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 (PubMed:29301961). 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). {ECO:0000255|HAMAP-Rule:MF_03226, ECO:0000269|PubMed:22952453, ECO:0000269|PubMed:29301961}. This is the function of YJU2 (YJU2 splicing factor homolog, Ensembl gene identifier ENSG00000105248).