Studies on a functional coordination among amino acid sensing, TORC1 and autophagy in Saccharomyces cerevisiae

Eukaryotic cell growth is tightly linked with the availability of nutrients like as amino acids and glucose. When the intracellular amino acid level is sufficient for cell growth, the target of rapamycin (TOR) signaling pathway is activated and plays crucial roles in cell growth by regulating translation, ribosome biogenesis and so on. In contrast, when cells are starved from amino acids, macroautophagy is induced and functions for maintaining cellular homeostasis under starvation. In the first part, I studied a crosstalk between TOR complex 1 (TORC1) pathway and amino acid signaling. Using a genome-wide protein localization study, I found that Stp1 disappeared from the nucleus upon inactivation of TORC1 by rapamycin, suggesting the involvement of Stp1 in the TOR signaling pathway. Supporting this notion, knockout mutant for the STP1 gene was found to be hypersensitive to rapamycin, and overexpression of Stp1 conferred resistance to rapamycin. Interestingly, I found that the rapamycin-induced disappearance of Stp1 from the nucleus resulted from Stp1 degradation, which was dependent on the activity of a protein phosphatase 2A (PP2A)-like phosphatase Sit4, a well-known downstream effector of TORC1. Taken together, these findings highlight an intimate connection between the amino acid-sensing pathway and the rapamycin-sensitive TOR signaling pathway. In the second part, I investigated an interesting bidirectional regulation between TORC1 and autophagy. It has been reported in various model organisms that autophagy and TORC1 signaling are strongly involved in eukaryotic cell aging, and decreasing TORC1 activity extends longevity by an autophagy-dependent mechanism. Thus, to expand our knowledge of the regulation of eukaryotic cell aging, it is important to understand the relationship between TORC1 signaling and autophagy. I showed that mutant cells with weak TORC1 activity maintain autophagy longer than wild-type cells, and TORC1 is partially reactivated under ongoing nitrogen starvation by an autophagy-dependent mechanism. In addition, I found that Atg13 is gradually rephosphorylated during prolonged nitrogen starvation, and the kinase activity of Atg1 is required for Atg13 rephosphorylation. These data suggest that TORC1 can be substantially, but not fully, reactivated in an autophagy-dependent manner under ongoing starvation, and reactivated TORC1 eventually plays a role in the attenuation of autophagy. In conclusion, I have shown a fine tuning between TORC1 and autophagy in accordance with the availability of amino acids. This bidirectional regulation makes cells respond properly to their extracellular environment and maintain cellular homeostasis even under amino acid starvation.Docto