MODULATION OF THE GOLGI-APPARATUS IN SACCHAROMYCES-CEREVISIAE SEC7 MUTANTS AS SEEN BY 3-DIMENSIONAL ELECTRON-MICROSCOPY

The three-dimensional configuration of the Golgi apparatus has been ex amined with the electron microscope in thick Golgi sections of Sacchar omyces cerevisiae prepared from a wild-type strain and from sec7 mutan ts maintained for various periods of time at the nonpermissive tempera ture of 37 degrees C and then returned to the permissive temperature o f 24 degrees C. Reduced osmium postfixation of glutaraldehyde fixed sp ecimens stained intensely the content of Golgi elements and thus facil itated their three-dimensional characterization. In wild-type S. cerev isiae, the Golgi elements usually appeared as isolated networks of mem branous tubules dispersed throughout the cytoplasm. Along such network s, distensions filled with stained material were similar in size to ne arby secretory granules, suggesting that the latter formed by fragment ation of the Golgi elements. In sec7 mutants maintained at 37 degrees C in low (0.1%) glucose medium, secretion granules progressively decre ased in number and soon disappeared. Concomitantly the networks of Gel s tubules increased in size and complexity, lost their distensions, an d then transformed into flattened saccules forming stacks of up to sev en or eight saccules that were similar to the Golgi stacks seen in mam malian cells. However in contrast to the latter, connections between t he saccules were evident and Golgi-associated small vesicles were gene rally absent. Following return to the permissive temperature (24 degre es C), secretion granules reappeared, the Golgi stacks progressively d ecreased in size, and resumed their initial state of separated small t ubular networks. Thus in sec7 mutant, grown at 37 degrees C in low glu cose medium, segregation of secretory granules is blocked. As a result , Golgi membranes accumulate to form a continuous system of stacked an d interconnected saccules. (C) 1993 Wiley-Liss, Inc.