In situ localization of beta-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; ho wever, the three-dimensional interactions of those macromolecules have not yet been clarified. Transmission electron microscopy reveals a three-layere d 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 reveal s a surface completely filled with alpha -galactomannan particles. To bette r understand the organization of the cell wall and to complement our previo us studies, we set out to locate the three different types of beta -glucan by immuno-electron microscopy. Our results suggest that the less dense laye r of the cell wall contains mainly beta -1,6-branched beta -1,3-glucan. Occ asionally a line of gold particles can be seen, labelling fine filaments ra diating from the cell membrane to the alpha -galactomannan layer, suggestin g that some of the radial filaments contain beta -1,6-branched beta -1,3-gl ucan. beta -1,6-glucan is preferentially located underneath the alpha -gala ctomannan layer. Linear beta -1,3-glucan is exclusively located in the prim ary septum of dividing cells. beta -1,6-glucan only labels the secondary se ptum 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-gluca n is present from early stages of septum formation and persists until the s eptum is completely formed; then just before cell division the label disapp ears. From these results we suggest that linear beta -1,3-glucan is involve d 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 appa ratus, on small vesicles and underneath the cell membrane. These results gi ve fresh evidence for the hypothesis that P-1,6-glucan is synthesized in th e endoplasmic reticulum-Golgi system and exported to the cell membrane. Cop yright (C) 2001 John Wiley & Sons, Ltd.