Activation of mammalian IRE1[alpha] upon ER stress depends on dissociation of BiP rather than on direct interaction with unfolded proteins

IRE1, an ER-localized transmembrane protein, plays a central role in the unfolded protein response. Upon ER stress, IRE1 senses the accumulation of unfolded proteins in the ER, and transfers signal from the ER to the cytosol. Recently, it was reported that the luminal domain of yeast Ire1 senses the unfolded proteins via a two-step mechanism, namely dissociation of BiP and direct interaction with unfolded proteins. However, it has been unclear whether a similar mechanism is applicable to mammalian IRE1[alpha]. To address this point, we analyzed luminal-domain mutants of mammalian IRE1[alpha] in cells, and evaluated the anti-aggregation activity of the luminal fragment of IRE1[alpha] in vitro. We generated a mutant that has low affinity for BiP, and this mutant was significantly activated even under normal conditions. Moreover, the luminal fragments of mammalian IRE1[alpha] did not exhibit anti-aggregation activity. These results suggest that in contrast to yeast Ire1, the regulation of mammalian IRE1[alpha] strongly depends on the dissociation of BiP. [PUBLICATION ABSTRACT]