A Role for Phospholipase D (Pld1p) in Growth, Secretion, and Regulation of Membrane Lipid Synthesis in Yeast*

The SEC14 gene encodes a phosphatidylinositol/phosphatidylcholine transfer protein essential for secretion and growth in yeast (1). Mutations (cki1, cct1, and cpt1) in the CDP-choline pathway for phosphatidylcholine synthesis suppress the sec14 growth defect (2), permitting sec14 ts cki1, sec14 ts cct1, andsec14 ts cpt1 strains to grow at thesec14 ts restrictive temperature. Previously, we reported that these double mutant strains also excrete the phospholipid metabolites, choline and inositol (3). We now report that these choline and inositol excretion phenotypes are eliminated when theSPO14 (PLD1) gene encoding phospholipase D1 is deleted. In contrast to sec14 ts cki1 strains, sec14 ts cki1 pld1 strains are not viable at thesec14 ts restrictive temperature and exhibit a pattern of invertase secretion comparable withsec14 ts strains. Thus, the PLD1gene product appears to play an essential role in the suppression of the sec14 ts defect by CDP-choline pathway mutations, indicating a role for phospholipase D1 in growth and secretion. Furthermore, sec14 ts strains exhibit elevated Ca2+-independent, phophatidylinositol 4,5-bisphosphate-stimulated phospholipase D activity. We also propose that phospholipase D1-mediated phosphatidylcholine turnover generates a signal that activates transcription of INO1, the structural gene for inositol 1-phosphate synthase.