GEOapplication/xmlftp://ftp.ncbi.nlm.nih.gov/geo/series/GSE312nnn/GSE312839/primaryOK200OtherHomo sapiensOtherhttps://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE312839GEOGSEfalseRBM39 Modulates UPR Signaling Through Alternative Splicing of IRE1α/ERN1 [CRASP-Seq]The unfolded protein response (UPR) preserves endoplasmic reticulum proteostasis through coordinated signaling pathways, including the IRE1α-XBP1 axis, which promotes adaptive transcriptional programs via non-canonical XBP1 mRNA splicing. However, upstream mechanisms regulating this pathway remain incompletely defined. Here, we apply CRASP-Seq, a scalable RNA-coupled CRISPR screening platform, to systematically identify regulators of XBP1 splicing. We uncover the U2 snRNP auxiliary factor RBM39 as a critical positive regulator of this process. Perturbation of RBM39 or U2 snRNP components induces alternative splicing of ERN1, leading to exon-18 skipping and the production of an unstable transcript subject to nonsense-mediated decay, as well as a truncated IRE1α isoform that acts in a dominant-negative manner to suppress XBP1 splicing. Mechanistically, we show that heat shock reduces RBM39 functional activity and promotes ERN1 exon-18 skipping, thereby attenuating IRE1α–XBP1 signaling. Functionally, hyperactivation of this pathway is detrimental under proteotoxic stress, suggesting that exon-18 skipping serves as a stress-adaptive mechanism to limit UPR output. Together, our findings reveal a previously unrecognized regulatory axis linking the canonical splicing machinery to UPR signaling and establish alternative splicing of ERN1 as a key modulator of cellular stress responses.2026/04/27GSE312839GSM9355320GSM9355321GSM9355319GSM9355317GSM9355318GSM935531624676312839Homo sapiens