The oxysterol binding protein Kes1p regulates Golgi apparatus phosphatidylinositol-4-phosphate function

The Saccharomyces cerevisiae phosphatidylcholine/phosphatidylinositol transfer protein Sec14p is required for Golgi apparatus-derived vesicular transport through coordinate regulation of phospholipid metabolism. Sec14p is normally essential. The essential requirement for SEC14 can be bypassed by inactivation of (i) the CDP-choline pathway for phosphatidyllcholine synthesis or (ii) KES1, which encodes an oxysterol binding protein. A unique screen was used to determine genome-wide genetic interactions for the essential gene SEC14 and to assess whether the two modes of '' sec14 bypass '' were similar or distinct. The results indicate that inactivation of the CDP-choline pathway allows cells with inactivated SEC14 to live through a mechanism distinct from that of inactivation of KES1. We go on to demonstrate an important biological function of Kes1p. Kes1p regulates Goigi apparatus-derived vesicular transport by inhibiting the function of Pik1p-generated Golgi apparatus phosphatidylinositol-4-phosphate (Pl-4P). Kes1p affects both the availability and level of Golgi apparatus PI-4P. A set of potential PI-4P-responsive proteins that include the Rab GTPase Ypt31p and its GTP exchange factor are described.