The malaria parasite Plasmodium falciparum Sortilin is essential for merozoite formation and apical complex biogenesis

Role of Rabenosyn-5 and Rab5b in host cell cytosol uptake reveals conservation of endosomal transport in malaria parasites

Vesicular trafficking, including secretion and endocytosis, plays fundamental roles in the unique biology of Plasmodium falciparum blood-stage parasites. Endocytosis of host cell cytosol (HCC) provides nutrients and room for parasite growth and is critical for the action of antimalarial drugs and parasite drug resistance. Previous work showed that PfVPS45 functions in endosomal transport of HCC to the parasite's food vacuole, raising the possibility that malaria parasites possess a canonical endolysosomal system. However, the seeming absence of VPS45-typical functional interactors such as rabenosyn 5 (Rbsn5) and the repurposing of Rab5 isoforms and other endolysosomal proteins for secretion in apicomplexans question this idea. Here, we identified a parasite Rbsn5-like protein and show that it functions with VPS45 in the endosomal transport of HCC. We also show that PfRab5b but not PfRab5a is involved in the same process. Inactivation of PfRbsn5L resulted in PI3P and PfRab5b decorated HCC-filled vesicles, typical for endosomal compartments. Overall, this indicates that despite the low sequence conservation of PfRbsn5L and the unusual N-terminal modification of PfRab5b, principles of endosomal transport in malaria parasite are similar to that of model organisms. Using a conditional double protein inactivation system, we further provide evidence that the PfKelch13 compartment, an unusual apicomplexa-specific endocytosis structure at the parasite plasma membrane, is connected upstream of the Rbsn5L/VPS45/Rab5b-dependent endosomal route. Altogether, this work indicates that HCC uptake consists of a highly parasite-specific part that feeds endocytosed material into an endosomal system containing more canonical elements, leading to the delivery of HCC to the food vacuole.

Toxoplasma sortilin interacts with secretory proteins and it is critical for parasite proliferation

BACKGROUND

The human sortilin protein is an important drug target and detection marker for cancer research. The sortilin from Toxoplasma gondii transports proteins associated with the apical organelles of the parasite. In this study, we aimed to determine the intracellular localization and structural domains of T. gondii sortilin, which may mediate protein transportation. Approaches to the functional inhibition of sortilin to establish novel treatments for T. gondii infections were explored.

METHODS

A gene encoding the sortilin protein was identified in the T. gondii genome. Immunoprecipitation and mass spectrometry were performed to identify the protein species transported by T. gondii sortilin. The interaction of each structural domain of sortilin with the transported proteins was investigated using bio-layer interferometry. The binding regions of the transported proteins in sortilin were identified. The effect of the sortilin inhibitor AF38469 on the infectivity of T. gondii was investigated. The binding site of AF38469 on sortilin was determined.

RESULTS

The subdomains Vps10, sortilin-C, and sortilin-M of the sortilin were identified as the binding regions for intracellular transportation of the target proteins. The sortilin inhibitor AF38469 bound to the Vps10 structural domain of T. gondii sortilin, which inhibited parasite invasion, replication, and intracellular growth in vitro and was therapeutic in mice infected with T. gondii.

CONCLUSION

The Vps10, sortilin-C, and sortilin-M subdomains of T. gondii sortilin were identified as functional regions for intracellular protein transport. The binding region for the sortilin inhibitor AF38469 was also identified as the Vps10 subdomain. This study establishes sortilin as a promising drug target against T. gondii and provides a valuable reference for the development of anti-T. gondii drug-target studies.

Functional screening reveals Toxoplasma prenylated proteins required for endocytic trafficking and rhoptry protein sorting

