Biogenesis of the 20S Proteasome

Proteasome jS-subunits are synthesized with N-terminal extensions and are inactive because a free N-terminal threonine is required for peptide-bond hydrolysis [130]. The precursor jS-subunits assemble with a-subunits to form half proteasomes com- 

The N-terminal extensions are removed thereby generating a new unblocked N-terminal threonine in the catalytically active yS-subunits. A small accessory protein called Umpl in yeast or proteassemblin in mammalian cells assists in the final assembly of the 20S proteasome [132], Interestingly Umpl/POMP is apparently trapped in the proteasome s central chamber and degraded upon maturation of the enzyme [133]. 

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