Structural basis for the recognition of spliceosomal SmN/B/B' proteins by the RBM5 OCRE domain in splicing regulation.

Abstract

The multi-domain splicing factor RBM5 regulates the balance between antagonistic isoforms of the apoptosis-control genes <i>FAS/CD95</i>, <i>Caspase-2</i> and <i>AID</i>. An OCRE (OCtamer REpeat of aromatic residues) domain found in RBM5 is important for alternative splicing regulation and mediates interactions with components of the U4/U6.U5 tri-snRNP. We show that the RBM5 OCRE domain adopts a unique β-sheet fold. NMR and biochemical experiments demonstrate that the OCRE domain directly binds to the proline-rich C-terminal tail of the essential snRNP core proteins SmN/B/B'. The NMR structure of an OCRE-SmN peptide complex reveals a specific recognition of poly-proline helical motifs in SmN/B/B'. Mutation of conserved aromatic residues impairs binding to the Sm proteins <i>in vitro</i> and compromises RBM5-mediated alternative splicing regulation of FAS/CD95. Thus, RBM5 OCRE represents a poly-proline recognition domain that mediates critical interactions with the C-terminal tail of the spliceosomal SmN/B/B' proteins in <i>FAS/CD95</i> alternative splicing regulation.