Trafficking of STEVOR to the Maurer's clefts in Plasmodium falciparum-infected erythrocytes

PEXEL is a proteolytic maturation site for both exported and non-exported Plasmodium proteins

Obligate intracellular malaria parasites dramatically remodel their erythrocyte host through effector protein export to create a niche for survival. Most exported proteins contain a pentameric Plasmodium export element (PEXEL)/host-targeting motif that is cleaved in the parasite ER by the aspartic protease Plasmepsin V (PMV). This processing event exposes a mature N terminus required for translocation into the host cell and is not known to occur in non-exported proteins. Here, we report that the non-exported parasitophorous vacuole protein UIS2 contains a bona fide PEXEL motif that is processed in the P. falciparum blood stage. While the N termini of exported proteins containing the PEXEL and immediately downstream ~10 residues are sufficient to mediate translocation into the RBC, the equivalent UIS2 N terminus does not promote the export of a reporter. Curiously, the UIS2 PEXEL contains an unusual aspartic acid at the fourth position, which constitutes the extreme N-terminal residue following PEXEL cleavage (P1', RIL↓DE). Using a series of chimeric reporter fusions, we show that Asp at P1' is permissive for PMV processing but abrogates export. Moreover, mutation of this single UIS2 residue to alanine enables export, reinforcing that the mature N terminus mediates export, not PEXEL processing per se. Prompted by this observation, we further show that PEXEL sequences in the N termini of other non-exported rhoptry proteins are also processed, suggesting that PMV may be a more general secretory maturase than previously appreciated, similar to orthologs in related apicomplexans. Our findings provide new insight into the unique N-terminal constraints that mark proteins for export.IMPORTANCEHost erythrocyte remodeling by malaria parasite-exported effector proteins is critical to parasite survival and disease pathogenesis. In the deadliest malaria parasite Plasmodium falciparum, most exported proteins undergo proteolytic maturation via recognition of the pentameric Plasmodium export element (PEXEL)/host-targeting motif by the aspartic protease Plasmepsin V, which exposes a mature N terminus that is conducive for export into the erythrocyte host cell. While PEXEL processing is considered a unique mark of exported proteins, we demonstrate that PEXEL motifs are present and processed in non-exported proteins. Importantly, we show that specific residues at the variable fourth position of the PEXEL motif inhibit export despite being permissive for processing, reinforcing that features of the mature N terminus, and not PEXEL cleavage, identify cargo for export. This opens the door to further inquiry into the nature and evolution of the PEXEL motif.

Sequence elements within the PEXEL motif and its downstream region modulate PTEX-dependent protein export in Plasmodium falciparum

The parasite Plasmodium falciparum causes the most severe form of malaria and to invade and replicate in red blood cells (RBCs), it exports hundreds of proteins across the encasing parasitophorous vacuole membrane (PVM) into this host cell. The exported proteins help modify the RBC to support rapid parasite growth and avoidance of the human immune system. Most exported proteins possess a conserved Plasmodium export element (PEXEL) motif with the consensus RxLxE/D/Q amino acid sequence, which acts as a proteolytic cleavage recognition site within the parasite's endoplasmic reticulum (ER). Cleavage occurs after the P1 L residue and is thought to help release the protein from the ER so it can be putatively escorted by the HSP101 chaperone to the parasitophorous vacuole space surrounding the intraerythrocytic parasite. HSP101 and its cargo are then thought to assemble with the rest of a Plasmodium translocon for exported proteins (PTEX) complex, that then recognises the xE/D/Q capped N-terminus of the exported protein and translocates it across the vacuole membrane into the RBC compartment. Here, we present evidence that supports a dual role for the PEXEL's conserved P2 ' position E/Q/D residue, first, for plasmepsin V cleavage in the ER, and second, for efficient PTEX mediated export across the PVM into the RBC. We also present evidence that the downstream 'spacer' region separating the PEXEL motif from the folded functional region of the exported protein controls cargo interaction with PTEX as well. The spacer must be of a sufficient length and permissive amino acid composition to engage the HSP101 unfoldase component of PTEX to be efficiently translocated into the RBC compartment.

Novel secretory organelles of parasite origin - at the center of host-parasite interaction

Reorganization of cell organelle-deprived host red blood cells by the apicomplexan malaria parasite Plasmodium falciparum enables their cytoadherence to endothelial cells that line the microvasculature. This increases the time red blood cells infected with mature developmental stages remain within selected organs such as the brain to avoid the spleen passage, which can lead to severe complications and cumulate in patient death. The Maurer's clefts are a novel secretory organelle of parasite origin established by the parasite in the cytoplasm of the host red blood cell in order to facilitate the establishment of cytoadherence by conducting the trafficking of immunovariant adhesins to the host cell surface. Another important function of the organelle is the sorting of other proteins the parasite traffics into its host cell. Although the organelle is of high importance for the pathology of malaria, additional putative functions, structure, and genesis remain shrouded in mystery more than a century after its discovery. In this review, we highlight our current knowledge about the Maurer's clefts and other novel secretory organelles established within the host cell cytoplasm by human-pathogenic malaria parasites and other parasites that reside within human red blood cells.

Knock-sideways by inducible ER retrieval enables a unique approach for studying Plasmodium-secreted proteins

Malaria parasites uniquely depend on protein secretion for their obligate intracellular lifestyle but approaches for dissecting Plasmodium-secreted protein functions are limited. We report knockER, a unique DiCre-mediated knock-sideways approach to sequester secreted proteins in the ER by inducible fusion with a KDEL ER-retrieval sequence. We show conditional ER sequestration of diverse proteins is not generally toxic, enabling loss-of-function studies. We employed knockER in multiple Plasmodium species to interrogate the trafficking, topology, and function of an assortment of proteins that traverse the secretory pathway to diverse compartments including the apicoplast (ClpB1), rhoptries (RON6), dense granules, and parasitophorous vacuole (EXP2, PTEX150, HSP101). Taking advantage of the unique ability to redistribute secreted proteins from their terminal destination to the ER, we reveal that vacuolar levels of the PTEX translocon component HSP101 but not PTEX150 are maintained in excess of what is required to sustain effector protein export into the erythrocyte. Intriguingly, vacuole depletion of HSP101 hypersensitized parasites to a destabilization tag that inhibits HSP101-PTEX complex formation but not to translational knockdown of the entire HSP101 pool, illustrating how redistribution of a target protein by knockER can be used to query function in a compartment-specific manner. Collectively, our results establish knockER as a unique tool for dissecting secreted protein function with subcompartmental resolution that should be widely amenable to genetically tractable eukaryotes.

