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:19935684, PubMed:27445308, PubMed:27459055, PubMed:29146871). Plays a role (via N-terminus) 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 (PubMed:23438600, PubMed:27459055). Further stabilizes spliceosome conformation for 3'-splice site docking (via C-terminus) promoting exon ligation (PubMed:29146871). 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 (PubMed:19935684, PubMed:27445308, PubMed:27459055, PubMed:29146871). Interacts (via C-terminus) with CLF1 (PubMed:23438600). Interacts (via N-terminus) with SYF1 (PubMed:23438600). Interacts with U2 snRNA; this interaction is direct (PubMed:23438600). Identified in 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 (PubMed:11884590). Expressed in exponential-phase cells grown in rich medium. Present with 2250 molecules/cell in log phase SD medium. Belongs to the CWC16 family. YJU2 subfamily.