Involved in pre-mRNA splicing as a component of the splicing factor SF3B complex (PubMed:27720643). 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). 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 (By similarity). 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). Component of the B-WICH complex, at least composed of SMARCA5/SNF2H, BAZ1B/WSTF, SF3B1, DEK, MYO1C, ERCC6, MYBBP1A and DDX21 (PubMed:16603771). Identified in the spliceosome C complex (PubMed:11991638). 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). Interacts directly with the splicing factor U2AF (PubMed:12234937). Within the SF3B complex interacts directly (via HEAT domain) with SF3B3, SF3B5 and (via HEAT domain) with PHF5A (PubMed:27720643). Interacts directly with SF3B6 (PubMed:16432215, Ref.37, PubMed:21062891). The SF3B complex composed of SF3B1, SF3B2, SF3B3, SF3B4, SF3B5, SF3B6 and PHF5A interacts with U2AF2 (PubMed:27720643). Phosphorylated form interacts with PPP1R8 (PubMed:12105215). Interacts with PQBP1 (PubMed:23512658). Interacts with RBM17 (PubMed:17589525). Interacts with RBM39 (PubMed:24795046). Interacts with SETX (PubMed:21700224). Interacts with RBM15 (PubMed:26575292). Interacts with USH1G (PubMed:34023904). During mitosis, transiently dispersed from the nuclear speckles to the cytoplasm. Phosphorylated. Phosphorylation occurs concomitantly with the splicing catalytic steps. Phosphorylation on Thr-244, Thr-248 and Thr-313 by cyclin-dependent kinases promotes interaction with PPP1R8 during mitosis. Citrullinated by PADI4. Belongs to the SF3B1 family.