The crystal structural analysis of the 20S proteasome from bovine liver


      Proteasome, a multisubunit proteolytic complex, is the central enzyme of nonlysosomal protein degradation. It is involved in various biological processes such as the degradation of misfolded proteins and the degradation of short-lived regulatory proteins.
   The proteolytic functions are controlled by a ubiquitin system (Fig. 1). In this system, the 26S proteasome breaks down polyubiquitinated substrates into short peptides. The eukaryotic 20S proteasome with a molecular weight of about 700 KDa is the catalytic portion of the 26S proteasome with a molecular weight of about 2000 KDa together with two PA700 regulatory complexes. The archaeon cells have a 20S proteasome functioning without the regulatory complex.

Fig. 1 Ubiquitin system

       Only higher eukaryotic cells gained the immune system to protect their cells from intracellular pathogens like viruses. The higher eukaryotic proteasome have a job to generate antigenic peptides (Fig. 2).

Fig. 2 Immune system and proteasomes

 The crystal structure of the archaeon T.acidophilum proteasome and that of the yeast S. cervisiae has been determined at 3.4 ? and 2.4 ? resolution in 1995 and 1997, respectively. These enzymes have cylindrical shapes and are composed of 28 subunits arranged in a particle as four heptameric rings. Archaeon proteasomes consists of 14 copies each of 2 distinct but related polypeptides,???and ?. On the other way eukaryotic proteasomes are built of 2 copies of each of 7 distinct ?- and ?-type subunits. Mammals contain seven different ? and ten different ? proteasome subunit genes. Despite the diversity in numbers of different subunits, the molecular shape of 20S proteasome is strongly conserved from archaea to human enzymes. In higher eukaryotes, the 20S proteasome that contains three inducible subunits, ?1i, ?2i and ?5i, is mentioned as an immunoproteasome and enhances proteasomal generation of MHC class I-binding peptides (Fig. 3).


Fig. 3 Caricatures of the 20S proteasomes

     PA28, an activator of 20S proteasome, associates with the 20S proteasome to form a football-like structure (Fig. 4) and hybrid proteasome with PA700, and the counterpart genes of PA28s are not present in budding yeast. 20S proteasomes in higher eukaryotes have special roles in the immune system. To clarify their immunological role and the mechanism underlying the interaction with PA28 that does not present in the yeast and to understand catalytic systems different from the yeast, we have worked on analyzing the crystal structure of the mammalian 20S proteasome.


Fig. 4 Composite models of the 26S proteasome (a)
and the football-like proteasome (b) based on electron microscopy.


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Fig. 6 Seven α-type subunits (α ring)

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Fig. 7 Seven β-type subunits (β ring)