Plasmodium falciparum: molecular analysis of a putative protective antigen, the thermostable 96-kDa protein

Food vacuole proteome of the malarial parasite Plasmodium falciparum

The Plasmodium falciparum food vacuole (FV) is a lysosome-like organelle where erythrocyte hemoglobin digestion occurs. It is a favorite target in the development of antimalarials. We have used a tandem mass spectrometry approach to investigate the proteome of an FV-enriched fraction and identified 116 proteins. The electron microscopy analysis and the Western blot data showed that the major component of the fraction was the FV and, as expected, the majority of previously known FV markers were recovered. Of particular interest, several proteins involved in vesicle-mediated trafficking were identified, which are likely to play a key role in FV biogenesis and/or FV protein trafficking. Recovery of parasite surface proteins lends support to the cytostomal pathway of hemoglobin ingestion as a FV trafficking route. We have identified 32 proteins described as hypothetical in the databases. This insight into FV protein content provides new clues towards understanding the biological function of this organelle in P. falciparum.

A GBP 130 derived peptide from Plasmodium falciparum binds to human erythrocytes and inhibits merozoite invasion in vitro

The malarial GBP 130 protein binds weakly to intact human erythrocytes; the binding sites seem to be located in the repeat region and this region's antibodies block the merozoite invasion. A peptide from this region (residues from 701 to 720) which binds to human erythrocytes was identified. This peptide named 2220 did not bind to sialic acid; the binding site on human erythrocyte was affected by treatment with trypsin but not by chymotrypsin. The peptide was able to inhibit Plasmodium falciparum merozoite invasion of erythrocytes. The residues F701, K703, L705, T706, E713 (FYKILTNTDPNDEVERDNAD) were found to be critical for peptide binding to erythrocytes.

Reconstitution of protein transport across the vacuolar membrane in Plasmodium falciparum-infected permeabilized erythrocytes

The parasite Plasmodium falciparum induces morphological and biochemical alterations of its host erythrocyte. Some of these changes are mediated by parasite proteins that are transported to specific destinations within the erythrocyte or to the erythrocyte plasma membrane. The pathways underlying this transport are still unknown. We anticipate that at least some aspects of these pathways may be biologically unique and therefore potential targets for chemotherapeutic intervention. We have utilized bacterial pore-forming proteins to establish an experimental system that allows selective permeabilization of the erythrocyte plasma membrane, without affecting the integrity of the vacuolar membrane and the parasite plasma membrane, in order to study protein transport from the parasite into the host erythrocyte. Physiological properties of the parasite within permeabilized erythrocytes, such as the ability to synthesize proteins, will be described. The permeabilization of infected erythrocytes has allowed the dissection of individual steps in protein transport from the parasite surface across the vacuolar membrane. Possible pathways involved in the trafficking of parasite proteins within the erythrocyte cytosol, i.e. in a cell that normally has no need to transport proteins, will be discussed.

Plasmodium falciparum asparagine and aspartate rich protein 2 is an evolutionary conserved protein whose repeats identify a new family of parasite antigens

We describe here a new Plasmodium falciparum antigen, asparagine and aspartate rich protein 2 (PfAARP2) of 150 kDa, which is encoded by a unique gene on chromosome 1. PfAARP2 is first expressed 12 h post-invasion and accumulates in trophozoites and schizonts. Immunofluorescence studies indicate that PfAARP2 is translocated into the red blood cell cytoplasm. The central region of Pfaarp2 contains blocks of repetitions encoding asparagine and aspartate residues, which define a new family of related genes dispersed on different chromosomes, and two members of this family have also been identified. Interestingly, the non-repeated N- and C-termini of PfAARP2 display significant similarity to two yeast and human predicted proteins, and its possible function is discussed.

