A conserved region in the EBL proteins is implicated in microneme targeting of the malaria parasite Plasmodium falciparum

Molecular Characterization and Immuno-Reactivity Patterns of a Novel Plasmodium falciparum Armadillo-Type Repeat Protein, PfATRP

Nearly half of the genes in the Plasmodium falciparum genome have not yet been functionally investigated. We used homology-based structural modeling to identify multiple copies of Armadillo repeats within one uncharacterized gene expressed during the intraerythrocytic stages, PF3D7_0410600, subsequently referred to as P. falciparum Armadillo-Type Repeat Protein (PfATRP). Soluble recombinant PfATRP was expressed in a bacterial expression system, purified to apparent homogeneity and the identity of the recombinant PfATRP was confirmed by mass spectrometry. Affinity-purified α-PfATRP rabbit antibodies specifically recognized the recombinant protein. Immunofluorescence assays revealed that α-PfATRP rabbit antibodies reacted with P. falciparum schizonts. Anti-PfATRP antibody exhibited peripheral staining patterns around the merozoites. Given the localization of PfATRP in merozoites, we tested for an egress phenotype during schizont arrest assays and demonstrated that native PfATRP is inaccessible on the surface of merozoites in intact schizonts. Dual immunofluorescence assays with markers for the inner membrane complex (IMC) and microtubules suggest partial colocalization in both asexual and sexual stage parasites. Using the soluble recombinant PfATRP in a screen of plasma samples revealed that malaria-infected children have naturally acquired PfATRP-specific antibodies.

PfCERLI1 is a conserved rhoptry associated protein essential for Plasmodium falciparum merozoite invasion of erythrocytes

The disease-causing blood-stage of the Plasmodium falciparum lifecycle begins with invasion of human erythrocytes by merozoites. Many vaccine candidates with key roles in binding to the erythrocyte surface and entry are secreted from the large bulb-like rhoptry organelles at the apical tip of the merozoite. Here we identify an essential role for the conserved protein P. falciparum Cytosolically Exposed Rhoptry Leaflet Interacting protein 1 (PfCERLI1) in rhoptry function. We show that PfCERLI1 localises to the cytosolic face of the rhoptry bulb membrane and knockdown of PfCERLI1 inhibits merozoite invasion. While schizogony and merozoite organelle biogenesis appear normal, biochemical techniques and semi-quantitative super-resolution microscopy show that PfCERLI1 knockdown prevents secretion of key rhoptry antigens that coordinate merozoite invasion. PfCERLI1 is a rhoptry associated protein identified to have a direct role in function of this essential merozoite invasion organelle, which has broader implications for understanding apicomplexan invasion biology.

Rhoptries are essential organelles for invasion of erythrocytes by Plasmodium. Here, the authors characterize the rhoptry-associated protein CERLI1 using quantitative super-resolution microscopy, showing that it is important for parasite invasion and secretion of rhoptry proteins including vaccine antigens.

Plasmodium yoelii Erythrocyte-Binding-like Protein Modulates Host Cell Membrane Structure, Immunity, and Disease Severity

Malaria is a deadly parasitic disease that continues to afflict hundreds of millions of people every year. Infections with malaria parasites can be asymptomatic, with mild symptoms, or fatal, depending on a delicate balance of host immune responses. Malaria parasites enter host red blood cells (RBCs) through interactions between parasite ligands and host receptors, such as erythrocyte-binding-like (EBL) proteins and host Duffy antigen receptor for chemokines (DARC). Plasmodium yoelii EBL (PyEBL) is known to play a role in parasite invasion of RBCs. Here, we show that PyEBL also affects disease severity through modulation of host immune responses, particularly type I interferon (IFN-I) signaling. This discovery assigns a new function to PyEBL and provides a mechanism for developing disease control strategies.

Erythrocyte-binding-like (EBL) proteins are known to play an important role in malaria parasite invasion of red blood cells (RBCs); however, any roles of EBL proteins in regulating host immune responses remain unknown. Here, we show that Plasmodium yoelii EBL (PyEBL) can shape disease severity by modulating the surface structure of infected RBCs (iRBCs) and host immune responses. We identified an amino acid substitution (a change of C to Y at position 741 [C741Y]) in the protein trafficking domain of PyEBL between isogenic P. yoelliinigeriensis strain N67 and N67C parasites that produce different disease phenotypes in C57BL/6 mice. Exchanges of the C741Y alleles altered parasite growth and host survival accordingly. The C741Y substitution also changed protein processing and trafficking in merozoites and in the cytoplasm of iRBCs, reduced PyEBL binding to band 3, increased phosphatidylserine (PS) surface exposure, and elevated the osmotic fragility of iRBCs, but it did not affect invasion of RBCs in vitro. The modified iRBC surface triggered PS-CD36-mediated phagocytosis of iRBCs, host type I interferon (IFN-I) signaling, and T cell differentiation, leading to improved host survival. This study reveals a previously unknown role of PyEBL in regulating host-pathogen interaction and innate immune responses, which may be explored for developing disease control strategies.

Dissecting the Gene Expression, Localization, Membrane Topology, and Function of the Plasmodium falciparum STEVOR Protein Family

During its intraerythrocytic development, the malaria parasite Plasmodium falciparum exposes variant surface antigens (VSAs) on infected erythrocytes to establish and maintain an infection. One family of small VSAs is the polymorphic STEVOR proteins, which are marked for export to the host cell surface through their PEXEL signal peptide. Interestingly, some STEVORs have also been reported to localize to the parasite plasma membrane and apical organelles, pointing toward a putative function in host cell egress or invasion. Using deep RNA sequencing analysis, we characterized P. falciparumstevor gene expression across the intraerythrocytic development cycle, including free merozoites, in detail and used the resulting stevor expression profiles for hierarchical clustering. We found that most stevor genes show biphasic expression oscillation, with maximum expression during trophozoite stages and a second peak in late schizonts. We selected four STEVOR variants, confirmed the expected export of these proteins to the host cell membrane, and tracked them to a secondary location, either to the parasite plasma membrane or the secretory organelles of merozoites in late schizont stages. We investigated the function of a particular STEVOR that showed rhoptry localization and demonstrated its role at the parasite-host interface during host cell invasion by specific antisera and targeted gene disruption. Experimentally determined membrane topology of this STEVOR revealed a single transmembrane domain exposing the semiconserved as well as variable protein regions to the cell surface.IMPORTANCE Malaria claims about half a million lives each year. Plasmodium falciparum, the causative agent of the most severe form of the disease, uses proteins that are translocated to the surface of infected erythrocytes for immune evasion. To circumvent the detection of these gene products by the immune system, the parasite evolved a complex strategy that includes gene duplications and elaborate sequence polymorphism. STEVORs are one family of these variant surface antigens and are encoded by about 40 genes. Using deep RNA sequencing of blood-stage parasites, including free merozoites, we first established stevor expression of the cultured isolate and compared it with published transcriptomes. We reveal a biphasic expression of most stevor genes and confirm this for individual STEVORs at the protein level. The membrane topology of a rhoptry-associated variant was experimentally elucidated and linked to host cell invasion, underlining the importance of this multifunctional protein family for parasite proliferation.

