Structural diversity of the major surface antigen of Plasmodium falciparum merozoites

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

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

A 22 kDa protein associated with the Plasmodium falciparum merozoite surface protein-1 complex

The Plasmodium falciparum merozoite surface protein-1 (MSP-1) is synthesized as a precursor of approximately 195 kDa and is processed to form a complex of polypeptides on the surface of free merozoites. As a result of a second processing event, the entire MSP-1 complex is shed from the surface, apart from a C-terminal fragment that remains anchored to the merozoite membrane. We have identified a 22 kDa protein (p22) on the surface of merozoites by cell surface radioiodination and indirect immunofluorescence assay on unfixed free merozoites. p22 is also a component of the shed MSP-1 complex where it is present in part as a 19 kDa form (p22(19)) as shown by immunochemical and peptide mapping analyses. The soluble complex contains MSP-1-derived polypeptides and p22 in approximately stoichiometrically equal amounts. N-terminal amino acid sequence analyses of p22/p22(19) showed that the protein is not derived from the MSP-1 precursor.

Structural analysis of the glycosyl-phosphatidylinositol membrane anchor of the merozoite surface proteins-1 and -2 of Plasmodium falciparum

Plasmodium falciparum accumulates the two merozoite surface proteins-1 and -2 during schizogony. Both proteins are proposed to be anchored in membranes by glycosyl-phosphatidylinositol membrane anchors. In this report the identity of these GPI-anchors is confirmed by labelling with tritiated precursors and additionally by specific enzymatic and chemical treatments. Detailed structural analysis of the core-glycans showed that the GPI-anchors of both proteins possess an extra alpha 1-2 linked mannose at the conserved trimannosyl-core-glycan. MSP-1 and MSP-2 labelled with tritiated myristic acid possess primarily radioactive myristic acid at inositol rings in both GPI-anchors. Additionally the hydrophobic fragments released from [3H]myristic acid labelled GPI-anchors were identified as diacyl-glycerols, carrying preferentially [3H]palmitic acid in an ester-linkage.

Use of a recombinant baculovirus product to measure naturally-acquired human antibodies to disulphide-constrained epitopes on the P. falciparum merozoite surface protein-1 (MSP1)

An enzyme-linked immunosorbent assay (ELISA) has been developed to measure antibody levels in human sera to a candidate vaccine antigen, merozoite surface protein-1 (MSP1), of the malaria parasite Plasmodium falciparum. To ensure the detection of antibodies reactive with important conformational epitopes, antigens used in the ELISA were obtained from either in vitro parasite cultures, or from a baculovirus expression system in which correct folding of recombinant MSP1-derived polypeptides has been previously demonstrated. The specificity of the ELISA was confirmed using a novel antibody affinity select method. The assay was used to investigate the pattern of acquisition of anti-MSP1 antibodies in a cross-sectional survey of 387 3-8 year old residents of a malaria endemic area of The Gambia. A significant positive correlation between anti-MSP1 antibody levels and age was evident, though individual responses to two antigens corresponding to two distinct domains of the MSP1 varied widely.

Synthesis and secretion of proteins by released malarial parasites

Controlled mechanical homogenization of Plasmodium falciparum-infected erythrocytes releases parasites of a quality sufficient for studying the export of newly synthesized plasmodial proteins. Protein synthesis occurs within intact released parasites as defined by resistance of acid-insoluble incorporation of radiolabel to high levels of exogenously added EDTA, hexokinase, and RNaseA. While exogenously added ATP and erythrocyte cytosol were not essential for biosynthetic activity at levels comparable to that seen in infected erythrocytes, the addition of an extracellular ATP regenerating system (ARS) stimulated the synthesis of parasite proteins. Conversely, parasite viability and biosynthetic activity are decreased by the addition of a non-hydrolyzable ATP analogue (ATP gamma S), ADP, or ATP in the absence of a regenerating system. These data suggest a metabolic interdependence between extracellular energy metabolism and biosynthetic functions within the parasite. The export of a predominant subset of proteins was retarded in the presence of Brefeldin A, indicating the existence of a classical secretory pathway characteristic of that seen in higher eukaryotic cells. Interestingly, a Brefeldin A-insensitive component of export was also consistently observed; this may suggest the existence of an additional alternative secretory mechanism in malaria.

