Explanation:
The Exon Junction Complex (EJC) is a eukaryotic molecular machine that interacts with spliced mRNA upstream of exon-exon junctions, providing a binding platform for other trans-acting proteins that determine the fate of the mRNA. The spliceosome deposits the ~335kD EJC in a non-sequence specific manner 20-24 nucleotides upstream of an exon-junction. Functionally, the EJC aids in nuclear export of spliced mRNAs, assists in nonsense-mediated decay of incorrectly spliced mRNAs containing premature stop codons, and enhances translation efficiency.
Pre-mRNA bound by a spliceosome is usually not exported from the nucleus, so as to make sure that only fully-processed mRNA travels to the cytoplasm to be translated. A protein called the mRNP exporter binds to the EJC, both through RNA interactions and interactions with the EJC-associated protein REF (RNA export factor) to help pre-mRNA exit the nuclear pore complex.
Interestingly, the efficiency of unspliced mRNA export is dependent on the length; longer mRNAs are exported more efficiently than shorter mRNAs. In spliced mRNAs, however, once the 5' exon is long enough to bind the EJC, the length of the spliced mRNA does not affect the export efficiency.
There are a certain number of EJCs in a cell, and they must be recycled in order to continue tagging mature mRNAs. Once in the cytoplasm, the ribosome-associated regulator protein (PYM) acts as a dissociation factor.
