Clarification:
The Exon Junction Complex (EJC) functions as a eukaryotic molecular assembly that interacts with spliced mRNA near the exon-exon junctions, establishing a binding site for additional trans-acting proteins that influence the mRNA's fate. The spliceosome positions the approximately 335kD EJC in a non-sequence specific way, approximately 20-24 nucleotides upstream of an exon junction. Functionally, the EJC plays a role in the nuclear export of spliced mRNAs, facilitates nonsense-mediated decay of improperly spliced mRNAs containing premature stop signals, and increases translation efficiency.
Pre-mRNA that is bound by a spliceosome is typically not exported from the nucleus, ensuring that only fully-processed mRNA moves to the cytoplasm for translation. A protein called mRNP exporter attaches to the EJC, via interactions with RNA and with the EJC-related protein REF (RNA export factor), to assist pre-mRNA in departing the nuclear pore complex.
Interestingly, the efficiency of unspliced mRNA export relies on its length; longer mRNAs are exported more effectively than shorter mRNAs. Conversely, in spliced mRNAs, once the 5' exon reaches a sufficient length to bind the EJC, the length does not impact the efficiency of export.
A specific quantity of EJCs exists within a cell, necessitating their recycling to continuously tag mature mRNAs. Upon entering the cytoplasm, the ribosome-associated regulatory protein (PYM) acts to promote dissociation.
