During the splicing reaction, introns are removed from the pre-mRNA and exons are joined together. This process occurs in a stepwise manner and is carried out by the spliceosome, which consists of five snRNPs and hundreds of proteins. Splicing Factor 1 (SF1) acts at the onset of spliceosome assembly by recognizing the intron branch-point sequence (BPS) in the early complex E. SF1 is then displaced from the pre-mRNA and the BPS is recognized by U2 snRNP in complex A. In this study, we identified novel SF1-interacting partners by performing yeast-two-hybrid screens and co-immunoprecipitation coupled to mass spectrometry. Our results show that SF1 interacts both in vitro and in HeLa nuclear extracts with SURP domains of the U2 snRNP-associated proteins SF3a120 and CHERP. This interaction is required for efficient formation of complex A by promoting the association of U2 snRNP with the spliceosome.