Part of the spliceosome which catalyzes two sequential transesterification reactions, first the excision of the non-coding intron from pre-mRNA and then the ligation of the coding exons to form the mature mRNA. Plays a role in stabilizing the structure of the spliceosome catalytic core and docking of the branch helix into the active site, producing 5'-exon and lariat intron-3'-intermediates. Component of the spliceosome (PubMed:11884590). Present in the activated B complex, the catalytically activated B* complex which catalyzes the branching, the catalytic step 1 C complex catalyzing the exon ligation, and the postcatalytic P complex containing the ligated exons (mRNA) and the excised lariat intron (By similarity). Belongs to the 40S cdc5-associated complex (or cwf complex), a spliceosome sub-complex reminiscent of a late-stage spliceosome composed of the U2, U5 and U6 snRNAs and at least brr2, cdc5, cwf2/prp3, cwf3/syf1, cwf4/syf3, cwf5/ecm2, spp42/cwf6, cwf7/spf27, cwf8, cwf9, cwf10, cwf11, cwf12, prp45/cwf13, cwf14, cwf15, cwf16, cwf17, cwf18, cwf19, cwf20, cwf21, cwf22, cwf23, cwf24, cwf25, cwf26, cyp7/cwf27, cwf28, cwf29/ist3, lea1, msl1, prp5/cwf1, prp10, prp12/sap130, prp17, prp22, sap61, sap62, sap114, sap145, slu7, smb1, smd1, smd3, smf1, smg1 and syf2 (PubMed:11884590). Belongs to the CWC16 family. YJU2 subfamily.