Blood-Stage Malaria Parasite Antigens: Structure, Function, and Vaccine Potential

Plasmodium parasites are the causative agent of malaria, a disease that kills approximately 450,000 individuals annually, with the majority of deaths occurring in children under the age of 5 years and the development of a malaria vaccine is a global health priority. Plasmodium parasites undergo a complex life cycle requiring numerous diverse protein families. The blood stage of parasite development results in the clinical manifestation of disease. A vaccine that disrupts the blood stage is highly desired and will aid in the control of malaria. The blood stage comprises multiple steps: invasion of, asexual growth within, and egress from red blood cells. This review focuses on blood-stage antigens with emphasis on antigen structure, antigen function, neutralizing antibodies, and vaccine potential.

Moderately Neutralizing Epitopes in Nonfunctional Regions Dominate the Antibody Response to Plasmodium falciparum EBA-140

Plasmodium falciparum erythrocyte-binding antigen 140 (EBA-140) plays a role in tight junction formation during parasite invasion of red blood cells and is a potential vaccine candidate for malaria. Individuals in areas where malaria is endemic possess EBA-140-specific antibodies, and individuals with high antibody titers to this protein have a lower rate of reinfection by parasites. The red blood cell binding segment of EBA-140 is comprised of two Duffy-binding-like domains, called F1 and F2, that together create region II. The sialic acid-binding pocket of F1 is essential for binding, whereas the sialic acid-binding pocket in F2 appears dispensable. Here, we show that immunization of mice with the complete region II results in poorly neutralizing antibodies. In contrast, immunization of mice with the functionally relevant F1 domain of region II results in antibodies that confer a 2-fold increase in parasite neutralization compared to that of the F2 domain. Epitope mapping of diverse F1 and F2 monoclonal antibodies revealed that the functionally relevant F1 sialic acid-binding pocket is a privileged site inaccessible to antibodies, that the F2 sialic acid-binding pocket contains a nonneutralizing epitope, and that two additional epitopes reside in F1 on the opposite face from the sialic acid-binding pocket. These studies indicate that focusing the immune response to the functionally important F1 sialic acid binding pocket improves the protective immune response of EBA-140. These results have implications for improving future vaccine designs and emphasize the importance of structural vaccinology for malaria.

PfCDPK1 mediated signaling in erythrocytic stages of Plasmodium falciparum

Calcium Dependent Protein Kinases are key effectors of calcium signaling in malaria parasite. PfCDPK1 is critical for asexual development of Plasmodium falciparum, but its precise function and substrates remain largely unknown. Using a conditional knockdown strategy, we here establish that this kinase is critical for the invasion of host erythrocytes. Furthermore, using a multidisciplinary approach involving comparative phosphoproteomics we gain insights into the underlying molecular mechanisms. We identify substrates of PfCDPK1, which includes proteins of Inner Membrane Complex and glideosome-actomyosin motor assembly. Interestingly, PfCDPK1 phosphorylates PfPKA regulatory subunit (PfPKA-R) and regulates PfPKA activity in the parasite, which may be relevant for the process of invasion. This study delineates the signaling network of PfCDPK1 and sheds light on mechanisms via which it regulates invasion.Calcium dependent protein kinase 1 (CDPK1) plays an important role in asexual development of Plasmodium falciparum. Using phosphoproteomics and conditional knockdown of CDPK1, the authors here identify CDPK1 substrates and a cross-talk between CDPK1 and PKA, and show the role of CDPK1 in parasite invasion.

Hierarchical phosphorylation of apical membrane antigen 1 is required for efficient red blood cell invasion by malaria parasites

Central to the pathogenesis of malaria is the proliferation of Plasmodium falciparum parasites within human erythrocytes. Parasites invade erythrocytes via a coordinated sequence of receptor-ligand interactions between the parasite and host cell. One key ligand, Apical Membrane Antigen 1 (AMA1), is a leading blood-stage vaccine and previous work indicates that phosphorylation of its cytoplasmic domain (CPD) is important to its function during invasion. Here we investigate the significance of each of the six available phospho-sites in the CPD. We confirm that the cyclic AMP/protein kinase A (PKA) signalling pathway elicits a phospho-priming step upon serine 610 (S₆₁₀), which enables subsequent phosphorylation in vitro of a conserved, downstream threonine residue (T₆₁₃) by glycogen synthase kinase 3 (GSK3). Both phosphorylation steps are required for AMA1 to function efficiently during invasion. This provides the first evidence that the functions of key invasion ligands of the malaria parasite are regulated by sequential phosphorylation steps.

Pellicle formation in the malaria parasite

The intraerythrocytic developmental cycle of Plasmodium falciparum is completed with the release of up to 32 invasive daughter cells, the merozoites, into the blood stream. Before release, the final step of merozoite development is the assembly of the cortical pellicle, a multi-layered membrane structure. This unique apicomplexan feature includes the inner membrane complex (IMC) and the parasite's plasma membrane. A dynamic ring structure, referred to as the basal complex, is part of the IMC and helps to divide organelles and abscises in the maturing daughter cells. Here, we analyze the dynamics of the basal complex of P. falciparum. We report on a novel transmembrane protein of the basal complex termed BTP1, which is specific to the genus Plasmodium. It colocalizes with the known basal complex marker protein MORN1 and shows distinct dynamics as well as localization when compared to other IMC proteins during schizogony. Using a parasite plasma membrane marker cell line, we correlate dynamics of the basal complex with the acquisition of the maternal membrane. We show that plasma membrane invagination and IMC propagation are interlinked during the final steps of cell division.

