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In vertebrates, a nuclear cap-binding complex (CBC) formed by two cap- binding proteins, CBP20 and CBP80, is involved in several steps of RNA metabolism, including pre-mRNA splicing and nuclear export of some RNA polymerase II-transcribed U snRNAs. The CBC is highly conserved, and antibodies against human CBP20 cross-react with the CBP20 counterpart in the dipteran Chironomus tentans. Using immunoelectron microscopy, the in situ association of CBP20 with a specific pre-mRNP particle, the Balbiani ring particle, has been analyzed at different stages of pre-mRNA synthesis, maturation, and nucleo-cytoplasmic transport. We demonstrate that CBP20 binds to the nascent pre-mRNA shortly after transcription initiation, stays in the RNP particles after splicing has been completed, and remains attached to the 5' domain during translocation of the RNP through the nuclear pore complex (NPC). The rapid …