Required for pre-spliceosome formation, which is the first step of pre-mRNA splicing. 2 commitment complexes, CC1 and CC2, have been defined. 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 snRNP U1-associated protein PRP40, bridging the U1 snRNP-associated 5'-splice site and the MSL5-associated branch point 3' intron splice site. Involved in nuclear retention of pre-mRNA