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Eucaryotic Protein Synthesis. Eukaryotic mRNAs. See Figure 30.26 for the structure of the typical mRNA transcript Note the 5'-methyl-GTP cap and the poly A tail Cap is essential for mRNA binding and stabilizes mRNA by preventing degradation

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eukaryotic mrnas
Eukaryotic mRNAs

See Figure 30.26 for the structure of the typical mRNA transcript

  • Note the 5'-methyl-GTP cap and the poly A tail
  • Cap is essential for mRNA binding and stabilizes mRNA by preventing degradation
  • Poly A tail enhances stability and translational efficiency of mRNAs
  • Shine-Dalgarno sequence not present
initiation in eucaryotes
Initiation in eucaryotes
  • Family of at least 14 eukaryotic initiation factors
  • The initiator tRNA is a special one that carries only Met and functions only in initiation - it is called tRNAiMet but it is not formylated
eukaryotic initiation
Eukaryotic Initiation
  • Begins with formation of ternary complex of eIF-2, GTP and Met-tRNAiMet
  • 1) This binds to 40S ribosomal subunit:eIF-3:eIF1A complex to form the 43S preinitiation complex
    • Note no mRNA yet, so no codon association with Met-tRNAiMet
  • 2) mRNA then adds with several other factors, forming the 48Sinitiation complex (Fig. 33.23)
    • 48S initiation complex scans to find the first AUG (start) codon
  • 3) At AUG, 60S subunit adds to make 80S initiation complex (GTP is hydrolyzed)
regulation of initiation
Regulation of Initiation

Phosphorylation is the key, as usual

  • At least two proteins involved in initiation (Ribosomal protein S6 and eIF-4F) are activated by phosphorylation
  • But phosphorylation of eIF-2a causes it to bind all available eIF-2B and sequesters it, therefore translation is down-regulated by phosphorylation
elongation and termination
Elongation and Termination
  • Elongation is similar to procaryotic elongation:
    • EF1A homolog to EF-Tu, EF1B homolog to EF-Ts, EF2 homolog to EF-G
  • Termination even simpler: only one RF, binds with GTP at the termination codon
inhibitors of protein synthesis
Inhibitors of Protein Synthesis

Two important purposes to biochemists

  • These inhibitors (Figure 30.30) have helped unravel the mechanism of protein synthesis
  • Those that affect prokaryotic but not eukaryotic protein synthesis are effective antibiotics
  • Streptomycin - an aminoglycoside antibiotic - induces mRNA misreading. Resulting mutant proteins slow the rate of bacterial growth
  • Puromycin - binds at the A site of both prokaryotic and eukaryotic ribosomes, accepting the peptide chain from the P site, and terminating protein synthesis
diphtheria toxin
Diphtheria Toxin

An NAD+-dependent ADP ribosylase

  • One target of this enzyme is EF2
  • EF2 has a diphthamide (see Figure 33.27)
  • Toxin-mediated ADP-ribosylation of EF2 allows it to bind GTP but makes it inactive in protein synthesis
  • One toxin molecule ADP-ribosylates many EF2s, so just a little is lethal!
ricin
Ricin

from Ricinus communis (castor bean)

  • One of the most deadly substances known
  • A glycoprotein that is a disulfide-linked heterodimer of 30 kD subunits
  • The B subunit is a lectin (a class of proteins that binds specifically to glycoproteins & glycolipids)
  • Endocytosis followed by disulfide reduction releases A subunit, which catalytically inactivates the large subunit of ribosomes
ricin a subunit mechanism
Ricin A subunit mechanism
  • Ricin A chain specifically attacks a single, highly conserved adenosine near position 4324 in eukaryotic 28S RNA
  • N-glycosidase activity of A chain removes the adenosine base
  • Removal of this A (without cleaving the RNA chain) inactivates the large subunit of the ribosome
  • One ricin molecules can inactivate 50,000 ribosomes, killing the eukaryotic cell!