Protein sorting in Plasmodium falciparum-infected red blood cells permeabilized with the pore-forming protein streptolysin O

Plasmodium falciparum is an intracellular parasite of human red blood cells (RBCs). Like many other intracellular parasites, P. falciparum resides and develops within a parasitophorous vacuole which is bound by a membrane that separates the host cell cytoplasm from the parasite surface. Some parasite proteins are secreted into the vacuolar space and others are secreted, by an as yet poorly defined pathway, into the RBC cytosol. The transport of proteins from the parasite has been followed mainly using morphological methods. In search of an experimental system that would allow (i) dissection of the individual steps involved in transport from the parasite surface into the RBC cytosol, and (ii) an assessment of the molecular requirements for the process at the erythrocytic side of the vacuolar membrane, we permeabilized infected RBCs with the pore-forming protein streptolysin O using conditions which left the vacuole intact. The distribution of two parasite proteins which served as markers for the vacuolar space and the RBC cytosol respectively was analysed morphologically and biochemically. In permeabilized RBCs the two marker proteins were sorted to the same compartments as in intact RBCs. The protein which was destined for the RBC cytosol traversed the vacuolar space before it was translocated across the vacuolar membrane. Protein transport could be arrested in the vacuole by removing the RBC cytosol. Translocation across the vacuolar membrane required ATP and a protein source at the erythrocytic face of the membrane, but it was independent of the intracellular ionic milieu of the RBC.

Characterization of membrane proteins exported from Plasmodium falciparum into the host erythrocyte

Plasmodium falciparum is an intracellular parasite of the red blood cell. During development it exports proteins which are transported to specific locations within the host erythrocyte. We have begun to identify and characterize exported membrane proteins of P. falciparum in order to obtain specific marker molecules for the study of the mechanisms involved in the distribution of parasite-derived proteins within the host cell. In this report we describe the characterization of a 35 kDa protein which is recognized by a monoclonal antibody. The protein is tightly associated with membranes isolated from infected erythrocytes; it is resistant to extraction with alkali and soluble after treatment with detergents. It is located at the membrane of the parasitophorous vacuole and in membrane-bound compartments which appear in the cytoplasm of the infected erythrocyte. The protein co-localizes with the previously described exported protein-1 (exp-1). Considering its localization and physical similarities to exp-1, we name the 35 kDa protein the exported protein-2 (exp-2).

Brefeldin A inhibits transport of the glycophorin-binding protein from Plasmodium falciparum into the host erythrocyte

Plasmodium falciparum, a protozoan parasite of the human erythrocyte, causes the most severe form of malaria. During its intraerythrocytic development, the parasite synthesizes proteins which are exported into the host cell. The compartments involved in the secretory pathway of P. falciparum are still poorly characterized. A Golgi apparatus has not been identified, owing to the lack of specific protein markers and Golgi-specific post-translational modifications in the parasite. The fungal metabolite brefeldin A (BFA) is known to inhibit protein secretion in higher eukaryotes by disrupting the integrity of the Golgi apparatus. We have used the parasite-encoded glycophorin-binding protein (GBP), a soluble protein found in the host cell cytoplasm, as a marker to investigate the effects of BFA on protein secretion in the intracellular parasite. In the presence of BFA, GBP was not transported into the erythrocyte, but remained inside the parasite cell. The effect caused by BFA was reversible, and the protein could be chased into the host cell cytoplasm within 30 min. Transport of GBP from the BFA-sensitive site into the host cell did not require protein synthesis. Similar observations were made when infected erythrocytes were incubated at 15 degrees C. Incubation at 20 degrees C resulted in a reduction rather than a complete block of protein export. The relevance of our findings to the identification of compartments involved in protein secretion from the parasite cell is discussed.

