Skeleton binding protein 1 localizes to the Maurer's cleft and interacts with PfHSP70-1 and PfHSP70-x in Plasmodium falciparum gametocyte-infected erythrocytes.

Plasmodium falciparum accounts for the majority of malaria deaths, due to pathology provoked by the ability of infected erythrocytes to adhere to vascular endothelium within deep tissues. The parasite recognizes endothelium by trafficking and displaying protein ligands on the surface of asexual stage infected erythrocytes, such as members of the large family of pathogenic proteins, P. falciparum erythrocyte membrane protein 1 (PfEMP1). Parasite-encoded skeleton binding protein 1 (SBP1) plays an important role in the transport of these binding-related surface proteins, via cleft-like membranous structures termed Maurer's clefts, which are present within the cytoplasm of infected erythrocytes. Erythrocytes infected with gametocyte stages accumulate in the extravascular compartment of bone marrow; and it was suggested that their surface-expressed adhesion molecule profile and protein trafficking mechanisms might differ from those in asexual stage parasites. Protein trafficking mechanisms via Maurer's clefts have been well investigated in asexual stage parasite-infected erythrocytes; but little is known regarding the gametocyte stages. In this study, we characterized SBP1 during gametocyte maturation and demonstrated that SBP1 is expressed and localizes to dot-like Maurer's cleft structures in the cytoplasm of gametocyte-infected erythrocytes. Co-immunoprecipitation and mass spectrometry assays indicated that SBP1 interacts with the molecular chaperones PfHSP70-1 and PfHSP70-x. Localization analysis suggested that some PfHSP70-1 and/or PfHSP70-x localize in a dot-like pattern within the cytoplasm of immature gametocyte-infected erythrocytes. These findings suggest that SBP1 may interact with HSP70 chaperones in the infected erythrocyte cytoplasm during the immature gametocyte stages.

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.