Involved in pre-mRNA splicing as component of the spliceosome (PubMed:11867543, PubMed:28781166). Required for the assembly of the U4/U5/U6 tri-snRNP complex, one of the building blocks of the spliceosome (PubMed:11867543). Identified in the spliceosome B complex (PubMed:28781166). 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 (PubMed:11867543, PubMed:16723661, PubMed:26912367). Interacts with a complex formed by SNU13 and U4 snRNA, but not with SNU13 or U4 snRNA alone (PubMed:17412961, PubMed:21784869). The complex formed by SNU13 and PRPF31 binds also U4atac snRNA, a characteristic component of specific, less abundant spliceosomal complexes (PubMed:21784869). Interacts with PRPF6/U5 snRNP-associated 102 kDa protein (PubMed:11867543, PubMed:17412961, PubMed:26912367). 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 (PubMed:15960975). Interacts (via its NLS) with CTNNBL1 (PubMed:21385873). Interacts with USH1G (PubMed:34023904). Predominantly found in speckles and in Cajal bodies. Ubiquitously expressed. Interacts with the snRNP via the Nop domain. The coiled coil domain is formed by two non-contiguous helices. The disease is caused by variants affecting the gene represented in this entry. Belongs to the PRP31 family.