Exon junction complex

An exon junction complex (EJC) is a protein complex which forms on a pre-messenger RNA strand at the junction of two exons which have been joined together during RNA splicing. The EJC has major influences on translation, surveillance and localization of the spliced mRNA. It is first deposited onto mRNA during splicing and is then transported into the cytoplasm. There it plays a major role in post-transcriptional regulation of mRNA. It is believed that exon junction complexes provide a position-specific memory of the splicing event. The EJC consists of a stable heterotetramer core, which serves as a binding platform for other factors necessary for the mRNA pathway. The core of the EJC contains the protein eukaryotic initiation factor 4A-III (eIF4A-III; a DEAD-box RNA helicase) bound to an adenosine triphosphate (ATP) analog, as well as the additional proteins Magoh and Y14. The binding of these proteins to nuclear speckled domains has been measured recently and it may be regulated by PI3K/AKT/mTOR signaling pathways. In order for the binding of the complex to the mRNA to occur, the eIF4AIII factor is inhibited, stopping the hydrolysis of ATP. This recognizes EJC as an ATP dependent complex. EJC also interacts with a large number of additional proteins; most notably SR proteins. These interactions are suggested to be important for mRNA compaction.

The EJC is made up of several key protein components: RNPS1, Y14, SRm160, Aly/REF and Magoh, among others. RNPS1 can function as a coactivator of splicing, but along with Y14, it also takes part in the process of nonsense-mediated decay in eukaryotes. SRm160 is another coactivator that has been proposed to enhance mRNA 3’ end processing. The protein component Magoh is thought to facilitate the subcytoplasmic localization of mRNAs while Aly is engaged in nuclear mRNA export. Aly is believed to be recruited to the exon junction complex by the protein UAP56. UAP56 is recognized as an RNA helicase but acts as a splicing factor required for early splicesome assembly. Another factor involved in the EJC pathway is DEK. This component is known to take part in a variety of functions ranging from splicing to transcriptional regulation and chromatin structure.

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