Mutations that are synthetically lethal with a gas1Δ allele cause defects in the cell wall of Saccharomyces cerevisiae

The GAS1 -related genes of fungi encode GPI-anchored proteins with beta -1,3-glucanosyltransferase activity. Loss of this activity results in defects in the assembly of the cell wall. We isolated mutants that show a synthetic defect when combined with a gas1 Delta allele in Saccharomyces cerevisiae, and identified nine wild-type genes that rescue this defect. The indispensability of BIG1 and KRE6 for the viability of gas1 Delta cells confirmed the important role of beta -1,6-glucan in cells that are defective in the processing of beta -1,3-glucan. The identification of the Wsc1p hypo-osmotic stress sensor and components of the PKC signal transduction pathway in our screen also confirmed that the cell wall integrity response attenuates the otherwise lethal gas1 Delta defect. Unexpectedly, we found that the KEX2 gene is also required for the viability of the gas1 Delta mutant. Kex2p is a Golgi/endosome-membrane-anchored protease that processes secretory preproteins. A cell wall defect was also found in the kex2 Delta mutant, which was suppressible by multiple copies of the MKC7 or YAP3 gene, both of which encode other GPI-anchored proteases. Therefore, normal cell wall assembly requires proteolytic processing of secretory preproteins. Furthermore, the genes CSG2 and IPT1 were found to be required for normal growth of gas1 Delta cells in the presence of 1 M sorbitol. This finding suggests that complex sphingolipids play a role in the hyper-osmotic response.