Apparent lack of N-glycosylation in the asexual intraerythrocytic stage of Plasmodium falciparum

This study investigates protein glycosylation in the asexual intraerythrocytic stage of the malaria parasite, Plasmodium falciparum, and the presence in the infected erythrocyte of the respective precursors. In in vitro cultures, P. falciparum can be metabolically labeled with radioactive sugars, and its multiplication can be affected by glycosylation inhibitors, suggesting the capability of the parasite to perform protein-glycosylation reactions. Gel-filtration analysis of sugar-labeled malarial proteins before and after specific cleavage of N-glycans or O-glycans, respectively, revealed the majority of the protein-bound sugar label to be incorporated into O-glycans, but only little (7-12% of the glucosamine label) or no N-glycans were found. Analysis of the nucleotide sugar and sugar-phosphate fraction showed that radioactive galactose, glucosamine, fucose and ethanolamine were converted to their activated derivatives required for incorporation into protein. Mannose was mainly recovered as a bisphosphate, whereas the level of radiolabeled GDP-mannose was below the detection limit. The analysis of organic-solvent extracts of sugar-labeled cultures showed no evidence for the formation by the parasite of dolichol cycle intermediates, the dedicated precursors in protein N-glycosylation. Consistently, the amount of UDP-N-acetylglucosamine formed did not seem to be affected by the presence of tunicamycin in the culture. Oligosaccharyl-transferase activity was not detectable in a lysate of P. falciparum, using exogenous glycosyl donors and acceptors. Our studies show that O-glycosylation is the major form of protein glycosylation in intraerythrocytic P. falciparum, whereas there is little or no protein N-glycosylation. A part of these studies has been published in abstract form [Dieckmann-Schuppert, A., Hensel, J. and Schwarz, R. T. (1991) Biol. Chem. Hoppe-Seyler 372, 645].

Labeling and initial characterization of polar lipids in cultures of Plasmodium falciparum

The present report describes the radioactive labeling of polar lipids in in vitro cultures of Plasmodium falciparum as well as their extraction with organic solvents and their partial characterization by chemical and enzymatic methods. All substances detected could be cleaved by alkali, suggesting that they were esters rather than sphingolipids or compounds containing alkyl groups. Dolichol-cycle intermediates were not detected. Phosphatidylinositol, phosphatidylethanolamine, and phosphatidylcholine were labeled by fatty acids and inositol or ethanolamine, respectively, confirming their de novo synthesis by the parasite. Metabolic labeling with glucosamine and cleavage by phosphatidylinositol-specific phospholipase C provided evidence of the formation of N-acetyl-glucosaminyl-phosphatidylinositol, an obligate precursor in the biosynthesis of glycosylphosphatidylinositol membrane anchors of proteins.

Processing of the Plasmodium falciparum major merozoite surface protein-1: identification of a 33-kilodalton secondary processing product which is shed prior to erythrocyte invasion

We have previously shown that only a single 19-kDa fragment of the Plasmodium falciparum major merozoite surface protein (MSP1) is carried with an invading merozoite into the infected red cell. This fragment (MSP1(19] is derived from the C-terminal membrane-bound end of a major product, MSP1(42), of the primary stage of MSP1 proteolytic processing. Using a monoclonal antibody mapped to an epitope within the N-terminal region of MSP1(42), we have shown that a soluble 33-kDa polypeptide (MSP1(33) corresponding to the N-terminal region of MSP1(42) is shed into culture supernatants during merozoite release and erythrocyte invasion. These observations provide further evidence that the secondary processing of MSP1(42) involves a highly site-specific proteolytic activity.

Structural diversity of Plasmodium falciparum gp200 is detected by T cells

T lymphocyte clones (TLC) specific for P. falciparum gp200 (a glycoprotein precursor of the main merozoite surface component) were obtained from two individuals with past exposure to malaria. The 25 established TLC carried the CD4 antigen and proliferated in the presence of immunopurified gp200, crude lysate of the parasite and intact infected red blood cells. They were further tested in proliferation assays for their capacity to recognize the structural diversity displayed by gp200. The stimulating antigen used in these assays was either sonicated or viable preparations of schizonts from five P. falciparum isolates differing in their gp200. The majority of the TLC proliferated similarly in the presence of each of the isolates. One third of the TLC proliferated to a different extent depending on the isolate used for stimulation, while two clones gave isolate-specific responses. These results indicate that the majority of human TLC raised in vitro against gp200, is directed against common determinants. This also suggests that immunization with full length gp200 will not lead predominantly to T cell help restricted to isolate-specific determinant.

