Sir2 links the unfolded protein response and the heat shock response in a stress response network

The Heat Shock Response (HSR) in the cytosol and the Unfolded Protein Response (UPR) in the endoplasmic reticulum are major pathways of the cellular proteostasis network. In Saccharomyces cerevisiae, HSR is regulated by transcription factor Hsf1, and UPR Ire1 branch activates transcription factor Had. Here we demonstrate systemic regulation of proteostasis through a direct link between UPR and HSR. Hsf1 is activated by UPR and its HSR depends on intact UPR. This link is mediated by Sir2, which is not only essential for Hsf1 HSR but also required for Hsf1 activation by UPR. Excess Sir2 augments Hsf1 activation by UPR and can compensate for its impairment in UPR-defective strains. Sir2 is upregulated by UPR but, in turn, it also attenuates this pathway, ensuring that UPR functions only transiently. (C) 2015 The Authors. Published by Elsevier Inc.