Involved in pre-mRNA splicing as a component of the splicing factor SF3B complex. 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. Together with other U2 snRNP complex components may also play a role in the selective processing of microRNAs (miRNAs) from the long primary miRNA transcript, pri-miR-17-92 (PubMed:26255770). 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. Component of the B-WICH complex, at least composed of SMARCA5/SNF2H, BAZ1B/WSTF, SF3B1, DEK, MYO1C, ERCC6, MYBBP1A and DDX21. Identified in the spliceosome C 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). Interacts directly with the splicing factor U2AF. Within the SF3B complex interacts directly (via HEAT domain) with SF3B3, SF3B5 and (via HEAT domain) with PHF5A. Interacts directly with SF3B6. The SF3B complex composed of SF3B1, SF3B2, SF3B3, SF3B4, SF3B5, SF3B6 and PHF5A interacts with U2AF2. Phosphorylated form interacts with PPP1R8. Interacts with PQBP1. Interacts with RBM17. Interacts with RBM39. Interacts with SETX. Interacts with RBM15. Interacts with ZRSR1 (PubMed:29617656). Interacts with USH1G (By similarity). During mitosis, transiently dispersed from the nuclear speckles to the cytoplasm. Ubiquitous. Phosphorylated (By similarity). Phosphorylation occurs concomitantly with the splicing catalytic steps (By similarity). Phosphorylation on Thr-244, Thr-248 and Thr-313 by cyclin-dependent kinases promotes interaction with PPP1R8 during mitosis (By similarity). Citrullinated by PADI4. Belongs to the SF3B1 family.