Proteasome structural arrangement

Fig. 10.5. Molecular surface of the archaeal (A), the euka otic 20S (B) and the HsIV proteasome (C). The accessible surface is colored in blue, the clipped surface (along the cylinder axis) in white. To mark the position of the active sites, the complexes are shown with the bound inhibitor calpain (yellow). (A) The disorder of the first N-terminal residues in the archaeal a-subunits generates a channel in the structure of the CP, (B) whereas the asymmetric but well-defined arrangement of the a N-terminal tails seals the chamber in eukaryotic CPs. (C) The eubacterial "miniproteasome" has an open channel through which unfolded proteins and small peptides can access the proteolytic sites. (D) Ribbon plot of the free...

The 20S proteasome is a protease complex comprising 28 subunits. It is a barrelshaped structure formed by the axial stacking of four rings made up of two outer a rings and two inner / rings arranged in / / order. Proteasomes are members of the N-terminal nucleophile- (Ntn-) hydrolase superfamily [19]. Their N-terminal threonine residues are exposed as the nucleophile in peptide bond hydrolysis [20, 21]. In eukaryotic cells, three of the fS-type subunits have N-terminal threonine residues, are active and have specificities determined largely by the nature of their... 

Figure 23.5. 20S Proteasome. The 20S proteasome comprises 28 homologous subunits (a, red (5, blue), arranged in four rings of 7 subunits each. Some of the P subunits (highlighted in yellow) include protease active sites at the amino termini. The top view shows the approximate seven-fold symmetry of the structure.

Fig.I. Structure of the 20Sproteasome. (A) Low-resolution model (1.2 nm) ofthe20S proteasome derived from the crystal structure of the Thermoplasma proteasome (Lowe et al., 1995). The a subunits form the heptameric outer rings the /3 subunits, the inner rings. (B) The same structure cut open along the sevenfold axis to display the two antechambers (AC) and the central chamber (CC) with the 14 active sites (marked in black). The channel openings at the two ends of the cylinder are 1.3 nm in diameter. (C) Schematic representation of the arrangement of subunits within eukaryotic 20S pro-teasomes (Groll etal., 1997). Open boxes represent proteolytically inactive subunits filled boxes, proteolytically active subunits. The single C2 symmetry axis is shown. (D) Fold of a and /3 subunits of the Thermoplasma proteasome (Lowe et al, 1995). A pair of five-stranded /S sheets is flanked on both sides by a-helices. Helices (H) and strands (S) are numbered HO to H5 and SI toSlO. The ft subunits lack helix HO, which occupies the cleft on one side of the /3-sheet sandwich in the a subunits. The active-site threonine (Thr-1) of p subunits is shown in a ball-and-stick representation.

FIGURE 27-42 Three-dimensional structure of the eukaryotic proteasome. The 25S proteasome is highly conserved in all eukaryotes. The two subassemblies are the 20S core particle and the 1 9S regulatory particle, (a) (PDB ID 1 IRU) The core particle consists of four rings arranged to form a barrel-like structure. Each of the inner rings has seven different fj subunits (light blue), three of which have protease... 

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