An aspartyl protease directs malaria effector proteins to the host cell.

Plasmodium falciparum causes the virulent form of malaria and disease manifestations are linked to growth inside infected erythrocytes. To survive and evade host responses the parasite remodels the erythrocyte by exporting several hundred effector proteins beyond the surrounding parasitophorous vacuole membrane. A feature of exported proteins is a pentameric motif (RxLxE/Q/D) that is a substrate for an unknown protease. Here we show that the protein responsible for cleavage of this motif is plasmepsin V (PMV), an aspartic acid protease located in the endoplasmic reticulum. PMV cleavage reveals the export signal (xE/Q/D) at the amino terminus of cargo proteins. Expression of an identical mature protein with xQ at the N terminus generated by signal peptidase was not exported, demonstrating that PMV activity is essential and linked with other key export events. Identification of the protease responsible for export into erythrocytes provides a novel target for therapeutic intervention against this devastating disease.

Plasmodium falciparum erythrocyte membrane protein-1 specifically suppresses early production of host interferon-gamma.

Plasmodium falciparum erythrocyte membrane protein-1 (PfEMP-1) is a variable antigen expressed by P. falciparum, the malarial parasite. PfEMP-1, present on the surface of infected host erythrocytes, mediates erythrocyte binding to vascular endothelium, enabling the parasite to avoid splenic clearance. In addition, PfEMP-1 is proposed to regulate host immune responses via interactions with the CD36 receptor on antigen-presenting cells. We investigated the immunoregulatory function of PfEMP-1 by comparing host cell responses to erythrocytes infected with either wild-type parasites or transgenic parasites lacking PfEMP-1. We showed that PfEMP-1 suppresses the production of the cytokine interferon-gamma by human peripheral blood mononuclear cells early after exposure to P. falciparum. Suppression of this rapid proinflammatory response was CD36 independent and specific to interferon-gamma production by gammadelta-T, NK, and alphabeta-T cells. These data demonstrate a parasite strategy for downregulating the proinflammatory interferon-gamma response and further establish transgenic parasites lacking PfEMP-1 as powerful tools for elucidating PfEMP-1 functions.

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.

Re-defining the Golgi complex in Plasmodium falciparum using the novel Golgi marker PfGRASP.

Plasmodium falciparum, the causative agent of malaria, relies on a sophisticated protein secretion system for host cell invasion and transformation. Although the parasite displays a secretory pathway similar to those of all eukaryotic organisms, a classical Golgi apparatus has never been described. We identified and characterised the putative Golgi matrix protein PfGRASP, a homologue of the Golgi re-assembly stacking protein (GRASP) family. We show that PfGRASP is expressed as a 70 kDa protein throughout the asexual life cycle of the parasite. We generated PfGRASP-GFP-expressing transgenic parasites and showed that this protein is localised to a single, juxtanuclear compartment in ring-stage parasites. The PfGRASP compartment is distinct from the ER, restricted within the boundaries of the parasite and colocalises with the cis-Golgi marker ERD2. Correct subcellular localisation of this Golgi matrix protein depends on a cross-species conserved functional myristoylation motif and is insensitive to Brefeldin A. Taken together our results define the Golgi apparatus in Plasmodium and depict the morphological organisation of the organelle throughout the asexual life cycle of the parasite.

Characterisation of two novel proteins from the asexual stage of Plasmodium falciparum, H101 and H103.

The merozoite surface of the pathogenic malaria parasite Plasmodium falciparum is comprised of proteins that are important for the identification and invasion of human red cells. Merozoite surface protein (MSP)3 is a polymorphic protein associated with the surface of merozoites and is also a vaccine candidate. A distinct feature of the MSP3 sequence is three blocks of alanine-rich heptad repeats that are predicted to form an intramolecular coiled-coil. Three orthologues of MSP3 that also contain alanine-rich heptad repeats have been described in P. vivax and we therefore searched the P. falciparum genome database for MSP3 paralogues. We have identified two genes, H101 and H103 related to MSP3, however like another MSP3 paralogue, MSP6, H101 and H103 do not contain heptad repeats. H101 and H103 are expressed during the asexual cycle and immunofluorescence indicates H103 localises to the merozoite surface as a peripheral membrane protein. Transfected parasite lines that express truncated forms of H101 or H103 were viable and grew at the same rate as the parental parasite line. This result may reflect redundancy in function among members of the MSP3/MSP6 gene family as has been described for other families of paralogue genes in P. falciparum.

