Dimerization-dependent cis-autophosphorylation activates the UPR and ISR kinases

Abstract

Maintaining cellular homeostasis requires the ability to detect and respond to various forms of stress. Protein kinases of the unfolded protein response (UPR) monitor protein folding in the endoplasmic reticulum, while the integrated stress response (ISR) kinases sense viral infection, nutrient deprivation, mitochondrial oxidative stress and proteotoxic stress. However, the precise molecular mechanism by which these kinases are activated is still unknown. In this study, we show that the ISR kinases PERK, HRI, and Gcn2, as well as the UPR kinase Ire1, undergo dimerization-dependent cis-autophosphorylation of their activation loops. The back-to-back dimerization of the kinase domain allosterically activates each kinase. We derive a simple mathematical model for dimerization-dependent cis-autophosphorylation, which we use to obtain kinetic parameters of PERK autophosphorylation. We show that dimerization promotes not only activation loop phosphorylation of PERK, but also phosphorylation of its substrate, eIF2. In cells, kinase domain dimerization is necessary and sufficient for the activation of PERK. In summary, we propose a model in which a dimer is the minimal functional unit of all UPR and ISR kinases.