The integrated stress response (ISR) and the unfolded protein response (UPR) are conserved signaling networks governed by stress sensor kinases. Four ISR sensor kinases, GCN2, HRI, PERK, and PKR, phosphorylate eIF2α in response to multiple stresses, leading to a global protein synthesis shutdown coupled to the translation of select mRNAs, which results in an adaptive remodeling of the proteome. Besides PERK, which the ISR and the UPR share, the UPR relies on two additional ER stress sensors, the kinase/RNase IRE1 and the membrane-tethered transcription factor ATF6, to induce adaptive signaling programs that reinstate ER homeostasis. However, if the stress is unrelenting, both the ISR and the UPR can switch to drive cell death. The mechanistic commonalities and crosstalk between the ISR and UPR remain an active area of investigation. In this talk, I will share new results that support common signaling principles of the ISR and UPR sensor kinases and reveal a new layer of communication between the ISR and the UPR. Specifically, we found that dynamic clustering is a prominent feature of PKR activation reminiscent of the high-order assemblies of IRE1 and PERK observed during ER stress. Surprisingly, PKR clusters excluded eIF2α, and mutations in PKR that disrupt cluster assembly enhanced eIF2α phosphorylation, suggesting that PKR clusters act as enzyme sinks that control enzyme-substrate interactions by limiting PKR-eIF2α encounters. Moreover, stress-free activation of PKR induced a master cell death program dependent on ISR-driven expression of DR5 (death receptor 5), as occurs during unmitigable ER stress. Remarkably, stress-free activation of the ISR selectively activated IRE1 independent of sensing unfolded proteins in the ER lumen, and treatment with the small-molecule ISR inhibitor ISRIB reversed it. Our data provide new mechanistic insights into two fundamental aspects of stress sensor kinase signaling: (1) dynamic clustering of stress sensors may provide the means to fine-tune stress responses, and (2) the ISR selectively activates IRE1, thus coupling the ISR and the UPR outside their common node PERK.
Departmental start-up funds, Sponsored Research Agreement - Calico Life Sciences LLC
