Vesicle-mediated trafficking of parasite proteins to the host cell cytosol and erythrocyte surface membrane in Plasmodium falciparum infected erythrocytes

During the development of the asexual stage of the malaria parasite, Plasmodium falciparum, the composition, structure and function of the host cell membrane is dramatically altered, including the ability to adhere to vascular endothelium. Crucial to these changes is the transport of parasite proteins, which become associated with or inserted into the erythrocyte membrane. Protein and membrane targeting beyond the parasite plasma membrane must require unique pathways, given the parasites intracellular location within a parasitophorous vacuolar membrane and the lack of organelles and biosynthetic machinery in the host cell necessary to support a secretory system. It is not clear how these proteins cross the parasitophorous vacuolar membrane or how they traverse the erythrocyte cytosol to reach their final destinations. The identification of: (1) a P. falciparum homologue of the protein Sar1p, which is an essential component of the COPII-based secretory system in mammalian cells and yeast and (2) electron-dense, possibly coated, secretory vesicles bearing P. falciparum erythrocyte membrane protein 1 and P. falciparum erythrocyte membrane protein 3 in the host cell cytosol of P. falciparum infected erythrocytes recently provided the first direct evidence of a vesicle-mediated pathway for the trafficking of some parasite proteins to the erythrocyte membrane. The major advance in uncovering the parasite-induced secretory pathway was made by incubating infected erythrocytes with aluminium tetrafluoride, an activator of guanidine triphosphate-binding proteins, which resulted in the accumulation of the vesicles into multiple vesicle strings. These vesicle complexes were often associated with and closely abutted the erythrocyte membrane, but were apparently prevented from fusing by the aluminium fluoride treatment, making their capture by electron microscopy possible. It appears that malaria parasites export proteins into the host cell cytosol to support a vesicle-mediated protein trafficking pathway.