Chitin synthesis in a gas1 mutant of Saccharomyces cerevisiae

The existence of a compensatory mechanism in response to cell wall damage h as been proposed in yeast cells. The increase of chitin accumulation is par t of this response. In order to study the mechanism of the stress-related c hitin synthesis, we tested chitin synthase I (CSI), CSII, and CSIII in vitr o activities in the cell-wall-defective mutant gas1 Delta. CSI activity inc reased twofold with respect to the control, a finding in agreement with an increase in the expression of the CHS1 gene. However, deletion of the CHS1 gene did not affect the phenotype of the gas1 Delta mutant and only slightl y reduced the chitin content. Interestingly, in chs1 gas1 double mutants th e lysed-bud phenotype, typical of chs1 null mutant, was suppressed, althoug h in gas1 cells there was no reduction in chitinase activity. CHS3 expressi on was not affected in the gas1 mutant. deletion of the CHS3 gene severely compromised the phenotype of gas1 cells, despite the fact that CSIII activi ty, assayed in membrane fractions, did not change. Furthermore, in chs3 gas 1 cells the chitin level was about 10% that of gas1 cells. Thus, CSIII is t he enzyme responsible for the hyperaccumulation of chitin in response to ce ll wall stress. However, the level of enzyme or the in vitro CSIII activity does not change. This result suggests that an interaction with a regulator y molecule or a posttranslational modification, which is not preserved duri ng membrane fractionation, could be essential in vivo for the stress-induce d synthesis of chitin.