mRNA 3'UTRs chaperone intrinsically disordered regions to control protein activity.

More than 2,700 human mRNA 3'UTRs have hundreds of highly conserved (HC) nucleotides, but their biological roles are unclear. Here, we show that mRNAs with HC 3'UTRs mostly encode proteins with long intrinsically disordered regions (IDRs), including MYC, UTX, and JMJD3. These proteins are only fully active when translated from mRNA templates that include their 3'UTRs, raising the possibility of functional interactions between 3'UTRs and IDRs. Rather than affecting protein abundance or localization, we find that HC 3'UTRs control transcriptional or histone demethylase activity through co-translationally determined protein oligomerization states that are kinetically stable. 3'UTR-dependent changes in protein folding require mRNA-IDR interactions, suggesting that mRNAs act as IDR chaperones. These mRNAs are multivalent, a biophysical RNA feature that enables their translation in network-like condensates, which provide favorable folding environments for proteins with long IDRs. These data indicate that the coding sequence is insufficient for the biogenesis of biologically active conformations of IDR-containing proteins and that RNA can catalyze protein folding.