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      ‘Ribozoomin’ – Translation Initiation from the Perspective of the Ribosome-bound Eukaryotic Initiation Factors (eIFs)

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          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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          Most cited references201

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          Site-specific ribose methylation of preribosomal RNA: a novel function for small nucleolar RNAs.

          Eukaryotic cells contain many fibrillarin-associated small nucleolar RNAs (snoRNAs) that possess long complementarities to mature rRNAs. Characterization of 21 novel antisense snoRNAs from human cells followed by genetic depletion and reconstitution studies on yeast U24 snoRNA provides evidence that this class of snoRNAs is required for site-specific 2'-O-methylation of preribosomal RNA (pre-rRNA). Antisense sno-RNAs function through direct base-pairing interactions with pre-rRNA. The antisense element, together with the D or D' box of the snoRNA, provide the information necessary to select the target nucleotide for the methyltransfer reaction. The conclusion that sno-RNAs function in covalent modification of the sugar moieties of ribonucleotides demonstrates that eukaryotic small nuclear RNAs have a more versatile cellular function than earlier anticipated.
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            Regulation of translation via mRNA structure in prokaryotes and eukaryotes.

            The mechanism of initiation of translation differs between prokaryotes and eukaryotes, and the strategies used for regulation differ accordingly. Translation in prokaryotes is usually regulated by blocking access to the initiation site. This is accomplished via base-paired structures (within the mRNA itself, or between the mRNA and a small trans-acting RNA) or via mRNA-binding proteins. Classic examples of each mechanism are described. The polycistronic structure of mRNAs is an important aspect of translational control in prokaryotes, but polycistronic mRNAs are not usable (and usually not produced) in eukaryotes. Four structural elements in eukaryotic mRNAs are important for regulating translation: (i) the m7G cap; (ii) sequences flanking the AUG start codon; (iii) the position of the AUG codon relative to the 5' end of the mRNA; and (iv) secondary structure within the mRNA leader sequence. The scanning model provides a framework for understanding these effects. The scanning mechanism also explains how small open reading frames near the 5' end of the mRNA can down-regulate translation. This constraint is sometimes abrogated by changing the structure of the mRNA, sometimes with clinical consequences. Examples are described. Some mistaken ideas about regulation of translation that have found their way into textbooks are pointed out and corrected.
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              Hierarchical phosphorylation of the translation inhibitor 4E-BP1.

              In most instances, translation is regulated at the initiation phase, when a ribosome is recruited to the 5' end of an mRNA. The eIF4E-binding proteins (4E-BPs) interdict translation initiation by binding to the translation factor eIF4E, and preventing recruitment of the translation machinery to mRNA. The 4E-BPs inhibit translation in a reversible manner. Hypophosphorylated 4E-BPs interact avidly with eIF4E, whereas 4E-BP hyperphosphorylation, elicited by stimulation of cells with hormones, cytokines, or growth factors, results in an abrogation of eIF4E-binding activity. We reported previously that phosphorylation of 4E-BP1 on Thr 37 and Thr 46 is relatively insensitive to serum deprivation and rapamycin treatment, and that phosphorylation of these residues is required for the subsequent phosphorylation of a set of unidentified serum-responsive sites. Here, using mass spectrometry, we identify the serum-responsive, rapamycin-sensitive sites as Ser 65 and Thr 70. Utilizing a novel combination of two-dimensional isoelectric focusing/SDS-PAGE and Western blotting with phosphospecific antibodies, we also establish the order of 4E-BP1 phosphorylation in vivo; phosphorylation of Thr 37/Thr 46 is followed by Thr 70 phosphorylation, and Ser 65 is phosphorylated last. Finally, we show that phosphorylation of Ser 65 and Thr 70 alone is insufficient to block binding to eIF4E, indicating that a combination of phosphorylation events is necessary to dissociate 4E-BP1 from eIF4E.
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                Author and article information

                Journal
                Curr Protein Pept Sci
                Curr. Protein Pept. Sci
                CPPS
                Current Protein & Peptide Science
                Bentham Science Publishers
                1389-2037
                1875-5550
                June 2012
                June 2012
                : 13
                : 4
                : 305-330
                Affiliations
                Laboratory of Eukaryotic Gene Regulation, Institute of Microbiology AS CR, Videnska 1083, Prague, 142 20, the Czech Republic
                Author notes
                [* ]Address correspondence to this author at the Laboratory of Eukaryotic Gene Regulation, Institute of Microbiology AS CR, Videnska 1083, Prague, 142 20, the Czech Republic; Tel: +420 241 062 288; Fax: +420 241 062 501; E-mail: valasekl@ 123456biomed.cas.cz
                Article
                CPPS-13-305
                10.2174/138920312801619385
                3434475
                22708493
                9836877d-e141-443b-a71d-bb4e7962e96d
                © 2012 Bentham Science Publishers

                This is an open access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.5/), which permits unrestrictive use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 12 February 2011
                : 16 January 2012
                : 16 February 2012
                Categories
                Article

                Biochemistry
                translation initiation,ribosome,gcn4.,aug,mrna,eif,translational control,reinitiation
                Biochemistry
                translation initiation, ribosome, gcn4., aug, mrna, eif, translational control, reinitiation

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