Skeleton binding protein 1 (SBP1) of Plasmodium falciparum accumulates in electron-dense material before passing through the parasitophorous vacuole membrane

Plasmodium falciparum proteins involved in vascular endothelial cell adherence are transported to the surface of infected erythrocytes. These proteins are exported through parasite-derived membrane structures within the erythrocyte cytoplasm called Maurer's clefts. Skeleton binding protein 1 (SBP1) is localized in the Maurer's clefts and plays an important role in transporting molecules to the surface of infected erythrocytes. Details of the translocation pathway are unclear and in this study we focused on the subcellular localization of SBP1 at an early intraerythrocytic stage. We performed immunoelectron microscopy using specific anti-SBP1 antibodies generated by immunization with recombinant SBP1 of P. falciparum. At the early trophozoite (ring form) stage, SBP1 was detected within an electron dense material (EDM) found in the parasite cytoplasm and in the parasitophorous vacuolar (PV) space. These findings demonstrate that SBP1 accumulates in EDM in the early trophozoite cytoplasm and is transported to the PV space before translocation to the Maurer's clefts formed in the erythrocyte cytoplasm.