Structure of the 20S Proteasome

The structure of the 20S proteasome (fig. 2.16) from Thermoplasma acidophilum displays four rings stacked upon each other surroimding a central cavity in which pro-... 

The presence of the protease center in the central cavity ensures that the proteolysis is compartmentalized and shielded from the surrounding media. The substrate proteins are accessible to the proteolytic center only via a ring-shaped opening at the end of the 20S proteasome and require the assistance of the 19S particle. The structure of the 20S proteasome also indicates that proteins are accessible to the catalytic center only in the imfolded state. 

The structure of the 20S proteasome (Fig. 2.13) from Thermoplasma acidophilum displays four rings stacked upon each other surrounding a central cavity in which proteolysis takes place. An N-terminal threonine has been identified as an essential active site residue of the protease center. The OH-group of the threonine functions as a nucleophile during hydrolysis of the petide bond. A similar mechanism of hydrolysis has been shown for other hydrolases, which, because of this property, are now included in the family of N-terminal nucleophile hydrolases. For some /1-subunits of eucaryotes the N-terminal threonine is generated by autoproteolysis of an N-terminal prosequence. 

Groll, M., Nazif, T., Huber, R., and Bogyo, M. Probing structural determinants distal to the site of hydrolysis that control substrate specificity of the 20S proteasome. Chem. 

Whereas the structure and function of the 20S proteasome have been elucidated in great detail (for review see Baumeister et al 1998), the 19S regulator is understood only dimly at present. Structural studies are hampered by the low intrinsic stability of this assembly and extensive remodelling, which makes it notoriously difficult to obtain homogeneous preparations. Nevertheless, analyses of yeast. Drosophila and human 26S... 

In this chapter, we review the current knowledge of the structure, assembly and function of the 20S proteasome and its regulators in prokaryotic and eukaryotic cells. 

TMC-95A, a cydic peptide metabolite from Apiospra montagnei, is a potent competitive inhibitor of all active sites and forms characteristic hydrogen bonds with the protein backbone. The crystal structure of the yeast 20S proteasome in complex with TMC-95A indicates a non-covalent linkage to the active y -subunits the N-terminal threonine residues are not modified. The TMC-95A backbone adopts a -conformation and extends the -strand SI by the generation of an antiparallel P-sheet. This stmcture is similar to that seen with the aldehyde and epoxyketone inhibitors. An interactions of TMC-95A are formed with main-chain atoms and strictly conserved residues of the 20S proteasome. 

Groll, M., Koguchi, Y., Huber, R., Kohno, j.. Crystal structure of the 20 S proteasome TMC-95A complex a non-covalent proteasome inhibitor. J. Mol. 

Figure 19 The eukaryotic 26 S proteasome. Subunit compositions of the 19 S regulatory particle of Saccharomyces cerevisiae is shown on the left. The a and p rings of the 20 S proteasome, each of which consists of seven different subunits, are included to indicate how the base 19 S complex is linked to the core 20 S protease complex. The crystal structure of the 20s degradation chamber is shown in both side and top views (figure made using PDB 1RYP).

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.

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