Cross-intron bridging interactions in the yeast commitment complex are conserved in mammals

The commitment complex is the first defined step in the yeast (S. cerevisiae) splicing pathway. It contains U1 snRNP as well as Mud2p, which resembles human U2AF65. In a genetic screen, we identified the yeast gene MSL-5, which is a novel commitment complex component. Genetic and biochemical criteria indicate a direct interaction between MsI5p and both Mud2p and the U1 snRNP protein Prp40p. This defines a bridge between the two ends of the intron. MsI5p (renamed BBP for branchpoint bridging protein) has a mammalian ortholog, the splicing factor SF1. Our results show that SF1 interacts strongly with human U2AF65, and that SF1 is a bona fide E complex component. This implies that aspects of these novel cross-intron protein-protein interactions are conserved between yeast and mammals.