Complex Regulation of the Yeast Heat Shock Transcription Factor

Abstract

The yeast heat shock transcription factor (HSF) is regulated by posttranslational modification. Heat and superoxide can induce the conformational change associated with the heat shock response. Interaction between HSF and the chaperone hsp70 is also thought to play a role in HSF regulation. Here, we show that the Ssb1/2p member of the hsp70 family can form a stable, ATP-sensitive complex with HSF-a surprising finding because Ssb1/2p is not induced by heat shock. Phosphorylation and the assembly of HSF into larger, ATP-sensitive complexes both occur when HSF activity decreases, whether during adaptation to a raised temperature or during growth at low glucose concentrations. These larger HSF complexes also form during recovery from heat shock. However, if HSF is assembled into ATP-sensitive complexes (during growth at a low glucose concentration), heat shock does not stimulate the dissociation of the complexes. Nor does induction of the conformational change induce their dissociation. Modulation of the in vivo concentrations of the SSA and SSB proteins by deletion or overexpression affects HSF activity in a manner that is consistent with these findings and suggests the model that the SSA and SSB proteins perform distinct roles in the regulation of HSF activity. INTRODUCTIONIt has long been known that in cells of many species, including Escherichia coli and Saccharomyces cerevisiae, cell division rates are tightly coupled with the steady-state levels and rates of synthesis of ribosomal proteins and rRNA. For example, cells in rich media, with a short generation time, display higher abundance of rRNA and higher rates of synthesis of ribosomal proteins than do cells in poor media, with a long generation time. In E. coli, some of this regulation is achieved by transcriptional attenuation and translational regulation via binding of ribosomal proteins to the RNAs of target operons (Freedman et al., 1985;Cole and Nomura, 1986). In yeast, regulation is partly transcriptional, via RAP1 and its binding site, the upstream activating sequence (UAS) rpg (Herruer et al., 1987;Moehle and Hinnebusch, 1991;Kraakman et al., 1993), although much of the regulation of ribosomal protein abundance is achieved by a competition between the assembly into ribosomes and the rapid degradation of unassembled ribosomal proteins (Warner et al., 1985;Maicas et al., 1988).Growth rate control clearly modulates the level of the translational machinery, which in turn influences the overall rate of protein synthesis. Changes in the rates of protein synthesis must, in turn, have profound implications for the protein chaperone system. Newly synthesized polypeptides typically do not adopt their mature conformation immediately but instead follow a complex folding pathway. For many proteins, the cytoplasmic chaperone system plays an integral role in helping these polypeptides adopt their mature conformations (for review, see Hendrick and Hartl, 1995). The hsp70 proteins are generally believed to bind efficiently to nascent or newly synthesized polypeptides...

Transcriptional Regulation inSaccharomyces cerevisiae: Transcription Factor Regulation and Function, Mechanisms of Initiation, and Roles of Activators and Coactivators

“…The sensor for both transient and sustained response appears to be the DNA-binding and oligomerization domains of Hsf1 on the basis of heterologous fusion proteins and other studies (Bonner et al 1994(Bonner et al , 2000a. Temperatureresistant mutations near the DBD enable Hsf1 lacking its C-terminal AD to activate transcription (Hashikawa et al 2006).…”

“…These suppressors implicate the DBD in mediating the response, as did single point mutations that constitutively enhanced transcription (Bulman et al 2001). A conformational change accompanying activation can be detected by electrophoretic mobility shift in vitro and requires two trimers of Hsf1 bound to DNA (Bonner et al 2000a; S. , but determining whether a similar alteration occurs in vivo is challenging. Genetic evidence suggests that repression of DNA-bound Hsf1 may be mediated by chaperones (Duina et al 1998;Bonner et al 2000a).…”

“…A conformational change accompanying activation can be detected by electrophoretic mobility shift in vitro and requires two trimers of Hsf1 bound to DNA (Bonner et al 2000a; S. , but determining whether a similar alteration occurs in vivo is challenging. Genetic evidence suggests that repression of DNA-bound Hsf1 may be mediated by chaperones (Duina et al 1998;Bonner et al 2000a). Thus, mutations in Hsf1 that cause constitutive activity could alter its interactions with inhibitory chaperones.…”

Fungal Hsp90: a biological transistor that tunes cellular outputs to thermal inputs

“…Almost three decades ago Lindquist 2,50 and Didomenico et al 51 postulated that feedback components down-regulate the heat shock response. Initially, Hsp70 was thought to be the key HSF repressor in mammalian systems [52][53][54] , and Hsf1 interacts with Hsp70 family members in S. cerevisiae 55 . However, yeast Sse1 (an Hsp70) is required for Hsp90-dependent functions 56 , suggesting that Hsp90 is the Hsf1 repressor.…”

Induced Metastable Memory in Heat Shock Response

“…In most cells 1 , HSPs are commonly found at very low concentrations, but the production of some of them can be induced in response to external stimuli. This phenomenon could be explained by the complex regulation of Heat Shock Transcription Factor (HSF) [8] and its relationship with HSP and the misfolded proteins [3]. At the promoter site of Hsp70 is found the Heat Shock Element (HSE), that can be stimulated by the complexation with a trimerized protein, the Heat Shock Factor (HSF).…”