Role of the Plasmodium export element in trafficking parasite proteins to the infected erythrocyte

Flp/FRT-mediated disruption of ptex150 and exp2 in Plasmodium falciparum sporozoites inhibits liver-stage development

Plasmodium falciparum causes severe malaria and assembles a protein translocon (PTEX) complex at the parasitophorous vacuole membrane (PVM) of infected erythrocytes, through which several hundred proteins are exported to facilitate growth. The preceding liver stage of infection involves growth in a hepatocyte-derived PVM; however, the importance of protein export during P. falciparum liver infection remains unexplored. Here, we use the FlpL/FRT system to conditionally excise genes in P. falciparum sporozoites for functional liver-stage studies. Disruption of PTEX members ptex150 and exp2 did not affect sporozoite development in mosquitoes or infectivity for hepatocytes but attenuated liver-stage growth in humanized mice. While PTEX150 deficiency reduced fitness on day 6 postinfection by 40%, EXP2 deficiency caused 100% loss of liver parasites, demonstrating that PTEX components are required for growth in hepatocytes to differing degrees. To characterize PTEX loss-of-function mutations, we localized four liver-stage Plasmodium export element (PEXEL) proteins. P. falciparum liver specific protein 2 (LISP2), liver-stage antigen 3 (LSA3), circumsporozoite protein (CSP), and a Plasmodium berghei LISP2 reporter all localized to the periphery of P. falciparum liver stages but were not exported beyond the PVM. Expression of LISP2 and CSP but not LSA3 was reduced in ptex150-FRT and exp2-FRT liver stages, suggesting that expression of some PEXEL proteins is affected directly or indirectly by PTEX disruption. These results show that PTEX150 and EXP2 are important for P. falciparum development in hepatocytes and emphasize the emerging complexity of PEXEL protein trafficking.

DNA N-glycosylases Ogg1 and EndoIII as components of base excision repair in Plasmodium falciparum organelles

The integrity of genomes of the two crucial organelles of the malaria parasite - an apicoplast and mitochondrion in each cell - must be maintained by DNA repair mediated by proteins targeted to these compartments. We explored the localisation and function of Plasmodium falciparum base excision repair (BER) DNA N-glycosylase homologs PfEndoIII and PfOgg1. These N-glycosylases would putatively recognise DNA lesions prior to the action of apurinic/apyrimidinic (AP)-endonucleases. Both Ape1 and Apn1 endonucleases have earlier been shown to function solely in the parasite mitochondrion. Immunofluorescence localisation showed that PfEndoIII was exclusively mitochondrial. PfOgg1 was not seen clearly in mitochondria when expressed as a PfOgg1leader-GFP fusion, although chromatin immunoprecipitation assays showed that it could interact with both mitochondrial and apicoplast DNA. Recombinant PfEndoIII functioned as a DNA N-glycosylase as well as an AP-lyase on thymine glycol (Tg) lesions. We further studied the importance of Ogg1 in the malaria life cycle using reverse genetic approaches in Plasmodium berghei. Targeted disruption of PbOgg1 resulted in loss of 8-oxo-G specific DNA glycosylase/lyase activity. PbOgg1 knockout did not affect blood, mosquito or liver stage development but caused reduced blood stage infection after inoculation of sporozoites in mice. A significant reduction in erythrocyte infectivity by PbOgg1 knockout hepatic merozoites was also observed, thus showing that PbOgg1 ensures smooth transition from liver to blood stage infection. Our results strengthen the view that the Plasmodium mitochondrial genome is an important site for DNA repair by the BER pathway.

Unveiling potential repurposed drug candidates for Plasmodium falciparum through in silico evaluation: A synergy of structure-based approaches, structure prediction, and molecular dynamics simulations

The rise of drug resistance in Plasmodium falciparum, rendering current treatments ineffective, has hindered efforts to eliminate malaria. To address this issue, the study employed a combination of Systems Biology approach and a structure-based pharmacophore method to identify a target against P. falciparum. Through text mining, 448 genes were extracted, and it was discovered that plasmepsins, found in the Plasmodium genus, play a crucial role in the parasite's survival. The metabolic pathways of these proteins were determined using the PlasmoDB genomic database and recreated using CellDesigner 4.4.2. To identify a potent target, Plasmepsin V (PF13_0133) was selected and examined for protein-protein interactions (PPIs) using the STRING Database. Topological analysis and global-based methods identified PF13_0133 as having the highest centrality. Moreover, the static protein knockout PPIs demonstrated the essentiality of PF13_0133 in the modeled network. Due to the unavailability of the protein's crystal structure, it was modeled and subjected to a molecular dynamics simulation study. The structure-based pharmacophore modeling utilized the modeled PF13_0133 (PfPMV), generating 10 pharmacophore hypotheses with a library of active and inactive compounds against PfPMV. Through virtual screening, two potential candidates, hesperidin and rutin, were identified as potential drugs which may be repurposed as potential anti-malarial agents.

APEX2-based proximity proteomic analysis identifies candidate interactors for Plasmodium falciparum knob-associated histidine-rich protein in infected erythrocytes

The interaction of Plasmodium falciparum-infected red blood cells (iRBCs) with the vascular endothelium plays a crucial role in malaria pathology and disease. KAHRP is an exported P. falciparum protein involved in iRBC remodelling, which is essential for the formation of protrusions or "knobs" on the iRBC surface. These knobs and the proteins that are concentrated within them allow the parasites to escape the immune response and host spleen clearance by mediating cytoadherence of the iRBC to the endothelial wall, but this also slows down blood circulation, leading in some cases to severe cerebral and placental complications. In this work, we have applied genetic and biochemical tools to identify proteins that interact with P. falciparum KAHRP using enhanced ascorbate peroxidase 2 (APEX2) proximity-dependent biotinylation and label-free shotgun proteomics. A total of 30 potential KAHRP-interacting candidates were identified, based on the assigned fragmented biotinylated ions. Several identified proteins have been previously reported to be part of the Maurer's clefts and knobs, where KAHRP resides. This study may contribute to a broader understanding of P. falciparum protein trafficking and knob architecture and shows for the first time the feasibility of using APEX2-proximity labelling in iRBCs.

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.

Metabolism of host lysophosphatidylcholine in Plasmodium falciparum-infected erythrocytes

The human malaria parasite Plasmodium falciparum requires exogenous fatty acids to support its growth during the pathogenic, asexual erythrocytic stage. Host serum lysophosphatidylcholine (LPC) is a significant fatty acid source, yet the metabolic processes responsible for the liberation of free fatty acids from exogenous LPC are unknown. Using an assay for LPC hydrolysis in P. falciparum-infected erythrocytes, we have identified small-molecule inhibitors of key in situ lysophospholipase activities. Competitive activity-based profiling and generation of a panel of single-to-quadruple knockout parasite lines revealed that two enzymes of the serine hydrolase superfamily, termed exported lipase (XL) 2 and exported lipase homolog (XLH) 4, constitute the dominant lysophospholipase activities in parasite-infected erythrocytes. The parasite ensures efficient exogenous LPC hydrolysis by directing these two enzymes to distinct locations: XL2 is exported to the erythrocyte, while XLH4 is retained within the parasite. While XL2 and XLH4 were individually dispensable with little effect on LPC hydrolysis in situ, loss of both enzymes resulted in a strong reduction in fatty acid scavenging from LPC, hyperproduction of phosphatidylcholine, and an enhanced sensitivity to LPC toxicity. Notably, growth of XL/XLH-deficient parasites was severely impaired when cultured in media containing LPC as the sole exogenous fatty acid source. Furthermore, when XL2 and XLH4 activities were ablated by genetic or pharmacologic means, parasites were unable to proliferate in human serum, a physiologically relevant fatty acid source, revealing the essentiality of LPC hydrolysis in the host environment and its potential as a target for anti-malarial therapy.

