A Novel Multi‐domain Phosphatidylinositol Transfer Protein/Oxysterol Binding Protein Senses Specific Phosphoinositide Pools On Toxoplasma Dense Granules

Abstract

The Apicomplexan Toxoplasma gondii is a common parasite of humans and other mammals and causes severe disease in immunocompromised individuals. Apicomplexa are obligate intracellular pathogens, and their infection of host cells requires the orchestrated secretion of proteins from several highly specialized secretory organelles, the regulation of which is incompletely understood. Studies in other eukaryotic systems have established that membrane trafficking through distal stages of the secretory pathway is regulated by phosphoinositide (PIP) signaling networks and that phosphatidylinositol (PtdIns) transfer proteins (PITPs) are important components of these signaling circuits. We used PIP‐specific biosensors to map phosphoinositide pools on Toxoplasma membranes, and identified at least two distinct pools of PtdIns(4)P: one on the Golgi/early secretory membranes, and one associated with Apicomplexan‐specific secretory organelles called dense granules (GRA3(+)). Studies in yeast demonstrate that PITPs potentiate the activities of PtdIns kinases (PIKs) to produce privileged PIP pools on secretory membranes, and PtdIns(4)P‐mediated pro‐secretory activities are countered by the antagonistic activities of specific members of the oxysterol binding protein (OSBP) superfamily. We have identified a Toxoplasma multi‐domain protein, the PITP membrane protein (PIMP), that couples a START‐type PITP domain, a pleckstrin homology (PH) domain, and a C‐terminal OSBP domain. PIMP is a bona fide PITP that transfers PtdIns and phosphatidylcholine (PtdCho) between membranes in vitro, and potentiates PIK activity in vivo when expressed in yeast. Of the four known Toxoplasma PITP genes, it is the only one capable of doing so. The PIMP PH domain specifically binds PtdIns(4)P and PtdIns(4,5)P2 in vivo, and recognizes these pools on dense granules in intracellular parasites. Full length PIMP localizes to (GRA3+) dense granules, and this association is mediated by PH domain PIP binding. PIMP is a physical platform for integration of multiple lipid signals on specialized secretory membranes in Toxoplasma and is the first known example of a PITP coupled to an OSBP domain. As dense granules are unique to Apicomplexa and required to support intracellular proliferation of the parasites, dense granule signaling networks would make good therapeutic targets for Apicomplexan infections.Support or Funding InformationThis research was funded by NIH grant R01‐GM112591, RO1‐GM44530 and grant BE‐0017 from the Robert A. Welch Foundation.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.