Three-dimensional structure of tubular networks, presumably Golgi in nature, in various yeast strains: a comparative study.

BACKGROUND

In the yeast Saccharomyces cerevisiae, the Golgi apparatus consists of discrete units distributed throughout the cytoplasm. When such units are examined in three dimensions, in relatively thick sections prepared for the electron microscope, they usually appear as small tubular networks with a stained material accumulating in dilations located at the junctions of membranous tubules. To see whether such tubular networks are observed in other yeast species, the three-dimensional structure of organelles in eight additional yeast strains, endowed with diverse biological properties, are examined.

METHODS

Yeast strains were grown at 24 degrees C in YPD medium (2% Bactopeptone, 1% Bactoyeast extract, and 2% glucose). Cells that were examined by electron microscopy came from exponentially growing cultures grown in a shaking water bath and maintained at a OD 600 (optical density at 600 nm) of 0.5. Cells were fixed in a fixative containing 2% glutaraldehyde in 0.1 M cacodylate buffer pH 7.4 and 0.8 M sorbitol. They were then treated for 15 min in 1% sodium metaperiodate and postfixed for 1 hr in potassium ferrocyanide-osmic acid. They were preembedded in agarose prior to dehydration and finally embedded in Epon. In these conditions, the preservation of cell organelles was improved and the cytoplasmic retraction from the cell wall was minimized. Photographs of sections tilted at +/- 15 degrees from the 0 degrees position of the goniometric stage were used to prepare stereopairs from which the three-dimensional configuration of the organelles was visualized.

RESULTS

In all yeast strains, tubular networks appeared as separate elements or units disperse throughout the cytoplasm. Each unit consisted of anastomosed membranous tubules. In some strains such as Saccharomyces cerevisiae, Zygosaccharomyces rouxii, or Saccharomyces pombe, such units appeared mainly as polygonal networks of intensely stained membranous tubules. Along these networks, distensions filled with stained material were similar in size to nearby secretory granules, suggesting that the latter formed by fragmentation of the tubular networks. In Hansenula polymorpha, Pichia pastoris, and Debaryomyces hansenii, networks of anastomosed tubules were closely superposed to each other and formed parallel arrays reminiscent of the stacks of Golgi saccules seen in mammalian cells. However, in contrast to what is usually found in the latter, the layers making up the parallel arrays in yeasts, were clearly continuous to each other. In other strains, i.e., Kluyveromyces lactis, Candida albicans, and Candida parapsilosis, the situation was intermediate and their cytoplasm contained only arrays of small size with two or at most three superposed layers of membranous tubules. Small vesicles in the 30-50 nm range were rarely encountered in most yeast strains.

CONCLUSIONS

It is therefore concluded that tubular networks, presumably Golgi in nature, are present in all yeasts examined so far. Yet, in some strains, these tubular networks may be arranged in parallel arrays or stacks.