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. 2022 Mar 23;8(12):eabk2022. doi: 10.1126/sciadv.abk2022

Fig. 2. Deterministic mathematical model of the cellular ISR.

Fig. 2.

(A) Schematic representation of the parameters and reactions included in the mathematical model. The stress-sensing module represents the activation of stress kinases. PKR is activated by binding to dsRNA (viral stress, p-PKR*), HRI by arsenite treatment (oxidative stress, HRI*), and PERK by thapsigargin (ER stress, PERK*). Active stress kinases signal to the decision module, where upon crossing a p-eIF2α level threshold will trigger SG formation (SG-On). Elevated p-eIF2α levels activate the recovery module consisting of the GADD34 negative feedback loop, i.e., ppp15R1a promoter activation (POFF to PON) with time delay (clock symbol), GADD34 transcription (mGADD34), and protein synthesis (GADD34). In turn, GADD34 dephosphorylates eIF2α and thereby resumes translation. Gray arrow, basal eIF2α dephosphorylation by CReP, the constitutive regulatory subunit of PP1. Ø, degradation. (B and C) Absolute quantification of eIF2α and PKR mean molecule numbers in Huh7 YFP-TIA1 cells, in the absence and presence of IFN-α. (B) Representative quantitative Western blot analysis of eIF2α and PKR. (C) Estimated eIF2α and PKR mean molecule number per cell (±SD). Number of repeats (n) and statistical significance compared to untreated cells (untr.) are indicated; ****P < 0.0001. (D and E) Determination of protein half-lives by CHX pulse experiments and Western blot analyses. (D) PKR and eIF2α half-lives (n = 3). (E) GADD34 (n = 4). Black line, best nonlinear fit.