Re-defining the Golgi complex inPlasmodium falciparumusing the novel Golgi markerPfGRASP

Plasmodium falciparum erythrocyte membrane protein 1 diversity in seven genomes--divide and conquer

The var gene encoded hyper-variable Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family mediates cytoadhesion of infected erythrocytes to human endothelium. Antibodies blocking cytoadhesion are important mediators of malaria immunity acquired by endemic populations. The development of a PfEMP1 based vaccine mimicking natural acquired immunity depends on a thorough understanding of the evolved PfEMP1 diversity, balancing antigenic variation against conserved receptor binding affinities. This study redefines and reclassifies the domains of PfEMP1 from seven genomes. Analysis of domains in 399 different PfEMP1 sequences allowed identification of several novel domain classes, and a high degree of PfEMP1 domain compositional order, including conserved domain cassettes not always associated with the established group A–E division of PfEMP1. A novel iterative homology block (HB) detection method was applied, allowing identification of 628 conserved minimal PfEMP1 building blocks, describing on average 83% of a PfEMP1 sequence. Using the HBs, similarities between domain classes were determined, and Duffy binding-like (DBL) domain subclasses were found in many cases to be hybrids of major domain classes. Related to this, a recombination hotspot was uncovered between DBL subdomains S2 and S3. The VarDom server is introduced, from which information on domain classes and homology blocks can be retrieved, and new sequences can be classified. Several conserved sequence elements were found, including: (1) residues conserved in all DBL domains predicted to interact and hold together the three DBL subdomains, (2) potential integrin binding sites in DBLα domains, (3) an acylation motif conserved in group A var genes suggesting N-terminal N-myristoylation, (4) PfEMP1 inter-domain regions proposed to be elastic disordered structures, and (5) several conserved predicted phosphorylation sites. Ideally, this comprehensive categorization of PfEMP1 will provide a platform for future studies on var/PfEMP1 expression and function.

About one million African children die from malaria every year. The severity of malaria infections in part depends on which type of the parasitic protein PfEMP1 is expressed on the surface of the infected red blood cells. Natural immunity to malaria is mediated through antibodies to PfEMP1. Therefore hopes for a malaria vaccine based on PfEMP1 proteins have been raised. However, the large sequence variation among PfEMP1 molecules has caused great difficulties in executing and interpreting studies on PfEMP1. Here, we present an extensive sequence analysis of all currently available PfEMP1 sequences and show that PfEMP1 variation is ordered and can be categorized at different levels. In this way, PfEMP1 belong to group A–E and are composed of up to four components, each component containing specific DBL or CIDR domain subclasses, which in some cases form entire conserved domain combinations. Finally, each PfEMP1 can be described in high detail as a combination of 628 homology blocks. This dissection of PfEMP1 diversity also enables predictions of several functional sequence motifs relevant to the fold of PfEMP1 proteins and their ability to bind human receptors. We therefore believe that this description of PfEMP1 diversity is necessary and helpful for the design and interpretation of future PfEMP1 studies.

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

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

Evolution and diversity of the Golgi body

Often considered a defining eukaryotic feature, the Golgi body is one of the most recognizable and functionally integrated cellular organelles. It is therefore surprising that some unicellular eukaryotes do not, at first glance, appear to possess Golgi stacks. Here we review the molecular evolutionary, genomic and cell biological evidence for Golgi bodies in these organisms, with the organelle likely present in some form in all cases. This, along with the overwhelming prevalence of stacked cisternae in most eukaryotes, implies that the ancestral eukaryote possessed a stacked Golgi body, with at least eight independent instances of Golgi unstacking in our cellular history.

