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B. Goigi Apparatus The Golgi apparatus of Saccharomyces cerevisiae is difficult to visualize with the normal fixation procedures. One of the reasons for this might be the surprisingly high elasticity of the cell wall of Saccharomyces cerevisiae (De Nobel et al. 1990a; Secretion in Yeast Morris et al. 1986). The best results are obtained by freeze-substitution fixation (Baba and Osumi 1987; Baba et al. 1989) and freeze-etching (Moor and Miihlethaler 1963). The use of sphaeroplasts as starting material (Makarow 1988) or of osmotically supported fixation solution (Preuss et al.

The pro-part is removed in the late Golgi by the endoprotease Kex2p. Further proteolytic processing Py Golgi enzymes results in the formation of the mature pheromone consisting of only 13 amino acids (Kurjan and Herskowitz 1982); the mature pheromone is secreted into the medium. D. Secretory Pathway Mutauts The analysis of the secretory pathway has been greatly facilitated by the discovery of a large class of temperature-sensitive secretory mutants (sec mutants) by Novick et al. (1980). These are conditional mutants that are able to grow normally at the permissive temperature (usually 25°C) but in which a specific step of the secretory pathway becomes blocked at the restrictive temperature (usually 37°C), often resulting in accumulation of specific compartments of the secretory pathway before the cell dies (Schekman and Novick 1982).

3. O-Glycosylation........................ 4. Attachment of GPI Anchors . . . . . . . C. Protein Folding. . . . . . . . . . . . . . D. Retention Mechanisms of ER Resident Proteins. . . . . . . . . . . IV. Processing of Proteins in the Golgi Apparatus A. Elongation of N-Chains . . . . . . . . . . B. Elongation of O-Chains . . . . . . . . . . C. Proteolytic Processing ..................... V. Secretion of Cell Envelope Proteins. . . . . VI. Conclusions .