Identification of a non-exported Plasmepsin V substrate that functions in the parasitophorous vacuole of malaria parasites

IMPORTANCE

In the manuscript, the authors investigate the role of the protease Plasmepsin V in the parasite-host interaction. Whereas processing by Plasmepsin V was previously thought to target a protein for export into the host cell, the authors now show that there are proteins cleaved by this protease that are not exported but instead function at the host-parasite interface. This changes the view of this protease, which turns out to have a much broader role than anticipated. The result shows that the protease may have a function much more similar to that of related organisms. The authors also investigate the requirements for protein export by analyzing exported and non-exported proteins and find commonalities between the proteins of each set that further our understanding of the requirements for protein export.

Toxoplasma protein export and effector function

Toxoplasma gondii is a single-celled eukaryotic parasite with a considerable host range that must invade the cells of warm-blooded hosts to survive and replicate. The challenges and opportunities that such a strategy represent have been met by the evolution of effectors that are delivered into host cells, counter host defences and co-opt host cell functions for their own purposes. These effectors are delivered in two waves using distinct machinery for each. In this Review, we focus on understanding the architecture of these protein-export systems and how their protein cargo is recognized and selected. We discuss the recent findings on the role that host manipulation has in latent Toxoplasma infections. We also discuss how these recent findings compare to protein export in the related Plasmodium spp. (the causative agent of malaria) and how this can inform our understanding of host manipulation in the larger Apicomplexa phylum and its evolution.

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.

The mature N-termini of Plasmodium effector proteins confer specificity of export

IMPORTANCE

Malaria parasites export hundreds of proteins to the cytoplasm of the host red blood cells for their survival. A five amino acid sequence, called the PEXEL motif, is conserved among many exported proteins and is thought to be a signal for export. However, the motif is cleaved inside the endoplasmic reticulum of the parasite, and mature proteins starting from the fourth PEXEL residue travel to the parasite periphery for export. We showed that the PEXEL motif is dispensable for export as long as identical mature proteins can be efficiently produced via alternative means in the ER. We also showed that the exported and non-exported proteins are differentiated at the parasite periphery based on their mature N-termini; however, any discernible export signal within that region remained cryptic. Our study resolves a longstanding paradox in PEXEL protein trafficking.

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.

Metabolism of host lysophosphatidylcholine in Plasmodium falciparum-infected erythrocytes

The human malaria parasite Plasmodium falciparum requires exogenous fatty acids to support its growth during the pathogenic, asexual erythrocytic stage. Host serum lysophosphatidylcholine (LPC) is a significant fatty acid source, yet the metabolic processes responsible for the liberation of free fatty acids from exogenous LPC are unknown. Using a novel assay for LPC hydrolysis in P. falciparum-infected erythrocytes, we have identified small-molecule inhibitors of key in situ lysophospholipase activities. Competitive activity-based profiling and generation of a panel of single-to-quadruple knockout parasite lines revealed that two enzymes of the serine hydrolase superfamily, termed exported lipase (XL) 2 and exported lipase homolog (XLH) 4, are the dominant lysophospholipase activities in parasite-infected erythrocytes. The parasite ensures efficient exogenous LPC hydrolysis by directing these two enzymes to distinct locations: XL2 is exported to the erythrocyte, while XLH4 is retained within the parasite. While XL2 and XLH4 were individually dispensable with little effect on LPC hydrolysis in situ, loss of both enzymes resulted in a strong reduction in fatty acid scavenging from LPC, hyperproduction of phosphatidylcholine, and an enhanced sensitivity to LPC toxicity. Notably, growth of XL/XLH-deficient parasites was severely impaired when cultured in media containing LPC as the sole exogenous fatty acid source. Furthermore, when XL2 and XLH4 activities were ablated by genetic or pharmacologic means, parasites were unable to proliferate in human serum, a physiologically-relevant fatty acid source, revealing the essentiality of LPC hydrolysis in the host environment and its potential as a target for anti-malarial therapy.

A small cysteine-rich fungal effector, BsCE66 is essential for the virulence of Bipolaris sorokiniana on wheat plants

The Spot Blotch (SB) caused by hemibiotrophic fungal pathogen Bipolaris sorokiniana is one of the most devastating wheat diseases leading to 15-100% crop loss. However, the biology of Triticum-Bipolaris interactions and host immunity modulation by secreted effector proteins remain underexplored. Here, we identified a total of 692 secretory proteins including 186 predicted effectors encoded by B. sorokiniana genome. Gene Ontology categorization showed that these proteins belong to cellular, metabolic and signaling processes, and exhibit catalytic and binding activities. Further, we functionally characterized a cysteine-rich, B. sorokiniana Candidate Effector 66 (BsCE66) that was induced at 24-96 hpi during host colonization. The Δbsce66 mutant did not show vegetative growth defects or stress sensitivity compared to wild-type, but developed drastically reduced necrotic lesions upon infection in wheat plants. The loss-of-virulence phenotype was rescued upon complementing the Δbsce66 mutant with BsCE66 gene. Moreover, BsCE66 does not form homodimer and conserved cysteine residues form intra-molecular disulphide bonds. BsCE66 localizes to the host nucleus and cytosol, and triggers a strong oxidative burst and cell death in Nicotiana benthamiana. Overall, our findings demonstrate that BsCE66 is a key virulence factor that is necessary for host immunity modulation and SB disease progression. These findings would significantly improve our understanding of Triticum-Bipolaris interactions and assist in the development of SB resistant wheat varieties.

