Modifications of the Golgi apparatus in Saccharomyces cerevisiae lacking microtubules.

Abstract

Disassembly of cytoplasmic microtubules by nocodazole in cultured mammalian cells leads to the disruption of the continuous ribbonlike Golgi apparatus and dispersal of the Golgi elements from their normal juxtanuclear location, close to the microtubule-organizing center (MTOC), toward the cell periphery. Clearing of the drug induces reassembly of the microtubules from the MTOC and reorganization of the Golgi elements into a continuous ribbonlike juxtanuclear structure. In the yeast Saccharomyces cerevisiae, the Golgi apparatus does not form a continuous structure as in mammalian cells but instead constitutes independent units dispersed throughout the cytoplasm. It is the purpose of this article to investigate the role of microtubules in the structure and distribution of the Golgi elements in S. cerevisiae by studying the ultrastructure of cell organelles either in mutant cells deficient in beta-tubulin or in wild-type cells treated with the microtubule-depolymerizing drug nocodazole. Two S. cerevisiae yeast strains were used in this study: a control wild-type strain, CUY226 (ade2-101, his3-delta 200, leu2-delta 1, lys2-801, ura3-52 Mat alpha), and a mutant strain, CUY66 (tub2-401, ade2-101, ura3-52, Mat alpha). Nocodazole was added to the wild-type cells cultivated at 30 degrees C, and cells were fixed 5 min, 20 min, and 60 min, respectively, after adding the drug to the culture. Both strains were fixed and examined 5 min, 20 min, and 60 min after shifting the cultures from the permissive temperature of 30 degrees C to the restrictive temperature of 14 degrees C. Cells were fixed in 2% glutaraldehyde, treated for 15 min in 1% sodium metaperiodate, postfixed in reduced osmium, and embedded in Epon. To visualize the three-dimensional configuration of cell organelles, stereopairs were prepared from sections stained with lead citrate and tilted at +/- 15 degrees from the 0 degree position of the goniometric stage of the electron microscope. In mutant cells shifted to restrictive temperature and wild-type cells treated with nocodazole, the main ultrastructural modification was a fragmentation of networks of membranous tubules, which probably correspond to the yeast Golgi apparatus. Secretion granules were still present in growing buds, and they were dispersed in the cytoplasm, which contained in addition numerous small vesicles in the 30-60-nm diameter range. In normal cells, small vesicles may originate from the endoplasmic reticulum and fuse together to give rise to Golgi networks (Rambourg et al. 1994. Anat. Rec., 240:32-41). If this hypothesis is correct, the observations reported might indicate that intact microtubules orient the flow of small vesicles and favour their fusion into Golgi networks.