Plays a role in pre-mRNA splicing as a component of the U4/U6-U5 tri-snRNP, one of the building blocks of the precatalytic spliceosome (PubMed:11350945, PubMed:26912367). Regulates AURKB mRNA levels, and thereby plays a role in cytokinesis and in the spindle checkpoint. Does not have ubiquitin-specific peptidase activity (PubMed:18728397). The U4/U6-U5 tri-snRNP complex is a building block of the precatalytic spliceosome (spliceosome B complex) (PubMed:11350945, PubMed:26912367). 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, SNRPB, SNRPD1, SNRPD2, SNRPD3, SNRPE, SNRPF, SNRPG, DDX23, CD2BP2, PPIH, SNU13, EFTUD2, SART1 and USP39, plus LSM2, LSM3, LSM4, LSM5, LSM6, LSM7 and LSM8 (PubMed:16723661, PubMed:26912367). Belongs to the peptidase C19 family. Lacks the conserved His and Cys residues that are essential for the activity of de-ubiquitinating enzymes. Lacks ubiquitin C-terminal hydrolase activity (PubMed:18728397). Truncated N-terminus.