The nuclear effector ArPEC25 from the necrotrophic fungus Ascochyta rabiei targets the chickpea transcription factor CaβLIM1a and negatively modulates lignin biosynthesis, increasing host susceptibility

Fungal pathogens deploy a barrage of secreted effectors to subvert host immunity, often by evading, disrupting, or altering key components of transcription, defense signaling, and metabolic pathways. However, the underlying mechanisms of effectors and their host targets are largely unexplored in necrotrophic fungal pathogens. Here, we describe the effector protein Ascochyta rabiei PEXEL-like Effector Candidate 25 (ArPEC25), which is secreted by the necrotroph A. rabiei, the causal agent of Ascochyta blight disease in chickpea (Cicer arietinum), and is indispensable for virulence. After entering host cells, ArPEC25 localizes to the nucleus and targets the host LIM transcription factor CaβLIM1a. CaβLIM1a is a transcriptional regulator of CaPAL1, which encodes phenylalanine ammonia lyase (PAL), the regulatory, gatekeeping enzyme of the phenylpropanoid pathway. ArPEC25 inhibits the transactivation of CaβLIM1a by interfering with its DNA-binding ability, resulting in negative regulation of the phenylpropanoid pathway and decreased levels of intermediates of lignin biosynthesis, thereby suppressing lignin production. Our findings illustrate the role of fungal effectors in enhancing virulence by targeting a key defense pathway that leads to the biosynthesis of various secondary metabolites and antifungal compounds. This study provides a template for the study of less explored necrotrophic effectors and their host target functions.

An essential endoplasmic reticulum-resident N-acetyltransferase ortholog in Plasmodium falciparum

N-terminal acetylation is a common eukaryotic protein modification that involves the addition of an acetyl group to the N-terminus of a polypeptide. This modification is largely performed by cytosolic N-terminal acetyltransferases (NATs). Most associate with the ribosome, acetylating nascent polypeptides co-translationally. In the malaria parasite Plasmodium falciparum, exported effectors are thought to be translated into the endoplasmic reticulum (ER), processed by the aspartic protease plasmepsin V and then N-acetylated, despite having no clear access to cytosolic NATs. Here, we used inducible gene deletion and post-transcriptional knockdown to investigate the primary ER-resident NAT candidate, Pf3D7_1437000. We found that it localizes to the ER and is required for parasite growth. However, depletion of Pf3D7_1437000 had no effect on protein export or acetylation of the exported proteins HRP2 and HRP3. Despite this, Pf3D7_1437000 depletion impedes parasite development within the host red blood cell and prevents parasites from completing genome replication. Thus, this work provides further proof of N-terminal acetylation of secretory system proteins, a process unique to apicomplexan parasites, but strongly discounts a promising candidate for this post-translational modification.

The Medicines for Malaria Venture Malaria Box contains inhibitors of protein secretion in Plasmodium falciparum blood stage parasites

Plasmodium falciparum parasites which cause malaria, traffic hundreds of proteins into the red blood cells (RBCs) they infect. These exported proteins remodel their RBCs enabling host immune evasion through processes such as cytoadherence that greatly assist parasite survival. As resistance to all current antimalarial compounds is rising new compounds need to be identified and those that could inhibit parasite protein secretion and export would both rapidly reduce parasite virulence and ultimately lead to parasite death. To identify compounds that inhibit protein export we used transgenic parasites expressing an exported nanoluciferase reporter to screen the Medicines for Malaria Venture Malaria Box of 400 antimalarial compounds with mostly unknown targets. The most potent inhibitor identified in this screen was MMV396797 whose application led to export inhibition of both the reporter and endogenous exported proteins. MMV396797 mediated blockage of protein export and slowed the rigidification and cytoadherence of infected RBCs—modifications which are both mediated by parasite‐derived exported proteins. Overall, we have identified a new protein export inhibitor in P. falciparum whose target though unknown, could be developed into a future antimalarial that rapidly inhibits parasite virulence before eliminating parasites from the host.

Tryptophan C-mannosylation is critical for Plasmodium falciparum transmission

Tryptophan C-mannosylation stabilizes proteins bearing a thrombospondin repeat (TSR) domain in metazoans. Here we show that Plasmodium falciparum expresses a DPY19 tryptophan C-mannosyltransferase in the endoplasmic reticulum and that DPY19-deficiency abolishes C-glycosylation, destabilizes members of the TRAP adhesin family and inhibits transmission to mosquitoes. Imaging P. falciparum gametogenesis in its entirety in four dimensions using lattice light-sheet microscopy reveals defects in ΔDPY19 gametocyte egress and exflagellation. While egress is diminished, ΔDPY19 microgametes still fertilize macrogametes, forming ookinetes, but these are abrogated for mosquito infection. The gametogenesis defects correspond with destabilization of MTRAP, which we show is C-mannosylated in P. falciparum, and the ookinete defect is concordant with defective CTRP secretion on the ΔDPY19 background. Genetic complementation of DPY19 restores ookinete infectivity, sporozoite production and C-mannosylation activity. Therefore, tryptophan C-mannosylation by DPY19 ensures TSR protein quality control at two lifecycle stages for successful transmission of the human malaria parasite.

Here, Lopaticki et al. show that Plasmodium falciparum expresses a Dpy19 C-mannosyltransferase in the endoplasmic reticulum that glycosylates TSR domains. Functional characterization shows that PfDpy19 plays a critical role in transmission through mosquitoes as PfDpy19-deficiency abolishes C-glycosylation and destabilizes proteins relevant for gametogenesis and oocyst formation.

Implications of critical node-dependent unidirectional cross-talk of Plasmodium SUMO pathway proteins

The endoparasitic pathogen, Plasmodium falciparum (Pf), modulates protein-protein interactions to employ post-translational modifications like SUMOylation to establish successful infections. The interaction between E1 and E2 (Ubc9) enzymes governs species specificity in the Plasmodium SUMOylation pathway. Here, we demonstrate that a unidirectional cross-species interaction exists between Pf-SUMO and human E2, whereas Hs-SUMO1 failed to interact with Pf-E2. Biochemical and biophysical analyses revealed that surface-accessible aspartates of Pf-SUMO determine the efficacy and specificity of SUMO-Ubc9 interactions. Furthermore, we demonstrate that critical residues of the Pf-Ubc9 N terminus are responsible for diminished Hs-SUMO1 and Pf-Ubc9 interaction. Mutating these residues to corresponding Hs-Ubc9 residues restores electrostatic, π-π, and hydrophobic interactions and allows efficient cross-species interactions. We suggest that, in comparison with human counterparts, Plasmodium SUMO and Ubc9 proteins have acquired critical changes on their surfaces as nodes, which Plasmodium can use to exploit the host SUMOylation machinery.

Kinetic Tracking of Plasmodium falciparum Antigens on Infected Erythrocytes with a Novel Reporter of Protein Insertion and Surface Exposure

Intracellular malaria parasites export many proteins into their host cell, inserting several into the erythrocyte plasma membrane to enable interactions with their external environment. While static techniques have identified some surface-exposed proteins, other candidates have eluded definitive localization and membrane topology determination. Moreover, both export kinetics and the mechanisms of membrane insertion remain largely unexplored. We introduce Reporter of Insertion and Surface Exposure (RISE), a method for continuous nondestructive tracking of antigen exposure on infected cells. RISE utilizes a small 11-amino acid (aa) HiBit fragment of NanoLuc inserted into a target protein and detects surface exposure through high-affinity complementation to produce luminescence. We tracked the export and surface exposure of CLAG3, a parasite protein linked to nutrient uptake, throughout the Plasmodiumfalciparum cycle in human erythrocytes. Our approach revealed key determinants of trafficking and surface exposure. Removal of a C-terminal transmembrane domain aborted export. Unexpectedly, certain increases in the exposed reporter size improved the luminescence signal, but other changes abolished the surface signal, revealing that both size and charge of the extracellular epitope influence membrane insertion. Marked cell-to-cell variation with larger inserts containing multiple HiBit epitopes suggests complex regulation of CLAG3 insertion at the host membrane. Quantitative, continuous tracking of CLAG3 surface exposure thus reveals multiple factors that determine this protein’s trafficking and insertion at the host erythrocyte membrane. The RISE assay will enable study of surface antigens from divergent intracellular pathogens.

