Journal Article

Direct Association of Unfolded Proteins with Mammalian ER Stress Sensor, IRE1beta

Oikawa, Daisuke, Iwawaki, Takao, Kinjo, Masataka, Kitamura, Akira

2012

Overview

IRE1, an ER-localized transmembrane protein, plays a central role in the unfolded protein response (UPR). IRE1 senses the accumulation of unfolded proteins in its luminal domain and transmits a signal to the cytosolic side through its kinase and RNase domains. Although the downstream pathways mediated by two mammalian IRE1s, IRE1[alpha] and IRE1[beta], are well documented, their luminal events have not been fully elucidated. In particular, there have been no reports on how IRE1[beta] senses the unfolded proteins. In this study, we performed a comparative analysis to clarify the luminal event mediated by the mammalian IRE1s. Confocal fluorescent microscopy using GFP-fused IRE1s revealed that IRE1[beta] clustered into discrete foci upon ER stress. Also, fluorescence correlation spectroscopy (FCS) analysis in living cells indicated that the size of the IRE1[beta] complex is robustly increased upon ER stress. Moreover, unlike IRE1[alpha], the luminal domain of IRE1[beta] showed anti-aggregation activity in vitro, and IRE1[beta] was coprecipitated with the model unfolded proteins in cells. Strikingly, association with BiP was drastically reduced in IRE1[beta], while IRE1[alpha] was associated with BiP and dissociated upon ER stress. This is the first report indicating that, differently from IRE1[alpha], the luminal event mediated by IRE1[beta] involves direct interaction with unfolded proteins rather than association/dissociation with BiP, implying an intrinsic diversity in the sensing mechanism of mammalian sensors.

• Publisher: Public Library of Science
• Subject: Ribonuclease