Atomistic and Cellular Analysis of Start Phosphatidylinositol Transfer Proteins in the Context of the Phospholipid Extraction Mechanism and Phosphoinositide Signaling in the Parasite Toxoplasma

Phosphatidylinositol transfer proteins (PITPs) are essential regulators of the interface between phosphoinositide (PIP) signaling and membrane trafficking in eukaryotic cells. Genetic derangement of PITPs in multicellular organisms results in pathologies such as neurodegeneration, metabolic diseases, and vision and neurosensory defects. How PITPs sense multiple metabolic inputs to specify a membrane signaling event is key to our understanding of these pathologies and of PIP networks in general. To that end, we describe a novel multi-domain PITP of the START structural family: PITP Multidomain Protein (PIMP) is encoded by the eukaryotic parasite Toxoplasma, and associated with specialized parasite organelles called dense granules. Dense granules are secretory organelles that traffic Toxoplasma proteins during parasite infection of the mammalian host. PIMP links a PITP domain to an oxysterol binding protein (OSBP) domain. PITP and OSBP orthologs in other eukaryotes are expressed as separate proteins known to antagonize each other in specific PIP signaling pathways. We report that PIMP is a bona fide PITP in vivo and in vitro, and its binding and extraction of phosphatidylinositol (PtdIns) from membranes is required for this function. Furthermore, PIMP binds phosphatidylcholine as a counter-ligand to PtdIns, and senses specific higher order PIPs, PtdIns4P and PtdIns(4,5)P2 through a conserved plekstrin homology (PH) domain. We describe how Toxoplasma codes its secretory system with unique PtdIns4P pools, and define PIMP as a sensor of PIP pools on dense granules. Thus PIMP is a dense granule protein that represents a novel platform for the integration of multiple metabolic inputs with PtdIns4P signaling. The mechanism by which the PIMP PITP domain, or indeed any START PITP, extracts phospholipids from bilayers is unknown. We performed molecular dynamics simulations using the mammalian PITPα as a START PITP model. We report atomistic detail of the trajectory of phospholipids into the START PITP binding pocket. Overall, this work analyzes START PITP function on the atomistic and cellular levels, and offers possible explanations for how these proteins organize specific PIP signaling events.Doctor of Philosoph