and characterization of these P. falciparum invasion-associated proteins is the first step for unveiling potential new drug and vaccine target molecules to eradicate this deadly disease. Among the exclusive set of malarial vaccine candidates, the members of the rhoptry-associated protein (RAP) family have been associated with the parasite's binding to and invasion of RBCs. Remarkably, the third member of this family (named RAP-3) has been recently detected on the surface of non-infected RBCs exposed to free merozoites, therefore suggesting the participation of this protein during RBC infection. In this study, the sequence of RAP-3 was finely mapped using synthetic peptides in order to identify which are the specific binding regions involved in RAP3-RBC interactions. Two high-activity binding peptides (HABPs) established high affinity interactions with RBC surface molecules of about 27-90 kDa, which were differentially affected by different enzymatic treatments. RAP-1 and RAP-2 HABPs inhibited binding of RAP-3 HABPs to different extents, thus suggesting the recognition of similar binding sites on RBC membrane, as well as ability of RAP-3 HABPs to inhibit P. falciparum infection in vitro. Altogether, these functional analyses of RAP-3 HABPs strongly suggest a potential role for this protein in RBC invasion, and highlight its HABPs as potential targets to develop a fully protective minimal subunit-based malarial vaccine.

Family members stick together: multi-protein complexes of malaria parasites

Malaria parasites express a broad repertoire of proteins whose expression is tightly regulated depending on the life-cycle stage of the parasite and the environment of target organs in the respective host. Transmission of malaria parasites from the human to the anopheline mosquito is mediated by intraerythrocytic sexual stages, termed gametocytes, which circulate in the peripheral blood and are essential for the spread of the tropical disease. In Plasmodium falciparum, gametocytes express numerous extracellular proteins with adhesive motifs, which might mediate important interactions during transmission. Among these is a family of six secreted proteins with adhesive modules, termed PfCCp proteins, which are highly conserved throughout the apicomplexan clade. In P. falciparum, the proteins are expressed in the parasitophorous vacuole of gametocytes and are subsequently exposed on the surface of macrogametes during parasite reproduction in the mosquito midgut. One characteristic of the family is a co-dependent expression, such that loss of all six proteins occurs if expression of one member is disrupted via gene knockout. The six PfCCp proteins interact by adhesion domain-mediated binding and thus form complexes on the sexual stage surface having adhesive properties. To date, the PfCCp proteins represent the only protein family of the malaria parasite sexual stages that assembles to multimeric complexes, and only a small number of such protein complexes have so far been identified in other life-cycle stages of the parasite.

Mechanisms of malarial anaemia: potential involvement of the Plasmodium falciparum low molecular weight rhoptry-associated proteins

Plasmodium falciparum malaria is a major cause of morbidity and mortality throughout the tropics. Anaemia is a constant feature of the disease. Pregnant women mostly primigravidae and children below the age of 5 years are the most afflicted. Its pathogenesis is multifactorial and incompletely understood. Among several factors, the destruction of erythrocytes (RBCs) is the most frequently observed cause of severe malarial anaemia and the removal of non-parasitized RBCs (nEs) is thought to be the most important, accounting for approximately 90% of the reduction in haematocrit in acute malaria. Previous studies demonstrated that the tagging of nEs with the parasite antigen RAP-2 (rhoptry-associated protein-2; also designated RSP-2) due to either failed or aborted invasion by merozoites resulted in the destruction of these cells. In this study we further investigated the mechanisms mediating the destruction of nEs in the development of severe malarial anaemia and the possible involvement of RAP-2/RSP-2 and other members of the low molecular weight rhoptry complex (RAP-1: rhoptry-associated protein-1 and RAP-3: rhoptry-associated protein-3). Antibodies to the rhoptry-associated proteins were found to recognise the surface of nEs in a parasitaemia-dependent manner after merozoite release in P. falciparumin vitro cultures. These cells, as well as erythroblasts co-cultured with infected RBCs (IEs), could then be destroyed by either phagocytosis or lysis after complement activation. The ability of anti-rhoptry antibodies to mediate the destruction of RAP-2/RSP-2-tagged erythroblasts in the presence of effector cells was also investigated. Data obtained suggest that mouse monoclonal antibodies to the low molecular weight RAP proteins mediate the death of RAP-2/RSP-2-tagged erythroblasts on interaction with adherent monocytes. The mechanism of cell death is not yet fully known, but seems to involve primarily apoptosis. The above observations suggest that the antibody response against RAP-2/RSP-2 and other members of the complex could trigger the destruction of RAP-2/RSP-2-tagged host cells. Taken together it appears that during severe anaemia a defective bone marrow or dyserythropoiesis possibly due to erythroblast cell death, may overlap with the accelerated destruction of normal erythroid cells, either by opsonisation or complement activation further aggravating the anaemia which may become fatal. These observations could therefore have implications in the design, development and deployment of future therapeutic interventions against malaria.

