Disease-Associated Prion Protein Oligomers Inhibit the 26S Proteasome

• The mechanism of cell death in prion disease is unknown but is associated with the production of a misfolded conformer of the prion protein.
• We report that disease-associated prion protein specifically inhibits the proteolytic subunits of the 26S proteasome.
  
• Using reporter substrates, fluorogenic peptides, and an activity probe for the subunits, this inhibitory effect was demonstrated in pure 26S proteasome and three different cell lines.
• By challenge with recombinant prion and other amyloidogenic proteins, we demonstrate that only the prion protein in a nonnative sheet conformation inhibits the 26S proteasome at stoichiometric concentrations.
  
• Preincubation with an antibody specific for aggregation intermediates abrogates this inhibition, consistent with an oligomeric species mediating this effect. We also present evidence for a direct relationship between prion neuropathology and impairment of the ubiquitin-proteasome system (UPS) in prion-infected UPS-reporter mice.
  
• Together, these data suggest a mechanism for intracellular neurotoxicity mediated by oligomers of misfolded prion protein. new proteins. that recognize polyubiquitin tags attached to protein substrates and initiate the degradation process. The overall system of ubiquitination and proteasomal degradation is known as the

• Proteasomes are large protein complexes inside all eukaryotes and archaea, as well as in some bacteria.

1. In eukaryotes, they are located in the nucleus and the cytoplasm.^[1] The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Enzymes that carry out such reactions are called proteases.
   
2. Proteasomes are part of a major mechanism by which cells regulate the concentration of particular proteins and degrade misfolded proteins. The degradation process yields peptides of about seven to eight amino acids long, which can then be further degraded into amino acids and used in synthesizing^[2]
3. Proteins are tagged for degradation by a small protein called ubiquitin. The tagging reaction is catalyzed by enzymes called ubiquitin ligases. Once a protein is tagged with a single ubiquitin molecule, this is a signal to other ligases to attach additional ubiquitin molecules. The result is a polyubiquitin chain that is bound by the proteasome, allowing it to degrade the tagged protein.^[2]
4. In structure, the proteasome is a large barrel-like complex containing a "core" of four stacked rings around a central pore.
   
5. Each ring is composed of seven individual proteins. The inner two rings are made of seven β subunits that contain the six protease active sites. These sites are located on the interior surface of the rings, so that the target protein must enter the central pore before it is degraded.
   
6. The outer two rings each contain seven α subunits whose function is to maintain a "gate" through which proteins enter the barrel. These α subunits are controlled by binding to "cap" structures or regulatory particlesubiquitin-proteasome system.
7. The proteasomal degradation pathway is essential for many cellular processes, including the cell cycle, the regulation of gene expression, and responses to oxidative stress.
   
8. The importance of proteolytic degradation inside cells and the role of ubiquitin in proteolytic pathways was acknowledged in the award of the 2004 Nobel Prize in Chemistry to Aaron Ciechanover, Avram Hershko and Irwin Rose.^[3]

• amyloidogenic proteins:-The various protein deposits of brain amyloidosis share common ultrastructural, biophysical, and histological properties. These amyloidogenic deposits can be composed of distinct proteins, which are conceptually associated with different diseases.
  

1. Amyloidogenic proteins are typically soluble monomeric precursors, which undergo remarkable conformation changes associated with the polymerization into 8- to 10-nm wide fibrils, which culminate in the formation of amyloid aggregates.
2. Some amyloidogenic inclusions are extracellular, such as senile plaques of Alzheimer’s disease, which are composed of amyloid beta (Abeta) peptides.
3. Intracytoplasmic amyloid aggregates, such as neurofibrillary tangles in Alzheimer’s disease and Lewy bodies in Parkinson’s disease, are composed of the proteins tau and alpha-synuclein, respectively. These proteins are directly linked to the etiology of spectrum of neurodegenerative diseases, coining the terms "tauopathies" and "synucleinopathies."