The motor complex of Plasmodium falciparum: phosphorylation by a calcium-dependent protein kinase

Calcium-dependent protein kinases (CDPKs) of Apicomplexan parasites are crucial for the survival of the parasite throughout its life cycle. CDPK1 is expressed in the asexual blood stages of the parasite, particularly late stage schizonts. We have identified two substrates of Plasmodium falciparum CDPK1: myosin A tail domain-interacting protein (MTIP) and glideosome-associated protein 45 (GAP45), both of which are components of the motor complex that generates the force required by the parasite to actively invade host cells. Indirect immunofluorescence shows that CDPK1 localizes to the periphery of P. falciparum merozoites and is therefore suitably located to act on MTIP and GAP45 at the inner membrane complex. A proportion of both GAP45 and MTIP is phosphorylated in schizonts, and we demonstrate that both proteins can be efficiently phosphorylated by CDPK1 in vitro. A primary phosphorylation of MTIP occurs at serine 47, whereas GAP45 is phosphorylated at two sites, one of which could also be detected in phosphopeptides purified from parasite lysates. Both CDPK1 activity and host cell invasion can be inhibited by the kinase inhibitor K252a, suggesting that CDPK1 is a suitable target for antimalarial drug development.

Comparative testing of six antigen-based malaria vaccine candidates directed toward merozoite-stage Plasmodium falciparum

Immunogenicity testing of Plasmodium falciparum antigens being considered as malaria vaccine candidates was undertaken in rabbits. The antigens compared were recombinant baculovirus MSP-1(19) and five Pichia pastoris candidates, including two versions of MSP-1(19), AMA-1 (domains I and II), AMA-1+MSP-1(19), and fused AMA-1/MSP-1(19)). Animals were immunized with equimolar amounts of each antigen, formulated in Montanide ISA720. The specificities and titers of antibodies were compared using immunofluorescence assays and enzyme-linked immunosorbent assay (ELISA). The antiparasite activity of immunoglobulin G (IgG) in in vitro cultures was determined by growth inhibition assay, flow cytometry, lactate dehydrogenase assay, and microscopy. Baculovirus MSP-1(19) immunizations produced the highest parasite-specific antibody titers in immunofluorescence assays. In ELISAs, baculovirus-produced MSP-1(19) induced more antibodies than any other single MSP-1(19) immunogen and three times more MSP-1(19) specific antibodies than the AMA-1/MSP-1(19) fusion. Antibodies induced by baculovirus MSP-1(19) gave the highest levels of growth inhibition in HB3 and 3D7 parasite cultures, followed by AMA-1+MSP-1(19) and the AMA-1/MSP-1(19) fusion. With the FCR3 isolate (homologous to the AMA-1 construct), antibodies to the three AMA-1-containing candidates gave the highest levels of growth inhibition at high IgG concentrations, but antibodies to baculovirus MSP-1(19) inhibited as well or better at lower IgG concentrations. The two P. pastoris-produced MSP-1(19)-induced IgGs conferred the lowest growth inhibition. Comparative analysis of immunogenicity of vaccine antigens can be used to prioritize candidates before moving to expensive GMP production and clinical testing. The assays used have given discriminating readouts but it is not known whether any of them accurately reflect clinical protection.

Formation of the food vacuole in Plasmodium falciparum: a potential role for the 19 kDa fragment of merozoite surface protein 1 (MSP1(19))

