Changes in side-chain and backbone dynamics identify determinants of specificity in RNA recognition by human U1A...

The ribonucleoprotein (RNP) domain is one of the most common eukaryotic protein domains, and is found in many proteins involved in recognition of a wide variety of RNAs. Two structures of RNA complexes of human U1A protein have revealed important aspects of RNP-RNA recognition, but have also raised intriguing questions concerning how RNP domains discriminate between different RNAs. In this work, we extend the investigation of U1A-RNA recognition by comparing the dynamics of U1A protein both free and in complex with RNA. We have also investigated the trimolecular complex between two U1A proteins and the complete polyadenylation inhibition element to study the effect of RNA-dependent protein-protein interactions on protein conformational flexibility. We report that changes in backbone dynamics upon complex formation identify regions of the protein where conformational exchange processes are quenched in the RNA-bound conformation. Furthermore, amino acids whose side-chains experience significant changes in conformational flexibility coincide with residues particularly important for the specificity of the U1A protein/RNA interaction. This study adds a new dimension to the description of the coordinated changes in structure and dynamics that are critical to define the biological specificity of U1A and other RNP proteins.