INCREASE IN CHITIN AS AN ESSENTIAL RESPONSE TO DEFECTS IN ASSEMBLY OFCELL-WALL POLYMERS IN THE GGP1-DELTA MUTANT OF SACCHAROMYCES-CEREVISIAE

The GGP1/GAS1 gene codes for a glycosylphosphatidylinositol-anchored p lasma membrane glycoprotein of Saccharomyces cerevisiae. The ggp1 Delt a mutant shows morphogenetic defects which suggest changes in the cell wall matrix. In this work, we have investigated cell wall glucan leve ls and the increase of chitin in ggp1 Delta mutant cells. In these cel ls, the level of alkali-insoluble 1,6-beta-D-glucan was found to be 50 % of that of wild-type cells and was responsible for the observed decr ease in the total alkali-insoluble glucan. Moreover, the ratio of alka li-soluble to alkali-insoluble glucan almost doubled, suggesting a cha nge in glucan solubility. The increase of chitin in ggp1 Delta cells w as found to be essential since the chs3 Delta ggp1 Delta mutations det ermined a Severe reduction in the growth rate and in cell viability. E lectron microscopy analysis showed the loss of the typical structure o f yeast cell walls. Furthermore, in the chs3 Delta ggp1 Delta cells, t he level Of alkali-insoluble glucan was 57% of that of wild-type cells and the alkali-soluble/alkali-insoluble glucan ratio was doubled. We tested the effect of inhibition of chitin synthesis also by a differen t approach. The ggp1 Delta cells were treated with nikkomycin Z, a wel l-known inhibitor of chitin synthesis, and showed a hypersensitivity t o this drug. In addition, studies of genetic interactions with genes r elated to the construction of the cell wall indicate a synthetic letha l effect of the ggp1 Delta kre6 Delta and the ggp1 Delta pkc1 Delta co mbined mutations. Our data point to an involvement of the GGP1 gene pr oduct in the cross-links between cell wall glucans (1,3-beta-D-glucans with 1,6-beta-D-glucans and with chitin). Chitin is essential to comp ensate for the defects due to the lack of Ggp1p. Moreover, the activit ies of Ggp1p and Chs3p are essential to the formation of the organized structure of the cell wall in vegetative cells.