exon–exon junction – what's your function?
TRANSCRIPT
TRENDS in Cell Biology Vol.11 No.12 December 2001
http://tcb.trends.com 0962-8924/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
463News&Comment
The expression of aberrant proteins canhave disastrous consequences for a cell.Quality-control mechanisms that surveynewly synthesized mRNAs are thereforeessential to cellular well being. Unsplicedpre-mRNAs are retained in the nucleusbecause they fail to associate with theappropriate export factors. Anotherquality-control measure operates in thecytoplasm, where mRNAs containingpremature termination codons aredegraded via the nonsense-mediatedmRNA decay (NMD) pathway. In a prime example of complementary workfrom multiple laboratories, three recentpapers have coordinately reported a centralrole for the multiprotein exon–exonjunction complex (EJC) in coordinating these mRNA-surveillancemechanisms1–3.
When an intron is spliced out of apre-mRNA molecule, the EJC is depositedapproximately 20 nucleotides upstream ofthe resulting exon–exon junction,establishing a positional marker on themRNA. Inside the nucleus, the EJCconsists of at least five proteins, but, as
these papers report, it might better bethought of as a ‘dynamic post-splicingcomplex2’ that evolves along the mRNAprocessing pathway rather than being astatic landmark.
Moore and colleagues1 engineered apre-mRNA molecule with a very short (<20 nucleotide) 5’ exon that was efficientlyspliced but failed to provide a docking sitefor the EJC. Using this mRNA, theydemonstrated that the EJC is crucial forefficient nuclear export of spliced mRNAs.It performs this role through an interactionbetween the EJC component REF and themRNA export factor TAP/p15. In thecytoplasm, REF and TAP/p15 dissociatefrom the mRNA, whereas other EJCproteins remain bound and go on tofunction in co-translational NMD. Byartificially tethering EJC components to the3’UTRs of β-globin mRNAs, Steitz andcolleagues2 identified the protein RNPS1 as a key factor in NMD and alsodocumented a somewhat lesser role for the protein Y14. The association ofRNPS1 and Y14 with NMD seems tomediated through their interaction with the Upf3 protein2,3, which Dreyfussand coworkers show joins the EJC in the nuclear compartment3. According to the current model of NMD, if, during the first round of translation, atermination codon is encountered
upstream of the Upf3-marked EJC, NMD is initiated with the cooperation of the other NMD proteins Upf1 and Upf2.
In summary, these three papers havemade significant progress towardsunderstanding the integrative function ofthe EJC as an essential component ofmRNA quality control and defining thedynamic cast of characters that make up the quality-control machinery. Furtherstudies will elucidate the still-unknownmechanism by which the EJC is deposited on mRNA and the precisemRNA–protein and protein–proteininteractions that enable its proper function.
1 Le Hir, H. et al. (2001) The exon–exon junctioncomplex provides a binding platform for factors involved in mRNA export andnonsense-mediated mRNA decay. EMBO J.17, 4987–4997
2 Lykke-Andersen, J. et al. (2001) Communicationof the position of exon–exon junctions to the mRNA surveillance machinery by the protein RNPS1. Science 293, 1836–1839
3 Kim, V.N. et al. (2001) Role of thenonsense-mediated decay factor hUfp3 in the splicing-dependent exon–exon junctioncomplex. Science 293, 1832–1836
Jennifer Hood
Exon–exon junction – what’s your function?
MyoD is one of the four myogenicregulatory factors (MRFs) that can inducemyogenesis when transfected into non-myogenic cell lines. Several mechanisms,such as direct protein–protein interactionsand chromatin remodeling, might underliethe regulation of MRF activity. Rudnicki andcolleagues1 have examined how MyoDactivity is affected by the presence ofsignaling from the activated mitogen-activated protein kinase (MAPK) MEK1.
The authors first showed thatmyogenesis was inhibited when activatedMEK1 was coexpressed with the four MRFs(MyoD, Myf5, MRF 4 and myogenin).Transactivation of E boxes and of the myosinlight chain 1 or 3 promoter and enhancer byMRFs was repressed in the presence of
activated MEK1, but not with wild-type ordominant–negative MEK1. Myotubeformation and endogenous myogenin geneactivation were also inhibited in MyoD-treated 10T1/2 cells in the presence ofactivated MEK1. The authors also looked athow the rate of differentiation was affectedby MEK1 by using the MEK1 inhibitor U0126in C2C12 myoblasts and went on to show intransfected fibroblasts by immunostainingthat activated MEK1 (lacking its N-terminalnuclear export signal) localized in thenucleus. By using MyoD deletion mutants inthe presence or absence of activated MEK1,the N-terminal transactivation domain (TAD)of MyoD was shown to be required for thesuppressive effect of activated MEK1. Aftersubstituting the myoD TAD for a VP16 TAD,
the suppressive effect of MEK1 wasabolished.
These data suggest that activated MEK1suppresses MyoD activity by binding to itsTAD domain. It will be interesting to seewhether the ability of MEK1 to suppressother MRFs also occurs through use of theMyoD mechanism and whether the MyoDTAD can receive a dual signal for activationand activator destruction, as recentlyproposed for the VP16 TAD.
1 Perry, R.L.S. et al. (2001) Activated MEK1 bindsthe nuclear MyoD transcriptional complex torepress transactivation. Mol. Cell 8, 291–301
Chung L. Lau
The importance of being activated
‘The EJC might better be thought of as a
dynamic post-splicing complex rather than
being a static landmark’