Involved in pre-mRNA splicing as component of the spliceosome. Required for the assembly of the U4/U5/U6 tri-snRNP complex, one of the building blocks of the spliceosome. Identified in the spliceosome B 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 a complex formed by SNU13 and U4 snRNA, but not with SNU13 or U4 snRNA alone. The complex formed by SNU13 and PRPF31 binds also U4atac snRNA, a characteristic component of specific, less abundant spliceosomal complexes. Interacts with PRPF6/U5 snRNP-associated 102 kDa protein. Component of some MLL1/MLL complex, at least composed of the core components KMT2A/MLL1, ASH2L, HCFC1/HCF1, WDR5 and RBBP5, as well as the facultative components BAP18, CHD8, E2F6, HSP70, INO80C, KANSL1, LAS1L, MAX, MCRS1, MGA, KAT8/MOF, PELP1, PHF20, PRP31, RING2, RUVB1/TIP49A, RUVB2/TIP49B, SENP3, TAF1, TAF4, TAF6, TAF7, TAF9 and TEX10. Interacts (via its NLS) with CTNNBL1. Interacts with USH1G (By similarity). Predominantly found in speckles and in Cajal bodies. Interacts with the snRNP via the Nop domain. The coiled coil domain is formed by two non-contiguous helices. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the PRP31 family. Contaminating sequence. The C-terminus matches chromosome 19 region. Wrong choice of frame.