Plasmodium falciparum Exported Protein 1 is localized to dense granules in merozoites

Apical organellar proteins in Plasmodium falciparum merozoites play important roles upon invasion. To date, dense granule, the least studied apical organelle, secretes parasite proteins across the parasitophorous vacuole membrane (PVM) to remodel the infected erythrocyte. Although this phenomenon is key to parasite growth and virulence, only five proteins so far have been identified as dense granule proteins. Further elucidation of dense granule molecule(s) is therefore required. P. falciparum Exported Protein (EXP) 1, previously reported as a parasitophorous vacuole membrane (PVM) protein, is considered essential for parasite growth. In this study, we characterized EXP1 using specific anti-EXP1 antibodies generated by immunization of wheat germ cell-free produced recombinant EXP1. Immunofluorescence microscopy (IFA) demonstrated that EXP1 co-localized with RESA, indicating that the protein is initially localized to dense granules in merozoites, followed by translocation to the PVM. The EXP1 localization in dense granule of merozoites and its translocation to the PVM after invasion of erythrocytes were further confirmed by immunoelectron microscopy. Here, we demonstrate that EXP1 is one of the dense granule proteins in merozoites, which is then transported to the PVM after invasion.