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. 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 and of the U4/U6-U5 tri-snRNP complex. 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.