A revised mechanism for how Plasmodium falciparum recruits and exports proteins into its erythrocytic host cell
Authors: Gabriela Mikha , Matthews Kathryn M. , Boshoven Cas , Kouskousis Betty , Steer David L. , Jonsdottir Thorey K. , Bullen Hayley E. , Sleebs Brad E. , Crabb Brendan S. , de Koning-Ward Tania F. , Gilson Paul R. 
Published: September 2021

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 are 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.

Author Summary

Plasmodium falciparum, the most lethal species of human malaria parasite, infects erythrocytes and develops within a parasitophorous vacuole. To support rapid parasite growth and immune evasion, the parasite remodels its erythrocyte by exporting a myriad of proteins into the erythrocyte compartment. Parasite proteins destined for export are first imported into the endoplasmic reticulum (ER) and then secreted into the parasitophorous vacuole, where they are translocated across the parasitophorous vacuole membrane into the erythrocyte via a protein-conducting channel called PTEX. A missing link in the story has been how proteins destined for export are specifically guided from the ER to PTEX at the parasitophorous vacuole membrane. In this study, we found that one of the core PTEX components, HSP101, resides within the parasite’s ER, in addition to its PTEX-related location at the parasitophorous vacuole. We also found that ER-located HSP101 can interact transiently with cargo proteins en route to the parasitophorous vacuole membrane. Our findings support a model in which HSP101 forms an initial interaction with exported proteins in the ER and then chaperones them to the rest of PTEX at the parasitophorous vacuole membrane for export into the erythrocyte.