SKG1, a suppressor gene of synthetic lethality of kex2Deltagas1Delta mutations, encodes a novel membrane protein that affects cell wall composition.

The fungal GAS1-related genes encode GPI-anchored beta-1,3-glucanosyltransferase, and their loss causes a defect in the assembly of the cell wall. The KEX2 gene encodes a processing protease in the late Golgi compartment and its loss also results in defects in the cell wall. Simultaneous mutations of these genes are lethal in Saccharomyces cerevisiae. To understand the basis of this synthetic lethality, we screened for multicopy suppressors and identified 13 SKG (suppressor of kex2 gas1 synthetic lethality) genes. SKG1 encodes a transmembrane protein that localizes on the inner surface of the plasma membrane at the bud and in the daughter cell. The multicopy SKG1 increases the sensitivity of cells to zymolyase, and the skg1Delta null mutation increases resistance to it. This zymolyase susceptibility corresponds to an increase of alkali-soluble beta-1,3-glucan and a decrease of chitin in the cell wall. Thus SKG1 encodes a novel protein that affects the cell wall polymer composition in the growing region of the cell.