Regulated Ire1-dependent mRNA decay requires no-go mRNA degradation to maintain endoplasmic reticulum homeostasis in S. pombe

The unfolded protein response (UPR) continually monitors the protein folding capacity of the endoplasmic reticulum (ER). In S. pombe, Ire1, an ER membrane-resident kinase/endoribonuclease, utilizes a mechanism of selective degradation of ER-targeted mRNAs (RIDD) to maintain ER homeostasis. Here, we used a genetic screen to identify factors critical to the Ire1-mediated UPR response in S. pombe and found several proteins, Dom34, Hbs1 and SkiX, previously implicated in ribosome rescue and the no-go-decay (NGD) pathway. Ribosome profiling in ER-stressed cells lacking these factors revealed that Ire1-mediated cleavage on ER-associated mRNAs results in ribosome stalling on the cleaved transcript and, ultimately in full mRNA degradation. The process engages mechanisms of precise, iterated cleavage of the mRNAs and ribosome rescue. This clear signature allowed us to discover hundreds of novel mRNA targets of Ire1. Our results reveal that the UPR in S. pombe executes RIDD in an intricate interplay between Ire1, translation, and the NGD surveillance pathway, and establish a critical role for NGD in maintaining ER homeostasis. Overall design: 16 samples are included in the study (8 mRNA-Seq samples and 8 ribosome footprinting samples). Read sizes were examined for lengths of 15-18 nt and 25-34 nt for all profiling samples and reads were aligned by 5' ends for mRNA-Seq and 3' ends for profiling.