Knock-sideways by inducible ER retrieval reveals a novel extra-vacuolar function for Plasmodium PTEX component HSP101

Malaria parasites uniquely depend on protein secretion for their obligate intracellular lifestyle but approaches for dissecting Plasmodium secreted protein functions are limited. We report knockER, a novel DiCre-mediated knock-sideways approach to sequester secreted proteins in the ER by inducible fusion with a KDEL ER-retrieval sequence. We show conditional ER sequestration of diverse proteins is not generally toxic, enabling loss-of-function studies. We employed knockER in multiple Plasmodium species to interrogate the trafficking, topology and function of an assortment of proteins that traverse the secretory pathway to diverse compartments including the apicoplast (ClpB1), rhoptries (RON6), dense granules and parasitophorous vacuole (EXP2, PTEX150, HSP101). Taking advantage of the unique ability to redistribute secreted proteins from their terminal destination to the ER, we reveal vacuolar levels of the PTEX translocon component HSP101 but not PTEX150 are maintained in excess of what is required to sustain effector protein export into the erythrocyte. Intriguingly, vacuole depletion of HSP101 hypersensitized parasites to a destabilization tag that inhibits HSP101-PTEX complex formation but not to translational knockdown of the entire HSP101 pool, illustrating how redistribution of a target protein by knockER can be used to query function in a compartment-specific manner. Collectively, our results establish knockER as a novel tool for dissecting secreted protein function with sub-compartmental resolution that should be widely amenable to genetically tractable eukaryotes. SignificanceProtein trafficking and secretion through the endomembrane system is a defining feature of eukaryotes. The secretory pathway is central to the unique biology and pathology of the obligate intracellular malaria parasite, however tools for studying secreted protein function are limited. Knock-sideways is a powerful mutagenesis strategy that conditionally sequesters a protein away from its site of function but is generally not applicable to secreted proteins. We developed a simple approach to conditionally sequester Plasmodium secreted proteins in the ER by inducible C-terminal fusion with a KDEL ER-retrieval sequence that can be used for trafficking, topology and loss-of-function studies. The knockER strategy is broadly applicable to functional dissection of proteins that traverse the eukaryotic secretory pathway.