Proteasomes are large multi-subunit protease complexes that selectivel
y degrade intracellular proteins. Most of the proteins removed by thes
e proteases are tagged for destruction by ubiquitination. Proteasomes
have a role to play in controlling cellular processes, such as metabol
ism and the cell cycle, through signal-mediated proteolysis of key enz
ymes and regulatory proteins. They also operate in the stress response
, by removing abnormal proteins, and in the immune response, by genera
ting antigenic peptides.