Abstract
Intraerythrocytic malaria parasites reside within a parasitophorous vacuole membrane (PVM) that closely overlays the parasite plasma membrane (PPM) and constitutes the barrier between parasite and host compartments. The PVM is the site of several essential transport activities but the basis for organization of this membrane system is unknown. We utilized the second-generation promiscuous biotin ligase BioID2 fused to EXP2 or HSP101 to probe the content of the PVM, identifying known and novel candidate PVM proteins. Among the best represented hits were members of a group of single-pass integral membrane proteins that constitute a major component of the PVM proteome but whose function remains unclear. We investigated the function of EXP1, the longest known member of this group, by adapting a CRISPR/Cpf1 genome editing system to install the TetR-DOZI-aptamers system for conditional translational control. EXP1 knockdown was essential for intraerythrocytic development and accompanied by profound changes in vacuole ultrastructure, including increased separation of the PVM and PPM and formation of abnormal membrane structures in the enlarged vacuole lumen. While previous in vitro studies indicated EXP1 possesses glutathione S-transferase activity, a mutant version of EXP1 lacking a residue important for this activity in vitro still provides substantial rescue of endogenous exp1 knockdown in vivo. Intriguingly, while activity of the Plasmodium translocon of exported proteins was not impacted by depletion of EXP1, the distribution of the translocon pore-forming protein EXP2 was substantially altered. Collectively, our results reveal a novel PVM defect that indicates a critical role for EXP1 in maintaining proper PVM organization.
Importance Like other obligate intracellular apicomplexans, blood-stage malaria parasites reside within a membrane-bound compartment inside the erythrocyte known as the parasitophorous vacuole. Although the vacuole is the site of several transport activities essential to parasite survival, little is known about its organization. To explore vacuole biology, we adopted recently developed proteomic (BioID2) and genetic (CRISPR/Cpf1) tools for use in Plasmodium falciparum, which allowed us to query the function of the prototypical vacuole membrane protein EXP1.
Knockdown of EXP1 showed that a previously reported glutathione S-transferase activity cannot fully account for the essential function(s) of EXP1 and revealed a novel role for this protein in maintaining normal vacuole morphology and PVM protein arrangement. Our results provide new insight into vacuole organization and illustrate the power of BioID2 and Cpf1 (which utilizes a T-rich PAM uniquely suited to the P. falciparum genome) for proximity protein identification and genome editing in P. falciparum.