The Cellular and Molecular Interaction Between Erythrocytes and Plasmodium falciparum Merozoites

Plasmodium falciparum is the most lethal human malaria parasite, partly due to its genetic variability and ability to use multiple invasion routes via its binding to host cell surface receptors. The parasite extensively modifies infected red blood cell architecture to promote its survival which leads to increased cell membrane rigidity, adhesiveness and permeability. Merozoites are initially released from infected hepatocytes and efficiently enter red blood cells in a well-orchestrated process that involves specific interactions between parasite ligands and erythrocyte receptors; symptoms of the disease occur during the life-cycle’s blood stage due to capillary blockage and massive erythrocyte lysis. Several studies have focused on elucidating molecular merozoite/erythrocyte interactions and host cell modifications; however, further in-depth analysis is required for understanding the parasite’s biology and thus provide the fundamental tools for developing prophylactic or therapeutic alternatives to mitigate or eliminate Plasmodium falciparum-related malaria. This review focuses on the cellular and molecular events during Plasmodium falciparum merozoite invasion of red blood cells and the alterations that occur in an erythrocyte once it has become infected.

Naturally acquired antibody kinetics against Plasmodium vivax antigens in people from a low malaria transmission region in western Thailand

BACKGROUND

Plasmodium vivax (P. vivax) is the dominant Plasmodium spp. causing the disease malaria in low-transmission regions outside of Africa. These regions often feature high proportions of asymptomatic patients with sub-microscopic parasitaemia and relapses. Naturally acquired antibody responses are induced after Plasmodium infection, providing partial protection against high parasitaemia and clinical episodes. However, previous work has failed to address the presence and maintenance of such antibody responses to P. vivax particularly in low-transmission regions.

METHODS

We followed 34 patients in western Thailand after symptomatic P. vivax infections to monitor antibody kinetics over 9 months, during which no recurrent infections occurred. We assessed total IgG, IgG subclass and IgM levels to up to 52 P. vivax proteins every 2-4 weeks using a multiplexed Luminex® assay and identified protein-specific variation in antibody longevity. Mathematical modelling was used to generate the estimated half-life of antibodies, long-, and short-lived antibody-secreting cells.

RESULTS

Generally, an increase in antibody level was observed within 1-week post symptomatic infection, followed by an exponential decay of different rates. We observed mostly IgG1 dominance and IgG3 sub-dominance in this population. IgM responses followed similar kinetic patterns to IgG, with some proteins unexpectedly inducing long-lived IgM responses. We also monitored antibody responses against 27 IgG-immunogenic antigens in 30 asymptomatic individuals from a similar region. Our results demonstrate that most antigens induced robust and long-lived total IgG responses following asymptomatic infections in the absence of (detected) boosting infections.

CONCLUSIONS

Our work provides new insights into the development and maintenance of naturally acquired immunity to P. vivax and will guide the potential use of serology to indicate immune status and/or identify populations at risk.

A new mass spectral library for high-coverage and reproducible analysis of the Plasmodium falciparum-infected red blood cell proteome

Background

Plasmodium falciparum causes the majority of malaria mortality worldwide, and the disease occurs during the asexual red blood cell (RBC) stage of infection. In the absence of an effective and available vaccine, and with increasing drug resistance, asexual RBC stage parasites are an important research focus. In recent years, mass spectrometry–based proteomics using data-dependent acquisition has been extensively used to understand the biochemical processes within the parasite. However, data-dependent acquisition is problematic for the detection of low-abundance proteins and proteome coverage and has poor run-to-run reproducibility.

Results

Here, we present a comprehensive P. falciparum–infected RBC (iRBC) spectral library to measure the abundance of 44,449 peptides from 3,113 P. falciparum and 1,617 RBC proteins using a data-independent acquisition mass spectrometric approach. The spectral library includes proteins expressed in the 3 morphologically distinct RBC stages (ring, trophozoite, schizont), the RBC compartment of trophozoite-iRBCs, and the cytosolic fraction from uninfected RBCs. This spectral library contains 87% of all P. falciparum proteins that have previously been reported with protein-level evidence in blood stages, as well as 692 previously unidentified proteins. The P. falciparum spectral library was successfully applied to generate semi-quantitative proteomics datasets that characterize the 3 distinct asexual parasite stages in RBCs, and compared artemisinin-resistant (Cam3.II^R539T) and artemisinin-sensitive (Cam3.II^rev) parasites.

Conclusion

A reproducible, high-coverage proteomics spectral library and analysis method has been generated for investigating sets of proteins expressed in the iRBC stage of P. falciparum malaria. This will provide a foundation for an improved understanding of parasite biology, pathogenesis, drug mechanisms, and vaccine candidate discovery for malaria.

A revised mechanism for how Plasmodium falciparum recruits and exports proteins into its erythrocytic host cell

Plasmodium falciparum exports ~10% of its proteome into its host erythrocyte to modify the host cell's physiology. The Plasmodium export element (PEXEL) motif contained within the N-terminus of most exported proteins directs the trafficking of those proteins into the erythrocyte. To reach the host cell, the PEXEL motif of exported proteins is processed by the endoplasmic reticulum (ER) resident aspartyl protease plasmepsin V. Then, following secretion into the parasite-encasing parasitophorous vacuole, the mature exported protein must be unfolded and translocated across the parasitophorous vacuole membrane by the Plasmodium translocon of exported proteins (PTEX). PTEX is a protein-conducting channel consisting of the pore-forming protein EXP2, the protein unfoldase HSP101, and structural component PTEX150. The mechanism of how exported proteins are specifically trafficked from the parasite's ER following PEXEL cleavage to PTEX complexes on the parasitophorous vacuole membrane is currently not understood. Here, we present evidence that EXP2 and PTEX150 form a stable subcomplex that facilitates HSP101 docking. We also demonstrate that HSP101 localises both within the parasitophorous vacuole and within the parasite's ER throughout the ring and trophozoite stage of the parasite, coinciding with the timeframe of protein export. Interestingly, we found that HSP101 can form specific interactions with model PEXEL proteins in the parasite's ER, irrespective of their PEXEL processing status. Collectively, our data suggest that HSP101 recognises and chaperones PEXEL proteins from the ER to the parasitophorous vacuole and given HSP101's specificity for the EXP2-PTEX150 subcomplex, this provides a mechanism for how exported proteins are specifically targeted to PTEX for translocation into the erythrocyte.