Plasmodium falciparum Merozoite Surface Protein 1 (MSP1) is synthesized during schizogony as a 195-kDa precursor that is processed into four fragments on the parasite surface. Following a second proteolytic cleavage during merozoite invasion of the red blood cell, most of the protein is shed from the surface except for the C-terminal 19-kDa fragment (MSP1₁₉), which is still attached to the merozoite via its GPI-anchor. We have examined the fate of MSP1₁₉ during the parasite's subsequent intracellular development using immunochemical analysis of metabolically labeled MSP1₁₉, fluorescence imaging, and immuno-electronmicroscopy. Our data show that MSP1₁₉ remains intact and persists to the end of the intracellular cycle. This protein is the first marker for the biogenesis of the food vacuole; it is rapidly endocytosed into small vacuoles in the ring stage, which coalesce to form the single food vacuole containing hemozoin, and persists into the discarded residual body. The food vacuole is marked by the presence of both MSP1₁₉ and the chloroquine resistance transporter (CRT) as components of the vacuolar membrane. Newly synthesized MSP1 is excluded from the vacuole. This behavior indicates that MSP1₁₉ does not simply follow a classical lysosome-like clearance pathway, instead, it may play a significant role in the biogenesis and function of the food vacuole throughout the intra-erythrocytic phase.

Effective induction of high-titer antibodies by viral vector vaccines

Protein-in-adjuvant vaccines have shown limited success against difficult diseases such as blood-stage malaria. Here we show that a recombinant adenovirus-poxvirus prime-boost immunization regime (known to induce strong T cell immunogenicity) can also induce very strong antigen-specific antibody responses, and we identify a simple complement-based adjuvant to further enhance immunogenicity. Antibodies induced against a blood-stage malaria antigen by this viral vector platform are highly effective against Plasmodium yoelii parasites in mice and against Plasmodium falciparum in vitro.

Hyperreactive malarial splenomegaly is associated with low levels of antibodies against red blood cell and Plasmodium falciparum derived glycolipids in Yanomami Amerindians from Venezuela

The immunological basis of the aberrant immune response in hyperreactive malarial splenomegaly (HMS) is poorly understood, but believed to be associated with polyclonal B cell activation by an unidentified malaria mitogen, leading to unregulated immunoglobulin and autoantibody production. HMS has been previously reported in Yanomami communities in the Upper Orinoco region of the Venezuelan Amazon. To investigate a possible association between antibody responses against Plasmodium falciparum and uninfected red blood cell (URBC) glycolipids and splenomegaly, a direct comparison of the parasite versus host anti-glycolipid antibody responses was made in an isolated community of this area. The anti-P. falciparum glycolipid (Pfglp) response was IgG3 dominated, whereas the uninfected red blood cell glycolipid (URBCglp) response showed a predominance of IgG1. The levels of IgG1 against Pfglp, and of IgG4 and IgM against URBCglp were significantly higher in women, while the anti-Pfglp or URBCglp IgM levels were inversely correlated with the degree of splenomegaly. Overall, these results suggest differential regulation of anti-parasite and autoreactive responses and that these responses may be linked to the development and evolution of HMS in this population exposed to endemic malaria. The high mortality rates associated with HMS point out that its early diagnosis together with the implementation of malaria control measures in these isolated Amerindian communities are a priority.

Malaria vaccine-related benefits of a single protein comprising Plasmodium falciparum apical membrane antigen 1 domains I and II fused to a modified form of the 19-kilodalton C-terminal fragment of merozoite surface protein 1

We show that the smallest module of Plasmodium falciparum AMA1 (PfAMA1) that can be expressed in the yeast Pichia pastoris while retaining the capacity to induce high levels of parasite-inhibitory antibodies comprises domains I and II. Based on this, two fusion proteins, differing in the order of the modules, were developed. Each comprised one module of PfAMA1 (FVO strain, amino acids [aa] 97 to 442) (module A) and one module of PfMSP1(19) (Wellcome strain, aa 1526 to 1621) (module Mm) in which a cystine had been removed to improve immune responses. Both fusion proteins retained the antigenicity of each component and yielded over 30 mg/liter purified protein under fed-batch fermentation. Rabbits immunized with purified fusion proteins MmA and AMm had up to eightfold-higher immune responses to MSP1(19) than those of rabbits immunized with module Mm alone or Mm mixed with module A. In terms of parasite growth inhibition, fusion did not diminish the induction of inhibitory antibodies compared with immunization with module A alone or module A mixed with module Mm, and fusion outperformed antibodies induced by immunization with module M or Mm alone. When tested against parasites expressing AMA1 heterologous to the immunogen, antibodies to the fusion proteins inhibited parasite growth to a greater extent than did antibodies either to the individual antigens or to the mixture. These results suggest that compared with the individual modules delivered separately or as a mixture, fusion proteins containing these two modules offer the potential for significant vaccine-related advantages in terms of ease of production, immunogenicity, and functionality.

