How is the integrated stress response terminated?

Key mechanism for ISR termination revealed, answering a decades-old question about key cellular homeostasis process

The integrated stress response (ISR) is a key pathway to maintain cellular homeostasis, allowing cells to adapt to and survive various challenges in health and disease. Under strained conditions, such as viral infections, nutrient deficiency or accumulation of abnormal proteins, cells rely on the ISR to act as a traffic light system to slow production of new proteins, freeing up cellular resources to address the given issue or help the cell survive until conditions improve. The ISR pathway has been studied for several decades. However, how the ISR is terminated has proved elusive. Anne Bertolotti’s group, in the LMB’s Neurobiology Division, has finally resolved this longstanding question.

The eukaryotic translation initiation factor 2 (eIF2) is the ‘switch’ of the ISR. Key stress-sensing kinases monitor different aspects of cellular health and, when they sense disruption, they phosphorylate the alpha subunit of eIF2, activating the ISR switch. Phosphorylated eIF2 (P-eIF2) then binds to eIF2B, blocking it from its usual function of reactivating eIF2 to restart protein production in the ribosome. Therefore, termination of the ISR pathway requires relieving the inhibition of eIF2B.

In mammals there are two eIF2 phosphatases which reverse the activity of the monitor kinases; catalytic subunit PP1 and substrate recruiting subunits PPP1R15A (R15A) and PPP1R15B (R15B). Anne’s group found that in cells, R15B binds P-eIF2 in complex with eIF2B, a large, unexpected substrate of the phosphatase. This is a departure from previous assumptions which theorised that P-eIF2 is the substrate of the eIF2 phosphatases.

Claudia De Miguel and Sigurdur Thorkelsson used electron cryomicroscopy (cryo-EM), to solve the structures of native eIF2-eIF2B and P-eIF2-eIF2B complexes bound to R15B. This relied upon utilising the group’s previous characterisation of R15B’s binding mechanism to harness it as bait to capture native complexes from human cells. The resulting cryo-EM structures then guided biochemical investigations which demonstrated that R15B enables dephosphorylation of P-eIF2 on eIF2B. In the absence of R15B, the phosphate of P-eIF2 is inaccessible in the large complex and dephosphorylation cannot occur.

Once dephosphorylated on eIF2B, eIF2 transfers from the inhibitory to the active site of eIF2B, thereby relieving its blockage of eIF2B’s usual function and terminating the ISR. This therefore reveals R15B as the key factor which rescues eIF2 and eIF2B to resume translation of proteins – i.e. changing the red light of the ISR’s traffic light system back to green.

Beyond answering a key question about cellular health, this work holds wider implications as the ISR has been identified as a key target for therapeutic interventions to tackle a range of diseases and health conditions, including cancers and neurodegenerative diseases. To properly develop new therapies which harness the ISR, it is vital to possess a detailed understanding of the molecular mechanisms that govern it. This work resolves one of its remaining enigmas.

For her contributions to this project, Claudia received the 2025 Perutz Student Award, which is given annually by the LMB in recognition of outstanding research performed by an LMB PhD student.

This work was funded by UKRI MRC and the Wellcome Trust.

Further references

Termination of the integrated stress response. De Miguel, C., Thorkelsson, S.R., Fatalska, A., Hodgson, G., Wang, C. and Bertolotti, A. Science
Anne’s group page

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