Structural basis for the assembly of the Sxl-Unr translation regulatory complex

Genetic equality between males and females is established by chromosome-wide dosage-compensation mechanisms. In the fruit-fly Drosophila melanogaster, the dosage-compensation complex promotes two fold hypertranscription of the singlemale X-chromosome and is silenced in females by inhibition of the translation of msl2, which codes for the limiting component of the dosage-compensation complex(1,2). The female-specific protein Sex-lethal (Sxl) recruits Upstream of-N-ras(Unr) to the 39 untranslated region of msl2 messenger RNA, preventing the engagement of the small ribosomal subunit(3). Here we report the 2.8 angstrom crystal structure, NMR and small-angle X-ray and neutron scattering data of the ternary Sxl-Unr-msl2 ribonucleoprotein complex featuring unprecedented intertwined interactions of two Sxl RNA recognition motifs, a Unr cold-shock domain and RNA. Cooperative complex formation is associated with a 1,000-fold increase of RNA binding affinity for the Unr cold-shock domain and involves novel ternary interactions, as well as non-canonical RNA contacts by the alpha 1 helix of Sxl RNA recognition motif 1. Our results suggest that repression of dosage compensation, necessary for female viability, is triggered by specific, cooperative molecular interactions that lock a ribonucleoprotein switch to regulate translation. The structure serves as a paradigm for how a combination of general and widespread RNA binding domains expands the code for specific single-stranded RNA recognition in the regulation of gene expression.