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)