Yeast has numerous mechanisms to survive stress. Deletion of myosin type II (myo1Δ) in Saccharomyces cerevisiae results in a cell that has defective cytokinesis. To survive this genetically induced stress, this budding yeast up regulates the PKC1 cell wall integrity pathway (CWIP). More recently, our work indicated that TOR, another stress signaling pathway, was down regulated in myo1Δ strains. Since negative signaling by TOR is known to regulate PKC1, our objectives in this study were to understand the cross-talk between the TOR and PKC1 signaling pathways and to determine if they share upstream regulators for mounting the stress response in myo1Δ strains.
Here we proved that TORC1 signaling was down regulated in the myo1Δ strain. While a tor1Δ mutant strain had increased viability relative to myo1Δ, a combined myo1Δtor1Δ mutant strain showed significantly reduced cell viability. Synthetic rescue of the tor2-21ts lethal phenotype was observed in the myo1Δ strain in contrast to the chs2Δ strain, a chitin synthase II null mutant that also activates the PKC1 CWIP and exhibits cytokinesis defects very similar to myo1Δ, where the rescue effect was not observed. We observed two pools of Slt2p, the final Mitogen Activated Protein Kinase (MAPK) of the PKC1 CWIP; one pool that is up regulated by heat shock and one that is up regulated by the myo1Δ stress. The cell wall stress sensor WSC1 that activates PKC1 CWIP under other stress conditions was shown to act as a negative regulator of TORC1 in the myo1Δ mutant. Finally, the repression of TORC1 was inversely correlated with the activation of PKC1 in the myo1Δ strain.
Regulated expression of TOR1 was important in the activation of the PKC1 CWIP in a myo1Δ strain and hence its survival. We found evidence that the PKC1 and TORC1 pathways share a common upstream regulator associated with the cell wall stress sensor WSC1. Surprisingly, essential TORC2 functions were not required in the myo1Δ strain. By understanding how yeast mounts a concerted stress response, one can further design pharmacological cocktails to undermine their ability to adapt and to survive.
Keywords: PKC1, SLT2/MPK1, WSC1, Tor2-21, Fungal cell wall