Architecture of the Yeast Cell Wall

Abstract

In a previous study (Kollár, R., Petráková, E., Ashwell, G., Robbins, P. W., and Cabib, E. (1995) J. Biol. Chem. 270, 1170-1178), the linkage region between chitin and beta(1-->3)-glucan was solubilized and isolated in the form of oligosaccharides, after digestion of yeast cell walls with beta(1-->3)-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, beta(1-->3)-glucan, beta(1-->6)-glucan, chitin, and mannoprotein are linked together. Mannoprotein, with a protein moiety about 100 kDa in apparent size, is attached to beta(1-->6)-glucan through a remnant of a glycosylphosphatidylinositol anchor containing five alpha-linked mannosyl residues. The beta(1-->6)-glucan has some beta(1-->3)-linked branches, and it is to these branches that the reducing terminus of chitin chains appears to be attached in a beta(1-->4) or beta(1-->2) linkage. Finally, the reducing end of beta(1-->6)-glucan is connected to the nonreducing terminal glucose of beta(1-->3)-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 beta(1-->6)-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.