Large, multisubunit proteases (proteins which digest other proteins) which are found either free in the cytoplasm of the cell or attached to the surface of the endoplasmic reticulum. They are found in nearly all types of cells in high abundance, and have a fundamental role in the steady state recycling and turnover of proteins. It is important that cells have a way of destroying old proteins, because proteins eventually degrade and become non-functional. Additionally, certain proteins have only temporary usefulness and it behooves the cell to remove them after they are unnecessary.

Proteasomes recognize, unfold, and digest a protein that has been tagged for recycling by ubiquitin. The proteasome complex is made up of two functional parts:

  • the 19S (S - Svedburg, a unit of mass based on the sedimentation coefficient) capping subunit unfolds the protein and feeds it to the protease.
  • the 20S proteasome core contains the actual protease.

The proteasome core is believed to have evolved prior to the ubiquitin tagging machinery, suggesting that tagging evolved later to make digestion more efficient. The crystal structure is now available for 20S core proteasomes from several bacterial and a yeast source. The enzymes are cylindrical with active sites near the walls of a large inner cavity. The minimal proteasome is a homododecamer (12 subunits of the same type assembling together) made from two hexameric rings stacked head to head. In archaebacteria, each ring is made up of seven subunits instead of six. These have evolved into eukaryotic proteasomes, where each of the seven subunits comes from a different protein. High resolution microscopy data is available thus far for the 19S cap, but no direct atomic structures have been characterized.