Differential Requirement for the Cell Wall Integrity Sensor Wsc1p in Diploids Versus Haploids.

The primary role of the Cell Wall Integrity Pathway (CWI) in is monitoring the state of the cell wall in response to general life cycle stresses (growth and mating) and imposed stresses (temperature changes and chemicals). Of the five mechanosensor proteins monitoring cell wall stress, Wsc1p and Mid2p are the most important. We find that has a stringent requirement in zygotes and diploids, unlike haploids, and differing from 's role in shmoos. Diploids lacking die frequently, independent of mating type. Death is due to loss of cell wall and plasma membrane integrity, which is suppressed by osmotic support. Overexpression of several CWI pathway components suppress ∆ zygotic death, including , , and as well as the Rho-GAPS, and . Microscopic observations and suppression by and suggest that ∆ zygotes die during bud emergence. Downstream in the CWI pathway, overexpression of a hyperactive protein kinase C (Pkc1p-R398P) causes growth arrest, and blocks the pheromone response. With moderate levels of Pkc1p-R398P, cells form zygotes and the ∆ defect is suppressed. This work highlights functional differences in the requirement for Wsc1p in diploids Versus haploids and between Mid2p and Wsc1p during mating.