Strategies for developing multi-epitope, subunit-based, chemically synthesized anti-malarial vaccines

An anti-malarial vaccine against the extremely lethal Plasmodium falciparum is desperately needed. Peptides from this parasite's proteins involved in invasion and having high red blood cell-binding ability were identified; these conserved peptides were not immun genic or protection-inducing when used for immunizing Aotus monkeys. Modifying some critical binding residues in these high-activi binding peptides' (HABPs') attachment to red blood cells (RBC) allowed them to induce immunogenicity and protection against expermental challenge and acquire the ability to bind to specific HLA-DRp1* alleles. These modified HABPs adopted certain characterist structural configurations as determined by circular dichroism (CD) and ¹H nuclear magnetic resonance (NMR) associated with certain HLA-DRβ1* haplotype binding activities and characteristics, such as a 2-Å-distance difference between amino acids fitting into HLA-DRp1 Pockets 1 to 9, residues participating in binding to HLA-DR pockets and residues making contact with the TCR, suggesting haplotyp and allele-conscious TCR. This has been demonstrated in HLA-DR-like genotyped monkeys and provides the basis for designing high effective, subunit-based, multi-antigen, multi-stage, synthetic vaccines, for immediate human use, malaria being one of them.

Genetic analysis of the cytoplasmic domain of the PfRh2b merozoite invasion protein of Plasmodium falciparum

Apicomplexan parasites employ multiple adhesive ligands for recognition and entry into host cells. The Duffy binding-like (DBL) and the reticulocyte binding protein-like (RBL) families are central to the invasion of erythrocytes by the malaria parasite. These type-1 transmembrane proteins are composed of large ectodomains and small conserved cytoplasmic tail domains. The cytoplasmic tail domain of the micronemal DBL protein EBA-175 is required for a functional ligand-receptor interaction, but not for correct trafficking and localisation. Here we focus on the cytoplasmic tail domain of the rhoptry-localised Plasmodium falciparum RBL PfRh2b. We have identified a conserved sequence of six amino acids, enriched in acidic residues, in the cytoplasmic tail domains of RBL proteins from Plasmodium spp. Genetic analyses reveal that the entire cytoplasmic tail and the conserved motif within the cytoplasmic tail are indispensable for invasion P. falciparum. Site-directed mutagenesis of the conserved moiety reveals that changes in the order of the amino acids of the conserved moiety, but not the charge of the sequence, can be tolerated. Shuffling of the motif has no effect on either invasion phenotype or PfRh2b expression and trafficking. Although the PfRh2b gene can be readily disrupted, our results suggest that modification of the PfRh2b cytoplasmic tail results in strong dominant negative activity, highlighting important differences between the PfRh2b and EBA-175 invasion ligands.

Emerging rules for subunit-based, multiantigenic, multistage chemically synthesized vaccines