Site-specific N-terminal labelling of proteins in vitro and in vivo using N-myristoyl transferase and bioorthogonal ligation chemistry

N-Myristoyl transferase-mediated modification with azide-bearing substrates is introduced as a highly selective and practical method for in vitro and in vivo N-terminal labelling of a recombinant protein using bioorthogonal ligation chemistry.

Epigenetic silencing of Plasmodium falciparum genes linked to erythrocyte invasion

The process of erythrocyte invasion by merozoites of Plasmodium falciparum involves multiple steps, including the formation of a moving junction between parasite and host cell, and it is characterised by the redundancy of many of the receptor–ligand interactions involved. Several parasite proteins that interact with erythrocyte receptors or participate in other steps of invasion are encoded by small subtelomerically located gene families of four to seven members. We report here that members of the eba, rhoph1/clag, acbp, and pfRh multigene families exist in either an active or a silenced state. In the case of two members of the rhoph1/clag family, clag3.1 and clag3.2, expression was mutually exclusive. Silencing was clonally transmitted and occurred in the absence of detectable DNA alterations, suggesting that it is epigenetic. This was demonstrated for eba-140. Our data demonstrate that variant or mutually exclusive expression and epigenetic silencing in Plasmodium are not unique to genes such as var, which encode proteins that are exported to the surface of the erythrocyte, but also occur for genes involved in host cell invasion. Clonal variant expression of invasion-related ligands increases the flexibility of the parasite to adapt to its human host.

Plasmodium falciparum is responsible for the most severe forms of human malaria. Invasion of host erythrocytes is an essential step of the complex life cycle of this parasite. There is redundancy in many of the interactions involved in this process, such that the parasite can use different sets of receptor–ligand interactions to invade. Here, we demonstrate that the parasite can turn off the expression of some of the proteins that mediate invasion of erythrocytes. Expression can be turned off without alterations in the genetic information of the parasite by using a mechanism known as epigenetic silencing. This is far more flexible than genetic changes, and permits fast, reversible adaptation. Turning on or off the expression of these proteins did not affect the capacity of the parasite to invade normal or modified red cells, which suggests that the variant expression of these genes may be used by the parasite to escape immune responses from the host. Parasite proteins that participate in erythrocyte invasion are important vaccine candidates. Determining which proteins can be turned off is important because vaccines based on single antigens of the parasite that can be turned off without affecting its growth would have little chance of inducing protective immunity.

The importance of human FcgammaRI in mediating protection to malaria

The success of passive immunization suggests that antibody-based therapies will be effective at controlling malaria. We describe the development of fully human antibodies specific for Plasmodium falciparum by antibody repertoire cloning from phage display libraries generated from immune Gambian adults. Although these novel reagents bind with strong affinity to malaria parasites, it remains unclear if in vitro assays are predictive of functional immunity in humans, due to the lack of suitable animal models permissive for P. falciparum. A potentially useful solution described herein allows the antimalarial efficacy of human antibodies to be determined using rodent malaria parasites transgenic for P. falciparum antigens in mice also transgenic for human Fc-receptors. These human IgG1s cured animals of an otherwise lethal malaria infection, and protection was crucially dependent on human FcγRI. This important finding documents the capacity of FcγRI to mediate potent antimalaria immunity and supports the development of FcγRI-directed therapy for human malaria.

