Initiation of the TORC1-Regulated G₀ Program Requires Igo1/2, which License Specific mRNAs to Evade Degradation via the 5′-3′ mRNA Decay Pathway

Eukaryotic cell proliferation is controlled by growth factors and essential nutrients, in the absence of which cells may enter into a quiescent (G₀) state. In yeast, nitrogen and/or carbon limitation causes downregulation of the conserved TORC1 and PKA signaling pathways and, consequently, activation of the PAS kinase Rim15, which orchestrates G₀ program initiation and ensures proper life span by controlling distal readouts, including the expression of specific genes. Here, we report that Rim15 coordinates transcription with posttranscriptional mRNA protection by phosphorylating the paralogous Igo1 and Igo2 proteins. This event, which stimulates Igo proteins to associate with the mRNA decapping activator Dhh1, shelters newly expressed mRNAs from degradation via the 5′-3′ mRNA decay pathway, thereby enabling their proper translation during initiation of the G₀ program. These results delineate a likely conserved mechanism by which nutrient limitation leads to stabilization of specific mRNAs that are critical for cell differentiation and life span.