Evidences of G Coupled-Protein Receptor (GPCR) Signaling in the human Malaria Parasite Plasmodium falciparum for Sensing its Microenvironment and the Role of Purinergic Signaling in Malaria Parasites

The nucleotides were discovered in the early 19th century and a few years later, the role of such molecules in energy metabolism and cell survival was postulated. In 1972, a pioneer work by Burnstock and colleagues suggested that ATP could also work as a neurotransmitter, which was known as the "purinergic hypothesis". The idea of ATP working as a signaling molecule faced initial resistance until the discovery of the receptors for ATP and other nucleotides, called purinergic receptors. Among the purinergic receptors, the P2Y family is of great importance because it comprises of G proteincoupled receptors (GPCRs). GPCRs are widespread among different organisms. These receptors work in the cells' ability to sense the external environment, which involves: to sense a dangerous situation or detect a pheromone through smell; the taste of food that should not be eaten; response to hormones that alter metabolism according to the body's need; or even transform light into an electrical stimulus to generate vision. Advances in understanding the mechanism of action of GPCRs shed light on increasingly promising treatments for diseases that have hitherto remained incurable, or the possibility of abolishing side effects from therapies widely used today.

In silico validation of novel inhibitors of malarial aspartyl protease, plasmepsin V and antimalarial efficacy prediction

Plasmepsin V (Plm V) is an essential aspartic protease required for survival of the malaria parasite, Plasmodium falciparum (Pf). Plm V is required for cleaving the PEXEL motifs of many Pf proteins and its inhibition leads to a knockout effect, indicating its suitability as potential drug target. To decipher new inhibitors of PfPlm V, molecular docking of four HIV-1 protease inhibitors active against PfPlmV was performed on Glide module of Schrödinger suite that supported saquinavir as a lead drug, and therefore, selected as a control. Saquinavir contains an important hydroxyethylamine (HEA) pharmacophore, which was utilized as backbone coupled with piperazine scaffold to build new library of compounds. Newly designed HEA compounds were screened virtually against Plm V. Molecular docking led to a few hits (1 and 3) with higher docking score over the control drug. Notably, compound 1 showed the highest docking score (-11.90 kcal/mol) and XP Gscore (-11.948 kcal/mol). The Prime MMGBSA binding free energy for compound 1 (-60.88 kcal/mol) and 3 (-50.96 kcal/mol) was higher than saquinavir (-37.51 kcal/mol). The binding free energy for the last frame of molecular dynamic simulation supported compound 1 (-92.88 kcal/mol) as potent inhibitor of PfPlm V over saquinavir (-72.77 kcal/mol), and thus, deserves experimental validations in culture and subsequently in animal models.Communicated by Ramaswamy H. Sarma.

The Role of Malaria Parasite Heat Shock Proteins in Protein Trafficking and Remodelling of Red Blood Cells

The genus Plasmodium comprises intracellular eukaryotic parasites that infect many vertebrate groups and cause deadly malaria disease in humans. The parasites employ a suite of heat shock proteins to help traffic other proteins to different compartments within their own cells and that of the host cells they parasitise. This review will cover the role of these chaperones in protein export and host cell modification in the asexual blood stage of the human parasite P. falciparum which is the most deadly and well-studied parasite species. We will examine the role chaperones play in the import of proteins into the secretory pathway from where they are escorted to the vacuole space surrounding the intraerythrocytic parasite. Here, other heat shock proteins unfold protein cargoes and extrude them into the red blood cell (RBC) cytosol from where additional chaperones of parasite and possibly host origin refold the cargo proteins and guide them to their final functional destinations within their RBC host cells. The secretory pathway also serves as a launch pad for proteins targeted to the non-photosynthetic apicoplast organelle of endosymbiotic origin, and the role of heat shock proteins in trafficking proteins here will be reviewed. Finally, the function of chaperones in protein trafficking into the mitochondrion, the remaining organelle of endosymbiotic origin, will be discussed.

PV1 Protein from Plasmodium falciparum Exhibits Chaperone-Like Functions and Cooperates with Hsp100s

Plasmodium falciparum parasitophorous vacuolar protein 1 (PfPV1), a protein unique to malaria parasites, is localized in the parasitophorous vacuolar (PV) and is essential for parasite growth. Previous studies suggested that PfPV1 cooperates with the Plasmodium translocon of exported proteins (PTEX) complex to export various proteins from the PV. However, the structure and function of PfPV1 have not been determined in detail. In this study, we undertook the expression, purification, and characterization of PfPV1. The tetramer appears to be the structural unit of PfPV1. The activity of PfPV1 appears to be similar to that of molecular chaperones, and it may interact with various proteins. PfPV1 could substitute CtHsp40 in the CtHsp104, CtHsp70, and CtHsp40 protein disaggregation systems. Based on these results, we propose a model in which PfPV1 captures various PV proteins and delivers them to PTEX through a specific interaction with HSP101.

Inhibition of Plasmepsin V Activity Blocks Plasmodium falciparum Gametocytogenesis and Transmission to Mosquitoes

Plasmodium falciparum gametocytes infect mosquitoes and are responsible for malaria transmission. New interventions that block transmission could accelerate malaria elimination. Gametocytes develop within erythrocytes and activate protein export pathways that remodel the host cell. Plasmepsin V (PMV) is an aspartyl protease that is required for protein export in asexual parasites, but its function and essentiality in gametocytes has not been definitively proven, nor has PMV been assessed as a transmission-blocking drug target. Here, we show that PMV is expressed and can be inhibited specifically in P. falciparum stage I-II gametocytes. PMV inhibitors block processing and export of gametocyte effector proteins and inhibit development of stage II-V gametocytes. Gametocytogenesis in the presence of sublethal inhibitor concentrations results in stage V gametocytes that fail to infect mosquitoes. Therefore, PMV primes gametocyte effectors for export, which is essential for the development and fitness of gametocytes for transmission to mosquitoes.

Toxoplasma Mechanisms for Delivery of Proteins and Uptake of Nutrients Across the Host-Pathogen Interface

Many intracellular pathogens, including the protozoan parasite Toxoplasma gondii, live inside a vacuole that resides in the host cytosol. Vacuolar residence provides these pathogens with a defined niche for replication and protection from detection by host cytosolic pattern recognition receptors. However, the limiting membrane of the vacuole, which constitutes the host-pathogen interface, is also a barrier for pathogen effectors to reach the host cytosol and for the acquisition of host-derived nutrients. This review provides an update on the specialized secretion and trafficking systems used by Toxoplasma to overcome the barrier of the parasitophorous vacuole membrane and thereby allow the delivery of proteins into the host cell and the acquisition of host-derived nutrients.

