Virus Exploitation of Translation Flashcards
Why is it important to know how eukaryotes translate proteins?
Viruses need to compete with host mRNAs to make their own proteins. Understanding normal translation reveals how viruses subvert and hijack it for their own benefit
What are the key stages of translation in eukaryotes?
Initiation
Elongation
Termination
What is initiation driven by in eukaryotes?
90% of host mRNAs are cap-dependent, however some are IRES-driven
What is the 5’ cap?
A modified guanine nucleotide that is added to the 5’ end of mRNA during transcription
What is the role of 5’ cap?
- Prevents mRNA from degradation
- Essential for recognition and efficient translation of mRNA
What is the cap recognised by?
Eukaryotic initiation factor 4E, which is a part of the eIF4F complex
Summarise the initiation process for cap-dependent translation
- Recognition by eIF4E
- eIF4F complex binds to cap, and recruits small 40S ribosomal subunit
- 40s scans mRNA to find start codon (AUG)
- Start codon is found, and 60s ribosomal subunit joins to complete ribosome
- Translation begins
What does IRES-driven initiation allow for?
Cap-independent initiation (allows ribosomes to bind without a 5’ cap)
When is IRES-driven initiation mainly used?
When cap-dependent translation is shut down, such as during cellular stress.
Not all mRNAs have IRES elements, limiting its use
What is IRES?
A specific sequence within mRNA that can directly recruit the 40s subunit to the start codon
What is circularisation of mRNA?
The 5’ cap and 3’ poly-A tail are brought together by PABP, forming a closed-loop which is thought to increase translational efficiency
How does the elongation work in eukaryotic translation?
Ribosome moves codon to codon, with eukaryotic elongation factor 1A (eEF1A) delivering tRNAs, and eEF2 catalysing translocation
What is tRNA?
Transfer RNA, which bring amino acids to the ribosome
What are elongation factors?
eEF1A
eEF2
What is translocation during elongation?
The process by which the mRNA-tRNA moves through the ribosome one codon at a time, allowing each codon to be translated
How does termination occur in translation?
Eukaryotic termination factor 1 recognises stop codon, and the newly synthesised polypeptide is released
What are the different strategies by which viruses can hijack host translation?
- Cap snatching
- IRES exploitation
What is an example of a virus that uses cap snatching?
Influenza A
Why must influenza cap snatch?
Its genome lacks a 5’ cap, so it steals caps from host mRNAs to trick ribosomes
What is the process of cap snatching by Influenza A?
- Viral RNA-dependent RNA polymerase binds to 5’ cap of host cell pre-mRNA
- Host mRNA is cleaved a few nucleotides downstream of the cap by endonuclease from RdRp
- The capped RNA fragment is used as a primer to initiate the synthesis of viral mRNA by viral RdRp
What 3 subunits is RdRp made up of?
PB1-PB2-PA
What is the consequence of cap snatching?
- Host mRNAs are degraded, shutdown of host protein synthesis
- Viral mRNAs get preferentially translated (they’re the ones with caps)
What are two examples of viruses that use IRES exploitation?
- Poliovirus
- Hepatitis C virus (HCV)
How does poliovirus use IRES exploitation?
First:
- Viral 2A protease cleaves eIF4G, which decouples eIF4E from the ribosome
- Causes host cap-dependent translation to stop
Then:
- Poliovirus mRNA has an IRES that directly recruits ribosomes without eIF4E
- Uses a subset of eIFs (e.g. eIF3) to initiate translation
What does IRES stand for?
Internal ribosome entry site
How does hepatitis C virus use IRES exploitation?
Its IRES binds directly to the 40s ribosome, without the need for any eIFs.
It doesn’t necessarily shut down host mRNA translation and export (unlike poliovirus)
Where are the IRES found in polio and HCV?
In the 5’ untranslated region (UTR)
Why might IRES-mediated translation be advantageous for viruses?
- Continues to work during stress
- Faster initiation (bypasses cap-binding steps)
What are two ways in which viruses maximise their coding potential?
- Leaky scanning
- Ribosomal frame shifting
How do ribosomes normally decide where to start protein synthesis?
They scan the UTR from 5’ until they encounter the first AUG (start codon) with a strong Kozak sequence. This is where they translation begins.
When does leaky scanning occur?
If the first AUG has a weak Kozak context, some ribosomes will skip it and continue scanning.
Translation will then initiate at a downstream AUG, producing a different protein
Which viruses use leaky scanning and why?
Influenza A, HPV, HIV.
They use it to maximise their coding capacity, by producing multiple proteins from a single mRNA
What different proteins does Influenza A produce by leaky scanning?
Influenza A encodes PB1 (polymerase subunit) from the first AUG, and PB1-F2 (pre-apoptotic protein) from a downstream AUG via leaky scanning
What viruses primarily use ribosomal frameshifting?
Retroviruses (e.g. HIV)
How does ribosomal frame shifting work?
Where specific signals in the mRNA instruct the ribosome to change reading frame from 0 to +1 or -1.
This allows the production of different proteins from the same mRNA sequence, effectively through reading the mRNA from a different starting point.
Why is ribosomal frame shifting important for retroviruses?
The Gag gene is translated in the 0 frame, while the Pol gene is translated in the -1 frame.
A -1 ribosomal frameshift (-1PRF) allows for the production of Gag-Pol.