Profiling the humoral immune responses to Plasmodium vivax infection and identification of candidate immunogenic rhoptry-associated membrane antigen (RAMA)

Completion of sequencing of the Plasmodium vivax genome and transcriptome offers the chance to identify antigens among >5000 candidate proteins. To identify those P. vivax proteins that are immunogenic, a total of 152 candidate proteins (160 fragments) were expressed using a wheat germ cell-free system. The results of Western blot analysis showed that 92.5% (148/160) of the targets were expressed, and 96.6% (143/148) were in a soluble form with 67.7% of solubility rate. The proteins were screened by protein arrays with sera from 22 vivax malaria patients and 10 healthy individuals to confirm their immune profile, and 44 (27.5%, 44/160) highly reactive P. vivax antigens were identified. Overall, 5 candidates (rhoptry-associated membrane antigen [RAMA], Pv-fam-a and -b, EXP-1 and hypothetical protein PVX_084775) showed a positive reaction with >80% of patient sera, and 21 candidates with 50% to 80%. More than 23% of the highly immunoreactive proteins were hypothetical proteins, described for the first time in this study. One of the top immunogenic proteins, RAMA, was characterized and confirmed to be a serological marker of recent exposure to P. vivax infection. These novel immunoproteomes should greatly facilitate the identification of promising novel malaria antigens and may warrant further study.

BIOLOGICAL SIGNIFICANCE

The establishment of high-throughput cloning and expression systems has permitted the construction of protein arrays for proteome-wide study of Plasmodium vivax. In this study, high-throughput screening assays have been applied to investigate blood stage-specific immune proteomes from P. vivax. We identified 44 antigenic proteins from the 152 putative candidates, more than 23% of which were hypothetical proteins described for the first time in this study. In addition, PvRAMA was characterized further and confirmed to be a serological marker of exposure to infections. The expression of one-third of the selected antigenic genes were shifted between P. vivax and Plasmodium falciparum, suggesting that these genes may represent important factors associated with P. vivax selectivity for young erythrocytes and/or with immune evasion. These novel immune proteomes of the P. vivax blood stage provide a baseline for further prospective serological marker studies in malaria. These methods could be used to determine immunodominant candidate antigens from the P. vivax genome.

De novo assembly of a field isolate genome reveals novel Plasmodium vivax erythrocyte invasion genes

Recent sequencing of Plasmodium vivax field isolates and monkey-adapted strains enabled characterization of SNPs throughout the genome. These analyses relied on mapping short reads onto the P. vivax reference genome that was generated using DNA from the monkey-adapted strain Salvador I. Any genomic locus deleted in this strain would be lacking in the reference genome sequence and missed in previous analyses. Here, we report de novo assembly of a P. vivax field isolate genome. Out of 2,857 assembled contigs, we identify 362 contigs, each containing more than 5 kb of contiguous DNA sequences absent from the reference genome sequence. These novel P. vivax DNA sequences account for 3.8 million nucleotides and contain 792 predicted genes. Most of these contigs contain members of multigene families and likely originate from telomeric regions. Interestingly, we identify two contigs containing predicted protein coding genes similar to known Plasmodium red blood cell invasion proteins. One gene encodes the reticulocyte-binding protein gene orthologous to P. cynomolgi RBP2e and P. knowlesi NBPXb. The second gene harbors all the hallmarks of a Plasmodium erythrocyte-binding protein, including conserved Duffy-binding like and C-terminus cysteine-rich domains. Phylogenetic analysis shows that this novel gene clusters separately from all known Plasmodium Duffy-binding protein genes. Additional analyses showing that this gene is present in most P. vivax genomes and transcribed in blood-stage parasites suggest that P. vivax red blood cell invasion mechanisms may be more complex than currently understood. The strategy employed here complements previous genomic analyses and takes full advantage of next-generation sequencing data to provide a comprehensive characterization of genetic variations in this important malaria parasite. Further analyses of the novel protein coding genes discovered through de novo assembly have the potential to identify genes that influence key aspects of P. vivax biology, including alternative mechanisms of human erythrocyte invasion.

Plasmodium vivax is responsible for most malaria cases outside Africa, but is poorly understood, as the parasite is difficult to study in vitro. Genome sequencing studies offer a novel and exciting opportunity to better understand this parasite but, so far, have directly mapped reads onto the reference genome sequence generated from a single P. vivax strain. Here, we use sequence data generated from a field isolate to reconstruct long DNA sequences without relying on the reference genome. Our analyses reveal many P. vivax DNA sequences that are absent from the reference genome and contain 792 predicted genes. One of these novel genes encodes a predicted protein similar to known Plasmodium proteins involved in red blood cell invasion. This new gene is present in all P. vivax strains sequenced so far, except for the strain used to generate the reference genome, and is transcribed in blood-stage parasites. Overall, our analyses show that the catalogue of P. vivax genes was incomplete and that potentially important genes have been missed. We notably identified one putative invasion gene that seems functional and could dramatically change our understanding of the mechanisms determining red blood cell invasion by this important malaria parasite.

Molecular determinants for subcellular trafficking of the malarial sheddase PfSUB2

The malaria merozoite invades erythrocytes in the vertebrate host. Iterative rounds of asexual intraerythrocytic replication result in disease. Proteases play pivotal roles in erythrocyte invasion, but little is understood about their mode of action. The Plasmodium falciparum malaria merozoite surface sheddase, PfSUB2, is one such poorly characterized example. We have examined the molecular determinants that underlie the mechanisms by which PfSUB2 is trafficked initially to invasion-associated apical organelles (micronemes) and then across the surface of the free merozoite. We show that authentic promoter activity is important for correct localization of PfSUB2, likely requiring canonical features within the intergenic region 5' of the pfsub2 locus. We further demonstrate that trafficking of PfSUB2 beyond an early compartment in the secretory pathway requires autocatalytic protease activity. Finally, we show that the PfSUB2 transmembrane domain is required for microneme targeting, while the cytoplasmic domain is essential for surface translocation of the protease to the parasite posterior following discharge from micronemes. The interplay of pre- and post-translational regulatory elements that coordinate subcellular trafficking of PfSUB2 provides the parasite with exquisite control over enzyme-substrate interactions.

Vaccination with conserved regions of erythrocyte-binding antigens induces neutralizing antibodies against multiple strains of Plasmodium falciparum

Background

A highly effective vaccine against Plasmodium falciparum malaria should induce potent, strain transcending immunity that broadly protects against the diverse population of parasites circulating globally. We aimed to identify vaccine candidates that fulfill the criteria.

Methods

We have measured growth inhibitory activity of antibodies raised to a range of antigens to identify those that can efficiently block merozoite invasion for geographically diverse strains of P. falciparum.

Results

This has shown that the conserved Region III-V, of the P. falciparum erythrocyte-binding antigen (EBA)-175 was able to induce antibodies that potently inhibit merozoite invasion across diverse parasite strains, including those reliant on invasion pathways independent of EBA-175 function. Additionally, the conserved RIII-V domain of EBA-140 also induced antibodies with strong in vitro parasite growth inhibitory activity.

Conclusion

We identify an alternative, highly conserved region (RIV-V) of EBA-175, present in all EBA proteins, that is the target of potent, strain transcending neutralizing antibodies, that represents a strong candidate for development as a component in a malaria vaccine.

