RNA export factor (REF) is a component of the exon junction complex (EJC) that is deposited on mRNA in a splicing-dependent manner, and targets spliced mRNA for export. In this study, analysis of the RNA-binding protein complexes revealed that REF associates with β-globin mRNA at the region other than the EJC deposition site. Comparison between RNA polymerase II and T7 transcription and further analysis showed that the deposition of REF apart from the EJC is dependent on the 5′ cap structure, but not splicing. Excess amounts of m7 GpppGcap analog reduced REF binding to intronless mRNA, and a co-immunoprecipitation experiment revealed that REF interacts with the cap-binding protein CBP20. The export of Cy3-labeled intronless β-globinmRNA from nuclei of HeLa cells was enhanced by co-injection of CBP20 and REF. Thus, REF recruited by CBP20 may play a stimulatory role to export the capped intronless mRNAs.
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