The Biology and Biochemistry of Lipid Transfer Protein-Regulated Phosphoinositide Signaling

ABSTRACT/SUMMARY The overarching objective of the proposed research is to understand how eukaryotic cells organize major lipid signaling pathways, and how these do so in a manner that imparts both spatial and temporal specificity, and specificity of biological outcome. The system of interest is phosphoinositide signaling and the general question of how cells functionally channel a rather simple chemical code into a large diversity of biological activities. The experimental goal is to execute a detailed functional analysis of two under-investigated classes of proteins -- the phosphatidylinositol (PtdIns transfer proteins (PITPs) and the oxysterol binding related proteins (ORPs) -- in control of such functional channeling. To that end, two conceptually linked directions will be pursued that: (i) address cell biological, physiological and mechanistic questions related to the activities of yeast Sec14-like PITPs and the structurally unrelated mammalian StART-like Class 1 PITPs, and (ii) how the yeast oxysterol binding related protein (ORP) Kes1/Osh4 functions as a specific antagonist of Sec14-dependent phosphoinositide signaling in yeast and what role ORPs play in neural stem cell biology. The proposed studies will test specific hypotheses that relate to: (i) how fungal and mammalian PITPs bind membranes and exchange their lipid ligands, (ii) the mechanisms by which individual PITPs regulate specific steps of lipid metabolism in yeast and mammals (neural stem cells, melanoma and the obligate intracellular parasite Toxoplasma) to diversify the biological outcomes of phosphoinositide signaling, and (iii) how the oxysterol binding protein (OSBP)-related proteins work against Sec14- dependent PtdIns-4-phosphate signaling in regulating Golgi phosphatidylinositol-4-phosphate signaling. These studies will clarify key unanswered questions regarding the mechanism of function of PITPs, the mechanisms by which both Sec14-like and StART-like PITPs couple lipid metabolism to PtdIns kinase signaling, and more global ramifications of PITP functional interactions with ORPs. A growing number of inherited neurodegenerative and neurodevelopmental diseases, and diseases of proliferative disorders (e.g. cancer), are attributed to derangements in Sec14-like and StART-like PITP activities. Thus, the proposed studies will provide both new and fundamental information that bears directly on molecular mechanisms by which PITPs regulate and organize signal transduction in eukaryotes, and protect mammals from diseases of deranged cell proliferation (cancer), neurodegeneration and type 2 diabetes in humans.