The protein kinase C pathway is required for viability in quiescence in Saccharomyces cerevisiae

Protein kinase C, encoded by PKC1, regulates construction of the cell surface in vegetatively growing yeast cells. Pkc1 in part acts by regulating Mpk1, a MAP kinase [1, 2]. Mutants lacking Bck1, a component of the MAP kinase branch of the pathway, fail to respond normally to nitrogen starvation, which causes entry into quiescence [3, 4]. Given that the Tor1 and Tor2 proteins are key inhibitors of entry into quiescence [5], the Pkc1 pathway may regulate these proteins. We find that pkc1Delta and mpk1Delta mutants rapidly die by cell lysis upon carbon or nitrogen starvation. The Pkc1 pathway does not regulate the TOR proteins: transcriptional changes dependent on inhibition of the TORs occur normally in pkc1Delta and mpk1Delta mutants when starved for nitrogen; pkc1Delta and mpk1Delta mutants die rapidly upon treatment with rapamycin, an inhibitor of the TORs. We find that Mpk1 is transiently activated by rapamycin treatment via a novel mechanism. Finally, we find that rapamycin treatment or nitrogen starvation induces resistance to the cell wall-digesting enzyme zymolyase by a Pkc1-dependent mechanism. Thus, the Pkc1 pathway is not a nutrient sensor but acts downstream of TOR inhibition to maintain cell integrity in quiescence.