Involved in pre-mRNA splicing. Interacts with prp6 and prp13. May also be involved in the regulation of the G0-G1/G2 transition. Required for pre-spliceosome formation, which is the first step of pre-mRNA splicing. This protein is associated with snRNP U5. Has a role in branch site-3' splice site selection. Associates with the branch site-3' splice 3'-exon region. Interacts with brr2 and spp42.