Characterization of Host Cell Cytosol-filled Vesicles in the Human Malaria Parasite Plasmodium falciparum

The human malaria parasite Plasmodium falciparum (P. falciparum) goes through various blood stages, with endocytosis of host cell cytosol being a crucial process. This material, primarily comprising haemoglobin (Hb), is transported to the parasite's digestive vacuole (DV) for degradation. However, the specific endosomal transport pathway involved is not fully understood.

Inactivating certain proteins results in the presence of Hb-filled vesicles (HbVs) in the parasite's cytoplasm, preventing Hb from reaching the DV. This suggests these proteins are involved in endosomal transport. This report details a study focusing on the morphology and surface proteome of HbVs. CryoET revealed HbVs to be coatless, double-membraned vesicles with a narrow inter-membrane space lacking larger protein densities.

Surface proteomes generated by BioIDs predominantly detected DV proteases. Further investigation confirmed this protease as a bona fide HbV cargo. The BioID also identified several vesicle trafficking proteins, such as PfTBC8 and Rab11 proteins. Functional analysis of PfTBC8 highlighted its importance in Hb transport to the DV, as demonstrated by a novel phenotype characterized by Hb-filled structures accumulating in the cytoplasm and a decline in DV protease trafficking.

This was attributed to a defect possibly due to the disruption of Rab11a localization, which may act as a Rab11a effector in this endosomal transport pathway. The presence of VPS35 in the BioID and evidence of a disrupted inner HbV membrane in some vesicles supports a model where HbVs undergo maturation before fusing with the DV.

In conclusion, this study delineates the morphological and molecular characteristics of HbVs, identifies the cargo transported via this pathway, and uncovers a previously unrecognized Rab-regulatory component crucial for endosomal trafficking in P. falciparum blood-stage parasites.