Plays role in pre-mRNA splicing as core component of precatalytic, catalytic and postcatalytic spliceosomal complexes, both of the predominant U2-type spliceosome and the minor U12-type spliceosome (PubMed:10411133, PubMed:11971955, PubMed:28502770, PubMed:28781166, PubMed:28076346, PubMed:29361316, PubMed:30315277, PubMed:29360106, PubMed:29301961, PubMed:30728453, PubMed:30705154). Functions as a scaffold that mediates the ordered assembly of spliceosomal proteins and snRNAs. Required for the assembly of the U4/U6-U5 tri-snRNP complex, a building block of the spliceosome. Functions as scaffold that positions spliceosomal U2, U5 and U6 snRNAs at splice sites on pre-mRNA substrates, so that splicing can occur. Interacts with both the 5' and the 3' splice site. Part of the U5 snRNP complex (PubMed:2532307, PubMed:2527369). 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:2479028, PubMed:16723661, PubMed:26912367). Component of the U5.U4atac/U6atac snRNP complexes in U12-dependent spliceosomes (PubMed:11971955). Core component of U2-type precatalytic, catalytic and postcatalytic spliceosomal complexes (PubMed:10411133, PubMed:28502770, PubMed:28781166, PubMed:28076346, PubMed:29361316, PubMed:30315277, PubMed:29360106, PubMed:29301961, PubMed:30728453, PubMed:30705154). Found in a mRNA splicing-dependent exon junction complex (EJC) with SRRM1. Interacts with U5 snRNP proteins SNRP116 and SNRNP40. Interacts with EFTUD2 and SNRNP200. Interacts (via the MPN (JAB/Mov34) domain) with PRPF3 ('Lys-63'-linked polyubiquitinated); may stabilize the U4/U6-U5 tri-snRNP complex. Interacts (via RNase H homology domain) with AAR2 (PubMed:26527271). Interacts with RPAP3 and URI1 in a ZNHIT2-dependent manner (PubMed:28561026). Widely expressed. The MPN (JAB/Mov34) domain has structural similarity with deubiquitinating enzymes, but lacks the residues that would bind the catalytic metal ion. Contains a region with structural similarity to reverse transcripase, presenting the classical thumb, fingers and palm architecture, but lacks enzyme activity, since the essential metal-binding residues are not conserved. Contains a region with structural similarity to type-2 restriction endonucleases, but the residues that would bind catalytic metal ions in endonucleases are instead involved in hydrogen bonds that stabilize the protein structure. Contains a region with structural similarity to RNase H, but lacks RNase H activity. The disease is caused by variants affecting the gene represented in this entry.