Skeleton-binding protein 1 functions at the parasitophorous vacuole membrane to traffic PfEMP1 to the Plasmodium falciparum–infected erythrocyte surface

Abstract

A key feature of Plasmodium falciparum, the parasite causing the most severe form of malaria in humans, is its ability to export parasite molecules onto the surface of the erythrocyte. The major virulence factor and variant surface protein PfEMP1 (P falciparum erythrocyte membrane protein 1) acts as a ligand to adhere to endothelial receptors avoiding splenic clearance. Because the erythrocyte is devoid of protein transport machinery, the parasite provides infrastructure for trafficking across membranes it traverses. In this study, we show that the P falciparum skeleton-binding protein 1 (PfSBP1) is required for transport of PfEMP1 to the P falciparum-infected erythrocyte surface. We present evidence that PfSBP1 functions at the parasitophorous vacuole membrane to load PfEMP1 into Maurer clefts during formation of these structures. Furthermore, the major reactivity of antibodies from malaria-exposed multigravid women is directed toward PfEMP1 because this is abolished in the absence of PfSBP1.

Export of PfSBP1 to the Plasmodium falciparum Maurer’s Clefts

“…Mutating residues 2-9, 10-14, 21-26, 27-31 and 32-35, respectively, did not affect trafficking of the chimeric protein to the Maurer's clefts ( Figure 5B). In comparison, introducing alanines at positions 16-20 (A [16][17][18][19][20] seemed to reduce the trafficking efficiency. Now, fluorescence was seen in a dotted pattern surrounding the parasite body and in the Maurer's clefts ( Figure 5B).…”

“…In comparison to SBP full and the endogenous PfSBP1, the chimeric proteins A [16][17][18][19][20][21][22][23][24][25][26] and SBP Tm behaved differently. A [16][17][18][19][20][21][22][23][24][25][26] was exclusively present in the SLO pellet fraction ( Figure 8C). Some protein could be released into the supernatant after SLO/SAP treatment of the infected erythrocytes, albeit the vast majority of the protein was found in the SLO/SAP pellet fraction ( Figure 8C).…”

“…To test this hypothesis, we extended the alanine replacements creating A [10][11][12][13][14][15][16][17][18][19][20] and A [16][17][18][19][20][21][22][23][24][25][26] ( Figure 5A). A [10][11][12][13][14][15][16][17][18][19][20] exhibited the same partial phenotype as did A [16][17][18][19][20] ( Figure 5B). In comparison, substituting amino acids 16-26 by alanines ( Figure 5B) completely abrogated trafficking of the corresponding protein to the Maurer's clefts.…”

“…These proteins are all expressed early during intraerythrocytic development and seem to be accessory or structural components of the trafficking machinery. For example, knockout studies have shown that PfSBP1 and MAHRP-1 are necessary for the trafficking of the immunovariant adhesin PfEMP1 to the erythrocyte surface (18)(19)(20). PfREX-1 seems to play a role in Maurer's cleft morphology (21).…”

Maurer's clefts, the enigma of Plasmodium falciparum

“…Gene-deletion technology made it more possible to study the function of specific genes. First, knock out experiments eliminating the Maurer's clefts proteins MAHRP1 or SBP1 surprisingly revealed that PfEMP1 is not exported to the surface of the infected erythrocyte anymore (41,121,122). This finding was the first proof that Maurer's clefts have a function as an intermediate compartment in transport or sorting of proteins, at least for those destined for the erythrocyte membrane.…”