PEXEL is a proteolytic maturation site for both exported and non-exported Plasmodium proteins

Obligate intracellular malaria parasites dramatically remodel their erythrocyte host through effector protein export to create a niche for survival. Most exported proteins contain a pentameric P lasmodium ex port el ement (PEXEL)/Host Targeting Motif that is cleaved in the parasite ER by the aspartic protease Plasmepsin V (PMV). This processing event exposes a mature N-terminus required for translocation into the host cell and is not known to occur in non-exported proteins. Here we report that the non-exported parasitophorous vacuole protein UIS2 contains a bona fide PEXEL motif that is processed in the P. falciparum blood-stage. While the N-termini of exported proteins containing the PEXEL and immediately downstream ∼10 residues is sufficient to mediate translocation into the RBC, the equivalent UIS2 N-terminus does not promote export of a reporter. Curiously, the UIS2 PEXEL contains an unusual aspartic acid at the fourth position which constitutes the extreme N-terminal residue following PEXEL cleavage (P1', RILτDE). Using a series of chimeric reporter fusions, we show that Asp at P1' is permissive for PMV processing but abrogates export. Moreover, mutation of this single UIS2 residue to alanine enables export, reinforcing that the mature N-terminus mediates export, not PEXEL processing per se . Prompted by this observation, we further show that PEXEL sequences in the N-termini of other non-exported rhoptry proteins are also processed, suggesting that PMV may be a more general secretory maturase than previously appreciated, similar to orthologs in related apicomplexans. Our findings provide new insight into the unique N-terminal constraints that mark proteins for export.

Importance

Host erythrocyte remodeling by malaria parasite exported effector proteins is critical to parasite survival and disease pathogenesis. In the deadliest malaria parasite Plasmodium falciparum , most exported proteins undergo proteolytic maturation via recognition of the pentameric P lasmodium ex port el ement (PEXEL)/Host Targeting motif by the aspartic protease Plasmepsin V (PMV) which exposes a mature N-terminus that is conducive for export into the erythrocyte host cell. While PEXEL processing is considered a unique mark of exported proteins, we demonstrate PEXEL motifs are present and processed in non-exported proteins. Importantly, we show that specific residues at the variable fourth position of the PEXEL motif inhibit export despite being permissive for processing by PMV, reinforcing that features of the mature N-terminus, and not PEXEL cleavage, identify cargo for export cargo. This opens the door to further inquiry into the nature and evolution of the PEXEL motif.

Formation of ER-lumenal intermediates during export of Plasmodium proteins containing transmembrane-like hydrophobic sequences

During the blood stage of a malaria infection, malaria parasites export both soluble and membrane proteins into the erythrocytes in which they reside. Exported proteins are trafficked via the parasite endoplasmic reticulum and secretory pathway, before being exported across the parasitophorous vacuole membrane into the erythrocyte. Transport across the parasitophorous vacuole membrane requires protein unfolding, and in the case of membrane proteins, extraction from the parasite plasma membrane. We show that trafficking of the exported Plasmodium protein, Pf332, differs from that of canonical eukaryotic soluble-secreted and transmembrane proteins. Pf332 is initially ER-targeted by an internal hydrophobic sequence that unlike a signal peptide, is not proteolytically removed, and unlike a transmembrane segment, does not span the ER membrane. Rather, both termini of the hydrophobic sequence enter the ER-lumen and the ER-lumenal species is a productive intermediate for protein export. Furthermore, we show in intact cells, that two other exported membrane proteins, SBP1 and MAHRP2, assume a lumenal topology within the parasite secretory pathway. Although the addition of a C-terminal ER-retention sequence, recognised by the lumenal domain of the KDEL receptor, does not completely block export of SBP1 and MAHRP2, it does enhance their retention in the parasite ER. This indicates that a sub-population of each protein adopts an ER-lumenal state that is an intermediate in the export process. Overall, this suggests that although many exported proteins traverse the parasite secretory pathway as typical soluble or membrane proteins, some exported proteins that are ER-targeted by a transmembrane segment-like, internal, non-cleaved hydrophobic segment, do not integrate into the ER membrane, and form an ER-lumenal species that is a productive export intermediate. This represents a novel means, not seen in typical membrane proteins found in model systems, by which exported transmembrane-like proteins can be targeted and trafficked within the lumen of the secretory pathway.

Essential role of a Plasmodium berghei heat shock protein (PBANKA_0938300) in gametocyte development

The continued existence of Plasmodium parasites in physiologically distinct environments during their transmission in mosquitoes and vertebrate hosts requires effector proteins encoded by parasite genes to provide adaptability. Parasites utilize their robust stress response system involving heat shock proteins for their survival. Molecular chaperones are involved in maintaining protein homeostasis within a cell during stress, protein biogenesis and the formation of protein complexes. Due to their critical role in parasite virulence, they are considered targets for therapeutic interventions. Our results identified a putative P. berghei heat shock protein (HSP) belonging to the HSP40 family (HspJ62), which is abundantly induced upon heat stress and expressed during all parasite stages. To determine the role HspJ62, a gene-disrupted P. berghei transgenic line was developed (ΔHspJ62), which resulted in disruption of gametocyte formation. Such parasites were unable to form subsequent sexual stages because of disrupted gametogenesis, indicating the essential role of HspJ62 in gametocyte formation. Transcriptomic analysis of the transgenic line showed downregulation of a number of genes, most of which were specific to male or female gametocytes. The transcription factor ApiAP2 was also downregulated in ΔHspJ62 parasites. Our findings suggest that the downregulation of ApiAP2 likely disrupts the transcriptional regulation of sexual stage genes, leading to impaired gametogenesis. This finding also highlights the critical role that HspJ62 indirectly plays in the development of P. berghei sexual stages and in facilitating the conversion from the asexual blood stage to the sexual stage. This study characterizes the HspJ62 protein as a fertility factor because parasites lacking it are unable to transmit to mosquitoes. This study adds an important contribution to ongoing research aimed at understanding gametocyte differentiation and formation in parasites. The molecule adds to the list of potential drug targets that can be targeted to inhibit parasite sexual development and consequently parasite transmission.

Trafficking and Association of Plasmodium falciparum MC-2TM with the Maurer's Clefts

In this study, we investigated stage specific expression, trafficking, solubility and topology of endogenous PfMC-2TM in P. falciparum (3D7) infected erythrocytes. Following Brefeldin A (BFA) treatment of parasites, PfMC-2TM traffic was evaluated using immunofluorescence with antibodies reactive with PfMC-2TM. PfMC-2TM is sensitive to BFA treatment and permeabilization of infected erythrocytes with streptolysin O (SLO) and saponin, showed that the N and C-termini of PfMC-2TM are exposed to the erythrocyte cytoplasm with the central portion of the protein protected in the MC membranes. PfMC-2TM was expressed as early as 4 h post invasion (hpi), was tightly colocalized with REX-1 and trafficked to the erythrocyte membrane without a change in solubility. PfMC-2TM associated with the MC and infected erythrocyte membrane and was resistant to extraction with alkaline sodium carbonate, suggestive of protein-lipid interactions with membranes of the MC and erythrocyte. PfMC-2TM is an additional marker of the nascent MCs.

