The 20S CP presents a a7b7b7a7 barrel-like structure and was shown to exist in the eukaryotic cell as four different subtypes, depending on the subsets of incorporated catalytic beta subunits. In the standard proteasome (sP20S), the two b rings each contain three standard catalytic subunits, b1, b2, and b5, which are replaced by distinct immunosubunits, b1i, b2i, and b5i, in the immunoproteasome (iP20S), respectively. Two intermedi- ate 20S CP subtypes, b5i 20S proteasome (b5i P20S) and b1ib5i 20S proteasome (b1ib5i P20S), bearing a mixed incorporation of standard and immunosubunits, b1, b2, b5i and b1i, b2, b5i, respectively, have also been identified in a wide range of cell types and tissues (Guillaume et al, 2010). / a=alpha, b=beta , for proteasome subunit composition)
The catalytic subunits are responsible for the three proteasome proteolytic activities (trypsin like, chymotrypsin like, and caspase like), which can be modulated by the replacement of standard subunits by immunosubunits.
The iP20S is induced during the immune response in mammals but also exists in various amounts as constitutive proteasome complexes, depending on tissues or cell type
One 20S CP can interact at its two sides with either two identical regulators or two different ones, thus forming hybrid proteasomes (Tanahashi et al, 2000). The most studied regulator, the 19S RP, is involved in the recognition, the unfolding, and the translocation of poly-ubiquitinated substrates into the 20S CP for degradation. In addition to the 19S RP, PA28ab and PA28c RPs are abundant 20S proteasome-associated regulators pres- ent in the cytosol and the nucleus, respectively (Drews et al, 2007; Fabre et al, 2013). They catalyze protein degradation through an ubiquitin-independent pathway, which still needs to be completely clarified (Stadtmueller