Proteasomes

Proteasomes are large multi-enzyme complexes that digest endogenous proteins, such as...
transcription factors, cell cycle cyclins, virus coded proteins, improperly folded proteins due to translation errors (made by faulty genes) and proteins damaged by cytosol molecules... to short peptides, followed by --> hydrolysis of these peptides via cytoplasmic exopeptidases.
Protein Digestion... begins when cells add small polypeptide (ubiquitin) to a protein to be digested; addition of uniquitin targets a protein's entry into a Proteasome complex.
The Proteasome is a barrel-shaped structure made of a lid, a base and 4 stacked protein rings with trypsin, chymotrypsin, and caspace proteolytic activity.

At left is a side view of the 20S proteasome. Active sites are formed at the N-terminals of β1, β2, and β5, each of which has different substrate preferences.

At right is a cutaway stereoview showing how the active sites (yellow) are sequestered within the catalytic chamber.

(M. Rechsteiner, C.P. Hill, Trends Cell Biol, 15:27–33, 2005):
    from The Scientist: Vol. 9, Issue 9, page 26, May 9, 2005.

                        Mechanism of Action*

Following processing by an ubiquitin C-terminal hydrolase (UCH), Ub is conjugated to a specific substrate as a chain by the ubiquitin-activating (E1), -conjugating (E2), and -ligating (E3) enzymes. The chain targets the substrate to the 26S proteasome, which comprises one 20S proteolytic complex and two 19S regulatory complexes.

(from C.A. Ross, C.M. Pickart, Trends Cell Biol, 14:703–11, 2004)