Malaria rivals HIV and tuberculosis as the world's most deadly infection killing a child every 30 seconds. Antibodies and their receptors (Fc-receptors) have been shown to be vital for the development of protective immunity, and as such they act as correlates of protection in studies aimed at defining the best antigens to incorporate into current vaccines. Understanding antibody types and Fc-receptors that optimally induce immunity is therefore vital to developing the best vaccines. Surrogate markers of antibody efficacy currently rely on in vitro assays that are laborious and difficult to reproduce. It remains unclear if such in vitro assays are predictive of functional immunity in humans due to the lack of suitable animal models permissive for Plasmodium falciparum. Here, we create a transgenic in vivo mouse model that has significant advantage over the use of new world primates, the only other model for human malaria. We demonstrate that this model defines an Fc-dependent mechanism of parasite destruction that cannot be assessed in current in vitro assays. The model provides both a test for therapeutic antibody efficacy prior to clinical trials in humans and an important tool in malaria vaccine development.

Profiling the antibody immune response against blood stage malaria vaccine candidates

BACKGROUND

The complexity and diversity of the antibody immune response to the antigen repertoire of a pathogen has long been appreciated. Although it has been recognized that the detection of antibodies against multiple antigens dramatically improves the clinical sensitivity and specificity of diagnostic assays, the prognostic value of serum reactivity profiles against multiple microbial antigens in protection has not been investigated.

METHODS

Using malaria as a model we investigated whether antigen reactivity profiles in serum of children with different levels of clinical immunity to Plasmodium falciparum malaria correlated with protection. We developed a microarray immunoassay of 18 recombinant antigens derived from 4 leading blood-stage vaccine candidates for P. falciparum [merozoite surface protein 1 (MSP1), MSP2, MSP3, and apical membrane antigen (AMA)-1]. Associations between observed reactivity profiles and clinical status were sought using k-means clustering and phylogenetic networks.

RESULTS

The antibody immune response was unexpectedly complex, with different combinations of antigens recognized in different children. Serum reactivity to individual antigens did not correlate with immune status. By contrast, combined recognition of AMA-1 and allelic variants of MSP2 was significantly associated with protection against clinical malaria. This finding was confirmed independently by k-means clustering and phylogenetic networking.

CONCLUSIONS

The analysis of reactivity profiles provides a wealth of novel information about the immune response against microbial organisms that would pass unnoticed in analysis of reactivity to antigens individually. Extension of this approach to a large fraction of the proteome may expedite the identification of correlates of protection and vaccine development against microbial diseases.

Structural studies on Plasmodium vivax merozoite surface protein-1

Plasmodium vivax infection is the second most common cause of malaria throughout the world. Like other Plasmodium species, P. vivax has a large protein complex, MSP-1, located on the merozoite surface. The C-terminal MSP-1 sub-unit, MSP-1(42), is cleaved during red blood cell invasion, causing the majority of the complex to be shed and leaving only a small 15kDa sub-unit, MSP-1(19), on the merozite surface. MSP-1(19) is considered a strong vaccine candidate. We have determined the solution structure of MSP-1(19) from P. vivax using nuclear magnetic resonance (NMR) and show that, like in other Plasmodium species, it consists of two EGF-like domains that are oriented head-to-tail. The protein has a flat, disk-like shape with a highly charged surface. When MSP-1(19) is part of the larger MSP-1(42) precursor it exists as an independent domain with no stable contacts to the rest of the sub-unit. Gel filtration and analytical ultracentrifugation experiments indicate that P. vivax MSP-1(42) exists as a dimer in solution. MSP-1(19) itself is a monomer, however, 35 amino-acids immediately upstream of its N-terminus are sufficient to cause dimerization. Our data suggest that if MSP-1(42) exists as a dimer in vivo, secondary processing would cause the dissociation of two tightly linked MSP-1(19) proteins on the merozoite surface just prior to invasion.

