Subunit of the splicing factor SF3B required for 'A' complex assembly formed by the stable binding of U2 snRNP to the branchpoint sequence (BPS) in pre-mRNA. Sequence independent binding of SF3A/SF3B complex upstream of the branch site is essential, it may anchor U2 snRNP to the pre-mRNA. May also be involved in the assembly of the 'E' complex. Has been found in complex 'B' and 'C' as well (By similarity). Component of splicing factor SF3B which is composed of at least eight subunits. Belongs to the SF3B4 family.