Investigating the role of the ribosome associated chaperone Zuo1 under TORC1 inhibition

Protein homeostasis or proteostasis ensures the proteome is regulated in response to the requirements of the cell, which is essential for cell function and survival. Eukaryotic cells achieve proteostasis through regulation of the proteostasis network, a large collection of pathways involved in protein synthesis, folding and degradation, which maintain the integrity of the proteome. The proteostasis network is tightly regulated by the target of rapamycin complex 1 (TORC1) in response to nutrient availability and environmental signals. Upon TORC1 inhibition, degradative activity is increased, and protein synthesis is reduced through inhibition of translation initiation, to maintain cell viability in challenging conditions. The ribosome associated chaperones support proteostasis maintenance in basal conditions, by promoting protein folding, however a role for these chaperones under challenging conditions has not been demonstrated. In this work, I have demonstrated that Zuo1 is required for the reduction of translation, and proteostasis maintenance, upon TORC1 inhibition, and this is reliant on its role as a ribosome associated chaperone. Furthermore, the rapid degradation of eIF4G following TORC1 inhibition is prevented in zuo1Δ cells, contributing to decreased survival in these conditions. In addition to the loss of translational regulation, a defect in autophagy is uncovered in zuo1Δ cells, which underlies the failure to degrade eIF4G in response to TORC1 inhibition. In order to identify interactors of Zuo1 which could be facilitating its role in both the regulation of translation and autophagy, a quantitative proteomics approach was employed. Several novel interactors were identified and the preliminary characterisation of those which showed an altered interaction with Zuo1 following TORC1 inhibition, has revealed potential links to these processes. Overall, this work highlights that pleiotropic defects in zuo1Δ cells impairs cell survival under TORC1 inhibition and demonstrates the importance of Zuo1 for maintaining proteostasis and cell viability in response to changes in TORC1 signalling.