Seventeen million people die of transmittable diseases and 2/3 of the world's population suffer them annually. Malaria, tuberculosis, AIDS, hepatitis, and reemerging and new diseases are a great threat to humankind. A logical and rational approach for vaccine development is thus desperately needed. Protein chemistry provides the best tools for tackling these problems. The tremendous complexity of microbes, the different pathways they use for invading host cells, and the immune responses they induce can only be resolved by using the minimum subunit-based (chemically produced approximately 20-mer peptides), multiantigenic (most proteins involved in invasion), multistage (different invasion mechanisms) vaccine development approach. The most lethal form of malaria caused by Plasmodium falciparum (killing 3 million and affecting 500 million people worldwide annually) was used as target disease since many of its proteins, its invasion pathways, and its genome have been described recently. A New World primate (the Aotus monkey) is highly susceptibly to human malaria; its immune system molecules are 80-100% identical to those of its human counterpart, making it an excellent model for vaccine development. Chemically synthesized approximately 20-mer peptides, covering all the P. falciparum malaria proteins involved in red blood cell (RBC) invasion were synthesized by the classical t-Boc technology (based on synthetic SPf66 antimalarial vaccine information for identifying targets) and assayed in a highly sensitive, specific, and robust test for detecting receptor-ligand interactions between high-activity binding peptides (HABPs) and RBCs. HABPs were identified, some in which the molecule displays genetic variability (to be discarded due to their tremendous complexity) and elicits a strain-specific immune response and others that are conserved (no amino acid sequence variation). Conserved HABPs were synthesized in a polymeric form by adding cysteines at their N- and C-terminal ends to be used for monkey immunization. They became nonimmunogenic (no antibodies were induced) nonprotection inducers (monkeys were not protected against P. falciparum malaria challenge with a highly infective strain) suggesting a code of immunological silence or nonresponsiveness for these conserved HABPs. A large number of monkey trials involving a considerable number of Aotus monkeys were performed to break this code of immunological silence by replacing critical residues (determined by glycine peptide analogue scanning) to find that the following amino acid changes had to be made to render them antibody and protection inducing: F<-->R; W<-->Y; L<-->H; I<-->N; M<-->K; P<-->D; Q<-->E; C<-->T. The three-dimensional (3D) structure of >100 of these native modified HABPs (determined by (1)H NMR) revealed that the following structural changes had all to be achieved to allow a better fit into the major histocompatibility complex class II (MHC II)-peptide-TCR complex to properly activate the immune system: alpha-helix shortening, modifying their beta-turn, adopting segmental alpha-helix configuration, changing residue orientation, and increasing the distance of those residues fitting into the MHC II molecules from antigen-presenting cells. More than 100 such highly immunogenic, protection-inducing (against P. falciparum malaria) modified HABPs have been identified to date with this methodology, showing that it could lead to developing a highly effective subunit-based, multiantigenic, multistage synthetic vaccine against diseases scourging humankind, malaria being one of them.

Identification of protein complexes in detergent-resistant membranes of Plasmodium falciparum schizonts

Merozoite surface proteins of the human malaria parasite Plasmodium falciparum are involved in initial contact with target erythrocytes, a process that begins a cascade of events required for successful invasion of these cells. In order to identify complexes that may play a role in invasion we purified detergent-resistant membranes (DRMs), known to be enriched in merozoite surface proteins, and used blue native-polyacrylamide gel electrophoresis (BN-PAGE) to isolate high molecular weight complexes for identification by mass spectrometry. Sixty-two proteins were detected and these mostly belonged to expected DRM proteins classes including GPI-anchored, multi-membrane spanning and rhoptry proteins. Proteins from seven known complexes were identified including MSP-1/7, the low (RAP1/2 and RAP1/3), and high (RhopH1/H2/H3) molecular weight rhoptry complexes, and the invasion motor complex (GAP45/GAP50/myosinA). Remarkably, a large proportion of identified spectra were derived from only 4 proteins: the GPI-anchored proteins MSP-1 and Pf92, the putative GPI-anchored protein Pf113 and RAP-1, the core component of the two RAP complexes. Each of these proteins predominated in high molecular weight species suggesting their aggregation in much larger complexes than anticipated. To demonstrate that the procedure had isolated novel complexes we focussed on MSP-1, which predominated as a distinct species at approximately 500 kDa by BN-PAGE, approximately twice its expected size. Chemical cross-linking supports the existence of a stable MSP-1 oligomer of approximately 500 kDa, probably comprising a highly stable homodimeric species. Our observations also suggests that oligomerization of MSP-1 is likely to occur outside the C-terminal epidermal growth factor (EGF)-like domains. Confirmation of MSP-1 oligomerization, together with the isolation of a number of known complexes by BN-PAGE, makes it highly likely that novel interactions occur amongst members of this proteome.

