**2.2.2 The 20S proteasome subunit**

The 20S proteasome subunit consists of two outer and two inner rings that are stacked to form a cylindrical structure with three compartments [Lowe et al., 1995]. Each outer ring has seven alpha-subunits (α1 to α7), whereas each inner ring contains seven beta-subunits (β1 to β7) (Fig 4).

Fig. 4. The 20S proteasome.

Fig. 3. The 26S proteasome, composed of two regulatory 19S and one catalytic 20S subunits. is required for successful entering into the 20S subunit [Pickart, 2000]. A molecular gate (Nterminal tail of the α3-subunit) also guards the opening, but it is constitutively open when the 19S regulatory units are bound to the 20S proteasome [Groll, et al., 2000]. There are also multiple different 11S regulatory complexes that can replace the 19S regulator [Hill et al, 2002]. These alternate regulators do not have ATPase function and do not bind polyubiquitin chains. Proteasomes with 11S substitutions for 19S regulators have higher

The 20S proteasome subunit consists of two outer and two inner rings that are stacked to form a cylindrical structure with three compartments [Lowe et al., 1995]. Each outer ring has seven alpha-subunits (α1 to α7), whereas each inner ring contains seven beta-subunits (β1 to

levels of proteolytic activity [Cascio et al, 2002; Fruh et al, 1994].

**2.2.2 The 20S proteasome subunit** 

Fig. 4. The 20S proteasome.

β7) (Fig 4).

The 20S proteasome complex has chymotryptic, tryptic, and peptidylglutamyl-like activities [Ciechanover, 2005; Orlowski, 1990]. It is conformationally flexible with active catalytic sites located on the inner surface of the cylinder where protein substrates bind. Proteins unfolded and without Ub tag, enter the inner chamber, where they are hydrolyzed by six active proteolytic sites on the - β subunits (two sites each on the β1-, β2-, and β5-subunits) into small polypeptides ranging from three to 22 amino acids in length. Proteins cannot enter the inner cylinder through the outer walls of the 20S proteasome because the gaps between the rings are tight [Lowe et al., 1995; Stein et al., 1996]. In eukaryotic cells, 26S proteasome are localized both in the cytoplasm and in the nucleus. This distribution is tissue-specific [Lowe et al., 1995].