Co-option of Plasmodium falciparum PP1 for egress from host erythrocytes

Asexual proliferation of the Plasmodium parasites that cause malaria follows a developmental program that alternates non-canonical intraerythrocytic replication with dissemination to new host cells. We carried out a functional analysis of the Plasmodium falciparum homolog of Protein Phosphatase 1 (PfPP1), a universally conserved cell cycle factor in eukaryotes, to investigate regulation of parasite proliferation. PfPP1 is indeed required for efficient replication, but is absolutely essential for egress of parasites from host red blood cells. By phosphoproteomic and chemical-genetic analysis, we isolate two functional targets of PfPP1 for egress: a HECT E3 protein-ubiquitin ligase; and GCα, a fusion protein composed of a guanylyl cyclase and a phospholipid transporter domain. We hypothesize that PfPP1 regulates lipid sensing by GCα and find that phosphatidylcholine stimulates PfPP1-dependent egress. PfPP1 acts as a key regulator that integrates multiple cell-intrinsic pathways with external signals to direct parasite egress from host cells.

Rapid activation of distinct members of multigene families in Plasmodium spp

The genomes of Plasmodium spp. encode a number of different multigene families that are thought to play a critical role for survival. However, with the exception of the P. falciparum var genes, very little is known about the biological roles of any of the other multigene families. Using the recently developed Selection Linked Integration method, we have been able to activate the expression of a single member of a multigene family of our choice in Plasmodium spp. from its endogenous promoter. We demonstrate the usefulness of this approach by activating the expression of a unique var, rifin and stevor in P. falciparum as well as yir in P. yoelii. Characterization of the selected parasites reveals differences between the different families in terms of mutual exclusive control, co-regulation, and host adaptation. Our results further support the application of the approach for the study of multigene families in Plasmodium and other organisms.

Omelianczyk, Loh et al. activate the expression of a single member of a multigene family in Plasmodium spp. from its endogenous promoter, identifying differences between the different families. This study supports the application of the Selection Linked Integration method for studying multigene families in Plasmodium.

A patatin-like phospholipase functions during gametocyte induction in the malaria parasite Plasmodium falciparum

Patatin-like phospholipases (PNPLAs) are highly conserved enzymes of prokaryotic and eukaryotic organisms with major roles in lipid homeostasis. The genome of the malaria parasite Plasmodium falciparum encodes four putative PNPLAs with predicted functions during phospholipid degradation. We here investigated the role of one of the plasmodial PNPLAs, a putative PLA₂ termed PNPLA1, during blood stage replication and gametocyte development. PNPLA1 is present in the asexual and sexual blood stages and here localizes to the cytoplasm. PNPLA1-deficiency due to gene disruption or conditional gene-knockdown had no effect on intraerythrocytic growth, gametocyte development and gametogenesis. However, parasites lacking PNPLA1 were impaired in gametocyte induction, while PNPLA1 overexpression promotes gametocyte formation. The loss of PNPLA1 further leads to transcriptional down-regulation of genes related to gametocytogenesis, including the gene encoding the sexual commitment regulator AP2-G. Additionally, lipidomics of PNPLA1-deficient asexual blood stage parasites revealed overall increased levels of major phospholipids, including phosphatidylcholine (PC), which is a substrate of PLA₂ . PC synthesis is known to be pivotal for erythrocytic replication, while the reduced availability of PC precursors drives the parasite into gametocytogenesis; we thus hypothesize that the higher PC levels due to PNPLA1-deficiency prevent the blood stage parasites from entering the sexual pathway.

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.

A High-Resolution Map of SBP1 Interactomes in Plasmodium falciparum-infected Erythrocytes

The pathogenesis of malaria parasites depends on host erythrocyte modifications that are facilitated by parasite proteins exported to the host cytoplasm. These exported proteins form a trafficking complex in the host cytoplasm that transports virulence determinants to the erythrocyte surface; this complex is thus essential for malaria virulence. Here, we report a comprehensive interaction network map of this complex. We developed authentic, unbiased, highly sensitive proteomic approaches to determine the proteins that interact with a core component of the complex, SBP1 (skeleton-binding protein 1). SBP1 interactomes revealed numerous exported proteins and potential interactors associated with SBP1 intracellular trafficking. We identified several host-parasite protein interactions and linked the exported protein MAL8P1.4 to Plasmodium falciparum virulence in infected erythrocytes. Our study highlights the complicated interplay between parasite and host proteins in the host cytoplasm and provides an interaction dataset connecting dozens of exported proteins required for P. falciparum virulence.

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We used shotgun proteomics to identify SBP1-interacting factors

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System validation showed complex interplay between parasite and host proteins

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Our system can be used to explore protozoan parasite virulence in erythrocytes

Clipped histone H3 is integrated into nucleosomes of DNA replication genes in the human malaria parasite Plasmodium falciparum

Post-translational modifications of histone H3 N-terminal tails are key epigenetic regulators of virulence gene expression and sexual commitment in the human malaria parasite Plasmodium falciparum Here, we identify proteolytic clipping of the N-terminal tail of nucleosome-associated histone H3 at amino acid position 21 as a new chromatin modification. A cathepsin C-like proteolytic clipping activity is observed in nuclear parasite extracts. Notably, an ectopically expressed version of clipped histone H3, PfH3p-HA, is targeted to the nucleus and integrates into mononucleosomes. Furthermore, chromatin immunoprecipitation and next-generation sequencing analysis identified PfH3p-HA as being highly enriched in the upstream region of six genes that play a key role in DNA replication and repair: In these genes, PfH3p-HA demarcates a specific 1.5 kb chromatin island adjacent to the open reading frame. Our results indicate that, in P. falciparum, the process of histone clipping may precede chromatin integration hinting at preferential targeting of pre-assembled PfH3p-containing nucleosomes to specific genomic regions. The discovery of a protease-directed mode of chromatin organization in P. falciparum opens up new avenues to develop new anti-malarials.

Host immune evasion strategies of malaria blood stage parasite

Host immune evasion is a key strategy for the continual survival of many microbial pathogens including Apicomplexan protozoan: Plasmodium spp., the causative agent of Malaria. The malaria parasite has evolved a variety of mechanisms to evade the host immune responses within its two hosts: the female Anopheles mosquito vector and vertebrate host. In this review, we will focus on the molecular mechanisms of the immune evasion strategies used by the Plasmodium parasite at the blood stage which is responsible for the clinical manifestations of human malaria. We also aim to provide some insights on the potential targets for malaria interventions through the recent advancement in understanding the molecular biology of the parasite.

