Proteolytic Processing and Assembly of the C5 Subunit into the Proteasome Complex

Assembly of mammalian 20 S proteasomes from individual subunits is beginning to be investigated. Proteasomes are made of four heptameric rings in the configuration α7β7β7α7. By using anti-proteasome and anti-subunit-specific antibodies, we characterized the processing and assembly of the β subunit C5. The C5 precursor (25 kDa) remains as a free non-assembled polypeptide in the cell. The conversion of the C5 precursor to mature C5 (23 kDa) occurs concomitantly with its incorporation into 15 S proteasome intermediate and 20 S mature proteasome complexes. This processing is dependent on proteasome activity and takes place in the cytosol. These results are not fully compatible with the hypothesis that postulates that assembly of proteasomes takes place via a “half-proteasome” intermediate that contains one full α-ring and one full β-ring of unprocessed β subunit precursors.