Architecture of the proteasome

Proteasomes are abundant in the cytosol and nucleoplasm. The 20S core particles are cylinders with three proteolytic activities: chymotrypsin-like, trypsin-like and peptidylglutamyl-peptide hydrolysing (caspase-like). The active sites are inside the barrel, encoded by the β subunits. Each 20S core has 19S caps, regulatory particles (RP) at one or both ends.