'2TM proteins': an antigenically diverse superfamily with variable functions and export pathways

Malaria is a disease that affects millions of people annually. An intracellular habitat and lack of protein synthesizing machinery in erythrocytes pose numerous difficulties for survival of the human pathogen Plasmodium falciparum. The parasite refurbishes the infected red blood cell (iRBC) by synthesis and export of several proteins in an attempt to suffice its metabolic needs and evade the host immune response. Immune evasion is largely mediated by surface display of highly polymorphic protein families known as variable surface antigens. These include the two trans-membrane (2TM) superfamily constituted by multicopy repetitive interspersed family (RIFINs), subtelomeric variable open reading frame (STEVORs) and Plasmodium falciparum Maurer's cleft two trans-membrane proteins present only in P. falciparum and some simian infecting Plasmodium species. Their hypervariable region flanked by 2TM domains exposed on the iRBC surface is believed to generate antigenic diversity. Though historically named "2TM superfamily," several A-type RIFINs and some STEVORs assume one trans-membrane topology. RIFINs and STEVORs share varied functions in different parasite life cycle stages like rosetting, alteration of iRBC rigidity and immune evasion. Additionally, a member of the STEVOR family has been implicated in merozoite invasion. Differential expression of these families in laboratory strains and clinical isolates propose them to be important for host cell survival and defense. The role of RIFINs in modulation of host immune response and presence of protective antibodies against these surface exposed molecules in patient sera highlights them as attractive targets of antimalarial therapies and vaccines. 2TM proteins are Plasmodium export elements positive, and several of these are exported to the infected erythrocyte surface after exiting through the classical secretory pathway within parasites. Cleaved and modified proteins are trafficked after packaging in vesicles to reach Maurer's clefts, while information regarding delivery to the iRBC surface is sparse. Expression and export timing of the RIFIN and Plasmodium falciparum erythrocyte membrane protein1 families correspond to each other. Here, we have compiled and comprehended detailed information regarding orthologues, domain architecture, surface topology, functions and trafficking of members of the "2TM superfamily." Considering the large repertoire of proteins included in the 2TM superfamily and recent advances defining their function in malaria biology, a surge in research carried out on this important protein superfamily is likely.

Host cell remodeling by pathogens: the exomembrane system in Plasmodium-infected erythrocytes

Malaria is caused by infection of erythrocytes by parasites of the genus Plasmodium. To survive inside erythrocytes, these parasites induce sweeping changes within the host cell, one of the most dramatic of which is the formation of multiple membranous compartments, collectively referred to as the exomembrane system. As an uninfected mammalian erythrocyte is devoid of internal membranes, the parasite must be the force and the source behind the formation of these compartments. Even though the first evidence of the presence these of internal compartments was obtained over a century ago, their functions remain mostly unclear, and in some cases completely unknown, and the mechanisms underlying their formation are still mysterious. In this review, we provide an overview of the different parts of the exomembrane system, describing the parasitophorous vacuole, the tubovesicular network, Maurer's clefts, the caveola-vesicle complex, J dots and other mobile compartments, and the small vesicles that have been observed in Plasmodium-infected cells. Finally, we combine the data into a simplified view of the exomembrane system and its relation to the alterations of the host erythrocyte.

Plasmodium parasites remodel the host erythrocyte in various ways, including the formation of several membranous compartments, together referred to as the exomembrane system, within the erythrocyte cytosol that together are key to the sweeping changes in the host cell.

Shelph2, a bacterial-like phosphatase of the malaria parasite Plasmodium falciparum, is dispensable during asexual blood stage

During the erythrocytic cycle of the malaria parasite Plasmodium falciparum, egress and invasion are essential steps finely controlled by reversible phosphorylation. In contrast to the growing number of kinases identified as key regulators, phosphatases have been poorly studied, and calcineurin is the only one identified so far to play a role in invasion. PfShelph2, a bacterial-like phosphatase, is a promising candidate to participate in the invasion process, as it was reported to be expressed late during the asexual blood stage and to reside within an apical compartment, yet distinct from rhoptry bulb, micronemes, or dense granules. It was also proposed to play a role in the control of the red blood cell membrane deformability at the end of the invasion process. However, genetic studies are still lacking to support this hypothesis. Here, we take advantage of the CRISPR-Cas9 technology to tag shelph2 genomic locus while retaining its endogenous regulatory regions. This new strain allows us to follow the endogenous PfShelph2 protein expression and location during asexual blood stages. We show that PfShelph2 apical location is also distinct from the rhoptry neck or exonemes. We further demonstrate PfShelph2 dispensability during the asexual blood stage by generating PfShelph2-KO parasites using CRISPR-Cas9 machinery. Analyses of the mutant during the course of the erythrocytic development indicate that there are no detectable phenotypic consequences of Pfshelph2 genomic deletion. As this lack of phenotype might be due to functional redundancy, we also explore the likelihood of PfShelph1 (PfShelph2 paralog) being a compensatory phosphatase. We conclude that despite its cyclic expression profile, PfShelph2 is a dispensable phosphatase during the Plasmodium falciparum asexual blood stage, whose function is unlikely substituted by PfShelph1.

The N-terminal extension of the P. falciparum GBP130 signal peptide is irrelevant for signal sequence function

The malaria parasite P. falciparum exports a large number of proteins to its host cell, the mature human erythrocyte. Although the function of the majority of these proteins is not well understood, many exported proteins appear to play a role in modification of the erythrocyte following invasion. Protein export to the erythrocyte is a secretory process that begins with entry to the endoplasmic reticulum. For most exported proteins, this step is mediated by hydrophobic signal peptides found towards the N-terminal end of proteins. The signal peptides present on P. falciparum exported proteins often differ in length from those found in other systems, and generally contain a highly extended N-terminal region. Here we have investigated the function of these extended N-terminal regions, using the exported parasite protein GBP130 as a model. Surprisingly, several deletions of the extended N-terminal regions of the GBP130 signal peptide have no effect on the ability of the signal peptide to direct a fluorescent reporter to the secretory pathway. Addition of the same N-terminal extension to a canonical signal peptide does not affect transport of either soluble or membrane proteins to their correct respective subcellular localisations. Finally, we show that extended signal peptides are able to complement canonical signal peptides in driving protein traffic to the apicoplast of the parasite, and are also functional in a mammalian cell system. Our study is the first detailed analysis of an extended P. falciparum signal peptide and suggests that N-terminal extensions of exported Plasmodium falciparum proteins are not required for entry to the secretory system, and are likely to be involved in other, so far unknown, processes.

