Trends in Parasitology
Protein trafficking in Plasmodium falciparum-infected red blood cells
Section snippets
Trafficking of proteins to the parasitophorous vacuole
Trafficking of proteins within the confines of the parasite appears to involve most of the elements of a classical vesicle-mediated secretory pathway. Brefeldin A (BFA) is a drug that specifically blocks classical vesicle-mediated trafficking by interfering with the Plasmodium ADP ribosylation factor GDP–GTP exchange protein [a key molecule required for the assembly of coat protein (COP), which coats the transport vesicles] [6] and prevents secretion of most exported proteins examined so far.
Trafficking of proteins across the PVM
Attempts to analyze the signal sequences of exported parasite proteins have revealed some rather unusual motifs. Proteins destined for sites in the ER, parasite PM, PV, PVM and apical organelles appear to have classical hydrophobic N-terminal signal sequences (i.e. a stretch of ∼15 hydrophobic amino acids commencing three to 17 amino acids from the N-terminus) 24, 25, 26. By contrast, several proteins that are directed past the PVM to the RBC cytosol have a longer (up to 30 amino acids)
Exported membrane structures in the host RBC cytosol
The mature human RBC has no architecture of intracellular membranes; thus the parasite needs to set up its own transport system. Proteins, such as KAHRP, HRP-2 and MESA, have been observed in large membrane-free aggregates in the RBC cytosol 30, 39, 40. These proteins could transit the PV as unfolded polypeptides, then refold and form complexes that diffuse across the host RBC compartment, and spontaneously assemble at the appropriate destinations as a result of interactions with RBC proteins.
Trafficking machinery in the host RBC cytosol
The membranous structures in the RBC cytosol are assumed to be involved in trafficking of membrane-associated proteins. However, the mechanics of the trafficking process still need to be defined. Given that the RBC lacks the endogenous coat proteins needed for vesicle-mediated transport, the parasite must set up its own budding and fusion machinery. In this context, it is of some interest that components of the Plasmodium COPII complex have been reported to be exported to the host cell cytosol.
The role of Maurer's clefts in protein trafficking
MCs are thought to be an important transit depot for PfEMP-1 en route to the RBC membrane. Transit of newly synthesized protein to the pRBC surface requires about nine hours, and a significant proportion of the PfEMP-1 population appears to remain in an as yet poorly identified intracellular pool (most likely associated with MCs) [45]. PfEMP-1 is inserted into the MC membrane with the C-terminal domain exposed to the RBC cytoplasm and the N-terminal domain buried inside the cleft [45]. The
Future directions
Over the past few years, our understanding of the cell biological processes underlying host RBC modification by P. falciparum has advanced considerably. This is largely because of the complete sequencing and annotation of the P. falciparum genome, the development of transfection systems for asexual-stage parasites, highly sophisticated morphological analyses, and the identification of new parasite proteins exported into the RBC and their interactions with other RBC and parasite proteins.
Acknowledgements
We acknowledge funding from the National Health and Medical Research Council of Australia (B.M.C. and L.T.), The National Institutes of Health (B.M.C.), The Wellcome Trust (L.H.B.) and the Deutsche Forschungsgemeinschaft (K.L.). We thank John Hopkins for electron micrographic assistance.
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