New insights on human IRE1 tetramer structures based on molecular modeling.

Inositol-Requiring Enzyme 1α (IRE1α; hereafter IRE1) is a transmembrane kinase/ribonuclease protein related with the unfolded protein response (UPR) signaling. Experimental evidence suggests that IRE1 forms several three dimensional (3D) structural variants: dimers, tetramers and higher order oligomers, where each structural variant can contain different IRE1 conformers in different arrangements. For example, studies have shown that two sets of IRE1 dimers exist; a face-to-face dimer and a back-to-back dimer, with the latter considered the important unit for UPR signaling propagation. However, the structural configuration and mechanistic details of the biologically important IRE1 tetramers are limited. Here, we combine protein-protein docking with molecular dynamics simulations to derive human IRE1 tetramer models and identify a molecular mechanism of IRE1 activation. To validate the derived models of the human IRE1 tetramer, we compare the dynamic behavior of the models with the yeast IRE1 tetramer crystallographic structure. We show that IRE1 tetramer conformational changes could be linked to the initiation of the unconventional splicing of mRNA encoding X-box binding protein-1 (XBP1), which allows for the expression of the transcription factor XBP1s (XBP1 spliced). The derived IRE1 tetrameric models bring new mechanistic insights about the IRE1 molecular activation mechanism by describing the IRE1 tetramers as active protagonists accommodating the XBP1 substrate.