Nonclassical PITPs activate PLD via the Stt4p PtdIns-4-kinase and modulate function of late stages of exocytosis in vegetative yeast

Phospholipase D (PLD) is a PtdCho-hydrolyzing enzyme that plays central signaling functions in eukaryotic cells. We previously demonstrated that action of a set of four nonclassical and membrane-associated Sec14p-like phosphatidylinositol transfer proteins (PITPs) is required for optimal activation of yeast PLD in vegetative cells. Herein, we focus on mechanisms of Sfh2p and Sfh5p function in this regulatory circuit. We describe several independent lines of in vivo evidence to indicate these SFH PITPs regulate PLD by stimulating PtdIns-4,5-P-2 synthesis and that this stimulated PtdIns-4,5-P-2 synthesis couples to action of the Stt4p PtdIns 4-kinase. Furthermore, we provide genetic evidence to suggest that specific subunits of the yeast exocyst complex (i.e. a component of the plasma membrane vesicle docking machinery) and the Sec9p plasma membrane t-SNARE are regulated by PtdIns(4,5)P-2 and that Sfh5p helps regulate this interface in vivo. The collective in vivo and biochemical data suggest SFH-mediated stimulation of Stt4p activity is indirect, most likely via a substrate delivery mechanism.