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. 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. Belongs to the CWC16 family. YJU2 subfamily.