51
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Structure of a yeast 40S-eIF1-eIF1A-eIF3-eIF3j initiation complex. Nat Struct Mol Biol 2015; 22:269-71. [PMID: 25664723 DOI: 10.1038/nsmb.2963] [Citation(s) in RCA: 78] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2014] [Accepted: 12/24/2014] [Indexed: 11/09/2022]
Abstract
Eukaryotic translation initiation requires cooperative assembly of a large protein complex at the 40S ribosomal subunit. We have resolved a budding yeast initiation complex by cryo-EM, allowing placement of prior structures of eIF1, eIF1A, eIF3a, eIF3b and eIF3c. Our structure highlights differences in initiation-complex binding to the ribosome compared to that of mammalian eIF3, demonstrates a direct contact between eIF3j and eIF1A and reveals the network of interactions between eIF3 subunits.
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52
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Erzberger JP, Stengel F, Pellarin R, Zhang S, Schaefer T, Aylett CHS, Cimermančič P, Boehringer D, Sali A, Aebersold R, Ban N. Molecular architecture of the 40S⋅eIF1⋅eIF3 translation initiation complex. Cell 2015; 158:1123-1135. [PMID: 25171412 PMCID: PMC4151992 DOI: 10.1016/j.cell.2014.07.044] [Citation(s) in RCA: 162] [Impact Index Per Article: 18.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2014] [Revised: 05/29/2014] [Accepted: 07/29/2014] [Indexed: 11/25/2022]
Abstract
Eukaryotic translation initiation requires the recruitment of the large, multiprotein eIF3 complex to the 40S ribosomal subunit. We present X-ray structures of all major components of the minimal, six-subunit Saccharomyces cerevisiae eIF3 core. These structures, together with electron microscopy reconstructions, cross-linking coupled to mass spectrometry, and integrative structure modeling, allowed us to position and orient all eIF3 components on the 40S⋅eIF1 complex, revealing an extended, modular arrangement of eIF3 subunits. Yeast eIF3 engages 40S in a clamp-like manner, fully encircling 40S to position key initiation factors on opposite ends of the mRNA channel, providing a platform for the recruitment, assembly, and regulation of the translation initiation machinery. The structures of eIF3 components reported here also have implications for understanding the architecture of the mammalian 43S preinitiation complex and the complex of eIF3, 40S, and the hepatitis C internal ribosomal entry site RNA. X-ray structures of major yeast eIF3 components and subcomplexes Crosslinking coupled to mass-spectrometry analysis of 40S⋅eIF1⋅eIF3 complex Integrative modeling reveals architecture of 40S⋅eIF1⋅eIF3 complex
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Affiliation(s)
- Jan P Erzberger
- Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland.
| | - Florian Stengel
- Department of Biology, Institute of Molecular Systems Biology, ETH Zurich, Auguste-Piccard-Hof 1, 8093 Zurich, Switzerland
| | - Riccardo Pellarin
- Department of Bioengineering and Therapeutic Sciences, Department of Pharmaceutical Chemistry, and California Institute for Quantitative Biosciences (QB3), University of California, San Francisco, UCSF MC 2552, Byers Hall Room 503B, 1700 4th Street, San Francisco, CA 94158-2330, USA
| | - Suyang Zhang
- Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland
| | - Tanja Schaefer
- Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland
| | - Christopher H S Aylett
- Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland
| | - Peter Cimermančič
- Department of Bioengineering and Therapeutic Sciences, Department of Pharmaceutical Chemistry, and California Institute for Quantitative Biosciences (QB3), University of California, San Francisco, UCSF MC 2552, Byers Hall Room 503B, 1700 4th Street, San Francisco, CA 94158-2330, USA
| | - Daniel Boehringer
- Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland
| | - Andrej Sali
- Department of Bioengineering and Therapeutic Sciences, Department of Pharmaceutical Chemistry, and California Institute for Quantitative Biosciences (QB3), University of California, San Francisco, UCSF MC 2552, Byers Hall Room 503B, 1700 4th Street, San Francisco, CA 94158-2330, USA
| | - Ruedi Aebersold
- Department of Biology, Institute of Molecular Systems Biology, ETH Zurich, Auguste-Piccard-Hof 1, 8093 Zurich, Switzerland; Faculty of Science, University of Zurich, 8006 Zurich, Switzerland
| | - Nenad Ban
- Department of Biology, Institute of Molecular Biology and Biophysics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland.
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53
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Hershey JWB. The role of eIF3 and its individual subunits in cancer. BIOCHIMICA ET BIOPHYSICA ACTA-GENE REGULATORY MECHANISMS 2014; 1849:792-800. [PMID: 25450521 DOI: 10.1016/j.bbagrm.2014.10.005] [Citation(s) in RCA: 77] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/12/2014] [Revised: 10/28/2014] [Accepted: 10/28/2014] [Indexed: 12/15/2022]
Abstract
Specific individual subunits of eIF3 are elevated or reduced in numerous human tumors, and their ectopic overexpression in immortal cells can result in malignant transformation. The structure and assembly of eIF3 and its role in promoting mRNA and methionyl-tRNAi binding to the ribosome during the initiation phase of protein synthesis are described. Methods employed to detect altered levels of eIF3 subunits in cancers are critically evaluated in order to conclude rigorously that such subunits may cause malignant transformation. Strong evidence is presented that the individual overexpression of eIF3 subunits 3a, 3b, 3c, 3h, 3i and 3m may cause malignant transformation, whereas underexpression of subunits 3e and 3f may cause a similar outcome. Possible mechanisms to explain the malignant phenotypes are examined. The involvement of eIF3 in cancer reinforces the view that translational control plays an important role in the regulation of cell proliferation, and provides new targets for the development of therapeutic agents. This article is part of a Special Issue entitled: Translation and Cancer.
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Affiliation(s)
- John W B Hershey
- Department of Biochemistry and Molecular Medicine, University of California, Davis, CA 95616, United States.
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54
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Yue MM, Lv K, Meredith SC, Martindale JL, Gorospe M, Schuger L. Novel RNA-binding protein P311 binds eukaryotic translation initiation factor 3 subunit b (eIF3b) to promote translation of transforming growth factor β1-3 (TGF-β1-3). J Biol Chem 2014; 289:33971-83. [PMID: 25336651 DOI: 10.1074/jbc.m114.609495] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
P311, a conserved 8-kDa intracellular protein expressed in brain, smooth muscle, regenerating tissues, and malignant glioblastomas, represents the first documented stimulator of TGF-β1-3 translation in vitro and in vivo. Here we initiated efforts to define the mechanism underlying P311 function. PONDR® (Predictor Of Naturally Disordered Regions) analysis suggested and CD confirmed that P311 is an intrinsically disordered protein, therefore requiring an interacting partner to acquire tertiary structure and function. Immunoprecipitation coupled with mass spectroscopy identified eIF3 subunit b (eIF3b) as a novel P311 binding partner. Immunohistochemical colocalization, GST pulldown, and surface plasmon resonance studies revealed that P311-eIF3b interaction is direct and has a Kd of 1.26 μm. Binding sites were mapped to the non-canonical RNA recognition motif of eIF3b and a central 11-amino acid-long region of P311, here referred to as eIF3b binding motif. Disruption of P311-eIF3b binding inhibited translation of TGF-β1, 2, and 3, as indicated by luciferase reporter assays, polysome fractionation studies, and Western blot analysis. RNA precipitation assays after UV cross-linking and RNA-protein EMSA demonstrated that P311 binds directly to TGF-β 5'UTRs mRNAs through a previously unidentified RNA recognition motif-like motif. Our results demonstrate that P311 is a novel RNA-binding protein that, by interacting with TGF-βs 5'UTRs and eIF3b, stimulates the translation of TGF-β1, 2, and 3.
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Affiliation(s)
| | | | - Stephen C Meredith
- From the Departments of Pathology and Biochemistry and Molecular Biology, The University of Chicago, Chicago, Illinois 60637 and
| | - Jennifer L Martindale
- the Laboratory of Genetics, NIA, National Institutes of Health, Baltimore, Maryland 21224
| | - Myriam Gorospe
- the Laboratory of Genetics, NIA, National Institutes of Health, Baltimore, Maryland 21224
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55
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Sokabe M, Fraser CS. Human eukaryotic initiation factor 2 (eIF2)-GTP-Met-tRNAi ternary complex and eIF3 stabilize the 43 S preinitiation complex. J Biol Chem 2014; 289:31827-31836. [PMID: 25246524 DOI: 10.1074/jbc.m114.602870] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The formation of a stable 43 S preinitiation complex (PIC) must occur to enable successful mRNA recruitment. However, the contributions of eIF1, eIF1A, eIF3, and the eIF2-GTP-Met-tRNAi ternary complex (TC) in stabilizing the 43 S PIC are poorly defined. We have reconstituted the human 43 S PIC and used fluorescence anisotropy to systematically measure the affinity of eIF1, eIF1A, and eIF3j in the presence of different combinations of 43 S PIC components. Our data reveal a complicated network of interactions that result in high affinity binding of all 43 S PIC components with the 40 S subunit. Human eIF1 and eIF1A bind cooperatively to the 40 S subunit, revealing an evolutionarily conserved interaction. Negative cooperativity is observed between the binding of eIF3j and the binding of eIF1, eIF1A, and TC with the 40 S subunit. To overcome this, eIF3 dramatically increases the affinity of eIF1 and eIF3j for the 40 S subunit. Recruitment of TC also increases the affinity of eIF1 for the 40 S subunit, but this interaction has an important indirect role in increasing the affinity of eIF1A for the 40 S subunit. Together, our data provide a more complete thermodynamic framework of the human 43 S PIC and reveal important interactions between its components to maintain its stability.
