Involved in pre-mRNA splicing as a component of the splicing factor SF3B complex, a constituent of the spliceosome (PubMed:18981222). SF3B complex is required for 'A' complex assembly formed by the stable binding of U2 snRNP to the branchpoint sequence (BPS) in pre-mRNA (PubMed:18981222). Sequence independent binding of SF3A/SF3B complex upstream of the branch site is essential, it may anchor U2 snRNP to the pre-mRNA (By similarity). May also be involved in the assembly of the 'E' complex (By similarity). Belongs also to the minor U12-dependent spliceosome, which is involved in the splicing of rare class of nuclear pre-mRNA intron (By similarity). Identified in the spliceosome A complex; remains associated with the spliceosome throughout the splicing process (By similarity). Component of the spliceosome B complex (By similarity). Identified in the spliceosome C complex (By similarity). Identified in the spliceosome E complex (By similarity). Component of the U11/U12 snRNPs that are part of the U12-type spliceosome (By similarity). Component of splicing factor SF3B complex (PubMed:18981222). Identified in the SAGA transcription regulatory histone acetylation (HAT) complex; the interaction is RNA-independent (PubMed:27185460). Belongs to the RSE1 family.