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Related Concept Videos

Regulated mRNA Transport02:22

Regulated mRNA Transport

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In eukaryotes, transcription and translation are compartmentalized; an mRNA is first synthesized in the nucleus and then selectively transported to the cytoplasm for protein synthesis. Before transport, a pre-mRNA undergoes several steps of post-transcriptional modifications including splicing, 5' capping, and the addition of a poly-adenine tail. Various proteins bind to the pre-mRNA during these modifications. The mRNA transport takes place with the help of multiple proteins playing...
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Initiation of Translation02:33

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Initiating translation is complex because it involves multiple molecules. Initiator tRNA, ribosomal subunits, and eukaryotic initiation factors (eIFs) are all required to assemble on the initiation codon of mRNA. This process consists of several steps that are mediated by different eIFs.
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Ribosome synthesis is a highly complex and coordinated process involving more than 200 assembly factors. The synthesis and processing of ribosomal components occurs not only in the nucleolus but also in the nucleoplasm and the cytoplasm of eukaryotic cells.
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Nuclear Export of mRNA02:31

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Before mRNAs are exported to the cytoplasm, it is crucial to check each mRNA for structural and functional integrity. Eukaryotic cells use several different mechanisms, collectively known as mRNA surveillance, to look for irregularities in mRNAs. Irregular or aberrant mRNA are rapidly degraded by various enzymes. If a defective mRNA escapes the surveillance, it would be translated into a protein which would either be non-functional or not function properly. One of the primary irregularities in...
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Nonsense-mediated mRNA Decay02:27

Nonsense-mediated mRNA Decay

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The Upf proteins that carry out nonsense-mediated decay (NMD) are found in all eukaryotic organisms, including humans. Each protein has an individual role, but they need to work in collaboration. Upf1 is an ATP-dependent RNA helicase that unwinds the RNA helix. Because Upf1 can unwind any RNA, Upf2 and Upf3 are required to help Upf1 discriminate between nonsense and normal mRNAs.
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Functional Cyclization of Eukaryotic mRNAs.

Olga M Alekhina1,2, Ilya M Terenin3,4, Sergey E Dmitriev3,5,6

  • 1Institute of Protein Research, Russian Academy of Sciences, Pushchino, Moscow Region 142290, Russia.

International Journal of Molecular Sciences
|March 4, 2020
PubMed
Summary

Eukaryotic translation involves mRNA circularization, enhancing protein synthesis. This study experimentally proves that recycled ribosomes can reinitiate translation on the same mRNA molecule via a novel closed-loop assisted reinitiation (CLAR) mechanism, independent of eIF4A.

Keywords:
5′ cap–poly(A)-tail interactioncell-free systemeukaryotic mRNA translationin vitro translationpolysomeprotein synthesisribosome recyclingtranslation reinitiation

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Area of Science:

  • Molecular Biology
  • Gene Expression
  • Biochemistry

Background:

  • The closed-loop model proposes mRNA circularization via cap-eIF4E-eIF4G-PABP-poly(A) interactions to enhance protein synthesis.
  • This model suggests circularization facilitates ribosome recycling for subsequent translation rounds on the same mRNA.
  • Experimental proof for this mRNA circularization hypothesis and its functional consequences has been lacking.

Purpose of the Study:

  • To experimentally validate the closed-loop model of eukaryotic translation.
  • To investigate the mechanism of enhanced protein synthesis through mRNA circularization.
  • To identify the factors and conditions influencing translation reinitiation on circularized mRNAs.

Main Methods:

  • Utilized continuous in situ monitoring of luciferase synthesis in a mammalian in vitro system.
  • Employed reporter mRNAs with varying 5' and 3' untranslated regions (UTRs) and poly(A) tail lengths.
  • Assessed the impact of poly(A) RNA fragments, cap analogs, and eIF4A mutants on translation rates.

Main Results:

  • Observed an increased rate of translation initiation after the first round of translation for capped and polyadenylated mRNAs.
  • Demonstrated that this acceleration is dependent on the poly(A) tail and abrogated by competing poly(A) or cap analogs.
  • Found that moderate UTR lengths and longer poly(A) tails optimize acceleration, and the inhibitory effect of an eIF4A mutant diminishes over time.

Conclusions:

  • Provided the first experimental evidence for mRNA circularization enhancing translation initiation.
  • Revealed a novel closed-loop assisted reinitiation (CLAR) mode where recycled ribosomes reinitiate translation independently of eIF4A.
  • Established that functionally circularized mRNAs are efficiently translated via this non-canonical reinitiation pathway.