B i P ‐bound and nonclustered mode of I re1 evokes a weak but sustained unfolded protein response

In eukaryotic cells under nonstressed conditions, the endoplasmic reticulum ( ER )‐located molecular chaperone B i P is associated with an ER ‐membrane protein I re1 to inhibit its self‐association. While ER stress leads I re1 to form transiently B i P ‐unbound clusters, which strongly evoke the unfolded protein response ( UPR ), here we propose an alternative activation status of I re1. 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, I re1 foci disappear, while I re1 continues to be self‐associated. Under these conditions, I re1 may be activated as a homo‐dimer, as it shows considerable activity even when carrying the W 426 A mutation, which allows I re1 to form homo‐dimers but not clusters. Unlike the I re1 clusters, the nonclustered active form seems to be associated with B i P . An I re1 mutant not carrying the B i P ‐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, I re1 is weakly and continuously activated in a nonclustered form through its (re)association with B i P , which disperses the I re1 clusters.