Regulation of the cell integrity pathway by rapamycin-sensitive TOR function in budding yeast

The TOR (target of rapamycin) pathway controls cell growth in response to nutrient availability in eukary- otic cells. Inactivation of TOR function by rapamycin or nutrient exhaustion is accompanied by triggering various cellular mechanisms aimed at overcoming the nutrient stress. Here we report that in Saccharomyces cerevisiae the protein kinase C (PKC)-mediated mito- gen-activated protein kinase pathway is regulated by TOR function because upon specific Tor1 and Tor2 in- hibition by rapamycin, Mpk1 is activated rapidly in a process mediated by Sit4 and Tap42. Osmotic stabiliza- tion of the plasma membrane prevents both Mpk1 acti- vation by rapamycin and the growth defect that occurs upon the simultaneous absence of Tor1 and Mpk1 func- tion, suggesting that, at least partially, TOR inhibition is sensed by the PKC pathway at the cell envelope. This process involves activation of cell surface sensors, Rom2, and downstream elements of the mitogen- activated protein kinase cascade. Rapamycin also in- duces depolarization of the actin cytoskeleton through the TOR proteins, Sit4 and Tap42, in an osmotically suppressible manner. Finally, we show that entry into stationary phase, a physiological situation of nutrient depletion, also leads to the activation of the PKC path- way, and we provide further evidence demonstrating that Mpk1 is essential for viability once cells enter G0