BiP‐bound and nonclustered mode of Ire1 evokes a weak but sustained unfolded protein response

In eukaryotic cells under nonstressed conditions, the endoplasmic reticulum (ER)‐located molecular chaperone BiP is associated with an ER‐membrane protein Ire1 to inhibit its self‐association. While ER stress leads Ire1 to form transiently BiP‐unbound clusters, which strongly evoke the unfolded protein response (UPR), here we propose an alternative activation status of Ire1. When yeast cells are physiologically ER‐stressed by inositol depletion for a prolonged time, the UPR is weakly activated in a sustained manner after a transient peak of activation. During persistent stress, Ire1 foci disappear, while Ire1 continues to be self‐associated. Under these conditions, Ire1 may be activated as a homo‐dimer, as it shows considerable activity even when carrying the W426A mutation, which allows Ire1 to form homo‐dimers but not clusters. Unlike the Ire1 clusters, the nonclustered active form seems to be associated with BiP. An Ire1 mutant not carrying the BiP‐association site continued to form clusters and to be activated strongly even after long‐term stress. Similar observations were obtained when cells were ER‐stressed by dithiothreitol. We thus propose that upon persistent ER stress, Ire1 is weakly and continuously activated in a nonclustered form through its (re)association with BiP, which disperses the Ire1 clusters.