Model for the coupling of mRNA transcription, splicing, and nuclear export. Messenger RNA is transcribed by RNA polymerase II (pol II). Except for UAP56 (yellow circle), association of the pre-mRNA with EJC factors of the outer shell (dark green circles), factors with export activity (light green circles), or factors loosely interacting with the EJC and RNA (blue circles) is very dynamic. EJC core proteins (red circles) stabilize the mRNP complex (EJC core assembly). Spliced mRNPs in a stable, export-competent protein–RNA conformation are exported immediately (path A), while other mRNA–protein complexes, which carry an incomplete or destablized EJC (path B) or feature many exons and introns and are thus not completely spliced (path C), accumulate in speckles due to the lack of outer-shell factors that drive export. Completion of splicing and (re)assembly of a functional EJC leads to a stable protein–RNA complex that is exported.
