Identification of a non-exported Plasmepsin V substrate that functions in the parasitophorous vacuole of malaria parasites

Malaria parasites alter multiple properties of the host erythrocyte by exporting proteins into the host cell. Many exported proteins contain a five-amino acid motif called the Plasmodium Export Element (PEXEL) that is cleaved by the parasite protease Plasmepsin V (PM V). The presence of a PEXEL is considered a signature of protein export and has been used to identify a large number of exported proteins. The export of proteins becomes essential midway through the intraerythrocytic cycle - preventing protein export blocks parasite development 18-24 h after invasion. However, a genetic investigation revealed that the absence of the PEXEL protein PFA0210c causes parasite development to arrest immediately after invasion. We now show that this protein (renamed PV6) is cleaved by PM V but not exported into the host erythrocyte and instead functions in the parasitophorous vacuole. Furthermore, we show that the lysine residue that becomes the N terminus of PV6 after processing by PM V prevents export. This is the first example of a native Plasmodium falciparum PM V substrate that remains in the parasitophorous vacuole. We also provide evidence that the parasite produces at least one additional essential, non-exported PM V substrate. These results reveal that the presence of a PEXEL, and hence processing of a protein by PM V, does not always target a protein for export and that PM V has a broader function in parasite growth beyond processing exported proteins. Furthermore, we utilized this finding to investigate possible requirements for protein export further.