Export of parasite proteins to the erythrocyte cytoplasm: secretory machinery and traffic signals

Ticket to ride: export of proteins to the Plasmodium falciparum-infected erythrocyte

The malaria parasite Plasmodium falciparum exports numerous proteins to its chosen host cell, the mature human erythrocyte. Many of these proteins are important for parasite survival. To reach the host cell, parasites must cross multiple membrane barriers and then furthermore be targeted to their correct sub-cellular localisation. This novel transport pathway has received much research attention in the past decades, especially as many of the mechanisms are expected to be parasite-specific and thus potential targets for drug development. In this article we summarize some of the most recent advances in this field, and highlight areas in which further research is needed.

[Protein traffic in Plasmodium infected-red blood cells]

To survive within erythrocytes, Plasmodium parasites have to put into place different membrane and sub-cellular compartments in order to import different nutrients and to export proteins/antigens. Infected cells pose not only a major world health risk by killing two million people per year, but also a very interesting cell biology problem, as within the erythrocyte the parasite resides inside a vacuole called the parasitophorous vacuole and as a consequence, it is separated from the blood stream by three membrane barriers, its own plasma membrane, the parasitophorous vacuole membrane and the erythrocyte plasma membrane. In spite of these three barriers the parasite is capable of secreting antigens and importing nutrients, and to do this, it has developed a complex vesicular system that extends into the red blood cell cytoplasm to the plasma membrane. Understanding how the parasite controls this extensive vesicular traffic has driven research into Plasmodium Rabs, whose potential role is discussed.

Correct promoter control is needed for trafficking of the ring-infected erythrocyte surface antigen to the host cytosol in transfected malaria parasites

Following invasion of human erythrocytes, the malaria parasite, Plasmodium falciparum, exports proteins beyond the confines of its own plasma membrane to modify the properties of the host red cell membrane. These modifications are critical to the pathogenesis of malaria. Analysis of the P. falciparum genome sequence has identified a large number of molecules with putative atypical signal sequences. The signals remain poorly characterized; however, a number of molecules with these motifs localize to the host erythrocyte. To examine the role of these atypical signal sequences in the export of parasite proteins, we have generated transfected parasites expressing a chimeric protein comprising the N-terminal region of the P. falciparum ring-infected erythrocyte surface antigen (RESA) appended to green fluorescent protein (GFP). This N-terminal region contains a hydrophobic stretch of amino acids that is presumed to act as a noncanonical secretory signal sequence. Modulation of the timing of transgene expression demonstrates that trafficking of malaria proteins into the host erythrocyte is dependent on both the presence of an appropriate transport signal and the timing of expression. Transgene expression under the control of a trophozoite-specific promoter mistargets the chimeric molecule to the parasitophorous vacuole surrounding the parasite. However, expression of RESA-GFP in schizont stages, under the control of the RESA promoter, enables correct trafficking of a population of the chimeric protein to the host erythrocyte.

Maurer’s clefts—a novel secretory organelle?

During intra-erythrocytic development, the human malarial parasite Plasmodium falciparum extensively remodels its adopted cellular home by exporting proteins beyond the confines of its own plasma membrane, but is, however, faced with a major problem: the lack of an endogenous protein trafficking machinery within the host erythrocyte. Thus, in order to export proteins the parasite has to install its own protein export system within the host erythrocyte. A growing body of evidence suggests that Maurer's clefts, parasite-derived membranous structures in the cytosol of the host cell, are a crucial component of this protein sorting and trafficking machinery. In this review we summarize our current understanding of the ultra-structure of Maurer's clefts and their role in protein transport process.

etramps, a new Plasmodium falciparum gene family coding for developmentally regulated and highly charged membrane proteins located at the parasite-host cell interface

After invasion of erythrocytes, the human malaria parasite Plasmodium falciparum resides within a parasitophorous vacuole and develops from morphologically and metabolically distinct ring to trophozoite stages. During these developmental phases, major structural changes occur within the erythrocyte, but neither the molecular events governing this development nor the molecular composition of the parasitophorous vacuole membrane (PVM) is well known. Herein, we describe a new family of highly cationic proteins from P. falciparum termed early transcribed membrane proteins (ETRAMPs). Thirteen members were identified sharing a conserved structure, of which six were found only during ring stages as judged from Northern and Western analysis. Other members showed different stage-specific expression patterns. Furthermore, ETRAMPs were associated with the membrane fractions in Western blots, and colocalization and selective permeabilization studies demonstrated that ETRAMPs were located in the PVM. This was confirmed by immunoelectron microscopy where the PVM and tubovesicular extensions of the PVM were labeled. Early expressed ETRAMPs clearly defined separate PVM domains compared with the negatively charged integral PVM protein EXP-1, suggesting functionally different domains in the PVM with an oppositely charged surface coat. We also show that the dynamic change of ETRAMP composition in the PVM coincides with the morphological changes during development. The P. falciparum PVM is an important structure for parasite survival, and its analysis might provide better understanding of the requirements of intracellular parasites.

Evidence for a role for a Plasmodium falciparum homologue of Sec31p in the export of proteins to the surface of malaria parasite-infected erythrocytes

The malaria parasite, Plasmodium falciparum, spends part of its life cycle inside the enucleated erythrocytes of its human host. The parasite modifies the cytoplasm and plasma membrane of its host cell by exporting proteins beyond the confines of its own plasma membrane. We have previously provided evidence that a plasmodial homologue of the COPII protein, Sar1p, is involved in the trafficking of proteins across the erythrocyte cytoplasm. We have now characterised an additional plasmodial COPII protein homologue, namely Sec31p. Recombinant proteins corresponding to the WD-40 and the intervening domains of the PfSec31p sequence were used to raise antibodies. The affinity-purified antisera recognised a protein with an apparent relative molecular mass of 1.6×105 on western blots of malaria parasite-infected erythrocytes but not on blots of uninfected erythrocytes. PfSec31p was shown to be largely insoluble in nonionic detergent, suggesting cytoskeletal attachment. Confocal immunofluorescence microscopy of malaria parasite-infected erythrocytes was used to show that PfSec31p is partly located within the parasite and partly exported to structures outside the parasite in the erythrocyte cytoplasm. We have also shown that PfSec31p and PfSar1p occupy overlapping locations. Furthermore, the location of PfSec31p overlaps that of the cytoadherence-mediating protein PfEMP1. These data support the suggestion that the malaria parasite establishes a vesicle-mediated trafficking pathway outside the boundaries of its own plasma membrane – a novel paradigm in eukaryotic biology.

The malaria-infected red blood cell: structural and functional changes

The asexual stage of malaria parasites of the genus Plasmodium invade red blood cells of various species including humans. After parasite invasion, red blood cells progressively acquire a new set of properties and are converted into more typical, although still simpler, eukaryotic cells by the appearance of new structures in the red blood cell cytoplasm, and new proteins at the red blood cell membrane skeleton. The red blood cell undergoes striking morphological alterations and its rheological properties are considerably altered, manifesting as red blood cells with increased membrane rigidity, reduced deformability and increased adhesiveness for a number of other cells including the vascular endothelium. Elucidation of the structural changes in the red blood cell induced by parasite invasion and maturation and an understanding of the accompanying functional alterations have the ability to considerably extend our knowledge of structure-function relationships in the normal red blood cell. Furthermore, interference with these interactions may lead to previously unsuspected means of reducing parasite virulence and may lead to the development of novel antimalarial therapeutics.