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.

Electron tomography of Plasmodium falciparum merozoites reveals core cellular events that underpin erythrocyte invasion

Erythrocyte invasion by merozoites forms of the malaria parasite is a key step in the establishment of human malaria disease. To date, efforts to understand cellular events underpinning entry have been limited to insights from non-human parasites, with no studies at sub-micrometer resolution undertaken using the most virulent human malaria parasite, Plasmodium falciparum. This leaves our understanding of the dynamics of merozoite sub-cellular compartments during infectionincomplete, in particular that of the secretory organelles. Using advances in P. falciparum merozoite isolation and new imaging techniques we present a three-dimensional study of invasion using electron microscopy, cryo-electron tomography and cryo-X-ray tomography. We describe the core architectural features of invasion and identify fusion between rhoptries at the commencement of invasion as a hitherto overlooked event that likely provides a critical step that initiates entry. Given the centrality of merozoite organelle proteins to vaccine development, these insights provide a mechanistic framework to understand therapeutic strategies targeted towards the cellular events of invasion.

The cellular and molecular basis for malaria parasite invasion of the human red blood cell

Malaria is a major disease of humans caused by protozoan parasites from the genus Plasmodium. It has a complex life cycle; however, asexual parasite infection within the blood stream is responsible for all disease pathology. This stage is initiated when merozoites, the free invasive blood-stage form, invade circulating erythrocytes. Although invasion is rapid, it is the only time of the life cycle when the parasite is directly exposed to the host immune system. Significant effort has, therefore, focused on identifying the proteins involved and understanding the underlying mechanisms behind merozoite invasion into the protected niche inside the human erythrocyte.

H2A.Z and H2B.Z double-variant nucleosomes define intergenic regions and dynamically occupy var gene promoters in the malaria parasite Plasmodium falciparum

Histone variants are important components of eukaryotic chromatin and can alter chromatin structure to confer specialized functions. H2B variant histones are rare in nature but have evolved independently in the phyla Apicomplexa and Trypanasomatida. Here, we investigate the apicomplexan-specific Plasmodium falciparum histone variant Pf H2B.Z and show that within nucleosomes Pf H2B.Z dimerizes with the H2A variant Pf H2A.Z and that Pf H2B.Z and Pf H2A.Z occupancy correlates in the subset of genes examined. These double-variant nucleosomes also carry common markers of euchromatin like H3K4me3 and histone acetylation. Pf H2B.Z levels are elevated in intergenic regions across the genome, except in the var multigene family, where Pf H2A.Z/Pf H2B.Z double-variant nucleosomes are only enriched in the promoter of the single active var copy and this enrichment is developmentally regulated. Importantly, this pattern seems to be specific for var genes and does not apply to other heterochromatic gene families involved in red blood cell invasion which are also subject to clonal expression. Thus, Pf H2A.Z/Pf H2B.Z double-variant nucleosomes appear to have a highly specific function in the regulation of P. falciparum virulence.

The upstream sequence segment of the C-terminal cysteine-rich domain is required for microneme trafficking of Plasmodium falciparum erythrocyte binding antigen 175

Erythrocyte invasion is a critical step for survival of Plasmodium parasites, the causative agents of malaria, in their host and recognition of the host cell receptors by Plasmodium erythrocyte-binding-like (EBL) proteins plays an important role. Although EBL subcellular localization was shown to be closely linked to parasite virulence in the rodent model of malaria, the trafficking of EBL to micronemes, the secretory organelle in the invasive parasite is not fully understood. In this study, we assessed the impact of the deletion and amino acid replacement of Plasmodium falciparum EBL (EBA-175) using transgenic P. falciparum lines expressing modified EBA-175. We found that, in addition to a signal peptide and a cysteine rich region (region 6) to the cytoplasmic tail, a previously unrecognized sequence segment in region 5 was required for correct microneme trafficking of EBA-175. Replacement of Arg or Phe residues in this segment altered microneme trafficking, suggesting that the sequence itself contained critical information. Based on these findings, we propose that the sequence segment in region 5 is also required for the recognition of EBA-175 by the trafficking machinery to direct this protein to the microneme. Our results provide key information to clarify an as yet unidentified EBA-175 trafficking mechanism.

Dissection of minimal sequence requirements for rhoptry membrane targeting in the malaria parasite

Rhoptries are specialized secretory organelles characteristic of single cell organisms belonging to the clade Apicomplexa. These organelles play a key role in the invasion process of host cells by accumulating and subsequently secreting an unknown number of proteins mediating host cell entry. Despite their essential role, little is known about their biogenesis, components and targeting determinants. Here, we report on a conserved apicomplexan protein termed Armadillo Repeats-Only (ARO) protein that we localized to the cytosolic face of Plasmodium falciparum and Toxoplasma gondii rhoptries. We show that the first 20 N-terminal amino acids are sufficient for rhoptry membrane targeting. This protein relies on both - myristoylation and palmitoylation motifs - for membrane attachment. Although these lipid modifications are essential, they are not sufficient to direct ARO to the rhoptry membranes. Mutational analysis revealed additional residues within the first 20 amino acids of ARO that play an important role for rhoptry membrane attachment: the positively charged residues R9 and K14. Interestingly, the exchange of R9 with a negative charge entirely abolishes membrane attachment, whereas the exchange of K14 (and to a lesser extent K16) alters only its membrane specificity. Additionally, 17 proteins predicted to be myristoylated and palmitoylated in the first 20 N-terminal amino acids were identified in the genome of the malaria parasite. While most of the corresponding GFP fusion proteins were trafficked to the parasite plasma membrane, two were sorted to the apical organelles. Interestingly, these proteins have a similar motif identified for ARO.

Evolution and architecture of the inner membrane complex in asexual and sexual stages of the malaria parasite

The inner membrane complex (IMC) is a unifying morphological feature of all alveolate organisms. It consists of flattened vesicles underlying the plasma membrane and is interconnected with the cytoskeleton. Depending on the ecological niche of the organisms, the function of the IMC ranges from a fundamental role as reinforcement system to more specialized roles in motility and cytokinesis. In this article, we present a comprehensive evolutionary analysis of IMC components, which exemplifies the adaptive nature of the IMCs' protein composition. Focusing on eight structurally distinct proteins in the most prominent "genus" of the Alveolata-the malaria parasite Plasmodium-we demonstrate that the level of conservation is reflected in phenotypic characteristics, accentuated in differential spatial-temporal patterns of these proteins in the motile stages of the parasite's life cycle. Colocalization studies with the centromere and the spindle apparatus reveal their discriminative biogenesis. We also reveal that the IMC is an essential structural compartment for the development of the sexual stages of Plasmodium, as it seems to drive the morphological changes of the parasite during the long and multistaged process of sexual differentiation. We further found a Plasmodium-specific IMC membrane matrix protein that highlights transversal structures in gametocytes, which could represent a genus-specific structural innovation required by Plasmodium. We conclude that the IMC has an additional role during sexual development supporting morphogenesis of the cell, which in addition to its functions in the asexual stages highlights the multifunctional nature of the IMC in the Plasmodium life cycle.

