Molecular Logic of TORC1 through the EGO-Complex and Pib2

Abstract

The ability to manage cell growth according to nutrient and energy availability is fundamental to all life. The key regulator of this process in eukaryotes is the Target of Rapamycin Complex I (TORC1), which adjusts cell growth in response to a wide variety of nutrient and environmental signals. Despite increasing knowledge of the different cues regulating TORC1, the mechanisms used by TORC1 to integrate this information and adopt appropriate signaling modalities remains poorly characterized. In fact, the reliance on a relatively small number of TORC1 reporters has hindered the development of a unified TORC1 signaling model which covers a range of activity states between full activation or inactivation. Furthermore, TORC1-dependent response pathways have been mostly studied in response to acute stress; less well understood is how TORC1 signaling adapts to more nuanced changes to the environment.Here, I show that TORC1 can adopt multiple signaling states in response to diverse conditions in the baker’s yeast Saccharomyces cerevisiae. I begin by showing how the two key yeast TORC1 regulators, the Exit from G0 complex (EGOC) and Pib2, appear to regulate TORC1 in a parallel model when measuring the common TORC1 activity marker, phospho-RPS6. I then expand this model using global phosphoproteomics, and in contrast to the previous data, I show that many TORC1 targets can be regulated cooperatively (not in parallel) by EGOC and Pib2. Among these data, I characterize the phosphoglycerate dehydrogenase Ser33 as a Pib2-dependent TORC1 target. I then use a set of different TORC1 outputs to show that the TORC1 network can adopt different signaling levels, with varying thresholds for phosphorylation and transcription factor behavior based on environmental conditions. I then establish how a combination of parallel and cooperative regulation between the EGOC and Pib2 maintain a state of intermediate TORC1 activity, driving both growth and stress responses in partial starvation conditions. These findings provide unique insight that TORC1, and potentially other signaling hubs, can adopt different states, leading to substrate specificity across diverse conditions.