Necessary for the splicing of pre-mRNA. The BPB1(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. U2AF large subunit (u2af59), U2AF small subunit (u2af23) and bpb1 interact to form a complex required for complex A formation. Belongs to the BBP/SF1 family.