Proteomic analysis of exported chaperone/co-chaperone complexes of P. falciparum reveals an array of complex protein-protein interactions

Malaria parasites modify their human host cell, the mature erythrocyte. This modification is mediated by a large number of parasite proteins that are exported to the host cell, and is also the underlying cause for the pathology caused by malaria infection. Amongst these proteins are many Hsp40 co-chaperones, and a single Hsp70. These proteins have been implicated in several processes in the host cell, including a potential role in protein transport, however the further molecular players in this process remain obscure. To address this, we have utilized chemical cross-linking followed by mass spectrometry and immunoblotting to isolate and characterize proteins complexes containing an exported Hsp40 (PFE55), and the only known exported Hsp70 (PfHsp70x). Our data reveal that both of these proteins are contained in high molecular weight protein complexes. These complexes are found both in the infected erythrocyte, and within the parasite-derived compartment referred to as the parasitophorous vacuole. Surprisingly, our data also reveal an association of PfHsp70x with components of PTEX, a putative protein translocon within the membrane of the parasitophorous vacuole. Our results suggest that the P. falciparum- infected human erythrocyte contains numerous high molecular weight protein complexes, which may potentially be involved in host cell modification.

Trafficking of the exported P. falciparum chaperone PfHsp70x

Plasmodium falciparum extensively modifies its chosen host cell, the mature human erythrocyte. This remodelling is carried out by parasite-encoded proteins that are exported into the host cell. To gain access to the human red blood cell, these proteins must cross the parasitophorous vacuole, a membrane bound compartment surrounding the parasite that is generated during the invasion process. Many exported proteins carry a so-called PEXEL/HT signal that directs their transport. We recently reported the unexpected finding of a species-restricted parasite-encoded Hsp70, termed PfHsp70x, which is exported into the host erythrocyte cytosol. PfHsp70x lacks a classical PEXEL/HT motif, and its transport appears to be mediated by a 7 amino acid motif directly following the hydrophobic N-terminal secretory signal. In this report, we analyse this short targeting sequence in detail. Surprisingly, both a reversed and scrambled version of the motif retained the capacity to confer protein export. Site directed mutagenesis of glutamate residues within this region leads to a block of protein trafficking within the lumen of the PV. In contrast to PEXEL-containing proteins, the targeting signal is not cleaved, but appears to be acetylated. Furthermore we show that, like other exported proteins, trafficking of PfHsp70x requires the vacuolar translocon, PTEX.

Exported Epoxide Hydrolases Modulate Erythrocyte Vasoactive Lipids during Plasmodium falciparum Infection

Erythrocytes are reservoirs of important epoxide-containing lipid signaling molecules, including epoxyeicosatrienoic acids (EETs). EETs function as vasodilators and anti-inflammatory modulators in the bloodstream. Bioactive EETs are hydrolyzed to less active diols (dihydroxyeicosatrienoic acids) by epoxide hydrolases (EHs). The malaria parasite Plasmodium falciparum infects host red blood cells (RBCs) and exports hundreds of proteins into the RBC compartment. In this study, we show that two parasite epoxide hydrolases, P. falciparum epoxide hydrolases 1 (PfEH1) and 2 (PfEH2), both with noncanonical serine nucleophiles, are exported to the periphery of infected RBCs. PfEH1 and PfEH2 were successfully expressed in Escherichia coli, and they hydrolyzed physiologically relevant erythrocyte EETs. Mutations in active site residues of PfEH1 ablated the ability of the enzyme to hydrolyze an epoxide substrate. Overexpression of PfEH1 or PfEH2 in parasite-infected RBCs resulted in a significant alteration in the epoxide fatty acids stored in RBC phospholipids. We hypothesize that the parasite disruption of epoxide-containing signaling lipids leads to perturbed vascular function, creating favorable conditions for binding and sequestration of infected RBCs to the microvascular endothelium.

The malaria parasite exports hundreds of proteins into the erythrocyte compartment. However, for most of these proteins, their physiological function is unknown. In this study, we investigate two “hypothetical” proteins of the α/β-hydrolase fold family that share sequence similarity with epoxide hydrolases (EHs)—enzymes that destroy bioactive epoxides. Altering EH expression in parasite-infected erythrocytes resulted in a significant change in the epoxide fatty acids stored in the host cell. We propose that these EH enzymes may help the parasite to manipulate host blood vessel opening and inflame the vessel walls as they pass through the circulation system. Understanding how the malaria parasite interacts with its host RBCs will aid in our ability to combat this deadly disease.

Ticket to ride: export of proteins to the Plasmodium falciparum-infected erythrocyte

The malaria parasite Plasmodium falciparum exports numerous proteins to its chosen host cell, the mature human erythrocyte. Many of these proteins are important for parasite survival. To reach the host cell, parasites must cross multiple membrane barriers and then furthermore be targeted to their correct sub-cellular localisation. This novel transport pathway has received much research attention in the past decades, especially as many of the mechanisms are expected to be parasite-specific and thus potential targets for drug development. In this article we summarize some of the most recent advances in this field, and highlight areas in which further research is needed.

J-dot targeting of an exported HSP40 in Plasmodium falciparum-infected erythrocytes

Plasmodium falciparum exports a large number of proteins to its host cell, the mature human erythrocyte, where they are involved in host cell modification. Amongst the proteins trafficked to the host cell, many are heat shock protein (HSP)40 homologues. We previously demonstrated that at least two exported PfHSP40s (referred to as PFE55 and PFA660) localise to mobile structures in the P. falciparum-infected erythrocyte (Kulzer et al., 2010), termed J-dots. The complete molecular content of these structures has not yet been completely resolved, however it is known that they also contain an exported HSP70, PfHSP70x, and are potentially involved in transport of the major cytoadherance ligand, PfEMP1, through the host cell. To understand more about the nature of the association of exported HSP40s with J-dots, here we have studied the signal requirements for recruitment of the proteins to these structures. By expressing various exported GFP chimeras, we can demonstrate that the predicted substrate binding domain is necessary and sufficient for J-dot targeting. This targeting only occurs in human erythrocytes infected with P. falciparum, as it is not conserved when expressing a P. falciparum HSP40 in Plasmodium berghei-infected murine red blood cells, suggesting that J-dots are P. falciparum-specific. This data reveals a new mechanism for targeting of exported proteins to intracellular structures in the P. falciparum-infected erythrocyte.

