Contacts pre-mRNA on both sides of the branch site early in spliceosome assembly. 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. Interacts with RDS3. Present with 521 molecules/cell in log phase SD medium. Belongs to the SF3B1 family.