Yeast phosphatidylinositol transfer protein (Sec14p) function is essential for production of Golgi-derived secretory vesicles, and this requirement is bypassed by mutations in at least seven genes. Analyses of such 'bypass Sec14p' mutants suggest that Sec14p acts to maintain an essential Golgi membrane diacylglycerol (DAG) pool that somehow acts to promote Golgi secretory function. SPO14 encodes the sole yeast phosphatidylinositol-4,5-bisphosphateactivated phospholipase D (PLD). PLD function, while essential for meiosis, is dispensable for vegetative growth. Herein, we report specific physiological circumstances under which an unanticipated requirement for PLD activity in yeast vegetative Golgi secretory function is revealed. This PLD involvement is essential in 'bypass Sec14p' mutants where normally Sec14p-dependent Golgi secretory reactions are occurring in a Sec14p-independent manner. PLD catalytic activity is necessary but not sufficient for 'bypass Sec14p', and yeast operating under 'bypass Sec14p' conditions are ethanol-sensitive. These data suggest that PLD supports 'bypass Sec14p' by generating a phosphatidic acid pool that is somehow utilized in supporting yeast Golgi secretory function.

Two genes in2؉ -independent hydrolysis of several isoprenoid phosphates by particulate fractions isolated from these cells. The particulate and cytosolic fractions from the double disruption (lpp1⌬ dpp1⌬) showed essentially complete loss of Mg 2؉ -independent hydrolytic activity toward dolichyl phosphate (dolichyl-P), dolichyl pyrophosphate (dolichyl-P-P), farnesyl pyrophosphate (farnesyl-P-P), and geranylgeranyl pyrophosphate (geranylgeranyl-P-P). However, a modest Mg 2؉ -stimulated activity toward PA and dolichyl-P was retained in cytosol from lpp1⌬ dpp1⌬ cells. The action of Dpp1p on isoprenyl pyrophosphates was confirmed by characterization of the hydrolysis of geranylgeranyl-P-P by the purified protein. These results indicate that LPP1 and DPP1 account for most of the hydrolytic activities toward dolichyl-P-P, dolichyl-P, farnesyl-P-P, and geranylgeranyl-P-P but also suggest that yeast contain other enzymes capable of dephosphorylating these essential isoprenoid intermediates.Phosphorylated lipids serve diverse roles in cellular metabolism, including signal transduction, membrane biosynthesis, and energy storage. These lipid phosphates are created through direct phosphorylation of lipids by lipid kinases such as diacylglycerol kinase and dolichol kinase which produce phosphatidic acid (PA) 1 and dolichyl monophosphate (dolichyl-P), respectively. Alternatively, these molecules can be formed by degradation of precursor molecules as in the hydrolysis of phospholipids by phospholipase D to form PA or the transfer of oligosaccharides from a dolichol carrier to produce dolichyl pyrophosphate (dolichyl-P-P) or dolichyl-P (1). The phosphorylated lipids can be metabolized by several phosphatases or used in synthetic reactions to produce a variety of phospholipids or dolichyl oligosaccharides (2-6).