Pumilio protects Xbp1 mRNA from regulated Ire1-dependent decay

The unfolded protein response (UPR) maintains homeostasis of the endoplasmic reticulum (ER). Residing in the ER membrane, the UPR mediator Ire1 deploys its cytoplasmic kinase-endoribonuclease domain to activate the key UPR transcription factor Xbp1 through non-conventional splicing of Xbp1 mRNA. Ire1 also degrades diverse ER-targeted mRNAs through regulated Ire1-dependent decay (RIDD), but how it spares Xbp1 mRNA from this decay is unknown. Here, we identify binding sites for the RNA-binding protein Pumilio in the 3'UTR Drosophila Xbp1. In the developing Drosophila eye, Pumilio binds both the Xbp1^unspliced and Xbp1^spliced mRNAs, but only Xbp1^spliced is stabilized by Pumilio. Furthermore, Pumilio displays Ire1 kinase-dependent phosphorylation during ER stress, which is required for its stabilization of Xbp1^spliced. hIRE1 can phosphorylate Pumilio directly, and phosphorylated Pumilio protects Xbp1^spliced mRNA against RIDD. Thus, Ire1-mediated phosphorylation enables Pumilio to shield Xbp1^spliced from RIDD. These results uncover an unexpected regulatory link between an RNA-binding protein and the UPR.