FUNCTION Plays a role in mRNA splicing as a component of the U4/U6-U5 tri-snRNP, one of the building blocks of the spliceosome. May also bind to DNA.SUBUNIT Identified in the spliceosome C complex. Component of the U4/U6-U5 tri-snRNP complex composed of the U4, U6 and U5 snRNAs and at least PRPF3, PRPF4, PRPF6, PRPF8, PRPF31, SNRNP200, TXNL4A, SNRNP40, DDX23, CD2BP2, PPIH, SNU13, EFTUD2, SART1 and USP39. Interacts with UBL5 (PubMed:25092792). Interacts with IVNS1ABP (via Kelch repeats) (PubMed:30538201).INTERACTION Found in the nucleus of mitogen-activated peripheral blood mononuclear cells (PBMCs), tumor cells, or normal cell lines, but not in normal tissues except testis and fetal liver or in unstimulated PBMCs, suggesting preferential expression in proliferating cells.ALTERNATIVE PRODUCTS A number of isoforms may be produced.TISSUE SPECIFICITY Ubiquitously expressed.PTM Sumoylated with SUMO2.ALLERGEN Causes an allergic reaction in human. Binds to IgE from atopic dermatitis (AD) patients. Identified as an IgE autoantigen in atopic dermatitis (AD) patients with severe skin manifestations.SIMILARITY Belongs to the SNU66/SART1 family.