Plays role in pre-mRNA splicing as component of the U4/U6-U5 tri-snRNP complex that is involved in spliceosome assembly, and as component of the precatalytic spliceosome (spliceosome B complex). Component of the precatalytic spliceosome (spliceosome B complex) (By similarity). Component of the U4/U6-U5 tri-snRNP complex, a building block of the precatalytic spliceosome (spliceosome B complex) (By similarity). The U4/U6-U5 tri-snRNP complex is 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 (By similarity). Interacts directly with PRPF18, PPIH and PRPF3 (By similarity). Part of a heteromeric complex containing PPIH, PRPF3 and PRPF4 that is stable in the absence of RNA (By similarity). Interacts with ERCC6 (By similarity).