Alternatively spliced isoforms of AUF1 regulate a miRNA-mRNA interaction differentially through their YGG motif.

Proper base-pairing of a miRNA with its target mRNA is a key step in miRNA-mediated mRNA repression. RNA remodelling by RNA-binding proteins (RBPs) can improve access of miRNAs to their target mRNAs. The largest isoform p45 of the RBP AUF1 has previously been shown to remodel viral or AU-rich RNA elements. Here, we show that AUF1 is capable of directly promoting the binding of the miRNA let-7b to its target site within the 3'UTR of the POLR2D mRNA. Our data suggest this occurs in two ways. First, the helix-destabilizing RNA chaperone activity of AUF1 disrupts a stem-loop structure of the target mRNA and thus exposes the miRNA target site. Second, the RNA annealing activity of AUF1 drives hybridization of the miRNA and its target site within the mRNA. Interestingly, the RNA remodelling activities of AUF1 were found to be isoform-specific. AUF1 isoforms containing a YGG motif are competent RNA chaperones, whereas isoforms lacking the YGG motif are not. Overall, our study demonstrates that AUF1 has the ability to modulate a miRNA-target site interaction, thus revealing a new regulatory function for AUF1 proteins during post-transcriptional control of gene expression. Moreover, tests with other RBPs suggest the YGG motif acts as a key element of RNA chaperone activity.