U1A RNA-binding domain at 1.8 Å resolution

The human U1A RNA-binding domain (RBD1) adopts one of the most common protein folds, the RNA-recognition motif, and is a paradigm for understanding RNA - protein interactions. A 2.8 Angstrom resolution structure of the unbound RBD1 has previously been determined [Nagai et al. ( 1990). Nature ( London), 348, 515 - 520] and revealed a well defined alpha/beta core with disordered termini. Using a longer construct, a 1.8 Angstrom resolution structure of the unbound domain was determined that reveals an ordered C-terminal helix. The presence of this helix is consistent with a solution structure of the free domain [ Avis et al. ( 1996). J. Mol. Biol. 257, 398 - 411]; however, in the solution structure the helix occludes the RNA-binding surface. In the present structure, the helix occupies a position similar to that seen in a 1.9 Angstrom resolution RNA - RBD1 complex structure [Oubridge et al. ( 1994). Nature ( London), 372, 432 - 438]. The crystals in this study were grown from 2.2 M sodium malonate. It is possible that the high salt concentration helps to orient the C-terminal helix in the RNA-bound conformation by strengthening hydrophobic interactions between the buried face of the helix and the alpha/beta core of the protein. Alternatively, the malonate ( several molecules of which are bound in the vicinity of the RNA-binding surface) may mimic RNA.