Delineation of stage specific expression of Plasmodium falciparum EBA-175 by biologically functional region II monoclonal antibodies

BACKGROUND

The malaria parasite Plasmodium falciparum EBA-175 binds its receptor sialic acids on glycophorin A when invading erythrocytes. The receptor-binding region (RII) contains two cysteine-rich domains with similar cysteine motifs (F1 and F2). Functional relationships between F1 and F2 domains and characterization of EBA-175 were studied using specific monoclonal antibodies (mAbs) against these domains.

METHODS AND FINDINGS

Five mAbs specific for F1 or F2 were generated. Three mAbs specific for F2 potently blocked binding of EBA-175 to erythrocytes, and merozoite invasion of erythrocytes (IC(50) 10 to 100 µg/ml IgG in growth inhibition assays). A mAb specific for F1 blocked EBA-175 binding and merozoite invasion less effectively. The difference observed between the IC(50) of F1 and F2 mAbs was not due to differing association and disassociation rates as determined by surface plasmon resonance. Four of the mAbs recognized conformation-dependent epitopes within F1 or F2. Used in combination, F1 and F2 mAbs blocked the binding of native EBA-175 to erythrocytes and inhibited parasite invasion synergistically in vitro. MAb R217, the most potent, did not recognize sporozoites, 3-day hepatocyte stage parasites, nor rings, trophozoites, gametocytes, retorts, ookinetes, and oocysts but recognized 6-day hepatocyte stage parasites, and schizonts. Even though efficient at blocking binding to erythrocytes and inhibiting invasion into erythrocytes, MAb R217 did not inhibit sporozoite invasion and development in hepatocytes in vitro.

CONCLUSIONS

The role of the F1 and F2 domains in erythrocyte invasion and binding was elucidated with mAbs. These mAbs interfere with native EBA-175 binding to erythrocyte in a synergistic fashion. The stage specific expression of EBA-175 showed that the primary focus of activity was the merozoite stage. A recombinant RII protein vaccine consisting of both F1 and F2 domains that could induce synergistic activity should be optimal for induction of antibody responses that interfere with merozoite invasion of erythrocytes.

Identification and characterization of the Plasmodium vivax thrombospondin-related apical merozoite protein

Background

Malaria caused by Plasmodium vivax is a major public health problem worldwide that affects 70-80 million people in the Middle East, Asia, Western Pacific, South America and the Caribbean. Despite its epidemiological importance, few antigens from this parasite species have been characterized to date compared to Plasmodium falciparum, due in part to the difficulties of maintaining an in vitro culture of P. vivax. This study describes the identification of the P. falciparum thrombospondin-related apical merozoite protein homologue in P. vivax (PvTRAMP) and examines its potential to be further evaluated as vaccine candidate.

Methods

The gene encoding PvTRAMP was identified through an extensive search of the databases hosting the genome sequence of P. vivax. Genes adjacent to pvtramp were identified in silico to determine the degree of similarity between the protein sequences encoded by equivalent chromosomic fragments in P. falciparum and Plasmodium knowlesi. The pvtramp gene was amplified from cDNA of P. vivax schizont stages, cloned and expressed in Escherichia coli. Anti-PvTRAMP antisera was obtained by inoculating rabbits with PvTRAMP B cell epitopes produced as synthetic peptides in order to assess its recognition in parasite lysates by Western blot and in intact parasites by indirect immunofluorescence. The recognition of recombinant PvTRAMP by sera from P. vivax-infected individuals living in endemic areas was also assessed by ELISA.

Results

The PfTRAMP homologue in P. vivax, here denoted as PvTRAMP, is a 340-amino-acid long antigen encoded by a single exon that could have a potential role in cytoadherence, as indicated by the presence of a thrombospondin structural homology repeat (TSR) domain. According to its transcription and expression profile, PvTRAMP is initially located at the parasite's apical end and later on the parasite surface. Recombinant PvTRAMP is recognized by sera from infected patients, therefore, indicating that it is targeted by the immune system during a natural infection with P. vivax.

Conclusions

The results of this work support conducting further studies with PvTRAMP to evaluate its immunogenicity and protection-inducing ability in the Aotus animal model.

Functional diversification between two related Plasmodium falciparum merozoite invasion ligands is determined by changes in the cytoplasmic domain

The pathogenesis of Plasmodium falciparum depends on efficient invasion into host erythrocytes. Parasite ligands encoded by multi-gene families interact with erythrocyte receptors. P. falciparum reticulocyte binding protein homologues (PfRhs) are expressed at the apical surface of invasive merozoites and have divergent ectodomains that are postulated to bind different erythrocyte receptors. Variant expression of these paralogues results in the use of alternative invasion pathways. Two PfRh proteins, PfRh2a and PfRh2b, are identical for 2700 N-terminal amino acids and differ only in a C-terminal 500 amino acid region, which includes a unique ectodomain, transmembrane domain and cytoplasmic domain. Despite their similarity, PfRh2b is required for a well-defined invasion pathway while PfRh2a is not required or sufficient for this pathway. Mapping the genomic region encoding these proteins revealed a recombinogenic locus with PfRh2a and PfRh2b in a head-to-head orientation. We have generated viable PfRh2a/2b chimeric parasites to identify the regions required for alternative invasion pathway utilization. We find that the differential ability to use these pathways is conferred by the cytoplasmic domains of PfRh2a and PfRh2b, not the ectodomain or transmembrane regions. Our results highlight the importance of the cytoplasmic domain for functional diversification of a major adhesive ligand family in malaria parasites.

A cyanobacterial serine protease of Plasmodium falciparum is targeted to the apicoplast and plays an important role in its growth and development

The prokaryotic ATP-dependent protease machineries such as ClpQY and ClpAP in the malaria parasite may represent potential drug targets. In the present study, we show that the orthologue of cyanobacterial ClpP protease in Plasmodium falciparum (PfClpP) is expressed in the asexual blood stages and possesses serine protease activity. The PfClpP was localized in the apicoplast using a GFP-targeting approach, immunoelectron microscopy and by immunofluorescence assays. A set of cell permeable β-lactones, which specifically bind with the active site of prokaryotic ClpP, were screened using an in vitro protease assay of PfClpP. A PfClpP-specific protease inhibitor was identified in the screen, labelled as U1-lactone. In vitro growth of the asexual stage parasites was significantly inhibited by U1-lactone treatment. The U1-treated parasites showed developmental arrest at the late-schizont stage. We further show that the U1-lactone treatment resulted in formation of abnormal apicoplasts which were not able to grow and segregate in the parasite progeny; these effects were also evident by blockage in the replication of the apicoplast genome. Overall, our data show that the PfClpP protease has confirmed localization in the apicoplast and it plays important role in development of functional apicoplasts.

