Loss of the plasma membrane-bound protein Gas1p in Saccharomyces cerevisiae results in the release of beta1,3-glucan into the medium and induces a compensation mechanism to ensure cell wall integrity.

Deletion of GAS1/GGP1/CWH52 results in a lower beta-glucan content of the cell wall and swollen, more spherical cells (L. Popolo, M. Vai, E. Gatti, S. Porello, P. Bonfante, R. Balestrini, and L. Alberghina, J. Bacteriol. 175:1879-1885, 1993; A. F. J. Ram, S. S. C. Brekelmans, L. J. W. M. Oehlen, and F. M. Klis, FEBS Lett. 358:165-170, 1995). We show here that gas1delta cells release beta1,3-glucan into the medium. Western analysis of the medium proteins with beta1,3-glucan- and beta1,6-glucan-specific antibodies showed further that at least some of the released beta1,3-glucan was linked to protein as part of a beta1,3-glucan-beta1,6-glucan-protein complex. These data indicate that Gas1p might play a role in the retention of beta1,3-glucan and/or beta-glucosylated proteins. Interestingly, the defective incorporation of beta1,3-glucan in the cell wall was accompanied by an increase in chitin and mannan content in the cell wall, an enhanced expression of cell wall protein 1 (Cwp1p), and an increase in beta1,3-glucan synthase activity, probably caused by the induced expression of Fks2p. It is proposed that the cell wall weakening caused by the loss of Gas1p induces a set of compensatory reactions to ensure cell integrity.

Retention of Saccharomyces cerevisiae cell wall proteins through a phosphodiester-linked beta-1,3-/beta-1,6-glucan heteropolymer.

Yeast cell wall proteins, including Cwp1p and alpha-agglutinin, could be released by treating the cell wall with either beta-1,3-or beta-1,6-glucanases, indicating that both polymers are involved in anchoring cell wall proteins. It was shown immunologically that both beta-1,3- and beta-1,6-glucan were linked to yeast cell wall proteins, including Cwp1p and alpha-agglutinin. It was further shown that beta-1,3-glucan was linked to the wall protein through a beta-1,6-glucan moiety. The beta-1,6-glucan moiety could be removed from Cwp1p and other cell wall proteins by cleaving phosphodiester bridges either enzymatically using phosphodiesterases or chemically using ice-cold aqueous hydrofluoric acid. These observations are consistent with the notion that cell wall proteins in Saccharomyces cerevisiae are linked to a beta-1,3-/beta-1,6-glucan heteropolymer through a phosphodiester linkage and that this polymer is responsible for anchoring cell wall proteins. It is proposed that this polymer is identical to the alkali-soluble beta-1,3-/beta-1,6-glucan heteropolymer characterized by Fleet and Manners (1976, 1977).