Contribution of the tyrosines to the structure and function of the human U1A N-terminal RNA binding domain.

RNA binding domains (RBDs) are members of a large family of proteins that share minimal sequence conservation but adopt an alpha beta sandwich global fold. Defining the contributions of specific amino acids to RBD structure and RNA binding is critical to understanding the functions of these proteins. In these experiments with the human U1A N-terminal RNA binding domain (RBD1), the contributions from each of its four tyrosines to protein structure, stability, and RNA binding were measured. Each tyrosine was substituted with phenylalanine and one other selected residue, and the resulting proteins were characterized by chemical denaturation to measure their unfolding free energy, by binding free energies to the wild-type RNA hairpin, and by 19F NMR to probe for structural changes. Features of the protein identified in these experiments include a possible tyrosine/lysine contact in an alpha-helix, which may be an example of an energetically favorable aromatic/amino side chain interaction. One long loop of the protein, which shows unusual 15N backbone and tyrosine side-chain dynamics, is implicated in protein:protein association. The diverse interactions of the four tyrosine residues in the organization of RBD1 illustrate how each member of this family of proteins will have unique molecular details that contribute to function.