A synthetic analysis of the Saccharomyces cerevisiae stress sensor Mid2p, and identification of a Mid2p-interacting protein, Zeo1p, that modulates the PKC1-MPK1 cell integrity pathway

Mid2p is a plasma membrane protein that functions in Saccharomyces cerevisiae as a sensor of cell wall stress, activating the PKC1–MPK1 cell integrity pathway via the small GTPase Rho1p during exposure to mating pheromone, calcofluor white, and heat. To examine Mid2p signalling, a global synthetic interaction analysis of a mid2 mutant was performed; this identified 11 interacting genes. These include WSC1 and ROM2, upstream elements in cell integrity pathway signalling, and FKS1 and SMI1, required for 1,3-β-glucan synthesis. These synthetic interactions indicate that the Wsc1p sensor acts through Rom2p to activate the Fks1p glucan synthase in a Mid2p-independent way. To further explore Mid2p signalling a two-hybrid screen was done using the cytoplasmic tail of Mid2p; this identified ZEO1 (YOL109w), encoding a 12 kDa peripheral membrane protein that localizes to the plasma membrane. Disruption of ZEO1 leads to resistance to calcofluor white and to a Mid2p-dependent constitutive phosphorylation of Mpk1p, supporting a role for Zeo1p in the cell integrity pathway. Consistent with this, zeo1-deficient cells suppress the growth defect of mutants in the Rho1p GDP–GTP exchange factor Rom2p, while exacerbating the growth defect of sac7Δ mutants at 37 °C. In contrast, mid2Δ mutants have opposing effects to zeo1Δ mutants, being synthetically lethal with rom2Δ, and suppressing an 18 °C growth defect of sac7Δ, while overexpression of MID2 rescues a rom2Δ 37 °C growth defect. Thus, MID2 and ZEO1 appear to play reciprocal roles in the modulation of the yeast PKC1–MPK1 cell integrity pathway.