Mycoplasma pulmonis V-1 surface protein variation: occurrence in vivo and association with lung lesions

The V-1 antigen of Mycoplasma pulmonis is exposed to the surface of the mycoplasma and has an immunoblot banding pattern that varies in vitro between and within strains. To determine if V-1 variation occurs in vivo, we infected C3H/HeNCr mice intranasally with 5 X 10(8) colony-forming units of M. pulmonis strain 5782C. We isolated M. pulmonis clones from the respiratory tracts of mice up to 28 days post-infection, then used anti-V-1 monoclonal antibody P39 to visualize their V-1 immunoblot banding patterns. By the 28th day following infection, 92% of the recovered clones had variant V-1 banding patterns. Additionally, there was a significant correlation between the severity of lung lesions and the percentage of V-1 variant clones recovered from the respiratory tracts of individual mice. These studies prove that V-1 variation does occur in vivo, and suggest that mice with more severe pulmonary lesions tend to have more V-1 variant clones as a percentage of the M. pulmonis population. Thus, variation in the V-1 protein may be a mechanism by which M. pulmonis persists in the in vivo environment, possibly by evasion of host immune surveillance or by alteration of its surface membrane to take better advantage of its environmental niche in the host.

An S antigen gene from Plasmodium falciparum contains a novel repetitive sequence

The complete sequence of the gene coding for the S antigen from the Wellcome (West African) strain of Plasmodium falciparum has been obtained. It contains a central repetitive region consisting of 65 copies of a partially degenerate 24 bp sequence, coding for a conserved 8 amino acid repeat (Gly Pro Asn Ser Asp Gly Asp Lys). The repeat sequence is different from those of S antigens characterised in other strains and thus represents a new S antigen serotype.

Topography of epitopes on a polymorphic schizont antigen of Plasmodium falciparum determined by the binding of monoclonal antibodies in a two-site radioimmunoassay

The topographic distribution of common and variant epitopes on two divergent allelic forms of the 185-205K schizont glycoprotein of Plasmodium falciparum were studied by a two-site radioimmunoassay using monoclonal antibodies. Similarities in the conformation of the two molecules were apparent. On both antigens two distinct regions were mapped, each comprising of both strain-common and polymorphic epitopes. Epitopes common to the two PSAs were found to be closely associated with different variable epitopes in tertiary structure. It is suggested that this may contribute to parasite evasion of the host immune response.

Characterization of gp195 processed products purified from Plasmodium falciparum culture supernates

Schizonts of the malaria parasite Plasmodium falciparum synthesize a 195 kDa surface glycoprotein (gp195) that is processed into several smaller products including one of 83 kDa, which, in the case of the Camp strain, is sequentially processed into 73 and 67 kDa products. gp195 and its processing intermediates larger than 83 kDa were not precipitated from culture supernates, but the 83 and 73 kDa products were precipitated by three monoclonal antibodies (McAbs). The 83 and 73 kDa products were affinity purified from culture supernates by adsorbing to McAb 7B2 coupled to Affigel 10 and eluting either with 0.2 N acetic acid, pH 2.8, or with 3 M potassium isothiocyanate (KSCN). The epitope recognized by McAb 7B2 was denatured by acid elution but could be regenerated by treating with 8 M urea followed by dialysis. The implications of renaturing antigens to regenerate epitopes should be considered in studies on the purification, function and immunogenicity of malaria antigens.

Allelic dimorphism in a surface antigen gene of the malaria parasite Plasmodium falciparum

Merozoites of the malaria parasite Plasmodium falciparum carry surface proteins processed from a precursor termed p190 or p195. Polymorphism has been reported in this protein. Since the protein is a candidate for a malaria vaccine, it is important to understand the nature of this polymorphism. We have determined the complete nucleotide sequence of the p190 gene from the MAD20 strain (a Papua New Guinea isolate). Comparisons of the gene with that from other strains of P. falciparum allowed us to study the genetic basis of the antigen's polymorphism. The gene consists of sequences distributed in variable blocks, which are separated by conserved or semi-conserved sequences. Variable sequences occur both in regions that code for tripeptide repeats and in regions with no apparent repeats. Interestingly, according to the present data, variable sequences are not widely polymorphic but fall into two distinct types. We argue that the p190 protein is encoded by dimorphic alleles capable of limited genetic exchange and present evidence at the nucleotide level documenting intragenic recombination in Plasmodium.

