A novel peptide recognition mode revealed by the X-ray structure of a core U2AF-35/U2AF65 heterodimer

U2 auxiliary factor (U2AF) is an essential splicing factor that recognizes the 3' splice site and recruits the U2 snRNP to the branch point. The X-ray structure of the human core U2AF heterodimer, consisting of the U2AF(35) central domain and a proline-rich region of U2AF(65), has been determined at 2.2 Angstrom resolution. The structure reveals a novel protein-protein recognition strategy, in which an atypical RNA recognition motif (RRM) of U2AF(35) and the U2AF(65) polyproline segment interact via reciprocal "tongue-in-groove" tryptophan residues. Complementary biochemical experiments demonstrate that the core U2AF heterodimer binds RNA, and that the interacting tryptophan side chains are essential for U2AF dimerization. Atypical RRMs in other splicing factors may serve as protein-protein interaction motifs elsewhere during spliceosome assembly.