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proteasome A large complex of proteins found in the cytoplasm of cells that degrades cell proteins into small peptide units. Proteasomes participate in the disposal of damaged proteins and the removal of proteins that have reached the end of their functional life in the cell. They thus play a vital part both in the cell's response to stress and in the day-to-day running of the cell. A proteasome consists of a hollow cylinder of protease enzymes, each with its catalytic site facing the internal cylinder wall. Another protein complex guards the entrance to the cylinder. Proteins destined for degradation are usually tagged by a small protein, ubiquitin, and unfolded before entering the proteasome, where they are cleaved into peptides. However, the proteasome is thought to identify 'normal' proteins with short half-lives according to their N-terminal amino acid sequence.