Component of the CBC complex, which binds co-transcriptionally to the 5'-cap of pre-mRNAs and is involved in maturation, export and degradation of nuclear mRNAs. The CBC complex is required for efficient pre-mRNA splicing through efficient commitment complex and spliceosome formation. Together with NPL3, the CBC complex is required for export of mRNAs out of the nucleus. The CBC complex is also involved in nuclear mRNA degradation, probably by directing the mRNAs to the sites of degradation. Affects replication of the positive-strand RNA virus BMV.