Involved in pre-mRNA splicing as a component of the splicing factor SF3B complex, a constituent of the spliceosome (PubMed:10490618, PubMed:10882114, PubMed:27720643, PubMed:28781166). 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 (PubMed:12234937). May also be involved in the assembly of the 'E' complex (PubMed:10882114). Belongs also to the minor U12-dependent spliceosome, which is involved in the splicing of rare class of nuclear pre-mRNA intron (PubMed:15146077). Identified in the spliceosome A complex; remains associated with the spliceosome throughout the splicing process (PubMed:10490618). Component of the spliceosome B complex (PubMed:28781166). Identified in the spliceosome C complex (PubMed:11991638). Identified in the spliceosome E complex (PubMed:10882114). Component of the U11/U12 snRNPs that are part of the U12-type spliceosome (PubMed:15146077). Component of splicing factor SF3B complex which is composed of at least eight subunits; SF3B1, SF3B2, SF3B3, SF3B4, SF3B5, SF3B6, PHF5A and DDX42 (PubMed:12234937, PubMed:12738865, PubMed:28541300, PubMed:27720643). 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 (PubMed:12234937). Within the SF3B complex interacts directly with SF3B1 (via HEAT domain), SF3B5 and PHF5A (PubMed:27720643). The SF3B complex composed of SF3B1, SF3B2, SF3B3, SF3B4, SF3B5, SF3B6 and PHF5A interacts with U2AF2 (PubMed:27720643). Associates with the STAGA transcription coactivator-HAT complex. Interacts with SUPT3H (PubMed:11564863). Interacts with TAF3 (PubMed:11438666). The core of the protein consists of three beta-propeller domains. Belongs to the RSE1 family.