The Toxoplasma Dense Granule Proteins GRA17 and GRA23 Mediate the Movement of Small Molecules between the Host and the Parasitophorous Vacuole

Abstract

Summary Toxoplasma gondii is a protozoan pathogen in the phylum Apicomplexa that resides within an intracellular parasitophorous vacuole (PV) that is selectively permeable to small molecules through unidentified mechanisms. We have identified GRA17 as a Toxoplasma-secreted protein that localizes to the parasitophorous vacuole membrane (PVM) and mediates passive transport of small molecules across the PVM. GRA17 is related to the putative Plasmodium translocon protein EXP2 and conserved across PV-residing Apicomplexa. The PVs of GRA17-deficient parasites have aberrant morphology, reduced permeability to small molecules, and structural instability. GRA17-deficient parasites proliferate slowly and are avirulent in mice. These GRA17-deficient phenotypes are rescued by complementation with Plasmodium EXP2. GRA17 functions synergistically with a related protein, GRA23. Exogenous expression of GRA17 or GRA23 alters the membrane conductance properties of Xenopus oocytes in a manner consistent with a large non-selective pore. Thus, GRA17 and GRA23 provide a molecular basis for PVM permeability and nutrient access.

Critical Steps in Protein Export of Plasmodium falciparum Blood Stages

“…So far, no functional data are available for the predicted pore component EXP2 in P. falciparum. However, very recent data showed that PfEXP2 can complement the role of T. gondii GRA17, a related protein implicated in the long-sought nutrient pore activity of apicomplexan PVMs [64]. As one explanation for this finding, it was proposed that PfEXP2 may be a part of both a nutrient and a protein-conducting pore [64].…”

A Toxoplasma gondii Gluconeogenic Enzyme Contributes to Robust Central Carbon Metabolism and Is Essential for Replication and Virulence

“…gondii tachyzoites reside within a unique, non-fusogenic parasitophorous vacuole within the host cell (Joiner et al, 1990) and have access to a variety of carbon sources that can permeate across the limiting membrane of the vacuole (Gold et al, 2015;Schwab et al, 1994). However, 13 C-labeling and genetic studies (Lin et al, 2011;MacRae et al, 2012;Ohsaka et al, 1982;Starnes et al, 2009) suggest that under normal growth conditions, intracellular tachyzoites primarily utilize glucose to generate energy and essential anabolic precursors via glycolysis, the pentose phosphate pathway, and mitochondrial respiration.…”

Plasmodium species: master renovators of their host cells

“…However, knockdown of the expression of GRA17 in T. gondii did not affect the export of GRA24 and the PEXEL-containing protein GRA16. Instead it led to a decrease in the movement of small molecules across the PVM 66 . Furthermore, membrane conductance was enhanced in Xenopus laevis oocytes in which GRA17 and EXP2 were overexpressed 66 , but the time that was required to alter the conductance was much longer than would be expected for proteins that have functions in pore formation.…”

“…The homologue in T. gondii, GRA17, seems to function as a nutrient pore and can be functionally substituted with EXP2 (REF. 66), which suggests that EXP2 may have been a nutrient pore in the ancestor of Plasmodium spp. before being repurposed as a protein channel following the recruitment of HSP101.…”