The Heat Shock Response as a Condensate Cascade

Abstract

The heat shock response (HSR) is a gene regulatory program controlling expression of molecular chaperones implicated in aging, cancer, and neurodegenerative disease. Long presumed to be activated by toxic protein aggregates, recent work suggests a new functional paradigm for the HSR in yeast. Rather than toxic aggregates, adaptive biomolecular condensates comprised of orphan ribosomal proteins (oRP) and stress granule components have been shown to be physiological chaperone clients. By titrating away the chaperones Sis1 and Hsp70 from the transcription factor Hsf1, these condensates activate the HSR. Upon release from Hsp70, Hsf1 forms spatially distinct transcriptional condensates that drive high expression of HSR genes. In this manner, the negative feedback loop controlling HSR activity - in which Hsf1 induces Hsp70 expression and Hsp70 represses Hsf1 activity - is embedded in the biophysics of the system. By analogy to phosphorylation cascades that transmit information via the dynamic activity of kinases, we propose that the HSR is organized as a condensate cascade that transmits information via the localized activity of molecular chaperones.

Keywords: Condensate; Heat shock; Hsf1; Hsp70; Signaling cascade.

Figure 1.. Structure and activation mechanism of budding yeast Hsf1

A) Domain architecture of yeast Hsf1. IDR: intrinsically disordered region; NE1: N-terminal element 1; CE2: conserved element 2; DBD: DNA binding domain; 3mer: trimerization domain B) Structure of homo-trimeric Hsf1 as predicted by CollabFold, color coded by confidence score. C) Schematic of the Hsf1 activation mechanism during acute heat shock, beginning with dissociation of Hsp70 and culminating in formation of intergenic transcriptional condensates.

Figure 2.. Formation and dispersal of adaptive stress-induced biomolecular condensates

A) Stress-induced protein aggregation can be adaptive through the formation of reversible condensates or maladaptive through formation of irreversible and potentially toxic inclusions. Proteostasis collapse may result in the transition from reversible condensates to irreversible aggregates with loss-of-function and toxic gain-of-function properties. B) Orphan ribosomal proteins (oRPs) form adaptive condensates at the periphery of the nucleolus upon heat shock that maintain their reversibility due to the activity of the chaperones Hsp70 and Sis1.

Figure 3.. Condensate cascade model of the heat shock response

A) Schematic of the HSR condensate cascade in which environmental conditions trigger the formation of condition specific adaptive condensates (CSACs) that titrate Sis1 and Hsp70 away from Hsf1, resulting in the formation of HSR transcriptional condensates. B) Upon heat shock, oRPs form condensates on the surface of the nucleolus. The oRP condensates recruit Sis1 and Hsp70 away from the nucleoplasm where they were repressing Hsf1. Free Hsf1 then forms additional and distinct condensates with the transcriptional machinery to activate the HSR target genes. C) Other stress-induced condensates like cytosolic stress granules, as well as unknown condensates that may form on the surface of organelles like the ER and mitochondria, may also initiate the condensate cascade to activate the HSR in response to other environmental inputs.