Passive immunization with a multicomponent vaccine against conserved domains of apical membrane antigen 1 and 235-kilodalton rhoptry proteins protects mice against Plasmodium yoelii blood-stage challenge infection

During malaria parasite invasion of red blood cells, merozoite proteins bind receptors on the surface of the erythrocyte. Two candidate Plasmodium yoelii adhesion proteins are apical membrane antigen 1 (AMA1) and the 235-kDa rhoptry proteins (P235). Previously, we have demonstrated that passive immunization with monoclonal antibodies (MAbs) 45B1 and 25.77 against AMA1 and P235, respectively, protects against a lethal challenge infection with P. yoelii YM. We show that MAb 45B1 recognizes an epitope located on a conserved surface of PyAMA1, as determined by phage display and analysis of the three-dimensional structure of AMA1, in a region similar to that bound by the P. falciparum AMA1-specific inhibitory antibody 4G2. The epitope recognized by 25.77 could not be assigned. We report here that MAbs 45B1 and 25.77 also protect against challenge with the nonlethal parasite line 17X, in which PyAMA1 has a significantly different amino acid sequence from that in YM. When administered together, the two MAbs acted at least additively in providing protection against challenge with the virulent YM parasite. These results support the concept of developing a multicomponent blood-stage vaccine and the inclusion of polymorphic targets such as AMA1, which these results suggest contain conserved domains recognized by inhibitory antibodies.

Extensive proteolytic processing of the malaria parasite merozoite surface protein 7 during biosynthesis and parasite release from erythrocytes

In Plasmodium falciparum, merozoite surface protein 7 (MSP7) was originally identified as a 22kDa protein on the merozoite surface and associated with the MSP1 complex shed during erythrocyte invasion. MSP7 is synthesised in schizonts as a 351-amino acid precursor that undergoes proteolytic processing. During biosynthesis the MSP1 and MSP7 precursors form a complex that is targeted to the surface of developing merozoites. In the sequential proteolytic processing of MSP7, N- and C-terminal 20 and 33kDa products of primary processing, MSP7(20) and MSP7(33) are formed and MSP7(33) remains bound to full length MSP1. Later in the mature schizont, MSP7(20) disappears from the merozoite surface and on merozoite release MSP7(33) undergoes a secondary cleavage yielding the 22kDa MSP7(22) associated with MSP1. In free merozoites, both MSP7(22) and a further cleaved product, MSP7(19) present only in some parasite lines, were detected; these two derivatives are shed as part of the protein complex with MSP1 fragments during erythrocyte invasion. Primary processing of MSP7 is brefeldin A-sensitive while secondary processing is resistant to both calcium chelators and serine protease inhibitors. Primary processing of MSP7 occurs prior to that of MSP1 in a post-Golgi compartment, whereas the secondary cleavage occurs on the surface of the developing merozoite, possibly at the time of MSP1 primary processing and well before the secondary processing of MSP1.

Immunoglobulin G antibodies to merozoite surface antigens are associated with recovery from chloroquine-resistant Plasmodium falciparum in Gambian children