In the apicomplexans, endocytosed cargos (e.g., hemoglobin) are trafficked to a specialized organelle for digestion. This follows a unique endocytotic process at the micropore/cytostome in these parasites. However, the mechanism underlying endocytic trafficking remains elusive, due to the repurposing of classical endocytic proteins for the biogenesis of apical organelles. To resolve this issue, we have exploited the genetic tractability of the model apicomplexan Toxoplasma gondii, which ingests host cytosolic materials (e.g., green fluorescent protein[GFP]). We determined an association between protein prenylation and endocytic trafficking, and using an alkyne-labeled click chemistry approach, the prenylated proteome was characterized. Genome editing, using clustered regularly interspaced short palindromic repaet/CRISPR-associated nuclease 9 (CRISPR/Cas9), was efficiently utilized to generate genetically modified lines for the functional screening of 23 prenylated candidates. This identified four of these proteins that regulate the trafficking of endocytosed GFP vesicles. Among these proteins, Rab1B and YKT6.1 are highly conserved but are non-classical endocytic proteins in eukaryotes. Confocal imaging analysis showed that Rab1B and Ras are substantially localized to both the trans-Golgi network and the endosome-like compartments in the parasite. Conditional knockdown of Rab1B caused a rapid defect in secretory trafficking to the rhoptry bulb, suggesting a trafficking intersection role for the key regulator Rab1B. Further experiments confirmed a critical role for protein prenylation in regulating the stability/activity of these proteins (i.e., Rab1B and YKT6.1) in the parasite. Our findings define the molecular basis of endocytic trafficking and reveal a potential intersection function of Rab1B on membrane trafficking in T. gondii. This might extend to other related protists, including the malarial parasites. IMPORTANCE The protozoan Toxoplasma gondii establishes a permissive niche, in host cells, that allows parasites to acquire large molecules such as proteins. Numerous studies have demonstrated that the parasite repurposes the classical endocytic components for secretory sorting to the apical organelles, leaving the question of endocytic transport to the lysosome-like compartment unclear. Recent studies indicated that endocytic trafficking is likely to associate with protein prenylation in malarial parasites. This information promoted us to examine this association in the model apicomplexan T. gondii and to identify the key components of the prenylated proteome that are involved. By exploiting the genetic tractability of T. gondii and a host GFP acquisition assay, we reveal four non-classical endocytic proteins that regulate the transport of endocytosed cargos (e.g., GFP) in T. gondii. Thus, we extend the principle that protein prenylation regulates endocytic trafficking and elucidate the process of non-classical endocytosis in T. gondii and potentially in other related protists.

Timing of dense granule biogenesis in asexual malaria parasites

Malaria is an important infectious disease that continues to claim hundreds of thousands of lives annually. The disease is caused by infection of host erythrocytes by apicomplexan parasites of the genus Plasmodium. The parasite contains three different apical organelles - micronemes, rhoptries and dense granules (DGs) - whose contents are secreted to mediate binding to and invasion of the host cell and the extensive remodelling of the host cell that occurs following invasion. Whereas the roles of micronemes and rhoptries in binding and invasion of the host erythrocyte have been studied in detail, the roles of DGs in Plasmodium parasites are poorly understood. They have been proposed to control host cell remodelling through regulated protein secretion after invasion, but many basic aspects of the biology of DGs remain unknown. Here we describe DG biogenesis timing for the first time, using RESA localization as a proxy for the timing of DG formation. We show that DG formation commences approximately 37 min prior to schizont egress, as measured by the recruitment of the DG marker RESA. Furthermore, using a bioinformatics approach, we aimed to predict additional cargo of the DGs and identified the J-dot protein HSP40 as a DG protein, further supporting the very early role of these organelles in the interaction of the parasite with the host cell.

An image-based high-content screening for compounds targeting Toxoplasma gondii repurposed inhibitors effective against the malaria parasite Plasmodium falciparum

Apicomplexa phylum includes numerous obligate intracellular protozoan parasites that are life threatening for humans and animals. In this context, Plasmodium falciparum and Toxoplasma gondii are of particular interest, as they are responsible for malaria and toxoplasmosis, respectively, for which efficient vaccines are presently lacking and therapies need to be improved. Apicomplexan parasites have a highly polarized morphology, with their apical end containing specific secretory organelles named rhoptries and micronemes, which depend on the unique receptor and transporter sortilin TgSORT for their biogenesis. In the present study, we took advantage of the subcellular polarity of the parasite to engineer a clonal transgenic Toxoplasma line that expresses simultaneously the green fluorescent protein TgSORT-GFP in the post-Golgi-endosome-like compartment and the red fluorescent protein rhoptry ROP1-mCherry near the apical end. We utilized this fluorescent transgenic T. gondii to develop a miniaturized image-based phenotype assay coupled to an automated image analysis. By applying this methodology to 1,120 compounds, we identified 12 that are capable of disrupting the T. gondii morphology and inhibiting intracellular replication. Analysis of the selected compounds confirmed that all 12 are kinase inhibitors and intramembrane pumps, with some exhibiting potent activity against Plasmodium falciparum. Our findings highlight the advantage of comparative and targeted phenotypic analysis involving two related parasite species as a means of identifying molecules with a conserved mode of action.

