Export of mRNA

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  • Export of mRNA
    • Nuclear pore complexes
      • Spans both membranes of nuclear envelope
      • Nuclear basket
      • Cytoplasmic filaments
      • Composed of nucleoporins
        • 8 identical subcomplexes. Some of nucleoporins are present once in each subcomplex, others repeated up to 7x
        • 3 classes
          • Anchoring nucleoporin
            • Anchor the nuclear pore to the nuclear envelope - these are the proteins embedded in the double membrane
          • FG-repeat nucleoporin
            • contian FG repeats of the hydrophobic phenylalanine and the neutral glycine
            • lie in the channel of NPC
            • form brush-like / gel-like structure; acts as sieve
              • this blocks export of larger molecules and provides binding sites for exporting proteins
            • act as stepping stones ut of nucleus
          • Structural nucleoporin
            • build up complex and position other proteins
    • Method of export differs between each type of RNA
    • Cotranscriptional export preparation
      • mRNAs coated with proteins during transcription to form mRNPs
        • this structure modified until mRNP is ready to be exported, then further modified after export
      • Proteins added to the 5' end
        • cap-binding complex attaches to 5' end of mRNA
          • protects transcript
          • important in splicing, export and stability
          • needs to be added correctly
            • during transcription pausing a 7-methylguanosine is added and bound by nuclear cap-binding protein
              • signals to RNAP to escape the promoter and continue to transcribe the gene
                • signal is phosphorylation of ser-2 of the CTD
      • Proteins added during transcription elongation
        • adaptor proteins added to transcript whilst it is being transcribed
        • proteins called the exon junction complex are added at intron removal sites
          • added after splicing but still cotranscriptionally
            • introns act a nuclear retention signals, which prevents premature escape of RNA
              • once introns spliced out the signal is gone, therefore splicing must be finished before an mRNA can be exported
          • exon junction complex is deposited by spliceosome 20-24 nucleotides upstream of exon junctions
          • stabilises attachment of RNA export adaptor proteins and stabilises RNA itself
      • proteins added to the 3' end
        • after transcription termination the transcript pauses at the transcription complex briefly whilst polyadenylation occurs
          • polyadenylation important but not essential for RNA export
            • demonstrated using yeast
              • it is the run of adenosine residues, not another aspect of the polyA process which is important for nuclear export
                • likely role to bind a protein called polyA-binding protein during nuclear export
    • Molecular export mechanism
      • 5 different classes of protein involved
        • bind sequentially to mRNA
        • lead to disassembly of the export complexes in cytoplasm
        • export adaptors
          • link mRNA with the transport apparatus
          • TREX complex SR proteins
          • present in active sites of gene expression
        • 3' end formation
          • release the mRNP transcript from the gene
          • polyadenylation proteins
          • present in active sites of gene expression
        • export receptors
          • bind to export adaptors and the nuclear pore
          • Mex67/Mtr2 (yeast) TAP/p!% (animals)
          • present in nuclear periphery
        • nuclear pore proteins
          • inner channel proteins which interface with the RNA export complex during export
          • FG-repeat nucleoporins
          • present in nuclear envelope
        • disassembly and recycling of export components
          • disassemble RNA export complexes once they reach the cytoplasm
          • Rat8 (yeast) DBP5 (animals)
          • present in cytoplasm
      • for more notes on steps see "steps in mRNA export document"
    • Post-nuclear trafficking
      • some mRNAs aren't translated immediately, some need to be transported to right place and wait for right time
        • e.g. ASH1 gene
          • in yeast, ASH1 mRNA is translocated to the tip of a growing cell by actin and myosin (can be visualised using GFP-tagged RNA binding proteins in vitro)
          • ASH1  mRNA is exported from the nucleus
            • She2 binds stem loops in mRNA
              • she3 binds
                • Myo4 binds to She3
                  • completed complex allows myosin to carry the ASH1 mRNA along actin filaments to the budding cell where it is required
          • see dan's diagram
      • euk cells are asymmetric. asymmetric localisation of mRNAs inside cell determines intracellular protein distribution
        • localisation of mRNAs after they have been exported is important during embryological development
          • e.g. the morphogen gradient created by bicoid mRNA in fruit flies
            • bicoid is transcription factor that activates different genes.
              • high conc = head develops
              • mid conc = tummy develops
              • low conc = posterior develops
      • for other genes, kinesins and dyneins are used to travel along microtubules
        • kinesins transport mRNA cargos (e.g. oskar in fruit flies) to the +Ve end of the microtubules, and dyneins work in the opposite direction
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