The following small summary on a protein complex called EJC was inspired by a lecture given by Hervé Le Hir (ENS, Paris) at the Friedrich Miescher Institute in Basel in November 2014:

After transcription an mRNA becomes processed, exported, stored or transported, translated and degraded. Several multimeric protein complexes carry out these tasks and readily transform the initially naked mRNA into a large messenger ribonucleoprotein (mRNP) complex. For a long time it was believed that the described functional steps are occurring sequentially and relatively independent of each other. However, more recently it became clearer that many events during the life of an mRNA leave permanent protein marks which can influence the efficiency or occurrence of subsequent functional events and which are dependent on the sequence context. One of the first mRNPs that forms in the nucleus after transcription is the splicing machinery. It splices introns out of the pre-mRNA molecule, thereby creating the mature mRNA. The splicing reaction, however, leaves a relatively stable mark on the newly created spliced mRNA: The Exon Junction Complex (EJC).

What is an EJC and where is it formed?

The EJC is a multiprotein complex that forms as a consequence of splicing upstream of exon-exon junctions. Although the EJC’s composition is dynamic it contains four core proteins: The RNA helicase and eukaryotic initiation factor 4A3 (eIF4A3), metastatic lymph node 51 (MLN51), and the heterodimer Magoh/Y14. eIF4A3 possesses two ATP-dependent RecA domains which bind RNA in a “clamp-like” fashion. Magoh/Y14 seems to prevent conformational changes of eIF4A3, while the conserved SELOR domain of MLN51 also binds to the RNA and in addition stabilizes the RecA clamps further (1). This tetrameric core now serves as a platform allowing for the binding of other factors that catalyze different regulative processes during export, transport and translation of the mRNP. By using both fluorescence and electron microscopy approaches it became possible to narrow down the assembly zone of the tetrameric EJC core to nuclear punctuate regions termed perispeckles (periphery of nuclear speckles). All EJC subunits are enriched and fully assembled in these structures while MLN51, Magoh, and Y14 mutants fail to localize to the perispeckle region. Furthermore, perispeckles seem to contain polyA mRNAs and transcripts which are actively undergoing splicing (2). These nuclear compartments had earlier already been described as storage and assembly cites for splicing factors which highlights the possibility that EJC proteins join in a co- and post-splicing manner.

Which processes does the EJC catalyze?
Splicing of certain long intron containing mRNAs is affected by EJCs and the complex also seems to be responsible for the catalysis of one form of alternative splicing. Furthermore, the EJC is implicated in mRNA transport and plays an important role during nonsense-mediated decay (NMD) of transcript possessing a premature stop-codon. When such an erroneous codon is present, some EJCs remain bound to the mRNA because they are not displaced by the progressing ribosome and become bound by the up-frameshift factors Upf1, Upf2, and Upf3. Together these proteins trigger mRNA decay (3). For a long time it has been known that the presence of introns enhances the translation of a construct when compared to a similar construct that is lacking introns. Another
important task of EJCs therefore seems to be the enhancement of translational efficiency of spliced mRNAs. This has mainly been demonstrated by tethering all four EJC components artificially to mRNAs in Xenopus oocytes (4). The molecular details of this process have, however, remained elusive until recently.

How does the EJC influence translation?
It has been described that the EJC is the functional linker between splicing and an enhanced translation efficiency. Recently it emerged that the EJC component MLN51 might mediate this relationship by interacting with the translation initiation factor eIF3 (5). First of all it was observed that overexpression of MLN51 enhances translation of spliced luciferase reporters versus identical non-spliced reporters. Furthermore, MLN51 also enhances translation if the remaining three EJC components are not present. Immunoprecipitations then showed that several translation initiation factors and ribosomal subunits can bind EJC components, but only MLN51 binds via its SELOR domain to the initiation factor eIF3. This interaction might lead to a stabilization of the mRNP complex so that translation can initiate successfully. One problem, however, persists: Several studies have described that the ribosome displaces the EJC from the mRNP complex during the first round of translation. The question whether an upregulation of the first round of translation is sufficient to explain the observed positive effect on translation efficiency by the EJC is therefore still open. One explanation could be that EJCs increase the absolute pool of translated mRNAs via MLN51. Alternatively, MLN51 might increase the total number of initiating ribosomes on the single mRNA before the EJCs become displaced. It might also be possible that MLN51 survives on the mRNA after displacement, and thereby is able to initiate subsequent rounds of translation. This hypothesis seems probable since the other three EJC components are not required for an increased translation efficiency. Since a large number of factors have been described that peripherally bind EJCs (1) the molecular mechanism of translation enhancement is likely be more complex and more functional interactions of MLN51 need to be identified. The past years of research have, however, shown that the sequence context and all lifecycle steps of an mRNA are closely linked and the EJC serves as an interesting example for the complexity of an mRNAs life.

1. Le Hir H, Andersen GR. Structural insights into the exon junction complex. Curr Opin Struct Biol. 2008 Feb;18(1):112–9.
2. Daguenet E, Baguet A, Degot S, Schmidt U, Alpy F, Wendling C, et al. Perispeckles are major assembly sites for the exon junction core complex. Mol Biol Cell. 2012 May 1;23(9):1765–82.
3. Gehring NH, Kunz JB, Neu-Yilik G, Breit S, Viegas MH, Hentze MW, et al. Exon-junction complex components specify distinct routes of nonsense-mediated mRNA decay with differential cofactor requirements. Mol Cell. 2005 Oct 7;20(1):65–75.
4. Wiegand HL, Lu S, Cullen BR. Exon junction complexes mediate the enhancing effect of splicing on mRNA expression. Proc Natl Acad Sci U S A. 2003 Sep 30;100(20):11327–32.
5. Chazal P-E, Daguenet E, Wendling C, Ulryck N, Tomasetto C, Sargueil B, et al. EJC core component MLN51 interacts with eIF3 and activates translation. Proc Natl Acad Sci. 2013 Apr 9;110(15):5903–8.