OST component RPN1 is a novel regulator of IRE1 RNase activity that interacts with multiple distinct IRE1 and PERK complexes

The unfolded protein response (UPR) is an essential cell signalling system that regulates ER protein homeostasis. IRE1 and PERK are receptor proteins that propagate the UPR signal from the ER to the cytosol. Both receptors are suggested to interact with various proteins from different biological pathways, although the scale and scope of such interactions are unclear. Previous reconstitution experiments have utilized purified isolated domains of IRE1 and PERK to understand mechanism. Here, we affinity purify full length IRE1 and PERK from mammalian cells and characterise the complexes they form by biochemical techniques and assess RNase function in vivo. We identify RPN1 as a novel interacting protein present in complexes with IRE1 and PERK. In the Drosophila eye, RNAi knockdown of RPN1 results in loss of IRE1 RNase activity. This work provides a basis for understanding of protein interaction networks for IRE1 and PERK and identifies OST subunit RPN1 as a novel regulator of IRE1 RNase activity.