Ubiquitin, protein degradation and beyond

• Ubiquitin:

  • Small protein - 76 amino acids
  • Found in all eukaryotes, highly conserved
• Ubiquitin acts as a covalent modifier of proteins:

  Side chain of selected lysine residues in the protein of interest form a covalent isopeptide bond with the C terminal of ubiquitin (glycine 76)
• Polyubiquitin chains form by the ubiquitination of

  Ubiquitin
• Ubiquitination enzymes:

  • E1 - ubiquitin activating enzyme
  • E2 - ubiquitin conjugating enzyme
  • E3 - ubiquitin ligase enzymes
• Ubiquitin forms a covalent thiolester linkage with E1 and E2 enzymes - binds to cystine residue
• Deubiquitinating enzymes (DUB) reverse ubiquitination
• Ubiquitin acts as a co-factor for selective protein degradation by the 26S proteasome
• Lysine 48 linked polyubiquitin chains are recognised by the large multi sub unit protease called 26S proteasome
• 26S proteasome substrate is degraded to short peptides and ubiquitin is recycled by proteasome associated deubiquitinating enzymes
• The 26S proteasome

  • Abundant multi subunit protease complex in the cytoplasm and nucleus
  • Assembly requires ATP
• 26S proteasome subunits

  • 19S cap - binds to the lysine 48 polyubiquitin chain and uses ATP to unfold the substrate
  • 20S core - contains threonine protease subunits which digest the substrate
• • In rare cases serine or threonine can be modified with ubiquitin - ester bonds
  • Phosphorylation (serine) and acetylation (lysine) of ubiquitin expands the code