Figure 1

Illustration of the basic set of reactions in protein modification by ubiquitin. Processing of precursor molecules is necessary for the exposure of the conjugation site in most cases. Following its activation by the action of an E1 enzyme, the mature molecule is transferred to an E2 enzyme, which catalyzes the conjugation to the target protein. The latter action may require another "ligating" E3 enzyme. This conjugation process is balanced by deconjugation, which is mediated by a number of different enzymes. Degradation occurs in the 26S core proteasome, which contains multiple proteolytic sites within its two central rings. Peptides produced by the proteasome are released and rapidly degraded to amino acids by peptidases in the cytoplasm or transported to the endoplasmic reticulum and used in the presentation of class I antigens. The ubiquitin is not degraded but is released and reused.