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Affiliation(s)
- Masaaki Sokabe
- Department of Molecular and Cellular Biology, College of Biological Sciences, University of California, Davis, California 95616
| | - Christopher S Fraser
- Department of Molecular and Cellular Biology, College of Biological Sciences, University of California, Davis, California 95616.
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56
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Wagner S, Herrmannová A, Malík R, Peclinovská L, Valášek LS. Functional and biochemical characterization of human eukaryotic translation initiation factor 3 in living cells. Mol Cell Biol 2014; 34:3041-52. [PMID: 24912683 PMCID: PMC4135593 DOI: 10.1128/mcb.00663-14] [Citation(s) in RCA: 59] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2014] [Accepted: 06/04/2014] [Indexed: 11/20/2022] Open
Abstract
The main role of the translation initiation factor 3 (eIF3) is to orchestrate formation of 43S-48S preinitiation complexes (PICs). Until now, most of our knowledge on eIF3 functional contribution to regulation of gene expression comes from yeast studies. Hence, here we developed several novel in vivo assays to monitor the integrity of the 13-subunit human eIF3 complex, defects in assembly of 43S PICs, efficiency of mRNA recruitment, and postassembly events such as AUG recognition. We knocked down expression of the PCI domain-containing eIF3c and eIF3a subunits and of eIF3j in human HeLa and HEK293 cells and analyzed the functional consequences. Whereas eIF3j downregulation had barely any effect and eIF3a knockdown disintegrated the entire eIF3 complex, eIF3c knockdown produced a separate assembly of the a, b, g, and i subunits (closely resembling the yeast evolutionary conserved eIF3 core), which preserved relatively high 40S binding affinity and an ability to promote mRNA recruitment to 40S subunits and displayed defects in AUG recognition. Both eIF3c and eIF3a knockdowns also severely reduced protein but not mRNA levels of many other eIF3 subunits and indeed shut off translation. We propose that eIF3a and eIF3c control abundance and assembly of the entire eIF3 and thus represent its crucial scaffolding elements critically required for formation of PICs.
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Affiliation(s)
- Susan Wagner
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska, Prague, Czech Republic
| | - Anna Herrmannová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska, Prague, Czech Republic
| | - Radek Malík
- Laboratory of Epigenetic Regulations, Institute of Molecular Genetics ASCR, Videnska, Prague, Czech Republic
| | - Lucie Peclinovská
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska, Prague, Czech Republic
| | - Leoš Shivaya Valášek
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska, Prague, Czech Republic
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57
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Phosphorylation stoichiometries of human eukaryotic initiation factors. Int J Mol Sci 2014; 15:11523-38. [PMID: 24979134 PMCID: PMC4139797 DOI: 10.3390/ijms150711523] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2014] [Revised: 04/11/2014] [Accepted: 04/29/2014] [Indexed: 12/26/2022] Open
Abstract
Eukaryotic translation initiation factors are the principal molecular effectors regulating the process converting nucleic acid to functional protein. Commonly referred to as eIFs (eukaryotic initiation factors), this suite of proteins is comprised of at least 25 individual subunits that function in a coordinated, regulated, manner during mRNA translation. Multiple facets of eIF regulation have yet to be elucidated; however, many of the necessary protein factors are phosphorylated. Herein, we have isolated, identified and quantified phosphosites from eIF2, eIF3, and eIF4G generated from log phase grown HeLa cell lysates. Our investigation is the first study to globally quantify eIF phosphosites and illustrates differences in abundance of phosphorylation between the residues of each factor. Thus, identification of those phosphosites that exhibit either high or low levels of phosphorylation under log phase growing conditions may aid researchers to concentrate their investigative efforts to specific phosphosites that potentially harbor important regulatory mechanisms germane to mRNA translation.
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58
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Walsh D, Mohr I. Coupling 40S ribosome recruitment to modification of a cap-binding initiation factor by eIF3 subunit e. Genes Dev 2014; 28:835-40. [PMID: 24736843 PMCID: PMC4003276 DOI: 10.1101/gad.236752.113] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Recruitment of ribosomes to the mRNA 5′ terminus involves the activity of a large number of functionally discrete eukaryotic translation initiation factors (eIFs). However, how multiple eIFs coordinate during translation initiation remains poorly understood. Walsh and Mohr now identify an eIF3 subunit that regulates eIF4F modification and show that eIF3e is required for inducible eIF4E phosphorylation. This study establishes a mechanism by which 40S ribosome loading imparts a phosphorylation mark on the cap-binding eIF4F complex that regulates selective mRNA translation. 40S ribosomes are loaded onto capped mRNAs via the multisubunit translation initiation factors eIF3 and eIF4F. While eIF4E is the eIF4F cap recognition component, the eIF4G subunit associates with 40S-bound eIF3. How this intricate process is coordinated remains poorly understood. Here, we identify an eIF3 subunit that regulates eIF4F modification and show that eIF3e is required for inducible eIF4E phosphorylation. Significantly, recruitment of the eIF4E kinase Mnk1 (MAPK signal-integrating kinase 1) to eIF4F depended on eIF3e, and eIF3e was sufficient to promote Mnk1-binding to eIF4G. This establishes a mechanism by which 40S ribosome loading imparts a phosphorylation mark on the cap-binding eIF4F complex that regulates selective mRNA translation and is synchronized by a specific eIF3 subunit.
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Affiliation(s)
- Derek Walsh
- Department of Microbiology, New York University School of Medicine, New York, New York 10016, USA
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59
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Translation initiation factor eIF3b contains a nine-bladed β-propeller and interacts with the 40S ribosomal subunit. Structure 2014; 22:923-30. [PMID: 24768115 DOI: 10.1016/j.str.2014.03.010] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2013] [Revised: 03/13/2014] [Accepted: 03/21/2014] [Indexed: 11/20/2022]
Abstract
The multisubunit eukaryotic translation initiation factor 3, among which the subunit b (eIF3b) is a major scaffold protein, plays essential roles in protein synthesis. Here, we report the crystal structure of the WD40 domain of Chaetomium thermophilum eIF3b, revealing a nine-bladed β-propeller fold. Sequence analysis indicates that this propeller architecture is common to all eIF3b orthologs. Revisiting the cryoelectron microscopy (cryo-EM) map of the 43S preinitiation complex suggests an interaction of the eIF3b with the 40S ribosomal subunit involving the ribosomal protein S9e and the 18S rRNA. This model is strongly supported by the direct binding of eIF3b to 40S ribosomes and to the isolated ribosomal protein rpS9e in vitro.
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60
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Abstract
In eukaryotes, the translation initiation codon is generally identified by the scanning mechanism, wherein every triplet in the messenger RNA leader is inspected for complementarity to the anticodon of methionyl initiator transfer RNA (Met-tRNAi). Binding of Met-tRNAi to the small (40S) ribosomal subunit, in a ternary complex (TC) with eIF2-GTP, is stimulated by eukaryotic initiation factor 1 (eIF1), eIF1A, eIF3, and eIF5, and the resulting preinitiation complex (PIC) joins the 5' end of mRNA preactivated by eIF4F and poly(A)-binding protein. RNA helicases remove secondary structures that impede ribosome attachment and subsequent scanning. Hydrolysis of eIF2-bound GTP is stimulated by eIF5 in the scanning PIC, but completion of the reaction is impeded at non-AUG triplets. Although eIF1 and eIF1A promote scanning, eIF1 and possibly the C-terminal tail of eIF1A must be displaced from the P decoding site to permit base-pairing between Met-tRNAi and the AUG codon, as well as to allow subsequent phosphate release from eIF2-GDP. A second GTPase, eIF5B, catalyzes the joining of the 60S subunit to produce an 80S initiation complex that is competent for elongation.
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Affiliation(s)
- Alan G Hinnebusch
- Laboratory of Gene Regulation and Development, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892;
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61
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Beznosková P, Cuchalová L, Wagner S, Shoemaker CJ, Gunišová S, von der Haar T, Valášek LS. Translation initiation factors eIF3 and HCR1 control translation termination and stop codon read-through in yeast cells. PLoS Genet 2013; 9:e1003962. [PMID: 24278036 PMCID: PMC3836723 DOI: 10.1371/journal.pgen.1003962] [Citation(s) in RCA: 75] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2013] [Accepted: 10/01/2013] [Indexed: 11/19/2022] Open
Abstract
Translation is divided into initiation, elongation, termination and ribosome recycling. Earlier work implicated several eukaryotic initiation factors (eIFs) in ribosomal recycling in vitro. Here, we uncover roles for HCR1 and eIF3 in translation termination in vivo. A substantial proportion of eIF3, HCR1 and eukaryotic release factor 3 (eRF3) but not eIF5 (a well-defined "initiation-specific" binding partner of eIF3) specifically co-sediments with 80S couples isolated from RNase-treated heavy polysomes in an eRF1-dependent manner, indicating the presence of eIF3 and HCR1 on terminating ribosomes. eIF3 and HCR1 also occur in ribosome- and RNA-free complexes with both eRFs and the recycling factor ABCE1/RLI1. Several eIF3 mutations reduce rates of stop codon read-through and genetically interact with mutant eRFs. In contrast, a slow growing deletion of hcr1 increases read-through and accumulates eRF3 in heavy polysomes in a manner suppressible by overexpressed ABCE1/RLI1. Based on these and other findings we propose that upon stop codon recognition, HCR1 promotes eRF3·GDP ejection from the post-termination complexes to allow binding of its interacting partner ABCE1/RLI1. Furthermore, the fact that high dosage of ABCE1/RLI1 fully suppresses the slow growth phenotype of hcr1Δ as well as its termination but not initiation defects implies that the termination function of HCR1 is more critical for optimal proliferation than its function in translation initiation. Based on these and other observations we suggest that the assignment of HCR1 as a bona fide eIF3 subunit should be reconsidered. Together our work characterizes novel roles of eIF3 and HCR1 in stop codon recognition, defining a communication bridge between the initiation and termination/recycling phases of translation.
