In situ localization of β-glucans in the cell wall of Schizosaccharomyces pombe

The chemical composition of the cell wall of Sz-. pombe is known as beta -1,3-glucan, beta -1,6-glucan, alpha -1,3-glucan and alpha -galactomannan; however, the three-dimensional interactions of those macromolecules have not yet been clarified. Transmission electron microscopy reveals a three-layered structure: the outer layer is electron-dense, the adjacent layer is less dense, and the third layer bordering the cell membrane is dense. In intact cells of Sc. pombe, the high-resolution scanning electron microscope reveals a surface completely filled with alpha -galactomannan particles. To better understand the organization of the cell wall and to complement our previous studies, we set out to locate the three different types of beta -glucan by immuno-electron microscopy. Our results suggest that the less dense layer of the cell wall contains mainly beta -1,6-branched beta -1,3-glucan. Occasionally a line of gold particles can be seen, labelling fine filaments radiating from the cell membrane to the alpha -galactomannan layer, suggesting that some of the radial filaments contain beta -1,6-branched beta -1,3-glucan. beta -1,6-glucan is preferentially located underneath the alpha -galactomannan layer. Linear beta -1,3-glucan is exclusively located in the primary septum of dividing cells. beta -1,6-glucan only labels the secondary septum and does not co-localize with linear beta -1,3-glucan, while beta -1,6-branched beta -1,3-glucan is present in both septa. Linear beta -1,3-glucan is present from early stages of septum formation and persists until the septum is completely formed; then just before cell division the label disappears. From these results we suggest that linear beta -1,3-glucan is involved in septum formation and perhaps the separation of the two daughter cells. In addition, rye frequently found beta -1,6-glucan label on the Golgi apparatus, on small vesicles and underneath the cell membrane. These results give fresh evidence for the hypothesis that P-1,6-glucan is synthesized in the endoplasmic reticulum-Golgi system and exported to the cell membrane. Copyright (C) 2001 John Wiley & Sons, Ltd.