Analysis of the high molecular weight rhoptry complex of Plasmodium falciparum using monoclonal antibodies

Twenty-one monoclonal antibodies, obtained after immunization of mice with erythrocytic stages of Plasmodium falciparum, produced a double dot image in IFA. Immunoelectronmicroscopy indicated that the mAbs reacted with the rhoptries. Rhoptries are pear-shaped apical organelles, believed to be involved in invasion of the host cell by the parasite. The mAbs all immunoprecipitated the high molecular weight antigen complex. Some mAbs recognized on immunoblots only 1 protein of this complex, whereas others reacted with RhopH1 and RhopH3, or RhopH2 and RhopH3 or with the 3 proteins. An additional antigen of 52 kDa was also recognized by some of the mAbs. The epitopes defined by the mAbs were present in most of the 40 P. falciparum strains or isolates studied by IFA. Interestingly, the mAbs also reacted with high titres on P. vivax and P. ovale, but produced images that did not indicate an apical location. The mAbs failed to react on the non-human malaria parasites studied, P. cynomolgi and P. inui. On P. berghei or P. chabaudi parasites, only 5 mAbs gave a positive reaction, labelling a large network outside the parasite. Finally, the mAbs did not react with P. falciparum sporozoites, indicating that the rhoptries of merozoites and sporozoites, the two invasive stages of the malaria life-cycle are equipped with distinct sets of proteins.

Immunogenicity and antigenicity of a Plasmodium falciparum protein fraction (90-110 kDa) able to protect squirrel monkeys against asexual blood stages

A monkey vaccination trial using a Plasmodium falciparum protein fraction containing antigens of 90-110 kDa is reported. The fraction was obtained by electroelution from preparative polyacrylamide gels. Three monkeys out of five resisted a heavy challenge dose of highly virulent parasites. Using specific antisera, several components of the fraction were identified, namely heat shock protein 90 (hsp90), Ag44/RhopH3, ABRA, 96tR/GBP130 and Pf96 protease. The fraction did not contain KAHRP, nor the SERP antigen. The antibody response of the monkeys was studied on these individual antigens purified by preparative immunoprecipitation. Surprisingly, hsp90 was found in the immunoprecipitates obtained with SERP antisera. Interestingly, the response to hsp90 correlated with protection, high antibody titres being found only in the protected monkeys. In contrast, no correlation with protection could be found for the response to the other antigens.

The virulent Saimiri-adapted Palo Alto strain of Plasmodium falciparum does not express the ring-infected erythrocyte surface antigen

The Palo Alto strain of Plasmodium falciparum is highly virulent for the Saimiri sciureus monkey. We have observed that these parasites do not express the Ring-infected erythrocyte surface antigene (RESA) gene. Immunoblots indicated that the Pf155/RESA protein was absent. The RESA mRNA could not be detected. Polymerase chain reaction and Southern blot analysis demonstrated that this lack of expression is due to gene rearrangements. The majority of the Palo Alto parasites have a deletion of the entire RESA gene, whereas in a minor fraction the RESA sequences remain detectable, but the 5' miniexon 1 is inverted. These data show that the RESA protein is dispensable for in vivo parasite growth, at least in Saimiri monkeys.

In vitro phagocytosis inhibition assay for the screening of potential candidate antigens for sub-unit vaccines against the asexual blood stage of Plasmodium falciparum

We have previously established a direct correlation between immune protection against the asexual blood stage Plasmodium falciparum infection and the presence of opsonizing antibodies promoting phagocytosis of parasitized red blood cells. In the present communication we describe an in vitro assay for measuring phagocytosis inhibition (PIA) specific for P. falciparum-infected erythrocytes. The phagocytosis inhibition assay is a simple procedure for screening potential candidates for sub-unit vaccines against P. falciparum based on the correlation between opsonizing antibodies and immunoprotection. The assay was used to analyse 18 recombinant molecules, corresponding to 11 distinct antigens of P. falciparum. Pre-incubation and selective antibody depletion experiments demonstrate the antigen-antibody specificity of the PIA. The presence of epitopes participating as targets of opsonic antibodies were demonstrated in six distinct polypeptide antigens.

Plasmodium falciparum: Characterization of gene R45 encoding a trophozoite antigen containing a central block of six amino acid repeats

We describe here an antigen, called R45, expressed by the young trophozoites of Plasmodium falciparum. This antigen contains a block of tandem repeats of six amino acids which are recognized by sera from humans living in endemic areas. The R45 gene is located on chromosome 3. It is present in all strains examined and shows limited size polymorphism. The C-terminal unique region of the protein shows a strong homology with the catalytic domain of the serine protein kinases. Interestingly, the central repeats contain a large number of putative phosphorylation sites. The implications of these features are discussed.

