mRNA splicing factors, PRP9, PRP11, and PRP21, are necessary for binding of the U2 snRNP to the pre-mRNA in an early step of 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. Present with 3920 molecules/cell in log phase SD medium. Belongs to the SF3A3 family.