Required for pre-spliceosome formation, which is the first step of pre-mRNA splicing. This protein is associated with snRNP U1. Two commitment complexes, CC1 and CC2, have been defined in yeast. CC1 is a basal complex dependent only on the 5' splice site. CC2 is a complex of lower mobility and is dependent on a branchpoint as well as a 5' splice site region. This protein is involved in CC2 formation where it binds to the branchpoint binding protein MSL5, bridging the U1 snRNP-associated 5' splice site and the MSL5-associated branch point 3' intron splice site. Interacts with CRM1, MSL5, PRP8, and the RNA polymerase II largest subunit (RPB1). MSL5, MUD2 and PRP40 interact to form the commitment complex 2 (CC2), a precursor of mature spliceosomes. The WW and FF domains bind to the phosphorylated carboxy-terminal domain of RPB1. Belongs to the PRPF40 family.