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10.7: Export of mRNA and Ribosomes from the Nucleus

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    88960
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    The synthesis and processing of rRNAs are coincident with the assembly of the ribosomal subunits (Figure 10.25).

    Screen Shot 2022-05-19 at 11.43.56 PM.png
    Figure 10.25: Cells coordinate the export of ribosomal subunits from nucleus to cytoplasm with their assembly from mature rRNAs and ribosomal proteins imported from the cytoplasm.

    The 45S pre-rRNAs initially bind to ribosomal proteins in the nucleolus (that big nuclear body!) to initiate assembly, and then serve as a scaffold for the continued addition of ribosomal proteins to both the small and the large ribosomal subunits. After the 5S rRNA is added to the nascent large ribosomal subunit, processing (cleavage) of 45S rRNA is completed, and the subunits are separated. The separated ribosomal subunits exit the nucleus to the cytoplasm, where they will associate with mRNAs to translate new proteins. To better understand what is going on, try summarizing what you see here in the correct order of steps. You can also see this process animated at this link: Ribosome Assembly & Nuclear Export.

    Messenger RNAs are independently exported from the nucleus. Their 5’ methyl guanosine caps and poly(A) tails collaborate to facilitate transport into the cytoplasm. We now understand that proteins in the nucleus participate in the export process: a nuclear transport receptor binds along the mature (or maturing) mRNA; a poly(A)-binding protein binds along the poly(A) tail of the message; and another protein binds at or near the methyl guanosine CAP itself. These interactions enable transport of the mRNA through nuclear pores. After the mRNA is in the cytoplasm, the nuclear transport receptor recycles back into the nucleus while a translation initiation factor replaces the protein bound to the CAP. Figure 10.26 summarizes the nuclear transport process for mRNAs.

    Screen Shot 2022-05-19 at 11.45.53 PM.png
    Figure 10.26: The 5' guanosine CAP and 3' Poly(A) tail on mRNAs function during mRNA export from nucleus to cytoplasm. A nuclear transport factor along with specific 5’ guanosine CAP and 3’ poly(A) binding proteins associate with the mRNA during export. After export, the CAP binding protein and nuclear transport protein are recycled to the nucleus.

    See a more detailed description of mRNA transport from the nucleus at mRNA Export from the Nucleus. The mature mRNA, now in the cytoplasm, is ready for translation, the process of protein synthesis mediated by ribosomes and a host of translation factors. The genetic code directs polypeptide synthesis during translation. We will discuss the details of translation shortly.


    This page titled 10.7: Export of mRNA and Ribosomes from the Nucleus is shared under a not declared license and was authored, remixed, and/or curated by Gerald Bergtrom.

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