Targeted mutagenesis of the ring-exported protein-1 ofPlasmodium falciparumdisrupts the architecture of Maurer's cleft organelles

Ultrastructure of the asexual blood stages of Plasmodium falciparum

Plasmodium falciparum is the most deadly of the human malaria parasites. The particular virulence of this species derives from its ability to subvert the physiology of its host during the blood stages of its development. The parasite grows and divides within erythrocytes, feeding on the hemoglobin, and remodeling its host cells so they adhere to blood vessel walls. The advent of molecular transfection technology, coupled with optical microscopy of fluorescent protein reporters, has greatly improved our understanding of the ways in which the malaria parasite alters its host cell. However, a full interpretation of the information from these studies requires similar advances in our knowledge of the ultrastructure of the parasite. Here we give an overview of different electron microscopy techniques that have revealed the fine structure of the parasite at different stages of development. We present data on some of the unusual organelles of P. falciparum, in particular, the membrane structures that are elaborated in the erythrocyte cytoplasm and are thought to play an important role in trafficking of virulence proteins. We present and discuss some of the exciting whole cell imaging techniques that represent a new frontier in the studies of parasite ultrastructure.

Protein export in malaria parasites: do multiple export motifs add up to multiple export pathways?

Intracellular malaria parasites export numerous proteins into their host cell, a process essential for parasite survival and virulence. Many of these proteins are defined by a short amino acid sequence motif termed PEXEL or VTS that mediates their export, suggesting a collective trafficking route. The existence of several PEXEL-negative exported proteins (PNEPs) indicates that alternative export pathways might also exist. We review recent data on the sequences mediating export of PNEPs and compare this process to PEXEL export taking into account novel findings on the function of this motif. Based on this we propose that, despite the lack of a PEXEL in PNEPs, both groups of proteins might converge in a single export pathway on their way into the host cell.

Synthetic peptides from conserved regions of the Plasmodium falciparum early transcribed membrane and ring exported proteins bind specifically to red blood cell proteins

Severe malaria pathology is directly associated with cytoadherence of infected red blood cells (iRBCs) to healthy RBCs and/or endothelial cells occurring during the intraerythrocytic development of Plasmodium falciparum. We synthesized, as 20-mer long peptides, the members of the ring exported (REX) protein family encoded in chromosome 9, as well as the early transcribed membrane proteins (E-TRAMP) 10.2 and 4, to identify specific RBC binding regions in these proteins. Twelve binding peptides were identified (designated as HABPs): three were identified in REX1, two in REX2, one in REX3, two in REX4 and four in E-TRAMP 10.2. The majority of these HABPs was conserved among different P. falciparum strains, according to sequence analysis. No HABPs were found in E-TRAMP 4. Bindings of HABPs were saturable and sensitive to the enzymatic treatment of RBCs and HABPs had different structural features, according to circular dichroism studies. Our results suggest that the REX and E-TRAMP families participate in relevant interactions with RBC membrane proteins, which highlight these proteins as potential targets for the development of fully effective immunoprophylactic methods.

The role of the Maurer's clefts in protein transport in Plasmodium falciparum

Maurer's clefts (MCs) are membranous structures that are formed by Plasmodium falciparum and used by the parasite for protein sorting and protein export. Virulence proteins, as well as other proteins used to remodel the erythrocyte, are exported. Discontinuity between major membrane compartments within the infected erythrocyte cytoplasm suggests multiple traffic routes for exported proteins. The sequences of the conserved Plasmodium export element seem insufficient for export of all parasite proteins. The parasite displays remarkable versatility in the types of proteins exported to the MCs and in the functions of the proteins within the MCs. In this Review, protein export to the MCs and the role of the MCs in the transport of proteins to the erythrocyte membrane are summarized.

Malaria parasite proteins that remodel the host erythrocyte

Exported proteins of the malaria parasite Plasmodium falciparum interact with proteins of the erythrocyte membrane and induce substantial changes in the morphology, physiology and function of the host cell. These changes underlie the pathology that is responsible for the deaths of 1-2 million children every year due to malaria infections. The advent of molecular transfection technology, including the ability to generate deletion mutants and to introduce fluorescent reporter proteins that track the locations and dynamics of parasite proteins, has increased our understanding of the processes and machinery for export of proteins in P. falciparum-infected erythrocytes and has provided us with insights into the functions of the parasite protein exportome. We review these developments, focusing on parasite proteins that interact with the erythrocyte membrane skeleton or that promote delivery of the major virulence protein, PfEMP1, to the erythrocyte membrane.

Functional alteration of red blood cells by a megadalton protein of Plasmodium falciparum

Proteins exported from Plasmodium falciparum parasites into red blood cells (RBCs) interact with the membrane skeleton and contribute to the pathogenesis of malaria. Specifically, exported proteins increase RBC membrane rigidity, decrease deformability, and increase adhesiveness, culminating in intravascular sequestration of infected RBCs (iRBCs). Pf332 is the largest (>1 MDa) known malaria protein exported to the RBC membrane, but its function has not previously been determined. To determine the role of Pf332 in iRBCs, we have engineered and analyzed transgenic parasites with Pf332 either deleted or truncated. Compared with RBCs infected with wild-type parasites, mutants lacking Pf332 were more rigid, were significantly less adhesive to CD36, and showed decreased expression of the major cytoadherence ligand, PfEMP1, on the iRBC surface. These abnormalities were associated with dramatic morphologic changes in Maurer clefts (MCs), which are membrane structures that transport malaria proteins to the RBC membrane. In contrast, RBCs infected with parasites expressing truncated forms of Pf332, although still hyperrigid, showed a normal adhesion profile and morphologically normal MCs. Our results suggest that Pf332 both modulates the level of increased RBC rigidity induced by P falciparum and plays a significant role in adhesion by assisting transport of PfEMP1 to the iRBC surface.