Architecture of the Yeast Cell Wall

Abstract

linkage region between chitin and ␤(133)-glucan was solubilized and isolated in the form of oligosaccharides, after digestion of yeast cell walls with ␤(133)-glucanase, reduction with borotritide, and subsequent incubation with chitinase. In addition to the oligosaccharides, the solubilized fraction contained tritium-labeled high molecular weight material. We have now investigated the nature of this material and found that it represents areas in which all four structural components of the cell wall, ␤(133)-glucan, ␤(136)-glucan, chitin, and mannoprotein are linked together. Mannoprotein, with a protein moiety about 100 kDa in apparent size, is attached to ␤(136)-glucan through a remnant of a glycosylphosphatidylinositol anchor containing five ␣-linked mannosyl residues. The ␤(136)-glucan has some ␤(133)-linked branches, and it is to these branches that the reducing terminus of chitin chains appears to be attached in a ␤(134) or ␤(132) linkage. Finally, the reducing end of ␤(136)-glucan is connected to the nonreducing terminal glucose of ␤(133)-glucan through a linkage that remains to be established. A fraction of the isolated material has three of the main components but lacks mannoprotein. From these results and previous findings on the linkage between mannoproteins and ␤(136)-glucan, it is concluded that the latter polysaccharide has a central role in the organization of the yeast cell wall. The possible mechanism of synthesis and physiological significance of the cross-links is discussed.Cell walls are essential for the survival of fungal cells. Digestion of cell walls in the absence of an osmotic protector leads to cell lysis due to the high internal turgor pressure. Thus, substances that interfere with cell wall synthesis may be considered as potential antifungal agents (1). Because of its rigidity, the cell wall determines the shape of fungal cells. For that reason, cell wall formation has been used as a model for morphogenesis (1).The major components of fungal cell walls are polysaccharides and glycoproteins (2). In the yeast, Saccharomyces cerevisiae, the cell wall contains ␤(133)-D-glucan, ␤(136)-D-glucan, chitin, and mannoprotein(s) (3). The polysaccharides appear to have a structural function, whereas the mannoprotein(s) may act as "filler" and are important for the permeability of the cell wall (4, 5). How can one explain the strength and resilience of the fungal cell wall? Recent results with S. cerevisiae suggest that the answer may be found in the existence of covalent linkages between the different components of the wall that would give rise to a continuous and consequently stronger fabric. Thus, previous studies in our laboratories showed the presence of linkages between chitin and ␤(133)-glucan (6) as well as among glycoproteins, ␤(136)-glucan, and ␤(133)-glucan (7).The strategy for the investigation of interconnections between chitin and ␤(133)-glucan consisted in the digestion of cell walls with ␤(133)-glucanase, followed by labeling of the exposed reducing ends with borotritide and enzymati...

Genome-wide Analysis of the Response to Cell Wall Mutations in the Yeast Saccharomyces cerevisiae

“…The increased chitin content in cell wall mutants was shown recently to be due to a concurrent increase of both Gfa1p and Chs3p activity (55). This higher level of chitin together with increased cross-linkages between this component and ␤-glucan probably contributes to a strengthening of the cell wall (92,93). Moreover, a higher number of cross-links between cell wall polymers is likely to occur as indicated by up-regulation of genes that encode glucosyl/glucanolytransferases (CRH1, BGL2, and SCW10).…”

The Candida albicans CHS4 gene complements a Saccharomyces cerevisiae skt5/chs4 mutation and is involved in chitin biosynthesis

“…Chitin is synthesized inside the cytoplasmic surface of plasma membranes, and is then extruded and deposited on the outer surface of the cell wall as microfibrils, which subsequently congregate to form crystalline structures in combination with glucan complex (Kollar et al, 1995). It is still not known, however, how chitin is built into the cell wall.…”

“…However, there is a more plausible reason. The cell wall is comprised of a network formed by linkage between mannoprotein via a GPI remnant to 1,6-p-glucan, 1,3-/?-glucan and finally chitin (Chaffin et al, 1998;Kollar et al, ,1995). Cell wall injury from diminished chitin synthesis may cause a fragile cell wall architecture with a consequent impairment in mannoprotein synthesis.…”

Modulation of Yeast Sln1 Kinase Activity by the Ccw12 Cell Wall Protein

“…This property of the wall is presumed to be due to the formation of interlinkages between the individual wall components to create a highly branched meshwork. A structure consisting of covalently linked ␤-1,3-and ␤-1,6-glucan, chitin, and mannoproteins has been reported (37,38), and a stable phosphodiester linkage has been proposed to connect the mannoproteins to the rest of the meshwork (39).…”