Bidirectional regulation between TORC1 and autophagy in Saccharomyces cerevisiae

It has been reported in various model organisms that autophagy and the target of rapamycin complex 1 (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. Many researchers have shown that TORC1 represses autophagy under normal growth conditions, and TORC1 inactivation contributes to the upregulation of autophagy. However, it is poorly understood how autophagy is regulated or terminated when starvation is prolonged. Here, we report that bidirectional regulation between autophagy and TORC1 exists in the yeast Saccharomyces cerevisiae. We show 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, we found that Atg13 is gradually rephosphorylated during prolonged nitrogen starvation, and the kinase activity of Atg1 is required for Atg13 rephosphorylation. Our data suggest that TORC1 can be substantially, if not fully, reactivated in an autophagy-dependent manner under ongoing starvation, and that partially reactivated TORC1 eventually plays a role in the attenuation of autophagy.