DISTINCT 19-S AND 20-S SUBCOMPLEXES OF THE 26-S PROTEASOME AND THEIR DISTRIBUTION IN THE NUCLEUS AND THE CYTOPLASM

The 26 S proteolytic complex (''26 S proteasome'') is a macromolecular assembly thought to be involved in ATP- and ubiquitin-dependent prote in degradation in the cytoplasm of higher eukaryotic cells. This compl ex is composed of one 20 S cylinder particle (multicatalytic proteinas e, 20 S proteasome) and two cap-shaped 19 S particles comprising a set of polypeptides in the M(r) range of 35,000-110,000. Here we show tha t cell supernatant fractions contain both these two subunit complexes as distinct particles as well as assembled to 26 S proteasomes. We hav e separated and purified all three forms from Xenopus laevis oocytes a nd have determined their peptidase and protease activities. Using vari ous antibodies specific for either a constitutive p52 polypeptide of t he 19 S cap complex or for proteins of the 20 S cylinder der particle, we have immunolocalized these complexes in both the cytoplasm and the nucleus of diverse species and cell types. The occurrence of all thre e forms, the 26 S proteasome, the 20 S cylinder particle, and the 19 S cap complex in the nucleoplasm has also been demonstrated in analyses of isolated giant nuclei from Xenopus oocytes. In addition, we show t hat the 19 S and 20 S subcomplexes can be released from 26 S proteasom es by ATP depletion and that readdition of ATP to 19 S and 20 S partic les in cell extracts leads to the reformation of 26 S proteasomes. We discuss that all three particles (19 S, 20 S, and 26 S) exist in a dyn amic equilibrium in both cell compartments and serve cytoplasmic as we ll as nucleus-specific functions.