The RGG/RG Motif of AUF1 Isoform P45 is a Key Modulator of the Protein's RNA Chaperone and RNA Annealing Activities

The RNA-binding protein AUF1 regulates post-transcriptional gene expression by affecting the steady state and translation levels of numerous target RNAs. Remodeling of RNA structures by the largest isoform AUF1 p45 was recently demonstrated in the context of replicating RNA viruses, and involves two RNA remodeling activities, i.e. an RNA chaperone and an RNA annealing activity. AUF1 contains two non-identical RNA recognition motifs (RRM) and one RGG/RG motif located in the C-terminus. In order to determine the functional significance of each motif to AUF1's RNA-binding and remodeling activities we performed a comprehensive mutagenesis study and characterized the wildtype AUF1, and several variants thereof. We demonstrate that each motif contributes to efficient RNA binding and remodeling by AUF1 indicating a tight cooperation of the RRMs and the RGG/RG motif. Interestingly, the data identify two distinct roles for the arginine residues of the RGG/RG motif for each RNA remodeling activity. First, arginine-mediated stacking interactions promote AUF1's helix-destabilizing RNA chaperone activity. Second, the electropositive character of the arginine residues is the major driving force for the RNA annealing activity. Thus, we provide the first evidence that arginine residues of an RGG/RG motif contribute to the mechanism of RNA annealing and RNA chaperoning.