ROLE OF THE YEAST PHOSPHATIDYLINOSITO PHOSPHATIDYLCHOLINE TRANSFER PROTEIN (SEC14P) IN PHOSPHATIDYLCHOLINE TURNOVER AND INO1 REGULATION/

In yeast, mutations in the CDP-choline pathway for phosphatidylcholine biosynthesis permit the cell to grow even when the SEC14 gene is comp letely deleted (Cleves, A., McGee, T., Whitters, E,, Champion, K,, Ait ken, J,, Dowhan, W,, Goebl, M,, and Bankaitis, V, (1991) Cell 64, 789- 800), We report that strains carrying mutations in the CDP-choline pat hway, such as cki1, exhibit a choline excretion phenotype due to produ ction of choline during normal turnover of phosphatidylcholine, Cells carrying cki1 in combination with sec14(ts), a temperature-sensitive a llele in the gene encoding the phosphatidylinositol/phosphatidylcholin e transporter, have a dramatically increased choline excretion phenoty pe when grown at the sec14(ts)-restrictive temperature, We show that t he increased choline excretion in sec14(ts) cki1 cells is due to incre ased turnover of phosphatidylcholine via a mechanism consistent with p hospholipase D-mediated turnover, We propose that the elevated rate of phosphatidylcholine turnover in sec14(ts) cki1 cells provides the met abolic condition that permits the secretory pathway to function when S ec14p is inactivated, As phosphatidylcholine turnover increases in sec 14(ts) cki1 cells shifted to the restrictive temperature, the INO1 gen e (encoding inositol-l-phosphate synthase) is also derepressed, leadin g to an inositol excretion phenotype (Opi(-)). Misregulation of the IN O1 gene has been observed in many strains with altered phospholipid me tabolism, and the relationship between phosphatidylcholine turnover an d regulation of INO1 and other coregulated genes of phospholipid biosy nthesis is discussed.