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Affiliation(s)
- Petra Beznosková
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska, Prague, the Czech Republic
| | - Lucie Cuchalová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska, Prague, the Czech Republic
| | - Susan Wagner
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska, Prague, the Czech Republic
| | - Christopher J. Shoemaker
- Howard Hughes Medical Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland, United States of America
| | - Stanislava Gunišová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska, Prague, the Czech Republic
| | | | - Leoš Shivaya Valášek
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska, Prague, the Czech Republic
- * E-mail:
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62
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Villa N, Do A, Hershey JWB, Fraser CS. Human eukaryotic initiation factor 4G (eIF4G) protein binds to eIF3c, -d, and -e to promote mRNA recruitment to the ribosome. J Biol Chem 2013; 288:32932-40. [PMID: 24092755 DOI: 10.1074/jbc.m113.517011] [Citation(s) in RCA: 114] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Recruitment of mRNA to the 40S ribosomal subunit requires the coordinated interaction of a large number of translation initiation factors. In mammals, the direct interaction between eukaryotic initiation factor 4G (eIF4G) and eIF3 is thought to act as the molecular bridge between the mRNA cap-binding complex and the 40S subunit. A discrete ∼90 amino acid domain in eIF4G is responsible for binding to eIF3, but the identity of the eIF3 subunit(s) involved is less clear. The eIF3e subunit has been shown to directly bind eIF4G, but the potential role of other eIF3 subunits in stabilizing this interaction has not been investigated. It is also not clear if the eIF4A helicase plays a role in stabilizing the interaction between eIF4G and eIF3. Here, we have used a fluorescence anisotropy assay to demonstrate that eIF4G binds to eIF3 independently of eIF4A binding to the middle region of eIF4G. By using a site-specific cross-linking approach, we unexpectedly show that the eIF4G-binding surface in eIF3 is comprised of the -c, -d and -e subunits. Screening multiple cross-linker positions reveals that eIF4G contains two distinct eIF3-binding subdomains within the previously identified eIF3-binding domain. Finally, by employing an eIF4G-dependent translation assay, we establish that both of these subdomains are required for efficient mRNA recruitment to the ribosome and stimulate translation. Our study reveals unexpected complexity to the eIF3-eIF4G interaction that provides new insight into the regulation of mRNA recruitment to the human ribosome.
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Affiliation(s)
- Nancy Villa
- From the Department of Molecular and Cell Biology, College of Biological Sciences, and
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63
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Human eIF4E promotes mRNA restructuring by stimulating eIF4A helicase activity. Proc Natl Acad Sci U S A 2013; 110:13339-44. [PMID: 23901100 DOI: 10.1073/pnas.1303781110] [Citation(s) in RCA: 127] [Impact Index Per Article: 11.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Elevated eukaryotic initiation factor 4E (eIF4E) levels frequently occur in a variety of human cancers. Overexpression of eIF4E promotes cellular transformation by selectively increasing the translation of proliferative and prosurvival mRNAs. These mRNAs possess highly structured 5'-UTRs that impede ribosome recruitment and scanning, yet the mechanism for how eIF4E abundance elevates their translation is not easily explained by its cap-binding activity. Here, we show that eIF4E possesses an unexpected second function in translation initiation by strongly stimulating eukaryotic initiation factor 4A (eIF4A) helicase activity. Importantly, we demonstrate that this activity promotes mRNA restructuring in a manner that is independent of its cap-binding function. To explain these findings, we show that the eIF4E-binding site in eukaryotic initiation factor 4G (eIF4G) functions as an autoinhibitory domain to modulate its ability to stimulate eIF4A helicase activity. Binding of eIF4E counteracts this autoinhibition, enabling eIF4G to stimulate eIF4A helicase activity. Finally, we have successfully separated the two functions of eIF4E to show that its helicase promoting activity increases the rate of translation by a mechanism that is distinct from its cap-binding function. Based on our results, we propose that maintaining a connection between eIF4E and eIF4G throughout scanning provides a plausible mechanism to explain how eIF4E abundance selectively stimulates the translation of highly structured proliferation and tumor-promoting mRNAs.
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64
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Bai Y, Zhou K, Doudna JA. Hepatitis C virus 3'UTR regulates viral translation through direct interactions with the host translation machinery. Nucleic Acids Res 2013; 41:7861-74. [PMID: 23783572 PMCID: PMC3763534 DOI: 10.1093/nar/gkt543] [Citation(s) in RCA: 56] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Abstract
The 3′ untranslated region (3′UTR) of hepatitis C virus (HCV) messenger RNA stimulates viral translation by an undetermined mechanism. We identified a high affinity interaction, conserved among different HCV genotypes, between the HCV 3′UTR and the host ribosome. The 3′UTR interacts with 40S ribosomal subunit proteins residing primarily in a localized region on the 40S solvent-accessible surface near the messenger RNA entry and exit sites. This region partially overlaps with the site where the HCV internal ribosome entry site was found to bind, with the internal ribosome entry site-40S subunit interaction being dominant. Despite its ability to bind to 40S subunits independently, the HCV 3′UTR only stimulates translation in cis, without affecting the first round translation rate. These observations support a model in which the HCV 3′UTR retains ribosome complexes during translation termination to facilitate efficient initiation of subsequent rounds of translation.
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Affiliation(s)
- Yun Bai
- Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA, Howard Hughes Medical Institute, University of California, Berkeley, CA 94720, USA, Department of Chemistry, University of California, Berkeley, CA 94720, USA and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
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65
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Querol-Audi J, Sun C, Vogan JM, Smith MD, Gu Y, Cate JHD, Nogales E. Architecture of human translation initiation factor 3. Structure 2013; 21:920-8. [PMID: 23623729 DOI: 10.1016/j.str.2013.04.002] [Citation(s) in RCA: 57] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2013] [Revised: 03/28/2013] [Accepted: 04/01/2013] [Indexed: 02/05/2023]
Abstract
Eukaryotic translation initiation factor 3 (eIF3) plays a central role in protein synthesis by organizing the formation of the 43S preinitiation complex. Using genetic tag visualization by electron microscopy, we reveal the molecular organization of ten human eIF3 subunits, including an octameric core. The structure of eIF3 bears a close resemblance to that of the proteasome lid, with a conserved spatial organization of eight core subunits containing PCI and MPN domains that coordinate functional interactions in both complexes. We further show that eIF3 subunits a and c interact with initiation factors eIF1 and eIF1A, which control the stringency of start codon selection. Finally, we find that subunit j, which modulates messenger RNA interactions with the small ribosomal subunit, makes multiple independent interactions with the eIF3 octameric core. These results highlight the conserved architecture of eIF3 and how it scaffolds key factors that control translation initiation in higher eukaryotes, including humans.
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Affiliation(s)
- Jordi Querol-Audi
- Department of Molecular and Cell Biology, California Institute for Quantitative Biosciences, University of California-Berkeley, Berkeley, CA 94720, USA
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66
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Valásek LS. 'Ribozoomin'--translation initiation from the perspective of the ribosome-bound eukaryotic initiation factors (eIFs). Curr Protein Pept Sci 2013; 13:305-30. [PMID: 22708493 PMCID: PMC3434475 DOI: 10.2174/138920312801619385] [Citation(s) in RCA: 103] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2011] [Revised: 01/16/2012] [Accepted: 02/16/2012] [Indexed: 02/05/2023]
Abstract
Protein synthesis is a fundamental biological mechanism bringing the DNA-encoded genetic information into
life by its translation into molecular effectors - proteins. The initiation phase of translation is one of the key points of gene
regulation in eukaryotes, playing a role in processes from neuronal function to development. Indeed, the importance of the
study of protein synthesis is increasing with the growing list of genetic diseases caused by mutations that affect mRNA
translation. To grasp how this regulation is achieved or altered in the latter case, we must first understand the molecular
details of all underlying processes of the translational cycle with the main focus put on its initiation. In this review I discuss
recent advances in our comprehension of the molecular basis of particular initiation reactions set into the context of
how and where individual eIFs bind to the small ribosomal subunit in the pre-initiation complex. I also summarize our
current knowledge on how eukaryotic initiation factor eIF3 controls gene expression in the gene-specific manner via reinitiation.
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Affiliation(s)
- Leos Shivaya Valásek
- Laboratory of Eukaryotic Gene Regulation, Institute of Microbiology AS CR, Prague, Czech Republic.
