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 (By similarity). May protect cells from TP53-dependent apoptosis upon dsDNA break damage through association with PRP19-CD5L complex (PubMed:22952453). 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. Morpholino knockdown of the protein causes small head and a curled tail by day 2. Knockdown causes increased sensitivity to IR-induced apoptosis (PubMed:22952453). In double morpholino knockdown of TP53 and YJU2, TP53 deficiency rescues animals from developmental neurodegeneration observed on YJU2 mutants and radiosensitivity (PubMed:22952453). Belongs to the CWC16 family. YJU2 subfamily.