The location of invasion-related protein MIC3 of Toxoplasma gondii and protective effect of its DNA vaccine in mice

The micronemal protein MIC3 of Toxoplasma gondii plays a predominant role in the early phase of the invasion process. In this research, the expression and location of protein MIC3 and fusion protein EGFP-MIC3 were observed in host cells and in T. gondii respectively. Protective experiments of DNA vaccine pcDNA3-MIC3 (pMIC3i) in animals showed that the vaccine could significantly prolong the survival time of the mice challenged by virulent RH strains of T. gondii. All mice vaccinated with plasmid pcDNA3-MIC3 have been elicited specific humoral immunity and cellular immunity. The cellular immune response was associated with the increase of the CD4(+) and CD8(+) T lymphocytes and the decrease of the CD4(+)/CD8(+) T lymphocyte ratio evidently. It indicated that the mic3 DNA vaccine could stimulate the host resisting Toxoplasma mainly by the way of CD8(+)CTL cells. In conclusion, a potent DNA vaccine pcDNA3-MIC3 could elicit a strong specific immune response and induce effective protection against T. gondii challenge in Kunming mice, suggesting that mic3 is a potential vaccine candidate against toxoplasmosis.

Plasmodium falciparumpossesses two GRASP proteins that are differentially targeted to the Golgi complex via a higher- and lower-eukaryote-like mechanism

Plasmodium falciparum, the causative agent of malaria, relies on a complex protein-secretion system for protein targeting into numerous subcellular destinations. Recently, a homologue of the Golgi re-assembly stacking protein (GRASP) was identified and used to characterise the Golgi organisation in this parasite. Here, we report on the presence of a splice variant that leads to the expression of a GRASP isoform. Although the first GRASP protein (GRASP1) relies on a well-conserved myristoylation motif, the variant (GRASP2) displays a different N-terminus, similar to GRASPs found in fungi. Phylogenetic analyses between GRASP proteins of numerous taxa point to an independent evolution of the unusual N-terminus that could reflect unique requirements for Golgi-dependent protein sorting and organelle biogenesis in P. falciparum. Golgi association of GRASP2 depends on the hydrophobic N-terminus that resembles a signal anchor, leading to a unique mode of Golgi targeting and membrane attachment.

Plasmodium falciparum Sec24 marks transitional ER that exports a model cargo via a diacidic motif

Exit from the endoplasmic reticulum (ER) often occurs at distinct sites of vesicle formation known as transitional ER (tER) that are enriched for COPII vesicle coat proteins. We have characterized the organization of ER export in the malaria parasite, Plasmodium falciparum, by examining the localization of two components of the COPII machinery, PfSec12 and PfSec24a. PfSec12 was found throughout the ER, whereas the COPII cargo adaptor, PfSec24a, was concentrated at distinct foci that likely correspond to tER sites. These foci were closely apposed to cis-Golgi sites marked by PfGRASP-GFP, and upon treatment with brefeldin A they accumulated a model cargo protein via a process dependent on the presence of an intact diacidic export motif. Our data suggest that the cargo-binding function of PfSec24a is conserved and that accumulation of cargo in discrete tER sites depends upon positive sorting signals. Furthermore, the number and position of tER sites with respect to the cis-Golgi suggests a co-ordinated biogenesis of these domains.

Plasmodium possesses dynein light chain classes that are unique and conserved across species

Plasmodium belongs to the phylum Apicomplexa. Within the Apicomplexa, Plasmodium, Toxoplasma and Cryptosporidium are parasites of considerable medical importance while Theileria and Eimeria are animal pathogens. P. falciparum is particularly important as it causes malaria, resulting in more than 1 million deaths each year. The malaria parasite actively invades the host cell in which it propagates and several proteins associated with the apical organelles have been implicated to be crucial in the invasion process. The biogenesis of the apical organelles is not well understood, but several studies indicate that microtubule-based vesicular transport is involved. Vesicular transport proteins are also present in Plasmodium and are presumed to be involved in transcellular transport in infected erythrocytes. Dynein is a multi-subunit motor protein involved in microtubule-based vesicular transport. In this study, we analyzed the cytoplasmic dynein light chains (Dlcs) of P. falciparum since they provide adaptor surface to the cargoes and are likely to be involved in differential transport. Dlcs consist of three different families: TcTex1/2, LC8 and LC7/roadblock. The data presented demonstrate that P. falciparum Dlcs sequences and functional domains show high sequence similarity within the species, but that only the Dlc group 1 (LC8) has a high similarity to human orthologues. TcTex1 and LC7/roadblock have low similarity to human orthologues. This sequence variation could be targeted for vaccine or drug development.