Protein Sorting in Plasmodium Falciparum

Plasmodium falciparum is a unicellular eukaryote with a very polarized secretory system composed of micronemes rhoptries and dense granules that are required for host cell invasion. P. falciparum, like its relative T. gondii, uses the endolysosomal system to produce the secretory organelles and to ingest host cell proteins. The parasite also has an apicoplast, a secondary endosymbiotic organelle, which depends on vesicular trafficking for appropriate incorporation of nuclear-encoded proteins into the apicoplast. Recently, the central molecules responsible for sorting and trafficking in P. falciparum and T. gondii have been characterized. From these studies, it is now evident that P. falciparum has repurposed the molecules of the endosomal system to the secretory pathway. Additionally, the sorting and vesicular trafficking mechanism seem to be conserved among apicomplexans. This review described the most recent findings on the molecular mechanisms of protein sorting and vesicular trafficking in P. falciparum and revealed that P. falciparum has an amazing secretory machinery that has been cleverly modified to its intracellular lifestyle.

Unique Endomembrane Systems and Virulence in Pathogenic Protozoa

Virulence in pathogenic protozoa is often tied to secretory processes such as the expression of adhesins on parasite surfaces or the secretion of proteases to assisted in tissue invasion and other proteins to avoid the immune system. This review is a broad overview of the endomembrane systems of pathogenic protozoa with a focus on Giardia, Trichomonas, Entamoeba, kinetoplastids, and apicomplexans. The focus is on unique features of these protozoa and how these features relate to virulence. In general, the basic elements of the endocytic and exocytic pathways are present in all protozoa. Some of these elements, especially the endosomal compartments, have been repurposed by the various species and quite often the repurposing is associated with virulence. The Apicomplexa exhibit the most unique endomembrane systems. This includes unique secretory organelles that play a central role in interactions between parasite and host and are involved in the invasion of host cells. Furthermore, as intracellular parasites, the apicomplexans extensively modify their host cells through the secretion of proteins and other material into the host cell. This includes a unique targeting motif for proteins destined for the host cell. Most notable among the apicomplexans is the malaria parasite, which extensively modifies and exports numerous proteins into the host erythrocyte. These modifications of the host erythrocyte include the formation of unique membranes and structures in the host erythrocyte cytoplasm and on the erythrocyte membrane. The transport of parasite proteins to the host erythrocyte involves several unique mechanisms and components, as well as the generation of compartments within the erythrocyte that participate in extraparasite trafficking.

Unraveling the Elusive Rhoptry Exocytic Mechanism of Apicomplexa

Apicomplexan parasites are unicellular eukaryotes that invade the cells in which they proliferate. The development of genetic tools in Toxoplasma, and then in Plasmodium, in the 1990s allowed the first description of the molecular machinery used for motility and invasion, revealing a crucial role for two different secretory organelles, micronemes and rhoptries. Rhoptry proteins are injected directly into the host cytoplasm not only to promote invasion but also to manipulate host functions. Nonetheless, the injection machinery has remained mysterious, a major conundrum in the field. Here we review recent progress in uncovering structural components and proteins implicated in rhoptry exocytosis and explain how revisiting early findings and considering the evolutionary origins of Apicomplexa contributed to some of these discoveries.

Vesicle dynamics during the egress of malaria gametocytes from the red blood cell

Malaria parasites are obligate intracellular pathogens that live in human red blood cells harbored by a parasitophorous vacuole. The parasites need to exit from the red blood cell to continue life-cycle progression and ensure human-to-mosquito transmission. Two types of blood stages are able to lyse the enveloping red blood cell to mediate egress, the merozoites and the gametocytes. The intraerythrocytic parasites exit the red blood cell via an inside-out mode during which the membrane of the parasitophorous vacuole ruptures prior to the red blood cell membrane. Membrane rupture is initiated by the exocytosis of specialized secretory vesicles following the perception of egress triggers. The molecular mechanisms of red blood cell egress have particularly been studied in malaria gametocytes. Upon activation by external factors, gametocytes successively discharge at least two types of vesicles, the osmiophilic bodies needed to rupture the parasitophorous vacuole membrane and recently identified egress vesicles that are important for the perforation of the erythrocyte membrane. In recent years, important components of the signaling cascades leading to red blood cell egress have been investigated and several proteins of the osmiophilic bodies have been identified. We here report on the newest findings on the egress of gametocytes from the red blood cell. We further focus on the content and function of the egress-related vesicles and discuss the molecular machinery that might drive vesicle discharge.

