Ire1 Has Distinct Catalytic Mechanisms for XBP1/HAC1 Splicing and RIDD

• Cleavage of XBP1/HAC1 intron by IRE1 is a distinct and separable activity from RIDD
• Oligomerization of active IRE1 is required for XBP1/HAC1 cleavage but not RIDD
• Both activities use the same catalytic residues but different substrate binding sites
• Selective activation of RIDD promotes cell death, whereas XBP1/HAC1 splicing supports survival

SummaryAn evolutionarily conserved unfolded protein response (UPR) component, IRE1, cleaves XBP1/HAC1 introns in order to generate spliced mRNAs that are translated into potent transcription factors. IRE1 also cleaves endoplasmic-reticulum-associated RNAs leading to their decay, an activity termed regulated IRE1-dependent decay (RIDD); however, the mechanism by which IRE1 differentiates intron cleavage from RIDD is not well understood. Using in vitro experiments, we found that IRE1 has two different modes of action: XBP1/HAC1 is cleaved by IRE1 subunits acting cooperatively within IRE1 oligomers, whereas a single subunit of IRE1 performs RIDD without cooperativity. Furthermore, these distinct activities can be separated by complementation of catalytically inactive IRE1 RNase and mutations at oligomerization interfaces. Using an IRE1 RNase inhibitor, STF-083010, selective inhibition of XBP1 splicing indicates that XBP1 promotes cell survival, whereas RIDD leads to cell death, revealing modulation of IRE1 activities as a drug-development strategy.

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