The proteasome is a large multisubunit complex of approximately 2.5 MDa that mediates a wide range of physiological and pathological cellular processes by selectively degrading unnecessary proteins in eukaryotes. The structure of a proteasome is comprised of the catalytic core particle (CP) and two terminal regulatory particles (RPs). The CP (also known as the 20S proteasome) is a barrel shaped multisubunit complex (approximately 750 kDa) consisting of four axially stacked heptameric rings (two outer α-rings and two inner β-rings) with 7 subunits in each ring, where three β subunits (β1, β2 and β5) contain catalytically active threonine residues and are associated with caspase-like, trypsin-like and chymotrypsin-like activities respectively.