A CHARACTERISTIC ARRANGEMENT OF AROMATIC AMINO-ACID-RESIDUES IN THE SOLUTION STRUCTURE OF THE AMINO-TERMINAL RNA-BINDING DOMAIN OF DROSOPHILA SEX-LETHAL

The Sex-lethal (Sxl) protein from Drosophila melanogaster has two RNA- binding domains (RBDs). As the amino-terminal RBD (RBD1) of the Sxl pr otein exhibits low sequence homology to the typical RBDs, particularly at the putative functional residues, it was difficult to unambiguousl y locate the RNP1 and RNP2 motifs. Therefore, in the present study, we defined the amino and carboxy-terminal borders of the first RNA-bindi ng domain (RBD1) of the Sxl protein by limited tryptic digestion. By r eplacement of Phe166 by Tyr, we constructed a highly soluble mutant, w hich exhibits the same RNA-binding properties as those of the wild-typ e. Using this mutant protein, we performed NMR measurements, and eluci dated the secondary and tertiary structures of the Sxl RBD1 in solutio n. The beta alpha beta beta alpha beta folding pattern is conserved in the solution structure of the Sxl RBD1, as in other reported RBD stru ctures. This allowed us to identify both the RNP1 and RNP2 motifs of t he Sxl RBD1 unambiguously. Intriguingly, the RNP2 motif of the Sxl RBD 1 has an he residue at the second position, which is generally occupie d by an aromatic amino acid residue in RBDs and has been suggested to be involved in their RNA binding. Furthermore, the loop region between beta 2 and beta 3 of the Sxl RBD1 has an exceptional cluster of aroma tic amino acid residues, in place of the normal basic amino acid clust er. In contrast, the second RBD of Sxl does not exhibit these characte ristic features. (C) 1997 Academic Press Limited.