The structure and target identification of the tandem RRM of yeast Npl3 bound to RNA reveal a cooperative sequence-specific recognition and an RNA chaperone role in splicing

The conserved SR-like protein Npl3 promotes the splicing of diverse pre-mRNAs. However, the RNA sequence(s) recognized by the RNA Recognition Motifs (RRM1 & RRM2) of Npl3 during the splicing reaction remain elusive. Here, we developed a split-iCRAC approach in vivo to determine the consensus sequence bound to each RRM in yeast. High-resolution NMR structures show that RRM2 recognizes a 5´-GNGG-3´ motif leading to an unusual mille-feuille topology. In addition, our data indicate a non-specific interaction of the RS domain with RNA. These structures reveal how RRM1 preferentially interacts with a CC-dinucleotide upstream of this motif, and how the inter-RRM linker and the region C-terminal to RRM2 contributes to cooperative RNA-binding. Structure-guided studies show that Npl3 genetically interacts with U2 snRNP specific factors and we provide evidence that Npl3 melts U2 snRNA stem-loop I, a prerequisite for U2/U6 duplex formation within the catalytic centre of the Bact spliceosomal complex. Our findings provide a mechanistic role for Npl3 during spliceosome active site formation.