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Eif4aiii Enhances Translation of Nuclear Cap-Binding Complex

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Eif4aiii Enhances Translation of Nuclear Cap-Binding Complex eIF4AIII enhances translation of nuclear cap-binding PNAS PLUS complex–bound mRNAs by promoting disruption of secondary structures in 5′UTR Junho Choea,1,2, Incheol Ryua,1, Ok Hyun Parka, Joori Parka, Hana Choa,3, Jin Seon Yoob, Sung Wook Chib,c, Min Kyung Kima, Hyun Kyu Songa, and Yoon Ki Kima,4 aDivision of Life Sciences, Korea University, Seoul 136-701, Republic of Korea; bDepartment of Health Sciences and Technology, Samsung Advanced Institute for Health Sciences and Technology, Sungkyunkwan University, Seoul 135-710, Republic of Korea; and cSamsung Research Institute for Future Medicine, Samsung Medical Center, Seoul 135-710, Republic of Korea Edited by Gerhard Wagner, Harvard Medical School, Boston, MA, and approved September 22, 2014 (received for review May 24, 2014) It has long been considered that intron-containing (spliced) mRNAs eIF4H to disrupt possible RNA secondary structures in the are translationally more active than intronless mRNAs (identical 5′UTR of mRNA, as reviewed previously (7, 8). On the AUG mRNA not produced by splicing). The splicing-dependent trans- codon, the large ribosomal subunit 60S joins the 40S subunit and lational enhancement is mediated, in part, by the exon junction forms the 80S ribosome complex. In general, translation of eIF4E- complex (EJC). Nonetheless, the molecular mechanism by which bound mRNA is responsible for the high expression of poly- each EJC component contributes to the translational enhancement peptides in the cytoplasm. remains unclear. Here, we demonstrate the previously unappreci- It has long been thought that nonsense-mediated mRNA de- ated role of eukaryotic translation initiation factor 4AIII (eIF4AIII), cay (NMD), which drives selective destruction of aberrant or a component of EJC, in the translation of mRNAs bound by the physiological mRNAs containing a premature termination codon nuclear cap-binding complex (CBC), a heterodimer of cap-binding (9–11), occurs during the first round of translation driven by the protein 80 (CBP80) and CBP20. eIF4AIII is recruited to the 5′-end of CBC (1). The above view, however, was recently challenged by mRNAs bound by the CBC by direct interaction with the CBC- two reports showing that NMD occurs in the cytoplasm, re- dependent translation initiation factor (CTIF); this recruitment of gardless of whether the CBC or eIF4E drives the first round of eIF4AIII is independent of the presence of introns (deposited EJCs translation (12, 13). In addition, several recent studies revealed after splicing). Polysome fractionation, tethering experiments, and that translation of CBC-bound mRNAs may occur multiple in vitro reconstitution experiments using recombinant proteins show times (3, 14–16) and that, more intriguingly, the translation of that eIF4AIII promotes efficient unwinding of secondary structures replication-dependent histone mRNAs mainly occurs on CBC- in 5′UTR, and consequently enhances CBC-dependent translation bound mRNA (15). Therefore, for simplicity and clarity, we will in vivo and in vitro. Therefore, our data provide evidence that eIF4AIII hereafter refer to the two types of translation as CBC-dependent is a specific translation initiation factor for CBC-dependent translation. translation (CT), which takes place on CBC-bound mRNA, and BIOCHEMISTRY eIF4AIII | cap-binding complex | translation | CTIF | nonsense-mediated mRNA decay Significance n mammalian cells, translation initiation is driven by two major Two major cap-binding components can drive mammalian Icap-binding proteins (CBPs). One is the nuclear cap-binding translation initiation: the nuclear cap-binding complex (CBC) complex (CBC), which is a heterodimer of nuclear CBP80 and and eukaryotic translation initiation factor 4E (eIF4E). Although CBP20 (CBP80/20) (1), and the other is the eukaryotic translation eIF4E-dependent translation has been well characterized, the initiation factor (eIF) 4E, which is a major cytoplasmic CBP mechanism of CBC-dependent translation remains unclear. Here, (2). The CBC binds the 5′-cap structure of a newly synthesized we demonstrate the previously unappreciated role of eIF4AIII mRNA in the nucleus and then directly interacts with an eIF4G- in CBC-dependent translation. eIF4AIII is traditionally consid- like scaffold protein called CBP80/20-dependent translation ered a component of the exon junction complex loaded onto mRNAs after splicing. In addition, we found that eIF4AIII can be initiation factor (CTIF) (3). Because CTIF is mostly present on recruited to the 5′-end of CBC-associated mRNA and promotes the cytoplasmic side of the nuclear envelope and is found in the the translation of CBC-associated mRNA by helping to unwind nucleus (3), the interaction between the CBC and CTIF