Toxoplasma gondii transmembrane microneme proteins and their modular design

Host cell invasion by the Apicomplexa critically relies on regulated secretion of transmembrane micronemal proteins (TM-MICs). Toxoplasma gondii possesses functionally non-redundant MIC complexes that participate in gliding motility, host cell attachment, moving junction formation, rhoptry secretion and invasion. The TM-MICs are released onto the parasite's surface as complexes capable of interacting with host cell receptors. Additionally, TgMIC2 simultaneously connects to the actomyosin system via binding to aldolase. During invasion these adhesive complexes are shed from the surface notably via intramembrane cleavage of the TM-MICs by a rhomboid protease. Some TM-MICs act as escorters and assure trafficking of the complexes to the micronemes. We have investigated the properties of TgMIC6, TgMIC8, TgMIC8.2, TgAMA1 and the new micronemal protein TgMIC16 with respect to interaction with aldolase, susceptibility to rhomboid cleavage and presence of trafficking signals. We conclude that several TM-MICs lack targeting information within their C-terminal domains, indicating that trafficking depends on yet unidentified proteins interacting with their ectodomains. Most TM-MICs serve as substrates for a rhomboid protease and some of them are able to bind to aldolase. We also show that the residues responsible for binding to aldolase are essential for TgAMA1 but dispensable for TgMIC6 function during invasion.

Erythrocyte binding ligands in malaria parasites: intracellular trafficking and parasite virulence

The intracellular trafficking of an Erythrocyte Binding Like (EBL) ligand has recently been shown to dramatically affect the multiplication rate and virulence of the rodent malaria parasite Plasmodium yoelii yoelii. In this review, we describe the current understanding of the role of EBL and other erythrocyte binding ligands in erythrocyte invasion, and discuss the mechanisms by which they may control multiplication rates and virulence in malaria parasites.

Sialic acids: key determinants for invasion by the Apicomplexa

Sialic acids are ubiquitously found on the surface of all vertebrate cells at the extremities of glycan chains and widely exploited by viruses and bacteria to enter host cells. Carbohydrate-bearing receptors are equally important for host cell invasion by the obligate intracellular protozoan parasites of the phylum Apicomplexa. Host cell entry is an active process relying crucially on proteins that engage with receptors on the host cell surface and promote adhesion and internalisation. Assembly into complexes, proteolytic processing and oligomerization are important requirements for the functionality of these adhesins. The combination of adhesive proteins with varying stringency in specificity confers some flexibility to the parasite in face of receptor heterogeneity and immune pressure. Sialic acids are now recognised to critically contribute to selective host cell recognition by various species of the phylum.

A novel Plasmodium falciparum erythrocyte binding protein associated with the merozoite surface, PfDBLMSP

Proteins on the surface of the merozoite, the invasive form of the malaria parasite Plasmodium falciparum,and those secreted from its apical secretory organelles are promising vaccine candidates against blood stage malaria. In the present study, we have identified a novel parasite protein (PfDBLMSP; Gene IDPF10_0348), that harbors a predicted signal sequence, a central Duffy binding-like (DBL) domain and a secreted polymorphic antigen associated with merozoites (SPAM) domain in its C-terminal half. Transcription and translation of pfdblmsp is up-regulated specifically in schizont stage parasites, similar to other well-chararacterized merozoite proteins involved in invasion of red blood cells (RBCs). PfDBLMSPwas localized on the merozoite surface with a GFP targeting approach using schizont-stage specific expression systems, and by immunofluorescence assays of the endogenous protein. PfDBLMSP expressed on the surface of mammalian cells (COS-7) showed binding with human RBCs and this binding was sensitive to trypsin and neuraminidase treatments. The recombinant proteins corresponding to the DBL and SPAM domains showed reactivity with immune sera from individuals residing in P. falciparum endemic areas. Polymorphism in PfDBLMSP sequences from different P. falciparum strains and field isolates suggested that its DBL domain is under natural immune pressure. Our data on localization and functional assays suggest a possible role of PfDBLMSP in binding of merozoites with erythrocytes during invasion.

Exoerythrocytic Plasmodium parasites secrete a cysteine protease inhibitor involved in sporozoite invasion and capable of blocking cell death of host hepatocytes

Plasmodium parasites must control cysteine protease activity that is critical for hepatocyte invasion by sporozoites, liver stage development, host cell survival and merozoite liberation. Here we show that exoerythrocytic P. berghei parasites express a potent cysteine protease inhibitor (PbICP, P. berghei inhibitor of cysteine proteases). We provide evidence that it has an important function in sporozoite invasion and is capable of blocking hepatocyte cell death. Pre-incubation with specific anti-PbICP antiserum significantly decreased the ability of sporozoites to infect hepatocytes and expression of PbICP in mammalian cells protects them against peroxide- and camptothecin-induced cell death. PbICP is secreted by sporozoites prior to and after hepatocyte invasion, localizes to the parasitophorous vacuole as well as to the parasite cytoplasm in the schizont stage and is released into the host cell cytoplasm at the end of the liver stage. Like its homolog falstatin/PfICP in P. falciparum, PbICP consists of a classical N-terminal signal peptide, a long N-terminal extension region and a chagasin-like C-terminal domain. In exoerythrocytic parasites, PbICP is posttranslationally processed, leading to liberation of the C-terminal chagasin-like domain. Biochemical analysis has revealed that both full-length PbICP and the truncated C-terminal domain are very potent inhibitors of cathepsin L-like host and parasite cysteine proteases. The results presented in this study suggest that the inhibitor plays an important role in sporozoite invasion of host cells and in parasite survival during liver stage development by inhibiting host cell proteases involved in programmed cell death.

Transcriptional profiling of growth perturbations of the human malaria parasite Plasmodium falciparum

Functions have yet to be defined for the majority of genes of Plasmodium falciparum, the agent responsible for the most serious form of human malaria. Here we report changes in P. falciparum gene expression induced by 20 compounds that inhibit growth of the schizont stage of the intraerythrocytic development cycle. In contrast with previous studies, which reported only minimal changes in response to chemically induced perturbations of P. falciparum growth, we find that approximately 59% of its coding genes display over three-fold changes in expression in response to at least one of the chemicals we tested. We use this compendium for guilt-by-association prediction of protein function using an interaction network constructed from gene co-expression, sequence homology, domain-domain and yeast two-hybrid data. The subcellular localizations of 31 of 42 proteins linked with merozoite invasion is consistent with their role in this process, a key target for malaria control. Our network may facilitate identification of novel antimalarial drugs and vaccines.

