Abstract
The secretory pathway in Plasmodium falciparum has evolved to transport proteins to the host cell membrane and to an endosymbiotic organelle, the apicoplast. The latter can occur via the ER or the ER-Golgi route. Here, we study these three routes using proteins Erythrocyte Membrane Protein-1 (PfEMP1), Acyl Carrier Protein (ACP) and glutathione peroxidase-like thioredoxin peroxidase (PfTPxGl) and inhibitors of vesicular transport. As expected, the G protein-dependent vesicular fusion inhibitor AlF4− and microtubule destabilizing drug vinblastine block the trafficking of PfEMP-1, a protein secreted to the host cell membrane. However, while both PfTPxGl and ACP are targeted to the apicoplast, only ACP trafficking remains unaffected by these treatments. This implies that G protein-dependent vesicles do not play a role in classical apicoplast protein targeting. Unlike the soluble protein ACP, we show that PfTPxGl is localized to the outermost membrane of the apicoplast. Thus, the parasite apicoplast acquires proteins via two different pathways: first, the vesicular trafficking pathway appears to handle not only secretory proteins, but an apicoplast membrane protein, PfTPxGl; second, trafficking of apicoplast luminal proteins appear to be independent of G protein-coupled vesicles.
Highlights
Plasmodium falciparum parasites export proteins to the plasma membrane of host erythrocytes, cells that do not possess their own trafficking machinery
The nature of the signals on these proteins and the signals on different types of vesicles that dictate the choice of the trafficking routes emanating from the endoplasmic reticulum (ER) is an avenue for future research. One such signal to direct apicoplast proteins through the Golgi could be membrane localization: here we show that PfTPxGl isassociated with the outermost membrane of apicoplasts, suggesting that, unlike luminal proteins, the protein is trafficked on vesicular membranes
AlF4− binds to the Gα subunit of G proteins by mimicking the γ -phosphate group of GTP; as a result, the heterotrimeric G protein remains in an active state even after GTP is hydrolysed to GDP (Chabre, 1990; Finazzi et al, 1994; Kahn, 1991)
Summary
Plasmodium falciparum parasites export proteins to the plasma membrane of host erythrocytes, cells that do not possess their own trafficking machinery. In order to do so, the parasite extensively modifies the host cell to make a favorable niche for survival (Moxon, Grau & Craig, 2011). The parasite can, be considered a major secretory cell. Proteins are targeted to their destinations by the endomembrane system, starting with the proteins’ entry into the endoplasmic reticulum (ER), a process facilitated by N-terminal signal sequences that are usually hydrophobic in nature. From the ER, proteins are sent to the Golgi and further to their final destinations. In P. falciparum, the ER consists of a tubular, interconnected network that surrounds the nucleus, while the
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