Required for pre-mRNA splicing and spliceosome assembly. As part of the pre-mRNA retention and splicing (RES) complex, required for nuclear pre-mRNA retention and efficient splicing. Required for MER1-activated splicing. Component of the 45S U1.U2.U4/U6.U5 penta-snRNP particle, a subcomplex of the spliceosome. Belongs to 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. Belongs to the pre-mRNA retention and splicing (RES) complex composed of at least BUD13, IST3 and PML1. Subunit of the U2 snRNP. Interacts with RDS3. Cells have a slow growth phenotype that is exacerbated at 37 degrees Celsius. Deletion mutants have a significant pre-mRNA leakage at 25 degrees Celsius. Present with 922 molecules/cell in log phase SD medium. Belongs to the IST3 family.