Involved in pre-mRNA splicing as a component of the splicing factor SF3B complex, a constituent of the spliceosome. SF3B complex is 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. Belongs also to the minor U12-dependent spliceosome, which is involved in the splicing of rare class of nuclear pre-mRNA intron. Identified in the spliceosome A complex; remains associated with the spliceosome throughout the splicing process. Component of the spliceosome B complex. Identified in the spliceosome C complex. Identified in the spliceosome E complex. Component of the U11/U12 snRNPs that are part of the U12-type spliceosome. Component of splicing factor SF3B complex which is composed of at least eight subunits; SF3B1, SF3B2, SF3B3, SF3B4, SF3B5, SF3B6, PHF5A and DDX42. SF3B associates with the splicing factor SF3A and a 12S RNA unit to form the U2 small nuclear ribonucleoproteins complex (U2 snRNP). Interaction between SF3B3 and SF3B1 is tighter than the interaction between SF3B3 and SF3B2. Within the SF3B complex interacts directly with SF3B1 (via HEAT domain), SF3B5 and PHF5A. The SF3B complex composed of SF3B1, SF3B2, SF3B3, SF3B4, SF3B5, SF3B6 and PHF5A interacts with U2AF2. Associates with the STAGA transcription coactivator-HAT complex. Interacts with SUPT3H. Interacts with TAF3. The core of the protein consists of three beta-propeller domains. Belongs to the RSE1 family.