Export of malaria proteins requires co-translational processing of the PEXEL motif independent of phosphatidylinositol-3-phosphate binding

Plasmodium falciparum exports proteins into erythrocytes using the Plasmodium export element (PEXEL) motif, which is cleaved in the endoplasmic reticulum (ER) by plasmepsin V (PMV). A recent study reported that phosphatidylinositol-3-phosphate (PI(3)P) concentrated in the ER binds to PEXEL motifs and is required for export independent of PMV, and that PEXEL motifs are functionally interchangeable with RxLR motifs of oomycete effectors. Here we show that the PEXEL does not bind PI(3)P, and that this lipid is not concentrated in the ER. We find that RxLR motifs cannot mediate export in P. falciparum. Parasites expressing a mutated version of KAHRP, with the PEXEL motif repositioned near the signal sequence, prevented PMV cleavage. This mutant possessed the putative PI(3)P-binding residues but is not exported. Reinstatement of PEXEL to its original location restores processing by PMV and export. These results challenge the PI(3)P hypothesis and provide evidence that PEXEL position is conserved for co-translational processing and export.

Malaria Parasite Proteins and Their Role in Alteration of the Structure and Function of Red Blood Cells

Malaria, caused by Plasmodium spp., continues to be a major threat to human health and a significant cause of socioeconomic hardship in many countries. Almost half of the world's population live in malaria-endemic regions and many of them suffer one or more, often life-threatening episodes of malaria every year, the symptoms of which are attributable to replication of the parasite within red blood cells (RBCs). In the case of Plasmodium falciparum, the species responsible for most malaria-related deaths, parasite replication within RBCs is accompanied by striking alterations to the morphological, biochemical and biophysical properties of the host cell that are essential for the parasites' survival. To achieve this, the parasite establishes a unique and extensive protein export network in the infected RBC, dedicating at least 6% of its genome to the process. Understanding the full gamut of proteins involved in this process and the mechanisms by which P. falciparum alters the structure and function of RBCs is important both for a more complete understanding of the pathogenesis of malaria and for development of new therapeutic strategies to prevent or treat this devastating disease. This review focuses on what is currently known about exported parasite proteins, their interactions with the RBC and their likely pathophysiological consequences.

Fractionation of Plasmodium-infected human red blood cells to study protein trafficking

Subcellular fractionation is a valuable tool to follow protein traffic between cellular compartments. Here we detail a procedure for fractionating erythrocytes infected with the human malaria parasite P. falciparum using the bacterial pore-forming protein Streptolysin O (SLO). Additionally we describe an experimental protocol to determine protein topology by carrying out a protease protection assay on SLO-lysed infected erythrocytes.

STEVOR is a Plasmodium falciparum erythrocyte binding protein that mediates merozoite invasion and rosetting

Variant surface antigens play an important role in Plasmodium falciparum malaria pathogenesis and in immune evasion by the parasite. Although most work to date has focused on P. falciparum Erythrocyte Membrane Protein 1 (PfEMP1), two other multigene families encoding STEVOR and RIFIN are expressed in invasive merozoites and on the infected erythrocyte surface. However, their role during parasite infection remains to be clarified. Here we report that STEVOR functions as an erythrocyte-binding protein that recognizes Glycophorin C (GPC) on the red blood cell (RBC) surface and that its binding correlates with the level of GPC on the RBC surface. STEVOR expression on the RBC leads to PfEMP1-independent binding of infected RBCs to uninfected RBCs (rosette formation), while antibodies targeting STEVOR in the merozoite can effectively inhibit invasion. Our results suggest a PfEMP1-independent role for STEVOR in enabling infected erythrocytes at the schizont stage to form rosettes and in promoting merozoite invasion.

A comparative study of the localization and membrane topology of members of the RIFIN, STEVOR and PfMC-2TM protein families in Plasmodium falciparum-infected erythrocytes

Background

Variant surface antigens (VSA) exposed on the membrane of Plasmodium falciparum infected erythrocytes mediate immune evasion and are important pathogenicity factors in malaria disease. In addition to the well-studied PfEMP1, the small VSA families RIFIN, STEVOR and PfMC-2TM are assumed to play a role in this process.

Methods

This study presents a detailed comparative characterization of the localization, membrane topology and extraction profile across the life cycle of various members of these protein families employing confocal microscopy, immunoelectron microscopy and immunoblots.

Results

The presented data reveal a clear association of variants of the RIFIN, STEVOR and PfMC-2TM proteins with the host cell membrane and topological studies indicate that the semi-conserved N-terminal region of RIFINs and some STEVOR proteins is exposed at the erythrocyte surface. At the Maurer’s clefts, the semi-conserved N-terminal region as well as the variable stretch of RIFINs appears to point to the lumen away from the erythrocyte cytoplasm. These results challenge the previously proposed two transmembrane topology model for the RIFIN and STEVOR protein families and suggest that only one hydrophobic region spans the membrane. In contrast, PfMC-2TM proteins indeed seem to be anchored by two hydrophobic stretches in the host cell membrane exposing just a few, variable amino acids at the surface of the host cell.

Conclusion

Together, the host cell surface exposure and topology of RIFIN and STEVOR proteins suggests members of these protein families may indeed be involved in immune evasion of the infected erythrocyte, whereas members of the PfMC-2TM family seem to bear different functions in parasite biology.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0784-2) contains supplementary material, which is available to authorized users.

Shared elements of host-targeting pathways among apicomplexan parasites of differing lifestyles

Apicomplexans are a diverse group of obligate parasites occupying different intracellular niches that require modification to meet the needs of the parasite. To efficiently manipulate their environment, apicomplexans translocate numerous parasite proteins into the host cell. Whereas some parasites remain contained within a parasitophorous vacuole membrane (PVM) throughout their developmental cycle, others do not, a difference that affects the machinery needed for protein export. A signal-mediated pathway for protein export into the host cell has been characterized in Plasmodium parasites, which maintain the PVM. Here, we functionally demonstrate an analogous host-targeting pathway involving organellar staging prior to secretion in the related bovine parasite, Babesia bovis, a parasite that destroys the PVM shortly after invasion. Taking into account recent identification of a similar signal-mediated pathway in the coccidian parasite Toxoplasma gondii, we suggest a model in which this conserved pathway has evolved in multiple steps from signal-mediated trafficking to specific secretory organelles for controlled secretion to a complex protein translocation process across the PVM.

