Necessary for the splicing of pre-mRNA. The SF1-U2AF59-U2AF23 complex has a role in the recognition of the branch site (5'-UACUAAC-3'), the pyrimidine tract and the 3'-splice site at the 3'-end of introns. Forms a heterodimer with the U2AF small subunit. Can also form a homodimer. U2AF large subunit (U2AF59), U2AF small subunit (U2AF23) and SF1 (bpb1) interact to form a complex required for complex A formation. Interacts with wat1/pop3. Belongs to the splicing factor SR family.