1-2-3. 3 ubiquitin-ligases (E1, E2, E3) attach a ubiquitin polypeptide to a protein targeted for digestion:  HSP-70 binds any misfolds & recruits E3-ligase to tag misfolded proteins.

4. The ubiquitinized protein binds to proteasome lid and enters central chamber where it is cleaved into peptides via tryptic, chymotryptic, & caspace proteolytic-like activity. The ubiquitins are recycled.

5. Cytosolic peptidases further digest the degraded peptides. 
     proteasome process: figure 1 and  figure 2. 
 

 protein life cycle animation (chaperonin & proteasomes)*^view@home