Figure 2: Various assemblies of proteasome. The proteasome is a mono-capped or bi-capped cylindrical structure. The cylindrical core (20S or CP) is formed by two different types of protein subunits, α and β, which are arranged in four stacked heptameric rings enclosing a central cavity. The proteasome core particle can be capped with 19S or 11S activator complexes (a third activator complex Blm10/PA200 is not shown).