Molecules incorporating a benzothiazole core scaffold inhibit the N-myristoyltransferase of Plasmodium falciparum

Recombinant N-myristoyltransferase of Plasmodium falciparum (termed PfNMT) has been used in the development of a SPA (scintillation proximity assay) suitable for automation and high-throughput screening of inhibitors against this enzyme. The ability to use the SPA has been facilitated by development of an expression and purification system which yields considerably improved quantities of soluble active recombinant PfNMT compared with previous studies. Specifically, yields of pure protein have been increased from 12 microg x l(-1) to >400 microg x l(-1) by use of a synthetic gene with codon usage optimized for expression in an Escherichia coli host. Preliminary small-scale 'piggyback' inhibitor studies using the SPA have identified a family of related molecules containing a core benzothiazole scaffold with IC50 values <50 microM, which demonstrate selectivity over human NMT1. Two of these compounds, when tested against cultured parasites in vitro, reduced parasitaemia by >80% at a concentration of 10 microM.

The twists and turns of Maurer's cleft trafficking in P. falciparum-infected erythrocytes

The malaria parasite, Plasmodium falciparum, invades the red blood cells (RBCs) of its human host and initiates a series of morphological rearrangements within the host cell cytoplasm. The mature RBC has no endogenous trafficking machinery; therefore, the parasite generates novel structures to mediate protein transport. These include compartments called the Maurer's clefts (MC), which play an important role in the trafficking of parasite proteins to the surface of the host cell. Recent electron tomography studies have revealed MC as convoluted flotillas of flattened discs that are tethered to the RBC membrane, prompting speculation that the MC could, in one respect, represent an extracellular equivalent of the Golgi apparatus. Visualization of both resident and cargo proteins has helped decipher the signals and routes for trafficking of parasite proteins to the MC and beyond.

Building the perfect parasite: cell division in apicomplexa

Apicomplexans are pathogens responsible for malaria, toxoplasmosis, and crytposporidiosis in humans, and a wide range of livestock diseases. These unicellular eukaryotes are stealthy invaders, sheltering from the immune response in the cells of their hosts, while at the same time tapping into these cells as source of nutrients. The complexity and beauty of the structures formed during their intracellular development have made apicomplexans the darling of electron microscopists. Dramatic technological progress over the last decade has transformed apicomplexans into respectable genetic model organisms. Extensive genomic resources are now available for many apicomplexan species. At the same time, parasite transfection has enabled researchers to test the function of specific genes through reverse and forward genetic approaches with increasing sophistication. Transfection also introduced the use of fluorescent reporters, opening the field to dynamic real time microscopic observation. Parasite cell biologists have used these tools to take a fresh look at a classic problem: how do apicomplexans build the perfect invasion machine, the zoite, and how is this process fine-tuned to fit the specific niche of each pathogen in this ancient and very diverse group? This work has unearthed a treasure trove of novel structures and mechanisms that are the focus of this review.

Evolution of the eukaryotic membrane-trafficking system: origin, tempo and mode

The emergence of an endomembrane system was a crucial stage in the prokaryote-to-eukaryote evolutionary transition. Recent genomic and molecular evolutionary analyses have provided insight into how this critical system arrived at its modern configuration. The apparent relative absence of prokaryotic antecedents for the endomembrane machinery contrasts with the situation for mitochondria, plastids and the nucleus. Overall, the evidence suggests an autogenous origin for the eukaryotic membrane-trafficking machinery. The emerging picture is that early eukaryotic ancestors had a complex endomembrane system, which implies that this cellular system evolved relatively rapidly after the proto-eukaryote diverged away from the other prokaryotic lines. Many of the components of the trafficking system are the result of gene duplications that have produced proteins that have similar functions but differ in their subcellular location. A proto-eukaryote possessing a very simple trafficking system could thus have evolved to near modern complexity in the last common eukaryotic ancestor (LCEA) via paralogous gene family expansion of the proteins encoding organelle identity. The descendents of this common ancestor have undergone further modification of the trafficking machinery; unicellular simplicity and multicellular complexity are the prevailing trend, but there are some remarkable counter-examples.