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67
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Nürenberg E, Tampé R. Tying up loose ends: ribosome recycling in eukaryotes and archaea. Trends Biochem Sci 2012; 38:64-74. [PMID: 23266104 DOI: 10.1016/j.tibs.2012.11.003] [Citation(s) in RCA: 57] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2012] [Revised: 11/15/2012] [Accepted: 11/16/2012] [Indexed: 01/29/2023]
Abstract
Ribosome recycling is the final - or first - step of the cyclic process of mRNA translation. In eukaryotes and archaea, dissociation of the two ribosomal subunits proceeds in a fundamentally different way than in bacteria. It requires the ABC-type ATPase ABCE1 [previously named RNase L inhibitor (Rli)1 or host protein (HP)68], but the reaction and its regulation remain enigmatic. Here, we focus on ribosome recycling in its physiological context, including translation termination and reinitiation. The regulation of this crucial event can only be described by a systems biology approach, involving a network of proteins modulating mRNA translation. The key role of ABCE1, and what is known about the structure and function of this versatile protein, is discussed.
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Affiliation(s)
- Elina Nürenberg
- Institute of Biochemistry, Biocenter, Goethe-University Frankfurt, Max-von-Laue-Str. 9, D-60438 Frankfurt/M., Germany
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68
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Kouba T, Dányi I, Gunišová S, Munzarová V, Vlčková V, Cuchalová L, Neueder A, Milkereit P, Valášek LS. Small ribosomal protein RPS0 stimulates translation initiation by mediating 40S-binding of eIF3 via its direct contact with the eIF3a/TIF32 subunit. PLoS One 2012; 7:e40464. [PMID: 22792338 PMCID: PMC3390373 DOI: 10.1371/journal.pone.0040464] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/08/2012] [Accepted: 06/07/2012] [Indexed: 01/02/2023] Open
Abstract
The ribosome translates information encoded by mRNAs into proteins in all living cells. In eukaryotes, its small subunit together with a number of eukaryotic initiation factors (eIFs) is responsible for locating the mRNA's translational start to properly decode the genetic message that it carries. This multistep process requires timely and spatially coordinated placement of eIFs on the ribosomal surface. In our long-standing pursuit to map the 40S-binding site of one of the functionally most complex eIFs, yeast multisubunit eIF3, we identified several interactions that placed its major body to the head, beak and shoulder regions of the solvent-exposed side of the 40S subunit. Among them is the interaction between the N-terminal domain (NTD) of the a/TIF32 subunit of eIF3 and the small ribosomal protein RPS0A, residing near the mRNA exit channel. Previously, we demonstrated that the N-terminal truncation of 200 residues in tif32-Δ8 significantly reduced association of eIF3 and other eIFs with 40S ribosomes in vivo and severely impaired translation reinitiation that eIF3 ensures. Here we show that not the first but the next 200 residues of a/TIF32 specifically interact with RPS0A via its extreme C-terminal tail (CTT). Detailed analysis of the RPS0A conditional depletion mutant revealed a marked drop in the polysome to monosome ratio suggesting that the initiation rates of cells grown under non-permissive conditions were significantly impaired. Indeed, amounts of eIF3 and other eIFs associated with 40S subunits in the pre-initiation complexes in the RPS0A-depleted cells were found reduced; consistently, to the similar extent as in the tif32-Δ8 cells. Similar but less pronounced effects were also observed with the viable CTT-less mutant of RPS0A. Together we conclude that the interaction between the flexible RPS0A-CTT and the residues 200–400 of the a/TIF32-NTD significantly stimulates attachment of eIF3 and its associated eIFs to small ribosomal subunits in vivo.
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Affiliation(s)
- Tomáš Kouba
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, v.v.i., Prague, The Czech Republic
| | - István Dányi
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, v.v.i., Prague, The Czech Republic
| | - Stanislava Gunišová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, v.v.i., Prague, The Czech Republic
| | - Vanda Munzarová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, v.v.i., Prague, The Czech Republic
| | - Vladislava Vlčková
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, v.v.i., Prague, The Czech Republic
| | - Lucie Cuchalová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, v.v.i., Prague, The Czech Republic
| | - Andreas Neueder
- Institut für Biochemie, Genetik und Mikrobiologie, University of Regensburg, Regensburg, Germany
| | - Philipp Milkereit
- Institut für Biochemie, Genetik und Mikrobiologie, University of Regensburg, Regensburg, Germany
| | - Leoš Shivaya Valášek
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, v.v.i., Prague, The Czech Republic
- * E-mail:
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69
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Jackson RJ, Hellen CUT, Pestova TV. Termination and post-termination events in eukaryotic translation. ADVANCES IN PROTEIN CHEMISTRY AND STRUCTURAL BIOLOGY 2012; 86:45-93. [PMID: 22243581 DOI: 10.1016/b978-0-12-386497-0.00002-5] [Citation(s) in RCA: 158] [Impact Index Per Article: 13.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Translation termination in eukaryotes occurs in response to a stop codon in the ribosomal A-site and requires two release factors (RFs), eRF1 and eRF3, which bind to the A-site as an eRF1/eRF3/GTP complex with eRF1 responsible for codon recognition. After GTP hydrolysis by eRF3, eRF1 triggers hydrolysis of the polypeptidyl-tRNA, releasing the completed protein product. This leaves an 80S ribosome still bound to the mRNA, with deacylated tRNA in its P-site and at least eRF1 in its A-site, which needs to be disassembled and released from the mRNA to allow further rounds of translation. The first step in recycling is dissociation of the 60S ribosomal subunit, leaving a 40S/deacylated tRNA complex bound to the mRNA. This is mediated by ABCE1, which is a somewhat unusual member of the ATP-binding cassette family of proteins with no membrane-spanning domain but two essential iron-sulfur clusters. Two distinct pathways have been identified for subsequent ejection of the deacylated tRNA followed by dissociation of the 40S subunit from the mRNA, one executed by a subset of the canonical initiation factors (which therefore starts the process of preparing the 40S subunit for the next round of translation) and the other by Ligatin or homologous proteins. However, although this is the normal sequence of events, there are exceptions where the termination reaction is followed by reinitiation on the same mRNA (usually) at a site downstream of the stop codon. The overwhelming majority of such reinitiation events occur when the 5'-proximal open reading frame (ORF) is short and can result in significant regulation of translation of the protein-coding ORF, but there are also rare examples, mainly bicistronic viral RNAs, of reinitiation after a long ORF. Here, we review our current understanding of the mechanisms of termination, ribosome recycling, and reinitiation after translation of short and long ORFs.
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Affiliation(s)
- Richard J Jackson
- Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom
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70
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Wilson DN, Doudna Cate JH. The structure and function of the eukaryotic ribosome. Cold Spring Harb Perspect Biol 2012; 4:4/5/a011536. [PMID: 22550233 DOI: 10.1101/cshperspect.a011536] [Citation(s) in RCA: 203] [Impact Index Per Article: 16.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
Structures of the bacterial ribosome have provided a framework for understanding universal mechanisms of protein synthesis. However, the eukaryotic ribosome is much larger than it is in bacteria, and its activity is fundamentally different in many key ways. Recent cryo-electron microscopy reconstructions and X-ray crystal structures of eukaryotic ribosomes and ribosomal subunits now provide an unprecedented opportunity to explore mechanisms of eukaryotic translation and its regulation in atomic detail. This review describes the X-ray crystal structures of the Tetrahymena thermophila 40S and 60S subunits and the Saccharomyces cerevisiae 80S ribosome, as well as cryo-electron microscopy reconstructions of translating yeast and plant 80S ribosomes. Mechanistic questions about translation in eukaryotes that will require additional structural insights to be resolved are also presented.
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71
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Jia W, Andaya A, Leary JA. Novel mass spectrometric method for phosphorylation quantification using cerium oxide nanoparticles and tandem mass tags. Anal Chem 2012; 84:2466-73. [PMID: 22304650 DOI: 10.1021/ac203248s] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
The stoichiometry of protein phosphorylation significantly impacts protein function. The development of quantitative techniques in mass spectrometry has generated the ability to systematically monitor the regulation levels of various proteins. This study reports an integrated methodology using cerium oxide nanoparticles and isobaric tandem mass tag (TMT) labeling to assess absolute stoichiometries of protein phosphorylation. This protocol was designed to directly measure the dephosphorylation levels for a known phosphorylation site, therefore allowing for quantification of phosphosites. Both the accuracy and precision of the method were verified using standard peptides and protein tryptic digests. This novel method was then applied to quantify phosphorylations on eukaryotic initiation factor 3H (eIF3H), a protein integral to overall eukaryotic protein translation initiation. To date, this is the first report of assessment of protein phosphorylation quantification on eIF3.
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Affiliation(s)
- Weitao Jia
- Department of Molecular and Cellular Biology, University of California, Davis, One Shields Avenue, Davis, California 95616, USA
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72
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Molecular mechanism of scanning and start codon selection in eukaryotes. Microbiol Mol Biol Rev 2012; 75:434-67, first page of table of contents. [PMID: 21885680 DOI: 10.1128/mmbr.00008-11] [Citation(s) in RCA: 281] [Impact Index Per Article: 23.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
The correct translation of mRNA depends critically on the ability to initiate at the right AUG codon. For most mRNAs in eukaryotic cells, this is accomplished by the scanning mechanism, wherein the small (40S) ribosomal subunit attaches to the 5' end of the mRNA and then inspects the leader base by base for an AUG in a suitable context, using complementarity with the anticodon of methionyl initiator tRNA (Met-tRNAiMet) as the key means of identifying AUG. Over the past decade, a combination of yeast genetics, biochemical analysis in reconstituted systems, and structural biology has enabled great progress in deciphering the mechanism of ribosomal scanning. A robust molecular model now exists, describing the roles of initiation factors, notably eukaryotic initiation factor 1 (eIF1) and eIF1A, in stabilizing an "open" conformation of the 40S subunit with Met-tRNAiMet bound in a low-affinity state conducive to scanning and in triggering rearrangement into a "closed" conformation incompatible with scanning, which features Met-tRNAiMet more tightly bound to the "P" site and base paired with AUG. It has also emerged that multiple DEAD-box RNA helicases participate in producing a single-stranded "landing pad" for the 40S subunit and in removing the secondary structure to enable the mRNA to traverse the 40S mRNA-binding channel in the single-stranded form for base-by-base inspection in the P site.
