Splicing factor that contacts pre-mRNA directly and is a component of the pre-mRNA-U1 snRNP complex (commitment complex 2) that forms during early spliceosome assembly in yeast extracts. MSL5, MUD2 and PRP40 interact to form the commitment complex 2 (CC2), a precursor of mature spliceosomes. Present with 4170 molecules/cell in log phase SD medium.