Export of mRNA
- Created by: natasha8sherry
- Created on: 10-01-14 11:48
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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
- Anchoring nucleoporin
- 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
- signals to RNAP to escape the promoter and continue to transcribe the gene
- during transcription pausing a 7-methylguanosine is added and bound by nuclear cap-binding protein
- cap-binding complex attaches to 5' end of mRNA
- 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
- introns act a nuclear retention signals, which prevents premature escape of RNA
- 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
- added after splicing but still cotranscriptionally
- 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
- it is the run of adenosine residues, not another aspect of the polyA process which is important for nuclear export
- demonstrated using yeast
- polyadenylation important but not essential for RNA export
- after transcription termination the transcript pauses at the transcription complex briefly whilst polyadenylation occurs
- mRNAs coated with proteins during transcription to form mRNPs
- 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"
- 5 different classes of protein involved
- 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
- Myo4 binds to She3
- she3 binds
- She2 binds stem loops in mRNA
- see dan's diagram
- e.g. ASH1 gene
- 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
- bicoid is transcription factor that activates different genes.
- e.g. the morphogen gradient created by bicoid mRNA in fruit flies
- localisation of mRNAs after they have been exported is important during embryological development
- 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
- some mRNAs aren't translated immediately, some need to be transported to right place and wait for right time
- Nuclear pore complexes
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