A novel role for the mating type (MAT) locus in the maintenance of cell wall integrity in Saccharomyces cerevisiae

The cell wall and stress response component (Wsc) protein family in the yeast Saccharomyces cerevisiae is encoded by at least three genes, WSC1, WSC2, and WSC3. The Wsc proteins are putative upstream activators of the RHO1-regulated PKC1-MAP kinase cascade, and are required for maintenance of cell wall integrity and the stress response. Deletion of WSC1 causes a cell lysis defect that is exacerbated by deleting WSC2 or WSC3. This eel lysis defect can be rescued by adding osmotic stabilizers, such as 1 M sorbitol, to the medium, and by overexpressing PKC1 or RHO1. To advance our understanding of the function of the WSC genes, we performed a genetic screen to identify other components of the pathways they regulate. Here we report our findings. MA Tal and MAT alpha 2 were identified as dosage-dependent suppressors of the lysis defect of a wsc Delta mutant. Overexpression of MA Tal or MAT alpha 2 was found to suppress the heat shock sensitivity, in addition to the lysis defect, of the wsc Delta mutant. Phenotypic suppression by these two genes, MATa1 and MAT alpha 2, is significantly stronger when they are overexpressed in cells of the opposite mating type. Deletion of MATa1 exacerbates the lysis defect of haploid and diploid wsc Delta strains. Our results suggest that the MAT locus plays a role in responses similar to those regulated by WSC and provide evidence for a regulatory effect of the MAT locus outside the realm of cell type determination.