Multiple autophosphorylations significantly enhance the endoribonuclease activity of human inositol requiring enzyme 1α

Background: Endoplasmic reticulum stress, caused by the presence of misfolded proteins, activates the stress sensor inositol-requiring enzyme 1 alpha (IRE1 alpha). The resulting increase in IRE1 alpha RNase activity causes sequence-specific cleavage of X-box binding protein 1 (XBP1) mRNA, resulting in upregulation of the unfolded protein response and cellular adaptation to stress. The precise mechanism of human IRE1 alpha activation is currently unclear. The role of IRE1 alpha kinase activity is disputed, as results from the generation of various kinase-inactivating mutations in either yeast or human cells are discordant. Kinase activity can also be made redundant by small molecules which bind the ATP binding site. We set out to uncover a role for IRE1 alpha kinase activity using wild-type cytosolic protein constructs. Results: We show that concentration-dependent oligomerisation is sufficient to cause IRE1 alpha cytosolic domain RNase activity in vitro. We demonstrate a role for the kinase activity by showing that autophosphorylation enhances RNase activity. Inclusion of the IRE1 alpha linker domain in protein constructs allows hyperphosphorylation and further enhancement of RNase activity, highlighting the importance of kinase activity. We show that IRE1 alpha phosphorylation status correlates with an increased propensity to form oligomeric complexes and that forced dimerisation causes great enhancement in RNase activity. In addition we demonstrate that even when IRE1 alpha is forced to dimerise, by a GST-tag, phospho-enhancement of activity is still observed. Conclusions: Taken together these experiments support the hypothesis that phosphorylation is important in modulating IRE1 alpha RNase activity which is achieved by increasing the propensity of IRE1 alpha to dimerise. This work supports the development of IRE1 alpha kinase inhibitors for use in the treatment of secretory cancers.