Role of inositol-containing sphingolipids in Saccharomyces cerevisiae during inositol starvation

The in vitro lipid requirements of UDP-N-acetylglucosamine-dolichol phosphate N-acetylglucosamine-1-phosphotransferase for the inositol-containing sphingolipids from Saccharomyces cerevisiae were characterized in terms of concentration and specificity. The effects of combinations of lipids, especially phosphatidylinositol and the inositol-containing sphingolipids, were also tested on the transferase. Phosphatidylinositol and phosphatidylglycerol stimulated the enzyme 3.3- and 2.8-fold, respectively. The inositol-containing sphingolipids, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine did not stimulate the activity of the transferase. Phosphatidylcholine and phosphatidylethanolamine in combination with phosphatidylinositol had no effect on the transferase activity; however, the inositol-containing sphingolipids markedly inhibited the stimulation of the transferase by phosphatidylinositol. This inhibition by the sphingolipids was prevented if phosphatidylcholine, in addition to the other lipids, was present in the assay mixture. In addition, changes due to inositol starvation in the in vivo membrane lipid environment, i.e., phosphatidylinositol and the inositol-containing sphingolipids, were analyzed to determine whether they corresponded to the observed in vitro effects. Three hours after the beginning of inositol starvation, there were 9- and 14-fold reductions in the accumulation of phosphatidylinositol in membrane fractions IIA (vesicles) and IV (endoplasmic reticulum), respectively, although there was only a 6-fold reduction in membrane fraction I (plasma membrane). The accumulation of [14C]inositol into inositol-containing sphingolipids also reflected the differences in the cellular location of membranes.