A revised mechanism for how Plasmodium falciparum recruits and exports proteins into its erythrocytic host cell

Abstract

Plasmodium falciparum exports ~10% of its proteome into its host erythrocyte to modify the host cell’s physiology. The Plasmodium export element (PEXEL) motif contained within the N-terminus of most exported proteins directs the trafficking of those proteins into the erythrocyte. To reach the host cell, the PEXEL motif of exported proteins is processed by the endoplasmic reticulum (ER) resident aspartyl protease plasmepsin V. Then, following secretion into the parasite-encasing parasitophorous vacuole, the mature exported protein must be unfolded and translocated across the parasitophorous vacuole membrane by the Plasmodium translocon of exported proteins (PTEX). PTEX is a protein-conducting channel consisting of the pore-forming protein EXP2, the protein unfoldase HSP101, and structural component PTEX150. The mechanism of how exported proteins are specifically trafficked from the parasite’s ER following PEXEL cleavage to PTEX complexes on the parasitophorous vacuole membrane is currently not understood. Here, we present evidence that EXP2 and PTEX150 form a stable subcomplex that facilitates HSP101 docking. We also demonstrate that HSP101 localises both within the parasitophorous vacuole and within the parasite’s ER throughout the ring and trophozoite stage of the parasite, coinciding with the timeframe of protein export. Interestingly, we found that HSP101 can form specific interactions with model PEXEL proteins in the parasite ER, irrespective of their PEXEL processing status. Collectively, our data suggest that HSP101 recognises and chaperones PEXEL proteins from the ER to the parasitophorous vacuole and given HSP101’s specificity for the EXP2-PTEX150 subcomplex, this provides a mechanism for how exported proteins are specifically targeted to PTEX for translocation into the erythrocyte.

Plasmodium falciparum Molecular Chaperones: Guardians of the Malaria Parasite Proteome and Renovators of the Host Proteome

“…However, there is some evidence emerging that suggests that PfHsp70-2 and PfHsp70-x (and potentially other chaperones/co-chaperones such as PfJDPs) may collaborate with the core threading machinery of PTEX, a class I AAA + ATPase (PfHsp101; Russo et al, 2010 ; Matthews et al, 2019 ) in the chaperoning of exported P. falciparum proteins. For example, it has been shown that PfHsp101 is localized to the ER and the PV ( Russo et al, 2010 ), and is able to preferentially associate with certain PEXEL-containing proteins within these compartments ( Gabriela et al, 2022 ). PfHsp70-2 has been shown to not only interact with proteins secreted into the PV, but also with exported proteins, including the main virulence factor P. falciparum erythrocyte membrane protein 1 (PfEMP1; Saridaki et al, 2008 ; Batinovic et al, 2017 ; Cortés et al, 2020 ).…”

“…As mentioned in the previous section, two of the exported PfJDPs (PFA0660w and PFE0055c) associate with PfHsp70-x in J-dots within the erythrocyte cytosol ( Külzer et al, 2010 ; Külzer et al, 2012 ; Grover et al, 2013 ; Petersen et al, 2016 ), and have been shown to be co-chaperones of PfHsp70-x ( Daniyan et al, 2016 ; Dutta et al, 2021b ). It has been proposed that these J-dots play a role in the trafficking and folding of exported proteins ( Külzer et al, 2012 ; Behl et al, 2019 ; Gabriela et al, 2022 ). Interestingly, one of the J-Dot PfJDPs, PFE0055c, was found to be essential ( Zhang et al, 2018 ), while the other (PFA0660w) was not.…”

“…Furthermore, a key phase in the pathology of malaria, is the invasion of host erythrocytes by the parasite, which it completely remodels by exporting over 400 parasite proteins, including a substantial proportion (∼5%) of molecular chaperones ( Cortés et al, 2020 ). This massive renovation of the host cell potentially requires unique protein folding pathways involving both parasite and host molecular chaperones ( Pesce and Blatch, 2014 ; Gabriela et al, 2022 ). This review will critique the evidence indicating that heat shock proteins serving as molecular chaperones, especially Hsp70 and Hsp40 (also called J domain proteins, JDPs) families, are highly adapted to maintaining the structural and functional integrity of the proteomes of the parasite and potentially the host erythrocyte.…”

The Medicines for Malaria Venture Malaria Box contains inhibitors of protein secretion in Plasmodium falciparum blood stage parasites

“…As MMV396797 appeared by microscopy to block protein export within the parasite, it seems unlikely that this compound targets PTEX150 or EXP2, which reside at the PVM. While HSP101 is dually located at the PVM and the parasite ER 46 , the relative lack of PEXEL protein blocked in the PV space after MMV396797 treatment suggests that it is unlikely to also be the target of this compound.…”

“…The importance of protein export for parasite survival is demonstrated by knocking-down expression of PTEX proteins which results in rapid parasite death 37,39 . HSP101 appears to first recognise protein cargos in the ER and accompany them to PTEX for translocation into the RBC indicating HSP101 inhibitors might result in protein cargoes becoming trapped in the ER and parasitophorous vacuole (PV) 45,46 .…”