Proteolytic processing and assembly of the C5 subunit into the proteasome complex

Assembly of mammalian 20 S proteasomes from individual subunits is beginnin g to be investigated. Proteasomes are made of four heptameric rings in the configuration alpha 7 beta 7 beta 7 alpha 7. By using anti-proteasome and a nti-subunit-specific antibodies, we characterized the processing and assemb ly of the beta subunit C5. The C5 precursor (25 kDa) remains as a free non- assembled polypeptide in the cell. The conversion of the C5 precursor to ma ture C5 (23 kDa) occurs concomitantly with its incorporation into 15 S prot easome intermediate and 20 S mature proteasome complexes. This processing i s dependent on proteasome activity and takes place in the cytosol. These re sults are not fully compatible with the hypothesis that postulates that ass embly of proteasomes takes place via a "half-proteasome" intermediate that contains one full alpha-ring and one full beta-ring of unprocessed beta sub unit precursors.