Control of eukaryotic translation

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Which step in translation is rate limiting?
initiation - highly regulated and involves < 25 proteins.
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What mediates translation?
The ribosome
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What does each subunit do?
Large - joins amino acids to the polypeptide chain. Small - reads the RNA
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In what direction does the ribosome move?
5' to 3' end.
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Draw the features of a typical mRNA
5' methylated GTP cap. Hairpin, Internal riboome entry site.][Poly A tail]
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What is the methylated GTP cap for?
regulates nuclear export of mRNA, mRNA stability. Critical for translation initiation as it binds eLF4E
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what does the IRES do?
allow translation independantly of the cap recognition
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Describe the first step in the formation of the 43s subunit
40s associates with elf3 and elf1a.
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What is the effect of these elfs binding?
prevents the reassociation of the 60s subunit
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What is the next thing to join the 40s subunit
elF2, with met-tRNA attached
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What completes the 43S pre-initation complex
Elf4f and elf5.
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What makes the 48s complex
elf4f binds to the 7'methyl guanosine cap of mRNA
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What does the 48s complex do/
Scan the mRNA for an AUG start codon
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What happens once the AUG is found
the 60s ribosomal sub-unit associates and all factors dissociate
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Two sites of regulation of initation
elf4f and elf2.
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Describe the elf4f complex
large elf4g, with elf4e, elf4b and elf4a attached.
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What might be the rate limiting factor?
elF4E because there is relatively low concentrations compared to numbers of ribosomes and other factors.
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What binds the 5'cap of mRNA
elF4E
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What does elF4G bind?
polyA binding protien - PABP
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Why is PABPs action useful
it circularises the DNA, allowing the ribosome to transfer form 3' end to 5' end without dissociatinf
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describe the structure of elf2
a, b and y subunit. y subunit is gtp bound.
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How is elF2 involved in regulation of translation
phosphorylated by a-kinase on the ser51, which INHIBITS translation
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When is a-kinase active?
in viral infection, metabolite starvation, hypoxia.
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Elongation - where does the energy required to move the ribosome come from
Hydrolysis of GTP several times
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Why might elongation be stalled
miRNA (+risc) binding to 5'UTR
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How can elongation be regulated?
eEF2 can be phosphorylated which inhibits elongation.
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Why would eef2 be phosphorylated
by MAP kinase cascade, in response to starvation hypoxia and stress via protien kinase A, or via cell cycle regulatory protiens
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When does termination occur?
Once one of the stop codons appears - UAA, UAG, UGA.
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What happens when a stop codon is in the A site
eRF1 binds, and eRF3 acts as a GTPase to stimulate polypeptide release
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Four times when translation is regulated
in response to growth factors and hormones, in the cell cycle, in viral infection, in structures with IRES and IRE
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An example of growth factors/hormones regulating translation
insulin --> insulin receptor --> Ras phosphorylates RAF --> MEK --> ERK --> Mnk1 --> elF4E = promotion of translation
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The Cell cycle and translational regulation
at G2/M, cap-dependant translation is inhibited. elF4E phosphorylation is reduced, and there is increase phosphorylation of elf2A and eEF2.
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Is all translation stopped at G2/M?
No- specific genes with IRES sequences are still translated
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Example of viruses regulating translation
picornaviruses - eg, poliovirus and encephalomyocarditis, cleave elf4g and PAMP
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How does viral mRNA escape inhibition?
they all have internal ribosomal entry sites.
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How do IRES control translation?
forms stem loop structures that allow translation even when cap dependant initation is blocked
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IRE-IRP interactions when the iron concentration is high
When iron concentration is HIGH - it binds IRP, preventing IRE binding, allowing translation of ferritin (storage) but decreasing transferrin receptors.
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Card 2

Front

What mediates translation?

Back

The ribosome

Card 3

Front

What does each subunit do?

Back

Preview of the front of card 3

Card 4

Front

In what direction does the ribosome move?

Back

Preview of the front of card 4

Card 5

Front

Draw the features of a typical mRNA

Back

Preview of the front of card 5
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