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Proteasome

Cartoon representation of a proteasome. Its active sites are sheltered inside the tube (blue). The caps (red; in this case, 11S regulatory particles) on the ends regulate entry into the destruction chamber, where the protein is degraded.
Top view of the proteasome above.

Proteasomes are protein complexes which degrade ubiquitin-tagged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Enzymes that help such reactions are called proteases.

Proteasomes are part of a major mechanism by which cells regulate the concentration of particular proteins and degrade misfolded proteins. Proteins are tagged for degradation with 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.[1] The degradation process yields peptides of about seven to eight amino acids long, which can then be further degraded into shorter amino acid sequences and used in synthesizing new proteins.[1]

Proteasomes are found inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, proteasomes are located both in the nucleus and in the cytoplasm.[2]

In structure, the proteasome is a cylindrical complex containing a "core" of four stacked rings forming a central pore. Each ring is composed of seven individual proteins. The inner two rings are made of seven β subunits that contain three to seven 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. 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 particles 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 ubiquitin–proteasome system.[3]

The proteasomal degradation pathway is essential for many cellular processes, including the cell cycle, the regulation of gene expression, and responses to oxidative stress. 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.[4]

  1. ^ a b Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipursky SL, Darnell J (2004). "3". Molecular cell biology (5th ed.). New York: W.H. Freeman and CO. pp. 66–72. ISBN 978-0-7167-4366-8.
  2. ^ Peters JM, Franke WW, Kleinschmidt JA (March 1994). "Distinct 19 S and 20 S subcomplexes of the 26 S proteasome and their distribution in the nucleus and the cytoplasm". The Journal of Biological Chemistry. 269 (10): 7709–18. doi:10.1016/S0021-9258(17)37345-3. PMID 8125997.
  3. ^ Nassif, Nicholas D.; Cambray, Samantha E.; Kraut, Daniel A. (May 2014). "Slipping up: Partial substrate degradation by ATP-dependent proteases". IUBMB Life. 66 (5): 309–317. doi:10.1002/iub.1271. PMID 24823973. S2CID 29860298.
  4. ^ Nobel Prize Committee (2004). "Nobel Prize Awardees in Chemistry, 2004". Retrieved 11 December 2006.

Proteasome

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