The alanine-rich heptad repeats are intact in the processed form of Plasmodium falciparum MSP3.

The potential of Plasmodium falciparum merozoite surface protein 3 as a component of an asexual-stage malaria vaccine is currently being assessed. The precursor form of MSP3 undergoes cleavage during schizogony to generate a mature processed form. It is unknown if this cleavage event is necessary for MSP3 function, but it may be an important consideration for assessing and developing MSP3 as an asexual-stage vaccine candidate. We have therefore determined the cleavage site in MSP3 by sequencing the N-terminus of the processed form of MSP3, which was isolated from parasite material. The position of the cleavage site indicates that the processed form of MSP3 retains the three blocks of alanine-rich heptad repeats, which are predicted to provide the structural framework for an intramolecular coiled-coil. The cleavage-site motif has many features in common with the published cleavage sites of MSP1(30), MSP6(36), and MSP7(22), which are all located on the merozoite surface and are implicated in the erythrocyte invasion process. The common cellular location and similar cleavage-site motifs suggest that these merozoite proteins may be cleaved by the same or related proteases.

Plasmodium falciparum merozoite surface protein 6 is a dimorphic antigen.

Merozoite surface protein 1 (MSP1) is a highly polymorphic Plasmodium falciparum merozoite surface protein implicated in the invasion of human erythrocytes during the asexual cycle. It forms a complex with MSP6 and MSP7 on the merozoite surface, and this complex is released from the parasite around the time of erythrocyte invasion. MSP1 and many other merozoite surface proteins contain dimorphic elements in their protein structures, and here we show that MSP6 is also dimorphic. The sequences of eight MSP6 genes indicate that the alleles of each dimorphic form of MSP6 are highly conserved. The smaller 3D7-type MSP6 alleles are detected in parasites from all malarious regions of the world, whereas K1-type MSP6 alleles have only been detected in parasites from mainland Southeast Asia. Cleavage of MSP6, which produces the p36 fragment in 3D7-type MSP6 and associates with MSP1, also occurs in K1-type MSP6 but at a different site in the protein. Anti-3D7 MSP6 antibodies weakly inhibited erythrocyte invasion by homologous 3D7 merozoites but did not inhibit a parasite line expressing the K1-type MSP6 allele. Antibodies from hyperimmune individuals affinity purified on an MSP3 peptide cross-reacted with MSP6; therefore, MSP6 may also be a target of antibody-dependent cellular inhibition.

Truncation of merozoite surface protein 3 disrupts its trafficking and that of acidic-basic repeat protein to the surface of Plasmodium falciparum merozoites.

Merozoite surface protein 3 (MSP3), an important vaccine candidate, is a soluble polymorphic antigen associated with the surface of Plasmodium falciparum merozoites. The MSP3 sequence contains three blocks of heptad repeats that are consistent with the formation of an intramolecular coiled-coil. MSP3 also contains a glutamic acid-rich region and a putative leucine zipper sequence at the C-terminus. We have disrupted the msp3 gene by homologous recombination, resulting in the expression of a truncated form of MSP3 that lacks the putative leucine zipper sequence but retains the glutamic acid-rich region and the heptad repeats. Here, we show that truncated MSP3, lacking the putative leucine zipper region, does not localize to the parasitophorous vacuole or interact with the merozoite surface. Furthermore, the acidic-basic repeat antigen (ABRA), which is present on the merozoite surface, also was not localized to the merozoite surface in parasites expressing the truncated form of MSP3. The P. falciparum merozoites lacking MSP3 and ABRA on the surface show reduced invasion into erythrocytes. These results suggest that MSP3 is not absolutely essential for blood stage growth and that the putative leucine zipper region is required for the trafficking of both MSP3 and ABRA to the parasitophorous vacuole.