A ROLE FOR PHOSPHOLIPASE-D (PLD1P) IN GROWTH, SECRETION, AND REGULATION OF MEMBRANE LIPID-SYNTHESIS IN YEAST

The SEC14 gene encodes a phosphatidylinositol/phosphatidylcholine tran sfer protein essential for secretion and growth in yeast (1). Mutation s (cki1, cct1, and cpt1) in the CDP-choline pathway for phosphatidylch oline synthesis suppress the sec14 growth defect (2), permitting sec14 (ts) cki1, sec14(ts) cct1, and sec14(ts) cpt1 strains to grow at the s ec14(ts) restrictive temperature. Previously, we reported that these d ouble mutant strains also ex crete the phospholipid metabolites, choli ne and inositol (3). We now report that these choline and inositol exc retion phenotypes are eliminated when the SPO14 (PLD1) gene encoding p hospholipase DI is deleted. In contrast to sec14(ts) cki1 strains, sec 14(ts) cki1 pld1 strains are not viable at the sec14(ts) restrictive t emperature and exhibit a pattern of invertase secretion comparable wit h sec14(ts) strains, Thus, the PLD1 gene product appears to play an es sential role in the suppression of the sec14(ts) defect by CDP-choline pathway mutations, indicating a role for phospholipase D1 in growth a nd secretion. Furthermore, sec14(ts) strains exhibit elevated Ca2+-ind ependent, phophatidylinositol 4,5-bisphosphate-stimulated phospholipas e D activity, We also propose that phospholipase D1-mediated phosphati dylcholine turnover generates a signal that activates transcription of INO1, the structural gene for inositol l-phosphate synthase.