Re-assessing the locations of components of the classical vesicle-mediated trafficking machinery in transfected Plasmodium falciparum

The malaria parasite, Plasmodium falciparum, exports proteins beyond the confines of its own plasma membrane, however there is debate regarding the machinery used for these trafficking events. We have generated transgenic parasites expressing chimeric proteins and used immunofluorescence studies to determine the locations of plasmodial homologues of the COPII component, Sar1p, and the Golgi-docking protein, Bet3p. The P. falciparum Sar1p (PfSar1p) chimeras bind to the endoplasmic reticulum surface and define a network of membranes wrapped around parasite nuclei. As the parasite matures, the endomembrane systems of individual merozoites remain interconnected until very late in schizogony. Antibodies raised against plasmodial Bet3p recognise two foci of reactivity in early parasite stages that increase in number as the parasite matures. Some of the P. falciparum Bet3p (PfBet3p) compartments are juxtaposed to compartments defined by the cis Golgi marker, PfGRASP, while others are distributed through the cytoplasm. The compartments defined by the trans Golgi marker, PfRab6, are separate, suggesting that the Golgi is dispersed. Bet3p-green fluorescent protein (GFP) is partly associated with punctate structures but a substantial population diffuses freely in the parasite cytoplasm. By contrast, yeast Bet3p is very tightly associated with immobile structures. This study challenges the view that the COPII complex and the Golgi apparatus are exported into the infected erythrocyte cytoplasm.

The Golgi-Associated Protein GRASP Is Required for Unconventional Protein Secretion during Development

During Dictyostelium development, prespore cells secrete acyl-CoA binding protein (AcbA). Upon release, AcbA is processed to generate a peptide called spore differentiation factor-2 (SDF-2), which triggers terminal differentiation of spore cells. We have found that cells lacking Golgi reassembly stacking protein (GRASP), a protein attached peripherally to the cytoplasmic surface of Golgi membranes, fail to secrete AcbA and, thus, produce inviable spores. Surprisingly, AcbA lacks a signal sequence and is not secreted via the conventional secretory pathway (endoplasmic reticulum-Golgi-cell surface). GRASP is not required for conventional protein secretion, growth, and the viability of vegetative cells. Our findings reveal a physiological role of GRASP and provide a means to understand unconventional secretion and its role in development.

Falcipain cysteine proteases require bipartite motifs for trafficking to the Plasmodium falciparum food vacuole

The Plasmodium falciparum cysteine proteases falcipain-2 and falcipain-3 hydrolyze hemoglobin in an acidic food vacuole to provide amino acids for erythrocytic malaria parasites. Trafficking to the food vacuole has not been well characterized. To study trafficking of falcipains, which include large membrane-spanning prodomains, we utilized chimeras with portions of the proteases fused to green fluorescent protein. The prodomains of falcipain-2 and falcipain-3 were sufficient to target green fluorescent protein to the food vacuole. Using serial truncations, deletions, and point mutations, we showed that both a 20-amino acid stretch of the lumenal portion and a 10-amino acid stretch of the cytoplasmic portion of the falcipain-2 prodomain were required for efficient food vacuolar trafficking. Mutants with altered trafficking were arrested at the plasma membrane, implicating trafficking via this structure. Our results indicate that falcipains utilize a previously undescribed bipartite motif-dependent mechanism for targeting to a hydrolytic organelle, suggesting inhibition of this unique mechanism as a new means of antimalarial chemotherapy.

Illuminating Plasmodium falciparum-infected red blood cells

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

The human malaria parasite Plasmodium falciparum expresses an atypical N-terminally extended pyrophosphokinase with specificity for thiamine

Vitamin B(1) is an essential cofactor for key enzymes such as 2-oxoglutarate dehydrogenase and pyruvate dehydrogenase. Plants, bacteria and fungi, as well as Plasmodium falciparum, are capable of synthesising vitamin B(1)de novo, whereas mammals have to take up this cofactor from their diet. Thiamine, a B(1) vitamer, has to be pyrophosphorylated by thiamine pyrophosphokinase (TPK) to the active form. The human malaria parasite P. falciparum expresses an N-terminally extended pyrophosphokinase throughout the entire erythrocytic life cycle, which was analysed by Northern and Western blotting. The recombinant enzyme shows a specific activity of 27 nmol min(-1) mg(-1) protein and specificity for thiamine with a K(m) value of 73 microM, while thiamine monophosphate is not accepted. Mutational analysis of the N-terminal extension of the plasmodial TPK showed that it influences thiamine binding as well as metal dependence, which suggests N-terminal participation in the conformation of the active site. Protein sequences of various plasmodial TPKs were analysed for their phylogeny, which classified the Plasmodium TPKs to a group distinct from the mammalian TPKs. To verify the location of the parasite TPK within the cell, immunofluorescence analyses were performed. Co-staining of PfTPK with a GFP marker visualised its cytosolic localisation.