Cross-species reactivity of antibodies against Plasmodium vivax blood-stage antigens to Plasmodium knowlesi

Malaria is caused by multiple different species of protozoan parasites, and interventions in the pre-elimination phase can lead to drastic changes in the proportion of each species causing malaria. In endemic areas, cross-reactivity may play an important role in the protection and blocking transmission. Thus, successful control of one species could lead to an increase in other parasite species. A few studies have reported cross-reactivity producing cross-immunity, but the extent of cross-reactive, particularly between closely related species, is poorly understood. P. vivax and P. knowlesi are particularly closely related species causing malaria infections in SE Asia, and whilst P. vivax cases are in decline, zoonotic P. knowlesi infections are rising in some areas. In this study, the cross-species reactivity and growth inhibition activity of P. vivax blood-stage antigen-specific antibodies against P. knowlesi parasites were investigated. Bioinformatics analysis, immunofluorescence assay, western blotting, protein microarray, and growth inhibition assay were performed to investigate the cross-reactivity. P. vivax blood-stage antigen-specific antibodies recognized the molecules located on the surface or released from apical organelles of P. knowlesi merozoites. Recombinant P. vivax and P. knowlesi proteins were also recognized by P. knowlesi- and P. vivax-infected patient antibodies, respectively. Immunoglobulin G against P. vivax antigens from both immune animals and human malaria patients inhibited the erythrocyte invasion by P. knowlesi. This study demonstrates that there is extensive cross-reactivity between antibodies against P. vivax to P. knowlesi in the blood stage, and these antibodies can potently inhibit in vitro invasion, highlighting the potential cross-protective immunity in endemic areas.

In recent years, malaria initiatives have increasingly shifted focus from achieving malaria control to achieving malaria elimination. However, the interventions used are leading to drastic changes in the proportions of different Plasmodium species causing clinical infection, particularly within Southeast Asia. Little is known about how these different parasite species interact/compete in nature or whether exposure to one species could cause some level of protection against another. We examined cross-reactive antibody responses to key parasite proteins with roles in red blood cell invasion and identified novel cross-species reactivity among the closest of malaria affecting the human population (P. vivax and P. knowlesi). This comprehensive analysis provides evidence that cross-reactive immunity could play an important role in areas where species distributions are perturbed by malaria control measures, and future efforts to identify the specific cross-reactive epitopes involved would be invaluable both to our understanding of malaria immunity and vaccine development.

Malaria parasite plasmepsins: More than just plain old degradative pepsins

Plasmepsins are a group of diverse aspartic proteases in the malaria parasite Plasmodium Their functions are strikingly multifaceted, ranging from hemoglobin degradation to secretory organelle protein processing for egress, invasion, and effector export. Some, particularly the digestive vacuole plasmepsins, have been extensively characterized, whereas others, such as the transmission-stage plasmepsins, are minimally understood. Some (e.g. plasmepsin V) have exquisite cleavage sequence specificity; others are fairly promiscuous. Some have canonical pepsin-like aspartic protease features, whereas others have unusual attributes, including the nepenthesin loop of plasmepsin V and a histidine in place of a catalytic aspartate in plasmepsin III. We have learned much about the functioning of these enzymes, but more remains to be discovered about their cellular roles and even their mechanisms of action. Their importance in many key aspects of parasite biology makes them intriguing targets for antimalarial chemotherapy. Further consideration of their characteristics suggests that some are more viable drug targets than others. Indeed, inhibitors of invasion and egress offer hope for a desperately needed new drug to combat this nefarious organism.

Assessment of Biological Role and Insight into Druggability of the Plasmodium falciparum Protease Plasmepsin V

Upon infecting a red blood cell (RBC), the malaria parasite Plasmodium falciparum drastically remodels its host by exporting hundreds of proteins into the RBC cytosol. This protein export program is essential for parasite survival. Hence export-related proteins could be potential drug targets. One essential enzyme in this pathway is plasmepsin V (PMV), an aspartic protease that processes export-destined proteins in the parasite endoplasmic reticulum (ER) at the Plasmodium export element (PEXEL) motif. Despite long-standing interest in this enzyme, functional studies have been hindered by the inability of previous technologies to produce a regulatable lethal depletion of PMV. To overcome this technical barrier, we designed a system for stringent post-transcriptional regulation allowing a tightly controlled, tunable knockdown of PMV. Using this system, we found that PMV must be dramatically depleted to affect parasite growth, suggesting the parasite maintains this enzyme in substantial excess. Surprisingly, depletion of PMV arrested parasite growth immediately after RBC invasion, significantly before the death from exported protein deficit that has previously been described. The data suggest that PMV inhibitors can halt parasite growth at two distinct points in the parasite life cycle. However, overcoming the functional excess of PMV in the parasite may require inhibitor concentrations far beyond the enzyme's IC50.

Proteomic Analysis of Plasmodium Merosomes: The Link between Liver and Blood Stages in Malaria

The pre-erythrocytic liver stage of the malaria parasite, comprising sporozoites and the liver stages into which they develop, remains one of the least understood parts of the lifecycle, in part owing to the low numbers of parasites. Nonetheless, it is recognized as an important target for antimalarial drugs and vaccines. Here we provide the first proteomic analysis of merosomes, which define the final phase of the liver stage and are responsible for initiating the blood stage of infection. We identify a total of 1879 parasite proteins, and a core set of 1188 proteins quantitatively detected in every biological replicate, providing an extensive picture of the protein repertoire of this stage. This unique data set will allow us to explore key questions about the biology of merosomes and hepatic merozoites.

Uncoupling the Threading and Unfoldase Actions of Plasmodium HSP101 Reveals Differences in Export between Soluble and Insoluble Proteins

The Plasmodium parasites that cause malaria export hundreds of proteins into their host red blood cell (RBC). These exported proteins drastically alter the structural and functional properties of the RBC and play critical roles in parasite virulence and survival. To access the RBC cytoplasm, parasite proteins must pass through the Plasmodium translocon of exported proteins (PTEX) located at the membrane interfacing the parasite and host cell. Our data provide evidence that HSP101, a component of PTEX, serves to unfold protein cargo requiring translocation. We also reveal that addition of a transmembrane domain to soluble cargo influences its ability to be translocated by parasites in which the HSP101 motor and unfolding activities have become uncoupled. Therefore, we propose that proteins with transmembrane domains use an alternative unfolding pathway prior to PTEX to facilitate export.

Plasmodium parasites must export proteins into their erythrocytic host to survive. Exported proteins must cross the parasite plasma membrane (PPM) and the parasitophorous vacuolar membrane (PVM) encasing the parasite to access the host cell. Crossing the PVM requires protein unfolding and passage through a translocon, the Plasmodium translocon of exported proteins (PTEX). In this study, we provide the first direct evidence that heat shock protein 101 (HSP101), a core component of PTEX, unfolds proteins for translocation across the PVM by creating transgenic Plasmodium parasites in which the unfoldase and translocation functions of HSP101 have become uncoupled. Strikingly, while these parasites could export native proteins, they were unable to translocate soluble, tightly folded reporter proteins bearing the Plasmodium export element (PEXEL) across the PVM into host erythrocytes under the same conditions. In contrast, an identical PEXEL reporter protein but harboring a transmembrane domain could be exported, suggesting that a prior unfolding step occurs at the PPM. Together, these results demonstrate that the export of parasite proteins is dependent on how these proteins are presented to the secretory pathway before they reach PTEX as well as their folded status. Accordingly, only tightly folded soluble proteins secreted into the vacuolar space and not proteins containing transmembrane domains or the majority of erythrocyte-stage exported proteins have an absolute requirement for the full unfoldase activity of HSP101 to be exported.

