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7.17B: The Initiation Complex and Translation Rate

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    9333
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    The first step of translation is ribosome assembly, which requires initiation factors.

    LEARNING OBJECTIVES

    Discuss how eukaryotes assemble ribosomes on the mRNA to begin translation

    KEY TAKEAWAYS

    Key Points

    • The components involved in ribosome assembly are brought together by the help of proteins called initiation factors which bind to the small ribosomal subunit.
    • Initiator tRNA is used to locate the start codon AUG (the amino acid methionine) which establishes the reading frame for the mRNA strand.
    • GTP carried by eIF2 is the energy source used for loading the initiator tRNA carried by the small ribosomal subunit on the correct start codon in the mRNA.
    • GTP carried by eIF5 is the energy source for assembling the large and small ribosomal subunits together.

    Key Terms

    • reading frame: either of three possible triplets of codons in which a DNA sequence could be transcribed
    • phosphorylation: the addition of a phosphate group to a compound; often catalyzed by enzymes

    Ribosome Assembly and Translation Rate

    Like transcription, translation is controlled by proteins that bind and initiate the process. In translation, before protein synthesis can begin, ribosome assembly has to be completed. This is a multi-step process.

    In ribosome assembly, the large and small ribosomal subunits and an initiator tRNA (tRNAi) containing the first amino acid of the final polypeptide chain all come together at the translation start codon on an mRNA to allow translation to begin. First, the small ribosomal subunit binds to the tRNAwhich carries methionine in eukaryotes and archaea and carries N-formyl-methionine in bacteria. (Because the tRNAi is carrying an amino acid, it is said to be charged.) Next, the small ribosomal subunit with the charged tRNAi still bound scans along the mRNA strand until it reaches the start codon AUG, which indicates where translation will begin. The start codon also establishes the reading frame for the mRNA strand, which is crucial to synthesizing the correct sequence of amino acids. A shift in the reading frame results in mistranslation of the mRNA. The anticodon on the tRNAi then binds to the start codon via basepairing. The complex consisting of mRNA, charged tRNAi, and the small ribosomal subunit attaches to the large ribosomal subunit, which completes ribosome assembly. These components are brought together by the help of proteins called initiation factors which bind to the small ribosomal subunit during initiation and are found in all three domains of life. In addition, the cell spends GTP energy to help form the initiation complex. Once ribosome assembly is complete, the charged tRNAis positioned in the P site of the ribosome and the empty A site is ready for the next aminoacyl-tRNA. The polypeptide synthesis begins and always proceeds from the N-terminus to the C-terminus, called the N-to-C direction.

    In eukaryotes, several eukaryotic initiation factor proteins (eIFs) assist in ribosome assembly. The eukaryotic initiation factor-2 (eIF-2) is active when it binds to guanosine triphosphate (GTP). With GTP bound to it, eIF-2 protein binds to the small 40S ribosomal subunit. Next, the initiatior tRNA charged with methionine (Met-tRNAi) associates with the GTP-eIF-2/40S ribosome complex, and once all these components are bound to each other, they are collectively called the 43S complex.

    Eukaryotic initiation factors eIF1, eIF3, eIF4, and eIF5 help bring the 43S complex to the 5′-m7G cap of an mRNA be translated. Once bound to the mRNA’s 5′ m7G cap, the 43S complex starts travelling down the mRNA until it reaches the initiation AUG codon at the start of the mRNA’s reading frame. Sequences around the AUG may help ensure the correct AUG is used as the initiation codon in the mRNA.

    Once the 43S complex is at the initiation AUG, the tRNAi-Met is positioned over the AUG. The anticodon on tRNAi-Met basepairs with the AUG codon. At this point, the GTP bound to eIF2 in the 43S complexx is hydrolyzed to GDP + phosphate, and energy is released. This energy is used to release the eIF2 (with GDP bound to it) from the 43S complex, leaving the 40S ribosomal subunit and the tRNAi-Met at the translation start site of the mRNA.

    Next, eIF5 with GTP bound binds to the 40S ribosomal subunit complexed to the mRNA and the tRNAi-Met. The eIF5-GTP allows the 60S large ribosomal subunit to bind. Once the 60S ribosomal subunit arrives, eIF5 hydrolyzes its bound GTP to GDP + phosphate, and energy is released. This energy powers assembly of the two ribosomal subunits into the intact 80S ribosome, with tRNAi-Met in its P site while also basepaired to the initiation AUG codon on the mRNA. Translation is ready to begin.

    The binding of eIF-2 to the 40S ribosomal subunit is controlled by phosphorylation. If eIF-2 is phosphorylated, it undergoes a conformational change and cannot bind to GTP. Therefore, the 43S complex cannot form properly and translation is impeded. When eIF-2 remains unphosphorylated, it binds the 40S ribosomal subunit and actively translates the protein.

    image

    Translation Initiation Complex: Gene expression can be controlled by factors that bind the translation initiation complex.

    The ability to fully assemble the ribosome directly affects the rate at which translation occurs. But protein synthesis is regulated at various other levels as well, including mRNA synthesis, tRNA synthesis, rRNA synthesis, and eukaryotic initiation factor synthesis. Alteration in any of these components affects the rate at which translation can occur.

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