The Ins and Outs of Plasmodium Rhoptries, Focusing on the Cytosolic Side

Parasites of the genus Plasmodium cause human and animal malaria, leading to significant health and economic impacts. A key aspect of the complex life cycle of Plasmodium parasites is the invasion of the parasite into its host cell, which is mediated by secretory organelles. The largest of these organelles, the rhoptry, undergoes rapid and profound physiological changes when it secretes its contents during merozoite and sporozoite invasion of the host erythrocyte and hepatocyte, respectively. Here we discuss recent advancements in our understanding of the dynamic rhoptry biology during the parasite's invasive stages, with a focus on the roles of cytosolically exposed rhoptry-interacting proteins (C-RIPs). We explore potential similarities between the molecular mechanisms driving merozoite and sporozoite rhoptry function.

Biogenesis and discharge of the rhoptries: Key organelles for entry and hijack of host cells by the Apicomplexa

Rhoptries are specialized secretory organelles found in the Apicomplexa phylum, playing a central role in the establishment of parasitism. The rhoptry content includes membranous as well as proteinaceous materials that are discharged into the host cell in a regulated fashion during parasite entry. A set of rhoptry neck proteins form a RON complex that critically participates in the moving junction formation during invasion. Some of the rhoptry bulb proteins are associated with the membranous materials and contribute to the formation of the parasitophorous vacuole membrane while others are targeted into the host cell including the nucleus to subvert cellular functions. Here, we review the recent studies on Toxoplasma and Plasmodium parasites that shed light on the key steps leading to rhoptry biogenesis, trafficking, and discharge.

Identification of a Novel RAMA/RON3 Rhoptry Protein Complex in Plasmodium falciparum Merozoites

Malaria causes a half a million deaths annually. The parasite intraerythrocytic lifecycle in the human bloodstream is the major cause of morbidity and mortality. Apical organelles of merozoite stage parasites are involved in the invasion of erythrocytes. A limited number of apical organellar proteins have been identified and characterized for their roles during erythrocyte invasion or subsequent intraerythrocytic parasite development. To expand the repertoire of identified apical organellar proteins we generated a panel of monoclonal antibodies against Plasmodium falciparum schizont-rich parasites and screened the antibodies using immunofluorescence assays. Out of 164 hybridoma lines, 12 clones produced monoclonal antibodies yielding punctate immunofluorescence staining patterns in individual merozoites in late schizonts, suggesting recognition of merozoite apical organelles. Five of the monoclonal antibodies were used to immuno-affinity purify their target antigens and these antigens were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Two known apical organelle protein complexes were identified, the high-molecular mass rhoptry protein complex (PfRhopH1/Clags, PfRhopH2, and PfRhopH3) and the low-molecular mass rhoptry protein complex (rhoptry-associated proteins complex, PfRAP1, and PfRAP2). A novel complex was additionally identified by immunoprecipitation, composed of rhoptry-associated membrane antigen (PfRAMA) and rhoptry neck protein 3 (PfRON3) of P. falciparum. We further identified a region spanning amino acids Q₂₂₁-E₄₈₁ within the PfRAMA that may associate with PfRON3 in immature schizonts. Further investigation will be required as to whether PfRAMA and PfRON3 interact directly or indirectly.

Antimalarial Peptide and Polyketide Natural Products from the Fijian Marine Cyanobacterium Moorea producens

A new cyclic peptide, kakeromamide B (1), and previously described cytotoxic cyanobacterial natural products ulongamide A (2), lyngbyabellin A (3), 18E-lyngbyaloside C (4), and lyngbyaloside (5) were identified from an antimalarial extract of the Fijian marine cyanobacterium Moorea producens. Compounds 1 and 1 exhibited moderate activity against Plasmodium falciparum blood-stages with EC50 values of 0.89 and 0.99 µM, respectively, whereas 3 was more potent with an EC50 value of 0.15 nM, respectively. Compounds 1, 4, and 5 displayed moderate liver-stage antimalarial activity against P. berghei liver schizonts with EC50 values of 1.1, 0.71, and 0.45 µM, respectively. The threading-based computational method FINDSITEcomb2.0 predicted the binding of 1 and 2 to potentially druggable proteins of Plasmodiumfalciparum, prompting formulation of hypotheses about possible mechanisms of action. Kakeromamide B (1) was predicted to bind to several Plasmodium actin-like proteins and a sortilin protein suggesting possible interference with parasite invasion of host cells. When 1 was tested in a mammalian actin polymerization assay, it stimulated actin polymerization in a dose-dependent manner, suggesting that 1 does, in fact, interact with actin.

