Genetic inactivation of essentialHSF1reveals an isolated transcriptional stress response selectively induced by protein misfolding

Abstract

ABSTRACTHeat Shock Factor 1 (Hsf1) in yeast drives the basal transcription of key proteostasis factors and its activity is induced as part of the core heat shock response. Exploring Hsf1 specific functions has been challenging due to the essential nature of theHSF1gene and the extensive overlap of target promoters with environmental stress response (ESR) transcription factors Msn2 and Msn4 (Msn2/4). In this study, we constructed a viablehsf1Δ strain by replacing theHSF1open reading frame with genes that constitutively express Hsp40, Hsp70 and Hsp90 from Hsf1-independent promoters. Phenotypic analysis showed that thehsf1Δ strain grow slowly, is sensitive to heat as well as protein misfolding and accumulates protein aggregates. Transcriptome analysis revealed that the transcriptional response to protein misfolding induced by azetidine-2-carboxylic acid is fully dependent of Hsf1. In contrast, thehsf1Δ strain responded to heat shock through the ESR. Following HS, Hsf1 and Msn2/4 showed functional compensatory induction with stronger activation of the remaining stress pathway when the other branch was inactivated. Thus, we provide a long overdue genetic test of the function of Hsf1 in yeast using the novelhsf1Δ construct. Our data highlight that the accumulation of misfolded proteins is uniquely sensed by Hsf1-Hsp70 chaperone titration inducing a highly selective transcriptional stress response.