Yield improvement and enzymatic dissection of Plasmodium falciparum plasmepsin V

To survive within a red blood cell (RBC), malaria parasites establish striking modifications to the permeability, rigidity and cytoadherence properties of the host cell. This is mediated by the export of hundreds of proteins from the parasite into the erythrocyte. Plasmodium falciparum plasmepsin V (PfPMV), is an ER resident aspartic protease that processes proteins for export into the host erythrocyte, plays a crucial role in parasite virulence and survival and is considered a potential malaria drug target. Most attempts at its heterologous expression in Escherichia coli have resulted in mainly the production of insoluble proteins. In this study, we employed a multipurpose fusion tag to improve the production of PfPMV in E. coli. Recombinant PfPMVm, comprising residues 84-521, was substantially obtained in soluble form and could be purified in a single step, yielding a 3.7-fold increase in purified PfPMVm compared to previous reports. Additionally, we have mutated the catalytic residues (D118N and D365N), individually and together, and the unpaired cysteine residue C178 to evaluate the effects on catalytic efficiency. Mutation of D365 had more pronounced effects on the catalytic efficiency than that of D118, suggesting that the D365 may act as a catalytic nucleophile to activate the water molecule. The importance of C178 was also confirmed by the inhibition by metal ions, indicating that C178 is partially involved in the substrate recognition. Collectively, our results describe an improved system to produce recombinant PfPMVm in E. coli and dissect the amino acids involved in catalysis and substrate recognition.

Guided STED nanoscopy enables super-resolution imaging of blood stage malaria parasites

Malaria remains a major burden world-wide, but the disease-causing parasites from the genus Plasmodium are difficult to study in vitro. Owing to the small size of the parasites, subcellular imaging poses a major challenge and the use of super-resolution techniques has been hindered by the parasites' sensitivity to light. This is particularly apparent during the blood-stage of the Plasmodium life cycle, which presents an important target for drug research. The iron-rich food vacuole of the parasite undergoes disintegration when illuminated with high-power lasers such as those required for high resolution in Stimulated Emission Depletion (STED) microscopy. This causes major damage to the sample precluding the use of this super-resolution technique. Here we present guided STED, a novel adaptive illumination (AI) STED approach, which takes advantage of the highly-reflective nature of the iron deposit in the cell to identify the most light-sensitive parts of the sample. Specifically in these parts, the high-power STED laser is deactivated automatically to prevent local damage. Guided STED nanoscopy finally allows super-resolution imaging of the whole Plasmodium life cycle, enabling multicolour imaging of blood-stage malaria parasites with resolutions down to 35 nm without sample destruction.

Illuminating how malaria parasites export proteins into host erythrocytes

Plasmodium parasites that cause the disease malaria have developed an elaborate trafficking pathway to facilitate the export of hundreds of effector proteins into their host cell, the erythrocyte. In this review, we outline how certain effector proteins contribute to parasite survival, virulence, and immune evasion. We also highlight how parasite proteins destined for export are recognised at the endoplasmic reticulum to facilitate entry into the export pathway and how the effector proteins are able to transverse the bounding parasitophorous vaculoar membrane via the Plasmodium translocon of exported proteins to gain access to the host cell. Some of the gaps in our understanding of the export pathway are also presented. Finally, we examine the degree of conservation of some of the key components of the Plasmodium export pathway in closely related apicomplexan parasites, which may provide insight into how the diverse apicomplexan parasites have adapted to survival pressures encountered within their respective host cells.

Essentiality of Plasmodium falciparum plasmepsin V

The malaria parasite replicates within erythrocytes. The pathogenesis of clinical malaria is in large part due to the capacity of the parasite to remodel its host cell. To do this, intraerythrocytic stages of Plasmodium falciparum export more than 300 proteins that dramatically alter the morphology of the infected erythrocyte as well as its mechanical and adhesive properties. P. falciparum plasmepsin V (PfPMV) is an aspartic protease that processes proteins for export into the host erythrocyte and is thought to play a key role in parasite virulence and survival. However, although standard techniques for gene disruption as well as conditional protein knockdown have been previously attempted with the pfpmv gene, complete gene removal or knockdown was not achieved so direct genetic proof that PMV is an essential protein has not been established. Here we have used a conditional gene excision approach combining CRISPR-Cas9 gene editing and DiCre-mediated recombination to functionally inactivate the pfpmv gene. The resulting mutant parasites displayed a severe growth defect. Detailed phenotypic analysis showed that development of the mutant parasites was arrested early in the ring-to-trophozoite transition in the erythrocytic cycle following gene excision. Our findings are the first to elucidate the effects of PMV gene disruption, showing that it is essential for parasite viability in asexual blood stages. The mutant parasites can now be used as a platform to further dissect the Plasmodium protein export pathway.

Aspartyl Protease 5 Matures Dense Granule Proteins That Reside at the Host-Parasite Interface in Toxoplasma gondii

Toxoplasma gondii is one of the most successful human parasites. Central to its success is the arsenal of virulence proteins introduced into the infected host cell. Several of these virulence proteins require direct maturation by the aspartyl protease ASP5, and all require ASP5 for translocation into the host cell, yet the true number of ASP5 substrates and complete repertoire of effectors is currently unknown. Here we selectively enrich N-terminally derived peptides using Terminal Amine Isotopic Labeling of Substrates (TAILS) and use quantitative proteomics to reveal novel ASP5 substrates. We identify, using two different enrichment techniques, new ASP5 substrates and their specific cleavage sites. ASP5 substrates include two kinases and one phosphatase that reside at the host-parasite interface, which are important for infection.

Toxoplasma gondii infects approximately 30% of the world’s population, causing disease primarily during pregnancy and in individuals with weakened immune systems. Toxoplasma secretes and exports effector proteins that modulate the host during infection, and several of these proteins are processed by the Golgi-associated aspartyl protease 5 (ASP5). Here, we identify ASP5 substrates by selectively enriching N-terminally derived peptides from wild-type and Δasp5 parasites. We reveal more than 2,000 unique Toxoplasma N-terminal peptides, mapping to both natural N termini and protease cleavage sites. Several of these peptides mapped directly downstream of the characterized ASP5 cleavage site, arginine-arginine-leucine (RRL). We validate candidates as true ASP5 substrates, revealing they are not processed in parasites lacking ASP5 or in wild-type parasites following mutation of the motif from RRL to ARL. All identified ASP5 substrates are dense granule proteins, and interestingly, none appear to be exported, thus differing from the analogous system in related Plasmodium spp. Instead we show that the majority of substrates reside within the parasitophorous vacuole (PV), and its membrane (the PVM), including two kinases and one phosphatase. We show that genetic deletion of WNG2 leads to attenuation in a mouse model, suggesting that this putative kinase is a new virulence factor in Toxoplasma. Collectively, these data constitute the first in-depth analyses of ASP5 substrates and shed new light on the role of ASP5 as a maturase of dense granule proteins during the Toxoplasma lytic cycle.

