Functions as a scaffold that mediates the ordered assembly of spliceosomal proteins and snRNAs. Required for association of BRR2 with the spliceosomal U5 snRNP, and the subsequent 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, as well as the branch region. Has a role in branch site-3' splice site selection. Associates with the branch site-3' splice 3'-exon region. Also has a role in cell cycle. Component of the U4/U6-U5 tri-snRNP complex composed of the U4, U6 and U5 snRNAs and at least PRP3, PRP4, PRP6, PRP8, PRP18, PRP31, PRP38, SNU13, SNU23, SNU66, SNU114, SPP381, SMB1, SMD1, SMD2, SMD3, SMX2, SMX3, LSM2, LSM3, LSM4, LSM5, LSM6, LSM7, LSM8, BRR2 and DIB1. Belongs to the CWC complex (or CEF1-associated complex), a spliceosome sub-complex reminiscent of a late-stage spliceosome composed of the U2, U5 and U6 snRNAs and at least BUD13, BUD31, BRR2, CDC40, CEF1, CLF1, CUS1, CWC2, CWC15, CWC21, CWC22, CWC23, CWC24, CWC25, CWC27, ECM2, HSH155, IST3, ISY1, LEA1, MSL1, NTC20, PRP8, PRP9, PRP11, PRP19, PRP21, PRP22, PRP45, PRP46, SLU7, SMB1, SMD1, SMD2, SMD3, SMX2, SMX3, SNT309, SNU114, SPP2, SYF1, SYF2, RSE1 and YJU2. Interacts with PRP40 and SNP1. Interacts (via SCwid domain) with CWC21. Interacts (via SCwid domain) with SNU114 (via N-terminus). Interacts (via RNase H homology domain and MPN domain) with BRR2; this modulates BRR2 ATPase and helicase activity. Interacts (via RNase H homology domain) with AAR2. AAR2 and BRR2 compete for PRP8 binding, and during U5 snRNP maturation BRR2 displaces the initially bound AAR2. Is associated with snRNP U5, together with SNU114 and BRR2. 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 a highly conserved loop. Contains a region with structural similarity to RNase H, but lacks RNase H activity. Present with 468 molecules/cell in log phase SD medium.