Deciphering Human Heat Shock Transcription Factor 1 Regulation via Post-Translational Modification in Yeast

Liliana Batista-Nascimento,Daniel W Neef,Phillip C C Liu,Dennis J Thiele,Claudina Rodrigues-Pousada,Mick F Tuite

doi:10.1371/journal.pone.0015976

Abstract

Heat shock transcription factor 1 (HSF1) plays an important role in the cellular response to proteotoxic stresses. Under normal growth conditions HSF1 is repressed as an inactive monomer in part through post-translation modifications that include protein acetylation, sumoylation and phosphorylation. Upon exposure to stress HSF1 homotrimerizes, accumulates in nucleus, binds DNA, becomes hyper-phosphorylated and activates the expression of stress response genes. While HSF1 and the mechanisms that regulate its activity have been studied for over two decades, our understanding of HSF1 regulation remains incomplete. As previous studies have shown that HSF1 and the heat shock response promoter element (HSE) are generally structurally conserved from yeast to metazoans, we have made use of the genetically tractable budding yeast as a facile assay system to further understand the mechanisms that regulate human HSF1 through phosphorylation of serine 303. We show that when human HSF1 is expressed in yeast its phosphorylation at S303 is promoted by the MAP-kinase Slt2 independent of a priming event at S307 previously believed to be a prerequisite. Furthermore, we show that phosphorylation at S303 in yeast and mammalian cells occurs independent of GSK3, the kinase primarily thought to be responsible for S303 phosphorylation. Lastly, while previous studies have suggested that S303 phosphorylation represses HSF1-dependent transactivation, we now show that S303 phosphorylation also represses HSF1 multimerization in both yeast and mammalian cells. Taken together, these studies suggest that yeast cells will be a powerful experimental tool for deciphering aspects of human HSF1 regulation by post-translational modifications.

Highlights

All organisms are exposed to proteotoxic stresses that result in the accumulation of misfolded proteins
Because S303 and S307 are constitutively phosphorylated in mammalian cells and alanine substitution of S303 or S307 promotes constitutive activation of Heat shock transcription factor 1 (HSF1) in mammalian cells in reporter assays [17,18] we tested whether S303 and/or S307 contribute to HSF1 repression in yeast
In this report we show that evaluation of the mechanisms that regulate HSF1 activity in yeast via posttranslational modifications can lead to important insights into the mechanisms that regulate HSF1 in mammalian cells

Summary

Introduction

All organisms are exposed to proteotoxic stresses that result in the accumulation of misfolded proteins In response to these stresses cells have evolved adaptive responses to protect and stabilize cellular proteins until more favorable conditions for cell proliferation are encountered [1]. The heat shock transcription factor, HSF, is a homotrimeric transcription factor that activates gene expression in response to a variety of stresses including heat and oxidative stress, as well as inflammation and infection [2]. Studies suggest that during the initial phase of the stress response, the inactive HSF1 monomer dissociates from Hsp, homotrimerizes, is transported to the nucleus and binds to heat shock elements (HSE) found in the promoters of HSF target genes [10,11]. Stress-dependent hyperphosphorylation of HSF1 by potentially multiple protein kinases has been proposed to, in part, promote HSF1 dependent transactivation [13,14,15]

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Conclusion