Alternative Protein Secretion in the Malaria Parasite Plasmodium falciparum

Plasmodium falciparum invades human red blood cells, residing in a parasitophorous vacuole (PV), with a parasitophorous vacuole membrane (PVM) separating the PV from the host cell cytoplasm. Here we have investigated the role of N-myristoylation and two other N-terminal motifs, a cysteine potential S-palmitoylation site and a stretch of basic residues, as the driving force for protein targeting to the parasite plasma membrane (PPM) and subsequent translocation across this membrane. Plasmodium falciparum adenylate kinase 2 (Pf AK2) contains these three motifs, and was previously proposed to be targeted beyond the parasite to the PVM, despite the absence of a signal peptide for entry into the classical secretory pathway. Biochemical and microscopy analyses of PfAK2 variants tagged with green fluorescent protein (GFP) showed that these three motifs are involved in targeting the protein to the PPM and translocation across the PPM to the PV. It was shown that the N-terminal 37 amino acids of PfAK2 alone are sufficient to target and translocate GFP across the PPM. As a control we examined the N-myristoylated P. falciparum ADP-ribosylation factor 1 (PfARF1). PfARF1 was found to co-localise with a Golgi marker. To determine whether or not the putative palmitoylation and the cluster of lysine residues from the N-terminus of PfAK2 would modulate the subcellular localization of PfARF1, a chimeric fusion protein containing the N-terminus of PfARF1 and the two additional PfAK2 motifs was analysed. This chimeric protein was targeted to the PPM, but not translocated across the membrane into the PV, indicating that other features of the N-terminus of PfAK2 also play a role in the secretion process.

Experimental determination of the membrane topology of the Plasmodium protease Plasmepsin V

The malaria parasite exports hundreds of proteins into its host cell. The majority of exported proteins contain a Host-Targeting motif (also known as a Plasmodium export element) that directs them for export. Prior to export, the Host-Targeting motif is cleaved by the endoplasmic reticulum-resident protease Plasmepsin V and the newly generated N-terminus is N-α-acetylated by an unidentified enzyme. The cleaved, N-α-acetylated protein is trafficked to the parasitophorous vacuole, where it is translocated across the vacuole membrane. It is clear that cleavage and N-α-acetylation of the Host-Targeting motif occur at the endoplasmic reticulum, and it has been proposed that Host-Targeting motif cleavage and N-α-acetylation occur either on the luminal or cytosolic side of the endoplasmic reticulum membrane. Here, we use self-associating ‘split’ fragments of GFP to determine the topology of Plasmepsin V in the endoplasmic reticulum membrane; we show that the catalytic protease domain of Plasmepsin V faces the endoplasmic reticulum lumen. These data support a model in which the Host-Targeting motif is cleaved and N-α-acetylated in the endoplasmic reticulum lumen. Furthermore, these findings suggest that cytosolic N-α-acetyltransferases are unlikely to be candidates for the N-α-acetyltransferase of Host-Targeting motif-containing exported proteins.

PFB0595w is a Plasmodium falciparum J protein that co-localizes with PfHsp70-1 and can stimulate its in vitro ATP hydrolysis activity

Heat shock proteins, many of which function as molecular chaperones, play important roles in the lifecycle and pathogenesis of the malaria parasite, Plasmodium falciparum. The P. falciparum heat shock protein 70 (PfHsp70) family of chaperones is potentially regulated by a large complement of J proteins that localize to various intracellular compartments including the infected erythrocyte cytosol. While PfHsp70-1 has been shown to be an abundant cytosolic chaperone, its regulation by J proteins is poorly understood. In this study, we characterized the J protein PFB0595w, a homologue of the well-studied yeast cytosolic J protein, Sis1. PFB0595w, similarly to PfHsp70-1, was localized to the parasite cytosol and its expression was upregulated by heat shock. Additionally, recombinant PFB0595w was shown to be dimeric and to stimulate the in vitro ATPase activity of PfHsp70-1. Overall, the expression, localization and biochemical data for PFB0595w suggest that it may function as a cochaperone of PfHsp70-1, and advances current knowledge on the chaperone machinery of the parasite.

A conserved domain targets exported PHISTb family proteins to the periphery of Plasmodium infected erythrocytes

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Multiple P. falciparum PHISTb proteins localise to the erythrocyte periphery.

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Solubility profiling indicates that these proteins associate with the red cell cytoskeleton.

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The PRESAN domain and a preceding N-terminal sequence is a novel targeting domain.

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A protein targeted to the red cell periphery is essential for parasite survival.

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P. knowlesi and P. vivax homologous domains also confer similar localisation.

During blood-stage infection, malaria parasites export numerous proteins to the host erythrocyte. The Poly-Helical Interspersed Sub-Telomeric (PHIST) proteins are an exported family that share a common ‘PRESAN’ domain, and include numerous members in Plasmodium falciparum, Plasmodium vivax and Plasmodium knowlesi. In P. falciparum, PHIST proteins have been implicated in protein trafficking and intercellular communication. A number of PHIST proteins are essential for parasite survival. Here, we identify nine members of the PHISTb sub-class of PHIST proteins, including one protein known to be essential for parasite survival, that localise to the erythrocyte periphery. These proteins have solubility characteristics consistent with their association with the erythrocyte cytoskeleton. Together, an extended PRESAN domain, comprising the PRESAN domain and preceding sequence, form a novel targeting-domain that is sufficient to localise a protein to the erythrocyte periphery. We validate the role of this domain in RESA, thus identifying a cytoskeleton-binding domain in RESA that functions independently of its known spectrin-binding domain. Our data suggest that some PHISTb proteins may act as cross-linkers of the erythrocyte cytoskeleton. We also show for the first time that peripherally-localised PHISTb proteins are encoded in genomes of P. knowlesi and vivax indicating a conserved role for the extended PRESAN domain of these proteins in targeting to the erythrocyte periphery.

Role of calcium signaling in the transcriptional regulation of the apicoplast genome of Plasmodium falciparum

Calcium is a universal second messenger that plays an important role in regulatory processes in eukaryotic cells. To understand calcium-dependent signaling in malaria parasites, we analyzed transcriptional responses of Plasmodium falciparum to two calcium ionophores (A23187 and ionomycin) that cause redistribution of intracellular calcium within the cytoplasm. While ionomycin induced a specific transcriptional response defined by up- or downregulation of a narrow set of genes, A23187 caused a developmental arrest in the schizont stage. In addition, we observed a dramatic decrease of mRNA levels of the transcripts encoded by the apicoplast genome during the exposure of P. falciparum to both calcium ionophores. Neither of the ionophores caused any disruptions to the DNA replication or the overall apicoplast morphology. This suggests that the mRNA downregulation reflects direct inhibition of the apicoplast gene transcription. Next, we identify a nuclear encoded protein with a calcium binding domain (EF-hand) that is localized to the apicoplast. Overexpression of this protein (termed PfACBP1) in P. falciparum cells mediates an increased resistance to the ionophores which suggests its role in calcium-dependent signaling within the apicoplast. Our data indicate that the P. falciparum apicoplast requires calcium-dependent signaling that involves a novel protein PfACBP1.