A Plasmodium falciparum blood stage antigen highly homologous to the glycophorin binding protein GBP

We have isolated a gene coding for a protein highly homologous to an antigen known as the glycophorin binding protein (GBP) which was therefore called GBPH. The gene consists of 2 exons interrupted by an intron located at a position corresponding to that of the GBP gene. The deduced amino acid sequence of GBPH comprises 427 residues and is characterized by a signal sequence and by an extended repeat region consisting of 8 units of 40 amino acid residues. The comparison of the amino acid sequences of GBPH and GBP reveals an identity of 69%. Antisera raised against a GBPH fragment that carries part of the repetitive region cross-react with GBP (105 kDa) and additionally detect some bands between 40 and 70 kDa, one of which may correspond to GBPH. The genes coding for GBP and GBPH are located on chromosomes 10 and 14, respectively. The GBP gene is transcribed as a highly abundant 6.5 kb mRNA in the blood-stage form, whereas Northern blot analysis using a GBPH specific probe detects 2 less abundant mRNAs of 2.3 kb and 2.7 kb. Southern blot analysis of P. falciparum DNA identifies a third member of the GBP gene family.

A study of the genomic diversity of Plasmodium falciparum in Senegal. 2. Typing by the use of the polymerase chain reaction

The genomic polymorphism of Plasmodium falciparum was investigated in a series of samples collected in Senegal during one transmission season. PCR analysis was performed on several genes coding for blood-stage antigens: the gene for the major merozoite surface antigen P190, the gene for the second merozoite surface antigen MSA2 and the gene coding for antigen 96tR/GBP130. In each case, several distinct forms of the genes studied were observed. Both the MAD20 and K1 allelic families of P190 genes were observed. PCR analysis of a single variable region did not differentiate each isolate. However, when the data obtained for several markers are combined, each isolate had a specific genotype. Thus, using PCR to study in parallel several loci is a useful tool to genetically type strains.

In vivo and in vitro derived Palo Alto lines of Plasmodium falciparum are genetically unrelated

The Uganda Palo Alto strain of Plasmodium falciparum (FUP) is routinely used as a reference isolate in a number of laboratories. It is one of the few P. falciparum strains that can both be propagated in vivo in monkeys and maintained in culture. The Palo Alto parasites have been characterized for several biochemical and molecular markers, but many of the data reported so far are contradictory. We have analyzed and compared by Southern blotting, PCR and DNA sequencing, several DNA preparations obtained from different FUP lines and from the FCR3 strain. We show here that FUP lines propagated in Saimiri monkeys (FUP/S) and those maintained in culture (FUP/C) for many years in our laboratory differ in the various genetic markers investigated (P190, MSA2, S-Ag, KAHRP, 96 tR, pPFPA rep 20 and pPF 11.1). Therefore, at the present, two genetically unrelated strains of P. falciparum widely distributed over numerous laboratories are designated FUP/Palo Alto. When the Saimiri-propagated FUP/S line was used to initiate an in vitro culture in human red blood cells, no evidence of instability or genetic drift was obtained. The growth rate and genomic characteristics remained constant for several months. Likewise, the FUP/C line was found unchanged after three transfers in Saimiri monkeys. FUP/CP parasites were shown to be genetically closely related to FCR3. In addition, a subline of FUP/C strain selected by repeated flotation on gelatin was found to differ in several characters such as KHARP, P190 and S-antigen genes, which are known to be located on different chromosomes.