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73
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Functional reconstitution of human eukaryotic translation initiation factor 3 (eIF3). Proc Natl Acad Sci U S A 2011; 108:20473-8. [PMID: 22135459 DOI: 10.1073/pnas.1116821108] [Citation(s) in RCA: 90] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Protein fate in higher eukaryotes is controlled by three complexes that share conserved architectural elements: the proteasome, COP9 signalosome, and eukaryotic translation initiation factor 3 (eIF3). Here we reconstitute the 13-subunit human eIF3 in Escherichia coli, revealing its structural core to be the eight subunits with conserved orthologues in the proteasome lid complex and COP9 signalosome. This structural core in eIF3 binds to the small (40S) ribosomal subunit, to translation initiation factors involved in mRNA cap-dependent initiation, and to the hepatitis C viral (HCV) internal ribosome entry site (IRES) RNA. Addition of the remaining eIF3 subunits enables reconstituted eIF3 to assemble intact initiation complexes with the HCV IRES. Negative-stain EM reconstructions of reconstituted eIF3 further reveal how the approximately 400 kDa molecular mass structural core organizes the highly flexible 800 kDa molecular mass eIF3 complex, and mediates translation initiation.
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74
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Sokabe M, Fraser CS, Hershey JWB. The human translation initiation multi-factor complex promotes methionyl-tRNAi binding to the 40S ribosomal subunit. Nucleic Acids Res 2011; 40:905-13. [PMID: 21940399 PMCID: PMC3258154 DOI: 10.1093/nar/gkr772] [Citation(s) in RCA: 65] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The delivery of Met-tRNAi to the 40S ribosomal subunit is thought to occur by way of a ternary complex (TC) comprising eIF2, GTP and Met-tRNAi. We have generated from purified human proteins a stable multifactor complex (MFC) comprising eIF1, eIF2, eIF3 and eIF5, similar to the MFC reported in yeast and plants. A human MFC free of the ribosome also is detected in HeLa cells and rabbit reticulocytes, indicating that it exists in vivo. In vitro, the MFC-GTP binds Met-tRNAi and delivers the tRNA to the ribosome at the same rate as the TC. However, MFC-GDP shows a greatly reduced affinity to Met-tRNAi compared to that for eIF2-GDP, suggesting that MFC components may play a role in the release of eIF2-GDP from the ribosome following AUG recognition. Since an MFC–Met-tRNAi complex is detected in cell lysates, it may be responsible for Met-tRNAi–40S ribosome binding in vivo, possibly together with the TC. However, the MFC protein components also bind individually to 40S ribosomes, creating the possibility that Met-tRNAi might bind directly to such 40S-factor complexes. Thus, three distinct pathways for Met-tRNAi delivery to the 40S ribosomal subunit are identified, but which one predominates in vivo remains to be elucidated.
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Affiliation(s)
- Masaaki Sokabe
- Department of Biochemistry and Molecular Medicine, University of California, Davis, CA 95616, USA
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75
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Duplex unwinding and ATPase activities of the DEAD-box helicase eIF4A are coupled by eIF4G and eIF4B. J Mol Biol 2011; 412:674-87. [PMID: 21840318 DOI: 10.1016/j.jmb.2011.08.004] [Citation(s) in RCA: 79] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2011] [Revised: 07/31/2011] [Accepted: 08/01/2011] [Indexed: 01/23/2023]
Abstract
Eukaryotic initiation factor (eIF) 4A is a DEAD-box helicase that stimulates translation initiation by unwinding mRNA secondary structure. The accessory proteins eIF4G, eIF4B, and eIF4H enhance the duplex unwinding activity of eIF4A, but the extent to which they modulate eIF4A activity is poorly understood. Here, we use real-time fluorescence assays to determine the kinetic parameters of duplex unwinding and ATP hydrolysis by these initiation factors. To ensure efficient duplex unwinding, eIF4B and eIF4G cooperatively activate the duplex unwinding activity of eIF4A. Our data reveal that eIF4H is much less efficient at stimulating eIF4A unwinding activity than eIF4B, implying that eIF4H is not able to completely substitute for eIF4B in duplex unwinding. By monitoring unwinding and ATPase assays under identical conditions, we demonstrate that eIF4B couples the ATP hydrolysis cycle of eIF4A with strand separation, thereby minimizing nonproductive unwinding events. Using duplex substrates with altered GC contents but similar predicted thermal stabilities, we further show that the rate of formation of productive unwinding complexes is strongly influenced by the local stability per base pair, in addition to the stability of the entire duplex. This finding explains how a change in the GC content of a hairpin is able to influence translation initiation while maintaining the overall predicted thermal stability.
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76
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Elucidating mechanistic principles underpinning eukaryotic translation initiation using quantitative fluorescence methods. Biochem Soc Trans 2011; 38:1587-92. [PMID: 21118131 DOI: 10.1042/bst0381587] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Eukaryotic translation initiation is an intricate process involving at least 11 formally classified eIFs (eukaryotic initiation factors), which, together with the ribosome, comprise one of the largest molecular machines in the cell. Studying such huge macromolecular complexes presents many challenges which cannot readily be overcome by traditional molecular and structural methods. Increasingly, novel quantitative techniques are being used to further dissect such complex assembly pathways. One area of methodology involves the labelling of ribosomal subunits and/or eIFs with fluorophores and the use of techniques such as FRET (Förster resonance energy transfer) and FA (fluorescence anisotropy). The applicability of such techniques in such a complex system has been greatly enhanced by recent methodological developments. In the present mini-review, we introduce these quantitative fluorescence methods and discuss the impact they are beginning to have on the field.
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77
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Park JH, Dias CAO, Lee SB, Valentini SR, Sokabe M, Fraser CS, Park MH. Production of active recombinant eIF5A: reconstitution in E.coli of eukaryotic hypusine modification of eIF5A by its coexpression with modifying enzymes. Protein Eng Des Sel 2011; 24:301-9. [PMID: 21131325 PMCID: PMC3038461 DOI: 10.1093/protein/gzq110] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2010] [Revised: 11/03/2010] [Accepted: 11/03/2010] [Indexed: 11/12/2022] Open
Abstract
Eukaryotic translation initiation factor 5A (eIF5A) is the only cellular protein that contains the polyamine-modified lysine, hypusine [N(ε)-(4-amino-2-hydroxybutyl)lysine]. Hypusine occurs only in eukaryotes and certain archaea, but not in eubacteria. It is formed post-translationally by two consecutive enzymatic reactions catalyzed by deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH). Hypusine modification is essential for the activity of eIF5A and for eukaryotic cell proliferation. eIF5A binds to the ribosome and stimulates translation in a hypusine-dependent manner, but its mode of action in translation is not well understood. Since quantities of highly pure hypusine-modified eIF5A is desired for structural studies as well as for determination of its binding sites on the ribosome, we have used a polycistronic vector, pST39, to express eIF5A alone, or to co-express human eIF5A-1 with DHS or with both DHS and DOHH in Escherichia coli cells, to engineer recombinant proteins, unmodified eIF5A, deoxyhypusine- or hypusine-modified eIF5A. We have accomplished production of three different forms of recombinant eIF5A in high quantity and purity. The recombinant hypusine-modified eIF5A was as active in methionyl-puromycin synthesis as the native, eIF5A (hypusine form) purified from mammalian tissue. The recombinant eIF5A proteins will be useful tools in future structure/function and the mechanism studies in translation.
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Affiliation(s)
- Jong Hwan Park
- Oral and Pharyngeal Cancer Branch, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892-4340, USA
| | - Camila A. O. Dias
- Department of Biological Sciences, School of Pharmaceutical Sciences, University of Estadual Paulista – UNESP, Bldg 30 Rm 211, Araraquara, SP, Brazil
| | - Seung Bum Lee
- Oral and Pharyngeal Cancer Branch, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892-4340, USA
| | - Sandro R. Valentini
- Department of Biological Sciences, School of Pharmaceutical Sciences, University of Estadual Paulista – UNESP, Bldg 30 Rm 211, Araraquara, SP, Brazil
| | | | - Christopher S. Fraser
- Department of Molecular and Cellular Biology, University of California, Davis, CA 95616, USA
| | - Myung Hee Park
- Oral and Pharyngeal Cancer Branch, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892-4340, USA
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78
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Mitchell SF, Walker SE, Algire MA, Park EH, Hinnebusch AG, Lorsch JR. The 5'-7-methylguanosine cap on eukaryotic mRNAs serves both to stimulate canonical translation initiation and to block an alternative pathway. Mol Cell 2010; 39:950-62. [PMID: 20864040 DOI: 10.1016/j.molcel.2010.08.021] [Citation(s) in RCA: 84] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2010] [Revised: 05/20/2010] [Accepted: 08/02/2010] [Indexed: 10/19/2022]
Abstract
Translational control is frequently exerted at the stage of mRNA recruitment to the initiating ribosome. We have reconstituted mRNA recruitment to the 43S preinitiation complex (PIC) using purified S. cerevisiae components. We show that eIF3 and the eIF4 factors not only stabilize binding of mRNA to the PIC, they also dramatically increase the rate of recruitment. Although capped mRNAs require eIF3 and the eIF4 factors for efficient recruitment to the PIC, uncapped mRNAs can be recruited in the presence of eIF3 alone. The cap strongly inhibits this alternative recruitment pathway, imposing a requirement for the eIF4 factors for rapid and stable binding of natural mRNA. Our data suggest that the 5' cap serves as both a positive and negative element in mRNA recruitment, promoting initiation in the presence of the canonical group of mRNA handling factors while preventing binding to the ribosome via an aberrant, alternative pathway requiring only eIF3.
