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. 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 (By similarity). 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 (By similarity). Component of the U5.U4atac/U6atac snRNP complexes in U12-dependent spliceosomes (By similarity). Core component of U2-type precatalytic, catalytic and postcatalytic spliceosomal complexes (By similarity). Found in a mRNA splicing-dependent exon junction complex (EJC) with SRRM1 (By similarity). Interacts with U5 snRNP proteins SNRP116 and SNRNP40 (By similarity). Interacts with EFTUD2 and SNRNP200 (By similarity). Interacts (via the MPN (JAB/Mov34) domain) with PRPF3 ('Lys-63'-linked polyubiquitinated); may stabilize the U4/U6-U5 tri-snRNP complex (By similarity). Interacts (via RNase H homology domain) with AAR2 (By similarity). Interacts with RPAP3 and URI1 in a ZNHIT2-dependent manner (By similarity). Strongly expressed in testis (preferentially in the outer cell layer), and moderately in ovary (preferentially in granulosa cells). During embryogenesis, is highly expressed at day 9.5 of gestation, and its expression decreases progressively during embryogenesis. 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. Truncated N-terminus.