Part 3: Post-transcriptional regulation of translation

Question 1

apolipoprotein B (apoB):

Answer 1

• Made in the intestine and liver
• shorter form of protein required in the intestine
  
  • apoB mRNA is edited to introduce a “premature stop codon.”

Question 2

How is the apoB altered in order to make its short form, which is required in the intestine?

Answer 2

• RNA base identity is enzymatically “edited” to create the short form of ApoB
  
  • via splicing polyadenylation: C is deaminated to a U

Question 3

micro RNAs (miRNAs) are:

Answer 3

• short RNA molecules that base pair with specific mRNA sequences and regulate translation/gene expression
  
  • the mRNA the miRNA base pairs with does not have to be a perfect match

Question 4

When miRNAs hybridize (base pair) with a specific mRNA sequence, what is complex is recruited?

Answer 4

RISC

• RISC complexes are thought to physically impede translation initiation

Question 5

What part of an mRNA strand do miRNAs usually bind to?

Answer 5

3’ UTR

Question 6

If an mRNA does not get translated, what usually happens to it?

Answer 6

• it gets degraded
• miRNA binding to the 3’ UTR of mRNAs and the blockage of transcription by RISC can lead to mRNA degradation

Question 7

The concentration of iron in a cell regulates the transcription of what gene?

Answer 7

• ferretin
  
  • binds to iron - preventing it from reacting
  • only transcribed when iron is in excess

Question 8

IRP:

Answer 8

• iron regulatory protein
• regulates translation or mRNA stability depending on cellular iron concentrations
  • absence of iron: binds to IRE and blocks transcription of ferritin
  • presence of iron: binds to iron and ferritin is transcribed

Question 9

IRE:

Answer 9

• iron response element
  
  • stem-loop mRNA sequence in front of the start codon of ferretin
  • binds to IRP in the absence of iron to prevent ferretin transcription

Question 10

Example of post-transcriptional regulation on the 3’ end of mRNA:

Answer 10

• miRNA/RISC complex
  
  • sterically blocks initiation of translation

Question 11

Example of post-transcriptional regulation on the 5’ end of mRNA:

Answer 11

• IRP/IRE
  
  • IRE is a stem-loop structure on the 5’ end of ferretin mRNA, right before the start codon
  • binding of IRP to IRE blocks formation of the preinitation complex

Question 12

Phosphorylation of initiation factor eIF2 causes GLOBAL translation inhibition in response to:

Answer 12

• Low amino acid concentration (poor nutrition)
• Cellular stress (e.g. oxidative stress)
• Immune response
• Unfolded proteins

Question 13

eIF2 has a recycling factor.

What is its name?

Answer 13

eIF2B

• the Guanine Nucleotide Exchange Factor (GEF) for eIF2
• required because the affinity for GDP is much higher than that for GTP
• Coverts eIF2-GDP back to eIF2-GTP

Question 14

Is there more eIF2 or eIF2B in a cell?

Answer 14

• more eIF2.
  
  • phosphorylated eIF2 irreversibly binds to eIF2B in states of poor nutrition.
  • eIF2 then cannot be recycled, active eIF2 levels drop, protein synthesis is reduced

Question 15

Steps in the regulation of eIF2 by nutritional state:

Answer 15

In poor nutrition state:

1. eIF2 is phosphorylated
2. phosphorylated eIF2 irreversibly binds to its recycling factor, eIF2B
3. eIF2B becomes unavailable, eIF2 can no longer be recycled
4. Protein synthesis drops

Question 16

The G-protein eIF2 needs _____ to assist in re-binding GTP after hydrolysis.

Answer 16

• a guanine nucleotide exchange factor (eIF2B)

Question 17

Role of mTOR pathway:

Answer 17

• upregulates translation during growth
• downregulates translation during stress
• constitutively in an active state in order to keep growth in check

Question 18

The mTOR signaling pathway represses translation in response to hypoxia by regulating the function of:

Answer 18

eIF4E

(protein that binds to the 5’ cap of mRNAs to initiate translation)

Question 19

mTOR signaling pathway regulates the function of: eIF4E via what protein:

Answer 19

4EBP

• 4EBP binding to eIF4E inhibitory
  
  • represses translation

Question 20

mTOR pathway during hypoxic state:

Answer 20

• mTOR pathway inhibited:
  
  • reduced 4EBP phosphorylation
  • 4EBP BINDS to eIF4E
  • translation INHIBITED

Question 21

Does phosphorylated or unphosphorylated 4EBP bind to eIF4E?

Answer 21

• unphosphorylated 4EBP binds to eIF4E
  
  • under hypoxic conditions
  • mTOR pathway inhibited
  • = translation inhibited

Question 22

What is the result of high mTOR activity?

Answer 22

• all 4EBP phosphorylated
  
  • no inhibition of eIF4E
  • no inhibition of translation
  • uncontrolled growth (cancer)

Question 23

4EBPs are proteins that:

Answer 23

• bind to eIF4E and inhibit translation.
  
  • When 4EBPs are phosphorylated, they are released from eIF4E and translation inhibition is relieved.