Intermolecular 3'UTR-3'UTR interactions drive Wnt gene activation through heteromeric protein assembly

Stem cell differentiation depends on transcription factors that are often encoded by mRNAs with highly conserved 3'UTRs. To determine their functional roles, we performed 3'UTR loss-of-function studies. Partial deletion of endogenous 3'UTRs altered stem cell differentiation efficiency in 7/10 cases. As 6/7 3'UTR deletions did not affect expression level of the encoded proteins, we reveal widespread abundance-independent regulatory roles of 3'UTRs. For example, 3'UTR deletion of CTNNB1, an mRNA that encodes the essential Wnt co-activator {beta}-catenin, keeps {beta}-catenin levels unaffected but impairs zebrafish embryogenesis and induction of the Wnt transcriptional program during human stem cell differentiation. We show that long intermolecular 3'UTR-3'UTR interactions between Wnt transcription factor mRNAs and CTNNB1 enable co-translational protein complex assembly of these transcription factors with {beta}-catenin. As antisense oligonucleotide-mediated blocking of 3'UTR interactions impairs Wnt program induction, our findings indicate that transcriptional regulators can form functional units during protein biogenesis to be fully active.