Protein-RNA and protein-protein interactions of the Drosophila sex-lethal mediated by its RNA-binding domains.

The Drosophila Sex-lethal (Sxl) contains two RNA-binding domains (RBDs) which belong to the RNA recognition motif (RRM) group. Sxl binds to a specific uridine-rich sequence which is believed to be the major cis-acting element for the splicing regulation of the transformer (tra) mRNA precursor. Here we show evidence supporting the previous suggestion that Sxl recognizes the sequence context downstream of the uridine-rich sequence. In addition, by means of UV-crosslinking assays with Sxl deletion constructs, we have demonstrated that Sxl RNA binding requires both of its RBDs for specificity and strength. Moreover, by the yeast two-hybrid analysis, we found that homodimeric interaction occurs between two Sxl molecules. Interestingly, the amino- and carboxy-terminal regions outside of the Sxl RBDs are dispensable for such dimerization, indicating that the protein-protein interaction is also mediated by RBDs. Coprecipitation experiments in vitro showed that the protein-protein interaction seems to be RNA-dependent but greatly enhanced by addition of the specific RNA containing the Sxl binding site, suggesting that the conformational change which is induced on binding to RNA may facilitate the interaction between Sxl molecules.