We examined the hypothesis that recovery from uncomplicated malaria in patients carrying drug-resistant Plasmodium falciparum is a measure of acquired functional immunity and may therefore be associated with humoral responses to candidate vaccine antigens. Gambian children with malaria were treated with chloroquine in 28-day trials, and recovery was defined primarily as the absence of severe clinical malaria at any time and absence of parasitemia with fever after 3 days. Plasma samples from these children were assayed by enzyme-linked immunosorbent assay for immunoglobulin G (IgG) to recombinant merozoite antigens: apical membrane antigen 1 (AMA-1) and the 19-kDa C-terminal region of merozoite surface protein 1 (MSP-1(19)), including antigenic variants of MSP-1(19) with double and triple substitutions. Antigen-specific IgG was more frequent in children who recovered, particularly that for MSP-1(19) (age-adjusted odds ratios: 0.32 [95% confidence interval, 0.05, 1.87; P = 0.168] for AMA-1, 0.19 [0.03, 1.11; P = 0.019] for recombinant MSP-1(19), 0.24 [0.04, 1.31; P = 0.032] for the recombinant MSP-1(19) double variant, and 0.18 [0.03, 0.97; P = 0.013] for the triple variant). IgG titers to MSP-1(19) and to the triple variant were higher in plasma samples taken 7 days after chloroquine treatment from children who carried resistant parasites but recovered and remained parasite free. Moreover, in children who were parasitemic on day 14 or day 28, there was an age-independent relationship between parasite density and IgG to both MSP-1(19) and the triple variant (coefficients of -0.550 and -0.590 and P values of 0.002 and 0.001, respectively). The results validate the use of this approach to identify antigens that are associated with protection from malaria.

Opinion: antibody-based therapies for malaria

Antibodies are multifunctional glycoproteins that are found in blood and tissue fluids, and can protect against malaria by binding and neutralizing malaria parasites and preparing them for destruction by immune cells. Important technical advances mean that it is now possible to synthesize antibodies against important Plasmodium antigens that could be used for therapeutic purposes. These reagents could be designed to act like a drug and kill parasites directly, or could be used in vaccine strategies to protect individuals from infection. In this article, we discuss the possible therapeutic uses of antibodies in the treatment and prevention of malaria.

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.

The MTIP-myosin A complex in blood stage malaria parasites

Parasites of the Apicomplexa phylum use an actomyosin motor to drive invasion of host cells. The motor complex is located at the parasite's periphery between the plasma membrane and an inner membrane complex. A crucial component of this complex is myosin tail domain interacting protein (MTIP) identified in the murine malaria parasite Plasmodium yoelii. Here, we show that MTIP is expressed in Plasmodium falciparum merozoites, localises to the periphery of the cell and is present in a complex with myosin A. The MTIP-myosin A tail interaction has a Kd of 235 nM and calcium ions do not play a role in modulating the binding affinity of the two molecules, despite reports of a predicted EF-hand in MTIP. Antibodies to MTIP were used to immobilise the MTIP-myosin A complex, allowing actin binding and motility to be examined. Measurement of actin filament velocities powered by myosin A revealed a velocity of 3.51 microm s(-1), a speed comparable to fast muscle myosins. A short peptide derived from the tail of myosin A (C-MyoA) bound to MTIP and was able to disrupt the association of MTIP and myosin A in parasite lysates. C-MyoA peptidomimetic compounds that disrupt the MTIP-myosin A interaction are predicted to inhibit parasite motility and host cell invasion, which may be targets for new therapeutic approaches.

Variation in the relationship between anti-MSP-1(19) antibody response and age in children infected with Plasmodium falciparum during the dry and rainy seasons

