Molecular chaperone BiP controls activity of the ER stress sensor Ire1 through interactions with its oligomers

The complex multistep activation cascade of Ire1, the most evolutionary conserved stress-sensor in the endoplasmic reticulum (ER), involves changes in the Ire1 conformation and oligomeric state. Ire1 activation enhances ER folding capacity, in part by overexpressing the ER Hsp70 molecular chaperone BiP; in turn, BiP provides tight negative control of Ire1 activation. This study demonstrates that one of the mechanisms by which BiP regulates Ire1 activation is through a direct BiP interaction with Ire1 oligomers. Particularly, we demonstrated that the binding of Ire1 luminal domain (LD) to unfolded protein substrates does not only trigger conformational changes in Ire1-LD that favour the formation of Ire1-LD oligomers but also exposes BiP binding motifs, enabling the molecular chaperone BiP to directly bind to Ire1-LD in a chaperone-like, ATP-dependent manner. These transient interactions between BiP and two short motifs in the disordered region of Ire1-LD that destabilize Ire1-LD oligomers and trigger their de-oligomerisation are reminiscent of interactions between clathrin and another Hsp70, cytoplasmic Hsc70. BiP binding to substrate-bound Ire1-LD oligomers enables unfolded protein substrates and BiP to synergistically and dynamically control Ire1-LD oligomerisation. These findings reveal a new mechanism by which BiP acts to return Ire1 to its deactivated state when an ER stress response is no longer required.