Processing of the precursor to the major merozoite surface antigens of Plasmodium falciparum

Specific sequences derived from the gene for the precursor to the major merozoite surface antigens (PMMSA) of Plasmodium falciparum have been expressed in Escherichia coli and the products have been used to produce antibodies. These antibodies, together with monoclonal antibodies, have been used to investigate the form of the PMMSA protein associated with merozoites. Polypeptide fragments derived by processing from the PMMSA protein have been detected in extracts of merozoites and assigned to locations within the PMMSA coding sequence.

Fragments of the polymorphic Mr 185,000 glycoprotein from the surface of isolated Plasmodium falciparum merozoites form an antigenic complex

Merozoites of the human malaria parasite Plasmodium falciparum express on their surface several antigens derived from a polymorphic glycoprotein precursor of Mr 185,000 synthesised earlier on by trophozoites and schizonts. A panel of 18 monoclonal antibodies against a range of different specificities of the precursor was used to characterise its mature products in spontaneously released merozoites. Merozoites released by [35S]methionine or [14C]glucosamine-labelled schizonts, or surface 125I-labelled purified merozoites, were extracted in detergents, and the antigens were detected by immunoprecipitation or Western blotting. We show that a nonglycosylated peptide of Mr 80,000 and two glycosylated fragments of Mr 40,000 and Mr 16,000, all derived from the precursor, are exposed on the surface of the mature merozoite. Precipitations from extracts in different detergents indicate that the 80 and 40 kDa fragments can form a non-covalent complex with each other and two additional major surface antigens of 36 and 22 kDa. Several antibodies react strongly with the complex but not with its dissociated subunits, thus indicating presence of conformational epitopes. Other epitopes are positively mapped on different dissociated subunits by immunoprecipitation and Western blotting. The 80 and 40 kDa antigens each carry a different polymorphic marker epitope, and both of these markers are absent on the 16 kDa fragment. The 40 and 16 kDa glycoproteins share common epitopes, and the latter may be derived from the former fragments. Only epitopes present on the 16 kDa antigen, but not those specific for the larger fragments, are detectable by immunofluorescence in the ring-stage. This indicates that the whole or a part of the 16 kDa antigen remains on the parasite through the invasion process.

Strain variation in the circumsporozoite protein gene of Plasmodium falciparum

The complete nucleotide sequence of the cloned circumsporozoite protein gene of the Plasmodium falciparum Wellcome (West African) isolate has been determined. The sequence shows two striking differences from that of the published Brazilian strain; the total number of tandem 12 base pair repeats is 46 compared to 41, and the 5' coding region contains an additional 30 nucleotides. From Southern blot experiments, two out of four cloned Thai lines also have a similar, higher number of repeats. Heterogeneity in the CSP gene repeat region and in the length of the 5' coding region allows the strains to be classed into three groups, with the Wellcome strain being indistinguishable from the Thai line T9-94.

Conservation and antigenicity of N-terminal sequences of GP185 from different Plasmodium falciparum isolates

Complementary DNA (cDNA) clones for GP185, a major antigenically diverse glycoprotein of Plasmodium falciparum, were isolated from a cDNA library of the Honduras I/CDC (Honduras I) isolate, and 1052 bp were sequenced. The expression of cDNA fragments in Escherichia coli using the vector pCQV2 allowed verification of the reading frame. This GP185 cDNA sequence, like the cDNA sequence for a homologous gene of the K1 isolate [Hall et al., Nature 311 (1984) 379-382], codes for a polypeptide which is truncated due to multiple, in-frame stop codons. This polypeptide corresponds to the N-terminal 15% of the proposed coding region of the GP185 gene [Holder et al., Nature 317 (1985) 270-273]. Comparison of the nucleotide sequences for the GP185 gene of Honduras I and five other isolates indicated that there are two areas of conserved DNA sequence, one of 310 bp (beginning 181 bp upstream from the proposed initiation codon) and the other of greater than or equal to 360 bp (located entirely within the coding region), separated by a region encoding isolate-specific tandem amino acid repeats. Rat antiserum was raised to a fusion protein derived from the conserved regions and the intervening repeat region of this Honduras I protein. This antiserum bound GP185 on immunoblots of the homologous Honduras I isolate and the heterologous K1 isolate, which has different tandem repeats. Serum from owl monkeys and humans previously infected with P. falciparum reacted with the fusion protein on immunoblots demonstrating that determinants in the N-terminal 15% of GP185 were immunogenic in infected individuals and suggesting that some of these sites are conserved among isolates.(ABSTRACT TRUNCATED AT 250 WORDS)