A set of glycosylphosphatidyl inositol-anchored membrane proteins of Plasmodium falciparum is refractory to genetic deletion

Targeted gene disruption has proved to be a powerful approach for studying the function of important ligands involved in erythrocyte invasion by the extracellular merozoite form of the human malaria parasite, Plasmodium falciparum. Merozoite invasion proceeds via a number of seemingly independent alternate pathways, such that entry can proceed with parasites lacking particular ligand-receptor interactions. To date, most focus in this regard has been on single-pass (type 1) membrane proteins that reside in the secretory organelles. Another class of merozoite proteins likely to include ligands for erythrocyte receptors are the glycosylphosphatidyl inositol (GPI)-anchored membrane proteins that coat the parasite surface and/or reside in the apical organelles. Several of these are prominent vaccine candidates, although their functions remain unknown. Here, we systematically attempted to disrupt the genes encoding seven of the known GPI-anchored merozoite proteins of P. falciparum by using a double-crossover gene-targeting approach. Surprisingly, and in apparent contrast to other merozoite antigen classes, most of the genes (six of seven) encoding GPI-anchored merozoite proteins are refractory to genetic deletion, with the exception being the gene encoding merozoite surface protein 5 (MSP-5). No distinguishable growth rate or invasion pathway phenotype was detected for the msp-5 knockout line, although its presence as a surface-localized protein was confirmed.

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.

The Plasmodium vivax rhoptry-associated protein 1

Rhoptries are cellular organelles localized at the apical pole of apicomplexan parasites. Their content is rich in lipids and proteins that are released during target cell invasion. Plasmodium falciparum rhoptry-associated protein 1 (RAP1) has been the most widely studied among this parasite species' rhoptry proteins and is considered to be a good anti-malarial vaccine candidate since it displays little polymorphism and induces antibodies in infected humans. Monoclonal antibodies directed against RAP1 are also able to inhibit target cell invasion in vitro and protection against P. falciparum experimental challenge is induced when non-human primates are immunized with this protein expressed in its recombinant form. This study describes identifying and characterizing RAP1 in Plasmodium vivax, the most widespread parasite species causing malaria in humans, producing more than 80 million infections yearly, mainly in Asia and Latin America. This new protein is encoded by a two-exon gene, is proteolytically processed in a similar manner to its falciparum homologue and, as observed by microscopy, the immunofluorescence pattern displayed is suggestive of its rhoptry localization. Further studies evaluating P. vivax RAP1 protective efficacy in non-human primates should be carried out taking into account the relevance that its P. falciparum homologue has as an anti-malarial vaccine candidate.

A conserved molecular motor drives cell invasion and gliding motility across malaria life cycle stages and other apicomplexan parasites

Apicomplexan parasites constitute one of the most significant groups of pathogens infecting humans and animals. The liver stage sporozoites of Plasmodium spp. and tachyzoites of Toxoplasma gondii, the causative agents of malaria and toxoplasmosis, respectively, use a unique mode of locomotion termed gliding motility to invade host cells and cross cell substrates. This amoeboid-like movement uses a parasite adhesin from the thrombospondin-related anonymous protein (TRAP) family and a set of proteins linking the extracellular adhesin, via an actin-myosin motor, to the inner membrane complex. The Plasmodium blood stage merozoite, however, does not exhibit gliding motility. Here we show that homologues of the key proteins that make up the motor complex, including the recently identified glideosome-associated proteins 45 and 50 (GAP40 and GAP50), are present in P. falciparum merozoites and appear to function in erythrocyte invasion. Furthermore, we identify a merozoite TRAP homologue, termed MTRAP, a micronemal protein that shares key features with TRAP, including a thrombospondin repeat domain, a putative rhomboid-protease cleavage site, and a cytoplasmic tail that, in vitro, binds the actin-binding protein aldolase. Analysis of other parasite genomes shows that the components of this motor complex are conserved across diverse Apicomplexan genera. Conservation of the motor complex suggests that a common molecular mechanism underlies all Apicomplexan motility, which, given its unique properties, highlights a number of novel targets for drug intervention to treat major diseases of humans and livestock.