Malaria remains a major parasitic disease in Africa, with 300-500 million new infections each year. There is therefore an urgent need for the development of new effective measures, including vaccines. Plasmodium falciparum merozoite surface protein-1(19) (MSP-1(19)) is a prime candidate for a blood-stage malaria vaccine. Blood samples were collected from children aged 10 days to 15 years in the months of January-March (N = 351) and October-November (N = 369) corresponding to the dry and rainy seasons, respectively. P. falciparum infection was determined by microscopy and enzyme linked immunosorbent assay (ELISA) was used to determine the total IgG and IgG subclasses. There was a significant increase in the mean anti-MSP-1(19) antibody titre in the dry season (p < 0.05), compared to the rainy season. A significantly positive correlation between the anti-MSP-1(19) antibody titre and parasite density (p < 0.01, r = 0.138) was observed. In the rainy season, unlike in the dry season, P. falciparum positive children had higher anti-MSP-1(19) antibody titres than P. falciparum negative children and this difference was significant (p < 0.05). When all individuals were grouped together, the anti-MSP-1(19) antibody titre increased with age in both seasons (r = 0.186 and 0.002), this increase was more apparent in the dry season. However, when the study population was divided into P. falciparum positive and negative groups, it was observed that in the rainy season, there was a negative correlation between anti-MSP-1(19) titre and age in P. falciparum positive individuals, while those who were P. falciparum negative had a positive correlation between anti-MSP-1(19) titre and age. Analysis of anti-MSP-1(19) IgG subclass showed that IgG1 and IgG3 mean titres were highest in both the dry and rainy seasons with an increase in the mean antibody titres for IgG1, IgG2 and IgG3 in the rainy season. In the dry season there was a positive correlation between IgG1, IgG2, and IgG3 titres with age, while IgG4 was negative, whereas in the rainy season there was a positive correlation between IgG2 and IgG4 (non-cytophilic antibodies) with age and a negative correlation for IgG1 and IgG3 (cytophilic antibodies) with age. Seasonal differences in the level of MSP-1(19) IgG subclass titres were observed for P. falciparum negative and positive individuals. Only samples, which were positive for IgG2 and IgG4, showed positive correlation between parasitemia and total IgG. The incidence of P. falciparum infection, which increases during the rainy season, might be an important determinant of anti-MSP-1(19) antibody levels in children living in Igbo-Ora and the results point to the fact that non-cytophilic antibodies to MSP-1(19) in children might be associated with an increase in total IgG and parasitemia.

Apical expression of three RhopH1/Clag proteins as components of the Plasmodium falciparum RhopH complex

The Plasmodium falciparum high molecular mass rhoptry protein ('PfRhopH') complex is important for parasite growth and comprises three distinct gene products: RhopH1, RhopH2 and RhopH3. We have previously shown that P. falciparum RhopH1 is encoded by either PFC0110w (clag3.2) or PFC0120w (clag3.1), members of the previously-named clag (cytoadherence-linked asexual gene) multigene family. In this report, we have further characterized rhoph1/clag members in terms of gene structure, transcription and protein expression. The cDNA sequences for all five rhoph1/clag members were determined, confirming previous in silico predictions of intron-exon boundaries. All member genes were transcribed in HB3 and 3D7 parasite lines, but clag3.2 was not transcribed in Dd2 parasites. The peak abundance of transcripts for all genes was observed during the late schizont stage. Antisera specific to Clag2 and Clag3.1 localized these proteins to the apical end of merozoites in segmented schizonts, and both proteins are found to be components of the PfRhopH complex. PfRhopH complex that was immunoprecipitated with anti-Clag9 antibody contained neither Clag2 nor Clag3.1, thereby suggesting that PfRhopH complexes contain only individual rhoph1/clag gene products. Since the PfRhopH complex binds the erythrocyte surface, and RhopH2 and RhopH3 are encoded by single copy genes, the RhopH1/Clag proteins may serve to confer some degree of specificity to the roles of the individual complexes.

A novel Sushi domain-containing protein of Plasmodium falciparum

Using bioinformatics analyses of the completed malaria genome sequence, we have identified a novel protein with a potential role in erythrocyte invasion. The protein (PFD0295c, ) has a predicted signal sequence and transmembrane domain and a sequence near the C-terminus of the protein shows significant similarity with Sushi domains. These domains, which exist in a wide variety of complement and adhesion proteins, have previously been shown to be involved in protein-protein and protein-ligand interactions. Orthologous genes have also been identified in the genomes of several other Plasmodium species, suggesting a conserved function for this protein in Plasmodium. Our results show that this protein is located in apical organelles and we have therefore designated the protein apical Sushi protein (ASP). We show that the expression of ASP is tightly regulated in the intraerythrocytic stages of the parasite and that it undergoes post-translational proteolytic processing. Based on our observations of timing of expression, location and proteolytic processing, we propose a role for ASP in erythrocyte invasion.