The evolutionarily conserved C-terminal tail of the Dib1 protein is required for spliceosome efficacy and Dib1 protein stability

The spliceosome catalyzes pre-messenger RNA splicing by facilitating two transesterification reactions that excise introns and ligate exons together. To accomplish this process, the spliceosome assembles and rearranges its components through a series of sequentially regulated steps. A key step in this pathway is the transition to the catalytically active B act complex. In the course of its catalytic activation, the spliceosome must release Dib1, an essential, conserved 143 amino acid protein that resides in the U4/U6-U5 tri-snRNP complex. However, the mechanism of Dib1 departure is not fully characterized. To further this understanding, this study determined the importance of the flexible C-terminus of Dib1 in the spliceosome. The C-terminus of Dib1 sits in proximity to the catalytic regions of both U5 and U6 snRNAs and interacts with Prp8 and Prp31 in the spliceosome. We find that the length of Dib1's C-terminus is critical for the stability of the protein and its removal results in loss of cell viability and splicing capabilities. Additionally, we have identified residues in Dib1's C-terminal tail that we postulate are part of stabilizing protein-protein interactions in the spliceosome, including key areas between Dib1 and Prp8. Together, these findings further our understanding of a critical step in spliceosome assembly and activation.