Surface antigens of Plasmodium falciparum-infected erythrocytes as immune targets and malaria vaccine candidates

Understanding the targets and mechanisms of human immunity to malaria caused by Plasmodium falciparum is crucial for advancing effective vaccines and developing tools for measuring immunity and exposure in populations. Acquired immunity to malaria predominantly targets the blood stage of infection when merozoites of Plasmodium spp. infect erythrocytes and replicate within them. During the intra-erythrocytic development of P. falciparum, numerous parasite-derived antigens are expressed on the surface of infected erythrocytes (IEs). These antigens enable P. falciparum-IEs to adhere in the vasculature and accumulate in multiple organs, which is a key process in the pathogenesis of disease. IE surface antigens, often referred to as variant surface antigens, are important targets of acquired protective immunity and include PfEMP1, RIFIN, STEVOR and SURFIN. These antigens are highly polymorphic and encoded by multigene families, which generate substantial antigenic diversity to mediate immune evasion. The most important immune target appears to be PfEMP1, which is a major ligand for vascular adhesion and sequestration of IEs. Studies are beginning to identify specific variants of PfEMP1 linked to disease pathogenesis that may be suitable for vaccine development, but overcoming antigenic diversity in PfEMP1 remains a major challenge. Much less is known about other surface antigens, or antigens on the surface of gametocyte-IEs, the effector mechanisms that mediate immunity, and how immunity is acquired and maintained over time; these are important topics for future research.

Maurer's clefts, the enigma of Plasmodium falciparum

Plasmodium falciparum, the causative agent of malaria, completely remodels the infected human erythrocyte to acquire nutrients and to evade the immune system. For this process, the parasite exports more than 10% of all its proteins into the host cell cytosol, including the major virulence factor PfEMP1 (P. falciparum erythrocyte surface protein 1). This unusual protein trafficking system involves long-known parasite-derived membranous structures in the host cell cytosol, called Maurer's clefts. However, the genesis, role, and function of Maurer's clefts remain elusive. Similarly unclear is how proteins are sorted and how they are transported to and from these structures. Recent years have seen a large increase of knowledge but, as yet, no functional model has been established. In this perspective we review the most important findings and conclude with potential possibilities to shed light into the enigma of Maurer's clefts. Understanding the mechanism and function of these structures, as well as their involvement in protein export in P. falciparum, might lead to innovative control strategies and might give us a handle with which to help to eliminate this deadly parasite.

The multifunctional autophagy pathway in the human malaria parasite, Plasmodium falciparum

Autophagy is a catabolic pathway typically induced by nutrient starvation to recycle amino acids, but can also function in removing damaged organelles. In addition, this pathway plays a key role in eukaryotic development. To date, not much is known about the role of autophagy in apicomplexan parasites and more specifically in the human malaria parasite Plasmodium falciparum. Comparative genomic analysis has uncovered some, but not all, orthologs of autophagy-related (ATG) genes in the malaria parasite genome. Here, using a genome-wide in silico analysis, we confirmed that ATG genes whose products are required for vesicle expansion and completion are present, while genes involved in induction of autophagy and cargo packaging are mostly absent. We subsequently focused on the molecular and cellular function of P. falciparum ATG8 (PfATG8), an autophagosome membrane marker and key component of the autophagy pathway, throughout the parasite asexual and sexual erythrocytic stages. In this context, we showed that PfATG8 has a distinct and atypical role in parasite development. PfATG8 localized in the apicoplast and in vesicles throughout the cytosol during parasite development. Immunofluorescence assays of PfATG8 in apicoplast-minus parasites suggest that PfATG8 is involved in apicoplast biogenesis. Furthermore, treatment of parasite cultures with bafilomycin A 1 and chloroquine, both lysosomotropic agents that inhibit autophagosome and lysosome fusion, resulted in dramatic morphological changes of the apicoplast, and parasite death. Furthermore, deep proteomic analysis of components associated with PfATG8 indicated that it may possibly be involved in ribophagy and piecemeal microautophagy of the nucleus. Collectively, our data revealed the importance and specificity of the autophagy pathway in the malaria parasite and offer potential novel therapeutic strategies.

Remodeling of human red cells infected with Plasmodium falciparum and the impact of PHIST proteins

In an infected erythrocyte (iRBC), renovation and decoration are crucial for malarial parasite survival, pathogenesis and reproduction. Host cell remodeling is mediated by an array of diverse parasite-encoded export proteins that traffic within iRBC. These remodeling proteins extensively modify the membrane and cytoskeleton of iRBC and help in formation of parasite-induced novel organelles such as 'Maurer's Cleft (MC), tubulovesicular network (TVN) and parasitophorous vacuole membrane (PVM) inside the iRBC. The genome sequence of Plasmodium falciparum shows expansion of export proteins, which suggests a complex requirement of these export proteins for specific pathogenesis and erythrocyte remodeling. Plasmodium helical intersperse sub-telomeric (PHIST) is a family of seventy-two small export proteins and many of its recently discovered functional characteristics suggest an intriguing putative role in modification of an iRBC. This review highlights the recent advances in parasite genomics, proteomics, and cell biology studies unraveling the host cell modification; providing a speculation on the impact of PHIST proteins in modification of the iRBC.

Protein export in malaria parasites: many membranes to cross

The continuous multiplication of Plasmodium parasites in red blood cells leads to a rapid increase in parasite numbers and is responsible for the disease symptoms of malaria. Survival and virulence of the parasite are linked to parasite-induced changes of the host red blood cells. These alterations require export of a large number of parasite proteins that are trafficked across multiple membranes to reach the host cell. Two classes of exported proteins are known, those with a conserved Plasmodium export element (PEXEL/HT) or those without this motif (PNEPs). Recent work has revealed new aspects of the determinants required for export of these 2 protein classes, shedding new light on the mode of trafficking during the different transport steps en route to the host cell.

The C-terminal portion of the cleaved HT motif is necessary and sufficient to mediate export of proteins from the malaria parasite into its host cell

The malaria parasite exports proteins across its plasma membrane and a surrounding parasitophorous vacuole membrane, into its host erythrocyte. Most exported proteins contain a Host Targeting motif (HT motif) that targets them for export. In the parasite secretory pathway, the HT motif is cleaved by the protease plasmepsin V, but the role of the newly generated N-terminal sequence in protein export is unclear. Using a model protein that is cleaved by an exogenous viral protease, we show that the new N-terminal sequence, normally generated by plasmepsin V cleavage, is sufficient to target a protein for export, and that cleavage by plasmepsin V is not coupled directly to the transfer of a protein to the next component in the export pathway. Mutation of the fourth and fifth positions of the HT motif, as well as amino acids further downstream, block or affect the efficiency of protein export indicating that this region is necessary for efficient export. We also show that the fifth position of the HT motif is important for plasmepsin V cleavage. Our results indicate that plasmepsin V cleavage is required to generate a new N-terminal sequence that is necessary and sufficient to mediate protein export by the malaria parasite.