Hong, D.; Kim, N.; Jo, Y.; Jeong, J.; Sohn, I.; Koo, T.; Kang, K.; Jeong, S.

The protein-coding sequence has long been considered the primary determinant of protein function. Alternative splicing within 3'UTRs (AS-3'UTRs) generates multiple transcript isoforms from a single gene, yet their roles in protein function and disease relevance remain largely unexplored. Through systematic transcriptome-wide identification of cancer-associated AS-3'UTRs, we uncover that AS-3'UTRs of {beta}-catenin mRNA direct distinct subcellular localization of {beta}-catenin mRNA isoforms. Specifically, an aberrantly spliced 3'UTR isoform promotes cytoplasmic mRNA condensate formation through FUS binding to a cancer-associated alternative exon (Exon 16A). Because {beta}-catenin function is exquisitely dependent on its subcellular distribution between adherens junctions and the nucleus, this aberrant 3'UTR isoform reprograms {beta}-catenin localization. By sequestering {beta}-catenin in the cytoplasm, the aberrant 3'UTR isoform prevents its incorporation into E-cadherin-based adherens junctions, thereby inducing epithelial-mesenchymal transition (EMT)-associated transcriptional programs. Notably, the expression signature of the aberrant 3'UTR isoform robustly correlates with poor clinical outcomes in colorectal cancer patients. Together, our findings reveal that AS-3'UTRs operate as a previously unrecognized post-transcriptional regulatory mechanism through which the untranslated region of mRNA, without altering a single amino acid, reprograms protein subcellular fate to drive oncogenic phenotypes.