Chitin Synthesis in a gas1 Mutant of Saccharomyces cerevisiae

The existence of a compensatory mechanism in response to cell wall damage has been proposed in yeast cells. The increase of chitin accumulation is part of this response. In order to study the mechanism of the stress-related chitin synthesis, we tested chitin synthase I (CSI), CSII, and CSIII in vitro activities in the cell-wall-defective mutantgas1 Δ. CSI activity increased twofold with respect to the control, a finding in agreement with an increase in the expression of theCHS1 gene. However, deletion of theCHS1 gene did not affect the phenotype of thegas1 Δ mutant and only slightly reduced the chitin content. Interestingly, inchs1 gas1 double mutants the lysed-bud phenotype, typical ofchs1 null mutant, was suppressed, although ingas1 cells there was no reduction in chitinase activity.CHS3 expression was not affected in thegas1 mutant. Deletion of theCHS3 gene severely compromised the phenotype ofgas1 cells, despite the fact that CSIII activity, assayed in membrane fractions, did not change. Furthermore, inchs3 gas1 cells the chitin level was about 10% that ofgas1 cells. Thus, CSIII is the enzyme responsible for the hyperaccumulation of chitin in response to cell wall stress. However, the level of enzyme or the in vitro CSIII activity does not change. This result suggests that an interaction with a regulatory molecule or a posttranslational modification, which is not preserved during membrane fractionation, could be essential in vivo for the stress-induced synthesis of chitin.