Studies on Pf155/RESA and other soluble antigens from in vitro cultured Plasmodium falciparum

Spent culture medium from in vitro cultures of Plasmodium falciparum was used as the source for immunoadsorbent enrichment of soluble parasite antigens. IgG obtained from P. falciparum-hyperimmune Liberian serum was used as the ligand in the immunoadsorbent. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the immunoadsorbent isolated material revealed the presence of at least 15 antigenic and parasite-derived polypeptides. Immunoblotting after SDS-PAGE showed greater than 20 antigenic polypeptides in the molecular weight range of 15-250 kDa. Several of these were heat-stable (100 degrees C for 5 min), and six of them were detected even after heating for 60 min. Among the latter antigens was Pf155/RESA, a merozoite antigen deposited in the erythrocyte membrane at invasion. Pf155/RESA can be detected using a modified erythrocyte membrane immunofluorescence (EMIF) assay. Inhibition of EMIF with heated parasite material showed that the antigenic activity was intact after heating for up to 30 min. The immunofluorescence-inhibitory activity was separated into two fractions, one containing Pf155/RESA and one containing polypeptides with molecular weights of 135 and 120 kDa. All three of these antigenic polypeptides bound selectively, albeit nonspecifically, to aminoethyl-BioGel resins.

Chemical crosslinking of Plasmodium falciparum glycoprotein, Pf200 (190-205 kDa), to the S-antigen at the merozoite surface

Merozoites were isolated from Plasmodium falciparum cultures labeled with [3H]mannose and [35S]methionine and treated with a cleavable homobifunctional crosslinker, dithiobis(succinimidyl) propionate. The crosslinked complexes were immunoprecipitated with Mab.5B1 directed against the major merozoite surface glycoprotein. Pf200 (MW 190-205), and reduced with dithiothreitol. Crosslinked immunocomplexes did not contain the second major merozoite surface glycoprotein, Pf50 (MW 45-55 kDa), or other major [35S]methionine-labeled proteins, except for a weakly labeled protein of 150 kDa. Crosslinked complexes immunoprecipitated with Mab.5B1 and then reduced with DTT were immunoblotted with antibody directed against three soluble P. falciparum antigens, a serine-rich antigen known as Pf126 or SERA, the S-antigen, and GBP-130. The 150-kDa S-antigen was readily detected in crosslinked immunocomplexes with Pf200. The SERA antigen, although crosslinked under these conditions, was not detected in association with Pf200 nor was GBP-130.

Specificities of antibodies that inhibit merozoite dispersal from malaria-infected erythrocytes

When malaria schizont-infected erythrocytes are cultured with immune serum, antibodies prevent dispersal of merozoites, resulting in the formation of immune clusters of merozoites (ICM) and inhibition of parasite growth. Antigens recognized by these antibodies were identified by probing two dimensional immunoblots of Plasmodium falciparum antigens with antibodies dissociated from immune complexes present at the surface of merozoites in ICM. Total immune serum recognized 88 of the 135 protein spots detected by colloidal gold staining, but antibodies dissociated from immune complexes recognized only 15 protein spots attributable to no more than eight distinct antigens. Antigens recognized by antibodies that inhibit merozoite dispersal include the precursor to the major merozoite surface antigens (gp195), a 126-kDa serine-repeat antigen (SERA), the 130-kDa protein that appears to bind to glycophorin (GBP130), and the approx. 45-kDa merozoite surface antigen. One other antigen (230/215-kDa doublet) was identified by using antibodies affinity purified from recombinant expression proteins. The identities of the other three antigens (150 kDa, 127 kDa and less than 30 kDa) were not determined. This approach provides a strategy for identifying epitopes accessible at the merozoite surface which may be important components of a multivalent vaccine against blood stages of P. falciparum.

Plasmodium falciparum: an intervening sequence in the GBP 130/96 tR gene

The 96 tR antigen is a heat stable protein produced during the late stages of the intraerythrocytic development of the malaria parasite Plasmodium falciparum and is released into the culture supernatant or the sera of infected patients at the time of schizont rupture. This antigen, identified as a putative protective antigen, was shown to be identical to the glycophorin-binding protein GBP 130 (Perkins 1988, Bonnefoy et al. 1988). We report here that the gene contains a small undescribed intervening sequence located immediately after the sequence coding for the signal sequence. This shows that in P. falciparum, all the genes described so far coding for proteins exported outside the parasitophorous vacuole share a common organization.