A complex of three related membrane proteins is conserved on malarial merozoites

Invasion of human red blood cells by the malaria parasite Plasmodium falciparum is a coordinated, multi-step process. Here, we describe three novel integral membrane proteins that colocalize on the inner membrane complex immediately beneath the merozoite plasma membrane. Each has six predicted transmembrane domains and is conserved in diverse apicomplexan parasites. Immunoprecipitation studies using specific antibodies reveal that these proteins assemble into a heteromeric complex. Each protein was also expressed on insect cells using the baculovirus vector system with a truncated SUMO tag that facilitates maximal expression and protein purification while permitting cleavage with SUMO protease to release unmodified parasite protein. The expressed proteins were successfully reconstituted into artificial liposomes, but were not recognized by human immune sera. Because all three genes are highly conserved in apicomplexan parasites, the complex formed by their encoded proteins likely serves an essential role for invasive merozoites.

Single amino acid substitution in Plasmodium yoelii erythrocyte ligand determines its localization and controls parasite virulence

The major virulence determinant of the rodent malaria parasite, Plasmodium yoelii, has remained unresolved since the discovery of the lethal line in the 1970s. Because virulence in this parasite correlates with the ability to invade different types of erythrocytes, we evaluated the potential role of the parasite erythrocyte binding ligand, PyEBL. We found 1 amino acid substitution in a domain responsible for intracellular trafficking between the lethal and nonlethal parasite lines and, furthermore, that the intracellular localization of PyEBL was distinct between these lines. Genetic modification showed that this substitution was responsible not only for PyEBL localization but also the erythrocyte-type invasion preference of the parasite and subsequently its virulence in mice. This previously unrecognized mechanism for altering an invasion phenotype indicates that subtle alterations of a malaria parasite ligand can dramatically affect host-pathogen interactions and malaria virulence.

Identification of rhoptry trafficking determinants and evidence for a novel sorting mechanism in the malaria parasite Plasmodium falciparum

The rhoptry of the malaria parasite Plasmodium falciparum is an unusual secretory organelle that is thought to be related to secretory lysosomes in higher eukaryotes. Rhoptries contain an extensive collection of proteins that participate in host cell invasion and in the formation of the parasitophorous vacuole, but little is known about sorting signals required for rhoptry protein targeting. Using green fluorescent protein chimeras and in vitro pull-down assays, we performed an analysis of the signals required for trafficking of the rhoptry protein RAP1. We provide evidence that RAP1 is escorted to the rhoptry via an interaction with the glycosylphosphatidyl inositol-anchored rhoptry protein RAMA. Once within the rhoptry, RAP1 contains distinct signals for localisation within a sub-compartment of the organelle and subsequent transfer to the parasitophorous vacuole after invasion. This is the first detailed description of rhoptry trafficking signals in Plasmodium.

Functional analysis of the leading malaria vaccine candidate AMA-1 reveals an essential role for the cytoplasmic domain in the invasion process

A key process in the lifecycle of the malaria parasite Plasmodium falciparum is the fast invasion of human erythrocytes. Entry into the host cell requires the apical membrane antigen 1 (AMA-1), a type I transmembrane protein located in the micronemes of the merozoite. Although AMA-1 is evolving into the leading blood-stage malaria vaccine candidate, its precise role in invasion is still unclear. We investigate AMA-1 function using live video microscopy in the absence and presence of an AMA-1 inhibitory peptide. This data reveals a crucial function of AMA-1 during the primary contact period upstream of the entry process at around the time of moving junction formation. We generate a Plasmodium falciparum cell line that expresses a functional GFP-tagged AMA-1. This allows the visualization of the dynamics of AMA-1 in live parasites. We functionally validate the ectopically expressed AMA-1 by establishing a complementation assay based on strain-specific inhibition. This method provides the basis for the functional analysis of essential genes that are refractory to any genetic manipulation. Using the complementation assay, we show that the cytoplasmic domain of AMA-1 is not required for correct trafficking and surface translocation but is essential for AMA-1 function. Although this function can be mimicked by the highly conserved cytoplasmic domains of P. vivax and P. berghei, the exchange with the heterologous domain of the microneme protein EBA-175 or the rhoptry protein Rh2b leads to a loss of function. We identify several residues in the cytoplasmic tail that are essential for AMA-1 function. We validate this data using additional transgenic parasite lines expressing AMA-1 mutants with TY1 epitopes. We show that the cytoplasmic domain of AMA-1 is phosphorylated. Mutational analysis suggests an important role for the phosphorylation in the invasion process, which might translate into novel therapeutic strategies.

Malaria is one of the most lethal parasitic diseases worldwide, causing more than 1 million fatalities per annum. Drug resistance is widespread, and a vaccine is not available. One of the leading blood stage vaccine candidates is the apical membrane antigen 1 (AMA-1), which is well-conserved among apicomplexan parasites. Although this protein plays an essential role in the invasion of human red blood cells, little is known about the molecular mechanisms underlying its function. In this study we use live video microscopy in the presence and absence of an invasion-inhibiting molecule to investigate the function of AMA-1 in real time. We establish a complementation assay based on strain-specific AMA-1 inhibition that allows functional characterization of this protein on a molecular level. Using this approach, we provide evidence that the intracellular cytoplasmic domain of AMA-1 is directly involved in the invasion process and that this domain is phosphorylated. We conclude that the phosphorylation of AMA-1 might represent an attractive target for novel therapeutic drug strategies independent of the polymorphic extracellular domain.

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.

Receptor-based identification of an inhibitory peptide against blood stage malaria

Plasmodium falciparum uses multiple host receptors to attach and invade human erythrocytes. Glycophorins have been implicated as receptors for parasite invasion in human erythrocytes. Here, we screened a phage display cDNA library of P. falciparum (FCR3, a sialic acid-dependent strain) using purified glycophorins and erythrocytes as bait. Several phage clones were identified that bound to immobilized glycophorins and contained the same 74 bp insert encoding the 7-amino acids sequence ETTLKSF. A similar screen using intact human erythrocytes in solution identified additional phage clones containing the same 7-amino acids sequence. Using ELISA and immunofluorescence, direct binding of ETTLKSF peptide to glycophorins and erythrocytes was confirmed. Pull-down and protease treatment assays suggest that ETTLKSF peptide specifically interacts with glycophorin C. The synthetic ETTLKSF peptide partially blocks merozoite invasion in human erythrocytes. Further characterization of ETTLKSF peptide could lead to the development of a novel class of inhibitors against the blood stage malaria.

