Ultracytochemical evidence of Golgi functions in microvesicles at all phases of cell cycle in Saccharomyces cerevisiae

Cell cycle-dependent protein secretion by Saccharomyces cerevisiae

Synchronized Saccharomyces cerevisiae cell populations were used to examine secretion rates of a heterologous protein as a function of cell cycle position. The synchronization procedure had a profound effect on the type and quality of data obtained. When cell synchrony was induced by cell cycle-arresting drugs, a significant physiological perturbation of cells was observed that obscured representative secretion data. In contrast, synchronization with centrifugal elutriation resulted in synchronized first-generation daughter cells with undetectable perturbation of the physiological state. The synchronized cells did not secrete significant amounts of protein until they reached cell division, suggesting that the secretion process in these cells is strongly cell cycle dependent. However, the maximum secretion rate of the synchronized culture (7-14 molecules/cell/second) was significantly lower than that of an asynchronous culture (29-51 molecules/cell/second). This result indicates that young daughter cells isolated in the synchronization process exhibit different protein secretion behavior than older mother cells that are absent in the synchronized cell population but present in the asynchronous culture.

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.

Cytochemical images of secretion in Saccharomyces cerevisiae and animal cells are different

Like in animal cells, the major secretory pathway of the ascomycetous budding yeast Saccharomyces (s.) cerevisiae consists of membrane-bound compartments which transport soluble and membrane (glyco)peptides to lysosomal vacuoles, cell wall, or out of the cell. The established model of the cellular machinery of the yeast secretory pathway was deduced largerly from molecular ex situ analyses and for budding yeast cells it was assumed to be identical with that of secretory animal cells. Interphase yeast cells were never considered. Glycosylation of peptides was detected in the endoplasmic reticulum (ER) and the putative Golgi cisternae. Coated membrane vesicles were assumed to transport intermediates into and within the Golgi cascade. Proteolytic trimming would occur in the last Golgi compartment. Golgi-derived membrane vesicles would serve for exocytosis or fuse with lysosomal vacuoles. In contrast to this notion, yeast cytologists showed specific features of secretion in S. cerevisiae and other Ascomycetes. Cytochemical observations in situ of both dividing and interphase yeast showed direct communication between nuclear envelope, ER and segregated Golgi cisternae. A new class of constitutive conveyors, coated protein globules smaller than membrane vesicles, was shown to exist throughout the cell cycle. The function of Golgi-derived membrane vesicles was constrained to promotion of exocytosis in budding yeast. Some of the Golgi apparatus functions were detected in both these classes of exocytotic conveyors. Uptake (phagocytosis) of transport conveyors and lipoprotein condensates has been shown to deliver enzymes and secretory compounds into vacuoles. This simplified machinery of secretion, postulated for S. cerevisiae, does not include the Golgi cascade.

The ultrastructure of yeast: cell wall structure and formation

Yeasts are unicellular eukaryotes, and are used widely as a model system in basic and applied fields of life science, medicine, and biotechnology. The ultrastructure of yeast cells was first studied in 1957 and the techniques used have advanced greatly in the 40 years since then; an overview of these methods is first presented in this review. The ultrastructure of budding and dimorphic yeast cells observed with a scanning electron microscope (SEM) and a transmission electron microscope (TEM) after thin sectioning and freeze-etching are then described, followed by discussion of the regeneration of the cell wall of Candida albicans protoplasts detected by cryosectioning. C. albicans protoplasts are regenerated to synthesize microfibrils on their surface. They are aggregated into thicker bundles which are intermeshed, forming a wide-meshed network of long fibrils. These microfibrillar structures are chains of beta-1,3-glucan which are broken down after treatment with beta-1,3-glucanase. Morphologically identical microfibrils are synthesized in vitro by a cell-free system in which the active cell membrane fraction as a source of beta-1,3-glucan synthetase and UDP glucose as the sole substrate are used. The diameter of an elemental fibril of beta-glucan is estimated to be 2.8 nm from the pattern of autocorrelation of the image obtained by computer processing. In contrast, in the presence of aculeacin A the formation of normal fibrillar nets or bundles is significantly inhibited, resulting in the occurrence of short fibrils. These electron microscopic data suggest that aculeacin A inhibits not only the synthesis of beta-1,3-glucan but the aggregation of microfibrils of this polysaccharide, allowing formation of the crystalline structure. On the basis of the cumulative data obtained from the electron microscopic studies, we are led to the assumption that de novo synthesized beta-glucan chains might initially form fine particles which are then transformed into thin fibrils with single to multiple strands which appear to be oriented parallel to each other so that they develop into fibrillar structures. This process of assembly of beta-glucan molecules leads to the development of a fibrous network within the regenerating Candida cell wall. Third, the mechanism of cell wall formation is shown by low-voltage (LV) SEM and TEM, using various techniques and computer graphics, of the regeneration system of Schizosaccharomyces pombe protoplasts: after 10 min of regeneration, the protoplasts begin to grow fibrillar substances of a beta-glucan nature, and a fibrillar network covers the surface of all protoplasts. The network is originally formed as fine particles on the protoplast surface and these are subsequently lengthened to microfibrils 2 nm thick. The microfibrils twist around each other and develop into 8 nm thick fibrils forming flat bundles 16 nm thick. Interfibrillar spaces are gradually filled with amorphous particles of an alpha-galactomannan nature and, finally, the complete cell wall is formed after 12 h. Treatment of reverting protoplasts with RuO4 provided clear TEM images of glucan fibrils with high electron density. The relationship between cell wall regeneration and intracellular organelles was examined by using serial thin sections stained with PATAg and computer-aided three-dimensional reconstruction. The secretory vesicles in a protoplast had increased markedly by 1.4, 3.4, and 5.8 times at 1.5, 3.0, and 5 h, respectively. Three-dimensional analysis indicates that Golgi apparatuses are located close together in the nucleus of the protoplast and are dispersed into the cytoplasm during the progress of cell wall formation.