RBM25 selectively stabilizes XBP1s mRNA independently of splicing factor activity

The unfolded protein response (UPR) preserves endoplasmic reticulum (ER) proteostasis through coordinated transcriptional and translational programs that promote adaptation and prevents apoptosis. A central component of this response is the IRE1-XBP1 signaling axis, in which IRE1-mediated unconventional splicing of XBP1 mRNA generates the active transcription factor XBP1s. While the mechanisms governing IRE1 activation and XBP1 splicing have been extensively characterized, the post-transcriptional processes that sustain XBP1s expression remain poorly understood. Here, we identify RNA-binding motif protein 25 (RBM25) as a key post-transcriptional regulator of the UPR that selectively stabilizes spliced XBP1 mRNA during ER stress. RBM25 binds the XBP1s 3′ untranslated region and prolongs XBP1s mRNA half-life without affecting unspliced XBP1u transcripts or IRE1-dependent regulated decay (RIDD) targets. Consequently, RBM25 depletion diminishes XBP1 transcriptional output, compromises adaptive UPR signaling, and sensitizes cells to ER stress-induced death across multiple stress paradigms. Together, our findings uncover a previously unrecognized RNA-based regulatory checkpoint that amplifies and sustains XBP1 signaling, thereby tuning the magnitude of the adaptive UPR and cellular survival under proteotoxic stress.