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

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

Characterization of a conserved rhoptry-associated leucine zipper-like protein in the malaria parasite Plasmodium falciparum

One of the key processes in the pathobiology of the malaria parasite is the invasion and subsequent modification of the human erythrocyte. In this complex process, an unknown number of parasite proteins are involved, some of which are leading vaccine candidates. The majority of the proteins that play pivotal roles in invasion are either stored in the apical secretory organelles or located on the surface of the merozoite, the invasive stage of the parasite. Using transcriptional and structural features of these known proteins, we performed a genomewide search that identified 49 hypothetical proteins with a high probability of being located on the surface of the merozoite or in the secretory organelles. Of these candidates, we characterized a novel leucine zipper-like protein in Plasmodium falciparum that is conserved in Plasmodium spp. This protein is expressed in late blood stages and localizes to the rhoptries of the parasite. We demonstrate that this Plasmodium sp.-specific protein has a high degree of conservation within field isolates and that it is refractory to gene knockout attempts and thus might play an important role in invasion.

Plasmodium in the postgenomic era: new insights into the molecular cell biology of malaria parasites

In this review, we bring together some of the approaches toward understanding the cellular and molecular biology of Plasmodium species and their interaction with their host red blood cells. Considerable impetus has come from the development of new methods of molecular genetics and bioinformatics, and it is important to evaluate the wealth of these novel data in the context of basic cell biology. We describe how these approaches are gaining valuable insights into the parasite-host cell interaction, including (1) the multistep process of red blood cell invasion by the merozoite; (2) the mechanisms by which the intracellular parasite feeds on the red blood cell and exports parasite proteins to modify its cytoadherent properties; (3) the modulation of the cell cycle by sensing the environmental tryptophan-related molecules; (4) the mechanism used to survive in a low Ca(2+) concentration inside red blood cells; (5) the activation of signal transduction machinery and the regulation of intracellular calcium; (6) transfection technology; and (7) transcriptional regulation and genome-wide mRNA studies in Plasmodium falciparum.

Mononeme: a new secretory organelle in Plasmodium falciparum merozoites identified by localization of rhomboid-1 protease

Compartmentalization of proteins into subcellular organelles in eukaryotic cells is a fundamental mechanism of regulating complex cellular functions. Many proteins of Plasmodium falciparum merozoites involved in invasion are compartmentalized into apical organelles. We have identified a new merozoite organelle that contains P. falciparum rhomboid-1 (PfROM1), a protease that cleaves the transmembrane regions of proteins involved in invasion. By immunoconfocal microscopy, PfROM1 was localized to a single, thread-like structure on one side of the merozoites that appears to be in close proximity to the subpellicular microtubules. PfROM1 was not found associated with micronemes, rhoptries, or dense granules, the three identified secretory organelles of invasion. Release of merozoites from schizonts resulted in the movement of PfROM1 from the lateral asymmetric localization to the merozoite apical pole and the posterior pole. We have named this single thread-like organelle in merozoites, the mononeme.

Malarial EBA-175 region VI crystallographic structure reveals a KIX-like binding interface

The malaria parasite proliferates in the bloodstream of its vertebrate host by invading and replicating within erythrocytes. To achieve successful invasion, a number of discrete and essential events need to take place at the parasite-host cell interface. Erythrocyte-binding antigen 175 (EBA-175) is a member of a family of Plasmodium falciparum erythrocyte-binding proteins involved in the formation of a tight junction, a necessary step in invasion. Here we present the crystal structure of EBA-175 region VI (rVI), a cysteine-rich domain that is highly conserved within the protein family and is essential for EBA-175 trafficking. The structure was solved by selenomethionine single-wavelength anomalous dispersion at 1.8 A resolution. It reveals a homodimer, containing in each subunit a compact five-alpha-helix core that is stabilized by four conserved disulfide bridges. rVI adopts a novel fold that is likely conserved across the protein family, indicating a conserved function. It shows no similarity to the Duffy-binding-like domains of EBA-175 involved in erythrocyte binding, indicating a distinct role. Remarkably, rVI possesses structural features related to the KIX-binding domain of the coactivator CREB-binding protein, supporting the binding and trafficking roles that have been ascribed to it and providing a rational basis for further experimental investigation of its function.

Illuminating Plasmodium falciparum-infected red blood cells

The malaria parasite undergoes a remarkable series of morphological transformations, which underpin its life in both human and mosquito hosts. The advent of molecular transfection technology coupled with the ability to introduce fluorescent reporter proteins that faithfully track and expose the activities of parasite proteins has revolutionized our view of parasite cell biology. The greatest insights have been realized in the erythrocyte stages of Plasmodium falciparum. P. falciparum invades and remodels the human erythrocyte: it feeds on haemoglobin, grows and divides, and subverts the physiology of its hapless host. Fluorescent proteins have been employed to track and dissect each of these processes and have revealed details and exposed new paradigms.

The Plasmodium falciparum RhopH2 promoter and first 24 amino acids are sufficient to target proteins to the rhoptries

The rhoptry secretory organelles of the malaria parasite, Plasmodium falciparum, contain a RhopH complex, which is composed of the proteins RhopH1, RhopH2, and RhopH3. RhopH1 is encoded by the rhoph1/clag multi-gene family, whereas RhopH2 and RhopH3 are encoded by single-copy genes. The precise function of the RhopH complex has not been identified, but it has been shown that the component proteins are involved in erythrocyte binding and perhaps participate in the formation of the parasitophorous vacuolar membrane. In this study, we have isolated pfrhoph2 promoter plus the signal peptide encoding sequence and generated transgene expression constructs to evaluate a trafficking and the RhopH complex formation in transgenic P. falciparum parasite lines. Interestingly, we found that the N-terminal 24 amino acids of RhopH2, including signal peptide sequence, were sufficient to target GFP to the rhoptries under the rhoph2 promoter. Because it was previously shown that the timing of the expression alone could not target proteins to the apical organelles, this targeting is likely mediated via a unique mechanism that is dependent on N-terminal 24 amino acids of RhopH2 early in the secretory pathway. The N-terminal one third of Clag3.1, which contains a distinct conserved domain with Toxoplasma gondii RON2, can not associate the RhopH complex as a GFP chimera, but a c-Myc-Clag3.1 chimera lacking the C-terminus successfully associates the RhopH complex indicating that cooperation of middle region is likely required but the C-terminus is not necessary.