STRUCTURE AND ASSEMBLY OF THE 20S PROTEASOME

The barrel-shaped 20S proteasome is one of the two components of a lar ger 26S particle, the multicatalytic 2000-kDa protease complex. The pr oteolytic sites are located in the inner chamber of the 20S particle a nd are only accessible via narrow entrances. This paper reviews the cu rrent knowledge concerning proteasome formation? proteolytic activitie s, structural aspects and assembly. Eukaryotic proteasomes are made up by four rings each of which contains seven different subunits occurri ng at fixed positions. While the outer rings contain alpha-type subuni ts, the inner ones comprise beta-type subunits. The current assembly m odel for eukaryotic 20S proteasomes is based upon the detection of 13S and 16S intermediates, respectively, in addition to previous findings with archaebacterial and eubacterial proteasome assembly. The availab le data suggest a cooperative assembly of the alpha-type and beta-type subunits into half proteasome-like complexes followed by dimerization into proteasomes. During or after dimerization of half proteasomes, t he beta-type subunits are processed. The prosequence of the beta-type subunits is essential for the assembly process and prevents protease a ctivity of immature proteasomes.