Increased sterol formation in Saccharomyces cerevisiae. Analysis of cell components and ultrastructure of vacuoles

Functional morphology of the secretory pathway organelles in yeast

The glycoprotein secretory pathway of yeast serves mainly for cell surface growth and cell division. It involves a centrifugal transport of transit macromolecules among organelles, whose membranes contain resident proteins needed for driving the transport. These resident membrane proteins return by retrograde vesicular transport. Apart from this, the pathway involves endocytosis. The model yeast Saccharomyces cerevisiae and vertebrate cells were found to contain very similar gene products regulating the molecular mechanism of glycoprotein transport, and the cellular mechanism of their secretion pathways was therefore also presumed to be identical. Biochemists have postulated that, in S. cerevisiae, the translocation of peptides through the endoplasmic reticulum membranes into the lumen of ER cisternae and the core glycosylation is followed by a vector-mediated transport into the functional cascade of the Golgi system cisternae and between them. This is the site of maturation and sorting of glycoproteins, before the ultimate transport by other vectors involving either secretion from the cells (exocytosis across the plasmalemma into the cell wall) or transport into the lysosome-like vacuole via a prevacuolar compartment, which serves at the same time as a primary endosome. The established cellular model of secretion deals with budding yeast; interphase yeast cells, in which the secretion is limited and which predominate in exponential cultures, have not been taken into consideration. The quality of organelle imaging in S. cerevisiae ultra-thin sections depends on the fixation technique used and on specimen contrasting by metals. The results achieved by combinations of different techniques differ mostly in the imaging of bilayers of membrane interfaces and the transparence of the matrix phase. Fixation procedures are decisive for the results of topochemical localisations of cellular antigenic components or enzyme activities, which form the basis of the following survey of functional morphology of organelles involved in the yeast secretory pathway. The existing results of these studies do not confirm all aspects of the vertebrate model of the Golgi apparatus proposed by molecular geneticists to hold for S. cerevisiae, and alternative models of the cellular mechanism of secretion in this yeast are, therefore, also discussed.

Regulation of sterol biosynthesis in Saccharomyces cerevisiae

Sterol synthesis in Saccharomyces cerevisiae was primarily controlled by the growth rate. At low specific growth rates the intermediates of ergosterol biosynthesis prevailed in cells. At the same time, the total sterol content reached about 6% of dry matter whereas the content of ergosterol was only 2-2.5%, which seems to be the maximum value for S. cerevisiae. After esterification with fatty acids these sterol intermediates are stored in lipid globules together with reserve triacylglycerols. The sporulating S. cerevisiae cells contained 3.5% sterols and 1.5% ergosterol of dry matter.

Effect of nitrogen limitation and sporulation on sterol and lipid formation in Saccharomyces cerevisiae

The content of sterols and lipids was compared in the cells of Saccharomyces cerevisiae cultivated in sporulation and the sterol-induction nitrogen-limited media. After 24 h the measured values in the two cultivations did not significantly differ. However, after subsequent 24 h, further formation of lipid globules and a corresponding increase of lipid and sterol content was detected only in the sterol-induction medium. To demonstrate the similarity of physiological state during the first day of the two cultivations, the combined cultivations were performed. Maximum sporulation, suggesting maximum similarity, of the two processes was achieved when the cells were grown in the sterol-induction medium for 15 h and then transferred to a sporulation medium.