Proteasome: A protein degradation "machine" within the cell that can digest a variety of proteins into short polypeptides and amino acids. The proteasome is itself made up of proteins. It requires ATP to work. It is hollow and has openings at both ends to allow entry of the protein to be digested.
A human cell contains about 30,000 proteasomes. These barrel-formed structures can break down practically all proteins to 7-9-amino-acid-long peptides. The active surface of the proteasome is within the barrel where it is shielded from the rest of the cell. The only way in to the active surface is via the "lock", which recognises proteins tagged with ubiquitin, denatures these proteins and admits them to the barrel for disassembly once the ubiquitin label has been removed. The peptides formed are released from the other end of the proteasome.
Proteasomes digest mainly endogenous proteins, those synthesized within the cell, as opposed to extracellular proteins such as the proteins in blood plasma. The digestion of protein removes excess enzymes and transcription factors and supplies amino acids for new protein synthesis.