Application of exoantigens of Babesia and Plasmodium in vaccine development

The University of Illinois malaria vaccine programme uses culture-derived soluble exoantigens of Plasmodium falciparum and the squirrel monkey as an experimental model. Exoantigens are soluble polypeptides naturally released into the blood plasma of animals infected with Babesia or Plasmodium species, or into the supernatant medium of in vitro cultures of these organisms. Immunization with soluble B. bovis and B. bigemina exoantigens prepared from culture supernatant fluids protected cattle against homologous and heterologous challenge. Similarly, vaccination of squirrel monkeys with supernatant fluids from P. falciparum cultures containing exoantigen induced protective immunity against acute clinical malaria. Susceptible monkeys have been vaccinated with an aluminium hydroxide-fortified antigenic fraction partially purified from supernatants of P. falciparum strains Indochina I and Genève/SGE-1; this conferred significant clinical protection against needle challenge with the homologous Indochina I strain, and a moderate degree of immunity to the heterologous strain. Following sequential purification by high performance liquid chromatography, the N-terminal amino acid sequences of P. falciparum 100 kDa, 83 kDa and 70 kDa exoantigens were determined. A 29 amino acid peptide constructed from the N-terminal sequence of the P. falciparum (Genève strain) 83 kDa exoantigen has been synthesized. When coupled to a carrier protein, the peptide was immunogenic in rabbits, mice and squirrel monkeys, inducing antibodies which were trophozoite-specific, reactive to native parasite proteins in a two-site enzyme immunoassay (EIA) and in Western blots, and which inhibited P. falciparum growth in vitro. Using this synthetic peptide, EIAs are being developed for the detection of antibodies to P. falciparum blood-phase parasites in individuals living in malaria-endemic areas of Africa, Asia and South America.

Cross-reactive antigenic determinants present on different Plasmodium falciparum blood-stage antigens

A gene encoding a previously undescribed antigen of Plasmodium falciparum has been isolated from a genomic expression library by use of a pool of human immune sera. Northern blot analysis indicated that the gene is expressed at the late stages of the intra-erythrocytic cycle. This antigen, 332, contains a series of degenerated amino acid repeats. Human antibodies affinity-purified on the 332 recombinant antigen reacted with a family of parasite proteins that are products of different genes. We identified antigens 11.1 and Pf155-RESA as members of this family and confirmed, using a human monoclonal antibody, the presence of cross-reacting determinants. The sequences of these antigens also share some structural homologies. The significance of this family of blood-stage antigens is discussed.

Stage-dependent processing and localization of a Plasmodium falciparum protein of 130,000 molecular weight

A Plasmodium falciparum protein of 130,000 molecular weight (m.w.) has been identified, cloned in Escherichia coli, and completely sequenced (Kochan et al. 1986). The protein appeared to bind to soluble glycophorin, a host erythrocyte surface protein. In the present study, extracts of parasites from different intraerythrocytic stages were immunoblotted with antibodies, raised against a 30,000 m.w. fusion protein corresponding to the 3' end of the 130,000 m.w. protein. It was demonstrated that the protein is synthesized at the trophozoite stage, accumulates at the schizont stage, and is processed at the merozoite stage to a triplet of three polypeptides. The processed proteins are present in the culture supernatant at the time of merozoite burst from the red cell. Immunofluorescent staining of the parasite at different intracellular stages indicates that the protein is localized on the parasite at the trophozoite stage. At late trophozoite stage, it appears to be transported to the erythrocyte cytoplasm, where it is present in small vesicles or inclusions. In mature schizonts the protein accumulates around the plasma membrane of the erythrocyte. At the segmenter stage, just prior to merozoite release, it appears also to surround the intracellular merozoite, as well as the erythrocyte plasma membrane. The soluble 130,000 m.w. protein binds to erythrocytes but binds significantly greater to erythrocyte membranes, suggesting it binds to an internal domain of glycophorin rather than the domain exposed on the surface. The 130,000 m.w. protein is present in 11 different geographic isolates of P. falciparum from diverse geographic origins. Its molecular weight is similar in all isolates.