Structural Basis for Activation of HRI by the DELE1 C-terminal domain

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Abstract
Haem-Regulated Inhibitor (HRI, aka EIF2AK1) is one of four stress-sensing kinases that phosphorylate eIF2α as part of the integrated stress response (ISR). HRI was first characterized as a haem-sensing kinase where it is inhibited when bound to haem. Recent studies have revealed another role for HRI in sensing mitochondrial stress via the mitochondrial protein DELE1. Upon stress, DELE1 is cleaved, and its C-terminal fragment (DELE1 CTD ) is released from the mitochondria to the cytoplasm, where it interacts with HRI, to trigger the ISR. This pathway is critical for mitochondrial quality control and neuronal health. Here, we perform biophysical analysis to demonstrate that purified recombinant DELE1 CTD binds with high affinity to the N-terminal region of HRI, competing with and displacing haem to activate HRI and pointing to a shared binding site. Using Hydrogen Deuterium Exchange Mass Spectrometry (HDX-MS), we map the binding footprint of DELE1 CTD on HRI and confirm that it overlaps with the haem-binding site. These findings support a simple activation mechanism: DELE1 CTD activates HRI by excluding inhibitory haem from its binding site.
Funding Information
  • Royal Society (RGS\R1\231147)
  • Medical Research Council (MR/R01579/1)
  • Medical Research Council (UKRI2546)
  • Biotechnology and Biological Sciences Research Council (BB/V019635/1)

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