Proteasomes (multi-protease complexes) as 20 S ring-shaped particles in a variety of eukaryotic cells

Latent multicatalytic protease complexes, named proteasomes, were purified to apparent homogeneity from various eukaryotic sources, such as human, rat, and chicken liver, Xenopus laevis ovary, and yeast (Saccharomyces cerevisiae), and their functional and structural properties were compared. They showed latency in breakdown of [methyl-3H]casein, but were greatly activated in various ways, such as by addition of polylysine. They all degraded three types of fluorogenic oligopeptides at the carboxyl side of basic, neutral, and acidic amino acids, and the three cleavage reactions showed different spectra for inhibition, suggesting that they had three distinct active sites. The proteasomes all seemed to be seryl endopeptidases with similar pH optima in the weakly alkaline region. Their physiochemical properties, such as their sedimentation coefficients (19 S to 22 S), diffusion coefficients (2.0-2.6 X 10(-7) cm2 s-1), molecular masses (700-900 kDa), and circular dichroic spectra, were similar. Their amino acid compositions were also very similar. Electron microscopy showed that they had similar well-defined symmetrical morphology, appearing to be ring-shaped particles with a small hole in the center. All the proteasomes seemed to be multisubunit complexes consisting of 15-20 polypeptides with molecular masses of 22-33 kDa and isoelectric points of pH 3-10, but they showed species-specific differences in subunit multiplicity. Moreover, they differed immunologically, as shown by Ouchterlony tests and immunoblotting analyses, although cross-immunoreactivities of some subunits or domains were observed. These results indicate that the sizes and shapes of these proteasomes have been highly conserved during evolution, but that they show species-specific differences in immunoreactivities and subunit structures. Thus proteasomes with similar structure and function seem to be ubiquitously distributed in eukaryotic organisms ranging from man to yeast. This distribution implies the general importance of these proteasomes for proteolysis.