Pumilio protects Xbp1 mRNA from regulated Ire1-dependent decay

Abstract

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. We identified binding sites for the RNA-binding protein Pumilio in the 3 UTR Drosophila Xbp1. In the developing Drosophila eye, Pumilio bound both the Xbp1unspliced and Xbp1spliced mRNAs, but only Xbp1spliced was stabilized by Pumilio. Furthermore, Pumilio displayed Ire1 kinase-dependent phosphorylation during ER stress, which was required for its stabilization of Xbp1spliced. Human IRE1 could directly phosphorylate Pumilio, and phosphorylated Pumilio protected Xbp1spliced mRNA against RIDD. Thus, Ire1-mediated phosphorylation enables Pumilio to shield Xbp1spliced from RIDD. These results uncover an important and unexpected regulatory link between an RNA-binding protein and the UPR.

Newly synthesized mRNA escapes translational repression during the acute phase of the mammalian unfolded protein response

“…Identifying the mechanism of co-translational deadenylation during the chronic UPR will suggest better experimental approaches to address the relationship between poly(A) tail length and efficiency of translation. Additional mechanisms for shortening of the poly(A) tails during the chronic phase could involve cis-and trans-regulatory elements such as miRNA and RNA-binding proteins recruited to mRNAs during ER stress, or release of PABP from stress granules (Backlund et al 2020;Duan et al 2020;Good and Stoffers 2020;Malhi 2014;Cairrao et al 2021) We described here the temporal regulation of poly(A) tail length of the newly synthesized XBP1 mRNA during the UPR. A biphasic response to ER stress has previously been described by us and by others (Krokowski et al 2013;Hetz and Papa 2018;Han et al 2013).…”