Identification and characterization of Ynl187, a novel factor that promotes stable association of the U1 snRNP with the 5' ss during pre-messenger RNA splicing

This dissertation describes the identification of a novel splicing factor, Ynl187p, and characterization of its role in precursor-messenger RNA splicing in the budding yeast, Saccharomyces cerevisiae. The discovery that loss of function mutations in YNL187w can bypass the requirement for Prp28p, an otherwise essential DExH/D-box protein and splicing factor, suggests that Ynl187p is a potential target of Prp28p. Because Prp28p is required for dislodging the U1 snRNP from the intron 5' splice site during pre-mRNA splicing, its targets are predicted to promote stable U1 snRNP/5' ss association during 5' ss recognition and commitment complex formation.;I show here that Ynl187p is synthetic-lethal with loss of function mutations in a specific subset of factors that stabilize the U1/5' ss interaction, most of which also bypass Prp28p. Deletion of Ynl187p is sufficient to overcome hyperstabilization of the U1 snRNA/5' ss interaction, indicating that it makes a significant contribution to U1/5' ss stability. Cross-linking studies revealed that contacts between specific proteins and the pre-mRNA are diminished in the absence of Ynl187p and that commitment complexes are unstable. Ynl187p localizes primarily to the nucleus and interacts directly with all five major spliceosomal snRNAs, demonstrating that it interacts with the spliceosome, but is not a snRNP-specific component. Taken together, these findings indicate an important role for Ynl187p in 5' ss recognition and in promoting proper assembly of the first commitment complex in splicing.