may secondary structures in 5′UTR. Therefore, our data provide occur either in the nucleus or during mRNA export through the evidence that eIF4AIII is a translation initiation factor specifi- nuclear pore complex. In the cytoplasm, at the 5′-end of mRNA, cally required for translation of CBC-associated mRNAs. the CBC–CTIF complex recruits the eIF3 complex (4) and then the 40S small subunit of the ribosome, thereby directing the so- Author contributions: J.C., I.R., O.H.P., J.P., H.C., J.S.Y., S.W.C., M.K.K., H.K.S., and Y.K.K. called “first” (or pioneer) round of translation (1). designed research; J.C., I.R., O.H.P., J.P., H.C., J.S.Y., and M.K.K. performed research; J.C., Translation of a CBC-bound mRNA precedes that of the I.R., O.H.P., J.P., H.C., S.W.C., M.K.K., H.K.S., and Y.K.K. analyzed data; and J.C., I.R., S.W.C., eIF4E-bound mRNA, because the CBC-bound mRNA is a pre- and Y.K.K. wrote the paper. cursor form of the eIF4E-bound mRNA (5). The timing of the The authors declare no conflict of interest. replacement of the CBC by eIF4E has not yet been elucidated; This article is a PNAS Direct Submission. however, it is known that the replacement is mediated by the Freely available online through the PNAS open access option. action of importin α/β in a translation-independent manner (6). 1J.C. and I.R. contributed equally to this work. The replaced eIF4E at the 5′-end of mRNA recruits a scaffold 2Present address: Stem Cell Program, Boston Children’s Hospital, Boston, MA 02115. protein, eIF4GI or eIF4GII, which recruits eIF3 and eIF4AI/II 3Department of Biochemistry and Biophysics, School of Medicine and Dentistry, University and, eventually, the 40S ribosomal subunit. Then, the recruited 40S of Rochester, Rochester, NY 14642. subunit scans the mRNA until it reaches the authentic translation 4To whom correspondence should be addressed. Email: [email protected]. initiation codon AUG. Efficient ribosome scanning requires either This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. eIF4AI or eIF4AII helicase and the accessory factors eIF4B and 1073/pnas.1409695111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1409695111 PNAS | Published online October 13, 2014 | E4577–E4586 Downloaded by guest on September 27, 2021 eIF4E-dependent translation (ET), which occurs on eIF4E-bound mRNA, irrespective of the number of rounds of translation. The critical difference between CBC-bound and eIF4E-bound CAG or A ATG TAG TAA Rβ-SL0- mRNAs is that the former is imprinted by the exon junction pCMV RLuc ∼ – Norm or Ter complex (EJC), which is deposited 20 24 nt upstream of an Stem loop exon–exon junction as a consequence of splicing (17). The EJC (ΔG = 0 kcal/mol) CAG or contains a heterotetrameric protein core that includes eIF4AIII, ATG TAG TAA Rβ-SL9.7- pCMV RLuc Magoh, Y14, metastatic lymph node 51 (MLN51; also known as Norm or Ter Barentsz), and peripheral proteins. The EJC plays various roles Stem loop in posttranscriptional regulation, including mRNA export from (ΔG = -9.7 kcal/mol) CAG or the nucleus, subcellular mRNA localization, translation, and ATG TAG TAA Rβ-SL87.8- pCMV RLuc NMD, as reported previously (18, 19). Norm or Ter EJCs are largely found in CBC-bound mRNA but not in Stem loop (ΔG = -87.8 kcal/mol) eIF4E-bound mRNA (5), and the deposited EJCs are displaced by a translocating ribosome (20). Accordingly, it is believed that B D Rβ-SL9.7-Norm 150 the EJC promotes efficiency of CT rather than ET. In line with ** ** mRNA siRNA this view, translational efficiency of intron-containing mRNA 100 Control eIF4AI eIF4AII eIF4AIII (spliced mRNA) is generally higher than that of intronless MUP eIF4AI mRNA (identical mRNA not produced by splicing) (21–23). 50 eIF4AII Such a difference is attributable, in part, to the EJC (deposited Norm) (% of 0 eIF4AIII as a consequence of splicing) rather than to splicing per se. -SL mRNA / β Ter Ter Ter Ter Ter Ter R Norm Norm Norm Norm Norm Furthermore, insulin treatment increases the enhanced trans- Norm β-actin lation of intron-containing mRNA compared with intronless Rβ-SL 0 9.7 87.8 0 9.7 87.8 mRNA through recruitment of activated ribosomal protein S6 kinase 1 (S6K1) to newly spliced mRNAs via the EJC compo- pcDNA4 PDCD4 nent S6K1 Aly/REF-like substrate (SKAR) (24). Despite these * C 150 * E 150 advances in understanding of the role of EJCs in CT, molecular mechanisms of the possible communication between the CT 100 100 complex and the EJC are poorly understood. mRNA 50 50 Here, we demonstrate that the core EJC component, eIF4AIII, -SL mRNA (%) -SL9.7 mRNA MUP (% of Norm) (% of β / β R is a constituent of the CT complex. eIF4AIII is preferentially / R 0 ′ Relative RLuc activity 0 Ter Ter Ter recruited to the 5 -end of CBC-bound mRNAs via its direct in- Rβ-SL9.7- Ter Norm Norm Norm Norm teraction with CTIF. Interestingly, the recruitment of eIF4AIII PDCD4 occurs irrespective of the presence of introns.
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