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Affiliation(s)
- Sarah F Mitchell
- Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
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79
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Lindqvist L, Robert F, Merrick W, Kakeya H, Fraser C, Osada H, Pelletier J. Inhibition of translation by cytotrienin A--a member of the ansamycin family. RNA (NEW YORK, N.Y.) 2010; 16:2404-2413. [PMID: 20943818 PMCID: PMC2995401 DOI: 10.1261/rna.2307710] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/07/2010] [Accepted: 09/17/2010] [Indexed: 05/30/2023]
Abstract
The ansamycins are a diverse and often physiologically active group of compounds that include geldanamycin and rifamycin, inhibitors of heat shock protein 90 and prokaryotic DNA-dependent RNA synthesis, respectively. Cytotrienin A is an ansamycin-type small molecule with potent antiproliferative and proapoptotic properties. Here, we report that this compound inhibits eukaryotic protein synthesis by targeting translation elongation and interfering with eukaryotic elongation factor 1A function. We also find that cytotrienin A prevents HUVEC tube formation and diminishes microvessel formation in the chorioallantoic membrane assay. These results provide a molecular understanding into cytotrienin A's previously reported properties as an anticancer apoptosis-inducing drug.
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Affiliation(s)
- Lisa Lindqvist
- Department of Biochemistry, McGill University, Montreal, Quebec H3G 1Y6, Canada
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80
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Hershey JWB. Regulation of protein synthesis and the role of eIF3 in cancer. Braz J Med Biol Res 2010; 43:920-30. [PMID: 20922269 DOI: 10.1590/s0100-879x2010007500098] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2010] [Accepted: 09/08/2010] [Indexed: 02/06/2023] Open
Abstract
Maintenance of cell homeostasis and regulation of cell proliferation depend importantly on regulating the process of protein synthesis. Many disease states arise when disregulation of protein synthesis occurs. This review focuses on mechanisms of translational control and how disregulation results in cell malignancy. Most translational controls occur during the initiation phase of protein synthesis, with the initiation factors being the major target of regulation through their phosphorylation. In particular, the recruitment of mRNAs through the m⁷G-cap structure and the binding of the initiator methionyl-tRNA(i) are frequent targets. However, translation, especially of specific mRNAs, may also be regulated by sequestration into processing bodies or stress granules, by trans-acting proteins or by microRNAs. When the process of protein synthesis is hyper-activated, weak mRNAs are translated relatively more efficiently, leading to an imbalance of cellular proteins that promotes cell proliferation and malignant transformation. This occurs, for example, when the cap-binding protein, eIF4E, is overexpressed, or when the methionyl-tRNA(i)-binding factor, eIF2, is too active. In addition, enhanced activity of eIF3 contributes to oncogenesis. The importance of the translation initiation factors as regulators of protein synthesis and cell proliferation makes them potential therapeutic targets for the treatment of cancer.
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Affiliation(s)
- John W B Hershey
- Department of Biochemistry and Molecular Medicine, University of California, Davis, 95616, USA.
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81
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Cai Q, Todorovic A, Andaya A, Gao J, Leary JA, Cate JHD. Distinct regions of human eIF3 are sufficient for binding to the HCV IRES and the 40S ribosomal subunit. J Mol Biol 2010; 403:185-96. [PMID: 20816988 DOI: 10.1016/j.jmb.2010.07.054] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/01/2010] [Revised: 06/30/2010] [Accepted: 07/27/2010] [Indexed: 02/05/2023]
Abstract
Translation of the hepatitis C virus (HCV) genomic RNA initiates from an internal ribosome entry site (IRES) in its 5' untranslated region and requires a minimal subset of translation initiation factors to occur, namely eukaryotic initiation factor (eIF) 2 and eIF3. Low-resolution structural information has revealed how the HCV IRES RNA binds human eIF3 and the 40S ribosomal subunit and positions the start codon for initiation. However, the exact nature of the interactions between the HCV IRES RNA and the translational machinery remains unknown. Using limited proteolysis and mass spectrometry, we show that distinct regions of human eIF3 are sufficient for binding to the HCV IRES RNA and the 40S subunit. Notably, the eIF3 subunit eIF3b is protected by HCV IRES RNA binding, yet is exposed in the complex when compared to subunits eIF3e, eIF3f, eIF3h, and eIF3l. Limited proteolysis reveals that eIF3 binding to the 40S ribosomal subunit occurs through many redundant interactions that can compensate for each other. These data suggest how the HCV IRES binds to specific regions of eIF3 to target the translational machinery to the viral genomic RNA and provide a framework for modeling the architecture of intact human eIF3.
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Affiliation(s)
- Qi Cai
- California Institute for Quantitative Biosciences, University of California, Berkeley, CA 94720, USA
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82
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Berry KE, Waghray S, Doudna JA. The HCV IRES pseudoknot positions the initiation codon on the 40S ribosomal subunit. RNA (NEW YORK, N.Y.) 2010; 16:1559-69. [PMID: 20584896 PMCID: PMC2905755 DOI: 10.1261/rna.2197210] [Citation(s) in RCA: 61] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/04/2023]
Abstract
The hepatitis C virus (HCV) genomic RNA contains an internal ribosome entry site (IRES) in its 5' untranslated region, the structure of which is essential for viral protein translation. The IRES includes a predicted pseudoknot interaction near the AUG start codon, but the results of previous studies of its structure have been conflicting. Using mutational analysis coupled with activity and functional assays, we verified the importance of pseudoknot base pairings for IRES-mediated translation and, using 35 mutants, conducted a comprehensive study of the structural tolerance and functional contributions of the pseudoknot. Ribosomal toeprinting experiments show that the entirety of the pseudoknot element positions the initiation codon in the mRNA binding cleft of the 40S ribosomal subunit. Optimal spacing between the pseudoknot and the start site AUG resembles that between the Shine-Dalgarno sequence and the initiation codon in bacterial mRNAs. Finally, we validated the HCV IRES pseudoknot as a potential drug target using antisense 2'-OMe oligonucleotides.
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Affiliation(s)
- Katherine E Berry
- Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720, USA
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83
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The C-terminal region of eukaryotic translation initiation factor 3a (eIF3a) promotes mRNA recruitment, scanning, and, together with eIF3j and the eIF3b RNA recognition motif, selection of AUG start codons. Mol Cell Biol 2010; 30:4415-34. [PMID: 20584985 DOI: 10.1128/mcb.00280-10] [Citation(s) in RCA: 79] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The C-terminal domain (CTD) of the a/Tif32 subunit of budding yeast eukaryotic translation initiation factor 3 (eIF3) interacts with eIF3 subunits j/Hcr1 and b/Prt1 and can bind helices 16 to 18 of 18S rRNA, suggesting proximity to the mRNA entry channel of the 40S subunit. We have identified substitutions in the conserved Lys-Glu-Arg-Arg (KERR) motif and in residues of the nearby box6 element of the a/Tif32 CTD that impair mRNA recruitment by 43S preinitiation complexes (PICs) and confer phenotypes indicating defects in scanning and start codon recognition. The normally dispensable CTD of j/Hcr1 is required for its binding to a/Tif32 and to mitigate the growth defects of these a/Tif32 mutants, indicating physical and functional interactions between these two domains. The a/Tif32 CTD and the j/Hcr1 N-terminal domain (NTD) also interact with the RNA recognition motif (RRM) in b/Prt1, and mutations in both subunits that disrupt their interactions with the RRM increase leaky scanning of an AUG codon. These results, and our demonstration that the extreme CTD of a/Tif32 binds to Rps2 and Rps3, lead us to propose that the a/Tif32 CTD directly stabilizes 43S subunit-mRNA interaction and that the b/Prt1-RRM-j/Hcr1-a/Tif32-CTD module binds near the mRNA entry channel and regulates the transition between scanning-conducive and initiation-competent conformations of the PIC.
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84
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Jackson RJ, Hellen CUT, Pestova TV. The mechanism of eukaryotic translation initiation and principles of its regulation. Nat Rev Mol Cell Biol 2010; 11:113-27. [PMID: 20094052 DOI: 10.1038/nrm2838] [Citation(s) in RCA: 1862] [Impact Index Per Article: 133.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Protein synthesis is principally regulated at the initiation stage (rather than during elongation or termination), allowing rapid, reversible and spatial control of gene expression. Progress over recent years in determining the structures and activities of initiation factors, and in mapping their interactions in ribosomal initiation complexes, have advanced our understanding of the complex translation initiation process. These developments have provided a solid foundation for studying the regulation of translation initiation by mechanisms that include the modulation of initiation factor activity (which affects almost all scanning-dependent initiation) and through sequence-specific RNA-binding proteins and microRNAs (which affect individual mRNAs).
