NMR SOLUTION STRUCTURE OF A DSRNA BINDING DOMAIN FROM DROSOPHILA STAUFEN PROTEIN REVEALS HOMOLOGY TO THE N-TERMINAL DOMAIN OF RIBOSOMAL-PROTEIN S5

The double-stranded RNA binding domain (dsRBD) is an similar to 65 ami no acid motif that is found in a variety of proteins that interact wit h double-stranded (ds) RNA, such as Escherichia coil RNase III and the dsRNA-dependent kinase, PKR. Drosophila staufen protein contains five copies of this motif, and the third of these binds dsRNA in vitro. Us ing multinuclear/multidimensional NMR methods, we have determined that staufen dsRBD3 forms a compact protein domain with an alpha-beta-beta -beta-alpha structure in which the two alpha-helices lie on one face o f a three-stranded anti-parallel beta-sheet. This structure is very si milar to that of the N-terminal domain of a prokaryotic ribosomal prot ein S5. Furthermore, the consensus derived from all known S5p family s equences shares several conserved residues with the dsRBD consensus se quence, indicating that the two domains share a common evolutionary or igin, Using in vitro mutagenesis, we have identified several surface r esidues which are important for the RNA binding of the dsRBD, and thes e all lie on the same side of the domain. Two residues that are essent ial for RNA binding, F32 and K50, are also conserved in the S5 protein family, suggesting that the two domains interact with RNA in a simila r way.