EXPORT OF STERYL ESTERS FROM LIPID PARTICLES AND RELEASE OF FREE STEROLS IN THE YEAST, SACCHAROMYCES-CEREVISIAE

Fatty acyl esters of the yeast specific sterol, ergosterol, are exclus ively stored in lipid particles. Under conditions of sterol deficiency , e.g., in the presence of terbinafine, an inhibitor of fungal squalen e epoxidase, steryl esters are hydrolyzed, and sterols are set free fa r membrane formation. Lipid particles do not contain steryl-ester hydr olase activity themselves; the highest specific activity of this enzym e is found in the plasma membrane. Therefore, steryl esters have to be exported from lipid particles to their site of hydrolytic cleavage. T his process of translocation and metabolic conversion was studied in v ivo. Addition of nocodazole to terbinafine-treated cells did not distu rb the mobilization of steryl esters, indicating that this process is not mediated by microtubuli-dependent vesicle flux. Under the influenc e of inhibitors of cellular energy production (azide and fluoride) and protein biosynthesis (cycloheximide) mobilization of steryl esters ca me to an halt. These results support the view that ongoing membrane pr oliferation may be a driving force for the release of sterols from ste ryl esters of lipid particles.