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Affiliation(s)
- Richard J Jackson
- Department of Biochemistry, University of Cambridge, Cambridge, CB2 1GA, UK.
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85
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Elantak L, Wagner S, Herrmannová A, Karásková M, Rutkai E, Lukavsky PJ, Valásek L. The indispensable N-terminal half of eIF3j/HCR1 cooperates with its structurally conserved binding partner eIF3b/PRT1-RRM and with eIF1A in stringent AUG selection. J Mol Biol 2010; 396:1097-116. [PMID: 20060839 PMCID: PMC2824034 DOI: 10.1016/j.jmb.2009.12.047] [Citation(s) in RCA: 73] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2009] [Revised: 12/17/2009] [Accepted: 12/22/2009] [Indexed: 11/13/2022]
Abstract
Despite the recent progress in our understanding of the numerous
functions of individual subunits of eukaryotic translation initiation factor 3
(eIF3), there is still only little known on the molecular level. Using NMR
spectroscopy, we determined the first solution structure of an interaction
between eIF3 subunits. We revealed that a conserved tryptophan residue in the
human eIF3j N-terminal acidic domain (NTA) is held in the helix α1
– loop L5 hydrophobic pocket of the human eIF3b-RRM. Mutating the
corresponding “pocket” residues in its yeast orthologue reduces
cellular growth rate, eliminates eIF3j/HCR1 association with eIF3b/PRT1
in vitro and in vivo, affects
40S-occupancy of eIF3, and produces a leaky scanning defect indicative of a
deregulation of the AUG selection process. Unexpectedly, we found that the
N-terminal half (NTD) of eIF3j/HCR1 containing the NTA motif is indispensable
and sufficient for wild-type growth of yeast cells. Furthermore, we demonstrate
that deletion of either j/HCR1 or its NTD only, or mutating the key tryptophan
residues results in the severe leaky scanning phenotype partially suppressible
by overexpressed eIF1A, which is thought to stabilize properly formed
pre-initiation complexes at the correct start codon. These findings indicate
that eIF3j/HCR1 remains associated with the scanning pre-initiation complexes
and does not dissociate from the small ribosomal subunit upon mRNA recruitment
as previously believed. Finally, we provide further support for earlier mapping
of the ribosomal binding site for human eIF3j by identifying specific
interactions of eIF3j/HCR1 with small ribosomal proteins RPS2 and RPS23 located
in the vicinity of the mRNA entry channel. Taken together we propose that
eIF3j/HCR1 closely co-operates with eIF3b/PRT1-RRM and eIF1A on the ribosome to
ensure proper formation of the scanning-arrested conformation required for
stringent AUG recognition.
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Affiliation(s)
- Latifa Elantak
- Structural Studies Division, MRC-Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, England, UK
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86
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Hasenöhrl D, Fabbretti A, Londei P, Gualerzi CO, Bläsi U. Translation initiation complex formation in the crenarchaeon Sulfolobus solfataricus. RNA (NEW YORK, N.Y.) 2009; 15:2288-2298. [PMID: 19861425 PMCID: PMC2779686 DOI: 10.1261/rna.1662609] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/26/2009] [Accepted: 09/09/2009] [Indexed: 05/28/2023]
Abstract
The function of initiation factors in and the sequence of events during translation initiation have been intensively studied in Bacteria and Eukaryotes, whereas in Archaea knowledge on these functions/processes is limited. By employing chemical probing, we show that translation initiation factor aIF1 of the model crenarchaeon Sulfolobus solfataricus binds to the same area on the ribosome as the bacterial and eukaryal orthologs. Fluorescence energy transfer assays (FRET) showed that aIF1, like its eukaryotic and bacterial orthologs, has a fidelity function in translation initiation complex formation, and that both aIF1 and aIF1A exert a synergistic effect in stimulating ribosomal association of the Met-tRNAi(Met) binding factor a/eIF2. However, as in Eukaryotes their effect on a/eIF2 binding appears to be indirect. Moreover, FRET was used to analyze for the first time the sequence of events toward translation initiation complex formation in an archaeal model system. These studies suggested that a/eIF2-GTP binds first to the ribosome and then recruits Met-tRNAi(Met), which appears to comply with the operational mode of bacterial IF2, and deviates from the shuttle function of the eukaryotic counterpart eIF2. Thus, despite the resemblance of eIF2 and a/eIF2, recruitment of initiator tRNA to the ribosome is mechanistically different in Pro- and Eukaryotes.
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Affiliation(s)
- David Hasenöhrl
- Max F. Perutz Laboratories, Department of Microbiology, Immunobiology and Genetics, University of Vienna, 1030 Vienna, Austria.
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87
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Fraser CS. The molecular basis of translational control. PROGRESS IN MOLECULAR BIOLOGY AND TRANSLATIONAL SCIENCE 2009; 90:1-51. [PMID: 20374738 DOI: 10.1016/s1877-1173(09)90001-1] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Our current understanding of eukaryotic protein synthesis has emerged from many years of biochemical, genetic and biophysical approaches. Significant insight into the molecular details of the mechanism has been obtained, although there are clearly many aspects of the process that remain to be resolved. Importantly, our understanding of the mechanism has identified a number of key stages in the pathway that contribute to the regulation of general and gene-specific translation. Not surprisingly, translational control is now widely accepted to play a role in aspects of cell stress, growth, development, synaptic function, aging, and disease. This chapter reviews the mechanism of eukaryotic protein synthesis and its relevance to translational control.
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Affiliation(s)
- Christopher S Fraser
- Department of Molecular and Cellular Biology, University of California at Davis, Davis, California 95616, USA
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88
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Iwahashi C, Tassone F, Hagerman RJ, Yasui D, Parrott G, Nguyen D, Mayeur G, Hagerman PJ. A quantitative ELISA assay for the fragile x mental retardation 1 protein. J Mol Diagn 2009; 11:281-9. [PMID: 19460937 DOI: 10.2353/jmoldx.2009.080118] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Non-coding (CGG-repeat) expansions in the fragile X mental retardation 1 (FMR1) gene result in a spectrum of disorders involving altered neurodevelopment (fragile X syndrome), neurodegeneration (late-onset fragile X-associated tremor/ataxia syndrome), or primary ovarian insufficiency. While reliable and quantitative assays for the number of CGG repeats and FMR1 mRNA levels are now available, there has been no scalable, quantitative assay for the FMR1 protein (FMRP) in non-transformed cells. Using a combination of avian and murine antibodies to FMRP, we developed a sensitive and highly specific sandwich enzyme-linked immunosorbent assay (ELISA) for FMRP in peripheral blood lymphocytes. This ELISA method is capable of quantifying FMRP levels throughout the biologically relevant range of protein concentrations and is specific for the intact FMRP protein. Moreover, the ELISA is well-suited for replicate protein determinations across serial dilutions in non-transformed cells and is readily scalable for large sample numbers. The FMRP ELISA is potentially a powerful tool in expanding our understanding of the relationship between FMRP levels and the various FMR1-associated clinical phenotypes.
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Affiliation(s)
- Christine Iwahashi
- Department of Biochemistry and Molecular Medicine, UC Davis, School of Medicine, Davis, CA 95616, USA
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89
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Fraser CS, Hershey JWB, Doudna JA. The pathway of hepatitis C virus mRNA recruitment to the human ribosome. Nat Struct Mol Biol 2009; 16:397-404. [PMID: 19287397 PMCID: PMC2689074 DOI: 10.1038/nsmb.1572] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2008] [Accepted: 02/03/2009] [Indexed: 01/29/2023]
Abstract
Eukaryotic protein synthesis begins with mRNA positioning in the ribosomal decoding channel in a process typically controlled by translation-initiation factors. Some viruses use an internal ribosome entry site (IRES) in their mRNA to harness ribosomes independently of initiation factors. We show here that a ribosome conformational change that is induced upon hepatitis C viral IRES binding is necessary but not sufficient for correct mRNA positioning. Using directed hydroxyl radical probing to monitor the assembly of IRES-containing translation-initiation complexes, we have defined a crucial step in which mRNA is stabilized upon initiator tRNA binding. Unexpectedly, however, this stabilization occurs independently of the AUG codon, underscoring the importance of initiation factor-mediated interactions that influence the configuration of the decoding channel. These results reveal how an IRES RNA supplants some, but not all, of the functions normally carried out by protein factors during initiation of protein synthesis.
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Affiliation(s)
- Christopher S Fraser
- Howard Hughes Medical Institute, University of California, Berkeley, California 94720, USA
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90
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Abstract
Translational control in eukaryotic cells is critical for gene regulation during nutrient deprivation and stress, development and differentiation, nervous system function, aging, and disease. We describe recent advances in our understanding of the molecular structures and biochemical functions of the translation initiation machinery and summarize key strategies that mediate general or gene-specific translational control, particularly in mammalian systems.