Who Needs a Contractile Actomyosin Ring? The Plethora of Alternative Ways to Divide a Protozoan Parasite

Cytokinesis, or the division of the cytoplasm, following the end of mitosis or meiosis, is accomplished in animal cells, fungi, and amoebae, by the constriction of an actomyosin contractile ring, comprising filamentous actin, myosin II, and associated proteins. However, despite this being the best-studied mode of cytokinesis, it is restricted to the Opisthokonta and Amoebozoa, since members of other evolutionary supergroups lack myosin II and must, therefore, employ different mechanisms. In particular, parasitic protozoa, many of which cause significant morbidity and mortality in humans and animals as well as considerable economic losses, employ a wide diversity of mechanisms to divide, few, if any, of which involve myosin II. In some cases, cell division is not only myosin II-independent, but actin-independent too. Mechanisms employed range from primitive mechanical cell rupture (cytofission), to motility- and/or microtubule remodeling-dependent mechanisms, to budding involving the constriction of divergent contractile rings, to hijacking host cell division machinery, with some species able to utilize multiple mechanisms. Here, I review current knowledge of cytokinesis mechanisms and their molecular control in mammalian-infective parasitic protozoa from the Excavata, Alveolata, and Amoebozoa supergroups, highlighting their often-underappreciated diversity and complexity. Billions of people and animals across the world are at risk from these pathogens, for which vaccines and/or optimal treatments are often not available. Exploiting the divergent cell division machinery in these parasites may provide new avenues for the treatment of protozoal disease.

The Plasmodium falciparum rhoptry bulb protein RAMA plays an essential role in rhoptry neck morphogenesis and host red blood cell invasion

The malaria parasite Plasmodium falciparum invades, replicates within and destroys red blood cells in an asexual blood stage life cycle that is responsible for clinical disease and crucial for parasite propagation. Invasive malaria merozoites possess a characteristic apical complex of secretory organelles that are discharged in a tightly controlled and highly regulated order during merozoite egress and host cell invasion. The most prominent of these organelles, the rhoptries, are twinned, club-shaped structures with a body or bulb region that tapers to a narrow neck as it meets the apical prominence of the merozoite. Different protein populations localise to the rhoptry bulb and neck, but the function of many of these proteins and how they are spatially segregated within the rhoptries is unknown. Using conditional disruption of the gene encoding the only known glycolipid-anchored malarial rhoptry bulb protein, rhoptry-associated membrane antigen (RAMA), we demonstrate that RAMA is indispensable for blood stage parasite survival. Contrary to previous suggestions, RAMA is not required for trafficking of all rhoptry bulb proteins. Instead, RAMA-null parasites display selective mislocalisation of a subset of rhoptry bulb and neck proteins (RONs) and produce dysmorphic rhoptries that lack a distinct neck region. The mutant parasites undergo normal intracellular development and egress but display a fatal defect in invasion and do not induce echinocytosis in target red blood cells. Our results indicate that distinct pathways regulate biogenesis of the two main rhoptry sub-compartments in the malaria parasite.

Despite improved control measures over recent decades, malaria is still a considerable health burden across much of the globe. The disease is caused by a single-celled parasite that invades and replicates within host cells. During invasion, the parasite discharges a set of flask-shaped secretory organelles called rhoptries, the contents of which are crucial for invasion as well as for modifications to the host cell that are important for parasite survival. Rhoptry discharge occurs through fusion of the relatively elongated rhoptry neck to the apical surface of the parasite. Different proteins reside within the bulbous rhoptry body and the neck regions, but how these proteins are selectively sent to their correct sub-compartments within the rhoptries and how the rhoptries are formed, is poorly understood. Here we show that a malaria parasite rhoptry bulb protein called rhoptry-associated membrane antigen (RAMA) plays an essential role in rhoptry neck formation and correct trafficking of certain rhoptry neck and bulb proteins. Parasites deficient in RAMA produce malformed rhoptries and–probably as a result—cannot invade host red blood cells. Our work sheds new light on how rhoptries are formed and reveals insights into the mechanism by which the correct sorting of proteins to distinct regions of the rhoptry is regulated.

