Regulation of the Unfolded Protein Response

IRE1's N-terminal domain senses misfolded protein accumulation in the ER lumen, triggering activation of its kinase domain. Activated IRE1 molecules form homodimers and phosphorylate each other, exposing endoribonuclease activity. This multimeric assembly initiates the unfolded protein response by splicing XBP1 mRNA, a key transcription factor that upregulates ER folding and degradation proteins.

XBP1 is a transcription factor produced when IRE1 splices its pre-mRNA. Once translated, XBP1 enters the nucleus and binds ER response elements on DNA, activating transcription of UPR target genes encoding chaperones. These chaperones are transported to the ER lumen where they refold misfolded proteins and restore ER protein homeostasis.

Chaperone-encoding mRNAs are exported from the nucleus and bound by ribosomes. The ribosomes carry these mRNAs to the ER membrane for cotranslational protein translocation, allowing chaperone proteins to be directly inserted into the ER lumen as they are synthesized. This process increases the influx of chaperone molecules needed to manage protein folding.

When IRE1 molecules dimerize, their cytosolic kinase domains phosphorylate each other in a trans-autophosphorylation process. This autophosphorylation exposes the endoribonuclease or RNA-splicing domains, enabling the formation of a rod-shaped complex with a surface capable of splicing XBP1 pre-mRNA in a spliceosome-independent manner.

RIDD, or regulated IRE1-dependent decay, is a mechanism where IRE1 cleaves specific mRNA substrates carrying consensus sequences that form stem-loop structures. By degrading these target mRNAs, RIDD reduces the number of nascent proteins directed to the ER lumen or membrane, decreasing the protein folding burden and quality control demands in the ER.

If the unfolded protein response cannot restore ER protein homeostasis and reduce protein overload, IRE1 initiates programmed cell death by decaying anti-apoptotic microRNAs. This protective mechanism prevents cells with irreversible ER dysfunction from surviving and potentially causing tissue damage or organismal harm.

IRE1 is a type I transmembrane protein kinase receptor with distinctive site-specific RNase activity that is highly conserved across eukaryotes. Its dual function—sensing misfolded proteins and executing both adaptive and apoptotic responses—makes it fundamental to cellular physiology and protein homeostasis maintenance across diverse organisms.