L7: Control of eukaryotic translation Flashcards
What are the points of regulation during replication, transcription, and translation in eukaryotic cells?
- DNA replication = regulated according to the cell cycle.
- DNA transcription = regulated by activation or repression and mRNA stability.
- RNA is translated into protein = regulated by mRNA stability, initiation of translation & blocking elongation
Protein synthesis is principally regulated at the initiation stage
Describe the structure and functions of ribosomes in translation
- composed of a small (40S) and large (60S) subunit, forming an 80S ribosome in eukaryotes.
- Small subunit decodes mRNA, while large subunit joins AAs onto the growing polypeptide chain
- ribosome’s association = reversible & regulated by eukaryotic initiation factors (eIFs)
what do eIFs stand for?
eukaryotic initiation factors
Outline the steps involved in eukaryotic translation initiation
- formation of 40S complex + eIFs + initiator tRNA(i) at AUG start codon = Assembly of 43S complex
- Formation of 48S complex from = binding of 43S to the mRNA’s 5’ end through interaction with initiation factors (eIF4F)
- Scanning for start codon, after recognition of it - joining of 60S ribosomal subunit to initiate translation.
forms 80S ribosome - ready for elongation
Outline the steps involved in eukaryotic translation elongation
- Codon recognition: New tRNA binds at A site with GTP and eEF1.
- GTP hydrolysis: Converts GTP to GDP, allowing correct amino acid attachment.
- Peptidyl transferase: Polypeptide chain transferred from tRNA in P site to A site.
- GTP hydrolysis: Ensures correct amino acid attachment.
- Translocation: Ribosomal complex moves one codon forward with GTP hydrolysis
Outline the steps involved in eukaryotic translation termination
- Ribosome encounters stop codon, and release factor (eRF1) binds to A site.
- eRF3 stimulates polypeptide release via GTP hydrolysis.
- Ribosome disassembles, releasing the polypeptide
How is the tRNA decoding process influenced by the wobble hypothesis?
-The “wobble” hypothesis explains how one tRNA can recognize multiple codons.
- The 3’ position of the codon (wobble codon base) allows non-Watson-Crick base pairing.
- reduces the number of tRNA species needed to decode codons
Explain the role of translation factors in eukaryotic translation initiation and elongation
- (eIFs) guide proper assembly of ribosomes at start codon.
- eIF2 brings initiator tRNA with GTP, eIF4F binds mRNA’s 5’ cap, eIF5 aids in fidelity.
Elongation factors (eEFs) help tRNAs decode codons and provide energy for peptide bond formation.
Mutations in eEF1 and eEF2 genes are linked to neurological diseases
- (eEFs) help tRNAs decode codons and provide energy for peptide bond formation.
- Mutations in eEF1 and eEF2 genes are linked to neurological diseases
Where does eukaryotic translation occur within the cell?
occurs in various cellular locations:
- Free ribosomes in the cytoplasm for cytoplasmic and most intracellular organelle proteins.
- Ribosomes on the endoplasmic reticulum for secreted proteins
How is eukaryotic translation regulated in the cell?
- Translation regulated by signal sequences, recognized by signal recognition particles (SRPs) that block elongation & localize ribosomes to the ER
Explain the regulation of translation initiation through eIF4F and eIF2
- eIF4F: Phosphorylated eIF4E binds 5’ cap efficiently, upregulating translation & cell proliferation.
- Growth factors and hormones upregulate translation via eIF4E-BP phosphorylation through PI3K-mTOR pathway.
- Ligand binding to cell membrane receptors initiates PI3K cascade → phosphorylation of eIF4E-BP.
- Phosphorylated eIF4E-BP prevents eIF4E sequestration, promoting translation and cell proliferation.
- eIF2: eIF-2 is composed of subunits (α, β, γ) and can be phosphorylated at eIF2α.
- Phosphorylated eIF2α inhibits initiation & protein synthesis.
Describe the regulation of initiation through eIF2 in response to stress and viral infection.
- Stress response induces eIF2α phosphorylation by kinases (mGCN2, PERK, PKR, HRI) under various stress conditions
- Phosphorylated eIF2α inhibits eIF2β, blocking GTP exchange & initiation.
- Reduced translation conserves resources, alters gene expression, and can induce apoptosis
- Phosphorylated eIF2α induces translation of specific TFs (ATF4, ATF3, CHOP, GADD153)
How is translation elongation regulated through eEF2 and microRNAs (miRNAs)?
- eEF2 phosphorylation inhibits elongation; it’s phosphorylated in response to stress or via cell cycle regulatory proteins.
- Phosphorylated eEF2 slows down protein synthesis & conserves energy
- miRNAs binding to the target mRNA’s 3’ UTR can inhibit elongation and induce mRNA degradation
Explain the concept of Internal Ribosome Entry Sites (IRES) and their role in translation regulation
- IRES forms stem-loop structures common in viral mRNAs & mRNAs needed during stress, mitosis, and apoptosis.
- IRES allows translation when cap-dependent initiation is blocked.
- Components of the 40S ribosome, such as eIF2, eIF3, eIF5, and eIF5B, bind directly to IRES sequences.
The complex scans mRNA for AUG codon → leading to translation initiation