A pan-apicomplexan phosphoinositide-binding protein acts in malarial microneme exocytosis

Invasion of human red blood cells by the malaria parasite Plasmodium falciparum is an essential step in the development of the disease. Consequently, the molecular players involved in host cell invasion represent important targets for inhibitor design and vaccine development. The process of merozoite invasion is a succession of steps underlined by the sequential secretion of the organelles of the apical complex. However, little is known with regard to how their contents are exocytosed. Here, we identify a phosphoinositide-binding protein conserved in apicomplexan parasites and show that it is important for the attachment and subsequent invasion of the erythrocyte by the merozoite. Critically, removing the protein from its site of action by knock sideways preferentially prevents the secretion of certain types of micronemes. Our results therefore provide evidence for a role of phosphoinositide lipids in the malaria invasion process and provide further insight into the secretion of microneme organelle populations, which is potentially applicable to diverse apicomplexan parasites.

Biogenesis and secretion of micronemes in Toxoplasma gondii

One of the hallmarks of the parasitic phylum of Apicomplexa is the presence of highly specialised, apical secretory organelles, called the micronemes and rhoptries that play critical roles in ensuring survival and dissemination. Upon exocytosis, the micronemes release adhesin complexes, perforins, and proteases that are crucially implicated in egress from infected cells, gliding motility, migration across biological barriers, and host cell invasion. Recent studies on Toxoplasma gondii and Plasmodium species have shed more light on the signalling events and the machinery that trigger microneme secretion. Intracellular cyclic nucleotides, calcium level, and phosphatidic acid act as key mediators of microneme exocytosis, and several downstream effectors have been identified. Here, we review the key steps of microneme biogenesis and exocytosis, summarising the still fractal knowledge at the molecular level regarding the fusion event with the parasite plasma membrane.

Characterization of a novel secretory spherical body protein in Babesia orientalis and Babesia orientalis-infected erythrocytes

Background

The spherical body, a membrane bound organelle localized in the apical organelle complex, is unique to Babesia and Theileria spp. The spherical body proteins (SBPs) secreted by spherical bodies include SBP1, SBP2, SBP3 and SBP4. Up to now, only SBP3 has been characterized in Babesia orientalis.

Methods

The BoSBP4 gene was amplified from cDNA and gDNA and cloned into the pGEX-6P-1 vector by homologous recombination, sequenced and analyzed by bioinformatics tools. The amino acid (aa) sequence of BoSBP4 was compared with that of Babesia bovis and Babesia bigemina as well as SBP3 of B. orientalis. The immunoreactivity was evaluated by incubating recombinant BoSBP4 (rBoSBP4) with the serum of B. orientalis-infected water buffalo. The native form of BoSBP4 was identified by incubating lysate of B. orientalis-infected water buffalo erythrocytes with the anti-rBoSBP4 mouse serum. The cellular localization of BoSBP4 was determined by indirect immunofluorescence assay.

Results

The full length of the BoSBP4 gene was estimated to be 945 bp without introns, encoding a 314 aa polypeptide with a predicted molecular weight of 37 kDa. The truncated recombinant protein was expressed from 70 to 945 bp as a GST fusion protein with a practical molecular weight of 70 kDa. BoSBP4 shared a 40% and 30% identity with B. bovis and B. bigemina, respectively. Furthermore, it was 31% identical to SBP3 of B. orientalis. BoSBP4 was identified in the lysate of B. orientalis-infected water buffalo erythrocytes with a molecular weight of 37 kDa, corresponding to the expected molecular mass of BoSBP4. The result of rBoSBP4 with positive serum revealed that BoSBP4 can elicit an immune response to B. orientalis-infected water buffalo. The cellular localization of BoSBP4 was detected to be adjacent to the merozoite nucleus in the intracellular phase, followed by the diffusion of the fluorescence of BoSBP4 into the cytoplasm of B. orientalis-infected erythrocytes as puncta-like specks and a gradual increase of the fluorescence.

Conclusions

In this study, SBP4 in B. orientalis was characterized for the first time. It may play a key role in interaction with the host cell by being secreted into the cytoplasm of the B. orientalis-infected erythrocytes to facilitate parasite growth and reproduction.