The PKC1-MPK1 pathway in yeast functions in the maintenance of cell wall integrity and in the stress response. We have identified a family of genes that are putative regulators of this pathway. WSC1, WSC2, and WSC3 encode predicted integral membrane proteins with a conserved cysteine motif and a WSC1-green f luorescence protein fusion protein localizes to the plasma membrane. Deletion of WSC results in phenotypes similar to mutants in the PKC1-MPK1 pathway and an increase in the activity of MPK1 upon a mild heat treatment is impaired in a wsc⌬ mutant. Genetic analysis places the function of WSC upstream of PKC1, suggesting that they play a role in its activation. We also find a genetic interaction between WSC and the RAS-cAMP pathway. The RAS-cAMP pathway is required for cell cycle progression and for the heat shock response. Overexpression of WSC suppresses the heat shock sensitivity of a strain in which RAS is hyperactivated and the heat shock sensitivity of a wsc⌬ strain is rescued by deletion of RAS2. The functional characteristics and cellular localization of WSC suggest that they may mediate intracellular responses to environmental stress in yeast.

The von Recklinghausen neurofibromatosis locus, NF1, encodes a protein with homology restricted to the catalytic region of the RAS GTPase-activating protein, GAP, and with extensive homology to the IRA1 and IRA2 gene products of the yeast S. cerevisiae. A segment of the NF1 cDNA gene, expressed in yeast, can complement loss of IRA function and can inhibit both wild-type and mutant activated human H-ras genes that are coexpressed in yeast. Yeast expressing the NF1 segment have increased H-ras GTPase-stimulating activity. These studies indicate that the NF1 gene product can interact with RAS proteins and demonstrate structural and functional similarities and differences among the GAP, IRA1, IRA2, and NF1 proteins.

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The interaction between RAS proteins and adenylyl cyclase was studied by using dominant interfering mutations of adenylyl cyclase from the yeast Saccharomyces cerevisiae. RAS proteins activate adenylyl cyclase in this organism. A plasmid expressing a catalytically inactive adenylyl cyclase was found to interfere dominantly with this activation. The interfering region mapped to the leucine-rich repeat region of adenylyl cyclase, which is homologous to domains present in several other proteins and is thought to participate in protein-protein interactions.