In vitro and in silico studies of naphthoquinones and peptidomimetics toward Plasmodium falciparum plasmepsin V

Plasmodium proteases play both regulatory and effector roles in essential biological processes in this important pathogen and have long been investigated as drug targets. Plasmepsin V from P. falciparum (PfPMV) is an essential protease that processes proteins for export into the host erythrocyte and is a focus of ongoing drug development efforts. In the present study, recombinant protein production, inhibition assays, binding studies as well as molecular docking and molecular dynamics simulation studies were used to investigate the mode of binding of a PEXEL-based peptidomimetic and naphthoquinone compounds to PfPMV. Consistent with our previous study, refolded PfPMVs were produced with functional characteristics similar to the soluble counterpart. Naphthoquinone compounds inhibited PfPMV activity by 50% at 50 μM but did not affect pepsin activity. The IC₅₀ values of compounds 31 and 37 against PfPMV were 22.25 and 68.94 μM, respectively. Molecular dynamics simulations revealed that PEXEL peptide interacted with PfPMV active site residues via electrostatic interactions while naphthoquinone binding preferred van der Waal interactions. P₁'-Ser of the PfEMP2 substrate formed an additional H-bond with Asp365 promoting the catalytic efficiency. Additionally, the effect of metal ions on the secondary structure of PfPMV was examined. Our results confirmed that Hg²⁺ ions reversibly induced the changes in secondary structure of the protein whereas Fe³⁺ ions induced irreversibly. No change was observed in the presence of Ca²⁺ ions. Overall, the results here suggested that naphthoquinone derivatives may represent another source of antimalarial inhibitors targeting aspartic proteases but further chemical modifications are required.

Cross-species analysis of apical asparagine-rich protein of Plasmodium vivax and Plasmodium knowlesi

The Plasmodium falciparum apical asparagine (Asn)-rich protein (AARP) is one of malarial proteins, and it has been studied as a candidate of malaria subunit vaccine. Basic characterization of PvAARP has been performed with a focus on its immunogenicity and localization. In this study, we further analyzed the immunogenicity of PvAARP, focusing on the longevity of the antibody response, cross-species immunity and invasion inhibitory activity by using the primate malaria parasite Plasmodium knowlesi. We found that vivax malaria patient sera retained anti-PvAARP antibodies for at least one year without re-infection. Recombinant PvAARP protein was strongly recognized by knowlesi malaria patients. Antibody raised against the P. vivax and P. knowlesi AARP N-termini reacted with the apical side of the P. knowlesi merozoites and inhibited erythrocyte invasion by P. knowlesi in a concentration-dependent manner, thereby suggesting a cross-species nature of anti-PvAARP antibody against PkAARP. These results can be explained by B cell epitopes predicted in conserved surface-exposed regions of the AARP N-terminus in both species. The long-lived anti-PvAARP antibody response, cross-reactivity, and invasion inhibitory activity of anti-PvAARP support a critical role of AARP during the erythrocyte invasion and suggest that PvAARP induces long-lived cross-species protective immunity against P. vivax and P. knowlesi.

PV1, a novel Plasmodium falciparum merozoite dense granule protein, interacts with exported protein in infected erythrocytes

Upon invasion, Plasmodium falciparum exports hundreds of proteins across its surrounding parasitophorous vacuole membrane (PVM) to remodel the infected erythrocyte. Although this phenomenon is crucial for the parasite growth and virulence, elucidation of precise steps in the export pathway is still required. A translocon protein complex, PTEX, is the only known pathway that mediates passage of exported proteins across the PVM. P. falciparum Parasitophorous Vacuolar protein 1 (PfPV1), a previously reported parasitophorous vacuole (PV) protein, is considered essential for parasite growth. In this study, we characterized PfPV1 as a novel merozoite dense granule protein. Structured illumination microscopy (SIM) analyses demonstrated that PfPV1 partially co-localized with EXP2, suggesting the protein could be a PTEX accessory molecule. Furthermore, PfPV1 and exported protein PTP5 co-immunoprecipitated with anti-PfPV1 antibody. Surface plasmon resonance (SPR) confirmed the proteins’ direct interaction. Additionally, we identified a PfPV1 High-affinity Region (PHR) at the C-terminal side of PTP5 where PfPV1 dominantly bound. SIM analysis demonstrated an export arrest of PTP5ΔPHR, a PTP5 mutant lacking PHR, suggesting PHR is essential for PTP5 export to the infected erythrocyte cytosol. The overall results suggest that PfPV1, a novel dense granule protein, plays an important role in protein export at PV.

Potential Sabotage of Host Cell Physiology by Apicomplexan Parasites for Their Survival Benefits

Plasmodium, Toxoplasma, Cryptosporidium, Babesia, and Theileria are the major apicomplexan parasites affecting humans or animals worldwide. These pathogens represent an excellent example of host manipulators who can overturn host signaling pathways for their survival. They infect different types of host cells and take charge of the host machinery to gain nutrients and prevent itself from host attack. The mechanisms by which these pathogens modulate the host signaling pathways are well studied for Plasmodium, Toxoplasma, Cryptosporidium, and Theileria, except for limited studies on Babesia. Theileria is a unique pathogen taking into account the way it modulates host cell transformation, resulting in its clonal expansion. These parasites majorly modulate similar host signaling pathways, however, the disease outcome and effect is different among them. In this review, we discuss the approaches of these apicomplexan to manipulate the host–parasite clearance pathways during infection, invasion, survival, and egress.

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.

An exported protein-interacting complex involved in the trafficking of virulence determinants in Plasmodium-infected erythrocytes

The malaria parasite, Plasmodium falciparum, displays the P. falciparum erythrocyte membrane protein 1 (PfEMP1) on the surface of infected red blood cells (RBCs). We here examine the physical organization of PfEMP1 trafficking intermediates in infected RBCs and determine interacting partners using an epitope-tagged minimal construct (PfEMP1B). We show that parasitophorous vacuole (PV)-located PfEMP1B interacts with components of the PTEX (Plasmodium Translocon of EXported proteins) as well as a novel protein complex, EPIC (Exported Protein-Interacting Complex). Within the RBC cytoplasm PfEMP1B interacts with components of the Maurer’s clefts and the RBC chaperonin complex. We define the EPIC interactome and, using an inducible knockdown approach, show that depletion of one of its components, the parasitophorous vacuolar protein-1 (PV1), results in altered knob morphology, reduced cell rigidity and decreased binding to CD36. Accordingly, we show that deletion of the Plasmodium berghei homologue of PV1 is associated with attenuation of parasite virulence in vivo.

Plasmodium-infected red blood cells export virulence factors, such as PfEMP1, to the cell surface. Here, the authors identify a protein complex termed EPIC that interacts with PfEMP1 during export, and they show that knockdown of an EPIC component affects parasite virulence.