A Role for the Saccharomyces cerevisiae Regulation of Ace2 and Polarized Morphogenesis Signaling Network in Cell Integrity

Saccharomyces cerevisiae RAM is a conserved signaling network that regulates maintenance of polarized growth and daughter-cell-specific transcription, the latter of which is critical for septum degradation. Consequently, cells defective in RAM function (designated ramΔ) are round in morphology, form feeble mating projections, and fail to separate following cytokinesis. It was recently demonstrated that RAM genes are essential in strains containing functional SSD1 (SSD1-v), which encodes a protein of unknown function that binds the RAM Cbk1p kinase. Here we investigated the essential function of RAM in SSD1-v strains and identified two functional groups of dosage suppressors for ramΔ lethality. We establish that all ramΔ mutants exhibit cell integrity defects and cell lysis. All dosage suppressors rescue the lysis but not the cell polarity or cell separation defects of ramΔ cells. One class of dosage suppressors is composed of genes encoding cell wall proteins, indicating that alterations in cell wall structure can rescue the cell lysis in ramΔ cells. Another class of ramΔ dosage suppressors is composed of ZRG8 and SRL1, which encode two unrelated proteins of unknown function. We establish that ZRG8 and SRL1 share similar genetic interactions and phenotypes. Significantly, Zrg8p coprecipitates with Ssd1p, localizes similarly to RAM proteins, and is dependent on RAM for localization. Collectively, these data indicate that RAM and Ssd1p function cooperatively to control cell integrity and suggest that Zrg8p and Srl1p function as nonessential inhibitors of Ssd1p.