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Affiliation(s)
- Nahum Sonenberg
- Department of Biochemistry and Goodman Cancer Centre, McGill University, Montreal, Quebec, H3G 1Y6, Canada
| | - Alan G. Hinnebusch
- Laboratory of Gene Regulation and Development, National Institute of Child Health and Human Development, NIH, Bethesda, MD 20892, USA
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91
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Pérard J, Rasia R, Medenbach J, Ayala I, Boisbouvier J, Drouet E, Baudin F. Human initiation factor eIF3 subunit b interacts with HCV IRES RNA through its N-terminal RNA recognition motif. FEBS Lett 2008; 583:70-4. [DOI: 10.1016/j.febslet.2008.11.025] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/22/2008] [Revised: 11/13/2008] [Accepted: 11/18/2008] [Indexed: 10/21/2022]
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92
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Szamecz B, Rutkai E, Cuchalová L, Munzarová V, Herrmannová A, Nielsen KH, Burela L, Hinnebusch AG, Valásek L. eIF3a cooperates with sequences 5' of uORF1 to promote resumption of scanning by post-termination ribosomes for reinitiation on GCN4 mRNA. Genes Dev 2008; 22:2414-25. [PMID: 18765792 DOI: 10.1101/gad.480508] [Citation(s) in RCA: 104] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Yeast initiation factor eIF3 (eukaryotic initiation factor 3) has been implicated in multiple steps of translation initiation. Previously, we showed that the N-terminal domain (NTD) of eIF3a interacts with the small ribosomal protein RPS0A located near the mRNA exit channel, where eIF3 is proposed to reside. Here, we demonstrate that a partial deletion of the RPS0A-binding domain of eIF3a impairs translation initiation and reduces binding of eIF3 and associated eIFs to native preinitiation complexes in vivo. Strikingly, it also severely blocks the induction of GCN4 translation that occurs via reinitiation. Detailed examination unveiled a novel reinitiation defect resulting from an inability of 40S ribosomes to resume scanning after terminating at the first upstream ORF (uORF1). Genetic analysis reveals a functional interaction between the eIF3a-NTD and sequences 5' of uORF1 that is critically required to enhance reinitiation. We further demonstrate that these stimulatory sequences must be positioned precisely relative to the uORF1 stop codon and that reinitiation efficiency after uORF1 declines with its increasing length. Together, our results suggest that eIF3 is retained on ribosomes throughout uORF1 translation and, upon termination, interacts with its 5' enhancer at the mRNA exit channel to stabilize mRNA association with post-termination 40S subunits and enable resumption of scanning for reinitiation downstream.
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Affiliation(s)
- Béla Szamecz
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AVCR, Prague, Videnska 1083, 142 20, The Czech Republic
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93
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Getting the message in protein synthesis. Keystone Symposium on Translational Regulatory Mechanisms. EMBO Rep 2008; 9:954-9. [PMID: 18758437 DOI: 10.1038/embor.2008.165] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2008] [Accepted: 07/17/2008] [Indexed: 11/08/2022] Open
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94
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Mass spectrometry reveals modularity and a complete subunit interaction map of the eukaryotic translation factor eIF3. Proc Natl Acad Sci U S A 2008; 105:18139-44. [PMID: 18599441 DOI: 10.1073/pnas.0801313105] [Citation(s) in RCA: 212] [Impact Index Per Article: 13.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
The eukaryotic initiation factor 3 (eIF3) plays an important role in translation initiation, acting as a docking site for several eIFs that assemble on the 40S ribosomal subunit. Here, we use mass spectrometry to probe the subunit interactions within the human eIF3 complex. Our results show that the 13-subunit complex can be maintained intact in the gas phase, enabling us to establish unambiguously its stoichiometry and its overall subunit architecture via tandem mass spectrometry and solution disruption experiments. Dissociation takes place as a function of ionic strength to form three stable modules eIF3(c:d:e:l:k), eIF3(f:h:m), and eIF3(a:b:i:g). These modules are linked by interactions between subunits eIF3b:c and eIF3c:h. We confirmed our interaction map with the homologous yeast eIF3 complex that contains the five core subunits found in the human eIF3 and supplemented our data with results from immunoprecipitation. These results, together with the 27 subcomplexes identified with increasing ionic strength, enable us to define a comprehensive interaction map for this 800-kDa species. Our interaction map allows comparison of free eIF3 with that bound to the hepatitis C virus internal ribosome entry site (HCV-IRES) RNA. We also compare our eIF3 interaction map with related complexes, containing evolutionarily conserved protein domains, and reveal the location of subunits containing RNA recognition motifs proximal to the decoding center of the 40S subunit of the ribosome.
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95
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Mitchell SF, Lorsch JR. Should I stay or should I go? Eukaryotic translation initiation factors 1 and 1A control start codon recognition. J Biol Chem 2008; 283:27345-27349. [PMID: 18593708 DOI: 10.1074/jbc.r800031200] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Start codon selection is a key step in translation initiation as it sets the reading frame for decoding. Two eukaryotic initiation factors, eIF1 and eIF1A, are key actors in this process. Recent work has elucidated many details of the mechanisms these factors use to control start site selection. eIF1 prevents the irreversible GTP hydrolysis that commits the ribosome to initiation at a particular codon. eIF1A both promotes and inhibits commitment through the competing influences of its two unstructured termini. Both factors perform their tasks through a variety of interactions with other components of the initiation machinery, in many cases mediated by the unstructured regions of the two proteins.
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Affiliation(s)
- Sarah F Mitchell
- Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
| | - Jon R Lorsch
- Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
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96
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Zhang L, Smit-McBride Z, Pan X, Rheinhardt J, Hershey JWB. An oncogenic role for the phosphorylated h-subunit of human translation initiation factor eIF3. J Biol Chem 2008; 283:24047-60. [PMID: 18544531 DOI: 10.1074/jbc.m800956200] [Citation(s) in RCA: 66] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Dysregulation of protein synthesis has been implicated in oncogenesis through a mechanism whereby "weak" mRNAs encoding proteins involved in cell proliferation are strongly translated when the protein synthesis apparatus is activated. Previous work has determined that many cancer cells contain high levels of eIF3h, a protein subunit of translation initiation factor eIF3, and overexpression of eIF3h malignantly transforms immortal NIH-3T3 cells. This is a general feature of eIF3h, as high levels also affect translation, proliferation, and a number of malignant phenotypes of CHO-K1 and HeLa cells and, most significantly, of a primary prostate cell line. Furthermore, overexpressed eIF3h inhibits Myc-dependent induction of apoptosis of primary prostate cells. eIF3h appears to function through translation, as the initial appearance of overexpressed eIF3h in rapidly induced NIH-3T3 cells correlates tightly with the stimulation of protein synthesis and the generation of malignant phenotypes. This oncogenic potential of eIF3h is enhanced by phosphorylation at Ser(183). Finally, reduction of eIF3h levels in breast and prostate cancer cell lines by short interfering RNA methods reduces their rates of proliferation and anchorage-independent growth in soft agar. The results provide compelling evidence that high eIF3h levels directly stimulate protein synthesis, resulting in the establishment and maintenance of the malignant state in cells.
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Affiliation(s)
- Lili Zhang
- Department of Biochemistry and Molecular Medicine, School of Medicine, University of California-Davis, CA 95616, USA
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97
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Abstract
The ribosome is a dynamic machine that undergoes many conformational rearrangements during the initiation of protein synthesis. Significant differences exist between the process of protein synthesis initiation in eubacteria and eukaryotes. In particular, the initiation of eukaryotic protein synthesis requires roughly an order of magnitude more initiation factors to promote efficient mRNA recruitment and ribosomal recognition of the start codon than are needed for eubacterial initiation. The mechanisms by which these initiation factors promote ribosome conformational changes during stages of initiation have been studied using cross-linking, footprinting, site-directed probing, cryo-electron microscopy, X-ray crystallography, fluorescence spectroscopy and single-molecule techniques. Here, we review how the results of these different approaches have begun to converge to yield a detailed molecular understanding of the dynamic motions that the eukaryotic ribosome cycles through during the initiation of protein synthesis.
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98
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Abstract
A truly groundbreaking paper by Pisarev et al. (2007), recently published in Cell, has solved a wide gap in our knowledge of eukaryotic mRNA translation, by elucidating how ribosomes are released from mRNA after the termination step.
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Affiliation(s)
- Richard J Jackson
- Department of Biochemistry, University of Cambridge, Cambridge CB2 1GA, UK.
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99
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Pisarev AV, Hellen CUT, Pestova TV. Recycling of eukaryotic posttermination ribosomal complexes. Cell 2008; 131:286-99. [PMID: 17956730 DOI: 10.1016/j.cell.2007.08.041] [Citation(s) in RCA: 171] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2007] [Revised: 08/15/2007] [Accepted: 08/28/2007] [Indexed: 11/16/2022]
Abstract
After translational termination, mRNA and P site deacylated tRNA remain associated with ribosomes in posttermination complexes (post-TCs), which must therefore be recycled by releasing mRNA and deacylated tRNA and by dissociating ribosomes into subunits. Recycling of bacterial post-TCs requires elongation factor EF-G and a ribosome recycling factor RRF. Eukaryotes do not encode a RRF homolog, and their mechanism of ribosomal recycling is unknown. We investigated eukaryotic recycling using post-TCs assembled on a model mRNA encoding a tetrapeptide followed by a UAA stop codon and report that initiation factors eIF3, eIF1, eIF1A, and eIF3j, a loosely associated subunit of eIF3, can promote recycling of eukaryotic post-TCs. eIF3 is the principal factor that promotes splitting of posttermination ribosomes into 60S subunits and tRNA- and mRNA-bound 40S subunits. Its activity is enhanced by eIFs 3j, 1, and 1A. eIF1 also mediates release of P site tRNA, whereas eIF3j ensures subsequent dissociation of mRNA.
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Affiliation(s)
- Andrey V Pisarev
- Department of Microbiology and Immunology, SUNY Downstate Medical Center, Brooklyn, NY 11203, USA
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