Subcellular localization of the J-protein Sis1 regulates the heat shock response

Abstract

Cells exposed to heat shock induce a conserved gene expression program, the heat shock response (HSR), encoding protein homeostasis (proteostasis) factors. Heat shock also triggers proteostasis factors to form subcellular quality control bodies, but the relationship between these spatial structures and the HSR is unclear. Here we show that localization of the J-protein Sis1, a cofactor for the chaperone Hsp70, controls HSR activation in yeast. Under nonstress conditions, Sis1 is concentrated in the nucleoplasm, where it promotes Hsp70 binding to the transcription factor Hsf1, repressing the HSR. Upon heat shock, Sis1 forms an interconnected network with other proteostasis factors that spans the nucleolus and the surface of the endoplasmic reticulum. We propose that localization of Sis1 to this network directs Hsp70 activity away from Hsf1 in the nucleoplasm, leaving Hsf1 free to induce the HSR. In this manner, Sis1 couples HSR activation to the spatial organization of the proteostasis network.

ReporterSeq reveals genome-wide dynamic modulators of the heat shock response across diverse stressors

“…Knockdown of SIS1 or SSA2 desensitized cells to every stressor except for the proteasome inhibitor bortezomib, a weak activator of the HSR. This is consistent with studies demonstrating direct binding between Hsp70 and Hsf1 ( Zheng et al, 2016 ) and suggesting that Sis1 is the primary Hsp40 facilitating this interaction ( Feder et al, 2021 ). We identified one strong regulator of the HSR, Asc1, as an HSR regulator independent of Ssa2/Sis1.…”

“…Hierarchical clustering of stressors revealed that cycloheximide clustered with late heat shock (40 and 120 min; Figure 5F ). Thus, many genetic perturbations that affect the HSR are dependent on translation, consistent with reports that the bulk of misfolded proteins in a cell are newly translated, nascent chains ( Medicherla and Goldberg, 2008 ) and the dependence of the HSR on translation in heat stress ( Feder et al, 2021 ). Translation-dependent HSR activation may explain blocking interactions observed in other stressors with low HSR induction, such as DTT, tunicamycin, and glucose starvation.…”

Essentiality of Sis1, a J-domain protein Hsp70 cochaperone, can be overcome by Tti1, a specialized PIKK chaperone

“…Sis1 has been implicated as a key regulator of heat shock transcription factor Hsf1 (Klaips et al, 2020;Alford et al, 2021;Feder et al, 2021). We took advantage of our ability to manipulate the amount of Sis1 in cells to probe the extent of its role.…”

“…Some, such as targeting certain proteins for degradation (Shiber et al, 2013;Summers et al, 2013;Prasad et al, 2018) and maintaining prions (Sondheimer et al, 2001), are unique to Sis1, but are not essential. Sis1 has also been identified as a regulator of heat shock transcription factor Hsf1 (Klaips et al, 2020;Feder et al, 2021), which drives expression from promoters having a Heat Shock Element (HSE). It is well established that Hsf1 activity is down regulated by Hsp70 binding (Masser et al, 2020;Pincus, 2020), but how critical and unique the role of Sis1 is as the J-domain protein cochaperone in this regulation by driving the Hsp70-Hsf1 interaction remains unresolved.…”

Transcriptional control of ribosome biogenesis in yeast: links to growth and stress signals

“…Numerous chemical and genetic perturbations of ribosome assembly only serve to augment the level of aggregation-prone orphan RPs and reveal a highly specific effect on transcription by RNAPII: up-regulation of heat shock factor 1 (Hsf1) target genes and down-regulation of RPGs. Activation of Hsf1 is probably due to relief of negative regulation by the Hsp70 proteins Ssa1/2, perhaps through effects on the essential Hsp40 co-chaperone Sis1 [ 76 , 77 ]. RP down-regulation, on the other hand, turns out to result from the rapid release of Ifh1 from RPG promoters, coincident with its accumulation in nuclear aggregates [ 44 ].…”