Identification and characterisation of the Plasmodium vivax rhoptry-associated protein 2

Plasmodium vivax is currently the most widespread of the four parasite species causing malaria in humans around the world. It causes more than 75 million clinical episodes per year, mainly on the Asian and American continents. Identifying new antigens to be further tested as anti-P. vivax vaccine candidates has been greatly hampered by the difficulty of maintaining this parasite cultured in vitro. Taking into account that one of the most promising vaccine candidates against Plasmodium falciparum is the rhoptry-associated protein 2, we have identified the P. falciparum rhoptry-associated protein 2 homologue in P. vivax in the present study. This protein has 400 residues, having an N-terminal 21 amino-acid stretch compatible with a signal peptide and, as occurs with its falciparum homologue, it lacks repeat sequences. The protein is expressed in asexual stage P. vivax parasites and polyclonal sera raised against this protein recognised a 46 kDa band in parasite lysate in a Western blot assay.

Proteomic analysis of rhoptry organelles reveals many novel constituents for host-parasite interactions in Toxoplasma gondii

Rhoptries are specialized secretory organelles that are uniquely present within protozoan parasites of the phylum Apicomplexa. These obligate intracellular parasites comprise some of the most important parasites of humans and animals, including the causative agents of malaria (Plasmodium spp.) and chicken coccidiosis (Eimeria spp.). The contents of the rhoptries are released into the nascent parasitophorous vacuole during invasion into the host cell, and the resulting proteins often represent the literal interface between host and pathogen. We have developed a method for highly efficient purification of rhoptries from one of the best studied Apicomplexa, Toxoplasma gondii, and we carried out a detailed proteomic analysis using mass spectrometry that has identified 38 novel proteins. To confirm their rhoptry origin, antibodies were raised to synthetic peptides and/or recombinant protein. Eleven of 12 of these yielded antibody that showed strong rhoptry staining by immunofluorescence within the rhoptry necks and/or their bulbous base. Hemagglutinin epitope tagging confirmed one additional novel protein as from the rhoptry bulb. Previously identified rhoptry proteins from Toxoplasma and Plasmodium were unique to one or the other organism, but our elucidation of the Toxoplasma rhoptry proteome revealed homologues that are common to both. This study also identified the first Toxoplasma genes encoding rhoptry neck proteins, which we named RONs, demonstrated that toxofilin and Rab11 are rhoptry proteins, and identified novel kinases, phosphatases, and proteases that are likely to play a key role in the ability of the parasite to invade and co-opt the host cell for its own survival and growth.

Plasmodium falciparum: red blood cell binding studies using peptides derived from rhoptry-associated protein 2 (RAP2)

Plasmodium falciparum rhoptry-associated proteins 1 (RAP1) and 2 (RAP2) are antigens presenting themselves as candidates for a subunit malaria vaccine. RAP2 protein, non-overlapping, consecutive peptides were synthesised and tested in red blood cell (RBC) binding assays to identify their receptor-ligand interaction in recognising RAP2 regions involved in the in vitro merozoite invasion process. Four high activity binding peptides (HABPs) were identified in the resulting 20 peptides. Peptides 26220 ((61)NHFSSADELIKYLEKTNINT(80)), 26225 ((161)IKKNPFLRVLNKASTTTHAT(180)) and 26229 ((241)RSVNNVISKNKTLGLRKRSS(260)) were located in the amino terminal and central part of the protein and HABP 26235 ((361)FLAEDFVELFDVTMDCYSRQ(380)) was located at the carboxy terminal. All these HABPs showed saturable binding and presented dissociation constants between 500 and 950 nM; the number of binding sites per RBC ranged from 48,000 to 160,000. High binding peptides' critical amino acids involved in RBC binding were determined by competition binding assays; their amino acids appear in bold in the sequences shown above. SDS-PAGE results showed that peptides 26220, 26225 and 26229 had at least two different sets of 62 and 42 kDa HABP receptors on RBCs and that peptide 26235 had at least two different sets of 77 and 62 kDa. HABPs inhibited in vitro merozoite invasion by between 54% and 94% at 200 microM, suggesting that these RAP2 peptides are involved in the in vitro P. falciparum invasion process.