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

The proliferation of the malaria parasite Plasmodium falciparum within the human host is dependent upon invasion of erythrocytes. This process is accomplished by the merozoite, a highly specialized form of the parasite. Secretory organelles including micronemes and rhoptries play a pivotal role in the invasion process by storing and releasing parasite proteins. The mechanism of protein sorting to these compartments is unclear. Using a transgenic approach we show that trafficking of the most abundant micronemal proteins (members of the EBL-family: EBA-175, EBA-140/BAEBL, and EBA-181/JSEBL) is independent of their cytoplasmic and transmembrane domains, respectively. To identify the minimal sequence requirements for microneme trafficking, we generated parasites expressing EBA-GFP chimeric proteins and analyzed their distribution within the infected erythrocyte. This revealed that: (i) a conserved cysteine-rich region in the ectodomain is necessary for protein trafficking to the micronemes and (ii) correct sorting is dependent on accurate timing of expression.

Protein targeting to destinations of the secretory pathway in the malaria parasite Plasmodium falciparum

The secretory pathway in the malaria parasite Plasmodium falciparum has many unique aspects in terms of protein destinations and trafficking mechanisms. Recently, several exciting insights into protein trafficking within this intracellular parasite have been unveiled: these include signals that are required for targeting of proteins to the red blood cell and the relict plastid (known as the apicoplast); and the elucidation of the pathways of the haemoglobin proteases targeted to the food vacuole. Protein-targeting to the apical organelles in P. falciparum, however, is still not very well understood, but available research offers a tantalising glimpse of the system.

Evidence for Golgi-independent transport from the early secretory pathway to the plastid in malaria parasites

The malaria parasite Plasmodium falciparum harbours a relict plastid (termed the apicoplast) that has evolved by secondary endosymbiosis. The apicoplast is surrounded by four membranes, the outermost of which is believed to be part of the endomembrane system. Nuclear-encoded apicoplast proteins have a two-part N-terminal extension that is necessary and sufficient for translocation across these four membranes. The first domain of this N-terminal extension resembles a classical signal peptide and mediates translocation into the secretory pathway, whereas the second domain is homologous to plant chloroplast transit peptides and is required for the remaining steps of apicoplast targeting. We explored the initial, secretory pathway component of this targeting process using green fluorescent reporter protein constructs with modified leaders. We exchanged the apicoplast signal peptide with signal peptides from other secretory proteins and observed correct targeting, demonstrating that apicoplast targeting is initiated at the general secretory pathway of P. falciparum. Furthermore, we demonstrate by immunofluorescent labelling that the apicoplast resides on a small extension of the endoplasmic reticulum (ER) that is separate from the cis-Golgi. To define the position of the apicoplast in the endomembrane pathway in relation to the Golgi we tracked apicoplast protein targeting in the presence of the secretory inhibitor Brefeldin A (BFA), which blocks traffic between the ER and Golgi. We observe apicoplast targeting in the presence of BFA despite clear perturbation of ER to Golgi traffic by the inhibitor, which suggests that the apicoplast resides upstream of the cis-Golgi in the parasite's endomembrane system. The addition of an ER retrieval signal (SDEL) - a sequence recognized by the cis-Golgi protein ERD2 - to the C-terminus of an apicoplast-targeted protein did not markedly affect apicoplast targeting, further demonstrating that the apicoplast is upstream of the Golgi. Apicoplast transit peptides are thus dominant over an ER retention signal. However, when the transit peptide is rendered non-functional (by two point mutations or by complete deletion) SDEL-specific ER retrieval takes over, and the fusion protein is localized to the ER. We speculate either that the apicoplast in P. falciparum resides within the ER directly in the path of the general secretory pathway, or that vesicular trafficking to the apicoplast directly exits the ER.